Benjamin Malvoisin

Anne-Line Auzende

Peter B Kelemen

Oman Drilling

Nanostructure of serpentinisation products: importance for water transport and low-temperature alteration

Nanostructure of serpentinisation products: Importance

for water transport and low-temperature alteration

Introduction

Hydration of mantle rocks below ~ 500°C (i.e. serpentinisation) is of current scientific interest because it substantially modifies the physical properties, the composition and the RedOx conditions in the oceanic lithosphere [START_REF] Escartín | Effects of serpentinization on the lithospheric strength and the style of normal faulting at slow-spreading ridges[END_REF][START_REF] Escartín | Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere[END_REF][START_REF] Oufi | Magnetic properties of variably serpentinized abyssal peridotites[END_REF][START_REF] Charlou | Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field[END_REF][START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF], and it provides an energy source for biological organisms [START_REF] Schulte | Serpentinization and its implications for life on the early Earth and Mars[END_REF][START_REF] Templeton | Formation and loss of metastable brucite: Does Fe(II)-bearing brucite support microbial activity in serpentinizing ecosystems[END_REF][START_REF] Kraus | Molecular Evidence for an Active Microbial Methane Cycle in Subsurface Serpentinite-Hosted Groundwaters in the Samail Ophiolite[END_REF]. This reaction often reaches completion while the solid volume increases by approximately 50 % during the replacement of primary olivine and pyroxene by serpentine [START_REF] Macdonald | Rate of serpentinization in seafloor environments[END_REF]. This is puzzling because reaction products are expected to precipitate in the pores leading to the clogging of the fluid pathways, consumption of fluid that forms a fast diffusion path, and armouring of reactive surfaces, all of which are negative feedbacks that might limit the extent of reaction [START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF][START_REF] Evans | Phase-Field Modeling of Reaction-Driven Cracking: Determining Conditions for Extensive Olivine Serpentinization[END_REF]. This process is probably responsible for a decrease in permeability over several orders of magnitude that has been measured in flowthrough serpentinisation experiments [START_REF] Godard | Incipient hydration of mantle lithosphere at ridges: A reactive-percolation experiment[END_REF][START_REF] Farough | Evolution of fracture permeability of ultramafic rocks undergoing serpentinization at hydrothermal conditions : An experimental study[END_REF]. However, the hypothesis that these negative feedbacks are self-limiting is inconsistent with the high extents of reaction generally observed in naturally serpentinised peridotites [START_REF] Macdonald | Rate of serpentinization in seafloor environments[END_REF][START_REF] Oufi | Magnetic properties of variably serpentinized abyssal peridotites[END_REF], suggesting that additional processes are at play to allow for complete serpentinisation.

It has been proposed that serpentinisation could occur at constant volume through the export of matter as aqueous species in the fluid [START_REF] Thayer | Serpentinization considered as a constant-volume metasomatic process[END_REF][START_REF] Carmichael | Induced Stress and Secondary Mass Transfer: Thermodynamic Basis for the Tendency toward Constant-Volume Constraint in Diffusion Metasomatism[END_REF][START_REF] Fletcher | Mineral growth in rocks: Kinetic-rheological models of replacement, vein formation, and syntectonic crystallization[END_REF]. However, some natural samples have recorded nearly no change in ratios of major elements during alteration (Mg/Si/Fe, e.g. [START_REF] Coleman | A chemical study of serpentinization-burro mountain[END_REF] suggesting that either hydration occurred with very little export of dissolved components with different solubilities, or that there was large-scale, congruent dissolution of silicate minerals. Recent work on microstructures indicates that serpentinisation in experiments [START_REF] Klein | Quantifying the volume increase and chemical exchange during serpentinization[END_REF] and in natural rocks [START_REF] Malvoisin | Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project[END_REF] is accompanied by large increases in solid volume (30 to 110 percent). Because serpentinisation and carbonation rapidly add mass to altered peridotite as a result of limited solubility of Mg, Si, Fe and Ca in aqueous fluids, thermodynamic models indicate that hydration and carbonation produce increases of 40 to 60% in solid mass and volume [START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF]de Obeso andKelemen, 2018, 2020;Kelemen, 2021aKelemen, , 2021b)). Because pore space in fresh and altered peridotites is limited (commonly less than 5%, mainly in fractures), these increases in solid volume must occur in an open system, and result in an increase in the total volume of serpentinised peridotites.

The preservation of fluid pathways during serpentinisation is essential to sustain reaction as the solid volume increases. Tectonic deformation and thermal cracking can generate fracture networks in peridotites [START_REF] Demartin | Experimental constraints on thermal cracking of peridotite at oceanic spreading centers[END_REF][START_REF] Rouméjon | Serpentinization and fluid pathways in tectonically exhumed peridotites from the southwest Indian ridge[END_REF]. Fractures can also be formed by the reaction itself. [START_REF] Macdonald | Rate of serpentinization in seafloor environments[END_REF] and [START_REF] O'hanley | Solution to the volume problem in serpentinization[END_REF] proposed that volume increase at the serpentine/olivine interface is accommodated through formation of expansion fractures, allowing for efficient fluid transport. The increase in volume during serpentinisation can increase differential stress to a level sufficient to generate fractures in the primary minerals, potentially maintaining or enhancing permeability, and increasing reactive surface area and reaction rates in a positive feedback mechanism [START_REF] Macdonald | Rate of serpentinization in seafloor environments[END_REF][START_REF] O'hanley | Solution to the volume problem in serpentinization[END_REF][START_REF] Jamtveit | Reaction induced fracturing during replacement processes[END_REF][START_REF] Rudge | A simple model of reaction-induced cracking applied to serpentinization and carbonation of peridotite[END_REF][START_REF] Plümper | The interface-scale mechanism of reaction-induced fracturing during serpentinization[END_REF][START_REF] Malvoisin | Serpentinization of oceanic peridotites: 2. Kinetics and processes of San Carlos olivine hydrothermal alteration[END_REF][START_REF] Malvoisin | Control of serpentinisation rate by reaction-induced cracking[END_REF][START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF][START_REF] Evans | Phase-Field Modeling of Reaction-Driven Cracking: Determining Conditions for Extensive Olivine Serpentinization[END_REF]Yoshida et al., 2020). However, this process alone may not be sufficient to prevent clogging of porosity. A recent numerical study has shown that the scale of mass transfer during reaction plays a key role for fluid pathway preservation [START_REF] Malvoisin | Achieving complete reaction while the solid volume increases: A numerical model applied to serpentinisation[END_REF]. If element transport occurs between the reaction front and the pores, porosity clogging leads to reaction shutdown. However, if the reaction products precipitate at the reaction front, pre-existing pores can be preserved allowing continuous fluid transport through time. This latter model assumes that fluid can be transported through the reaction products. Tutolo et al. (2016) measured a nanoscale porosity of several percent in serpentinised peridotites, probably associated with diffusive water transport in the serpentinisation products.

However, even if this nanoscale porosity plays a key role for reaction, its distribution and its relationship with the reaction products are still not known. The porous network formed during serpentinisation is also probably re-used during post-serpentinisation alteration at high water to rock ratio and low temperature (< 150°C). As for serpentinisation at higher temperature, this further alteration is thought to induce ferrous iron oxidation associated with water dissociation and molecular hydrogen generation [START_REF] Sleep | H2-rich fluids from serpentinization: Geochemical and biotic implications[END_REF][START_REF] Seyfried | Redox evolution and mass transfer during serpentinization: An experimental and theoretical study at 200 °C, 500 bar with implications for ultramafic-hosted hydrothermal systems at Mid-Ocean Ridges[END_REF][START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15°N on the Mid-Atlantic Ridge[END_REF]McCollom and Seewald, 2013;[START_REF] Miller | Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF][START_REF] Miller | Low temperature hydrogen production during experimental hydration of partiallyserpentinized dunite[END_REF][START_REF] Ellison | Low-Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite[END_REF]. Hydrogen production at low temperature probably supports microbial activity [START_REF] Templeton | Formation and loss of metastable brucite: Does Fe(II)-bearing brucite support microbial activity in serpentinizing ecosystems[END_REF]. Among the serpentinisation products, ferroan brucite ((Mg,Fe)(OH)2) appears to preferentially react by being extensively dissolved, by forming (hydro-)oxides [START_REF] Jöns | Reaction-induced porosity and onset of lowtemperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography[END_REF]Klein et al., 2020) or by reacting with SiO2,aq to form serpentine [START_REF] Tutolo | Serpentinization as a reactive transport process: The brucite silicification reaction[END_REF]. This difference in reactivity among the serpentinisation products may originate from their nanostructure.

Here we use high resolution imaging at the nanoscale to constrain the structure and the composition of the hydration products of olivine in an extensively serpentinised dunite collected during the Oman Drilling Project (Kelemen, 2021b). The observations suggest a mechanism for reaction product formation, and an explanation for brucite high reactivity during post-serpentinisation alteration.

Material and methods

Serpentinised dunite sample

The studied sample was collected in the "Multi-Borehole Observatory" (Kelemen, 2021b) during the Oman Drilling Project in the Wadi Tayi massif of the Samail Ophiolite (Kelemen et al., 2020). It comes from 215 m depth in Hole BA4A (sample BA4A-81-1-1-17, Kelemen et al., 2020). Hole BA4A is composed of an alternation of dunite (64 %) and minor harzburgite (26 %) with no systematic lithologic trends. We collected one of the freshest dunite in the middle of a 26 m thick body bound by 3 m and 0.4 m thick harzbugites at its top and bottom, respectively (see details in the visual core description in Kelemen et al., 2020).

Dunite is crosscut by ~ 30 gabbro and pyroxene dykes ranging in thickness from 0.05 to 1.5 m (Kelemen et al., 2020). The studied sample is dyke free. [START_REF] Malvoisin | Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project[END_REF] provide a full mineralogical and chemical description of the sample; a brief summary is given below.

The sample is cross-cut by a 100 μm-wide serpentine vein surrounded by a homogeneous mesh texture with olivine relicts in the centre of the meshes. X-ray microtomography reveals that it has been more than 80 % serpentinised. The reaction products form a submicron scale mixture of iron-rich brucite and serpentine. Olivine contains parallel inclusions of clinopyroxene + magnetite intergrowths which do not contain other serpentinization reaction products. This indicates that these inclusions have an orientation relationship with olivine acquired during a pre-serpentinization event, probably during symplectic exsolution in olivine at high temperature [START_REF] Moseley | Symplectic exsolution in olivine[END_REF]. [START_REF] Malvoisin | Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project[END_REF] measured the deformation of these inclusions during serpentinisation to estimate a change in volume during reaction close to 50 %. The major element composition indicates that there has not been significant mass transfer during reaction, except for the addition of H2O. This is confirmed by profiles in trace elements, showing that the pre-serpentinization zoning in Al, Ti, V, Sc and Cr of olivine is preserved during the reaction [START_REF] Malvoisin | Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project[END_REF]. The sample thus recorded isochemical serpentinisation with an associated solid volume increase corresponding to the value expected for reaction in a closed system. This mass transfer limited to the subgrain scale is characteristic of the incipient stage of serpentinization [START_REF] Evans | Using equilibrium thermodynamics in the study of metasomatic alteration, illustrated by an application to serpentinites[END_REF]. The observations performed here are thus relevant for this stage, and mineral-specific rather than rock-specific.

