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Introduction

Renewable energy from intermittent and variable sources such as wind, wave, biomass, and solar is expected to make an increasing contribution to worldwide energy supplies [1].

According to a current report from 2014 [2], global renewable electricity generation rose approximately 240 terawatt hours (TWh) (+5.0% year-on-year) to reach nearly 5070 TWh.

Moreover, it is accounted for almost 22% of total power generation [2]. However, renewable energy sources are rather very expensive comparing to the primitive energy sources such as coal, oil/natural gas [3] and nuclear power. Furthermore, intermittency of renewable sources such as the wind and solar (photovoltaic (PV)) presents a major obstacle to their extensive application into the electricity grid [4]. The cost of electricity storage is an additional concern.

To date, only the water pumped storage can retain electricity using hydroelectric damp (STEP in French) at low cost but potential convenient locations are rare. Sodium/Sulfur batteries using beta alumina ceramic membrane also provide an attractive solution, and more than hundreds of MW are installed by NGK/TEPCO worldwide. For example, since decades, the batteries are combined with renewable energy sources as in Reunion Island. Biomass energy also exhibits some significant disadvantages i.e. greenhouse gas emission and solid wastes combustion [5,6]. Furthermore, vast quantities of heat energy are being released into the atmosphere as waste on a daily basis. It is estimated that roughly 3×10 13 kWh of heat energy is being lost as dissipative waste energy each year from US manufacturing alone [7]. A part of the waste heat is generated in a power plant by incomplete recycling of fuel combustion or chemical reaction. The essential quality of heat is not its amount but its value. Such valuable heat wastes can still be reused for some useful and economic purpose. The strategy to recover this heat depends on the waste temperature and the financial implications [8]. For optimal utilization and to ensure safe operation with a renewable resource, technological improvements are required. These include management of heat loss, improvements of the connectivity and increasing the amont of intermittent renewable energy to electricity networks.

Over last two decades, the management of renewable resources by mean of an electrolyzer-based management (EBM) has been established [9]. EBM offers a versatile means for optimizing the process of harnessing energy supplies derived from variable and/or Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 4 intermittent renewable resources. The principal components of the EBM system, shown in Fig. 1, are an electrolyzer, a gas storage system, and a fuel cell. A heat engine/generator combination could be also used. Both electrolyzer and fuel cell are high-current, low-voltage DC devices [9]. Consequently, the energy management significantly yields the overall energy conversion efficiency. (EBM) system configured for renewable-resource inputs [9].

Hydrogen can be considered as an essential element in the fuel cell and electrolysis applications (EBM system) in the renewable power plant. Before theavailable of natural gas, hydrogen mixed methane was produced by the action of water on hot coal (coke). Hydrogen can potentially play a significant role in our future energy systems -By 2019, the hydrogen generation market is predicted to reach $138.2 billion [10]. Like electricity, hydrogen is an energy carrier and has one of the highest energy density values per mass, a yield of 122 -140 kJ/g. Such energy density value is about three times greater than that of hydrocarbon fuel [11]. Moreover, hydrogen is a non-toxic and extremely environment-friendly, since water is the only exhaust product when hydrogen is converted into electricity.

Hydrogen could be produced using excess electricity from the electrical power plant of renewable energy sources, through electrolysis of water as illustrated in Fig. 1. An electric current is used to split water into oxygen and hydrogen. Hydrogen can then be utilized by the fuel cell to generate power/electricity producing only water and heat as byproducts. Another interesting possibility of hydrogen application is the conversion of CO 2 gas into methane (CH 4 ) according to the Sabatier reaction:

O H CH H CO 2 4 2 2 2 4 + → + …(eq. 1)
The CO 2 conversion has already been established. A 250 kW Power-to-Gas plant in Werlte, Germany currently produces 300 m 3 of CH 4 per day [12].

Electrolysis and fuel cell technologies have shown tremendous application potential.

However, further intensive studies are necessary to improve their performance and especially their stability for long-term use. The material stability determines the operating cost.

Historical background of electrochemical conversion technology

The benefits of electrochemical energy conversion devices in connection with renewable power plants [13][14][15][16][17] are many as the various types of electrolyzer/fuel cell-based technologies demonstrate. Table 1 shows an operating condition of different electrolyzers/fuel cells over a wide range of temperature, from ambient to over 1000°C, and using diverse materials as electrolytes and electrodes.

The early 1800s, William Nicholson and Anthony Carlisle observed the evolution of gasses during the voltaic pile replication experiments. This allowed them to discover the electrolysis phenomenon leading to water splitting [18]. Water splitting is a general term for a chemical reaction in which water is separated into oxygen and hydrogen. In 1939, the first large-scale water electrolysis plant, with a hydrogen generation capacity of 10,000 m 3 h -1 , went into operation. In 1948, the first pressurized industrial electrolyzer was manufactured by Zdansky and Lonza [19], whereas the first solid-oxide water electrolysis unit was developed in 1972 [20].

In 1838, William Robert Grove invented the first fuel cell. He deployed a combination of sheet iron, copper and porcelain plates, and a solution of sulfate of copper and dilute acid. He called such first fuel cell the "gas battery". Later, Johnson Matthey [21] developed the cell with a platinum catalyst, and used hydrogen to generate an electric current. The innovation of the fuel cell was based on reversing chemical reaction to that occurring during electrolysis of Table 1: Technical characteristics of different electrolysis and fuel cells [22][23][24]. water. In 1853, solid oxide-based fuel cells (SOFCs) came into existence after the discovery of substantial galvanic electrolyte by J.-M. Gaugain [25]. Wiedemann reported that at the end of the 19th century, the expression "solid electrolyte" was used more frequently [26]. In the 1930s, some changes were made to fuel cells, mostly concerning their design such as shape and size. In the case of 'Bacon Cell', developed by Francis Thomas Bacon [22] in 1932, the modifications, related to the introduction of nickel gauze electrodes and the use of potassium hydroxide as an electrolyte, were carried out. The Bacon Cell was the forerunner of the alkaline fuel cell (AFC). Later in 1939, the first alkaline electrolyte fuel cell was built. Bacon spent almost thirty more years to achieve an operational fuel cell. By 1959, he demonstrated a fuel cell that could generate 5kW of energy. In the late 1950s and early 1960s, NASA in collaboration with industrial partners began the development of fuel cell generators for Gemini manned space missions. The first proton exchange membrane (PEM) fuel cell unit, developed with the help of Willard Thomas Grubb at General Electric (GE), was the result of this collaboration. Furthermore, another GE researcher, Leonard Niedrach improved Grubb's PEMFC by using platinum as a catalyst [24].

From the mid-1960s, Shell was involved in the development of direct methanol fuel cell (DMFC) as the use of liquid fuel was considered to be a great advantage for vehicle applications. More than fifty years of research allowed the PEMFC and DMFC devices to estrabish as commercial products e.g. perfluorinated ionomer (PFI) membranes, Nafion® (Dupont de Nemours), and Dow membrane (Dow Chemical Co.) [27,28]. Very rapidly they have dominated the small market share in the portable, stationary and transport sectors (Balard, Navigant Research, etc.). However, it is well known that several significant problems remain; e.g. high cost due to the use of platinum, liquid water management and durability of the polymer electrolyte.

In the 1970s, because of important fuel shortage and then very high oil prices, the socalled First Oil Crisis, many national governments and large companies initiated research projects to develop more efficient and low cost forms of energy generation. Significant advances in Phosphoric acid fuel cell (PAFC) technology, in particular concerning its stability and performance, appeared as a direct result of these undertakings. For example, large

Stationary PAFC units for prime, off-grid power, including a 1 MW unit were developed by International Fuel Cells (IFC, later UTC power).

Consequently, a recent (2015) review of Bi et al., [41] indicts that a number of published papers concerning proton conducting oxides (indexed in Scopus) has significantly increased in recent years, almost three times over ten years. The authors compared the study percentage between the proton conducting fuel cell and proton conducting electrolyzer. They show that the few studies devoted for electrolyzer application address dilemma in choosing electrolyte materials with proper features of proton conductivity and chemical stability. Hence, the researchers are continuously seeking a "good" material. This means chemically, mechanically and structurally stable proton-conducting oxide showing long lifetime, can be used as an electrolyte and/or as an electrode. Besides, zirconates and nickelates appear as the most promising candidates for proton conducting application, and as an electrolyte and an electrode material, respectively. Consequently, they are the subject of this thesis. A review of their actual "status" is given in chapter 2.

Electrochemistry of fuel cell and electrolyzer: the difference between oxide and proton conduction mechanism

The principal reaction of a fuel cell and water electrolyzer is chemically inverse, given by equation 2.

( ) ( ) ( )

g 2 1 2 2 2 O H g O g H ⇔ + … (eq. 2)
The forward reaction between hydrogen gas and oxygen gas appears in the fuel cell system. Applying thermodynamic potentials to the above equation (eq. 2) at room temperature and atmosphere pressure, the total energy (∆H, enthalpy) of -285.82 kJ is required. Excess heat (T∆S, entropy) of the reaction, about 48.7 kJ, can be expelled to the environment.

Consequently, the amount of energy (∆G =∆H-T∆S, the change of Gibbs free energy) per mole of hydrogen is -237.1 kJ, which can be provided as the electrical energy. In an ideal case, the fuel energy is converted to electrical energy at an efficiency of 83% (∆G/∆H x 100%) [42].

In contrast, the reverse reaction is visible in the water electrolyzer system. In addition, the system needs energy: first to dissociate the water molecule, and then to expand the produced gasses. Considering the system operating at room temperature and under atmosphere pressure, Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 10 the total electrical energy demand (∆G) and energy from environment (T∆S) are approximately 237.13 kJ and 48.7 kJ, respectively [42].

Figure 2 presents schematics of an oxygen-conducting SOFC and of a proton-conducting fuel cell and electrolyzer. Note: Proton Conducting Fuel Cell and Proton Conducting Electrolysis Cell are abbreviated as PCFC and PCEC, respectively. In keeping with the development of solid oxide-based fuel cell technology, oxygen-ion conducting cell will be discussed first. The solid oxide fuel cell, Fig. 2a consists of two electrodes generally cermets sandwiched around a dense ceramic electrolyte such as the most used zirconium oxide stabilized with yttrium oxide (YSZ). Fuel gas -hydrogen -is fed into the anode of the fuel cell. Oxygen, usually carrying by air, enters the cell via the cathode. The anode disperses the hydrogen gas equally over its whole surface and conducts the electrons that are freed from hydrogen molecule, to be used as power in the external circuit. The cathode contributes the oxygen fed to its surface and conducts the electrons back from the external circuit, where they can recombine with oxygen ions (eq. 3). The oxygen ions pass across the electrolyte and react with hydrogen to form water (eq. 4). The electrolyte determines the operating temperature of the fuel cell, separates two gases and is used to prevent two electrodes from coming into electronic contact blocking electrons. Typically, this system works at high temperature range (HT-SOFC), between 800ºC and 1000°C. The configuration of PCFC (Fig. 2b) and PCEC (Fig. 2c) is similar to oxygen ion conducting fuel cell. However, the principal operating mechanism is different. In the case of PCFC, Fig. 2b, hydrogen fuel gas is fed into the anode side and on oxidation releases the electrons (Eq. 6). The released electrons are transported to the cathode via an external circuit.

At the cathode, the oxidant air is reduced and consumes the transported electrons to form water with H 2 supplied by an electrolyte (Eq. 7). The PCFC electrolyte is typically a ceramic material e.g. Ln-doped BaZrO 3 perovskite, which shows high proton conductivity in the temperature range of 400-600°. The electrodes are commonly used of cermets.

In the case of PCEC, Fig. 2c, the water (steam) is fed into anode side, where it is electrochemically split into oxygen, and protons (eq. 9). The released electrons are transported to the cathode via an external circuit while the protons migrate through the dense electrolyte layer i.e. remarkable Ln-doped cerates/zirconates [43]. At the cathode side, the protons combine with electrons to form dry H 2 (eq. 10), which is the desired product. The oxygen gas as a byproduct is discharged at the anode side. Note, since the produced hydrogen is very reactive, it can be used to transform the CO 2 into CH 4 or other chemicals in the cathode chambers. The CO 2 stable cathodes are then requested to be used in PCEC/CO 2 converters.

The Lei Bi et al. [44] review describes the complexity of reaction steps occurring at a single-phase anode of proton-conducting electrolyzer (Fig. 3). The single-phase anode model provides both pathways for proton (H + ) and electron (e -), enabling the electrochemical reactions to occur on the whole electrode surface. describe the proton migration to triple phase boundary (TPB) sites, which are regarded as the Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 12 reaction active sites. Note, oxygen-ions do not participate in the reactions at TPB. The proton transfer process, including the proton transfer from decomposed H 2 O to TPB (step 8) and the proton migration to the electrolyte from the TPB sites (step 9), is the rate-limiting step. This mechanism is rather different from that of HT-SOFCs (and from that for proton-conducting SOFCs), in which oxygen vacancies also have to diffuse to the TPB area.

Step 1:

ad O H g O H 2 2 ) ( →
Step 2:

+ -+ → ad ad H OH O H 2
Step 3:

+ - - + → ad ad H O OH 2
Step 4:

- - → - ad ad O e O 2
Step 5:

ad ad O e O → - - Step 6: ) ( 2 2 ad O O ad →
Step 7: and electron conduction for proton-conducting PCECs after [44].

PCFCs and PCECs under water vapor pressure: advantages and drawbacks

In advanced applications, the proton-conducting PCFC and PCEC have operated under moderate to high temperature/pressure. Conventional electrolyzers, for example, which are capable to produce the hydrogen under 10-30 bar of H 2 O, are available on the market [45].

The management of water/gas pressure is well established in nuclear plant industry: for instance a boiling water (nuclear) reactor (BWR, ancient generation) operates at 70 bar and 285°C, and a pressurized water reactor (PWR, present generation) works at 150 bar and 320°C [46,47]. In the future in advanced nuclear plant, the supercritical water cooled reactor (SCWR) should operate at 450°C and under 250 bar. This could be considered as a high net power cycle efficiency system, providing not only the electricity need but also deliver relatively high-temperature process heat [47].

To understand the advantages and drawbacks of steam applications, the properties of water at elevated temperature and under pressure should be considered. As presented in Fig. 4 the supercritical water (T c = 373.95°C and P c = 220.5 bar), has both liquid-like and gas-like characteristics with high diffusivity and good heat-transporting properties. The water behaves as a non-polar solvent according to a significant decrease of dielectric constant [48]. The degree of hydrogen bonding decreases when the temperature increases [49]. As such supercritical water seen as a dense gas offering a medium with excellent transport properties, while possessing complete solvency for most of gases [50,51]. In a recent study, the chemical potential gradient (~3 x 10 -4 mol.s -1 .m -2 ) of mixed conducting ceramics (BZY) under water pressure at 750°C has been reported by Einar et al. [52]. (note: the diagram is not to scale and three data points of reactor's working condition in hydrogen production nuclear plant were collected from literature [46]).

However, the use of high-pressure steam can also have serious drawbacks such as high corrosion potential. In fact, water corrosion has received considerable attention during the past decades and convenient stable Ni/Cr-based steel have been developed [53,54]. The corrosion of stainless steel and nickel-based alloy during the process of supercritical water oxidation (SCWO) and aqueous solutions were discussed by Kritzer et al. [55,56]. They reported that the solubility and dissociation processes of both attacking inorganic ions (e.g.

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 14

- 2 3
CO , OH -, H + , F -, Cl -, Br -) and corrosion products (e.g. Cr 2 O 3 , CrOOH, NiCl 2 , NiBr 2 etc.) play the most important role in corrosion at high-temperature water. In the other words, with high density and high ionic product of the solvent, significant solubility and dissociation processes were observed. These accelerated electrochemical forms of corrosion. The characteristics of corrosion films were related to the physical and chemical properties of water: corrosion layer was lower by orders of magnitude, at the highest supercritical experimental of 500°C, compared with corrosion at subcritical 300°C [57][58][59][60][61]. The occurrence temperature of a typical corrosion, in the case of both general dissolution and stress corrosion cracking (SCC), was determined as 250-300°C. For pitting corrosion, a localized form of corrosion occurring in the passive state of a metal, the occurrence temperature was 150-200°C with inhibitors of hydroxide and/or carbonates ions. Due to its stochastic and non-predictive nature, pitting is a very dangerous form of corrosion.

In the case of ceramic materials, there are two potential corrosion mechanisms: ion exchange and chemical reactions.

In the case of ion-exchange process, some elements/cations for example K + , Ni + , Li + , Mg 2+ , Mn 4+ , etc. (ions with high diffusional mobility) exchanged with proton/protonic species, and were released into a solution. The species size mismatch led to cracks that increased the surface reaction and accelerated the corrosion. For example, the β-spodumene (LAS) matrix composite -a very interesting material because of its low and controlled thermal expansion -was corroded by proton ionic exchange reaction: (Na + )-H + giving rise to contractions/cracks and then crumbling [53].

External corrosion by chemical reactions, is often observed in the case of highly hydroscopic alkaline/alkaline-earths-based compounds, for example the Ln-doped (Ba/Sr)(Ce/Zr)O 3-δ electrolytes [62][63][64][65]. The authors show that treatment of the ceramics exposed to medium/high pressure (10-80 bar) in the middle-temperature range (300-500°C) gives rise to unexpected secondary phases, i.e. CeO 2 , Ba/SrO, Ba/Sr(OH) 2 and Ba/SrCO 3 present mostly on the surface of the anhydrous samples. In fact, such pressure/temperature treatment induces the surface hydroxylation (and carbonation reactions when CO 2 is dissolved in water), as shown in equation 12 and 13, respectively. I may be note that such corrosion layer can be present not only on the surface but also in the bulk. Different electrolytes/ electrodes show different degradation/corrosion levels [62][63][64][65] that allows dealing with their long-term stability determination.

Proton defects and diffusion in solid oxide-based anhydrous materials

Water molecules split at the surface of solid oxide ceramic, they enter or penetrate to the crystal lattice in the dissociated form according to Equation 14 based on the Kröger-Vink notation:

• • • + ′ ↔ + H H O V g O H 0 2 ) ( …(eq. 14)
where V is an oxygen vacancy; a positive charge is represented by a dot (•) and a negative charge by a prime (′).

The concentration and mobility of inserted protons determine the proton conductivity of a material. The hydroxide/proton ion concentration is proportional to the square root of the water partial pressure (or hydrogen partial pressure) in the surrounding atmosphere according to the Sievert's law behavior [66,67]. The proton concentration increases with increase in the water pressure up to a certain value. This varies depending on a material.

Due to the tiny size of a proton -just between that of electron and the smallest ion, Li +the physics and chemistry of a proton [68], in the solid state, are unique. The proton is very reactive with its environment. The most common case is of the proton's penetration of the covalent shell of neighboring acceptor ions, to form the classical XHn moieties (e.g. OH -, H 3 O + ). However, the proton can also interact with two acceptors giving rise to the Hydrogen bonds or even with more acceptors that lead to the presence of the so-called ionic, free proton.

The proton mobility in solid oxides may occur by protons hopping between adjacent oxygen ions at normal lattice sites (the so-called Grotthuss-type mechanism) or by diffusion of the whole XHn species [66]. The very local diffusion of protons between sites occurs mainly by rotational diffusion of a proton around oxygen ions. A proton transfer toward a neighboring oxide ion permits a long range diffusion [69]. The proton diffusion is characterized by its activation energy, the rotation diffusion being the faster reaction with lower activation energy, for instance, <0.2 eV [70][71][72][73][74]. In addition, the total activation energy of the proton conduction, can also be influenced (induced or not) by distortion of the solid oxide-based structure, depending on the materials.

Proton-conducting anhydrous materials and their properties

During the last two decades, perovskite-type oxides received significant attention due to their important application potential. The general formula of perovskite structure is ABO 3 , e.g. cerates (ACeO 3 ) and zirconates (AZrO 3 ), where A is usually a 'big' alkaline earth cation, Ca, Sr or Ba. The presence of proton is not intrinsic. They must be inserted. Proton insertion/incorporation is possible once an oxygen vacancy is present. Taking yttrium (Y)

doped BaCeO 3 as an example, oxygen vacancies are created due to the partial substitution of the tetravalent Ce at the B site by the trivalent Y, as described by equation 15:

• • + + ′ → O x o Ce V O Y O Y 3 2 3 2 … (eq. 15)
Proton conducting perovskite materials are firstly characterized by the occurrence of protonic conduction at high temperature. The potential of a perovskite ceramic as fuel cell electrolyte membrane was first demonstrated in 1964 by Forrat et al. [75]. Further research was undertaken by Iwahara et al. [76] at the time of the Oil crisis in 1981. They reported that Sc-doped SrCeO 3 , as an electrolyte, exhibited relatively high proton conduction at 700-1000°C. Furthermore, the current efficiency increased to 50-95 % in the 0.1-0.8 A.cm -2 current range, with water vapor supplied at 1 atm. It was obsearved that such high working temperature demands expensive materials for the cell interconnectors, caused thermal stresses, and limited the lifetime of proton conducting materials. Therefore, current researches are attempting to reduce the operating temperature below 700°C or better still to 600°C. The objective is to have operating temperatures sufficiently high to avoid the use of expensive catalyzers as in the case of RT Nafion®-based devices and sufficiently low to permit the use of standard steels for casing and piping. Since the ion transport mechanism in the ceramic electrolyte is a thermally activated process, the increasing electrolyte resistivity becomes another significant problem. To overcome this demerit, the search for electrolyte materials with high proton conductivity (low resistivity) and/or reducing the electrolyte thickness are necessary.

At LADIR laboratory, (actually MONARIS laboratory since january 2014) [64,[77][78][79][80][81][82][83],

proton-conducting perovskite electrolytes such as Ln-doped BZY and SZY have been investigated in order to better understand the phase stability, and especially to distinguish between the bulk and surface proton contents as well as to determine a true proton nature in perovskite host membrane. by a Ln 3+ element allows the oxygen vacancy formation, and hence the incorporation of protonic species at moderate temperature (300-600°C) and under water vapor pressure. Note, the insertion of proton species in the perovskite framework is strongly dependent on external parameters such as temperature and water partial pressure. The investigated samples were treated using an autoclave device, which allows the measurements as a function of high temperature (from RT to 620°C) and under high water vapor (up to 100 bar) pressure [63].

The utilized autoclave platform is unique. It was developed in direct relation to the study of water steam electrolyzer prototypes earlier developed, with low cost of hydrogen production, by AREVA NP [45,84]. These results show that protonation and proton conduction processes are in strict competition with hydroxylation. The hydroxylation is detected by the traces of secondary phases i.e. Ba(OH) 2 , Sr(OH) 2 in pristine powder materials and low ceramics density (high active surface area). Testing high dense ceramic pellets (more than 95% of theoretical density) under high temperature and high water vapor pressure conditions enhances enhances the homogenous distribution of bulk protons. The bulk-proton saturated state needs about 100 hours of protonation in the case of 1 mm thick ceramic [78,83,85].

