Keywords: Layered double hydroxides, nickel hydroxide, hydrothermal process, electrochemical deposition, supercapacitor, energy storage devices

cm -2 at a power density of 1.6 mW cm -2 .

In conclusion, all the results obtained in this thesis imply the promising potential application of Ni-based hydroxide composites as energy storage devices and provide valuable highlights to the exploration of new composite materials for supercapacitor electrodes in future works (Chapter 4).

ABSTRACT

Supercapacitors, as energy storage devices, have drawn great attention in our daily life to bridge the gap between batteries and capacitors. Therefore, the preparation of high-performance material electrodes for supercapacitors plays a vital role in the future technological developments. In this context, layered double hydroxides (LDHs) and Ni(OH)2 have been recognized as promising electrodes for supercapacitors, owing to their fast redox reaction and battery-type behavior.

The Chapter 1 of my PhD work gives a brief historic overview, principles and mechanism of energy storage, electrode materials of supercapacitors and the corresponding characterization methods.

In Chapter 2, after a brief introduction on LDHs and their investigation as electrode materials in supercapacitors, we summarize our results obtained on Ni-based LDHs as electrodes for supercapacitors. Firstly, NiFe LDHs on Ni foam (NF) coated with reduced graphene oxide (NiFe LDHs/rGO/NF) was prepared by electrochemical deposition method. NiFe LDHs/rGO/NF achieved enhanced specific capacity (585 C g -1 at a current density of 5 A g -1 ). Additionally, a flexible asymmetric supercapacitor was assembled using NiFe LDHs/rGO/NF as the cathode and mesoporous carbon (MC) coated on NF as the anode. The supercapacitor exhibited an energy density of 17.71 Wh kg -1 at a power density of 348.49 W kg -1 . Secondly, NiAl LDHs coated on carbon spheres (CS) supported by Ni foam (NiAl LDHs@CS/NF) electrodes were synthesized by a facile hydrothermal method. The performance of the prepared materials as binder-free electrodes in supercapacitors was assessed. The NiAl LDHs@CS/NF electrode achieved the largest areal capacity (1042 mC cm -2 ), as compared to the areal capacity values attained by NiFe LDHs@CS/NF (705.8 mC cm -2 ) and NiCr LDHs@CS/NF (814.9 mC cm -2 ) at 1 mA cm -2 . Therefore, a hybrid supercapacitor device comprising NiAl LDHs@CS/NF as the positive electrode and N-doped reduced graphene/NF as the negative electrode was assembled, which attained an energy density of 43 μWh cm -2 at a power density of 0.805 mW cm -2 . The hybrid supercapacitor was successfully applied to operate a windmill device continuously for 32 s. Finally, NiMnCr LDHscarbon spheres modified Ni foam (NiMnCr LDHs@CS/NF) nanocomposite was prepared using a two-step hydrothermal process and exhibited a high specific capacity of 569 C g -1 at 3 A g -1 with good reversibility and stability. Furthermore, a hybrid supercapacitor was fabricated using NiMnCr LDHs@CS/NF as the positive electrode and FeOOH coated on NF (FeOOH/NF) as the negative electrode. The energy storage device reached an energy density of 48 Wh kg -1 at a power density of 402.7 W kg -1 .