They are thus expected to apply to both dunite and harzburgite. We study here the contact between olivine and the serpentine/brucite mixture.

Methods

We first determined the composition and the texture of the sample at the micrometre scale with a field-emission scanning electron microscope (Tescan Mira II LMU) at the University of Lausanne. It was operated at 10 kV and 8 mm working distance. We then used imaging at the nanoscale to characterize the contacts.

Focused ion beam nanotomography

The 3-D nanostructure of the contact between olivine relics and its reaction products was determined with focused ion beam (FIB) tomography. A Zeiss Crossbeam 540 apparatus equipped with a field emission Scanning Electron Microscope (SEM) was used at the CIME-EPFL Laboratory (Lausanne) to sequentially mill 10 nm thick layers of a cuboid of size 10x20x13 μm. Inbetween milling stages, SEM images were acquired at 1.7 kV and 2.5 nA with a dwell time of 15 μs. The pixel size is of 10x10 nm. We used the MultiStackReg plug-in [START_REF] Thévenaz | A pyramid approach to subpixel registration based on intensity[END_REF] of the FIJI software to align the stack of images. We developed a Matlab code based on a median filter and on dilatation/erosion techniques to segment the images based on their back-scattered electron (BSE) intensity.

Transmission electron microscopy

We extracted disks from a 30 μm-thick petrographic thin section glued with CrystalBond epoxy for Transmission Electron Microscopy (TEM) analyses. The disks were then milled with an Ar+ ion polisher (PIPS II -Gatan, CMTC, Grenoble) operating at low incidence.

Then, the electron transparent bevels were coated with a 10 nm thick carbon film. We acquired high resolution images (HRTEM) with a JEOL JEM-2010 operated at 200 keV and having a 0.23 nm point to point resolution (CINaM, Marseille, France). We also obtained chemical maps at the nanoscale using a Field Electron Gun JEOL 2100F TEM operated at 200 keV and equipped with an EDX JEOL Centurio 1 detector (CMTC, Grenoble, France) in Scanning Transmission Electron mode (STEM) combined with Energy Dispersive Spectrometry (EDS) mapping.

In addition, an automated crystal orientation mapping tool (ASTAR ® ; NanoMEGAS) was used to map phase distribution and crystal orientation in the samples [START_REF] Rauch | Automated crystal orientation and phase mapping in TEM: Materials Characterization[END_REF]. 500x500 pixel maps were acquired at 200 keV with a probe size of 1.5 nm and by precessing the incident beam with an angle of 0.5°. Step sizes of 3 and 4 nm were used, leading to map sizes of 1.5x1.5 μm and 2x2 μm, respectively. The electron diffraction patterns acquired at each pixel were compared with template matching to a bank of patterns precomputed for the crystallographic structures of lizardite [START_REF] Mellini | Crystal structure of lizardite-1T from Elba[END_REF], brucite [START_REF] Chakoumakos | H/D isotope effects in brucite at low temperatures[END_REF] and olivine [START_REF] Angel | A century of mineral structures: How well do we know them[END_REF]. The reliability parameter defined in [START_REF] Rauch | Automated crystal orientation and phase mapping in TEM: Materials Characterization[END_REF] was used to generate a transparency mask which was superimposed on the phase and orientation maps to darken the pixels with a low reliability.

Results

The sample is highly homogeneous in microtexture. The reaction products around relict olivine grains are dominated by a mixture of serpentine and brucite with minor magnetite grains (Figure 1). At several micrometres to several tens of micrometres from the olivine surface, bright Fe-rich brucite appears to segregate in several micrometre wide zones with diffuse margins and in association with magnetite (Figures 1 A andC). Electron microprobe analyses always correspond to a mixture of serpentine and brucite, revealing that brucite segregation is never complete at the micrometre scale [START_REF] Malvoisin | Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project[END_REF]. Near the olivine surface, the serpentine/brucite mixture is more homogeneous in colour suggesting a more intimate mixing. Nervertheless, linear and parallel brighter zones reveal an organized structure at the submicrometre scale (Figures 1 B andD -black arrows). We investigated this structure at the nanoscale by focusing on the olivine/serpentine + brucite contact.

Morphology in three dimensions

We used FIB nanotomography to reveal the 3D structure at the nanoscale of the reaction products. We identified three reaction products of the serpentinisation reaction. The brightest regions have a platy habit, reaching a maximum size of 10 μm long and 400 nm wide (Figure 2). They correspond to the parallel and bright zones observed on SEM images. They are also regularly distributed in the reaction products, and have a preferential orientation with their longest axis roughly normal to the olivine surface, as shown on the three dimensional reconstructions on Figure 3. Sections parallel to the olivine surface show 1-2 μm wide polygons delimited by the brightest regions which form a honeycomb microstructure in 3D (Figures 2 and3). The columns filling this microstructure are composed of the darkest phase.

The third phase of intermediate grey tint in BSE images forms a ~ 200 nm thick continuous coating at the olivine surface (Figure 3 -orange region). We also observed connected pores forming up to 100 nm thick planes parallel to the olivine surface (Figure 2).

Structure at the nanoscale

EDS mapping in STEM mode confirmed the presence of three types of reaction products with different compositions (Figure 4). The reaction products are volumetrically dominated by the darkest phase on SEM images perpendicular to the olivine surface, with a homogeneous composition. The molar Mg/(Mg+Fe) and (Mg + Fe)/Si ratios measured in this phase are close to those measured at larger scale in lizardite (0.96 and 1.5, respectively; [START_REF] Malvoisin | Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project[END_REF]. This phase is hereafter referred as columnar lizardite. The brightest zones on SEM images appear as discontinuous domains with a low Mg/(Mg + Fe) ratio and a (Mg + Fe)/Si ratio higher than in olivine. This suggests that these interstitial zones are composed of a mixture of brucite and serpentine (hereafter interstitial mixture). The region coating the olivine surface has a rather homogeneous composition with Mg/(Mg + Fe) and (Mg + Fe)/Si ratios close to those of olivine. Such a composition again indicates a serpentine/brucite mixture (hereafter coating mixture) but richer in serpentine than the interstitial mixture (Figure 4 B andC). Graduations in chemical composition from the coating to the interstitial mixtures were observed at the base of the interstitial mixture, especially when the interstitial mixture is slightly oblique to the olivine surface (Figure 4 D andE). This change in composition probably corresponds to an increase in the brucite fraction in the brucite/serpentine mixture.

The automated crystal orientation maps confirmed the presence of lizardite in the columns occurring as ~ 1 μm wide and > 3 μm long grains with minor intercalated brucite lamellae (Figure 5). The orientations of lizardite and brucite lamellae in the columns are identical with their [001] axis radiating out from the olivine surface (Figure 5 B andD). Grains in the brucite/serpentine mixtures have a size below 100 nm and a random orientation.

Ordered (columnar lizardite) and disordered (coating and interstitial mixtures) domains were also observed on HRTEM images (Figure 6). The (001) basal planes of columnar lizardite are parallel to the contact between olivine and reaction products. The columnar lizardite displays sharp contacts with disordered brucite/serpentine mixtures (Figure 6 A and D). The coating mixture is continuous between olivine surface and lizardite columns, even though it varies in thickness from less than 50 nm to ~ 200 nm (Figure 6 D). Electron diffraction confirms that the disordered domains are composed of brucite and lizardite intermixed at a scale less than 100 nm (Figure 6B). The stack of differently oriented lizardite and brucite grains leads to extensive distortion and nanoscale pore formation along the (001) basal plane of both sheetlike minerals. The pores are approximately 10 nm wide and 1 nm high. Image analysis of Figure 6 A allowed a rough estimate of ~ 3 % porosity in the interstitial mixture. (Mg+Fe)/Si molar ratios measured with EDS point analyses are of ~ 2.0, 1.9, 1.6 and 2.4 in olivine, coating mixture, columnar lizardite and interstitial mixture, respectively (the locations of the analysed points are given in Figures 4 A and6 A). The higher (Mg+Fe)/Si ratios in the disordered domains compared to the ordered domains are in agreement with the EDS mapping and the automated crystal orientation maps. The higher (Mg+Fe)/Si ratio ratio in the interstitial mixture compared to the coating mixture also suggests a higher proportion of brucite inbetween the lizardite columns.

Discussion

Mechanism of fluid pathway preservation

The observation of a continuous disordered domain composed of a brucite/lizardite mixture is, to our knowledge, the first documentation of such a coating at the olivine surface. However, amorphous or poorly crystalline material (often called proto-serpentine) has been produced during olivine hydration in experiments [START_REF] Daval | Influence of amorphous silica layer formation on the dissolution rate of olivine at 90°C and elevated[END_REF][START_REF] Godard | Incipient hydration of mantle lithosphere at ridges: A reactive-percolation experiment[END_REF][START_REF] Lafay | Dissolution-reprecipitation and self-assembly of serpentine nanoparticles preceding chrysotile formation: Insights into the structure of proto-serpentine[END_REF] and observed in natural samples [START_REF] Cressey | Electron microscopy of serpentinite textures[END_REF][START_REF] Andreani | A microstructural study of a "crack-seal" type serpentine vein using SEM and TEM techniques[END_REF][START_REF] Rumori | Oriented, non-topotactic olivine → serpentine replacement in mesh-textured, serpentinized peridotites[END_REF][START_REF] Boudier | Serpentine Mineral Replacements of Natural Olivine and their Seismic Implications: Oceanic Lizardite versus Subduction-Related Antigorite[END_REF][START_REF] De Obeso | Fluid rock interactions on residual mantle peridotites overlain by shallow oceanic limestones: Insights from Wadi Fins[END_REF]. The location of the coating mixture at the olivine contact, and its continuity, suggest that it is the first product of olivine hydration (Figure 7). This first step involves local dissolution and precipitation as proposed for replacement reactions (Putnis, 2009). In such a process, the aqueous species formed during olivine dissolution precipitate locally at the olivine/coating mixture contact. The composition of the mixture with a (Mg+Fe)/Si ratio similar to olivine suggests that this first step is isochemical, with limited mass transfer and porosity clogging at a scale > 100 nm. Transport through the thin coating layer may occur by diffusion (Figure 7 B). In contrast to the thin coating layer, the reaction products are volumetrically dominated by the adjoining lizardite columns with a honeycomb microstructure. The pattern described here is similar to those formed by desiccation or magma cooling (i.e. columnar joints). The columns forming such patterns are oriented in parallel to the main gradient in temperature or concentration. Here, olivine dissolution controls the main gradient in aqueous species concentration. As a result, the columns are oriented perpendicularly to the olivine surface. [START_REF] Goehring | Nonequilibrium scale selection mechanism for columnar jointing[END_REF] proposed the following scaling law for columnar pattern formation:

Pe = (v Lc)/D ( 1
)
where Pe is the dimensionless Péclet number comparing advection and diffusion, v is the speed of pattern development, Lc is the characteristic size of the pattern, and D is the diffusivity. [START_REF] Goehring | Nonequilibrium scale selection mechanism for columnar jointing[END_REF] have shown that formation of patterns similar to the one described here requires Pe ~ 0.1. Using experimentally-determined serpentinisation rates [START_REF] Lafay | Mineral replacement rate of olivine by chrysotile and brucite under high alkaline conditions[END_REF][START_REF] Malvoisin | Serpentinization of oceanic peridotites: 2. Kinetics and processes of San Carlos olivine hydrothermal alteration[END_REF][START_REF] Mccollom | Temperature trends for reaction rates, hydrogen generation, and partitioning of iron during experimental serpentinization of olivine[END_REF] and a characteristic pattern size of ~ 1 μm (lizardite column width), equation ( 1) provides a rough estimate for diffusivity through the coating mixture between 10 -17 and 10 -20 m 2 /s. These values are consistent with the values estimated experimentally for intergranular diffusivity during serpentinization [START_REF] Malvoisin | Water diffusion-transport in a synthetic dunite: Consequences for oceanic peridotite serpentinization[END_REF] and approximately 10 orders of magnitude smaller than diffusivity in a free fluid (10 -9 m 2 /s; [START_REF] Oelkers | Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Aqueous tracer diffusion coefficients of ions to 1000°C and 5 kb[END_REF]. This may explain why the mineralogical processes described here occur at a scale below 100 nm, without mass transfer into the nanopores and porosity clogging.

Lizardite columns similar to the ones described here have also been identified by [START_REF] Boudier | Serpentine Mineral Replacements of Natural Olivine and their Seismic Implications: Oceanic Lizardite versus Subduction-Related Antigorite[END_REF] in a harzburgite from Oman, where they were interpreted as evidence for a nontopotaxial relationship between olivine and lizardite. [START_REF] Rumori | Oriented, non-topotactic olivine → serpentine replacement in mesh-textured, serpentinized peridotites[END_REF] and Rouméjon et al.

(2015) also reported the presence of columnar lizardite in mesh textures found in harzburgites from the Alps and the Southwest Indian Ridge. [START_REF] Cressey | Electron microscopy of serpentinite textures[END_REF] observed the presence of lizardite plates formed after olivine. This suggests that the formation of lizardite columns is a common process during mesh texture formation in both dunite and harzburgite.

The lizardite columns are separated by a 100-400 nm wide brucite/lizardite mixture with crystallographic characteristics similar to the coating mixture but with a higher proportion of brucite. To our knowledge, the fact that the lizardite columns is embedded in a brucite-rich mineralogical assemblage has not been previously reported. The composition of the interstitial mixture and the lizardite columns and their location at the contact with the coating mixture, suggest that their formation constitutes a second step in the serpentinisation process (Figure 7 B). The volumetric proportion of brucite estimated with image analysis in the reaction products is of 10 % and 22% in the phase maps of Figure 5A and 5C, respectively. These proportions are consistent with the proportion of 19 % expected for the isochemical serpentinisation of olivine, which was documented at larger scale in the same sample [START_REF] Malvoisin | Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project[END_REF], suggesting again that reaction occurs with mass transfer limited to a scale < 100 nm. The brucite grain size does not seem to evolve significantly during the second step of the reaction while lizardite forms micrometer-wide and well-ordered columns at the expense of nanometer-sized grains in the coating mixture. This difference in grain size evolution probably reflects a difference in solubility of the lizardite and brucite grains in the coating mixture at ~ 200 nm from the olivine surface where lizardite columns are formed.

Small grains are less stable than large grains due to their greater surface area. This is the driving force for Ostwald ripening during which small grains dissolve while large grains grow. The grain size at which the transition from dissolution to growth occurs (saturation grain size; d) can be defined as [START_REF] Steefel | A new kinetic approach to modeling water-rock interaction: The role of nucleation, precursors, and Ostwald ripening[END_REF]):

(2)

where is the surface free energy, is the molar volume, R is the gas constant, is the supersaturation defined as the ratio between the activity product for the dissolution reaction and the equilibrium constant. is not expected to be significantly different for brucite and lizardite in the coating mixture [START_REF] Malvoisin | Water diffusion-transport in a synthetic dunite: Consequences for oceanic peridotite serpentinization[END_REF]. However, is 1.5 J/m 2 for the (111) SiO2 surface and 0.05 J/m 2 for the (001) Mg(OH)2 surface [START_REF] Churakov | Ab initio study of dehydroxylationcarbonation reaction on brucite surface[END_REF][START_REF] Krasilin | Energy modeling of competition between tubular and platy morphologies of chrysotile and halloysite layers: Clays and Clay Minerals[END_REF]. These data indicates that the surface energy of lizardite could be approximately one order of magnitude higher than for brucite. For values calculated with SUPCRT92 [START_REF] Johnson | SUPCRT92: A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000[END_REF] by assuming thermodynamic equilibrium between lizardite and brucite and the same rate of Mg 2+ and SiO2,aq diffusion, the calculated d values are one to two orders of magnitude higher for chrysotile than for brucite (Figure 8). For a given supersaturation, lizardite grains have thus to be significantly larger than brucite grains to be preserved. Supersaturation is expected to decrease with distance from the olivine surface due to diffusion. At a distance of ~ 200 nm from the olivine surface, nanometre-sized lizardite grains are thus probably no more stable in the coating mixture where they dissolve to recrystallize in lizardite columns, as already suggested by [START_REF] Cressey | Electron microscopy of serpentinite textures[END_REF] and [START_REF] Rumori | Oriented, non-topotactic olivine → serpentine replacement in mesh-textured, serpentinized peridotites[END_REF]. The brucite nanograins are thermodynamically stable at this distance from olivine, and mechanically segregate at the column margins to form the interstitial mixture. This may not be always true, especially further away from the olivine surface where brucite layers are sometimes observed to be intercalated inbetween lizardite layers in the columns (Figure 5).

Brucite mechanical segregation in the interstitial mixture probably leads to significant viscous deformation and rotation that might contribute to formation of the nanoscale porosity observed at the column margins. Fully crystalline lizardite probably occupies a smaller volume in the well-ordered columns than in the polycrystalline coating mixture. Solid volume may thus decrease during the coating mixture → columnar lizardite + interstitial mixture reaction, also contributing to porosity formation. Tutolo et al. (2016) measured a porosity of several percent at the nanoscale in drill core samples from mid-ocean ridges. We probably image this same kind of porosity in our samples, ~ 3 % in interstitial mixture, and ~ 0.5 % when averaged over the entire alteration assemblage. In addition, the orientation of the interstitial mixture perpendicular to the olivine surface suggests that these domains can act as preferential fluid pathways at a scale of tens of microns (Figure 7). The size of the pores in the interstitial mixture and their low connectivity may not favor fluid transport driven by fluid pressure gradients but rather by diffusion under gradients in dissolved ions, electric potential [START_REF] Plümper | Fluiddriven metamorphism of the continental crust governed by nanoscale fluid flow[END_REF] or surface energy [START_REF] Evans | A Poroelastic Model of Serpentinization: Exploring the Interplay Between Rheology, Surface Energy, Reaction, and Fluid Flow[END_REF].

Our observations and interpretations regarding mass transfer limited to a scale of less than 100 nm are essential to constrain previously proposed reaction mechanisms. Numerical simulations have shown that limited mass transfer during reaction plays a key role for fluid pathway preservation and high extent of reaction to be achieved [START_REF] Malvoisin | Achieving complete reaction while the solid volume increases: A numerical model applied to serpentinisation[END_REF]. The solid volume increase is not accommodated by porosity clogging, but rather by deformation associated with tectonics or the reaction itself. Limited mass transfer during reaction favors stress-build up [START_REF] Malvoisin | Achieving complete reaction while the solid volume increases: A numerical model applied to serpentinisation[END_REF], which may lead to expansion or cracking [START_REF] Macdonald | Rate of serpentinization in seafloor environments[END_REF][START_REF] O'hanley | Solution to the volume problem in serpentinization[END_REF][START_REF] Jamtveit | Reaction induced fracturing during replacement processes[END_REF][START_REF] Rudge | A simple model of reaction-induced cracking applied to serpentinization and carbonation of peridotite[END_REF][START_REF] Malvoisin | Serpentinization of oceanic peridotites: 2. Kinetics and processes of San Carlos olivine hydrothermal alteration[END_REF][START_REF] Malvoisin | Control of serpentinisation rate by reaction-induced cracking[END_REF][START_REF] Kelemen | Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation[END_REF][START_REF] Evans | Phase-Field Modeling of Reaction-Driven Cracking: Determining Conditions for Extensive Olivine Serpentinization[END_REF]Yoshida et al., 2020).

Previous petrological observations also provide evidence for limited mass transfer during serpentinisation. [START_REF] Beard | Onset and progression of serpentinization and magnetite formation in Olivine-rich troctolite from IODP hole U1309D[END_REF] identified a first isochemical stage of serpentine + brucite formation at the expense of olivine at low water to rock ratio. [START_REF] Evans | Using equilibrium thermodynamics in the study of metasomatic alteration, illustrated by an application to serpentinites[END_REF] interpreted the presence of serpentine + brucite in harzburgite as evidence for limited silica transfer during the incipient stage of the reaction. The nanostructure described here could thus also be formed in harzburgite at this stage. As the reaction continues, the fluid flux can increase leading to mass transfer at larger scale with possible modification of the initially formed mineralogical assemblages. [START_REF] Frost | The process of serpentinization in dunite from new caledonia[END_REF] suggest that the initially formed brucite can react to form serpentine and magnetite. Such a reaction is not observed in the sample studied here but the observed nanostructure can give insights on serpentinisation product reactivity during further fluid/rock interaction.

Implication for post-serpentinisation alteration

As discussed above, the incipient stage of serpentinisation at low water to rock ratio is generally isochemical, aside from addition of H2O accompanied by very limited dissolution.