Furthermore, under such conditions, the structural phase transitions were observed. With the insertion of protons, the protonation decreased the distortion of the cubic perovskite unit-cell as well as its volume [64].

Objective of this work

The above description shows clearly that perovskite oxide-based materials are potential proton-conducting membranes, both electrolytes and electrodes, for fuel cell and electrolysis Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 18 applications. Prototypes have been tested by different groups [64,[77][78][79][80][81][82][83]. In order to go further than prototypes, the stable electrolytes and electrodes against vapor pressure (pH 2 O) are necessary. The structural and chemical stability of proton-conducting materials against CO 2 is also very important because of the possibility of CO 2 valorization due to its direct reaction with produced hydrogen.

The main purpose of the study presented here is to investigate the structural and chemical stability of perovskite-related oxide ceramics. Such ceramics were provided by different academic and industrial groups -AREVA NP, Imperial College, ICMCB-Université de Bordeaux (Table 3). According to previous studies performed at the laboratory, the protonation process as well as stability/ageing tests, were performed using the autoclave devices [83] 1 . Structural and chemical characterization of pristine and protonated samples were investigated using several analysis techniques such as Raman (RS) and infrared (IR) spectroscopy, thermogravimetry (TG), thermal expansion (TE), X-ray and Neutron diffraction.

This manuscript is divided into seven chapters.

Chapter 1 will introduce the fundamentals of protonic species and proton conduction.

This chapter will help us to describe the proton bonding and its localization in bulk and surface of solid-state frameworks. Proton content and proton conductivity measurement will be also briefly discussed.

In Chapter 2, fundamentals of perovskite-related oxide structure, phase transition and electrochemical properties will be concisely addressed. Raman and IR vibrational signatures of perovskite-based oxides and consequently mode assignments will be also described.

Chapter 3 presents the vibrational characterization of some secondary phases such as oxides, carbonates and hydroxides, expected to be formed in the corroded perovskite layer.

Chapter, 4 to 7 are in the form of published articles, as indicated below:

Chapter 4

C C H H A A P P T T E E R R 1 1

"Protonic species and proton conduction"

Chapter 1

Protonic species and proton conduction

In this chapter, we will first discuss the fundamentals of ionic and protonic conduction, in relation to solid chemistry defects. Then, we will address the protonic species throughout hydrogen bonds and mechanism/model of proton delocalization. Discrimination between bulk and surface protonic species in solid-state frameworks, their impact on thermal stability and phase transitions of perovskite frameworks as well as proton content measurement will also be explained. Finally, we will briefly discuss different types of proton conductors.

Fundamentals of ionic and proton conductions

Fundamentally, "Ionic conduction" is defined as the long range motion of an ion from one site to another through defects or specific pathway in a solid lattice or in an aqueous/ionic solution [1]. Alternatively, the term of "Proton conduction" is commonly employed when protons are transferred.

For ionic or protonic species with an activated diffusion, the conductivity (σ) is proportional to the product of the self-diffusion coefficient (D) and the concentration (C) of mobile species [2]. Hence, the conductivity (σ) can be expressed through the Nernst-Einstein relation as follows:

( ) kT e DC 2 ⋅ = σ … (eq. 1.1)
Since both D and C value are thermally activated, σ can be presented in the following way:

kT E kT E E k e C D T a d f exp exp 0 2 0 0 σ σ =       + -       ⋅ = … (eq. 1.2)
where E f and E d are enthalpies of formation and diffusion of charge carriers, respectively, E a is the activation energy of conduction and σ 0 is the associated pre-exponential factor.

The conductivity of a material is commonly measured at high temperature in isotherms or isobars using direct methods such as AC2. At higher frequency, the conductivity is determined using microwave relaxation and vibrational spectroscopy (THz range). As the frequency increases, the diffusion length decreases and the mobile species meet the discontinuity of the (periodic) potential. Consequently, the frequency-dependent monotonic behavior will disappear and different relaxations will determine the electric behavior.

In Infrared (IR) spectroscopy, the absorption coefficient α(ω) is related to σ(ω) by [3]:

( ) ( ) nc ω πσ ω α 4 = … (eq. 1.3)
where n is the refractive index and c is the velocity of light. The scattered intensity I(ω) measured in Raman spectroscopy (RS) is also related to frequency dependent conductivity by [4]:

( ) ( ) ( ) 1 + ∝ ω ω ω ω σ n I … (eq. 1.4)
where n(ω) is the Bose-Einstein population function.

According to their conductivity value, ionic compounds such as halides, sulfides, oxides etc. can be divided into three major groups [5]: i) insulators present the ionic conductivity lower than 10 -10 Ω -1 cm -1 at high temperature (500-1000C). and very large activation energy (>1 eV); ii) ionic conductors (IC) in which the presence of point defects leads to a conductivity up to 10 -5 Ω -1 .cm -1 at similar temperature and activation energy between 0.5 and 1 eV; and iii) superionic conductors (SIC) exhibit high conductivity (>10 -4 Ω -1 .cm -1 ) and a low activation energy (<0.3eV). Thus, the main difference between IC and SIC groups concerns the activation energy (E a ): lower than 0.3 or even 0.2 eV for SIC group.

Consequently temperature increase does not exert a significant effect on conductivity.

Defect chemistry

As mentioned above, the diffusion or conductivity (diffusion of charged species) in solids takes place because of defects in densely packed framework or because the availability of many empty sites along the 1D, 2D or 3D pathway of poorly packed framework. Indeed, defects determine a number of properties of crystalline solid [6]. The defects present in the Chapter 1: Protonic species and proton conduction 31 ionic solid can be considered as imperfections or deviations from an ideal structure. In other words, the ideal crystal structure is used as the reference state, and any deviation from this ideal state is termed as a defect. There are different types of structural defects: point defects, line defects, or plane defects. Furthermore, the point defects, especially related to this study, will be described.

Figure 1.1a illustrates point defects such as vacancies and interstitial atoms [7]. In the case of pure material, without any substitution, vacancies may be created by the lack of an atom.

When, an atom jumps to a new adjoining position on the surface, it leaves behind a vacancy.

A host atom in bulk diffuses to fill the vacancy thereby displacing the vacancy towards the bulk. An interstitial atom occupies the vacancy between the (bigger) host atoms. Vacancies and interstitial atoms appearing in a pure material are defined as "intrinsic". However, the point defects present in a crystal lattice can also be caused by foreign atoms such as subtituent/dopant (intentionally added to a material) and impurities (uncontrolled). These are called "extrinsic" point defects. In both intrinsic and extrinsic defects, the vacancies appear to maintain electro neutrality. For example, in the case of metal oxide solid (MO), Fig. 1.1b, the most common defects are the so-called Schottky and Frenkel ones. These two defects can be written using Kröger's notation:

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 32 ( )

defect Schottky V V O M O M O M + ⇔ + … (eq. 1.5) ( ) defect Frenkel M V M i M M + ⇔ … (eq. 1.6) where M M is atom M on site M, O O is atom O on site O, V M is vacant site M, V O is vacant site
O and M i is atom M on interstitial site. The excess charge associated with an M vacancy is balanced if there is an O vacancy nearby, called a Schottky defect. Alternatively, the charge in balance caused by the vacancy can be corrected by adding an interstitial of the same species;

an M vacancy is compensated by an M interstitial, called a Frenkel defect.

The formation of defects and equilibrium reactions in MO solid can be charged positively (•), negatively, (′) or they can remain neutral (x), giving rise to electronic defects, e.g.

• + ′ ⇔ h V V M x M … (eq. 1.7)
where h represents an electron hole.

Considering metal hydroxide solids M(OH) 2 i.e. Mg(OH) 2 , Cd(OH) 2 , the defects of OH x in such host lattice framework can be predicted: excess protons (H • ) and proton vacancies (O').

[ ]

O H OH x ′ + = • …(eq. 1.8)
The defect structures in both MO and M(OH) 2 compounds contain equilibrium concentrations of negatively and positively charged point defects. These are a result of internal equilibrium in the crystal and do not involve the surrounding reactions. In addition, if such metal oxide is exposed to a hydrogen-rich atmosphere, oxides may form extrinsic hydrogen defects. For example, in the case of interstitial protons ( • i H ) from water vapor which is considered in this thesis, the proton defect reaction can be written [8]:

( ) ( ) gas O e H gas O H i 2 2 4 1 2 1 + ′ + ⇔ • … (eq. 1.9)
Oxygen vacancies in metal oxides are created when oxygen atom in regular lattice sites depart for the surrounding in gas phase or by substitution of a cation M n+ with a cation M p+ , p<n, and then occupied by protons. It may be expressed as [9]: Chapter 1: Protonic species and proton conduction 33 ( )

• •• = + + O x O O OH O V g O H 2 2 .
..(eq. 1.10)

The introduction of protonic defects in oxides submitted to water-containing atmosphere has been intensively reviewed by Colomban and Novak [8].

Protonic species

Proton, noted as H + , without electrons outside the nucleus is a unique species among different ions. As noted previously, it has a very small size, intermediate between the electron and Li + ion (radius of the order 10 -13 cm, as compared with 10 -8 cm for all other ions) [10].

Because of these particular features proton can exert an enormous polarizing power on any molecule or ion in its neighborhood. In other words, a proton is very reactive with its environment. Generally protons can be distinguish as:

i. The proton that penetrates covalence sphere of acceptor ion. In most caseshydrogen is normally covalently attached to one atom, a proton donor, leading to common ions: hydroxyl group (OH -) with oxygen atom, oxonium (H 3 O + ) with water molecule, ammonium (NH 4 + ) with ammoniac, hydrazinium (N 2 H 5 + ) with hydrazine, etc. All these species easily solved themselves. Consequently they give more complex entities to form [H(H 2 O)] n + , or "cationic proton hydrates" [11].

ii. The proton that interacts with two acceptor ions creates the "hydrogen bonds". The hydrogen bonding occurs when two electronegative atoms, such as nitrogen and/or oxygen, interact with the same hydrogen giving rise to following entities: O-H…O or N-H…O. The proton (H + ) maybe centered in the hydrogen bond (Fig. 1.2b) or located closer to one of the acceptors (Fig. 1.2a)

The corresponding potential energy well of hydrogen bonds (Fig. 1.2d) is very anharmonic. Consequently it plays an important role on thermal expansion of a material and its vibrational signature. In the case of Brönsted AH acid, when a molecule is engaged in an A-H…B hydrogen bond, the basic group represents a supplementary attraction potential for the proton and can create a second (II) potential well, as dashed curves in Fig. Moreover, the AH stretching absorption bands are also characterized by an increase of intensity and band breadth on hydrogen bonding.

iii. In rare cases a proton interacts with more acceptors and becomes the "ionic or free proton" Here the lengthening of the bond gives rise to some ionic character with long distance coulombic interactions. In addition, the hydrogen species free of any covalent bonding may interact with more than 2 acceptors (Fig. 1.2c) [12][13][14]. hydrogen bond [12,15], Note: the red ball represents proton.

Different types of hydrogen bonds may be identified, i.e. weak, moderate and strong, according to the shape of the potential curve of the proton of an A-H…B system or the position of proton [16]. Asymmetric AH…B system, as blue dashed curve in Fig. 1.2d, contains a weak or moderate hydrogen bond. The potential barrier separating two minima is high and the proton is always closer to one atom than to the other (A or B). Red dashed curve (1.2d) is characteristic for the (A…H…B) system containing strong hydrogen bonds. In such a case, potential barrier is small with symmetric double minima. Table 1.1 gives approximate classification of O-H…O hydrogen bond system, the most common in protonic conductors. * The profile and bandwidth are also modified, the bands broaden and exhibit components (so called ABC components) when the H-bond is strengthened.

The distance of O…O (in Table 1.1) shows the distinction of hydrogen bond strengthweak (long) and strong (short). In addition, empirical relationship between O-H stretching frequencies and O…O distances is used to predict the hydrogen bond strength. However the band broadening could make the measurement difficult and no IR or Raman features are recorded for ionic proton. Only incoherent neutron scattering can be used to characterize these protonic features [17]. With respective variation of 10, 60 and 120 cm -1 /pm, the hydrogen bond strength changes from weak to medium-strong and strong, respectively [15]. It means that the variation of wavenumber with O..O distance is much higher for strong hydrogen bonds.

Proton transfer

The energy potential barrier determines the proton transfer. The proton can move easily (tunneling) only when the energy barrier between donor and acceptor sites is low [18].

Freund' study of Mg(OH) 2 offers a very didactic example [18]. The energy potential curve of hydrogen bond in Mg(OH) 2 was ascertained from the optical transition energies E 0n of the O-H stretching mode from zero level to the n-th excited level using the Morse potential equation [17]. a)

The existence of CB leads to the possibility of proton (H + ) conductivity measurement.

It implies that protons that reach this high-energy state cease to be bound to their parent O 2-, and exhibit 'delocalization form/state', as shown in Fig. 1.3c. The protons in excess (H • ) are highly mobile charge carriers. Proton vacancy carries a negative charge. Furthermore, the diffusion of proton vacancy from left to right occurs because a proton of neighboring OH - jumps from right to left as shown in Fig. 1.3e. Note, the proton jump in this Mg(OH) 2 case occurs within the four intermediate energy levels and/or energy of 0.4 eV.

Mechanism and model of proton transfer

Fundamentally, a proton is transferred through a solid/liquid framework in one of two distinct ways: i) By "vehicle mechanism" ("V-mechanism"), the proton is carried by a molecule such as H 2 O and gives rise to H 3 O. It is also rides on a carrier molecule of type such as NH 4 + . For example, H 3 O + is the mobile species in β-alumina like open layered structure [19]. The conduction depends on an excess of available sites.

In β-alumina, oxonium ions occupy only Beevers-Ross (BR) sites (σ RT ~10 -11

Ω -1 .cm -1 and E σ = 0.8 eV) while in β"-alumina, oxonium ions can occupy both prismatic and tetrahedral sites (σ RT ~10 -9 Ω -1 .cm -1 and E σ = 0.6 eV).

ii) By "Grotthuss mechanism" ("G-mechanism"), the proton jumps from a donor to a suitably placed acceptor molecule, for example from H 3 O + to H 2 O or from H 2 O to OH -as in Hydrogen Uranyl Phophate hydrate (HUP) and in water [19,20] This mechanism involves (a) displacement of H + along a hydrogen bond, with a transfer energy barrier (E bar ) and (b) transport of H + from this hydrogen bond to the next, with a bond breaking energy barrier E bond .

Diffusion of the water molecule and ions also takes place and the total conductivity is the superimposition of different mechanisms.

Beside the mechanism of proton transfer, the proton conductivity models were discussed by Colomban et al. [21]. Three main types of theoretical description can be made : i) "Liquid-like/Viscous models" for highly disordered solid electrolytes. These models are applicable when a proton hops through membranes such as glass and polymer. The corollary is that the conductivity increases when the viscosity decreases [21] and more complex laws than Arhenius' law are observed.

ii) "Hopping models". In these models the proton spends most of its time executing relatively low-amplitude vibrations in equilibrium lattice positions, and spends a very short time in jumping among permitted sites.

iii) "Lattice gas model". When the temperature is sufficiently high and allows the proton to reach the continuum, the species diffuses without strong interaction with the (low) potential wells. The proton is delocalized over many sites.

In the present study, with a very stable host framework (melting temperature > 1500°C)

and a low concentration of proton [2,17], the lattice gas model is considered. As presented in Consequently, it ensures that the two ions do not occupy the same lattice site.

Figure 1.4: Representation of different behavior of proton delocalization described by lattice gas models (Note: • symbol represents protonic species, H + ).

Chapter 1: Protonic species and proton conduction 39 Different assumptions of slow or fast movement of distinct protonic behavior, as presented in Fig. 1.4, are possible. In the case of short timescale, the species oscillates on a given site, while on long timescale, the proton diffuses. In fact, the slow or fast systems, yielding different regimes (hopping and diffusive), depend on two critical parameters: i) temperature versus hopping potential height (T/∆V) and ii) γ-damping to ω 0 -vibrational frequency ratio (γ/ω 0 ).

The proton conduction mechanism is significantly related to the definition of the local (meV energy range) and long range diffusion (µeV) proton motions [14]. Several studies using elastic and quasi-elastic neutron technique as a function of temperature and momentum transfer have been published for different perovskite-type compounds [22][23][24]. Hempelmann et al. [25] and Matzke et al. [26] postulate that the proton diffusion in Sr-cerate consists of a sequence of free diffusion and trapping/escape events, according to the common ion jump model. The mean jump length at 0.4 nm, is close to the unit-cell parameter value The mean distance between the traps ~1.7 nm is close to the distance between substituents. Previous studies [27,28] suggested that in the case of highly dense BaZr 0.97 Yb 0.03 O 3-δ H 0.0001 the fastest local motion of bulk proton observed at 500-600°C, is associated with the largest width of the quasi-elastic peak, consequently with subtle structural modifications and energy activation conductivity changes.

Protons in bulk and surface of solid-state frameworks

As discussed in section 1.2, the defects introduced from a water/hydrogen containing atmosphere, yield interstitial proton (H i • ), eq. 1.10. The interstitial protons can be incorporated with the host lattice defects i.e. oxygen vacancy. or not to oxygen ions detected on a material surface and in the bulk. Presumption product is expected to form either as "hydrogen bronzes" (H x MO n ) i.e. H 0.23 VO 2.5 , H 0.34 MoO 3 , H 0.33 UO 2.67 , or "oxy-hydroxides" (MO n-x (OH) x ) i.e. Al(OH) 3 , In(OH) 3 , VO(OH) 2 [29].

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 40 The water, in gas-phase, dissociates itself at the solid surface into a hydroxide ion (OH -) and a proton (H + ). Hydroxide ion, or protonic species, interact initially with oxygen defects on ionic-host surface lattice and the proton, forming a covalent bond with the host lattice oxygen, implied as 'surface proton". Although the surface species are usually observed in form of undesirable second phases i.e. hydroxides and/or carbonates, such surface proton materials can also be considered as proton conductors. Since the aim is to enhance proton conduction, an introduction of the bulk protons must be deployed. The presence of bulk defects facilitates the migration of protonic species or vacancies, called "bulk proton". The diffusion is associated with the local perturbation of the bulk structure related to the occurrence of protonic conductivity for example MnO 1.78 .nH 2 O gives σ RT ~10 -5 Ω -1 cm -1 [30].

In order to understand the properties of proton conductor, it is important to differentiate between surface and bulk protonic species. Colomban et al. [14] have suggested a method that compares materials having very different surface to volume ratio (S/V). The ultra dense single crystals/ ceramics have very low porosity, and the S/V ratio is low. On the contrary, ultra fine powders and highly porous materials can have a higher S/V ratio.

Another approach to distinguish between the bulk and the surface species is based on the different energies required for the elimination of proton species located in different lattice sites. The possible proton locations/sites are [14]: (i) species adsorbed at the external surface, (ii) adsorbed at the surface of internal pores, (iii) chemically bonded at the external or pore surface, and (iv) protons present in the bulk structure. Such varied proton locations formed instable species with difference of bonding energy. Thermal treatments at temperatures above the bonding energy, specifically eliminate the instable species. These are addressed in the next section.

Thermal stability and phase transition

The thermal stability of proton conducting material is directly related to the nature of the protonic species and to the strength of the H-bond. Table 1.2 summarizes the typical conducting protonic species and their stability range [12]. The variation of the bonding strength between a proton and its environment plays an important role. The proton conducting gels have the weakest H-bond strength and the lowest thermal stability (<80°C [12]. The H 3 O + ions lose their water at ~200-400°C as a function of the host framework. In the case of "ionic" protons and hydroxyl ions, it remains hosted in oxide frameworks above 500°C, sometime up to 1000°C [12,14]. Oxonium β-aluminas are good examples for the latter case. They are stable up to ~400°C and above 400°C. The evaluating protons destroy the β-aluminas framework, leading to the proton-containing crystalline transition alumina. Note, thermal stability of pristine Na β-aluminas in the lack of protons exceeds 1600°C.

Table 1.2: Typical conducting protonic species and their stability range [12].

Protonic species Typical stability range / °C

H 2 O, H + (H 2 O) n (gels) <80 H 3 O + , NH 4 + <400 OH - <600 H + <900
In order to increase the thermal stability range of a protonated material, one significant factor is the application of water pressure. Two alternative applications of partial water pressure are possible: i) the entire device itself must work under pressure i.e. autoclave, ii) the proton conductor is encapsulated by a host framework that kinetically retains the water and Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 42 hence increases the partial water pressure at a very local scale. According to the Jurin's law determining the equilibrium pressure close to a curveted surface [12] together with capillary condensation --(Kelvin equation: more condensation in a porous medium will occur by increasing the vapor pressure [31]), --the different size and the geometry (curvature radius) of the pores/ cavities of frameworks are also important factors.

The presence of protons and their diffusion can give rise to phase modifications /transitions as a function of the H-bond strength. For example, H 3 OUO 2 PO 4 .3H 2 O (HUP), ) n chains with the O…O distance of 0.257 nm and protons appear to be disordered between two equilibrium sites creating a statically symmetric hydrogen bond. The transition from phase I to phase II detected at 44.8°C, is interpreted in term of a conversion from infinite chains to cyclic dimmers with a subsequent weakening of the hydrogen bonds and increased orientational disorder HSO 4 -ions. In the case of the transition from phase II to phase III observed at 143°C, the structure exhibits high dynamic disorder. Raman bands, assigned as external mode, collapse into a broad wing near Rayleigh line. This is a characteristic of the plastic phase, and implies a free rotation of HSO 4 -ions on given sites. Phase III, furthermore, can be considered as a spectroscopic manifestation of a quasi-liquid state of the proton, the superionic phase. Under high partial pressure of water conditions, finally, the enthalpy of the transformation of chains (2-17 Cal.g -1 for phase I to phase II) into dimmers (15-33 Cal.g -1 for phase II to phase III) increase by more than a factor of 10 [8,20]. The phase transitions and local structural modifications are also observed for perovskites proton conduction. This problem will be discussed in Chapter 2.

Bulk proton (H + ) contents

Because of the absence of electron clouds and presence of rather high incoherent background, the location from X-ray/neutron diffraction refinements of protons in a disordered structure at room temperature and a fortiori at higher temperature is a difficult task. The thermal motion and the distribution of mobile protons across different sites for fast proton conductors or superionic conductors makes the proton location impossible.

Consequently, to detect hydrogen and protonic species in solids, it is mandatory to couple different methods to ensure a complete view of all hydrogen-containing species, and, further, to establish the relationship between the measured signal and H-species content.