In Chapter 3, Ni(OH)2@CuO@Cu foam binder-free electrodes were fabricated by a two-step process at room temperature with various deposition times (30, 50, 90, 150 and 200s). Among all the samples, Ni(OH)2@CuO@Cu-150 exhibited the largest areal capacity of 7063 mC cm -2 at 20 mA cm -2 , and was therefore chosen as the positive electrode in a hybrid supercapacitor. Using N-doped reduced graphene oxide on nickel foam (N-rGO/NF) as the negative electrode, a hybrid supercapacitor was assembled. It L'électrode NiFe HDL/rGO/NF présente une capacité spécifique de 585 C g -1 à une densité de courant de 5 A g -1 . De plus, un supercondensateur asymétrique flexible a été assemblé en utilisant NiFe HDL/rGO/NF comme cathode et du carbone mésoporeux (MC) déposé sur NF comme anode. Le supercondensateur présente une densité d'énergie de 17.71 Wh kg -1 et une densité de puissance de 348.49 W kg -1 . Dans la deuxième partie de ce chapitre, nous décrivons la synthèse hydrothermale d'HDLs à base de NiAl enrobées sur des sphères de carbone (CS) supportées par des électrodes en mousse de Ni (NiAl DHL@CS/NF). La performance des matériaux préparés en tant qu'électrodes sans liant dans des supercondensateurs a été évaluée. Le matériau d'électrode NiAl DHL@CS/NF présente une capacité surfacique de 1042 mC cm -2 à 1 mA cm -2 , beaucoup plus élevée comparée aux valeurs de capacités surfaciques de NiFe HDL@CS/NF (705.8 mC cm -2 ) et de NiCr LDHs@CS/NF (814.9 mC cm -2 ) à 1 mA cm - 2 . Par conséquent, un supercondensateur hybride comprenant NiAl HDL@CS/NF comme électrode positive et du graphène réduit dopé à l'azote (N-rGO)/NF comme électrode négative a été assemblé. Le dispositif présente une densité d'énergie de 43 μWh cm -2 à une densité de puissance de 0.805 mW cm -2 , et a été appliqué pour faire fonctionner un appareil éolien en continu pendant 32 s. Enfin, un matériau composite à base de NiMnCr sur un substrat de mousse de nickel recouvert de sphères de carbone (NiMnCr HDL@CS/NF) a été préparée en utilisant un processus hydrothermal en deux étapes. Le nanocomposite ainsi obtenu a été investigué comme électrode dans un supercondensateur et présente une capacité spécifique de 569 C g -1 à 3 A g -1 avec une bonne stabilité. De plus, un supercondensateur hybride a été fabriqué en utilisant NiMnCr HDL@CS/NF comme électrode positive et FeOOH déposé sur NF (FeOOH/NF) comme électrode négative. Le dispositif présente une densité d'énergie de 48 Wh kg -1 à une densité de puissance de 402.7 W kg -1 .

Dans le chapitre 3, des électrodes sans liant Ni(OH)2@CuO sur une mousse de cuivre ont été synthétisées par un processus en deux étapes à température ambiante. Nous avons étudié l'effet du temps de dépôt (30, 50, 90, 150 et 200 s) sur le comportement électrochimique des électrodes obtenues. Parmi tous les échantillons, Ni(OH)2@CuO@Cu-150 présente la plus grande capacité surfacique de 7063 mC cm - 2 à 20 mA cm -2 , et a donc été choisi comme électrode positive dans un supercondensateur hybride. En utilisant de l'oxyde de graphène réduit dopé à l'azote sur de la mousse de nickel (N-rGO/NF) comme électrode négative, un supercondensateur hybride a été assemblé. Il présente une bonne flexibilité, une stabilité cyclique et une densité d'énergie surfacique élevée de 130.4 μWh cm -2 à une densité de puissance de 1.6 mW cm -2 .

Une conclusion générale rappelle les principaux résultats obtenus dans ce travail de thèse sur l'application de composites d'hydroxydes doubles lamellaires à base de Ni en tant qu'électrodes dans des dispositifs de stockage d'énergie, et présente quelques perspectives envisageables à la lumière de ces travaux (chapitre 4). My heartfelt thanks also go to all my colleagues for all the help. I want to express my thanks to Alexandre Barras and Emerson Giovanelli for helping me out when I have trouble in the lab. To Yannick Coffinier, Mathias Dolci and Roxana Jijie, thank you for the help with SEM imaging. Also, many thanks to Abir Swaidan, Chengnan Li, Elizaveta Sviridova, Liuqing Pang, Yuan

Mots-clés:

GENERAL INTRODUCTION

Supercapacitors (SCs), as energy storage devices, have drawn great attention in our daily life to bridge the gap between batteries and capacitors owing to their advantages like enhanced power density, being almost maintenance-free, experiencing no memory effect, safe, short charge/discharge time, and low cost.