However, fluid influx can continue in serpentinised peridotites, favoring mass transfer at large scale. The switch from isochemical reaction (except for the addition of water) to open system conditions generally induces a decrease in the MgO/SiO2 ratio of the serpentinised peridotites by several percents [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF][START_REF] Niu | Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath Mid-Ocean ridges[END_REF][START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF]. This decrease could be associated with Si gain [START_REF] Malvoisin | Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical[END_REF][START_REF] De Obeso | Fluid rock interactions on residual mantle peridotites overlain by shallow oceanic limestones: Insights from Wadi Fins[END_REF][START_REF] Templeton | Formation and loss of metastable brucite: Does Fe(II)-bearing brucite support microbial activity in serpentinizing ecosystems[END_REF] and/or Mg loss [START_REF] Snow | Pervasive magnesium loss by marine weathering of peridotite[END_REF]de Obeso and Kelemen, 2020;[START_REF] De Obeso | A Mg Isotopic Perspective on the Mobility of Magnesium During Serpentinization and Carbonation of the Oman Ophiolite[END_REF]. Si-rich fluids can be formed during orthopyroxene serpentinisation [START_REF] Beard | Onset and progression of serpentinization and magnetite formation in Olivine-rich troctolite from IODP hole U1309D[END_REF][START_REF] Frost | The process of serpentinization in dunite from new caledonia[END_REF] or mafic rock alteration [START_REF] Bach | Seawater-peridotite interactions: First insights from ODP Leg 209[END_REF][START_REF] Paulick | Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 15[END_REF]. Mg loss is probably associated with brucite dissolution leading to the formation of voids in serpentinised peridotites [START_REF] Jöns | Reaction-induced porosity and onset of lowtemperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography[END_REF], and magnesite precipitation in veins [START_REF] Kelemen | situ carbonation of peridotite for CO2 storage[END_REF][START_REF] Kelemen | Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO 2 Capture and Storage[END_REF][START_REF] García Del Real | Clumped-isotope thermometry of magnesium carbonates in ultramafic rocks[END_REF].

Whether due to Mg-loss or Si-addition, it is apparent that brucite reacts rapidly during peridotite weathering. Comparing oceanic serpentinised peridotites collected by dredging versus drilling, Klein et al. (2020) demonstrated that there is a systematically lower brucite content in the dredged samples suggesting brucite alteration during low temperature alteration with seawater on the seafloor. Kelemen et al. (2020) and [START_REF] Ellison | Low-Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite[END_REF] reported that brucite is detectable via X-Ray diffraction only below 100 m depth in drill core from Oman Drilling Project Hole BA3A, despite the fact that the protolith was an Si-poor dunite, suggesting brucite was dissolved or reacted to form other minerals near the surface. Kelemen et al. (2020) also observed brucite absence in dunites from Oman Drilling Project Hole BA4A but in a more limited section of the core ranging from the surface to 18 m depth. Compilation of MgO/SiO2 data does not show a significant shift to low values compared to the terrestrial array in the peridotites from the Holes BA1B, BA3A and BA4A (Kelemen, 2021b). This probably indicates that brucite dissolution or silicification is spatially limited near the surface and thus only concerns a limited number of collected samples. An alternative explanation would be that brucite reaction near the surface does not involve mass transfer. Brucite could indeed react with serpentine according to the following reaction of iron oxidation:

Mg(OH)2 + 4 Fe 2+ (OH)2 + Mg3Si2O5(OH)4 = 2 Fe 3+ Mg2SiFe 3+ O5(OH)4 + H2O + 2 H2 (3)

Brucite + Amakinite + Mg-lizardite = Mg-cronstedtite + Water + Molecular Hydrogen Determining if mass transfer is involved in brucite removal in Oman will require further investigations of the rock microtexture and composition.

In the present study, brucite is found to occur as < 100 nm grains with a high surface area (Figure 5). Moreover, brucite occurs in regions rich in nanopores (Figure 6) where fluids may be preferentially transported (Figure 7). As a result, the reactive surface area and porosity are larger in brucite-rich than in lizardite-rich domains, which may help to explain the relatively high reactivity of brucite at low temperature.

Conclusion

We 2)) as a function of saturation for brucite (dashed line) and lizardite (plain line). The grains are expected to dissolve and grow below and above these lines, respectively. The maximum and minimum saturations correspond to the equilibria of the aqueous solution with olivine and lizardite/brucite mineralogical assemblage, respectively. There are calculated with SUPCRT92 [START_REF] Johnson | SUPCRT92: A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000[END_REF]) by assuming the same diffusivity for Mg 2+ and SiO2,aq. The dotted line indicates the measured width of the lizardite columns.

  studied the nanostructure of the products of serpentinisation in a sample from the Oman Drilling Project. The observations suggest a reaction in two steps. A brucite + serpentine mixture first precipitates at the olivine surface. The mixture then partly dissolves to form lizardite columns separated by a brucite-rich assemblage of brucite + serpentine grains containing a high density of nanopores. These observations indicate mass transfer limited to a scale below 100 nm during serpentinisation, preventing clogging of porosity and allowing for complete reaction to be achieved. Brucite occurs as nanograins in the fluid pathways, which favors its alteration at low temperature during further post-serpentinisation fluid/rock interaction. Tutolo, B.M., Mildner, D.F.R., Gagnon, C.V.L., Saar, M.O., and Seyfried, W.E., 2016, Nanoscale constraints on porosity generation and fluid flow during serpentinization:Geology, v. 44, p. 103-106, doi:10.1130/G37349.1. Yoshida, K., Okamoto, A., Shimizu, H., Oyanagi, R., and Tsuchiya, N., 2020, Fluid Infiltration Through Oceanic Lower Crust in Response to Reaction-Induced Fracturing:Insights From Serpentinized Troctolite and Numerical Models: Journal of Geophysical Research: Solid Earth, v. 125, p. e2020JB020268, doi:10.1029/2020JB020268.

Figure captions Figure 1 :

 captions1 Figure captionsFigure 1: Back-scattered electron images of the reaction products around relicts of olivine.

Figure 2 :

 2 Figure 2: Back-scattered electrons images reconstructed from Focused Ion Beam-Scanning

Figure 3 :

 3 Figure 3: Contact between olivine and its reaction products segmented in 3D. Olivine in

Figure 4 :

 4 Figure 4: Semi-quantitative compositional maps obtained with TEM-EDS mapping. A:

Figure 5 :

 5 Figure 5: Maps acquired with the automated crystal orientation tool at the olivine/serpentine

Figure 6 :

 6 Figure 6: TEM images of the reaction products (see location on Figure 4). A: Contact

Figure 7 :

 7 Figure 7: Sketch summarising the nanoscale observations. A: Olivine hydration forms three

Figure 8 :

 8 Figure 8: Saturation grain size (defined in equation (2)) as a function of saturation for brucite

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8

 12345678 Figure 1

Acknowledgments B.M. acknowledges support from the Swiss National Science Foundation (Grant No. PZ00P2_168083). P.B.K.'s effort is supported in part by the Arthur D. Storke Chair at Columbia University. G. Renou and D. Chaudenson are thanked for technical support. The Oman Drilling Project was funded by NASA -Astrobiology Institute (NNA15BB02A, Templeton PI), the German Research Foundation (DFG: KO 1723/21-1, Koepke PI), the Japanese Society for the Promotion of Science (JSPS no:16H06347, Michibayashi PI; and KAKENHI 16H02742, Takazawa PI), the European Research Council (Adv: no.669972; Jamtveit PI), the Swiss National Science Foundation (SNF: 20FI21_163073, Früh-Green PI), JAMSTEC, the TAMU-JR Science Operator, and contributions from the Sultanate of Oman Ministry of Regional Municipalities and Water Resources, the Oman Public Authority of Mining, Sultan Qaboos University, CNRS-Univ. Montpellier, Columbia University of New York, and the University of Southampton.


Rakesh K Sharma

I Khamidy

Laetitia Rapenne

Frédéric Charlot

Hamza Moussaoui

Jérôme Laurencin

Elisabeth Djurado
email: elisabeth.djurado@lepmi.grenoble-inp.fr

Highly efficient architectured Pr6O11 oxygen electrode for solid oxide fuel cell

Keywords: Pr6O11 oxygen electrode, hierarchical microstructure, electrostatic spray deposition, current collector, electrochemical impedance spectroscopy, FIB/SEM 3D reconstruction

An optimization of the oxygen electrode microstructure needs to be addressed to improve the performance of solid oxide fuel cell (SOFC) since the kinetics of the oxygen reduction reactions (ORR) in the cathode remains the main limiting factor. In this work, the electrochemical properties of the Pr6O11 architectured electrode are investigated as a function of the microstructure of the active functional layer (AFL) prepared by electrostatic spray deposition (ESD) on Ce0.9Gd0.1O2-δ (GDC) electrolyte. An optimization of the AFL microstructure is investigated varying sintering temperature from 600 °C to 1000 °C for 2 h in air. The effect of sintering temperature on the AFL microstructures is visualized and discussed thanks to FIB/SEM (focus ion beam/scanning electron microscopy) reconstructions. The composition and thickness of a CCL current collecting layer (CCL), deposited by screen-printing (SP) on the top of this AFL, are optimized as well. The nature of the CCL is either Pr6O11, La0.7Sr0.3MnO3-δ (LSM) or La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and the thickness is varied from 15 to 60 µm. Moreover, the compatibility of the Pr6O11 electrode with GDC is checked over 10 days in air at 800 °C.

Introduction

Solid Oxide Fuel Cells (SOFCs) have drawn much attention because they convert chemical energy into electrical energy with high efficiency and reduced emissions [START_REF] Wachsman | Lowering the temperature of solid oxide fuel cells[END_REF][START_REF] Liu | Rational SOFC material design: new advances and tools[END_REF]. The main objective of the current SOFC research is to reduce the operating temperatures down to intermediate range (500-700°C).

Lowering the operating temperature offers many advantages such as reduced materials cost, prolonged lifetime, flexible choice of cell materials and shortened start-up time [START_REF] Fabbri | Towards the Next Generation of Solid Oxide Fuel Cells Operating Below 600 °C with Chemically Stable Proton-Conducting Electrolytes[END_REF][START_REF] Lee | Tailoring gadolinium-doped ceria-based solid oxide fuel cells to achieve 2 W cm -2 at 550[END_REF][START_REF] Ma | Samarium-Doped Ceria Nanowires: Novel Synthesis and Application in Low-Temperature Solid Oxide Fuel Cells[END_REF]. However, a reduction of the operation temperature of conventional SOFCs is very challenging. The performance of SOFCs is decreased when the operating temperature is lowered due to a higher activation energy and lower oxygen reduction reaction (ORR) kinetics, especially at the cathodic side [START_REF] Yang | Perovskite Sr1-xCexCoO3-δ (0.05 ≤ x ≤ 0.15) as Superior Cathodes for Intermediate Temperature Solid Oxide Fuel Cells[END_REF]. Thus reducing the operating temperature requires a great improvement in cathode performance [START_REF] Wachsman | Lowering the temperature of solid oxide fuel cells[END_REF].