Intensive studies related to the determination of protonic species content have been performed by Colomban et al. [14,28]. The H + measurement techniques could be classified as i) direct methods and ii) indirect methods: The first-three techniques have been mostly used to study the proton location and dynamics and not to measure precisely the proton content [14]. For example, NMR study of (NH 4 ) 4 Fe(CN) 6 .1.5H 2 O (AFC) [33] showed that one of two non-equivalent NH 4 groups, together with the water protons, may be expected from the presence of three different, The Infrared (IR) spectroscopy is the most used direct technique to measure the hydroxyl ion and water content in solids. In IR technique, the huge anharmonicity of the X-H bond enhances the intensity of overtones and combination bands that can be detected by transmission measurements through matter. Owing to the low proton mass, its stretching vibrational signatures take place at high wavenumber: 1000-3800 cm -1 and combination/overtones are detected up to 10.000 cm -1 . For example, OH groups in H 6 V 6 O 13 presents bending modes in the range of 800-1100 cm -1 [34]. The H atom of hydroxyl groups is an independent oscillator that gives rise to very narrow IR stretching band. This shows a great ability of IR technique to detect weak narrow signatures of protonic species at low concentrations. The band width increases with electrical/mechanical coupling. The H-bond strength also increases and detection becomes difficult, even impossible. As such inelastic neutron spectroscopy should be preferred.

In the case of Raman technique, as indirect method, the low polarisability of the X-H bond leads to a poor Raman intensity of the protonic species signature. However, the narrow bandwidth of the stretching mode promotes their detection even when their intensity is very small [14]. The exact wavenumbers of the Raman modes depend on both the atomic masses and the bonding of the individual atoms. For example, Raman bands of lithium β-alumina hydrated with water at 3622, 3871, 3472 and 3409 cm -1 , were identified as OH -species from two different origins; (i) OH -species from the bonding of a proton to one of the bridging oxygen in conduction plane (ii) OH -species from dissociation of an H 2 O molecules [35].

Methods such as diffraction and vibrational spectroscopy give direct information if the Hcontent is sufficient to give a reliable signature. However, in many cases the low content combined with high static and dynamic disorder of the conduction protons hinder a reliable Chapter 1: Protonic species and proton conduction 45 analysis of the protonic species. Indirect measurement methods such as differential thermal/scanning calorimeter analysis (DTA/DSC), and thermal expansion (TE) give relative events (jumps, kinks) when matter including proton-containing species evolves, and the hosting framework is heated. Very small events can be detected. For example, thermal expansion (TE) measures the anharmonicity of the chemical bonds [14]. It is very sensitive to any phase transition of first order (thermal expansion jump), second order (kink or rate change) or even O'Keefee type (plot anomaly) [36]. The area delimited by the heating and cooling lines measures the stability of the material: For a stable material, the overlapping is nearly perfect. On the contrary, a significant area indicates a material evolution.

Here, we will further discuss a useful example of neutron scattering and thermal gravimetric analysis, which were employed to estimate the proton contents in previous study on similar compounds [14,37].

Due to the huge incoherent cross-section of hydrogen element ( H σ inc = 80.26 barns, [38,39]) neutron scattering techniques are among the best methods to study the proton containing materials. They allow firstly, to determine the proton content (elastic scattering), and secondly, to determine the local (rotation and translational jumps) and long range diffusive proton dynamics (quasi-elastic scattering from µeV to tenth of meV range). The ratio of incoherent neutron cross-sections (σ incoh ) calculated for protonated and deprotonated (or non-protonated) compounds is proportional to the ratio of elastic peak intensity (I) recorded for protonated and deprotonated (non-protonated) materials [38,39]).

For example, elastic neutron scattering has been used to determine the content of bulk protons in Sr-or Ba-based high dense zirconate ceramics as a function of temperatures [27,28]. Figure 1.6a shows the elastic peak intensity variation in the expected working temperature range in the case of SrZr 0.9 Ln 0.1 O 3-δ measured under dynamic high vacuum on dense ceramic pellets. The highest intensity at room temperature reveals the presence of the bulk and surface protonic species. At 400°C, the ceramic is cleaned out from the protonic surface species, while above 600°C, the departure of bulk protons appears. The intensity comparison allows a precise determination of the H content after calibration with reference materials i.e. non-absorbing vanadium and high-absorbing cadmium metals). Comparison must be made at about the same temperature with the same instrument/resolution/energy Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 46 window because the intensity of the elastic peak strongly depends on the H motion. Diffusing hydrogen contribute to the peak broadening if their energy is close to the resolution, or to the background if their energy is larger (long-range diffusion) [14,37]. Different instruments are necessary to study the different motions. The authors concluded that bulk proton amount depended on the surface area, i.e. the densification of ceramic. Very low H + content (0.001-0.005 mole H/mole perovskite) was observed in the case of high dense ceramic (>97%). Colomban et al. [14,37] showed that in the case of highly dense ceramic, the content of protonic moieties adsorbed at the surface was lower than that at the "bulk". On the other hand, the lower the ceramic density (higher porosity, higher active surface), the higher the content of surface protonic species. Studies by Hempelmann et al. [40] regarding cerate-based perovskite (SrCe 0.95 Yb 0.05 O 0.29 ), showed that the proton content in cerate ceramic (~95% theoretical density) remained low, about ~0.02, ten times higher than that of zirconate [37]). But no differentiation was noticedbetween surface and bulk H-content. The conductivity of cerates being higher than of zirconate, a higher bulk H content was expected. In order to measure the accuracy of the H-contents, it is necessary to combine different, direct and indirect, measurement techniques.

Proton conductors

Proton conductors can be classified according to the preparation method, structural dimensionality and conductivity mechanism [2].

The preparation method based classification can be divided into two: i) direct synthesis and ii) ion exchange. The direct synthesis is usually used for hydrates materials i.e. Second criterion practically concerns the nature of the mobile ion and the dimensionality of the potentially mobile ion sub-lattice. For example, polycrystalline materials (i.e. phase I of CsHSO 4 ) with a structure containing 1D conduction pathways, will have a very low overall conductivity due to an blocking effect at the grain boundaries. In other words, each grain boundary encountered along microscopic diffusion pathway is a wall, and creates a barrier to long-range diffusion.

The last classification criterion, the value of proton conductivity, allows distinguishing conductors and superionic conductors. The correlation between two parameters of proton conductivity (σ and E a value) is employed according to the proton conductivity mechanism.

The σ 0 term (eq. 1.

2) is directly related to the concentration of the conducting protonic species. Indeed, there are three ranges of σ value: high value for loosely packed structures (>10 -3 Ω -1 .cm -1 ), intermediate value for quasi-liquid like or gels (10 -7 -10 -3 Ω -1 .cm -1 ), and low value in case of extrinsic defect materials (<10 -7 Ω -1 .cm -1 ) [2]. Therefore, three types of proton conductors could be classified as: i) Anhydrous protonic conductors. -their activation energy is rather high and the proton conduction is related to the presence of defects. The conducting species are proton or proton vacancies. Typical examples for this protonic conductor type are KH 2 PO 4 (σ RT ~10 -8 Ω -1 .cm -1 and E a ~0.55 eV [41]) and ice (σ RT ~10 -7 /10 -8 Ω -1 .cm -1 and E a ~0.55 eV [42])

ii) Ionic conductors containing a loosely packed lattice with high concentration of potentially mobile species. iii) Compounds containing protonic species in quasi-liquid state. This protonic conductor type is considered either inside the structure (intrinsic or bulk conductors) or at the surface (gels or particle hydrates). Protonic species may jump from one acceptor to the other (G-mechanism). One good material for intrinsic or bulk protonic is, for example, CsHSO 4 [2] owing to σ RT ~10 -2 Ω -1 .cm -1 and E a ~0.33 eV. For surface protonic conductors, gels or particle hydrates such as V 2 O 5 .1.6H 2 O yield conductivity of ~10 -3

Ω -1 .cm -1 at room temperature with activation energy (E a ) of ~0.35 eV [2].

Perovskite-based oxide solid solution exhibits protonic conduction in an atmospheric containing hydrogen or steam from moderate to high temperature. For example, pure SrCeO 3

or BaCeO 3 ceramics owned the conductivities of which at least 10 -2 S cm -1 at 1000°C and at least 10 -3 S cm -1 at 600°C exposed to pure hydrogen gas [43]. However, the conduction in BaCeO 3 -based ceramics may be partly protonic and partly oxide ionic as a function of temperature range and water partial pressure [44]. Below 600°C, the oxygen vacancy conductivity remains very small with respect to the protonic conductivity [27].

Characterization of perovskite-based oxide structure as well as its electrical properties will be further discussed in next chapter.

C C H H A A P P T T E E R R 2 2 "Perovskite-Related Oxides"

Chapter 2

Perovskite-Related Oxides

In this Chapter, we will summarize the fundamentals of perovskite-related oxides structures, their phase transitions and electrochemical properties. This will assist in understanding the research undertaken by this author, subsequently. We will focus on the Raman and IR spectra, and consequently vibrational mode assignments, of perovskite-based oxides. All descriptions in this -Chapter -correspond-to our investigated samples, listed in Table 3 of the "Introduction".

Structure of perovskite-related oxides

The so-called perovskite name comprises large families of compound, including oxide compounds. The typical chemical formula of perovskite structure is ABO 3 , illustrated in Typical perovskite compounds such as SrTiO 3 [1] and BaZrO 3 [2] have a cubic lattice with m Pm3 space group at/close to room temperature. However, many perovskite-oxides are slightly distorted variants showing lower symmetry i.e. hexagonal, orthorhombic, etc. The structural variants result from the competition between the degree of freedom of each sublattice, which are strongly related to their properties, particularly piezo-or ferro-electric properties.

Perovskite compounds with different combinations of charged cations in A and B sites for example, Na 1+ Nb 5+ O 3 , Ba 2+ Ti 4+ O 3 , and La 3+ Al 3+ O 3 have been discovered. According to the simple ternary perovskite, the perovskite structure can also allow complex charge Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 56 compensated compositions formula such as A'A"B'B"O 3 , -called perovskite solid-solution.

A case study related to our research shows that because the presence of protons is not intrinsic to the ideal structure, they must be modified by substitution of trivalent cations e.g. lanthanides elements at B sites to form oxygen vacancies. The presence of oxygen vacancies promotes oxygen vacancy/ion diffusion and conductivity at high temperature [3,4]. Hence, this complex perovskite oxides can be formed as AB'B"O 3-δ i.e. SrZr 0.9 Yb 0.1 O 3-δ , SrZr 0.9 Yb 0.1 O 3-δ [3] . A further factor affecting the perovskite structure is large non-stoichiometry and substitutions. A number of perovskite-related structures, with perovskite building blocks, are formed when excesses, deficiencies, or substitutions beyond the solubility limits, create ordered defects and -new phases. -These comprise oxygen deficiency (Brownmillerite, A 2 B 2 O 5 ) and A atom access (Ruddelsden-Popper, A 2 BO 4 ).

In this project, Ruddelsden-Popper (RP) type structures were investigated. These are a series of structure having the general formula A n+1 B n O 3n+1 or AO(ABO 3 ) n . In addition, the RP type structure presented in Figure 2.1b consists of two units: the ABO 3 perovskite unit and AO rock salt unit, which are connected in series along the c-axis.

To understand the deviation from the ideal cubic structure, the ABO 3 oxides are first regarded as purely ionic crystals [3]. In the case of the ideal structure, the following relationship between the radii (r) of A, B, and O 2-ions may be proposed: )

O B O A r r r r + = + 2 …(eq. 2.1)
Therefore, the deviation from the ideal structure in perovskite oxides can be expressed through the following -"tolerance factor", t:

( ) ( ) O B O A r r r r t + + = 2 …(eq. 2.2)
In perovskite-type compounds, the value of t usually lies between 0.80 and 1.10. In the case of the ideal cubic structure, the value of t is close to 1 or at least grater than 0.89 [3]. In the case of some perovskite oxides with lower t values, the crystalline is in the illmenite structure -a polymorph of the perovskite structure where A and B have approximately the same size. In the case of Ruddelsden-Popper structure (A 2 BO 4 ), the tolerance factor is the measure of misfit between the ionic rock salt (A 2 O 2 ) and the covalent perovskite (BO 2 ) layers. For Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 58 example La 2 NiO 4 , the ideal tolerance factor is 0.866 < t < 1.0 [5,6]. Furthermore, the t value increases with increasing Ni 3+ content [5].

Structural phase transitions

Structural phase transition on perovskite-related oxides can be driven by many parameters such as chemical composition, oxygen stoichiometry (δ), temperature, pressure/stress, magnetic or electric fields etc. We recall that XRD and neutron diffraction 'see' the mean structure whereas Raman spectroscopy 'see' the superimposition of the contribution of the nanodomains [7][8][9][10][11][12].

Zirconate-and Cerate-based perovskite oxides

First, the structure phase transition of undoped zirconates and cerates oxides as a function of temperature would be discussed.

Pnma Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Ο I Ο I Ο I Ο I Figure 2.3:
The structural forms of undoped barium/strontium zirconates and cerates gathering data from literatures [13][14][15][16][17][18][19][20][21], each symbol represents the crystal phase: Ο -orthorhombic, Ο I -pseudo tetragonal,♦ -tetragonal, -cubic. [19] report that the structure of BaZrO 3 -remained in cubic lattice phase with Pm-3m space group. Similar to strontium cerate (SrCeO 3 ), the structure shows orthorhombic with Pbnm space group throughout the temperature range of 25-900°C [15].

In contrast, the SrZrO 3 and BaCeO 3 oxides undergo three steps of structural phase transitions. For SrZrO 3 , Howard et al. [18] report that the crystal structure varies from primitive orthorhombic (Pbnm) to pseudo-tetragonal (Imma)/body-centered orthorhombic (Ibnm) at 750°C, then to tetragonal phase (I4/mcm) at 840°C and finally to cubic phase (Pm-3m) at1070°C. More or less temperature value of each step in structural modification of such SrZrO 3 was observed by others [16,19,20]. In the case of BaCeO 3 , the structure undergoes from the orthorhombic (Pnma) to the pseudo-tetragonal (Immm) configuration at ~300°C, then to the rhombohedral phase (R-3c) at ~400°C, and the cubic (Pm-3m) at 900°C [13,14].

In Table 2.1, room temperature symmetry of substituted barium/strontium zirconate (AZrO 3+δ ) and cerate (ACeO 3+δ ) are summarized according to the cited literature [20,[22][23][24].

As reported by Giannici et al. [22], the structural modification of BaZrO 3+δ, from cubic to tetragonal occurs when yttrium (Y 3+ ) in amount of 6 %mole is substituted. Similarly, Kreuer et al. [25] concluded that such phase transition appears when the minimum amount of Ycontent of ~5 %mole, is substituted with 10 %mole of either gadolimium (Gd) or indium (In) in BaZrO 3+δ. The structure also exhibits the tetragonal phase. Such different trivalent substituent at B-site leads to a significant reduction of the mobility of protonic defects.

However, the authors concluded that the Y 3+ is found to be the most optimal as acceptor for BaZrO 3 due to its significantly higher proton conductivities.

In the case of doped SrZrO 3+δ , Koto et al. [23] studied the characterization of 5 and 10 %mole of Ytterbium (Yb) in SrZrO 3+δ using X-ray diffraction (XRD). The authors found that 10 % mole Yb-SrZrO 3 had orthorhombic symmetry with space group Pnma from room temperature to 900°C, which similar to that of 5 %mole Yb-doped. In addition, the unit-cell volume of the 10 %mole Yb-doped increases linearly with increasing temperature. However, a slightly bent of the unit-cell volume vs. temperature curve, between 500 and 600°C, was observed due to the loss of oxygen. For the doped cerate-based oxide, many authors [28,[30][31][32][33][34][35][36] have published that BaCe (1-y)-x Zr x Y y O 3-δ, (BCZY) material allows obtaining electrolyte with both high proton conductivity and good chemical stability. Structural study of barium cerate was first performed by Jacobson in 1972 [37]. Subsequently in 1992, Knight et al. [38] determined the structure of BaCe 0.9 Y 0.1 O 0.29 using a pulsed neutron source and time of light diffraction. The room temperature structure is orthorhombic with Pmcn space group. In the last decades, several studies focused on synthesis, characterization and chemical stability test of partially substituted BC by Zr and Y formed as BaCe (1-y)-x Zr x Y y O 3-δ [28,[39][40][41][42].

For example, the structure of BaCe 0.8-x Zr x Y 0.2 O 3-δ, with 0.2 ≤ Zr ≤ 0.6 [42] is cubic.

Chien et al. [39] reported that the BaCe 0.7 Zr 0.1 Y 0.2 O 3-δ (BZCY712) ceramic also shows cubic structure. The studies show that the first impact of zirconium substitution is the stabilization Chapter 2: Perovskite-Related Oxides 61 of the cubic phase of barium zirconate and secondly, a decrease in unit-cell parameters with increasing content of zirconium doping. Ricote et al. [40] and Koji et al. [27] studied the crystallographic phases of BaCe 0.9-x Zr x Y 0.1 O 3-δ (0 < Zr < 0.9) ceramics at room temperature.

They reported that the compounds with the Zr substitution between 0.3 ≤ Zr ≤ 0.9 show a pseudo cubic structure. The increasing of Zr concentration leads to a decrease of the lattice parameters, which reflects the substitution of small Zr 4+ for Ce 4+ -site. (0.72 and 0.87 Furthermore, a statistically distribution of Ce and Zr in BaCe 0.9-x Zr x Y 0.1 O 3-δ (0 ≤ x ≤ 0.9) was well observed.

Tao et al. [32] pointed out that zinc addition decreases the firing temperature and thus makes possible co-firing of both electrodes and electrolyte. 1 %mole of ZnO addition in barium cerium zirconate (BCZY) allows firing at 1325°C. The zinc compound is an unlikely B-site substituent because of its low charge (Zn 2+ ) and small size (radii 0.72 Å), which could reduce ionic conductivity. It may be noted that oxide additives with metal ions, having a stable oxidation state of 2 and an ionic radius similar to Zr 4+ , result in the best sinterability [43]. This synthesis method, extremely beneficial to sintering, yielded 97% dense sample. The effect of Zn addition on the structure of BaCe 0.9-x Zr x Y 0.1 O 3-δ ceramic has been intensively studied by Amsif et al [28]. The powders of BaCe 0.9-x Zr x Y 0.1 O 3-δ (x = 0.1, 0.3, 0.5, 0.7 and 0.9) were prepared by the freeze-drying precursor method. Then, a polycrystalline Zn powder was added to precursor powders and they were pressed into disks and sintered at 1200 and 1300°C for 10h. They observed that the BaCe 0.9-x Zr x Y 0.1 O 3-δ (x = 0.1, 0.3) samples with Zn addition exhibited an orthorhombic structure with Pnma space group. However, the Zn-doped BCZY samples in case of x > 0.3 showed a cubic structure with Pm-3m space group. The addition of 4 mol.% Zn into this phase permits reduction of the unit-cell volume. In fact, the Zn 2+ with radii of 0.74 Å replaces the Zr 4+ (radii 0.72 Å) than Ce 4+ , which results in smaller lattice distortion.

The phase transition behavior of Sr 1-x Ba x ZrO 3 were studied by Takayuki et al [19].

The results show that three phase transitions were observed with Ba-doped amount; a phase boundary from primitive orthorhombic (Pbnm) to body-centered orthorhombic (Ibmm) for x = 0.4 -0.5, from body-centered orthorhombic (Ibmm) to bode-centered tetragonal (I4/mcm) for x = 0.5 -0.6, and from the body-centered tetragonal (I4/mcm) to cubic phase (Pm3m) for x = Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 62 0.8 -0.9. According to the temperature impact Sr 0.5 Ba 0.5 ZrO 3 exhibits a phase transition from Ibmm to I4mcm between 100-200°C, and from I4mcm to Pm3m between 400-500°C. For Sr 0.4 Ba 0.6 ZrO 3 , the structural phase transition from I4mcm to Pmcm was between 300-400°C.

Nieto et al. [44] studied the phase transitions and proton transport in BaCe 0.95 Yb 0.05 O 3- δ perovskite. They reported that three phenomena of phase transitions appeared at RT-800°C temperature range: orthorhombic (Pnma) to tetragonal (Imma) at ~320°C, tetragonal (Imma)

to rhombohedral (R3c) at ~450°C, and rhobohedral (R3c) to cubic (Pm3m) at 800°C.

Protonic-conducting BaCe 0.8-x Zr x Y 0.2 O 2.9 ceramic powders fabricated by using glycine-nitrate (G/N) combustion method was investigated in terms of structural properties by Cien et al. [39]. They observed that calcined BZCY (x = 0.0-0.8) powders fabricated with G/N = ½ exhibited a single cubic structure and a structural transformation from cubic to rhombohedral for Zr/Ce < 2/6. Katahira et al. [27] showed that BaCe 0.9-x Zr x Y 0.1 O 3-α exhibited a single phase over the whole range 0 ≤ x ≤ 0.9. Symmetry of lattice was either orthorhombic (x ≤ 0.2) or cubic (0.3 ≤ x ≤ 0.9 Table 2.2 shows the comparison of our XRD analysis, of the three-lanthanide nickelate-based oxides (lattice parameters, space groups), with the literature data [6,[53][54][55][56][57][58][59].

Nickelate-based oxides

The most common feature is the orthorhombic distortion at room temperature. However, some materials have the tetragonal structure [6,54]. Several such symmetries of Ln 2 NiO 4+δ ceramics were variously observed for the lanthanide elements, oxygen stoichiometry (δ), synthesis route and temperature. For example, the La 2 NiO 4+δ (δ = 0.18) were refined in orthorhombic Fmmm space group, whereas stoichiometric La 2 NiO 4+δ (δ = 0) was in Bmab symmetry [55]. In addition, the tetragonal distortion with I4/mm space group was obtained by solid state reaction at 1200°C calcination temperature for 12h [6], while the orthorhombic phase (Bamb space group) was synthesized by Pechini-method at 1000°C [56]. Unfortunately, no data was taken for temperatures above room level.

After the study of Jorgensen et al. [55], numerous researchers focused on high temperature La 2 NiO 4+δ phase modification. Fernandez-Diaz et al. (1993) [60], reported that the unstrained tetragonal structure (I4/mmm) of La 2 NiO 4+δ remained in the measured temperature range of 25°C to 826.5°C. In contrast, Stephan [71] reported that nonstoichiometry La 2 NiO 4+δ (δ~0.18) exhibited Fmmm orthorhombic phase (at room temperature) and was transformed to I4/mm tetragonal phase at 150°C.

In the case of praseodymium nickelates, Buttrey et al. [67] show that the Pr 2 NiO 4+δ sample (δ = 0.06) was tetragonal (P4 2 /ncm) symmetry at low temperature (-268°C to 27°C).