Typically, SCs can generally be classified into three different types including electric double-layer capacitors (EDLCs), pseudocapacitors, and asymmetric supercapacitors (ASCs). The charge of an EDLC is physically stored by reversible electrostatic charge adsorption at the electrode-electrolyte interface, which is the simplest and most commercially available supercapacitor. Different from the EDLC, the key to store energy in pseudocapacitors is rapid reversible faradaic redox reactions at the surface or near surface of the electrode materials. In addition to EDLCs and pseudocapacitors, there is another configuration, called asymmetric supercapacitors (ASs). According to the type of the electrodes, hybrid supercapacitors and capacitive asymmetric supercapacitors are applied to distinguish the difference. It is generally accepted that a "hybrid supercapacitor" is composed by two electrodes with two different charge-storage mechanisms: one capacitive and one battery-type (Faradaic), whereas a capacitive asymmetric supercapacitor contains two capacitive electrodes of different materials.

In the last few years, to obtain high-performance supercapacitors, numerous electrode materials have been explored. Typically, carbon-based materials are the most widely used materials for EDLC electrodes and transition metal hydroxides/oxides like manganese dioxide (MnO2), nickel oxide (NiO), titanium dioxide (TiO2), layered double hydroxides (LDHs) and conducting polymers such as polypyrrole, polythiophene, etc are promising electrode materials for pseudocapacitors. Among all the materials, Ni-based hydroxides have drawn great attention and interest in the field of supercapacitors. Ni(OH)2 with large theoretical specific capacitance (2082 F g -1 ) has been extensively investigated for supercapacitors. Besides, Ni-based layered double hydroxides (LDHs), due to their well-defined and high redox activity, wide tunability of the types of metal cations and environmentally friendly nature, were also greatly explored for supercapacitor applications.

In the first part of my work, I have investigated the influence of different metal ions (Fe, Al, Mn, Cr) on the performance of Ni-based LDH electrode materials. In this context, NiFe, NiAl, NiMnCr LDH materials were coated on reduced graphene oxide (rGO)-or carbon spheres (CS)-modified nickel foam (NF) to produce NiFe LDHs/rGO/NF, NiAl LDHs@CS/NF and NiMnCr LDHs@CS/NF composites. The prepared electrode materials displayed improved specific/areal capacity than that of the same composites without carbon-based (reduced graphene oxide or carbon spheres) materials. NiFe LDHs/rGO/NF achieved a specific capacity of 585 C g -1 at 5 A g -1 , which is higher than that of NiFe LDHs/NF (530 C g -1 at 5 A g -1 ). NiAl LDHs@CS/NF electrode material attained an areal capacity of 1042.2 mC cm -2 at 1 mA cm -2 , which greater than that of NiAl LDHs/NF (851.9 mC cm -2 at 1 mA cm -2 ). NiMnCr LDHs@CS/NF composite exhibited a much larger specific capacity of 569 C g -1 at 3 A g -1 , as compared to 173.2 C g -1 at 3 A g -1 recorded for NiMnCr LDHs/NF.