One way to improve the cathode performance is to use mixed electronic and ionic conductors (MIEC) cathode such as a La(Sr)CoO3-δ (LSC), La(Sr)Fe(Co)O3-δ (LSFC) [START_REF] Wachsman | Lowering the temperature of solid oxide fuel cells[END_REF][START_REF] Liu | Rational SOFC material design: new advances and tools[END_REF] and nickelates (Ln2NiO4+δ, Ln = La, Pr, Nd) [START_REF] Minh | Ceramic fuel cells[END_REF][START_REF] Matsuzaki | Electrochemical properties of reduced-temperature SOFCs with mixed ionicelectronic conductors in electrodes and/or interlayers[END_REF][START_REF] Adler | Factors governing oxygen reduction in solid oxide fuel cell cathodes[END_REF][START_REF] Grenier | Novel cathodes for solid oxide fuel cells, Functional Materials for Sustainable Energy Applications[END_REF][START_REF] Marinha | Performance of (La, Sr)(Co, Fe) O3-x double-layer cathode films for intermediate temperature solid oxide fuel cell[END_REF][START_REF] Sharma | An innovative architectural design to enhance the electrochemical performance of La2NiO4+ δ cathodes for solid oxide fuel cell applications[END_REF][START_REF] Sharma | Design of interfaces in efficient Ln2NiO4+ δ (Ln= La, Pr) cathodes for SOFC applications[END_REF]. More recently, Clement et al. [START_REF] Nicollet | An innovative efficient oxygen electrode for SOFC: Pr6O11 infiltrated into Gd-doped ceria backbone[END_REF] have reported excellent transport properties for Pr6O11 and electrochemical properties for Pr6O11 infiltrated Ce0.9Gd0.1O2-δ (GDC) backbone. They have measured high oxygen diffusion (D * = 3.4 × 10 -8 ± 0.1 × 10 -8 cm 2 s -1 at 600 °C) and oxygen surface exchange coefficients (k * = 5.4 × 10 -7 ± 0.1 × 10 -7 cm s -1 at 600 °C) [START_REF] Nicollet | An innovative efficient oxygen electrode for SOFC: Pr6O11 infiltrated into Gd-doped ceria backbone[END_REF] with the lowest polarization resistance (0.028 at Ω cm 2 at 600 °C) for Pr6O11 infiltrated GDC cathodes [START_REF] Nicollet | An innovative efficient oxygen electrode for SOFC: Pr6O11 infiltrated into Gd-doped ceria backbone[END_REF]. In fact, there are only a few studies on Pr6O11 for SOFC applications which show an improvement in the electrochemical performance when Pr6O11 is used as an additive in composite electrodes [START_REF] Chiba | An SOFC cathode infiltrated with Pr6O11[END_REF][START_REF] Taguchi | LNF SOFC cathodes with an active layer using Pr6O11 or Pr-doped CeO2[END_REF][START_REF] Ding | Enhanced oxygen reduction activity on surface-decorated perovskite La0.6Ni0.4FeO3 cathode for solid oxide fuel cells[END_REF]. For example, Pr6O11 infiltrated LNF (LaNi0.6Fe0.4O3-δ) electrodes exhibits an improved performance over LNF (LaNi0.6Fe0.4O3δ) electrodes as reported by Chiba et al. and Ding et al [START_REF] Chiba | An SOFC cathode infiltrated with Pr6O11[END_REF][START_REF] Ding | Enhanced oxygen reduction activity on surface-decorated perovskite La0.6Ni0.4FeO3 cathode for solid oxide fuel cells[END_REF]. More recently, we have reported polarization resistances as low as 0.026 Ω cm 2 at 600 °C for a pure Pr6O11 columnar-type cathode prepared by ESD on GDC electrolyte followed by sintering at only 700 °C which shows good adhesion with the electrolyte and retains a fine microstructure [START_REF] Sharma | An efficient hierarchical nanostructured Pr6O11 electrode for solid oxide fuel cells[END_REF][START_REF] Chiba | High-temperature properties of Ce1-xPrxO2-δ as an active layer material for SOFC cathodes[END_REF][START_REF] Hyde | Kinetic Studies on Reactions of Praseodymium Oxides in an Oxygen Atmosphere: PrO1.83 + O2 -PrO2[END_REF]. Moreover, a low sintering temperature also avoids the problem of cracks and delamination. This is an advantage with respect to screen printing which requires a much higher sintering temperature (~1100 °C) to ensure a good adhesion of the electrode over the electrolyte.

Another way to enhance the cathode performance is to optimize the microstructure since, for a given electrode, electrode kinetics strongly depends upon the microstructural parameters such as the specific surface area, particle size, porosity and thickness [START_REF] Zhang | Simulation of sintering kinetics and microstructure evolution of composite solid oxide fuel cells electrodes[END_REF][START_REF] Hildenbrand | Influence of configuration and microstructure on performance of La2NiO4+ δ intermediate-temperature solid oxide fuel cells cathodes[END_REF][START_REF] Adler | Electrode kinetics of porous mixed-conducting oxygen electrodes[END_REF]. All these factors play a key role in the polarization resistance of SOFC electrodes [START_REF] Ma | Samarium-Doped Ceria Nanowires: Novel Synthesis and Application in Low-Temperature Solid Oxide Fuel Cells[END_REF][START_REF] Dieterle | Microstructure of Nanoscaled La0.6Sr0.4CoO3-δ Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells[END_REF][START_REF] Schneider | Percolation effects in functionally graded SOFC electrodes[END_REF]. This is the reason why hierarchically nanostructured porous electrodes have attracted an increasing interest in many applications for highperformance energy conversion and storage [START_REF] Sun | Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine[END_REF][START_REF] Lichtner | Processing of Hierarchical and Anisotropic Porosity LSM-YSZ Composites[END_REF]. It has been shown that macro-pore channels facilitate mass transport while nano-porous network enhances electrochemical reactions in the SOFC electrodes [START_REF] Chen | Low-temperature solid oxide fuel cells with hierarchically porous cathode nano-network[END_REF]. Nevertheless, hierarchical porosity should be well fine-tuned in order to have percolation of pore phases and thus high specific surface area. A large number of microstructures such as thin, coral and columnar-type morphologies with high active surface areas have been manufactured by ESD for various types of materials [START_REF] Sharma | Design of interfaces in efficient Ln2NiO4+ δ (Ln= La, Pr) cathodes for SOFC applications[END_REF][START_REF] Marinha | Influence of electrospraying parameters on the microstructure of La0.6Sr0.4Co0.2F0.8O3-δ films for SOFCs[END_REF][START_REF] Djurado | Electrostatic spray deposition of Ca3Co4O9+ δ layers to be used as cathode materials for IT-SOFC[END_REF][START_REF] Çelikbilek | Rational design of hierarchically nanostructured electrodes for solid oxide fuel cells[END_REF]. In addition, microstructural parameters such as pores and particle growth can be further tailored by simply controlling the sintering process and hence the ORR kinetics too.

Sintering kinetics are quite complex since it is influenced by numerous factors [START_REF] De Jonghe | Sintering of ceramics[END_REF]. However, a compromise in sintering temperature (Ts) is also needed in order to find an optimum microstructure because it affects other properties of the electrode such as the formation of necking between particles, adherence at the electrode/electrolyte interface, densification, mechanical strength, and conductivity [START_REF] Hwang | Electrochemical performance of LSCF-based composite cathodes for intermediate temperature SOFCs[END_REF][START_REF] Droushiotis | Effects of lanthanum strontium cobalt ferrite (LSCF) cathode properties on hollow fiber micro-tubular SOFC performances[END_REF]. Thus, our objective is to use an innovative, simple and low-cost ESD technique to prepare and study the hierarchical nanostructured porous Pr6O11 cathodes [START_REF] Marinha | Performance of (La, Sr)(Co, Fe) O3-x double-layer cathode films for intermediate temperature solid oxide fuel cell[END_REF][START_REF] Sharma | Design of interfaces in efficient Ln2NiO4+ δ (Ln= La, Pr) cathodes for SOFC applications[END_REF][START_REF] Djurado | Electrostatic spray deposition of Ca3Co4O9+ δ layers to be used as cathode materials for IT-SOFC[END_REF][START_REF] Çelikbilek | Rational design of hierarchically nanostructured electrodes for solid oxide fuel cells[END_REF].

Herein, the effect of sintering temperature on the microstructure of Pr6O11, deposited by ESD on GDC, is investigated in relationships with the electrochemical properties in a double-layer architecture. In addition, the effect of the composition and thickness of the CCL on electrochemical properties is discussed. Moreover, the reactivity and compatibility of Pr6O11 with GDC electrolyte are also studied.

Experimental

AFL preparation

For the active functional layers (AFLs), praseodymium nitrate hexahydrate [Pr(NO3)3•6H2O, 99.9%, Aldrich] based precursor solution of 0.02 M concentration was prepared into a mixture of ethanol (EtOH, CH3CH2OH, >99.9%, VWR Chemicals) and butyl carbitol (BC, diethylene glycol monobutyl ether CH3(CH2)3(OC2H4)2OH, Acros Organics, 99+%,), i.e. (EtOH: BC, 1:2) under vigorous stirring. These Pr6O11 layers have been deposited on Ce0.9Gd0.1O2-δ (GDC) substrates (19.6 mm in diameter, 1.2 mm thick) by electrostatic spray deposition (ESD) which is based on the principle of electrohydrodynamics laws [START_REF] Sharma | An innovative architectural design to enhance the electrochemical performance of La2NiO4+ δ cathodes for solid oxide fuel cell applications[END_REF][START_REF] Marinha | Influence of electrospraying parameters on the microstructure of La0.6Sr0.4Co0.2F0.8O3-δ films for SOFCs[END_REF][START_REF] Çelikbilek | Rational design of hierarchically nanostructured electrodes for solid oxide fuel cells[END_REF][START_REF] Sharma | La4Ni3O10δ as an efficient solid oxide fuel cell cathode: electrochemical properties versus microstructure[END_REF][START_REF] Chen | Electrostatic sol-spray deposition of nanostructured ceramic thin films[END_REF][START_REF] Marinha | Comprehensive review of current developments in IT-SOFCs[END_REF][START_REF] Constantin | Interface stability of thin, dense GDC film coating on YSZ for solid oxide fuel cells[END_REF]. A systematic study of the effect of the ESD deposition parameters on the microstructure of Pr6O11 has been reported in our previous work [START_REF] Sharma | An efficient hierarchical nanostructured Pr6O11 electrode for solid oxide fuel cells[END_REF]. In this paper, we have selected the columnar-type microstructure obtained from the EtOH: BC (1:2), at a nozzle-to-substrate distance, flow rate, substrate temperature and deposition time of, 20 mm, 1.5 mL h -1 , 300 °C and 3 h, respectively.