Biphasic phase (P4 2 /cnm tetragonal phase and Pccn orthorhombic phase) were proposed for Pr 2 NiO 4+δ sample (δ = 0.02) at above -156°C. For high temperature, Fernandez-Diaz et al. [60] found that the orthorhombic (Fmmm) to tetragonal (F4/mnm) phase transformation of Pr 2 NiO 4+δ (δ = 0.2) takes place at a temperature of ~420°C. Similar results to Allançon et al.

[68], they observed an orthorhombic (Bmab) to tetragonal (P42/cnm) transition at about 447°C for oxidized Pr 2 NiO 4+δ (δ = 0.28). decreased with increasing temperature. Consequently, the phase transformation for δ < 0.28 took place at a temperature of 516°C and tetragonal F4/mmm phase -occurred.

La 1-x Sr x Co 1-y Fe y O 3-δ materials

Lanthanum Strontium Cobalt Ferrite (La LSCF7382 Rhombohedral R-3c 5.447 5.447 13.224 [74] LSCF7355 Rhombohedral R-3c 5.459 5.459 13.368 [74] LSCF7355 Rhombohedral R-3c 5.481 5.481 13.270 [75] Note: cell parameters indicated here are characteristic of non-stoichiometry compounds (δ >0).

Tai et al. [72,73] studied crystal structure and thermal properties of La phases from the rhombohedral to the cubic at 450°C. The extension of cubic stability range to such low temperature was attributed to an increase in the ratio of A-site (La + Sr) to B-site (Co + Fe) cations, which resulted in A-site deficiency observed at higher temperature. The Asite deficiency extended not only the stability range of cubic structure, but also influenced transport properties: 2.5% A-site deficient induced a decrease in electrical conductivity of 33% and increased the formation of oxygen vacancies by 2%.

Conductivity properties

The aim of this thesis is not the characterization of the charge in proton conductivity.

However, data about conductivity observed under similar working conditions was consulted.

This was done in order to ascertain a global view of investigated ceramics. Hereafter it is summarized, based on anode/cathode/electrolyte reaction roles of perovskite-based oxides materials.

Electrolytes

Proton conductivities of various solid oxide-based materials were calculated from the data available on proton concentrations and mobilities for a wide range temperature, shown in Iwahara et al. [81][82][83] applied Yb-doped SrCeO 3 as the proton-conducting electrolyte in steam electrolyzer working at 700-1000°C temperature to produce hydrogen gas. The efficiencies for hydrogen production were 50-95% in the range of 0.1-0.8 A/cm 2 . BaCeO 3based ceramic were also developed [84] for similar application. The BaCeO 3 was found to be responsive in the presence of oxygen conduction when it exceeded 900°C. The protonic conductivity of alkaline earth zirconates such as CaZr 0.9 In 0.1 O 3 , SrZr 0.95 Y 0.05 O 3 and BaCe 0.95 Y 0.05 O 3 was successfully investigated [85][86][87]. These studies showed that barium and strontium zirconate ceramics exhibited higher conductivities than calcium zirconates.

The cerate (ACeO 3 ) and zirconate (AZrO 3 ) based-oxides have become well-established proton conducting electrolyte [27,[86][87][88][89][90]. The pure cerates and zirconates exhibit only low proton incorporation unless doped with subvalent cations [91]. The tetravalent cerium or zirconium ion is substituted by a lower-valent cation, thus constituting defects of effective negative charge. These defects may be charge compensated by oxygen vacancies (

• • O V ) or interstitial proton ( • i H ), as discussed in section 1.2.
In the literature consulted [92][93][94][95][96][97][START_REF] Chen | Perovskite-Related Oxides 89[END_REF][START_REF] Matskevich | [END_REF][100], most of the lanthanide elements were investigated as the substituents. Table 2. 4 shows proton conductivity values of some potential Ln (Nd, Sc, Er, Y, and Yb) -doped cerate and zirconate perovskites.

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 70 BaZr 0.9 Y 0.1 O 3-α 500-900 n/a 1.6x10 -3 to 6x10 -3 [27] Typical values of the activation energy of proton conductivity determined at intermediate temperature was around 0.6eV in Yb-doped SrCeO 3 [101], 0.3 to 0.5 eV in Ydoped BaZrO 3 , [106] and ~0.6 eV in Nd-doped BaCeO 3 -based electrolytes [103].

Accordingly, the proton-conducting perovskite oxides had advantages in intermediate to low temperature usage (400-700 °C). For example, BaCe 0.9 Y 0.1 O 3-α reached conductivity of ~10 -3 S.cm -1 at 400°C [102].

At intermediate SOCF/SOEC operating temperature (<700°C) --which allows the electrolyte highly conductive protons --the main obstacles were related to the increasing electrolyte resistivity. In order to reduce the electrolyte resistance, decreasing electrolyte's thickness was one of the solutions. In fact, the thinner electrolyte film reduced ohmic losses, and consequently allowed the electrolysis cells to operate at lower temperatures [107][108][109].

For example, relatively low applied voltages of 1.08 V at 0.4 A.cm -2 and 1.32 V at 0.6 A.cm -2

were required for SrCe 0.9 Sc 0.1 O 3-δ electrolyte PCEC with a thickness of 10 mm. [110]. In Chapter 2: Perovskite-Related Oxides 71 addition, to achieve the same current densities under the same working conditions voltages of 7.2 V and 8.8 V were required for the cell with a 500µm. thick SrCe 0.9 Sc 0.1 O 3-δ electrolyte.

Actually, thickness of 100 to 20 µm could be achieved in real devices.

Electrode materials for PCFCs and PCECs

Fundamentally, the electrolysis cell system (PCEC) is a reverse mode of fuel cell system (PCFC). To explain a function of electrodes of these two distinct systems, the cathode side of PCFC (Fig. 2.7a) exhibits oxygen dissociative adsorption, diffusion, reduction, and the formation and evaporation of water by-product. In a PCEC system the mode is reversed and the anode side (Fig. 2.7b) exhibits dissociation of water, oxidation, diffusion and the formation oxygen gas. In addition, the anode side of PCFC (Fig. 2.7a) exhibits oxidation of hydrogen gas, yielding hydrogen ions, which travel throughout the anode side in the direction to meet the electrolyte. The reverse mode occurs at the cathode side of PCEC (Fig. 2.7b).

Here the hydrogen ions leave the electrolyte and travel towards the cathode side and produce hydrogen gas. In some cases introduction of water on the dry side improved the device efficiency. In fact, some perovskite solid oxide materials used as cathode for PCFC system, could be used as cathode for PCEC. For example, the La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) has been Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 72 studied as a cathode for PCFC by Fabbri et al [111]. This materials has also been mentioned for use as anode for PCEC [112]. Similarly, the materials used as cathode for PCFC system were used as anode of PCEC. In some studies [111,113,114], Ni was researched as cathode for PCFC system and as anode of PCEC. Hence, for clearer comprehension, we will refer to "air electrodes" for the PCEC anodes and PCFC cathode, and "hydrogen (or fuel) electrodes"

for the PCEC cathodes and PCFC anodes.

As mentioned earlier, most of the protonic SOFCs and SOECs prototypes utilized Pt and Ni. However, Pt was not preferred due to its high cost. Ni was alternatively proposed as hydrogen electrode material, to reduce costs. Although both Pt and Ni performed well with BaCeO 3 -and SrCeO 3 -based proton-conducting electrolytes, their performance became poorer with increasing of Zr in the electrolyte composition. Large polarization losses such as SrZrO 3

and BaZrO 3 [115,116], were noticed. Sakai et al. [START_REF] Sakai | Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 90[END_REF] suggested that the poor performance of these electrodes was attributed to the limited interface area. They also proposed fabrication of plated electrodes, instead of the conventional pasted electrodes, to decrease the electrode particle size. As a matter of this fact, Ni-based composite materials have been investigated the most as hydrogen materials for PCFCs and PCECs.

Table 2.5 summarizes electrical characteristics of Ni-based cermets for hydrogen electrodes applications. The electronic conductivity (S cm -1 ) is typically measured by the 4 probes method, and the polarization resistance (R pol , Ω cm 2 ) obtained using EIS on symmetrical cells. The Ni-BZY and Ni-BCY demonstrated excellent catalytic properties and good current collection. Ni-based cermets tended to agglomerate after prolonged operation which led to a reduced three-phase boundary and increased cell resistance. Lei Bi et al. [113] suggested that the area specific resistance (ARS) of Ni-BZY was within the suitable range In case of air electrode applications, it is necessary to emphasize again that the two distinct systems involved different reactions; oxygen reduction for PCFC and water oxidation.

However, air electrodes for both systems played similar roles. They possessed a larger area of triple phases boundary (TPB, a meeting point of electrode, electrolyte and oxygen ion transfer into the electrolyte) to maximize electrochemical reactions [107,114,119] and to achieve a high electrochemical performance. In addition, Fabbri et al. [111] and He et al. [109] have investigated the air-electrode working mechanism. They indicated that both proton and oxygen ion transport to TPB were rate limiting. Only proton reactions at TPB were limiting in the electrolysis mode. Therefore, the ideal air electrode material for proton conducting electrolytes should also posses both protonic and electronic conduction.

In recent years, only a few materials having both protonic and electronic conductivities have been reported. Another example of mixed protonic and electronic conducting materials is Ln 2 NiO 4+δ (Ln = La, Pr, Nd) ceramics. They have attracted much attention as alternate air electrode materials for lower-temperature operations. These materials, also gave diffusivity of interstitial oxideions at intermediate temperatures. In addition, the thermal expansion coefficient (TEC) values of such Ln 2 NiO 4+δ cathodes exhibited a rather good mechanical compatibility with usual electrolytes (12.7x10 -6 K -1 , 13x10-6 K -1 , 13.6x10 -6 K -1 for Nd 2 NiO 4+δ , La 2 NiO 4+δ and Pr 2 NiO 4+δ respectively) [58]. These materials showed intense electrocatalytic activity for oxygen reduction reaction due to their high MIEC properties [56]. The polarization resistance (R pol ) of Ln 2 NiO 4+δ (Ln = La, Pr, Nd) compounds, obtained from impedance spectroscopy 43 Ω.cm 2 under water partial pressure at 750°C [125]. Other examples of composite cathodes are LSCF [127,128] and BSCF [123] oxide-based ceramics. The architecturedelectrode of BSFC-BCY10 gave lower polarization resistance of 0.19 Ω.cm 2 , while the LSCF with BZCY and LCC showed 0.19 Ω.cm 2 and 0.076 Ω.cm 2 , respectively. These complex perovskite materials are likely to be potential cathode materials for protonic ceramic fuel cell (PCFC) in the future.

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 76

Vibrational analysis of perovskite-related oxides

The analysis of atom/ion motions using the group theory is a standard procedure used by IR and Raman spectroscopists. It is known as factor group analysis. The symmetry properties of the crystal were determined by studying the effect of each symmetry operation in the factor group on each type of atom in the unit cell. Several symmetry operators were applied to the same vibrational unit. These constituted a "point group". At least one point (the center of gravity) remained unmoved, when the symmetry operations were carried out.

R e s u l t s o f factor group analysis, showing the IR and Raman active modes in the case of various expected point symmetry groups of perovskite-liked oxides, are summarized in Table 2.7. [140] The present group theory and vibrational modes were selected from the literature studied [74,129,130,[START_REF] De Andres | [END_REF][137][138][139][140]. The characters of the different irreducible representations were predicted from character table. The highest symmetry (Pm-3m) such as cubic structure had none of Raman active modes. Indeed, there was only one formula unit per unit cell and an infrared active 3F 1u phonon. In contrast, in the lower symmetry both Raman and IR modes were active.

This very important point underlined the fact that Raman scattering intensity would be very sensitive to any distortion of the cubic mean structure. Furthermore, because of the very local character of the Raman probe and the Polarisability of the chemical bond(s) involved in the vibrational mode, nanodomains could contribute independently to the spectrum [11,46,52].

Cerate and zirconate materials

For an ideal ABO 3 perovskite-like structure, a low wavenumber range (below ~200 cm -1 ) revealed the vibrations of the A ion network which coupled themselves with librations modes (T', R') of BO 6 ion-covalent entities leading to the lattice modes. The bending (δ) and stretching (υ) modes of BO 6 octahedra were expected in the 300-500 cm -1 and 600-900 cm -1 wavenumber range, respectively [START_REF] Zaafrani | Dotoral thesis "Protonation, distorsions structurales et espèces protoniques dans des pérovskites lacunaires[END_REF].

According to the orthorhombic phase with Pbnm space group for cerate perovskites, the irreducible representations yielded 24 Raman active modes and 25 IR active modes (Table 2.8) [13,[129][130][131][132][133][134][START_REF] Cavalcante | Perovskite-Related Oxides 91[END_REF]. For undoped BaCeO 3 at -196.15°C, Genet et al. [133] described the motions of each Raman mode below 400 cm -1 . They identified that there were 11 number of stretching coordinates; 3 number for Ce-O bonds and 8 number for Ba-O bonds. In addition, the authors considered 6 O-Ce-O and 36 O-Ba-O modes. In the higher wavenumber range, broad and weak bands were at about 520, 630 and 720 cm -1 . The authors suggested that this was due to the first-and/or second-order scattering induced by defects such as oxygen vacancies.

In the case of zirconates perovskites at room temperature, undoped SrZrO 3 presented an orthorhombic structure with a ZrO 6 octahedra tilted in the unit cell [START_REF] Cavalcante | Perovskite-Related Oxides 91[END_REF]. The SrZrO 3 presented 24 Raman active modes as indicated in Table 2.7. In the case of undoped BaZrO 3 , Ph. Colomban et al. [7] presented that this material had cubic symmetry and the Pm3m space group. It may be noted that such cubic symmetry in the first-order Raman activity was forbidden [7]. Yet the authors observed the presence of Raman spectra for such undoped BaZrO 3 with significant absolute intensity values induced by the short-range disorder. The Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 78 intensity increased after the partial substitution of Zr by lanthanide (Ln-doped BaZrO 3 ), due to the local distorstion induced by oxygen vacancies and Zr substitution. Table 2.8 presents the Raman and IR signatures characteristic of undoped zirconates perovskites studied by our group [12,52,[START_REF] Zaafrani | Protonation, distorsions structurales et espéces protoniques dans des perovskites lacunaires[END_REF]. The observed Raman wavenumbers (100-900 cm -1 ) of both pure barium zirconates and strontium zirconates could be distinguished in three ranges as discussed earlier. Ph. Colomban et al. [46,47] [ 12,49]).

Nickelate-based oxides

The lattice vibrational modes of stoichiometric and oxidized Ln 2 NiO 4+δ have been reported by Andres et al. [START_REF] De Andres | [END_REF]. In the case of stoichiometric Ln 2 NiO 4 (centrosymmetric D 2h point group and the primitive cell of the orthorhombic phase (Bmab)), 42 normal modes were expected (Table 2.7); 18 Raman active modes, 17 IR active modes and 7 modes were silent.

The Ni ions are surrounded by distorted oxygen octahedra but the structure could also be seen the lanthanide ionic radius combined with the mass effect. The same phonon, but associated with the Ln motions, would be shifted in a similar way but at a lower wavenumber at around

Ln O O Ln m m / ) 2 ( ω ω =
. The square root mass ratio was about 3 for the three materials. So the frequency would lie at around 150 -200 cm -1 region to the Ln stretching modes. Two Raman peaks, detected at the 220-275cm -1 region, were assigned to the bending modes of the O(2) and Ln ions. They were separated into motions along the x and y directions. Those parallel to the y-axis corresponded to the tetragonal-to-orthorhombic deformation and maintained a lower energy than the others. Moreover, the stretching and bending motions of the same ions in a vibrational unit were frequently related by the empiric relation, ω s = 2ω b .

The equivalent Ln bending mode to stretching mode was attributed to the 100-130 cm -1 . The highest-energy one-phonon peak was observed at around 680 cm -1 and was due to the Ni-O(1) stretching vibration. Small size of the rare-earth ions induced a more distorted structure with space for interstitial oxygen ions. This finally led to the presence of the tetragonal phase with many defects such as interstitial oxygen ions, or perhaps even to an orthorhombic Fmmm or even lower symmetry phase with extra oxygen ions. Table 2.9 indicates the observed IR and Raman characteristic phonon frequencies of tetragonal (I4/mmm) Ln 2 NiO 4 compounds and the mode assignments.

δ Ln-O(II) υ La-O(II) δ La-O(II)-Ni δ La-O(I)-Ni υ La-O(II)-Ni υ La-O(I)-Ni
The oxidized Ln 2 NiO 4+δ samples and/or high temperature phase had the tetragonal I4/mmm (D 4h ) symmetry with two formulas in the unit cell and one in the primitive cell.

Group theory predicted 2A 1g +2E g Raman active modes at the Γ point. However, there were 7 observed IR active modes and 6 observed Raman active modes mentioned in the literature consulted [START_REF] De Andres | [END_REF][137][138][139][START_REF] Lobo | [END_REF]144].

La 1-x Sr x Co 1-y Fe y O 3-δ materials

As we discussed in section 2.2.3, the LSC, LSF and LSCF compounds at room temperature, exhibited rhombohedral phase with the R-3c space group. Studies of these complex doubled perovskite are limited. Abrashev et al. [145] reported a case of LaMO 3

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 82 (M=Al, Mn) perovskite with R-3c space group [145]. In R-3c lattice, the La atoms occupied the 2a (

4 1 4 1 4 1 , ,
) positions and gave rise to four modes (

u g u g E E A A + + + 2 2
). The M atoms occupied the 2b ( 0 , 0 , 0

) positions and participated in four modes (

u u u E A A 2 2 2

+ +

). The oxygen atoms occupied the 6e (

4 1 2 1 , , + x x
) positions and took part in twelve modes

( u g u g u g E E A A A A 3 3 2 2 2 2 1 1 + + + + +
). Within these total 20 Γ-point modes, 18 corresponded to optical phonons. The A 1g +4E g modes were Raman active, the 3A 2u +5E u were IR active and the remaining 2A 1u +3A 2g were inactive. Ishikawa et al. [146] studied Raman spectra of the Orbital-Phonon coupling in LaCoO 3 with R-3c rhombohedral phase. The geometry of crystal structure was the same as that described by Abrashev et al. [145]. The Raman modes were located at 86, 172, 261, 432, and 584 cm -1 at -268.5°C. Their assignments are presented in Table 2.10.

Table 2.10:

Observed Raman frequencies of LSCF compounds [74,128,146,147]. Very recently, the LSC, LSF, and LSCF materials have been investigated using Raman spectroscopy by Rousseau et al. [74]. Figure 2.11 shows representative Raman spectra of LSCF sample containing the Co amount (5%). A typical Raman spectrum of LSCF consisted of two broad peaks centered at ~546 cm -1 and ~632 cm -1 , and one broad band at ~420 cm -1 , (Fig. 2.11g). Intensity of Raman spectrum increased remarkably at high temperature (>400°C). This indicated the presence of oxygen reduction of LSCF. Namely, the existence of two overlapping bands near 546 and 632 cm -1 on the spectrum was observed.

Samples Raman (RS) peak position / cm

Similarly, such overlapping bands result were observed by Siebert et al. [147].

At high temperature together with Jahn-Teller effect, the LSCF E g vibrational Raman mode at ~432 cm -1 , showed strong intensity decrease. Additionally, the broad band of existing

C C H H A A P P T T E E R R 3 3

"Potential secondary and corrosion phases"

In this Chapter, the potential secondary and corrosion phases characterized by vibrational spectroscopy will be addressed.

Chapter 3

Potential secondary and corrosion phases

Potential secondary phases

Alkaline earth metals are basic components that react easily with CO 2 [1][2][3][4]. A carbonation reaction for the perovskite-related oxides may be represented as:

2 3 2 3 BO ACO CO ABO + → + …(eq. 3.1)
where A, B are generally Ba, Sr, Ce, or Zr cations, respectively. Matsumoto [5] explained that this reaction occurs with 1 bars CO 2 at temperature less than a critical temperature -around 1025°C for Barium cerates and 575 -628°C for that of zirconates. Therefore, whenever hydrocarbon is used as fuel with barium cerate-based electrolyte, the cell should be operated at higher temperature than the critical temperature in order to avoid deterioration by the reaction with CO 2 .

Several studies suggest that cerate zirconate solid solutions are stable in CO 2 (~0.017 bar) [6][7][8]. For example, sintered perovskite SrCe 0.95 Yb 0.05 O 3-δ , SrZr 0.9 Y 0.1 O 3-δ and BaCe 0.6 Zr 0.3 Nd 0.1 O 3-δ ceramics were exposed to 800°C for 3 h in 100% CO 2 and the H 2 -CO-CO 2 mixture. In the case of SrCe 0.95 Yb 0.05 O 3-δ , CeO 2 and SrCO 3 were formed on exposure to both CO 2 and H 2 -CO-CO 2 mixture gases. The electrolyte reacted with CO 2 , as shown in Eq.

3.2-3.3, to form SrCO 3 , BaCo 3 and CeO 2 [5].

2 3 2 3 CeO SrCO CO SrCeO + → + …(eq. 3.2) 2 3 2 3 CeO BaCO CO BaCeO + → + …(eq. 3.3)

Raman/IR vibrational modes of carbonates

Fundamentally, for

- 2 3
CO ion in the free state, the molecule exhibits four normal modes [9]; υ 1 symmetric stretching mode ( 1 A′ , Raman active), υ 2 symmetric bending mode Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 96 Because of the covalent character of CO 3 vibrational unit, the stretching mode is well defined and narrow and thus easily detected, even at very low intensity. Although only oxygen atoms move in the symmetric stretching mode, the coupling with the cation is sufficiently strong to induce mass dependent shift and thus the carbonate composition can be identified. Librational and lattice modes are also rather strong and characteristic of each carbonate.

Table 3.1 summarizes the f Raman signature of carbonate compounds.. The Raman study of second phase occuring on surface electrolyte samples was verified by many authors [11][12][13][14]. Several type of electrolyte perovskite-oxide ceramics were exposed at intermediate temperature under low to high vapor pressure. In case of SrZr 0.9 Yb 0.1 O 3-δ ceramic, the Raman peak at ~1070 cm -1 was observed and assigned as traces of SrCO 3 . For different Ln-doped barium zirconates, Raman signature of BaCO 3 compounds was observed at 1060 cm -1 .

Chapter 3: Potential secondary and corrosion phases 97

Raman/IR vibrational modes of metal oxides

The common method to identify corrosion products is to compare Raman spectra of the corrosion layers with the reference spectra of expected phases [26][27][28][29][30][31].

Raman signature of iron oxides and other metal oxides are summarized in Table 3.2.