In the second part of my thesis work, Ni(OH)2@CuO@Cu foam binder-free electrodes were fabricated by a two-step process at room temperature with different deposition times. Ni(OH)2@CuO@Cu-150 exhibited large areal capacity of 7063 mC cm -2 at 20 mA cm -2 , and was therefore chosen as the positive electrode in a hybrid supercapacitor. Using N-doped reduced graphene oxide on nickel foam (N-rGO/NF) as the negative electrode, a hybrid supercapacitor was assembled. It displayed good flexibility, cycling stability and high areal energy density of 130.4 μWh.cm -2 at a power density of 1.6 mW.cm -2 . Furthermore, a hybrid supercapacitor device was assembled based on as-prepared NiFe LDHs/rGO/NF as the positive electrode and mesoporous carbon (MC)/NF as the negative electrode, exhibiting an energy density of 17.71 Wh kg -1 and a power density of 348.49 W kg -1 . Besides, a hybrid supercapacitor device comprising NiAl LDHs@CS/NF as the positive electrode and N-doped reduced graphene/NF as the negative electrode attained an energy density of 43 μWh cm -2 at a power density of 0.805 mW cm -2 . Finally, a hybrid supercapacitor was assembled using NiMnCr LDHs@CS/NF as the positive electrode and FeOOH coated on NF (FeOOH/NF) as the negative electrode, reaching an energy density of 48 Wh kg -1 at a power density of 402.7 W kg -1 .

CHAPTER 1 Introduction

Faced with a series of problems such as the ever-increasing exhaustion of fossil fuels, environmental pollution and global warming, people have realized the importance to develop green, pollution-free and sustainable energy supplies throughout the world [1][2][3][4]. Natural energies including wind energy, solar energy, water energy and so on are being explored. As a consequence, it is urgent to develop effective energy storage and conversion devices to make the best use of these energies [5][6][7][8]. Therefore, various energy-storage devices such as batteries, supercapacitors (SCs), fuel cells are being extensively investigated. As a bridge for energy-power difference between a traditional capacitor (having high power) and fuel cells/batteries (having high energy storage) illustrated in Figure 1.1, SCs have drawn wide attention, owing to their advantages like enhanced power density, being almost maintenance-free, experiencing no memory effect, safe, short charge/discharge time, and low cost [10][11][12][13].

On the one hand, SCs can be applied to offer high power density for electric and hybrid vehicles required for short-term acceleration along with recuperation of energy during braking (Figure 1.2). On the other hand, SCs can also be utilized as power supplies for portable devices like mobile phones, notebook computers, digital cameras etc. -being small, lightweight and flexible (Figure 1.2). Figure 1.2: Some application areas of asymmetric supercapacitor devices [14].

Compared with SCs, batteries with higher energy density have been widely utilized and commercialized; however, batteries tend to generate heat and dendrites when running at high power, which can lead to serious safety issues [15]. In contrast, supercapacitors can safely provide high power and fast charging, and have good cycle stability, which can replace batteries in some applications [16,17]. Recently, Tesla announced that a combination of batteries and SCs will be applied to provide power to its new electric car, offering a promising potential application of SCs in the future.

Brief Historic Overview

Capacitor technology was pioneered thanks to the invention of the Leyden Jar (1745-1746), which was made of glass containers with metal foil [18]. The metal foil served as the electrode, and the jar was used as the dielectric. During the charging process of the above device, the positive (+ ve) charge accumulated on one electrode and the negative (-ve) charge accumulated on the other electrode. When metal wires are used to connect these two charges, a discharge process occurs. In 1957, a patent for the first supercapacitor (Electric Double Layer Capacitor-EDLC) was granted to Becker, which used activated carbon as the electrode plate [19].

In EDLC, the charge storage occurs statically (non-Faradaic), that is, no charge transfer occurs between the electrode and the electrolyte (this makes it highly reversible and highly stable). After Becker, Sohio, based in Cleveland, Ohio, also utilized high-area carbon material for double-layer capacitors in a nonaqueous solvent electrolyte containing a dissolved tetraalkylammonium salt, achieving a higher operating voltage Generally, ASCs are classified into two types, namely, capacitive asymmetric supercapacitors with two capacitive electrodes and hybrid supercapacitors (HSCs) in which one electrode stores charge by a battery-type Faradaic process, while the other electrode stores charge based on a capacitive mechanism [26]. To better understand their energy storage mechanism, they are discussed separately.