During the ESD deposition, the voltage was fixed to approximately 5-7 kV. The prepared films were subsequently sintered at 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C, respectively for 2 h in air.

Symmetric cell preparation

Several symmetric cells (electrode/GDC/electrode) were prepared as given in Table 1. These electrodes consist of an AFL of Pr6O11 prepared by ESD, topped by a screen printing (SP) current collecting layer (CCL) of a different nature. For the screen-printing, inks were prepared by mixing the commercial powders such as LSM (La0.7Sr0.3MnO3-δ, SSC Inc., d50 = 0.2 μm), LSCF (La0.6Sr0.4Co0.2Fe0.8O3-δ, Marion Tech., d50 = 0.27 μm) and Pr6O11 (Materion (cerac INC.), d50 <5μm) into a KD 2921 solvent. Then these double-layer electrodes were sintered in a two-steps sintering: first, at 450 °C for 2 h in air with a ramp of 0.5 °C min -1 to remove the binders and ink solvents and then, at 600 °C for 2 h in air with a ramp of 2 °C min -1 in order to improve the adhesion onto the ESD layer. All samples were cooled down to room temperature at 3 °C min -1 rate. Area of each electrode was 1.54 cm 2 .

Microstructural and electrochemical characterization

The crystal structure and purity of the films were determined by X-ray diffraction (XRD, Philips X'Pert-MPD system, Cu Kα radiation, λ=1.54056 Å) in the Bragg-Brentano configuration. Fullprof software was used to determine the lattice parameters from the refined X-ray pattern by profile matching [START_REF] Rodriguez-Carvajal | Recent advances in magnetic structure determination by neutron powder diffraction[END_REF]. The aging test of the Pr6O11 films was performed in air at 800 °C and 900 °C for 10 days and the thermodynamic stability and compatibility with GDC was checked by XRD. Film surface and crosssectional morphology of the films were analyzed by scanning electron microscopy (ZEISS Ultra 55 instrument) with a field emission gun (FEG). High-Resolution TEM (HRTEM) was also performed on a JEOL 2010 LaB6 (JEOL, Japan) transmission electron microscope to evaluate the particle size. HRTEM samples were prepared by scratching the surface of Pr6O11 using the diamond tip and placed on a copper (Cu) holey carbon grid. The particle size of the electrodes was estimated using an image analysis tool (ImageJ software). Moreover, the AFL microstructures have been reconstructed by FIB-SEM tomography in order to characterize the effect of the sintering temperature on the morphology of the Pr6O11 active layer. The 3D reconstructions were performed using a dual column focused ion beam (FIB) (Ga) field emission gun scanning electron microscope, FEG-SEM NVISION 40 from Carl ZEISS. Images with a pixel size of 15 nm were acquired using backscattered electrons detector (BSE) with the accelerating voltage of 1.5 kV. Further details on sample preparation and data acquisition for 3D reconstructions are already described in the literature [START_REF] Çelikbilek | Rational design of hierarchically nanostructured electrodes for solid oxide fuel cells[END_REF]. Post-processing and 3D reconstruction were carried out by ImageJ and Avizo softwares, and by procedures, as reported in the literature [START_REF] Villanova | UsseglioViretta, 3D phase mapping of solid oxide fuel cell YSZ/Ni cermet at the nanoscale by holographic X-ray nanotomography[END_REF]. The specific surface area was then estimated by following a method from the literature [START_REF] Moussaoui | Stochastic geometrical modeling of solid oxide cells electrodes validated on 3D reconstructions[END_REF].

Electrochemical properties of the double-layer electrodes were studied by electrochemical impedance spectroscopy (EIS). For this purpose, symmetrical cells were characterized at open circuit potential (OCP)

in the temperature range 450 to 700 °C using an autolab potentiostat-galvanostat (PGSTAT 302N) with a signal amplitude of 0.02 V (from 0.05 Hz to 1 MHz). Gold grid (Heraeus, 1024 meshes cm -2 woven from 0.06 mm dia. wire) was used as the current collector. Prior to EIS measurement, samples (except sample 7 only treated at 600 °C) were heated up to 700 °C for 5 h to reach thermal stability and then EIS measurements were carried out from 700 °C to 450 °C. A model based on equivalent circuits with the ZView ® software (Scribner Associates) has been used to fit the EIS diagrams. The polarization resistance (Rpol) value was calculated from electrode impedance on the Nyquist plot which is equal to the difference between high frequency (HF) and low frequency (LF) intercept on the real axis. In addition, series resistance (RS) was also extracted from the Nyquist plot which is equal to the high frequency (HF) intercept on the real axis. Both polarization resistance (Rpol) and series resistance (RS) were normalized with the electrode area. The ESD films are further investigated after sintering for 2 h in air from 600 to 1000 °C by TEM, HRTEM and selected area electron diffraction (SAED). The TEM images in Fig. 2 reveal the presence of numerous interconnected nanoparticles of average ~ 20, 35, 50-100, 100-150 and 200 nm after sintering at 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C, respectively. There are discrepancies in the crystal size calculated from XRD and TEM measurements, especially at 1000 °C (200 nm by TEM versus >100 nm by X-ray). Scherrer's equation (XRD) gives only an estimate of the average crystallite size in the ESD coating and it could be misleading since this formula is valid as long as the crystalline domain size is smaller than 100 nm in the limit of resolution. Each technique does not measure the same length since the morphology, in particular, is not taken into account in the X-ray method. Figures . 2b, e, h, k, andn show distinct lattice fringes for all films which are significant of the high crystallinity of the films, irrespective of sintering conditions. The SAED patterns, as given in Fig. 2c, f, i, l ando, show more pronounced diffraction rings for the samples sintered up to 700 °C than for the ones sintered at a higher temperature. This observation is coherent with the presence of finer grains at the low sintering temperature. These rings are attributed to the (111), ( 200), ( 220), ( 311) and ( 222) crystal planes of the fluorite cubic structure of polycrystalline Pr6O11 phase. This is consistent with XRD data.

Results and discussion

Structural properties of the Pr6O11 AFL

Microstructural properties

In the ESD process, a positive high voltage is applied to the stainless steel nozzle from which positively charged droplets are generated and directed to the ground substrate. The microstructure of the ESD deposited films depends on the size of the droplets (d) impacting the heated substrate. This latter can be estimated and controlled by varying the solution properties and the deposition parameters, as per Gañan-Calvo's relationship [START_REF] Gañan-Calvo | Current and droplet size in the electrospraying of liquids[END_REF].

6 / 1 3 0 ~        K Q d γ ρε (Equation 1)
where ρ is the solvent density, ε0 the vacuum permeability, Q the flow rate, γ the surface tension and K the electrical conductivity of the solution.

According to Equation 1, the physicochemical properties of the precursor solution such as ρ, γ, boiling point (b.p.) and K play a crucial role on evaporation of the solvents during the transport and spreading of the droplets when they impact the substrate. Therefore, in this process, the composition of the solvent is one of the key parameters that affects the droplet size and hence controls the microstructure [START_REF] Sharma | An innovative architectural design to enhance the electrochemical performance of La2NiO4+ δ cathodes for solid oxide fuel cell applications[END_REF][START_REF] Chen | Electrostatic sol-spray deposition of nanostructured ceramic thin films[END_REF][START_REF] Marinha | Comprehensive review of current developments in IT-SOFCs[END_REF].

Moreover, for a given solvent composition, droplet size can be further controlled by varying the technical key parameters such as flow rate, substrate temperature, nozzle-to-substrate distance and deposition time.

According to our previous paper [START_REF] Sharma | An efficient hierarchical nanostructured Pr6O11 electrode for solid oxide fuel cells[END_REF] describing different and innovative microstructures of Pr6O11 varying the ESD parameters, we have selected the ESD parameters in order to get hierarchical morphologies. Thus, all films were deposited using EtOH: BC (1:2) solution flow rate, substrate temperature, at a nozzle-to-substrate distance and deposition time of 1.5 mL.h -1 , 300 °C, 20 mm and 3 h, respectively. In these conditions, droplet size was estimated around 4.8 µm in average for ρ, γ, b. p., and K of 0.9 g cm -3 , 0.027 N m -1 , 140 °C, 17.25 µS cm -1 , respectively. The resulting microstructures are shown in Fig. 3. Hierarchical nanostructured films, ~10 µm thick, can be observed after a sintering at only 600 °C (Fig. 3a-d) and 700 °C (Fig. 3e-h), respectively. They are composed of large interconnected space between the columnar blocks at the macro-scale (~ 1-2 µm large) (Fig. 3a and3e). A fine porous structure within the blocks is observed at the nano-scale (Fig. 3d and3h) whereas the surface of the columns seems to be fully dense. The columnar microstructure is originating from the numerous stresses during the drying process of quite large droplets impacting the heated substrate [START_REF] Sharma | An efficient hierarchical nanostructured Pr6O11 electrode for solid oxide fuel cells[END_REF]. Indeed, when these large wet droplets spread onto the heated substrate, the too large volume change occurs during the too fast drying process at 300 °C. Consequently, stresses are developed which cause the film cracking. Particle size as low as 20-35 nm can be detected in the columns sintered at 600-700 °C (Fig. 3b,3f). The presence of a thin dense layer (TDL, ~ 200 nm thick) has been also observed (Fig. 3d,3h) and plays the main role in the adherence of the AFL on the GDC electrolyte. Indeed, at the initial stage of the ESD process, the charged liquid droplets spread on the polished flat surface of the ground substrate and lead to the formation of a continuous thin dense layer. On further increasing the sintering temperature to 800 °C, 900 °C and 1000 °C, average intra-columnar particle size is increased (from 50 to 200 nm) (Fig. 3j,3n,3r) while a drastic reduction of the thickness is detected in the respective cross-sections (~ 10 µm down to 5 µm, as shown in Fig. 3g,3k,3o,3s).

The double-layered architecture of the electrodes is based on an AFL of Pr6O11 deposited by ESD topped by a CCL deposited by SP made of different electronic conductors (Table 1). The main objective of depositing a CCL by SP on the top of the rough surface of the AFL is to improve the electrical contact between the ESD layer and the current collecting grid. The microstructure of these double-layer electrodes (samples 1-3) is shown in Fig. 4 for three different SP layers based on Pr6O11, L0.6S0.4C0.2F0.8O3δ (LSCF) and La0.7Sr0.3MnO3-δ (LSM).