Furthermore, because of the lack of reliable literature, Raman study of La 2 O 3 -based and CoO.OH compounds will be discussed in the next section. Magnetite (Fe 3 O 4 ) crystallizes in the spinel structure with space group Fd3m. Iron ions occupy both tetrahedral and octahedral sites. It is an inverse spinel with metal conductivity at room temperature. Owing to the cubic ABCABC dense packing of the oxygen atoms, density reaches 5.18 [32,33]. The crystal structure of maghemite (γ-Fe 2 O 3 ) is closely related to the crystal structure of magnetite with vacancies (and possible proton insertion). Consequently, the density lowers to 4.87 [33]. The basic structure of γ-Fe 2 O 3 is cubic (space group: P4 3 32), whereas the ordered distribution of the cation vacancies results in a symmetry reduction (P4 1 2 1 2) [34]. The structure of α-Fe 2 O 3

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 98 corresponds to the R3c space group with a hexagonal ABAB dense packing of the oxygen atoms [33]. Density is therefore high (~5.23). Structures of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) are more complex. They can be roughly described as frameworks made of the dense oxygen atom packing presented above. This preserves a layered structure in which many oxygen atoms are missing and some others are replaced by OH -species. The density decreases, and the lowest value is measured for lepidocrocite (d=3). The second order mode at about 1320 cm -1 is generally the stronger one under green laser excitation and can be used for the detection of traces [28,35,36].

Lanthanum oxides/oxyhydroxide/oxycarbonates

Lanthanum oxide is a well-known compound that highly reacts with water and CO 2 , and hydroxide and carbonate compounds are produced [48,49]. This phenomenon also occurs with Recording the spectra as a function of the temperature guarantees the collection of convenient spectrum for each phase. The 458 nm line delivered by Ar + ion laser was used. The Raman scattering was performed under power of 2 mW throughout an objective of 50x (long working distance). The spectra were collected in the range of 60-4000 cm -1 . The spectra results, presented in Figure 3.2, are of 10 accumulations, each lasting 5 seconds.

The Raman scattering of La 2 O 3 powder, (Fig. 3.2a), was examined using Linkam device with the heating rate of 10°C.min -1 . When La 2 O 3 powder sample was kept in the air at room temperature (Fig. 3.2a), it rapidly hydrated Well-defined peaks at 143, 232, 287, 346, and 455 cm -1 , as well as a weak broadband at 612 cm -1 were observed. An intense signature at ~3606 cm -1 was shown due to symmetric stretching mode of O-H. These Raman signatures were rather similar to those in the relevant literature [15], and were attributed to La(OH) 3

phase, formed at the powder surface. of LaOOH were remained. However, bands at 300-500 cm -1 were modified: i) a broad band at 304 cm -1 appeared, and well-defined peaks at 310 and 343 cm -1 of spectrum at 350°C disappeared; ii) peaks at 384 and 425 cm -1 moved downshift to 371 and 398 cm -1 . Furthermore, peaks detected at 85 and 1071 cm -1 were observed. These could be assigned to the tetragonal phase of the La 2 O 2 CO 3 compound.

After heating the sample at 800°C for one hour, the Raman spectrum detected three welldefined peaks at 104, 181 and 391 cm -1 . It showed results similar to those in the literature available on the subject [15], and assigned to single phase of A-La 2 O 3 type. Consequently, we recorded Raman spectra during cooling step from 800°C to room temperature. It showed that the sample remained in the form of A-La 2 O 3 type. However, the Raman spectrum collected after opening the Linkam immediately showed the three peaks (104,192, and 408 cm -1 ) with slightly higher wavenumber. In our study, we obtained only a pure A-La 2 O 3 type from the sample of thermally treated La(OH) 3 powder. Orera et. al [15] cannot separate between Cand A-La 2 O 3 type by heating LaOOH phase to 400°C in a furnace under vacuum ambient for several hours. The Raman signatures showed at 103, 189, 312, and 404 cm -1 . Figure 3.2b shows well representative Raman spectra of La 2 O 2 CO 3 powder sample at room temperature. We observed peaks at 91,123,212,265,373,400,472,755, and 1095 cm -1 . These peaks were similar to those in the available literature, which assigned to II-La 2 O 2 CO 3 type [15]. However, a peak at 122 cm -1 was observed to be different from that in the literature -appeared two well-defined peaks at 112 and 125 cm -1 .

Cobalt oxides/oxyhydroxide

Heterogenite, a cobalt oxyhydroxide has been proposed as an alternative low cost natural material for numerous applications such as battery technology, catalyst, supraconductor and semiconductor sensor [46,[51][52][53][54]. This material has the formula CoO.OH with a brucite-like structure with layers of edge-sharing Co (III) oxohydroxo octahedral [46,54,55]. Two forms of heterogenite are identified, namely heterogenite-2H and heterogenite-3R. The heterogenite-2H has trigonal symmetry (R-3m) while the 3R possesses hexagonal symmetry (P6 3 /mmc).

The existence of such forms makes the formation of polytypes possible.

Burlet et al. [46] studied heterogenite (CoO.OH) using Raman spectroscopy (532 nm, 2 mW and x50 LWD). The authors showed four different types of Raman response of natural heterogenite samples; i) heterogenite, ii) dehydroxylated heterogenite, iii) Ni-bearing heterogenite, and iv) bured heterogenite. Show figures from the literature, the spectra of heterogenite exhibits two main peaks; at 495cm -1 , 623cm -1 and two minor peaks; at 1133 cm -1 and 1202 cm -1 . The authors also investigated the heterogenite as a function of temperature. A small flat piece of heterogenite formed in black mining was investigated by thermogravimetry (TG) and thermal expansion (TE). Experimental details are described in appendix D. The thermodynamic behavior, in particular the mass variation with temperature, (Fig. 3.3a), was observed. There were two mass losses at around 350°C and 850°C. This was similar to the study of Deliens [57]. He described the transformation corresponding to the following formula: The thermal expansion curve, (Fig. 3.3b), exhibits a huge expansion jump at 400°C, and a small one at 750°C. The abrupt jump at 400°C is due to dehydroxylation of hetrogenite into Co-spinel. This results in the structural phase transition from hexagonal to cubic phase of Cospinel. • Raman spectrum of heterogenite unaffected by laser heating (<0.5 mW) are characterized by a major peak at 498 cm -1 , and a broader peak centred at 623cm -1 .

The broad peak centered at 1144 cm -1 are similar to the result of the overlapping of the three minor peaks at 563 cm -1 , 623 cm -1 and 676 cm -1 . These positions are in agreement with those reported by Burlet et. al [46].

• Raman spectrum of laser-induced Co-spinel shows five visible peaks at 184, 463, 502, 605 cm -1 (shoulder) and 665 cm -1 (major peak). These positions are downshifted (~5-10 cm -1 ) because of the Raman response to the thermally treated heterogenite sample. This can be attributed to the very small size of the formed spinel crystals (nanometric scale), as has been discussed by Ai and Jiang [58].

• For the Co-spinel structure by artificial heating, the Raman spectrum shows a well-defined peaks at 689 cm -1 (main peak) and 616, 519, 482, 194 cm -1 . It exhibits a significant upward shift. In this chapter, three types of nickelates ceramics formula as Ln 2 NiO 4+δ (Ln = La, Pr, Nd), is the first investigated electrode series. We will report on its chemical and structural characterization against medium and high water vapor treatment (20 bar and 40 bar) at intermediate temperature. We will also discuss the ageing of materials against caustic ambient (CO 2 saturated water vapor pressure). The principal analysis techniques i.e. IR, Raman spectroscopy, TGA, TE, and X-ray diffraction are employed.

A.1 Preperation of wter use

According to the aim of study, we performed the protonation treatment using two kinds of water: i) CO 2 -free water or decarbonated water -we use of distilled water, and then deionized them by ion exchange resins "Milli-Q plus". Furthermore, a quantity of this deionized water was purged by nitrogen gas (N 2 ) for several hours in order to ensure of free CO 2 absorbed water.

ii) CO 2 -absorbed water or carbonated water-we use a commercial sparkling water "Perrier". The quantity of bicarbonate ( 3 HCO ) is 390 mg/l.

A.2 Procedure of "ex-situ" Autoclave treatment

The ex-situ autoclave treatment was performed using HTP (600°C/100bars) autoclave device, as shown in Figure A.2. The status of the seals has been importantly verified by our previous study 1 . The seal is an oval ring having a conical surface alloy Nimonic A80 with very high chromium content. The billet alloy Nimonic A80 has been widely investigated, and found in the USA. The "ring" tests were carried out by AREVA NP to specify the fracture strength and enable the sizing TOP Industry.

The procedure of protonation treatment is summarized in four steps: i) Preparing the sample and autoclave -placing the sample into the "goal-foil" holder in order to avoid direct contact between samples and autoclave chamber that may produce the contamination, then, put gently the samples into the autoclave chamber.

Introducing the prepared water for 5 ml into the autoclave chamber, and immediately close the autoclave device using 50 Nm force of ratchet. (Note: carefully close for 2-3 times in order to ensure that the device is tightly closed. Set the autoclave chamber with the conventional heater and controller, and carefully connect the right electric wire between the heater and controller to avoid any electrical circuit short. for fuel cells and electrolyzers ii) Start-up procedure -after setting up the autoclave in good condition, we then start the machine. First, set the target temperature as 75-100°C, and press the "green" button to start the machine. When the actual temperature is close to the target temperature, we increase the target temperature to 150°C, ... , 550°C. Note: increasing systematically the target temperature (interval step of 50-100°C) and observing carefully the pressure increase. The pressure can be limited below the limit of the saturation pressure by controlling the amount of water introduced. In case that the actual pressure is higher than target pressure, we can manually opening a safety valve (yellow) allowing the water vapor escapes, and adjust to the set pressure at investigated temperature (Table A.

2).

iii) Treatment observation -after the machine is stable at working conditions, we should observe at least 30-45 minutes. By then, the observation as well as data transfer should be regularly done every 1-2 days until the end of the treatment.

iv) Shutdown and opening -to shut down the treatment system, we first stop the machine by the "red button" and leave it cool down for several hours until the temperature of ( )

T L L T T L L L ∆ ∆ = - - = 0 1 2 0 1 2 α ...(eq. C.3)
where L 0 is the original length, L 2 is the length at temperature T 2 and L 1 is the length at temperature T 1 . The units of α are thus K -1 or °C-1 . Conversion factors are:

To convert to Multiply by 10 -6 /K 10 -6 /F 0.55556 10 -6 /°F 10 -6 /K 1.8 ppm/°C 10 -6 /K 1 10 -6 /°C 10 -6 /K 1 (µm/m)/°F 10 -6 /K 1.8 (µm/m)/°C 10 -6 /K 1 10 -6 /R 10 -6 /K 1.8

The coefficient of thermal expansion is also often defined as the fractional increase in length per unit rise in temperature. The exact definition varies, depending on whether it is specified at a precise temperature (true coefficient of thermal expansion or α or over a temperature range (mean coefficient of thermal expansion or α). The true coefficient is related to the slope of the tangent of the length versus temperature plot, while the mean coefficient is governed by the slope of the chord between two points on the curve. Variation in CTE values can occur according to the definition used. When α is constant over the temperature range then α = α . Finite-element analysis (FEA) software such as (MSC software) requires that α be input, not α .

Heating or cooling affects all the dimensions of a body of material, with a resultant change in volume. Volume changes may be determined:

T V V V ∆ = ∆ ∆ α 0 ...(eq. C.4)
Where ∆V and V 0 are the volume change and original volume, respectively, and α V represents the volume coefficient of thermal expansion. In many materials, the value of α V is anisotropic; that is, it depends on the crystallographic direction along which it is measured.

For materials in which the thermal expansion is isotropic, α V is approximately 3α 1 .

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 212 To determine the thermal expansion coefficient, two physical quantities (displacement and temperature) must be measured on a sample that is undergoing a thermal cycle. Three of the main techniques used for CTE measurement are dilatometry, interferometry, and thermomechanical analysis.

In this study, the mechanical dilatrometry technique was employed. With this technique, a specimen is heated in a furnace, and the displacement of the ends of the specimen is transmitted to a sensor by means of push rods. The precision of the test is lower than that of interferometry, and the test is generally applicable to materials with CTE above 5×10 -6 /K (2.8×10 -6 /°F) over the temperature range of -180 to 900 °C (-290 to 1650 °F). Push rods may be of the vitreous silica type, the high-purity alumina type, or the isotropic graphite type.

Alumina systems can extend the temperature range up to 1600 °C (2900 °F) and graphite systems up to 2500 °C (4500 °F). ASTM Test Method E 228 coves the determination of linear thermal expansion of rigid solid materials using vitreous silica push rod or tube dilatometers.

TE (Dilatrometry) instrument:

The TE measurements, in this study, were performed using a Setaram Setsys 1650 dilatometer instrument equipped with amorphous silica rod and support, as shown in Figure

D.4 The infrared spectrum

Infrared spectroscopy is based on the interaction of electromagnetic radiation with a molecular system, in most cases in the form of absorption of energy from the incident beam.

The frequencies of the absorption bands ν s are proportional to the energy difference between the vibrational ground and exited stated, Figure D.2.

Figure D.2: Principal of infrared absorption and its spectrum.

In a classical approach, the oscillating electric field E of the incident light with frequency ν 0 and amplitude E 0 can be written as where α is the polarizability of the lattice. If the lattice is vibrating with frequency ν m and amplitude q 0 , then the nuclear displacement can be written as:

t q q m πν 2 cos 0 = …(eq. D.8)
For small amplitudes, α is a linear function of q, and can be expanded as ...,

0 0 +         ∂ ∂ + = q q α α α …(eq. D.9)
where the partial derivative is evaluated at the equilibrium nuclear position. Combining equation D.6 -D.9, gives: Intensities of the X-ray diffraction peaks are affected by many variables such as the electron distribution around any atom or ion and the structure of the crystal. Scattering of the x-rays occur from the electron cloud surrounding the atom or ion, and, therefore, the intensities are a sum of all scattering from the electron cloud. Because the spacing within the electron cloud is smaller than the wavelength of the incident X-ray, partially destructive interference occurs from the sum. Moreover, an atomic scattering factor, f 0 , can be calculated for atoms and ions. The structure factor, F hkl , describes how the atomic arrangement of the crystal affects the x-ray diffraction peaks. The Rietveld refinement method, named for Le Bail Rietveld who developed the method, incorporates all of the variables that affect diffraction peaks, and involves the least squares fit to refine structural information of the sample.

( ) { } ( ) { } [ ] t t E q q t E P m m ν ν π ν ν π α πν α - + +         ∂ ∂ + = 0 0 0 0 0 0 0 0 2 cos 2 cos 2
It should be emphasized that the Bragg equation (Eq. E.

Crystal system Constraints

= 2 1 hkl d Cubic a = b = c α = β = γ = 90° 2 2 2 2 a l k h + + Tetragonal a = b ≠ c α = β = γ = 90° 2 2 2 2 2 c l a k h + + Orthorhombic a ≠ b ≠ c α = β = γ = 90° 2 2 2 2 2 2 c l b k a h + + Hexagonal a = b ≠ c α = β = 90° γ = 120° 2 2 2 2 2 3 4 c l a k hk h + + + Trigonal/ Rhombohedral a = b = c α = β = γ ( ) ( )( ) ( ) α α α α α 3 2 2 2 2 2 2 2 cos 2 cos 3 1 cos cos 2 sin + - - + + + + + a kl hl hk l k h Monoclinic α = γ = 90° β β β β

Appendix F Neutron Diffraction

Among most other methods, neutron is a powerful and direct method allows a detailed understanding of the static and dynamic properties on an atomic scale of materials that occur in our environment. Combined with x-ray scattering a very large range of momentum and energy transfer can be covered thanks to the high complementary of both techniques. The most relevant, unique character of neutrons that cannot be matched by any other techniques, can be summarized follows 1 :

-The neutron interacts with the atomic nucleus and not with the electrons as x-rays do. This has important consequences: i) the response of neutrons from light atoms like hydrogen or oxygen is much higher than for x-rays, ii) neutron can easily distinguish atoms of comparable atomic number, iii) neutrons distinguish isotopes: for example, deuteration of macromolecules allows to focus on specific aspects of their atomic arrangement or their motion.

-For the same wavelength as hard X-ray, the neutron energy is much lower and comparable to the energy of elementary excitations in matter. Therefore, neutrons do not only allow the determination of the 'static average' chemical structure, but also the investigation of the dynamic properties of the atomic arrangements that are directly related to the physical properties of materials.

-By virtue of its neutrality, the neutron is rather weakly interacting with matter, which means that there is almost no reaction damage to living objects under study. Also, the rather weak interaction with matter results in a large penetration depth and therefore the bulk properties of matter can be studied. This is also important for the investigation of materials under extreme conditions such as very low and high temperature, high pressure, high magnetic and electric fields, etc. Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 228 -The neutron carries a magnetic moment that makes it an excellent probe for the determination of magnetic structures and magnetic excitations.

F.1 Properties of Neutron

The neutron is a nuclear particle with a mass rather close to that of the proton: The neutron does not exist naturally in free form, but decays into a proton, an electron, and and anti-neutrino. The neutron lifetime, τ = 886 s, is much longer than the time of a neutron within a scattering experiment, where each neutron spends merely a fraction of a second.

Neutrons for scattering experiments are usually extracted from a moderator, which shows down neutrons produced in a spallation source or reactor. The thermalized neutron are extracted from the source through beam ports, which are either evacuated or helium filled tubes looking into moderator. If the moderator is designed correctly, the probability P(υ) of neutrons having a velocity between υ and υ+dυ follows a Maxwell-Boltzmann distribution:

( ) υ υ π π υ υ υ d e T k m d P T k m B B 1 2 2 2 2 4 ⋅ - ⋅ ⋅         ⋅ ⋅ = ⋅ … (Eq. F.2)
where m is the neutron mass, k B is the Boltzmann constant (1.381x10 -23 J.K -1 .mol -1 ) and T is the temperature of the moderator material. The maximum of P(υ) is reached for the neutron velocity m T k B 2 = υ . This relation shows the reason of using the popular terms hot, thermal and cold neutrons. The maximum neutrons fluxes measured by Laboratoire Léon Brillouin (LLB) is 17.5x10 9 neutrons.cm -2 .s -1 for the cold source, 7.52x10 9 neutrons.cm -2 .s -1 for the hot source, and 3.93x10 9 neutrons.cm -2 .s -1 for the thermal one 1 .

1 Experimental Facilities, Laboratoire Leon Brillouin, CEA Saclay, France, 2003.

F.2 Interaction of Neutrons with condensed matter

The neutron interaction with matter is weak. In the following distinguish between three different cases as shown in the -Absorption: the neutron beam intensity is reduced when crossing the sample, but the propagation direction stays unchanged.

-Refraction: the incident beam changes its direction, but is not weakened. The deflection angle (α) is small (a few degrees).

-Scattering: the incident neutron plane wave is scattered by the nuclei and yield an outgoing spherical wave of reduced intensity at the scattering angle 2θ.

F.2.1 Absorption

In this process, some neutrons lost by being absorbed in the nuclei, which then emit radiation or undergo fission. The intensity of the transmitted neutron beam is given by The values σ a have been determined experimentally; they are generally small with a few well-known exceptions. It is conventional to tabulate σ a for a neutron υ = 2200 m/s (corresponding to λ=1.8 Å). For most elements, the energy of thermal neutrons is far off from any absorption resonance; in this case, σ a is inversely proportional to the neutron velocity.

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 230 An important application of absorption is the detection of neutrons using materials with large σ a (e.g. 3 He). Neutron detectors are based on exothermic reactions. The energy released in these processes is orders of magnitudes higher than the kinetic energy of the thermal neutrons being detected. The pulse height generated by the ion pairs is therefore almost independent of the energy of the detected neutrons, unfortunately it hence cannot be used for energy analysis.

F.2.2 Refraction

For neutron technique, a refraction index defined by scattering densities may be defined instead. Assuming that an incoming neutron wave is scattered from a thin plate, Fig. The refraction index is then given by: where the glancing angle for neutrons with wavelength λ=1 Å is e.g. as small as 6' for natural nickel.

F.2.3 Scattering

The principal of the scattering of particles by a sample is the same for most kinds of In a general sample, the neutron scattering length varies from nucleus to nucleus. This can be due to the variation of the nuclear spin direction, or to variation between isotopes of the same element -or between different elements. The latter is a static disorder that for a macroscopic sample can be treated in the same way as a time-dependent disorder, since we can assume the sample is large enough to essentially represent an ensemble average.

List of Table

233

L LI IS ST T O OF F T TA AB BL LE E

Figure 1 :

 1 Figure 1: Schematic diagram of a stand-alone electrolyzer and fuel cell-based management

Figure 2 :

 2 Figure 2: Schematic diagram of a) solid oxide fuel cell (SOFC), b) proton conducting fuel cell (PCFC) and c) proton-conducting electrolysis cell (PCEC).

Figure 3 :

 3 Figure 3: Schematic reaction steps of single-phase anode material with simultaneous proton

Figure 4 :

 4 Figure 4: Phase diagram of the one component H 2 O and indication of the working conditions of different reactor types in nuclear power plant detailled in the text

Figure 1 . 1 :

 11 Figure 1.1: Scheme of (a) Point defects (b) Schottky and Frenkel defect forms in metal oxides solids (MO).

Figure 1 . 2 :

 12 Figure 1.2: Schematic of a single donor acceptor of hydrogen interaction (a-b), of an ionic proton (c); and d the potential energy well of proton involved in the A-H..B

  The experimental results helped to calculate the dissociation energy (D) of free OH -(OH -= O 2-+ H + ), i.e. 4.70 eV, which was similar to that value derived from the thermodynamic data (4.71 eV). Corresponding to the vibrational stretching excitation of the O-H oscillator, the O-H stretching peaks varied as a function of transition energy (E), and the observed overtones were then given by E 0n = E n -E 0 , with n=1, 2, 3…. In addition, the value Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 36 for E 0n was obtained from overtones of the O-H stretching mode which occurred at approximately twice, thrice, four times the frequency of the O-H fundamental peak (Fig.1.3a). Note, the fundamental O-H peak is very narrow, and detected near the high wavenumber, 3700 cm -1 for IR and 3655 cm -1 for Raman.

Figure 1 . 3 :

 13 Figure 1.3: a) overtone series of the stretching mode in Mg(OH) 2 b) Morse potential well ofMg(OH) 2 and its linear superposition schematically presented the formation of proton conduction band (CB) above the fifth excited level, c) delocalization of proton in CB, d) diffusion of a proton vacancy[18].

Figure 1 .