Principles and Charge Storage Mechanisms

Electric Double Layer Capacitors (EDLCs)

This capacitance mechanism is similar to a typical mechanism of dielectric capacitors; however, a supercapacitor can store more energy on the basis of EDL principle, owing to the large interfacial area of the electrodes. The charge of an EDLC is physically stored by reversible electrostatic charge adsorption at the electrodeelectrolyte interface, as depicted in Figure 1.6, which is the simplest and most commercially available supercapacitor [24,[27][28][29].

Figure 1.6:

The scheme of an electrical double layer capacitor (EDLC) [30].

In EDLC, the capacitance arises due to true capacitance effect that the charge storage takes place directly across the double layer of the electrode material without any charge transfer across the interface. According to the physical electrostatic process, the formation and relaxation of the electric double layer occur in a very short time range, on the order of 10 -8 s, which is less than that of the redox reaction of the pseudocapacitor (10 -2 ~10 -4 s). The electric double layer can immediately respond to potential changes, because there is no Faraday reaction during the charging/discharging process of EDLC.

The capacitance of an EDLC electrode can be generally estimated according to Equation 1.1:

C= 𝜀 𝑟 𝜀 0 ×A d (Eq. 1.1)
Where εr is the relative permittivity related to the liquid electrolyte used, ε0 is the permittivity of vacuum, A is the effective surface area of the electrode material which is accessible to the electrolyte ions, and d is the effective charge separation distance between the electrical double layer, i.e., the Debye length.

At the same time, according to Equation 1.1, the high effective surface area plays a vital role for the capacitance of an EDLC. Therefore, carbon-based materials with high specific surface areas were widespread investigated for EDLCs electrodes, such as activated carbons [31,32], carbon aerogels [33][34][35], carbon nanotubes [27,36,37] and graphene [38][39][40]. Theoretically, a carbon-based electrode with high specific surface area (1000~3000 m 2 g -1 ) can achieve an areal capacitance of 10~21 μF cm -2 [41][42][43], corresponding to a specific capacitances of 300~550 F g -1 [44]. However, the specific surface area of the electrode materials cannot be fully available experimentally, that is why the specific capacitance of pure carbon-based EDLCs practically achieved has generally been limited to about 100~250 F g -1 [27,45,46] and the energy density lies in the range of 3~10 Wh kg -1 [47].

Pseudocapacitors

Different from the EDLC, the key to store energy in pseudocapacitors is rapid reversible faradaic redox reactions in the surface or near surface of the electrode materials [48][49][50]. This process involves a valence state change, which can deliver (a) Underpotential deposition occurs when the metal ion forms an adsorbed monolayer on the surface of another metal substrate that is much higher than its redox potential.

However, pseudocapacitive materials normally work in aqueous electrolytes, limiting their widespread application.

(b) Redox pseudocapacitance is based on redox reactions generated at the surface or near-surface of electrode materials with ions from the electrolyte. Usually, in the redox process, cations in the electrolyte are electrochemically adsorbed on the surface oxidized species along with fast and reversible electron transfer across the interface between the electrode and the electrolyte, which can be described by Equation 1.3:

Ox + zB + + ze -↔ RedB z (Eq. 1.3)
where B + is the surface-absorbed electrolyte cation (H + , K + , Na + , ...) and z is the number of transferred electrons. The reactions depend upon the interface between the electrode and electrolyte. Therefore, the porosity and surface area of the electrode materials are of relevance to obtain high specific capacitance values.

MA y + xLi + + xe -↔ Li x MA y (Eq. 1.4)
where MAy is the layer-lattice intercalation host material (e.g., Nb2O5 [55,56]) and x is the number of transferred electrons. It is obvious that the metal valence must change to keep electric neutrality during the intercalation process.