As shown in Fig. 4, the SP layer with larger primary particle size than the ones estimated in the AFL (Pr6O11, d50 < 5μm; sample 1, LSCF, d50 = 0.27 μm; sample 2 and LSM, d50 = 0.2 μm; sample 3) is improving the thickness homogeneity of the Pr6O11 AFL. The thickness of the SP layer is ~30 µm for sample 1 to 3. In addition, the role of the thickness of the CCL has been observed to vary from 15 µm (sample 4), 45 µm (sample 5) to 60 µm (sample 6). Moreover, excellent adhesion between the AFL of Pr6O11 and SP layer has been observed for all CCL except for the Pr6O11 SP layer.

3D FIB-SEM reconstruction and image analyzes

Both microstructures of the active layer, sintered at 700 °C and 900 °C, have been further characterized by 3D FIB/SEM tomography. Fig. 5 shows the 3D rendering volumes for these two samples. Surprisingly, the visualization of 3D reconstructions for both samples reveals columns with a fine porosity in the bulk of the column, and a quasi-dense and continuous outer layer exhibiting only some disconnected pores. This 'core-shell' structure of the ESD deposit seems to be not affected by the sintering temperature. The average thickness of the shell is estimated by ImageJ software to 460 nm and 675 nm for sample sintered at 700 °C and 900 °C, respectively. Considering the SEM images, it therefore can be concluded that only the grain size (Fig. 3f, Fig. 3 n) is increased from 35 to 150 nm and the dimension of the columns (Fig. 3g and Fig. 3o) is changed with a typical height of 9 µm and 7 µm for the columns sintered at 700 °C and 900 °C, respectively. As a consequence, the free surface area of the ESD deposit is much larger when the sintering temperature is lower. The specific surface area is roughly estimated by following the procedure from literature [START_REF] Moussaoui | Stochastic geometrical modeling of solid oxide cells electrodes validated on 3D reconstructions[END_REF]. Since the inner porosities are completely closed, the computation of the specific surface area only considers the surface of the columns and ignores the inner porosities. From the calculation, it is found out that the specific surface area is 0.36 and 0.12 µm -1 for columns sintered at 700 °C and 900 °C, respectively, which means that the value is 3 times larger for the columns sintered at 700 °C.

Electrochemical properties

The electrochemical properties of these double-layer Pr6O11 electrodes were investigated versus the composition and thickness of the CCL and the sintering temperature of the AFL. EIS measurements were carried out on symmetrical cells from 450 to 700 °C in air at OCV. The temperature dependence of the polarization resistance, Rpol, is shown in Fig. 6. It is observed that the composition of the CCL has a strong impact of the electrode performances, especially for LSCF and LSM (samples 2 and 3, respectively) as shown in Fig. 6a. For example, the Rpol value is decreased drastically from 0.10 to 0.02 and 0.026 Ω cm 2 at 600 °C when the nature of the CCL is changing from Pr6O11 (sample1) to LSCF (sample 2) and LSM (sample 3), respectively. This significant difference in the performance of the double-layer electrode is certainly due to a much lower electronic conductivity of Pr6O11 (1.3 S cm -1 at [START_REF] Moriche | Chemical and electrical properties of LSM cathodes prepared by Mechanosynthesis[END_REF]. Consequently, one can probably assume that a larger homogenization of the current distribution is occurring along the cathode functional layer when LSCF and LSM CCLs are used than for Pr6O11 based CCL.

In both cases, the whole volume of the cathode is concerned avoiding any current constrictions. In fact, LSM being a pure electronic conducting material, it can contribute to the electrochemical reaction (ORR) through a limited number of triple phase boundary points (TPB). On the contrary to LSM, LSCF is a mixed ionicelectronic conductor and leads to an extension of the ORR through TPB surfaces. However, one can also notice that performance of the electrodes with screen-printed LSCF (sample 2) and LSM (sample 3) is similar which also rules out the active participation of the screen-printed CCL in the electrochemical reaction. Indeed, not all the cathode thickness is electrochemically active. The electrochemical active length has been reported to be typically within a few hundred nanometers from the electrode/electrolyte interface, especially for LSCF AFL deposited by ESD [START_REF] Çelikbilek | State-of-the-Art La0.6Sr0.4Co0.2Fe0.8O3-δ Cathode for SOFC: Microstructural and Electrochemical Properties[END_REF]. Therefore, the Pr6O11 columnar structure of few micrometers thick should be the only part of the electrode participating to the reactive mechanism and must roughly correspond to the active functional layer. Moreover, as reported in the literature [START_REF] Marinha | Performance of (La, Sr)(Co, Fe) O3-x double-layer cathode films for intermediate temperature solid oxide fuel cell[END_REF][START_REF] Bae | Properties of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) double-layer cathodes on gadolinium-doped cerium oxide (GDC) electrolytes: II. Role of oxygen exchange and diffusion[END_REF][START_REF] Hsu | Enhancement of solid oxide fuel cell performance by La0.6Sr0.4Co0.2Fe0.8O3δ double-layer cathode[END_REF][START_REF] Haanappel | Optimisation of processing and microstructural parameters of LSM cathodes to improve the electrochemical performance of anode-supported SOFCs[END_REF][START_REF] Kenney | Engineering of microstructure and design of a planar porous composite SOFC cathode: a numerical analysis[END_REF], double-layer electrodes which are much thicker than the electrochemical active thickness are more efficient than the single layer ones. This is probably due to a more homogeneous distribution of the current lines. In the following, the effect of the LSM CCL thickness on electrode performance has been investigated. As one can see in Fig. 6b, the thickness of the CCL layer ranging from 15 to 60 µm has no significant effect on the Rpol values which are found in the range 0.026 -0.032 Ω cm 2 at 600 °C. A minimum CCL thickness of 15 µm based on LSM is good enough for having an efficient double-layer electrode where the electrochemically active layer is Pr6O11 deposited by ESD. This minimum CCL thickness indicates that the in-plane conductivity is already optimized thanks to a typical nanostructured AFL deposited by ESD characterized by large surface area and a good contact at the electrode/electrolyte interface. Evaluations conducted by other researchers have reached similar conclusions. A minimum cathode thickness is required for the full utilization of the total surface area of the electrode/electrolyte interface, as reported by Haanappel et al. [START_REF] Haanappel | Optimisation of processing and microstructural parameters of LSM cathodes to improve the electrochemical performance of anode-supported SOFCs[END_REF].

The effect of the microstructure of the AFL on the electrode efficiency has been shown in Fig. 7a for double-layer architecture based on Pr6O11 AFL deposited by ESD and sintered at different temperatures (600°C -1000 °C) and then topped by LSM CCL, 30 µm thick. The cathode on which the AFL was sintered only at 600 °C shows the lowest Rpol value (0.02 Ω cm 2 at 600 °C, Table 2). A clear increase of Rpol values from 0.02 to 0.14 Ω cm 2 is observed in Fig. 7a (Table 2) when the sintering temperature of the AFL is passing from 600°C to 1000°C. This evolution is clearly correlated to the dimension of the columns which decreases with increasing of the sintering temperature (Fig. 3 and Fig. 5). The smaller dimension of columns sintered at higher temperature leads to smaller free surface area, as evident from the calculation that is presented in section 3.3. This statement suggests that the free surface area of the Pr6O11 columns is involved on the reactive mechanism. Therefore, because of the 'core-shell' microstructure of the ESD layer, it can be reasonably proposed that the reactions cannot take place in the inner porosity of the columns but mainly occur on the free surface of the material. This surface appears to be the key microstructural property that must be maximized to improve the electrode performances.

The linear behavior up to 600 °C in the Arrhenius plot indicates that a single ORR mechanism is active for all films, irrespective of the sintering temperature. Moreover, all the cathodes show a similar activation energy (Table 2) regardless of the sintering temperature which also indicates that the ORR mechanism is similar in the entire electrodes. The lowest Rpol value of 0.02 Ω cm 2 at 600 °C reported in this work (sample 7) and with comparison to the literature can be due to an optimized hierarchical microstructure (in terms of particle size, porosity and percolation) of the Pr6O11 AFL deposited by ESD, 6 µm thick, and sintered at only 600 °C. In addition, the enhancement of the ORR is suggested to be related to a good current collection performed by LSM, 30 µm thick, which further activates the electrode surface by avoiding the current constriction [START_REF] Sharma | An innovative architectural design to enhance the electrochemical performance of La2NiO4+ δ cathodes for solid oxide fuel cell applications[END_REF][START_REF] Sharma | La4Ni3O10δ as an efficient solid oxide fuel cell cathode: electrochemical properties versus microstructure[END_REF]. Lastly, impedance spectra (Fig. S1a-b) are composed of several processes which contribute to the total polarization resistance. The diagrams have been fitted by using a typical equivalent circuit model consisting of an inductance L, a series resistance Rs and two or three R/CPE (constant phase element) circuit elements. According to the analysis, two or three main contributions have been observed located at high frequency (HF, referred as P1), medium frequency (MF, referred as P2) and low frequency (LF, referred as P3) depending upon the microstructure of the cathodes and temperature. R1, R2, and R3 are the polarization resistances corresponding to contributions P1, P2 and P3, respectively. In detail, resistance, capacitance and frequency corresponding to each process are shown in Fig. 7b-d andS2, respectively, as a function of sintering and measuring temperature. The capacitances corresponding to process P2 are in the range 10 -3 -10 -1 F cm -2 (Fig. S2b). Thus the medium frequency arc (P2) could be associated with the global reaction of charge transfer taking place at the Pr6O11/gas interface. This process can be co-limited by various phenomena such as adsorption of gaseous oxygen O2, dissociation of O2 and oxygen incorporation into the electrode material [START_REF] Adler | Limitations of charge-transfer models for mixed-conducting oxygen electrodes[END_REF][START_REF] Escudero | A kinetic study of oxygen reduction reaction on La2NiO4 cathodes by means of impedance spectroscopy[END_REF]. The capacitances associated to process P1 (high frequency) are in the range 10 -2 -10 -1 F cm -2 and nearly temperature independent (Fig. S2a). The high frequency arc (P1) could be attributed to the ionic transfer across the GDC/Pr6O11 interface. As expected, both R1 and R2 decrease with increasing the operating temperature (Fig. 7b-c). It seems that the reaction at the Pr6O11/gas interface is the rate-limiting step for the ORR since R2 values are always larger than those of R1 [START_REF] Lou | Synthesis and characterization of aluminumdoped perovskites as cathode materials for intermediate temperature solid oxide fuel cells[END_REF][START_REF] Huang | Ba0.9Co0.7Fe0.2Mo0.1O3-δ: A Promising Single-Phase Cathode for Low-Temperature Solid Oxide Fuel Cells[END_REF][START_REF] Gu | Electrochemical properties of LaBaCo2O5+ δ-Sm0. 2Ce0. 8O1. 9 composite cathodes for intermediate-temperature solid oxide fuel cells[END_REF][START_REF] Mogni | Oxygen reduction reaction on Ruddlesden-Popper phases studied by impedance spectroscopy[END_REF]. Moreover, one can also notice that R2 values increase with increasing sintering temperature (Fig. 7c). This statement is in good agreement with the evolution of the gas/solid surface area of the columns with the sintering temperature (i.e. the surface area decreases with increasing the sintering temperature). This remark reinforces the proposition that the process P2 at intermediate frequencies could be ascribed to the oxygen exchange occurring at the surface of the columns. Besides, no significant variation in R1 values (Fig. 7b) has been observed with the sintering temperature. This result is consistent with the SEM observations and XRD characterizations. Indeed, whatever the sintering temperature, no delamination, and no secondary phase have been highlighted at the electrode/electrolyte interface. This analysis is also in good agreement with the apex frequency corresponding to processes P1 and P2. As once see in Fig S2d that apex frequency corresponding to process P1 does not depends significantly on sintering temperature. On the other hand, a significant variation in the apex frequency of process P2 has been observed (Fig. S2e). Apex frequency decreases with increasing sintering temperature due to lower surface area. The low frequency contribution (P3), appeared at T ≥ 550 °C, can be related to a gas impedance due to either porous electrodes or the experimental set-up [START_REF] Jacobsen | Diffusion and conversion impedance in solid oxide fuel cells[END_REF][START_REF] Bessler | Gas concentration impedance of solid oxide fuel cell anodes I. Stagnation point flow geometry[END_REF][START_REF] Pang | Characterization of Ba-deficient PrBa1-xCo2O5+ δ as cathode material for intermediate temperature solid oxide fuel cells[END_REF]. The AFL based on Pr6O11 deposited by ESD has been subjected to heat treatment in air at 800 °C and 900 °C for 10 days and then characterized by X-ray diffraction (Fig. 8) in order to detect the limit of the longterm chemical stability and compatibility with GDC electrolyte. For this, columnar-type Pr6O11 film has been prepared on GDC electrolyte by ESD and sintered at 700 °C in air for 2 h prior to these long-term heat treatments. At 800 °C, no decomposition or reactivity of Pr6O11 with the GDC electrolyte has been observed within the detection limit of XRD (Fig. 8a). However, a heat treatment at 900 °C for 10 days leads to the decomposition of Pr6O11 into Pr2O3 along with some unidentified secondary phase(s) (Fig. 8b). Finally, Pr6O11 AFL presents a quite good stability and compatibility with GDC for its potential use as IT-SOFC electrode since all the processing temperature is lower than or equal to 700 °C.