 1 Figure1.3b shows an interesting feature, namely that the overtone sequence appears to come to an abrupt end above the 5 th exited level. It gives a linear array of OH -ions separated by the same distance as in the Mg(OH) 2 structure. In fact, a series of deep wells, where the protons are located on the 0 level, is defined as the valence band (VB). Above the 05 th transition with energy of 2.0 eV, the superposition produces a continuum energy region, defined as a proton conduction band (CB).

figure 1 . 4 ,

 14 figure 1.4, generally, five different proton motion types are schematized: a) tunneling, b) jumping, c) ballistic or Kramer's regime, d) liquid-like diffusion and e) gas-like motion. The hydrogen normally hops from one site to another by thermal activation. This hopping activity requires sufficient screening between ions that host lattice potential to become dominant.

Fig. 1 .

 1 Fig. 1.5 presents the schematic of metal oxides (MO x ) with the hydrogen defects, bonded

Figure 1 . 5 :

 15 Figure 1.5: H + defect under water vapor pressure interacting on surface and bulk of metal oxide (MO) n framework [29].

CsHSO 4 and

 4 NH 4 H(S/Se)O 4 have been widely studied[8,20,32]. Hydrogen uranyl phosphase, H 3 OUO 2 PO 4 .3H 2 O (HUP), is an interesting case of phase transition[20]. Four phases defined by temperature --I (-3.15°C), II (-3.15 to -75.15°C), III (-75.15 to -123.1°C) and IV (below -123.1°C) --were observed. In phase I, a quasi-liquid state of water layer related to the rotational and translation disorder of H 2 O and H 3 O + species was observed with considerable orientational disorder of phosphase tetrahedra. In phase II, the structure became orthorhombic with drastically reduction of the translational disorder. In this phase, the protonic species could also be described in term of H 5 O 2 + and H 9 O 4 . In phase III, the rotation of water molecules hydrogen bonded to PO 4 groups and the structure became uniaxial and long range in the form of HUP sheets. Finally, in phase IV, the last degree of rotational freedom of the water molecules around the intra-square O...O directions was frozen,and the system appeared completely ordered.CsHSO 4 , is a good example to illustrate specific defects in protonic materials under partial pressure of water. The structure of room temperature phase I consists of infinite (HSO 4 -

Figure 1 . 6 :

 16 Figure 1.6: Determination of protonic species content : (a) elastic peak intensity variation as a function of temperature and (b) mass loss detected by TG analysis in the case of SrZrLnO 3-δ [14,37], Note: schema draw in (b) illustrates the distinction of bulk and surface protons.

Figure 2 .

 2 Figure 2.1a, where A is a large cation (e.g. akali, alkaline earth, rare earth ions) and B is a small or medium sized metal ion often a transition metal ion. The name corresponds to natural mineral CaTiO 3 . The ideal cubic structure (Pm-3m) is made of two sub-lattices: (i) large-sized 12-coordinated cations at the A site where Coulombian interactions are important (ii) small-sized 6-corordinated cations at the B site formed of BO 6 octahedral with covalent bond.

Figure 2 . 1 :

 21 Figure 2.1: a) Ideal perovskite structure (ABO 3 ) b) a perovskite-related structure in Ruddelsden-Popper phase, (A 2 BO 4 ) [3].

Chapter 2 :

 2 Perovskite-Related Oxides 57 (

Figure 2 .

 2 Figure 2.2 shows the crystal symmetries detected in the case of

Figure 2 . 2 :

 22 Figure 2.2:The effect of ionic size of A-and B-site cations on the observed distortions of the perovskite structure[3].

Figure 2 .

 2 3 illustrates the phase diagram of undoped BaZrO 3 , SrZrO 3 , BaCeO 3 and SrCeO 3 . (Note: the data were gathered and summarized from previously published literature[13][14][15][16][17][18][19][20][21]). The phase transitions in such perovskite-based oxides were performed in temperature range of 25 to 1200°C using X-ray diffraction (XRD) and/or high-resolution neutron powder diffraction.

Chapter 2 : 59 Figure 2 .

 2592 Figure 2.3 clearly shows that BaZrO 3 and SrCeO 3 undergo more phase transitions than SrZrO 3 and BaCeO 3 . Sugimoto et al.[19] report that the structure of BaZrO 3 -remained in

Ln 2

 2 NiO 4+δ (Ln = La, Pr, Nd) ceramic samples, namely nickelate oxides, exhibited a significant range of oxygen non-stoichiometric composition (δ > 0). An excess of oxygen and charge compensating Ni 3+ ions (holes) stabilized the Ruddlesden-Poper phase by reducing the instrinsic charge separation between the electropositive La 2 O 2 and NiO 2 layers and the structural strain due to the misfit between the La 2 O 2 and NiO 2 layers.

Figure 2 . 4 :

 24 Figure 2.4: Approximate structural phase diagram for La 2 NiO 4+δ[55].

Figure 2 . 5 :

 25 Figure 2.5: Summary of phase modification of the three lanthanides (La, Pr, Nd) nickelates.

Figure 2 .

 2 Figure 2.5 illustrates structural transition of the lanthanides (La, Pr, Nd) nickelate materials over low-and high-temperature range. Concerning non-stoichiometric Nd 2 NiO 4+δoxide, at room temperature, the structure was heavily dependent on oxygen excess[6]. The polycrystalline compounds exhibited biphasic phase between orthorhombic and tetragonal with small amount of excess oxygen (δ < 0.065). At high oxygen excess, the structure became orthorhombic phase. According to Fernandez-Diaz et al.[60], the presence of a distribution of unit-cell parameters in octahedral phase has been shown by neutron powder diffraction study. The orthorhombic strain defined as

  1-x Sr x Co 1- y Fe y O 3-δ powders. The authors observed a weight change in TG measurement (in air above 500°C. up to 1100°C) resulting from both the loss of lattice oxygen (reduction) and the uptake of oxygen (oxidation). On increasing the Sr content for La 1-x Sr x Co 0.2 Fe 0.8 O 3-δ compound, the temperature at which TG weight loss became significant decreased, and the magnitude of oxygen loss increased. Orthorhombic to rhombohedral phase transition was detected for Sr amount of ~ 0.1, at 170 -250°C.Thermal and chemical analysis of La 0.5 Sr 0.5 Fe 1-x Co x O 3-δ (x = 0, 0.5, 1) materials were reported by Lein et al.[76]. The samples were treated up to 1000°C under N 2 or Air atmosphere. They observed that the sintered materials exhibited the rhombohedral symmetry with R-3c space group at room temperature. The second order rhombohedral -cubic phase transition of La 0.5 Sr 0.5 Fe 0.5 Co 0.5 O 3-δ was located approximately at ~350°C. Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 68 Soldati et al.[77] investigated reduction and oxidation cycle of the nanocrystalline La 0.6 Sr 0.4 Fe 0.2 Co 0.8 O 3-δ oxides by X-ray absorption Near Edge Spectroscopy (XANES) and Xray powder diffraction (XPD). They confirmed that this material reacted differently under reductive or oxidative atmosphere. Structural modification was observed when treated for 24 hours under Ar atmosphere at 500°C. This resulted in the co-existence of Rhombohedral symmetry with another oxygen vacancy ordered phase. The reduction cycle explained the change in formal valance of the transition metal atoms to lower oxidation state (i.e. Fe 4.25+ to Fe 4.02+ and Co 3.36+ to Co 3.07+ ), together with the formation of oxygen vacancies.Emmerlich et al.[78], studied the effect of the ratio of A-site cations (La+Sr) to B-site cations (Co+Fe) enhancement on the defective structure of the La 1-x Sr x Co 1-y Fe y O 3-δ The use of magnetron sputtering from a designed target allowed the authors to control the chemical composition via synthesis temperature variations. The study showed -the formation of LSCF

Figure 2 . 6 .

 26 Figure 2.6. This data clearly illustrated that perovskite-type oxides had the highest proton

Figure 2 . 6 :

 26 Figure 2.6: Proton conductivities of oxide systems vs reduced inverse temperature [80].

Figure 2 . 7 :

 27 Figure 2.7: Working diagram of anode and cathode of PCFC (a) and PCEC -reverse mode of PCFC (b) Note: a part involve with fuel may call 'hydrogen electrode' and one involve with oxygen/air call 'air electrode'.

below 1 . 5 Ω

 15 cm 2 . Mather et al. [118], have studied Ni-CaZr 0.95 Y 0.05 O 2.975 (Ni-CZY) and Ni-SrZr 0.95 Y 0.05 O 2.975 (Ni-SZY) cermet anodes for PCFCs prepared by similar mathod. They suggested that the overall polarization resistance was sensitive to the level of proton conductivity in the ceramic-oxide phase. The Ni-CZY electrode was known to produce a polarization resistance higher than Ni-SZY electrode. Comparing chemical stability between Ni-BCY and Ni-BZY [119], the Ni-BCY cermet electrode showed instability in H 2 O and CO 2 containing atmosphere, which was unsuitable for practical operation in alternative Chapter 2: Perovskite-Related Oxides 73 carbonaceous fuels. On the contrary, Ni-BZY showed better chemical stability in such carbon dioxide atmosphere.

Chapter 2 :

 2 Perovskite-Related Oxides 75 measurements on symmetrical cell, is shown in Fig.2.8[126]. The Pr phases show good electrode materials according to their lowest polarization resistance values.

Figure 2 . 8 :

 28 Figure 2.8: Arrhenius plots of the polarization resistance (R pol ) of Ln 2 NiO 4+δ (Ln = La, Nd, Pr) [126].

  had also studied Raman scattering of pristine Ln-doped (Yb, Y and In) barium zirconate and barium cerate, Figure2.9a. A part of their work was conducted at room temperature. The results showed that the broadening of the Zr-O stretching mode in the case of pristine SZ:Yb was located at ~730 cm -1 , and was assigned to the octahedron modes perturbed by oxygen vacancies. Figure2.9b shows Raman spectra of Yand Zn-doped BaCeZrO 3-δ observed by Slodczyk et al.[12]. The author observed seven Raman broad lines, This spectrum could not be assigned to the monoclinic symmetry. In other words, the authors suggested that it was in a multiphasic, disordered material. Moreover, the presence of such nanodomains gave rise to the Raman activity depending on the B-site cation octahedra (BO 6 ) polarization. For instance, the contribution modes involving Ce-O and Zr-O bonds were much higher than those of Zn-O bond.

Figure 2 . 9 :

 29 Figure 2.9: Raman spectra characteristic of SZ, SZ:Ln, and protonated SZ:Ln (a) and BCZ:Y,Zn (b) (Note: reference data from our previous works, Slodczyk et al.

Figure 2 . 10 :

 210 Figure 2.10: Raman spectra of the stoichiometric Ln 2 NiO 4 compounds at temperature above low-temperature structural phase transition [136].

Figure 2 .

 2 Figure 2.10 shows Raman spectra of the stoichiometric Ln 2 NiO 4 compounds studied by Andres et al. [136]. The most intense peak at around 450 cm -1 is related to the O(2) stretching modes. The corresponding wavenumber was observed at 443 cm -1 for La 2 NiO 4 , 457 cm -1 for Pr 2 NiO 4 , and 470 cm -1 for Nd 2 NiO 4 . This was not due to a change in the Ni-O(2) distance which was nearly constant for Bmab symmetry compounds (La, ~2.235Å; Pr, ~2.23 Å; Nd, 2.25 Å). It was due to the shortening of the Ln-O(2) distance induced by decreasing

Chapter 2 :

 2 Perovskite-Related Oxides 83 two components was located at 540 cm -1 . An observation of a band near 650 cm -1 assigned to A 2g revealed structural distortion due to the Jahn-Teller effect.

Figure 2 . 11 :

 211 Figure 2.11: Characteristics of Raman signature at room temperature of (a) LaCoO 3 , (b) La 0.7 Sr 0.3 CoO 3 , (c) LaCo 0.8 Fe 0.2 O 3 , (d) La 0.7 Sr 0.3 Co 0.8 Fe 0.2 O 3 (-5% Co), (e) La 0.7 Sr 0.3 Co 0.8 Fe 0.2 O 3 , (f) La 0.7 Sr 0.3 Co 0.8 Fe 0.2 O 3 (+5% Co) and (g) La 0.7 Sr 0.3 Co 0.5 Fe 0.5 O 3 samples. *Bands assigned to Co 3 O 4 particles [74].

( 1 A′

 1 ′ , IR active), υ 3 asymmetric stretching mode ( E′ , IR and Raman active) and υ 4 asymmetric bending mode ( E ′ ′ , IR and Raman active). The normal modes of these four vibrations are shown in Fig. 3.1.

Figure 3 . 1 :

 31 Figure 3.1: Normal modes of vibration of pyramidal XY 3 molecules [9,10].

Chapter 3 : 3 ,

 33 Potential secondary and corrosion phases 99 Rare earth (RE)-rich compounds such as nickelates (Ln 2 NiO 4+δ ) perovskites. Raman scattering of lanthanum oxide-based materials was studied. Rare earth (RE) oxides can be formed in hexagonal phase, namely A-type RE oxide, with space group 1 3m P . The phonons can be classified according to irreducible representation of the D 3d point group: ( ) IR active. Another form, C-type RE sesquioxides, are body center cubic phase with space group Ia-3. This structure is related to fluorite structure with each RE ion located at the centre of the cube from which two of the eight nearest neighbor oxygen of the fluorite structure have been removed. Due to body-centered structure, the unit-cell contains the primitive structure twice. So, only eight unit formulas must be used to theoretically determine the number of vibrations. The irreducible representations for the optical and acoustical of Raman active are 22 modes of La 2 O 2 CO 3 and La 2 (CO 3 ) 3 .xH 2 O powders were collected from our collaboration with SPCTS Laboratory, UMR CNRS 7315, Limoges. Raman spectra were recorded at elevated temperature (30-800°C) using Labram HR800 Raman micro-spectrometer (Horiba Jobin Yvon SAS, Lonjumeau, France) equipped with Linkam (Linkam Scientific Instruments Ltd., Surrey, United Kingdom) heating stage. The above materials are very hydroscopic.

Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 100 Figure 3 . 2 :

 10032 Figure 3.2: Raman spectra of a) La 2 O 3 based sample at different temperature b) La 2 O 2 CO 3 at room temperature.

Figure 3 .

 3 Figure 3.3 shows thermal analysis result of a sample of cobalt oxyhydroxide (CoO.OH).

Figure 3 . 3 :

 33 Figure 3.3: Thermal analysis of CoOOH (black mining) a) TGA curve under He-atmosphere, 10°C.min -1 and b) Therman expansion curve under Ar-atmosphere, 10°C.min -1 .

Figure 3 . 4 shows 104 250Figure 3 . 4 :

 3410434 Figure 3.4 shows Raman spectra of heterogenite and Co-spinel phase. The Raman scattering was performed using 458 nm laser line. The Co-spinel phase was transformed from originated heterogenite material by i) laser-induced heating/transformation ii) thermal treatment at 800°C under argon atmosphere. The Raman spectra presented in Fig. 3.4 show that:

Chapter 7 : 6

 76 Chemical and structural stability of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ ceramic vs. medium/high water vapor pressure 145 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ δ δ δ ceramic vs. medium/high water vapor pressure" In this chapter, we will present the chemical and structural characterization of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF6428) ceramic samples, as our second investigated electrode series. The protonation treatments were under medium and high H 2 O pressure (20 bar and 40 bar) at intermediate temperature. The result will also be demonstrated ageing of the material against CO 2 -saturated water vapor pressure. The principal analysis techniques i.e. IR, Raman spectroscopy, TGA, TE, and X-ray diffraction are employed.

Figure A. 2 :

 2 Figure A.2: HTP (600°C/100bars) autoclave device; heater and autoclave chamber (left), pressure and temperature controller (right).

C. 4 .

 4 The very low thermal expansion of amorphous silica makes a direct comparison of asmeasured data possible. Flat pieces of the investigated perovskite-related oxides ceramics are measured in 30-700°C temperature range in both heating and cooling for three successive cycles with heating rate of 10°C. A ceramic piece with ~10 mm. was placed into the dilatometry instrument, and pressed by the am amorphous silica rod with the charge approximately 150 N. The measurement were then run under argon atmosphere. The argon gas is Alphagaz-2 grade (99.99% purity: H 2 O < 0.5 ppm vol., O 2 < 0.1 ppm vol., CO < 0.1 ppm vol., CO 2 < 0.1 ppm vol., H 2 < 0.1 ppm vol., C n H m < 0.5 ppm vol.) provided by Air Liquide company, France.

Figure C. 4 :

 4 Figure C.4: The thermal expansion (TE) Setaram Setsys 1650 dilatometer instrument equipped with amorphous silica rod and support.

6 )

 6 This electric field interacts with and distorts the electron cloud of the lattice, inducing an electric dipole P given by

  . D.10) where the first term n the right side of the equality in Eq. D.9 is the Rayleigh scattering term, which has a frequency equal to the incident frequency, ν 0 . The stoke and anti-Stokes terms, having frequencies shifted from the incident frequency, are partial derivatives of the polarizability act as coefficients for the Stokes and anti-Stokes terms, thus indicating that these Raman terms are non-zero only if there is a change in the polarizability ( molecule vibrates. If the polarizability does not change during a vibration, this coefficient is zero, and the vibration is not Ramanactive. for fuel cells and electrolyzers 224 atoms as is seen for the two darker atoms in the top plane and one plane beneath. The phase shift comprises of two shares, ∆ 1 and ∆ 2 , the sum of which equals 2dsinθ for any arbitrary angle θ. Constructive interference for the reflected wave, however, can only be achieved when the phase shift 2dsinθ is a multiple of the wavelength.

Figure E. 1 :

 1 Figure E.1: Visualization of the Bregg equation. Maximum scattered intensity is only observed when the phase shifts add to a multiple of the incident wavelength λ.

  1 A. Furrer, Neutron Scattering in Novel Materials, World Scientific Publishing, Singapore, 2000.

  Fig. F.1:

Figure F. 1 :

 1 Figure F.1: Schematic view of the three different types of neutron interaction with matter.

  is the intensity of the incoming neutrons, N 0 the density of the nuclei, σ a the corresponding absorption cross section and d the thickness of the sample.

F. 2 ,

 2 the refractive index n r can be derived by integrating the overlap of the non-scattered incoming wave with the scattered one over the radius element L of the plate with thickness ∆ yielding Fresnel integrals. If L is large compared to the neutron wavelength λ, the integral can be solved.

Figure F. 2 :

 2 Figure F.2: Refraction from a thin plate with thickness, ∆.

  ρ is the number of nuclei per unit volume and b is the mean value of the scattering length of the nuclei. Total refraction can be reached for glancing angles

  radiation. The geometry of a scattering experiment is shown in Fig. F.3. Incident particles (neutrons, photons, etc.) having a wavevector k i and a flux F (particles per cm 2 and s) are acattered by the sample into the element of solid angle dΩ = sin ΘdΘdΦ. then the number of particles scattered per second into dΩ with wavevector k f is given by . F.6)where dσ/dΩ is the differential cross-section with the dimension of an area.

Figure F. 3 :

 3 Figure F.3: Geometry for a scattering experiment. A plane wave of neutron that propagates along the z-direction is scattered by the sample into a solid angle dΩ.

F. 3

 3 Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 232 Instrumental aspects In this research, the neutron diffraction measurements were performed in the French neutron facility Orphée research at the CNRS-CEA Léon Brillouin Laboratory (Saclay, France). A cylindrical vanadium sample holder is employed. Neutron experiments were carried out on a G4.1 cold Neutron Two-Axis Diffractometer PYRRHIAS equipped with a vertical focusing pyrolytic graphite monocrometer and a 800-cells multi-detector. The wavelength of 0.24226 nm was used. The collecting neutron diffraction patterns were home-made furnace requiring high vacumm (10 -7 Torr).

Figure 1 : 4 Figure 2 : 11 Figure 3 : 13 Figure 1 . 1 : 31 Figure 1 . 2 : 34 Figure 1 . 3 : 36 Figure 1 . 4 : 38 Figure 1 . 5 : 40 Figure 1 . 6 : 46 Figure 2 Figure 2 . 2 : 57 Figure 2 . 3 : 58 Figure 2 . 4 : 65 Figure 2 . 5 : 66 Figure 2 . 6 : 69 Figure 2 . 7 : 71 Figure 2 . 8 : 75 Figure 2 . 9 : 79 Figure 2 . 10 : 80 Figure 2 . 11 : 7 83 Figure 3 . 1 : 96 Figure 3 . 1 : 100 Figure 3 . 3 : 103 List of Figure 237 Figure 3 . 4 :long working distance objective 104 Figure 8 . 1 : 163 Figure 8 . 2 :Figure 8 . 4 :Figure 8 . 5 :Figure 8 . 6 :Figure 8 . 7 :Figure 8 . 8 :Figure 8 . 9 :Figure 9 . 1 :Figure 9 . 2 :Figure 9 . 3 :Figure 9 . 4 : 187 Figure 9 . 5 :Figure 9 . 6 :Figure 9 . 2 :Figure 9 . 3 :Figure A. 1 :Figure A. 3 :Figure B. 1 :Figure B. 2 : 1 208Figure C. 2 :Figure C. 3 :Figure C. 4 :Figure D. 1 :Figure D. 2 :Figure D. 4 :Figure E. 1 :Figure F. 1 :Figure F. 2 :Figure F. 3 :

 1421131311311234133614381540164622257235824652566266927712875297921080211783319631100331032373410481163828485868788899192939418795969293131212341241123 Figure 1: Schematic diagram of a stand-alone electrolyzer and fuel cell-based management (EBM) system configured for renewable-resource inputs 4

Table 2 :

 2 Electrochemical reactions occurring in SOFC, PCFC, and PCEC system.

					SOFC					PCFC	PCEC
	Anode	H	+	O	-	→	H	O	+	2	e	-

  The research was focused on lanthanide-modified barium/ Zr 0.3 Y 0.16 Zn 0.04 O 3+δ ) materials. As mentioned above, the partial substitution of the Zr 4+

	strontium	zirconate	(BaZr 0.9 Yb 0.1 O 3+δ ,	SrZr 0.9 Yb 0.1 O 3+δ ,	BaZr 0.25 In 0.75 O 3+δ	and
	BaCe 0.5

Table 3 : Description of investigated perovskite-related oxides. Sample name Composition Abb. Application Collaborators/Providers

 3

	Zn,Y-doped barium	BaCe 0.5 Zr 0.3 Y 0.16 Zn 0.04 O 3-δ	BCZY:Zn electrolyte	Department of Materials,
	mixed cerate				Imperial College London
	zirconate
	Er-doped strontium	SrZr 0.9 Er 0.1 O 3-δ	SZE	electrolyte	AREVA NP
	zirconate
	Rare-earth nickelate	La 2 NiO 4+δ	LNO	cathode	ICMCB, Université de

Table 1 .1: Approximate

 1 classification of O-H…O hydrogen bonds[16].

	Hydrogen bond	R(O..O) distance A°	Wavenumber*/Examples
	Weak	2.70	>3200	H 2 O (ices, hydrates)
				R-OH (alcohols, phenols)
	Moderate	2.70-2.60	3200-2800	RCOOH (acids)
	Strong	2.60-2.40	2800-700	MH (RCOO) 2 (acid salts)

Table 1 .3: Direct

 1

	Direct methods	Indirect methods
	H-NMR, microwave relaxation, neutron	DTA/DSC, TE, Raman
	scattering, IR spectroscopy, TGA	spectroscopy
	The direct methods probing hydrogen atom and H-bonds commonly employ, (Table 1.3,
	H-NMR), microwave relaxation, neutron scattering, IR spectroscopy and TGA techniques.

and indirect measurement methods of H-content in solids.

  Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 44 narrowed NMR signals. In addition, the narrowing 1 H NMR signals resulted from complete isotope local motion with partial average of the intermolecular dipolar interaction by translational motion. Work on this subject is rare. Hellmut et al[34] have conducted experiments to precisely measure the proton content using H-NMR technique i.e. "water traces" in various silicate glasses Ca 2 Mg 5 Si 8 O 22 , Al 2 Si 4 O 10 NaAlSi 2 O 6 , CaSO 4 . The concept of quantitative NMR application is based on direct proportionality of the area under an NMR peak to the number of nuclear spins contributing to that peak.

  Sb 2 O 2 .5.4H 2 O, SnO 2 .2H 2 O, ZrO 2 .1.75H 2 O. The ion exchange is a substitution of conducting ions i.e. Na + in β-alumina.

  These materials have high σ and high activation energy at Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 48 low temperature. The proton conducting species ride on a carrier molecule such as H 3 O + or NH + (V-mechanism). In addition, good illustration materials for this proton conductor are oxonium or ammonium in β-alumina. For example β-Al 11 O 16 1.25O.(H 3 O + ) yield σ RT ~10 -10 Ω -1 .cm -1 and E a ~0.8 eV[2].

	Al 11 O 16 1.25O.(NH 4	+ ) yield σ RT 1.5x10 -7 Ω -1 .cm -1 and E a ~0.5 eV, β-

Table 2 . 1 :

 21 Symmetry and Unit-Cell parameters of barium and strontium zirconate-and cerate-based oxides at room temperature (25°C).

	Compound	Notation	Symmetry	Space Group	Lattice parameters / Å a b c	Ref.
	Zirconate-based oxides
	BaZrO 3-δ	BZ	Cubic	Pm-3m	4.1931 4.1931 4.1931	[22]
	BaZr 0.94 Y 0.06 O 3-δ	BZY6	Tetragonal	P4/mbm	5.9438 5.9438 4.1991	[22]
	BaZr 0.90 Y 0.10 O 3-δ	BZY10	tetragonal	P4mm	4.2151 4.2151 4.2047	[25]
	BaZr 0.90 In 0.10 O 3-δ	BZIn10	tetragonal	P4mm	4.1989 4.1989 4.2022	[25]
	BaZr 0.90 Gd 0.10 O 3-δ	BZGd10	tetragonal	P4mm	4.2205 4.2205 4.2069	[25]
	SrZrO 3-δ	SZ	Orthorhombic Pnma	5.816	8.225	5.813	[20,23,24]
	SrZr 0.9 Yb 0.1 O 3-δ	SZYb	Orthorhombic Pnma	5.8321 8.2221 5.8079	[23]
	Cerate-based oxides
	SrCeO 3	SC	Orthorhombic Pbnm	6.0117 6.1538 8.5886	[17]
	BaCeO 3-δ	BC	Orthorhombic Pmcn	8.791	6.252	6.227	[26]
	BaCe 0.9-x Zr x Y 0.1 O 3-δ	BCY : Zr ≤0.2	Orthorhombic n/a	8.712	6.134	6.18	[27]
		BCZY Zr>0.2	Cubic	n/a	4.209	4.209	4.209	[27]
	BaCe 0.4 Zr 0.5 Y 0.1 Zr 0.04 O 3-δ	BCZY :Zn	Cubic	Pm-3m	4.295	4.295	4.295	[28]
	BaCe 0.6 Zr 0.3 Y 0.1 Zr 0.04 O 3-δ	BCZY :Zn	Orthorhombic Pnma	8.663	6.137	81.237	[28]
	BaCe 0.5 Zr 0.35 Sc 0.1 Zr 0.05 O 3-δ BCZS :Zn	Orthorhombic Pbnm	6.085	6.073	8.596	[29]

  Grimaud et al.[45] studied in-situ Raman spectroscopy of BaCe 0.9 Y 0.1 O 3-δ water partial pressure of 0.03 bar. They reported that the structural transition from tetragonal (Imcm) SrZr 0.9 Ln 0.1 O 3-δ )[10,52] ceramics exposed under case of Yb-doped with barium zirconates (BZ) and with strontium zirconates (SZ), structure of both materials remained as pristine phase (cubic for BZ:Yb and orthorhombic for SZ:Yb) after treating the samples under 15 bar H 2 O at 200°C for 23 days[12]. However, in-situ Raman of SrZr 0.93 Yb 0.07 O 2.965 and SrZr 0.93 Yb 0.07 O 2.965 H 0.03[49], as a function of hydrostatic helium pressure (0.5 to 40 GPa), showed a sequence of structural modification at 10, 20 and 35 GPa. This result was similar to phase transition sequence as a function of temperature; orthorhombic (Pnma) 750°C to pseudo-tetragonal (Immm) 840°C to tetragonal (I4/mcm) to cubic (Pm3m). Change due to protonation was subtle but clear. For BaCe 0.5 Zr 0.3 Y 0.16 Zn 0.04 O 3-δ ceramic[12], a phase transformation from distorted rhombohedral to cubic was observed at 500°C under 10 bar H 2 O for 10h.

	to rhombohedral (R-3c) occurred between 400-500°C. According to previous works at
	MONARIS/LADIR [8,10,12,46-52] [27], the in-situ Raman spectroscopy equipped with
	autoclave device was employed to investigate structural phase transition of perovskite oxide
	ceramics under vapor pressure and at intermediate temperature (550-650°C). The protonic
	conducting Ln-doped strontium zirconates (

). The chemical stability test of sintered specimens was in CO 2 atmosphere (pH 2 O ~0.017 bar) at 900°C for 2h. It showed that Zr-substituted BaCeO 3 stability against CO 2 increased with increasing Zr-content. Similar to Pooja et al.

[42]

, chemical stability of BaCe 0.9-x Zr x Y 0.2 O 3-α (x = 0.4) in CO 2 atmosphere and in boiling water for several days were studied. The structural characterization was performed using powder XRD pattern. The result showed that doping Zr in lower amount than 0.2, exhibited low tolerance (<0.8659), which resulted in low chemical stability of the perovskite oxide samples under both H 2 O and CO 2 atmosphere.

Table 2 . 2 :

 22 Symmetry and Unit-Cell parameters of Ln 2 NiO 4+δ materials at room temperature (25°C). NiO 4+δ is described as a successive of NiO 2 layers alternating with Ln 2 O 2 bi-layers. At high temperature (e.g. >700°C for La 2 NiO 4+δ in Fig.2.4), the structure of nickelates is tetragonal (High Temperature Tetragonal phase, nominated as HTT-phase). Here the Ni 2+ ions are octahedrally coordinated with six oxygen atoms. As temperature goes down, the in-plane cell parameters for both kinds of layers become different (a.p. NiO 2 > a.p. La 2 O 2 ). In absence of oxidizing atmosphere the resulting stress is relieved by the rotation of the NiO 6 octahedral.

	Compound	Notation	Symmetry	Space Group	Lattice parameters / Å Ref. a b c
	La 2 NiO 4+δ	LNO	Orthorhombic Fmmm	5.467 5.462 12.693 [53]
			Tetragonal	I4/mmm	3.864 3.864 12.699 [54]
			Orthorhombic Fmmm	5.461 5.472 12.714 [55]
			Orthorhombic Fmmm	5.460 5.474 12.688 [57]
			Orthorhombic Fmmm	5.456 5.462 12.691 [56]
			Orthorhombic Fmmm	5.449 5.457 12.672 [58]
	Pr 2 NiO 4+δ	PNO	Orthorhombic Fmmm	5.462 5.385 12.479 [53]
			Orthorhombic Fmmm	5.391 5.452 12.439 [54]
			Orthorhombic Bmab	5.392 5.545 12.446 [56]
			Orthorhombic Bmab	5.389 5.454 12.443 [58]
	Nd 2 NiO 4+δ	NNO	Orthorhombic Fmmm	5.378 5.457 12.379 [53]
			Orthorhombic Fmmm	5.379 5.442 12.351 [54]
			Orthorhombic Bmab	5.374 5.458 12.376 [56]
			Orthorhombic Bmab	5.384 5.445 12.339 [59]
			Orthorhombic Fmmm	5.459 5.374 12.380 [6]
			Tetragonal	I4/mmm	3.866 3.866 12.197 [6]

Note: cell parameters indicated here are characteristic of non-stoichiometry compounds (δ >0).

Low and high temperature phase transformation of nickelate-based oxides (Ln 2 NiO 4+δ , Ln = La, Pr, Nd) have been intensively studied

[6,55,[57][58][59][60][61][62][63][64][65][66][67][68][69]

. The structure of ternary oxides Ln 2 4 , 1226.5°C for Pr 2 NiO 4 , and 1626.5°C for Nd 2 NiO 4

[60]

.

  phase was observed in the as-calcined powder with y ≤ 0.7. Otherwise it exhibited orthorhombic symmetry. Traces of CoO were detected in the sample with y ≤ 0.2[72,73]. In case of Sr variation (x) with y = 0.8, the as-calcined sample was in orthorhombic phase with x < 0.3. The rhombohedral structure was observed with 0.3 ≤ x ≤ 0.4. In addition, the crystal structure of La 1-x Sr x Co 0.2 Fe 0.8 O 3-δ with x > 0.8 exhibited coexisting phases of the cubic and the rhobohedral. The single phase of the cubic with Pm-3m space group was observed for LSCF6428 ceramic by Dailly et al.[56].

	1-x Sr x Co 1-y Fe y O 3-δ ; LSFC) can be considered
	as a solid solution between La 1-x Sr x CoO 3-δ (LSC) and La 1-x Sr x FeO 3-δ (LSF). The LSFC
	compound mostly resulted in rhombohedral structure with R-3c space group, in Table 2.3.
	Variation of Sr 2+ and Fe 3+ ions substitute in A and B cations respectively were factors which

caused-symmetry change. Synthesis and characterization of compositions in the system of La 0.8 Sr 0.2 Co 1-y Fe y O 3-δ and La 1-x Sr x Co 0.2 Fe 0.8 O 3-δ were reported

[72,73]

. At room temperature, the rhombohedral

Table 2 . 3 :

 23 Symmetry and Unit-Cell parameters of La 1-x Sr x Co 1-y Fe y O 3-δ materials at room temperature (25°C).

	Compound	Notation	Symmetry	Space Group	Lattice parameters / Å Ref. a b c
	La 1-x Sr x Co 1-y Fe y O 3-δ LSCF6428 Rhombohedral R-3c	5.498 5.498 13.378 [53]
		LSCF6428 Cubic	Pm-3m	5.484 5.484 5.484	[56]

Table 2 . 4 :

 24 Conductivity of potential proton-conducting electrolytes.

	Material	Working	Activation energy	Proton conductivity	Ref.
		temperature	eV	S.cm -1
		°C
	Cerate-base oxides
	SrCe 0.95 Yb 0.05 O 3	200	0.63	1.4x10 -6	[101]
	BaCe 0.9 Y 0.1 O 3-α (BC: Y)	500-900 400	n/a n/a	1.8x10 -2 to 7x10 -2 < 10 -3	[27] [102]
	BaCe 0.9-x Zr x Y 0.1 O 3-α	600-900	0.35	n/a
	(BCZY)
	BaCe 0.95 Nd 0.05 O 3	25-200	0.56	1.8x10 -6 at 112°C	[103]
	Zirconate-based oxides
	SrZr 0.95 Yb 0.05 O 3	100-900	0.4-0.6	~5x10 -3	[86,104]
	SrZr 0.95 Er 0.05 O 3	100-900	~0.55 at 100°C	~7x10 -3 at 400°C	[104]
	SrZr 0.95 Sc 0.05 O 3	100-900	0.63 at 100°C	~2.5x10 -3 at 400°C	[104]
	SrZr 0.95 Y 0.05 O 3	100-900	0.57 at 100°C	~9x10 -3 at 400°C	[104]
	Sr(Ce 1-x Zr x ) 0.95 Yb 0.05 O 3	800°C	0.6	4.5x10 -6	[105]
	BaZr 0.9 Y 0.1 O 2.95	>300	0.3-0.5	n/a	[106]

Table 2 . 5 :

 25 Electrical properties of Ni-based cermet for hydrogen electrode applications.

				Electrochemical Properties
	Materials	Temp. °C	Testing system	E a	R p	EIS	Ref.
				eV	Ω..cm 2	S.cm -1
	Ni-BaSr 0.90 Y 0.10 O 3 -δ	600	PCFC	n/a	~13	n/a	[120]
	Ni-SrCe 0.90 Yb 0.10 O 3 -δ 700	PCFC	~0.4	5	n/a	[121]
	Ni-SrZr 0.95 Y 0.05 O 2.975 700	PCFC	n/a	~1	n/a	[118]
	NiO-BaZr 0.8 Y 0.2 O 3-δ	600	PCFC	0.44-0.65	0.37	n/a	[113]
	NiO-BaCe 0.9 Y 0.1 O 3-δ	600-750	PCFC	~0.7	n/a	>40	[122]

Table 2

 2 .6 summarizes potential perovskite-based oxides for air electrode applications. There are two categories of the electrode: single materials and composite materials. The composite electrodes are generally a mixture component of electrode and electrolyte compound. This material is supposed to increase the ability of protonic transportation. The electrical performances of air electrodes depend on a compatible Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 74 electrolyte as well as a cell assembly design. The Ba 1-x Sr x Co 1-y Fe y O 3-δ (BSCF) and Pr 2 NiO 4+δ (PNO) in single air electrode was reported by F. Muavy et al.[123], indicating very low polarization resistance (<0.8 Ω.cm 2 ) in the PCFC system. Furthermore, the authors found that the used of composite electrodes allow an important decrease of polarization resistance for instance 0.23 Ω.cm 2 and 0.19 Ω.cm 2 for PNO-BCY10 and BSCF-BCY10, respectively. The results led many researchers to improve the performance of BSCF by adding silver as the current collectors.

Table 2 . 6 :

 26 Summary of electrical properties of perovskite-based oxide as air electrodes applications.

	Cathode	Electrolyte Testing	Temperature	R p	Ref.
			system	°C	Ω Ω Ω Ω cm 2
	Single air electrode
	Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ	BCY10	PCFC	600	<0.8	[123]
	Pr 2 NiO 4+δ	BCY10	PCFC	600	<0.8
	La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ	BCY10	PCFC	600	~6	[56]
	La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ	BCZY	PCFC	600	0.89	[124]
	Composite air electrode
	BCY10-BSCF BCY10-PNO	BCY10 BCY10	PCFC PCFC	600 600	0.19 0.23	[123]
	BZCYYb-NNO	BZCYYb	PCFC	750	0.43	[125]

Table 2 . 7 :

 27 Symmetry species and vibrational modes expected for the orthorhombic, tetragonal, rhombohedral and cubic symmetries.

	Factor group	Space group	Known Sample	Irreducible representations Raman active modes IR active modes Inactive modes	Ref.
	Orthorhombic structure
	D 2h	Pbnm	(Ba,Sr)CeO 3	7A g + 5B 1g	9B 1u +7B 2u +	8A u + B 1u	[129-
				+ 7B 2g +5B 3g	9B 3u	+ B 2u + B 3u	134]
		Pnma	SrZrO 3	7A g + 5B 1g	9B 1u +7B 2u +	8A u + B 1u	[135]
				+ 7B 2g +5B 3g	9B 3u	+ B 2u + B 3u
	D 2h	Bmab	Ln 2 NiO 4	5A g + 3B 1g	6B 1u +7B 2u	B 1u +B 2u +B 3u +4A u	[136]
			(low-T)	+ 4B 2g + 6B 3g	+ 4B 3u
	Tetragonal structure
	D 2h	I4/mmm	Ln 2 NiO 4	2A 1g + 2E g	3A 2u + 4E u	B 2u	[136-
			(high T)				139]
	Rhombohedral structure
	6 3 d D	R-3c	LSCF7382	A 1g +4E g	3A 2u +5E u	2A 1u +3A 2g	[74]
	Cubic structure
	O h	Pm-3m	SrTiO 3	none	3F 1u	3F 1u + F 2u

Table 2 . 8 :

 28 Raman and IR signatures and mode assignment of undoped BaZrO 3 and SrZrO 3[START_REF] Zaafrani | Protonation, distorsions structurales et espéces protoniques dans des perovskites lacunaires[END_REF].

	Sample	BaZrO 3	SrZrO 3	Assignments
	IR	Raman IR	Raman

Table 2 . 9 :

 29 Observed phonon frequencies of lanthanide (La, Pr, Nd) nickelate samples.

	Samples			Observed phonon peak position / cm -1			Ref.
					Infrared (IR)
	La2NiO4+δ	109	220	225	343	348	495	685	[136-138]
	Pr2NiO4+δ	112	213			320		679	[136,143]
	Nd2NiO4+δ	115	228			317		686	[136]
	Mode	Eu	Eu	A2u	A2u	Eu	A2u	Eu
	Motion	δ Ln-O(II)	δ Ln-O(II)-Ni	υ La-O(II)	υ Ni-O(I)	δ Ni-O(I)	υ La-O(II)-Ni	δ Ni-O(I)
					Raman (RS)
	La2NiO4+δ	87	151	263	335		443	685	[136-138]
	Pr2NiO4+δ	94	160	240	320		457	679	[136]
	Nd2NiO4+δ	93	170	254	317		470	686	[136,144]
	Mode	Eg	A1g	Eg	Eg		A1g	Eu
	Motion

  -1

	LaCoO 3	86	172	261	432	584
	La 0.7 Sr 0.3 CoO 3			298	416	540	646
	LaCo 0.8 Fe 0.2 CoO 3				485	545	692
	La 0.7 Sr 0.3 Co 0.8 Fe 0.2 O 3			298	417	550	650
	Mode	E g	E g	A 1g	E g	A 2g	A 2g

Table 3 . 1 :

 31 Raman signature of potential carbonate compounds (Note: the bold characters are strong bands).

	Formula	Raman wavenumber / cm -1	Ref.
	I-La 2 O 2 CO 3	427, 315, 670, 721, 867, 1064	[15]
	Ia-La 2 O 2 CO3	93, 106, 124, 137, 155, 223, 298, 319, 341, 441, 458, 671,	[15,16]
		680, 698, 716, 730, 858, 1050, 1060
	II-La 2 O 2 CO 3	82, 112, 125, 204, 256, 363, 393, 660, 675, 745, 1066, 1088	[15,16]
	LaCO 3 (OH)	149, 196, 247, 377, 702, 725, 725, 1087	[17]
	Nd 2 (CO 3 ) 3 •8H 2 O 162, 195, 226, 336, 684, 730, 761, 1091	[18]
	NdCO 3 (OH)	149, 198, 229, 250, 371, 702, 727, 1084	[19]
	SrCO 3	100, 148, 180, 236, 244, 258, 510, 710, 1071, 1445	[20-22]
	FeCO 3	194, 299, 731, 1088, 1434, 1738	[22-24]
	CoCO 3	180, 202, 304, 725, 1087,

Table 3 . 2 :

 32 Raman signature of potential metal oxide compounds (Note: the bold character is strong band).

	Formula Raman wavenumber / cm -1	Ref.
	A-La 2 O 3 103, 189, 405	[15,37,38]
	C-La 2 O 3 116, 270, 312, 356, 376	[15]
	Pr 2 O 3	226, 458, 598, 726	[39,40]
	A-Nd 2 O 3 105, 150, 190, 334, 427, 436	[37,41,42]
	SrO 2	863	[waal, 1998]
	SrO	520, 540, 560
	γ-Fe 2 O 3	358, 390, 499, 678, 710	[28,34,35,43]

α-Fe 2 O

3 226, 247, 292, 406, 495, 600, 700, 819

			[28,35,44]
	Fe 3 O 4	297, 319, 523, 666	[35,43,44]
	Co 3 O 4	184, 456, 510, 602, 669	[45,46]
	NiO	210, 406, 541, 723, 901, 1085, 1260	[47]

Table 3 .

 3 1 indicates the Raman signatures of L 2 O 3 -based materials as well as prepared II-La 2 O 2 CO 3 powder with the litearature data [15].

Table 3 . 3 :

 33 Raman signature (cm -1 ) of La 2 O 3 based and La 2 O 2 CO 3 samples.

				La2O3 based sample				La2O2CO3	II-La2O2CO3
	RT	La(OH)3 [15]	350°C	LaOOH [15]	500°C	800°C	RT (after)	A-La2O3 [15]	RT	A,Orera [15]
	144	137,8	85		85,8	101,1	105	103,7	91	82,7
	233,5	227		116,2w		180,6	193	189,7	122	112,5
	287,9	281,7	129	123,2	128,5	391,6	408	405,8	137 b	125,5
	346,2	339		135,4			845 w		166 w
	455,7	449		164,7w					212	204,7
	613,1 w	605,2 b	205,2	202,4	203,8				265	256,5
		648,8 b, w	226,8 b	216,1	224,8 b				372	363,5
			311,1	312,3	305,5 b				400	393,8
			343,4	344,6	371,6				472	660,3
			382,9	383	398				682 w	675,4
			424,2	424,5	693,1 w				753,5	745
			677 w		747,5 w					1066,5 vw
			714 w		868,6 w				1096	1088
			1069		1071
	Note: vs: very strong, s: strong, m: medium, w: weak, n: narrow, b: broad

Chapter 4 :

 4 Combined bulk and surface analysis of the BaCe 0.5 Zr 0.3 Y 0.16 Zn 0.04 O 3-δ (BCZYZ)

	ceramic proton-conducting electrolyte
	C C H H A A P P T T E E R R 4 4
	109

"Combined bulk and surface analysis of the BaCe 0.5 Zr 0.3 Y 0.16 Zn 0.04 O 3-δ (BCZYZ) ceramic proton-conducting electrolyte"

	Chapter 5: Structural stability of anhydrous proton conducting SrZr 0.9 Er 0.1 O 3-δ perovskite Chapter 6: Protonation and structural/chemical stability of Ln 2 NiO 4+δ ceramics vs. H 2 O/CO 2 :
	ceramic vs. protonation/deprotonation cycling: Neutron diffraction and Raman studies High temperature/water pressure ageing test
		C C H H A A P P T T E E R R 5 5 C C H H A A P P T T E E R R 6 6
	"Structural stability of anhydrous "Protonation and structural/chemical
	proton conducting SrZr 0.9 Er 0.1 O 3-δ δ δ δ stability of Ln 2 NiO 4+δ δ δ δ ceramics vs.
	H 2 O	perovskite ceramic vs.
	protonation/deprotonation cycling:
	Neutron diffraction and Raman studies"
	In this chapter, we will report the characterization of BaCe 0.5 Zr 0.3 Y 0.16 Zn 0.04 O 3-δ
	(BCZYZ) ceramic samples, which were exposed under H 2 O pressure (60 mbar
	and 10 bar) at intermediate temperature. The fundamental analysis techniques
	i.e. IR, Raman spectroscopy, TGA, TE, TEM, and X-ray diffraction are Here, we will report the structural stability of Er-doped strontium zirconate,
	employed. SrZr 0.9 Er .16 O 3-δ (SZ: Er), ceramic samples. The samples were treated under 40
	bar water vapor pressure at intermediate temperature. The techniques of neutron
		diffraction and Raman spectroscopy are mainly employed.
		117 131

/CO 2 : High temperature/water pressure ageing test"

  1) is valid for any lattice structure, not only the simple cubic one. The generalization is easily performed by just inserting the interplanar spacing dhkl of the crystal lattice under investigation. Table 1.1 gives the relation of dhkl and the unit cell parameters for different crystal classes.