Conclusions

In this work, we have successfully studied the effect of the sintering temperature on the microstructure of the hierarchical nanostructured porous Pr6O11 layer deposited on GDC electrolyte by ESD technique for its promising use as an active functional in double-layer architecture cathode for IT-SOFC application. Irrespective of the sintering temperature in this work, all AFLs are single phase and crystallize in an Fm 3m fluorite cubic structure with remarkable differences at the micro and nano-scales.

Grain sizes increase from 20 to 200 nm when the sintering temperature is increased from 600 to 1000 °C for 2 h in air. Interestingly, by 3D FIB-SEM tomography, 'core-shell' microstructures in which dense outer layer surrounds inner porosities in the columns are observed for both columns sintered at 700 o C and 900 o C. The column thickness and the specific surface area are decreased from 9 to 7 µm and from 0.36 to 0.12 µm -1 , respectively when the sintering temperature is increased from 700 to 900 o C. However, no reactivity between electrodes and GDC electrolyte is observed as a consequence of sintering since all the processing temperature is lower than or equal to 700 °C. Electrochemical measurements on double-layer electrodes (AFL by ESD + screen-printed LSM CCL) show an increase in Rpol with increasing AFL sintering temperature. From microstructural characterizations, it can be concluded that the key parameter is the specific surface area of the surface of the column. The composition of the current collector does not affect significantly the electrochemical performance, provided it a good electronic conductor. Similar Rpol values have been obtained in this work for LSCF (0.022 Ω cm 2 at 600 °C) and LSM (0.026 Ω cm 2 at 600 °C) CCLs whereas the larger Rpol value was obtained for Pr6O11 (0.10 Ω cm 2 at 600 °C) CCL.

Moreover, the thickness of a good electronic current collector has no significant role on electrochemical performance. A polarization resistance value of ~ 0.03 Ω cm 2 at 600 °C has been obtained irrespective of current collector thickness. To the best of our knowledge, cathode consisted of Pr6O11 as AFL, 6 µm thick, sintered at 600 °C topped by 30 µm SP LSM exhibits the lowest polarization resistance of 0.02 Ω cm 2 at 600 °C thanks to an optimized hierarchical microstructure. Columnar AFL Pr6O11 shows good long-term thermodynamic stability and compatibility with electrolyte GDC at a temperature inferior or equal to 800 °C.

Fig. 1

 1 Fig.1shows the X-ray diffraction patterns of Pr6O11 ESD films deposited on a GDC electrolyte and sintered in air for 2 h at 600 °C, 700 °C, 800 °C and 1000 °C, respectively. All diffraction peaks are indexed in the

Fig. 1 X

 1 Fig.1X-ray diffraction patterns of Pr6O11 ESD films deposited on a GDC electrolyte and sintered for 2 h in air at 600 °C, 700 °C, 800 °C, and 1000 °C, respectively.

Fig. 2

 2 Fig. 2 TEM, HRTEM and SAED patterns of powder scratched from columnar Pr6O11 ESD films (AFL) sintered in air for 2 h at different temperatures, respectively at : (a-c) 600°C, (d-f) 700 °C, (g-i) 800 °C, (jl) 900 °C and (m-o) 1000 °C.

Fig. 3

 3 Fig.3SEM micrographs of Pr6O11 ESD films deposited on GDC with an EtOH: BC (1:2) solution, 0.02 M deposited at 300 °C for a nozzle to substrate distance of 20 mm, a flow rate of 1.5 mL.h -1 and deposition time of 3 h and sintered for 2 h in air at different temperatures: 600 °C: (a, b) surface, (c) cross section, (d) interface; 700 °C: (e, f) surface, (g) cross section, (h) interface; 800 °C: (i, j) surface, (k) cross section, (l) interface; 900 °C: (m, n) surface, (o) cross section, (p) interface; 1000 °C: (q, r) surface, (s) cross section, (t) interface.

Fig. 4

 4 Fig. 4 SEM micrographs of double-layer electrodes based on an AFL of Pr6O11 deposited by ESD and topped by a SP CCL made of different electronic conductors such as: sample 1(Pr6O11) (a, b) surface, (c) cross section; sample 2 (LSCF): (d, e) surface, (f) cross section; sample 3 (LSM): (g, h) surface, (i) cross section. ESD films obtained with H2O: BC (1:2) solution, 0.02 M, deposited at 300 °C, 20 mm for a flow rate of 1.5 mL h -1 and for 3 h after calcination at 700 °C for 2 h in air.

Fig. 5

 5 Fig. 5 3D reconstructed images of the active functional layer of Pr6O11 sintered for 2 h in air at (a) 700 °C and (b) 900 °C. The dashed line represents the shell of one column.

Fig. 6

 6 Fig. 6 Arrhenius plot of Rpol in air at OCV for the double-layer electrodes on GDC: (a) effect of the CCL composition, (b) effect of the CCL thickness based on LSM.

Fig. 7

 7 Fig. 7 Arrhenius plot of the double-layer electrodes (in air at OCV conditions) on GDC electrolyte for different microstructures of the AFL based on Pr6O11 deposited by ESD, sintered at different temperatures (600°C -1000 °C) and then topped by LSM CCL, 30 µm thick; (a) Rpol, (b) R1, (c) R2 and (d) R3.

Table 1

 1 Sample identification, composition, and thickness of the current collecting layer (CCL), and sintering conditions for Pr6O11 ESD layer and ESD particle size (from TEM images); final sintering of the double-layer is at 600 °C for 2 h in air.

	Sample #	Composition/ Technique	Composition/ thickness of CCL	Sintering in air (after ESD)	ESD particle size (nm)
	sample 1	Pr6O11 ESD + Pr6O11 SP	Pr6O11/ 30 µm	700 °C / 2 h	35
	sample 2	Pr6O11 ESD + LSCF SP	LSCF/ 30 µm	700 °C / 2 h	35
	sample 3	Pr6O11 ESD + LSM SP	LSM/ 30 µm	700 °C / 2 h	35
	sample 4	Pr6O11 ESD + LSM SP	LSM/ 15µm	700 °C / 2 h	35
	sample 5	Pr6O11 ESD + LSM SP	LSM/ 45µm	700 °C / 2 h	35
	sample 6	Pr6O11 ESD + LSM SP	LSM/ 60µm	700 °C / 2 h	35
	sample 7	Pr6O11 ESD + LSM SP	LSM/ 30 µm	600 °C / 2 h	< 20
	sample 8	Pr6O11 ESD + LSM SP	LSM/ 30 µm	800 °C / 2 h	50-100
	sample 9	Pr6O11 ESD + LSM SP	LSM/ 30 µm	900 °C / 2 h	100-150
	sample 10	Pr6O11 ESD + LSM SP	LSM/ 30 µm	1000 °C / 2 h	200

Table 2

 2 Series resistance (RS), polarization resistance (Rpol) in (Ω cm 2 ) at 600 °C and activation energy (Ea) in (eV) of the electrodes.

		Double-layer (ESD (Pr6O11) + SP as CCL) samples
		CCL nature /	RS	Rpol	Ea	CCL nature	RS	Rpol	Ea
		thickness				/thickness
	sample 1 Pr6O11/30 µm 18.86 0.101 1.28 sample 6	LSM/60 µm	5.73 0.032 1.39
	sample 2 LSCF/30 µm	6.28 0.022 1.28 sample 7	LSM/30 µm	7.59 0.020 1.10
	sample 3	LSM/30 µm	8.68 0.026 1.39 sample 8	LSM/30 µm	7.97 0.035 1.33
	sample 4	LSM/15 µm	7.58 0.030 1.38 sample 9	LSM/30 µm	7.48 0.077 1.38
	sample 5	LSM/45 µm	7.45 0.032 1.38 Sample 10 LSM/30 µm	7.81 0.140 1.43
	3.5

. Chemical stability and compatibility with the GDC electrolyte Fig

. 8 XRD patterns of the Pr6O11 film prepared by ESD on GDC electrolyte after heat treatment for 10 days in air at (a) 800 °C and (b) 900 °C.

* * * * * 800 °C O O O O O O O O O O O O O O O O O O (111) * * * * * * * * * * * * * * * O O O

Acknowledgments

This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" n° AN-10-LABX-44-01funded by the "Investments for the Future" Program. The authors would like to thank S. Coindeau and T. Encinas for XRD and R. Martin for SEM and EDX analyses in CMTC (Grenoble INP, France).