Table E . 1 :

 E1 Interplanar spacing dhkl for different crystal systems and their dependency on Millar indices hkl. Parameters a, b abd c give the lengths of the crystallographic unit cell, while α, β and γ specify the angles between them.

Table 1 :

 1 Technical characteristics of different electrolysis and fuel cells[22][23][24] 6

Table 2 :

 2 Electrochemical reactions occurring in SOFC, PCFC, and PCEC system 11

Table 3 :

 3 Description of investigated perovskite-related oxides. 18

Table 1 .

 1 1: Approximate classification of O-H…O hydrogen bonds 35

Table 1 .

 1 2: Typical conducting protonic species and their stability range 41

Table 1 .

 1 3: Direct and indirect measurement methods of H-content in solids 43

Table 2 .

 2 1: Symmetry and Unit-Cell parameters of barium and strontium zirconate-and ceratebased oxides at room temperature (25°C) 60

Table 2 .

 2 2: Symmetry and unit-cell parameters of Ln 2 NiO 4+δ materials at room temperature 64

Table 2 .

 2 3 : Symmetry and Unit-Cell parameters of La 1-x Sr x Co 1-y Fe y O 3-δ materials at room

	temperature (25°C)

Table 2 .

 2 1: Conductivity of potential proton-conducting electrolytes 70

Table 2 .

 2 2: Electrical properties of Ni-based cermet for hydrogen electrode applications 73

Table 2 .

 2 3: Summary of electrical properties of perovskite-based oxide as air electrodes application 74

Table 2 .

 2 7: Symmetry species and vibrational modes expected for the orthorhombic, tetragonal, rhombohedral and cubic symmetries 76

Table 2 .

 2 4: Raman and IR signatures and mode assignment of undoped BaZrO 3 and SrZrO 3

	[142].

Table 2 .

 2 9: Observed phonon frequencies of lanthanide (La, Pr, Nd) nickelate samples Table 2.10: Observed Raman frequencies of LSCF compounds 82

Table 3 .

 3 3: Raman signature (cm -1 ) of La 2 O 3 based and La 2 O 2 CO 3 samples 101

Table 8 .

 8 1: Symmetry groups and unit cell volume of pristine and protonated LnNO ceramics 167

Table 8 .

 8 2: Symmetry groups and unit cell volume of pristine and protonated LSCF6428 ceramics 168

Table 8 .

 8 3: Unit-cell volume of pristine, protonated (H1, H2) and deprotonated (DH1, DH2) SZE ceramic 170 Tableau 9.1: Comparaison des volumes de maille (Å 3 ) des céramiques SrZr 0.9 Er 0.1 O 3-δ en fonction des cycles de protonation : non-protonée (pristine), protonée (H1, H2) et déprotonée (DH1, DH2) 187 Tableau 9.2: Groupe de symétrie et volume de maille des céramiques LnNO. En fonction des cycles de protonation 189 Tableau 9.3: Groupe de symétrie et volume de maille de la céramique LSCF6428 non-

	protonée et protonée

Table E .

 E 1: Interplanar spacing dhkl for different crystal systems and their dependency on

O. Zaafrani, Dotoral thesis "Protonation, distorsions structurales et espèces protoniques dans des pérovskites lacunaires", Université Pierre et Marie Curie, Paris, France,

The doctoral scholarship has funded by the Franco-Thai scholarship and Burapha University staff grant. The work was carried out at "De la molécule aux nano-objets: réactivité, interactions et spectroscopies" (MONARIS) laboratory -UMR8233 from September 2012 to October 2015 under the supervision of Dr. Philippe Colomban.

First of all, I would like to acknowledge my supervisor, Dr. Philippe Colomban, for stimulating discussions,

Appendix B Optical microscopy : grains and pores size measurement Appendix C Thermal Analysis Appendix D IR and Raman Spectroscopy Appendix E X-ray diffraction Appendix F Neutron diffraction List of Table List of Figure

Appendix A Protonation Treatment: autoclaves and procedures

The steam under medium and high pressure at moderate temperature is extremely reactive and dangerous. The design and implementation construction autoclaves route are required special procedures.

In previous study [1] , the principal of Autoclave's use has been was established. Table C.1 lists the principal characteristic of used Autoclave devices. Controlling the protonation of the ceramic (water vapor pressure treatment according to the temperature) led to install different autoclaves programmable temperature. The pressure can be limited to a value lower than the saturation pressure by manually opening a safety valve to allow water vapor to escape. In this research, the as-prepared Ln 2 NiO 4+δ and LSCF6428 ceramics were treated in different conditions as shown in Table A.2. The treatment under high vapor pressure (~40 bar) is to accelerate the material's ageing while the one under medium vapour pressure (~20 bar) is a working condition. Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers

202

The procedure of "in-situ" autoclave treatment is summarized in four steps:

i) Preparing the sample and autoclave -following the first step of "ex-situ" autoclave treatment, however, the sample is placed directly into the autoclave chamber (without the goal-foil holder) and the quantity of prepared water is only 0.5-1 ml. Ensure the connection of the optical cell to the thermal control unit; Connect the thermocouples, the heater for heating the cell, and the pressure sensor to the temperature regulation device. Verify that the heat regulation unit is at ambient temperature and pressure.

ii) Positioning of the optical head Horiba Jobin Yvon SuperHeads -to obtain a good Raman spectra while the sample is protonated, we first measure the distance between the optical head and sample surface. We noted the number of distance by the measuring tools. Note: this number of distance is fixed during the experiment.

iii) Cooling procedure -Open the valve of the compressed air at a maximum and turn on the suction pump for circulating liquid nitrogen coupled a Variac autotransformer to its maximum level. Adjust the autotransformer that is coupled to 80 volts and which enables it to adjust flow of liquid nitrogen to flow to 0-270 volts.

iv) Start-up and Raman measuring procedure -after setting up the autoclave in good condition, we then start the machine. First, we switch on the machine, and then set the target temperature i.e. 100°C. When the actual temperature reaches to the target temperature, we could examine the Raman scattering at the fixed position that we have set up in the step 2. Note: the step of target temperature should be designed before starting the experiment. While running the Raman scattering, we should observe and note carefully the actual temperature and pressure. In case that the actual pressure is higher than safety pressure, we can manually opening a safety valve (yellow) allowing the water vapor escapes.

v) Shutdown and opening -to shut down the treatment system, we first switch down the machine by the same button, and leave the autoclave are cooled for several hours until the temperature of the autoclave equilibrium to ambient temperature. Note: it is

necessary not to open the hot autoclave in order to avoid a damaging of screw and ring connectors.

Appendix B Optical Microscopy: grains and pores size measurement

Microscopes are instruments designed to produce magnified visual or photographic images of small objects. The microscope must accomplish three tasks: produce a magnified image of the specimen, separate the details in the image, and render the details visible to the human eye or camera. This group of instruments includes not only multiple-lens designs with objectives and condensers, but also very simple single lens devices that are often hand-held, such as a magnifying glass.

B.1 Olympus BX51 Instrument

The optical components contained within modern microscopes are mounted on a stable, ergonomically designed base that allows rapid exchange, precision centering, and careful alignment between those assemblies that are optically interdependent. Together, the optical and mechanical components of the microscope, including the mounted specimen on a glass micro slide and coverslip, form an optical train with a central axis that traverses the microscope stand and body.

The microscope optical train (BX51) typically consists of an illuminator (including the light source and collector lens), a substage condenser, specimen, objective, eyepiece, and detector, which is either some form of camera or the observer's eye. Research-level microscopes also contain one of several light-conditioning devices that are often positioned between the illuminator and condenser, and a complementary detector or filtering device that is inserted between the objective and the eyepiece or camera. The conditioning device(s) and

B.2 Cracking-line-intercept measurement (ASTM E112)

To determine the intercept distance between crack-network on the surface samples, the microscope with x20 objective (focalization of 100 µm), OLYMPUS was employed.

The intercept method derives a "G number" by superimposing a pattern over the image and counting how many times that pattern "intercepts" with a crack line. The cracking lineintercept analysis is simply described in 4 steps:

i. Draw a single dot at middle-left edge of the investigated micro-scale image.

ii. Draw testing lines from the dot to another edge (top, right and bottom) in fixed angle (θ ~25°).

iii. Measure the crack distance which the testing line drawn cross over, given d Ni where i is testing-line number and n = measuring distant number.

iv.

Statistical calculation for an average distance of each testing line:

B.3 Measurement of pore and grain size

In order to measure the size of grains and pore, a suitable magnification of image should be considered. In this test, we employed an objective x100, which provides magnification of 20 µm. The measurement procedure was following the four steps of line intercept analysis as discussed earlier.

Appendix C Thermal Analysis

Thermal analysis is a series technique where physical properties i.e. mass, dimension enthalpy, of a material are measured as a function of time, while the material is subjected to a temperature program. The physical properties are thus measured as a function of temperature.

This method plays an important role in the characterization of ceramics and inorganic electronic materials because the synthesis and applications of these materials generally involve such a wide range of temperature. Hence, the evaluation of their composition, properties, reactivity as a function of temperature and time is of vital importance.

To observe such characteristic of perovskite-oxide ceramics as a function of temperature, this research employed Thermogravimetry (TGA) and Thermal Expantion (TE).

C.1 Thermogravimetry (TG)

Thermogravimetric analysis (TGA) is an analytical technique used to determine a material's thermal stability and its fraction of volatile components by monitoring the weight change that occurs as a specimen is heated. The measurement is normally carried out in air or in an inert atmosphere, such as Helium or Argon, and the weight is recorded as a function of increasing temperature. Sometimes, the measurement is performed in a lean oxygen atmosphere (1 to 5% O2 in N2 or He) to slow down oxidation. In addition to weight changes, some instruments also record the temperature difference between the specimen and one or more reference pans (differential thermal analysis, or DTA) or the heat flow into the specimen pan compared to that of the reference pan (differential scanning calorimetry, or DSC). The latter can be used to monitor the energy released or absorbed via chemical reactions during the heating process. In particular case perovskite or perovskite-like materials, although the investigated materials are formed in nonstoichiometry, this measurement is viable and convenient method to follow the changes quantitative of materials.

During the TGA testing process, a sample of analyzed material is placed into an alumina cup, which supported on and analytical balance located outside the furnace chamber.

D.2 Thermal Expansion (TE)

Nearly all materials expand on heating due to increased atomic vibrations. For ceramics, the magnitude of this expansion is related to the phases present and the pore-size distribution of the ceramic. Thermal expansion is typically measured using a thermo dilatometer. The thermal expansion of refractory ceramics is related to the potential thermalshock resistance. For structural ceramics, thermal expansion is a critical design parameter for the design of expansion joints. Thermal expansion is commonly reported as the coefficient of linear thermal expansion (α) which is the slope of the thermal expansion curve. The linear coefficient of thermal expansion is calculated as:

Appendix D

IR and Raman spectroscopy

Molecules consist of atoms which have a certain mass and which are connected by elastic bonds. As a result, they can perform periodic motions, they have "vibrational degrees of freedom": all motions of the atoms in a molecules relative to each other are a superposition of so-called "normal vibrations", in which all atoms are vibrating with the same phase and "normal frequency". Their amplitudes are described by a "normal coordinate". Polyatomic molecules with n atoms possess 3n-6 normal vibrations (c.a. linear molecules have 3n-5 normal vibration), which define their "vibrational spectra". The spectra depend on the masses of the atoms, their geometrical arrangement, and the strength of their chemical bonds.

D.1 Theoretical of Molecular vibrations

The where k is the force constant of the spring, in N. m -1 , which is a measure of the strength of the bond between the two atoms. Equation D.1 is Hooke's law, and the resulting motion is simple harmonic. Based on these assumptions the ball-and-spring model is a harmonic oscillator. The Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 216 vibrational frequency, ν 0 , in Hz, of the harmonic oscillator in terms of classical mechanics is given by

where m is the reduced mass in kg In vibrational spectroscopy, it is common to use the wavenumber unit ν ~, which is expressed in cm -1 . This is the number of waves in a length of the centimeter, the reciprocal wavelength, and is given by the following relationship:

where λ is the wavelength and c is the velocity of the light in vacuum (2.9979 x 10 8 ms -1 ).

The wavenumber unit has the advantage of being linear with energy.

D.3 The symmetry of molecules and molecular vibrations

Most crystals exhibit macroscopically visible symmetry axes and planes. In order to utilize the symmetry of molecules and crystals for vibrational spectroscopy, the symmetry properties have to be defined conveniently.

For

where n i is the quantum number and ν i is the normal frequency associated with the normal

A crystal is capable of directly absorbing a photon that corresponds to an energy of vibration, given in Eq. D.5. Because the photons are polarized, the electric field of the photon must be capable of interesting with the vibration. An electric field will then create a force on charged particles, such as the positive and negative ions consisting the crystal. As the electric field points in one direction, the electric dipole may be stretched along an axis of the crystal.

However, when the electric field points in the opposite direction, the electric dipole becomes compressed. This results in a vibration of the system. The photon-dipole interaction leads to the selection rule for infrared absorption -vibrations are infrared-active if there is a change in the dipole moment during the vibration, further if the dipole is not changed during the vibration, an incident photon will not be capable of exciting the vibration.

The absorption bands due to the vibrational transitions are found in the wavelength of 2.5 … 1000 µm, which is equivalent to a wavenumber range of ν ~ = 4000 … 10 cm -1 . One of the most distinct and easily recognizable peaks in an IR spectrum is the broad O-H absorption of alcohols and phenols. The broad O-H peak is placed by a sharp signal around ~3600 cm -1 .

D.5 the Raman spectrum

The Raman effect, named for the Nobel prize work by Chandrasekhra Venkata Raman in 1928, is an inelastic light scattering effect resulting from the excitation or de-excitation of vibrational modes. In the simple case of a single molecule, Figure D.3, an incident photon can either scatter elastically, known as Rayleigh scattering, or scatter inelastically and excite or absorb a vibrational mode of the molecule. In a crystal lattice, the photon may scatter from a lattice phonon that will change the energy of the phonon. The energy loss or gain of the inelastically scattered photon corresponds to the energy of a vibrational level, or phonon level, Chemical and structural stability of proton conductor perovskite ceramic for fuel cells and electrolyzers 220 of the system. If a phonon is created at a higher energy state, then the scattered photon will have less energy than the incident photon. Alternatively, a phonon can be annihilated, resulting in a scattered photon having greater energy than the incident photon.

Raman scattering is a phenomenon with a low probability of occurrence. Only one in 106 of incident photons is inelastically scattered. Because the majority of incident light is Rayleigh scattered, Raman detector systems must remove this elastically scattered light from the collected light, or else the Rayleigh scattered light can overwhelm the weaker Raman signal. Raman scattering is typically performed non-resonantly, meaning that the energy of the incident photons does not equal any energy difference between any phonon levels of the lattice. In the most common conceptual description, the photon excites the system to a virtual state before decaying to the ground state (Rayleigh) or an excited vibrational state (Raman).

This can be seen in

D.2 Vibrational spectroscopy (IR and R) instruments

In this study, ATR FT-IR spectra were recorded between 600 cm -1 and 4000 cm -1 at room temperature. The Bruker IR-scope FTIR spectrometer (

Appendix E X-ray diffraction (XRD)

X-ray diffraction is a common and powerful tool for determining the crystal structure of a material. However, sample preparation is crucial, and the technique cannot examine all varieties of samples. The sample must be a single crystal, a uniform surface, or powder, all of which require a long-range order for coherent x-ray diffraction.

E.1 Theoretical

Atoms within a crystal lattice are formed regular planes with spacing d between each plane. Coherent x-rays incident upon these regular planes can reflect off these planes and constructively interfere if the path length difference is a multiple of the wave-length of the light. Knowing the angle at which the light scatters from these planes allows determination of the distance between each plane. Many different planes may exist in a regular crystal lattice, defined by coplanar atoms in the various crystal sites throughout the lattice. The incident xrays will, therefore, be capable of scattering off all these planes that lead to constructive interference at many different angles. Information about the crystal structure can be determined from this diffraction pattern.

The spacing between atomic planes, d, is determined using Bragg's law,

Where θ is the angle between the incident light and the lattice planes, m is an integer describing the order, and λ is the wavelength of the incident light. X-rays are used because, to achieve measurable diffraction, the wavelength must be on the order of d. Because the x-ray diffraction (XRD) spectrum and structure of spinel are well known, the lattice parameter can be calculated by knowing the Miller indices of each spinel diffraction peak.

E.2 Instrument and measurement conditions

In this study, XRD patterns of ground pellets were performed by Batocchi's group at ICMCB, Université de Bordeaux, France. The investigations were collected using a PANalytical X'pert MPD-PRO Bragg-Brentano.

The operating range is θ-2θ geometry diffractometer equipped with a germanium primary monochromator over an angular range of 2θ = 10-130°. Each acquisition lasted for 64 hours.

The Cu-Kα 1 (λ = 1.54056 Å) radiation was generated at 45 kV and 40 mA. Unit-cell parameters were calculated by Le Bail profile matching. The phase identification was carried out using the ICCD database and PANatycal X'pert high score software. The relative quantities of the different phases were estimated according to the ratio of the relative intensities of the major peaks recorded in the XRD patterns.

Abstract:

The chemical and structural stability of well-densified ceramics potentially used as H 2 /air fuel cell/electrolyzer (and perhaps in CO2/Hydrocabons Converter) electrolyte or electrodes vs. CO 2free/saturated pressurized water has been studied. The pressurization maximizes the efficiency of the energy conversion systems. Four types of pervoskite-related oxide ceramics were concerned: BaCe 0.4 Zr 0.5 Y 0.1 Zn 0.04 O 3-δ (BCZYZ), SrZr 0.9 Er 0.1 O 3-δ (SZE), Ln 2 NiO 4+δ (LNO, Ln = La, Pr, Nd), and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF6428). Dense ceramic samples were exposed at 550°C to water vapor pressure in an autoclave for days to weeks. The protonation treatments were performed in two different conditions: operating condition (≤20 bar of CO 2 -free water pressure, 550°C) and accelerated aging conditions (≥40 bar of CO 2 -free/CO 2 -saturated water pressure, 550°C). The pristine and 'protonated' samples were characterized using various analysis techniques: Optical Microscopy, Thermogravimetry, Thermal Expansion, (micro/macro) ATR FTIR, Raman micro-spectroscopy, X-ray and Neutron Scattering. The study reveals that under the operating condition (≤20 bar), the stability of LNO/LSCF6428 electrode materials and of SZE electrolyte appears good, while the BCZYZ electrolyte exhibit significant corrosion. The superior stability of LSFC6428 exposed in extreme CO 2water vapor atmosphere was demonstrated about 5 to 30 times better than LNO and SZE homologues. The surface secondary phases form at the grain boundary. The protonation modifies more or less the structure symmetry, the unit-cell volume/parameter and the phase transition sequence in relation with the modification of the oxygen vacancy distribution. The proton doping level for different samples is also discussed. Keywords: proton conductor, perovskite, hydrogen, autoclave, protonation, structure, stability, TGA, thermal expansion, Raman spectroscopy, ATR FTIR, X-ray diffraction, neutron diffraction, electrolyte, electrode, PC-SOFC, PC-SOEC

Résumé:

La stabilité structurale et chimique de céramiques bien densifiées candidates pour leur utilisation comme électrolyte ou matériau d'électrode de piles à combustible, électrolyseur H 2 /air ou même de convertisseur CO 2 /Hydrocarbures a été étudiée vis-à-vis de l'eau sous pression (autoclave, eau pauvre ou saturée en CO 2 ). La pressurisation maximise l'efficacité des dispositifs. Quatre familles de pérovskites ont été considérées: BaCe 0.4 Zr 0.5 Y 0.1 Zn 0.04 O 3-δ (BCZYZ), SrZr 0.9 Er 0.1 O 3-δ (SZE), Ln 2 NiO 4+δ (LNO, Ln = La, Pr, Nd), and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF6428). Les céramiques denses sont traitées à 550°C en autoclave plusieurs jours à plusieurs semaines, soit dans des conditions de fonctionnement (≤20 bar, eau sans CO 2 , 550°C), soit en vieillissement accéléré (≥40 bar eau sans ou saturée de CO 2 , 550°C). Les céramiques ont été analysées avant et après 'protonation' par Microscopie Optique, Thermogravimétrie, Expansion Thermique, (micro/macro) ATR FTIR, Raman micro-Spectroscopie, diffraction des rayons X et des neutrons. En condition de fonctionnement (≤20 bar), la stabilité des matériaux d'électrodes LNO/LSCF6428 et de l'électrolyte SZE est bonne, alors que la céramique BCZYZ se corrode. La céramique LSFC6428 soumis à des conditions sévères (eau saturée en CO 2 , 40 bar) est 5 à 30 fois moins corrodée que les composés LNO and SZE. La corrosion s'initie en surface, aux joints de grains. La protonation modifie plus ou moins la symétrie, le volume et les paramètres de la maille unitaire ainsi que les transitions de phase en relation avec la modification de la distribution/organisation des lacunes d'oxygène. Le niveau de dopage en proton des différents matériaux est aussi discuté.

Mots clés : conducteur protonique, pérovskite, hydrogène, autoclave, protonation, structure, stabilité, ATG, dilatation thermique, ATR FTIR, spectroscopie Raman, diffraction des rayons X, diffraction des neutrons, électrolyte, électrode, PC-SOFC, PC-SOEC