Asymmetric Supercapacitors (Capacitive Asymmetric Supercapacitors

vs. Hybrid Supercapacitors)

In addition to EDLCs and pseudocapacitors, there exists a special configuration, called asymmetric supercapacitors. They are energy storage devices, which consist of two different electrodes, separator and electrolyte. According to the type of the electrode, hybrid supercapacitors and capacitive asymmetric supercapacitors are applied to distinguish the difference. It is generally accepted that a "hybrid supercapacitor" is composed by two electrodes with two different charge-storage mechanisms: one capacitive and one battery-type Faradaic [44,[57][58][59], whereas a capacitive asymmetric supercapacitor contains two capacitive electrodes of different materials [28,[60][61][62][63].

It is universally acknowledged that it is an effective method to construct hybrid supercapacitors to enhance the energy density [64,65]. In a hybrid supercapacitor, the capacitive electrode typically acts as the negative electrode to supply high power density, and the battery-type pseudocapacitive component works as the positive electrode to impart high energy density [66,67]. Moreover, devices consisting of a Faradaic electrode (such as Ni(OH)2 or Co3O4) and a carbon electrode represent a typical type of hybrid supercapacitor device. This combination leads to higher working potential and yields higher capacitance value, which is two to three times higher than that of conventional capacitors as well as EDLC and pseudocapacitors.

For a hybrid supercapacitor, to acquire an optimal performance, a charge balance Q+ (charges stored in the positive electrode) = Q-(charges stored in the negative electrode)

between the two electrodes is necessary.

The charge stored (Q) is determined following the Equation 1.5:

Q = C × m (Eq. 1.5)
Where C (C g -1 ) is the specific capacity and m (g) is the mass of active material. So that the mass balancing follows the Equation 1.6:

m + m - = C - C + (Eq. 1.6)
Where m+ and mare the mass of positive and negative electrode materials, respectively, C+ and Care the specific capacity of the positive and negative electrodes, respectively.

Electrode materials for supercapacitors

In the last few years, to obtain high-performance supercapacitors, numerous electrode materials have been explored. Typically, carbon-based materials are the most widely used materials for EDLC electrodes. The unique stability of carbon in various electrolyte solutions under a wide range of potentials and temperatures enables the utilization of conductive carbon materials (e.g., activated carbon, porous carbon, carbon nanotubes, graphene and carbon quantum dots) for various electrochemical systems and devices [68][69][70]. Besides, to further improve the performance, carbon-based electrodes doped with heteroatom such as N, P and B atoms were also investigated [33,34,71,72].

Owing to the reversible faradaic redox reactions on the surface or near surface of the electrode materials, the pseudocapacitance (C) is 10~100 times larger than that achieved in EDLC [73]. As a result, pseudocapacitor electrode materials have attracted widespread attention. Generally, transition metal hydroxides/oxides like manganese dioxide (MnO2) [74,75], nickel oxide (NiO) [11,76,77], titanium dioxide (TiO2) [78,79], layered double hydroxides (LDHs) [80][81][82] and conducting polymers such as polypyrrole [83][84][85], polythiophene [86,87], etc are promising electrode materials for pseudocapacitors.

In the thesis, we will focus on typical carbon-based materials and transition metal oxides/ hydroxides as examples to discuss electrode materials.

Carbon-based materials

Carbon-based materials are attractive for electrochemical applications since carbon is the sixth most abundant element in the Earth's crust and hence its massive use is cost effective. Herein, a few representative carbon-based materials are discussed as follows.

Activated carbon (AC)

AC is a carbon material with well-developed pores formed by disorderly arrangement of graphite-like crystallites, which has the advantages of large specific surface area, abundant pores and simple preparation process, making it the earliest carbon electrode material used in supercapacitor electrodes [88][89][90]. At present, the common types of activated carbon mainly include: activated carbon powder, activated carbon cloth, activated carbon fibers, etc. [91][92][93]. The electric double layer capacitance of activated carbon in organic electrolytes can reach 100-120 F g -1 ; this value can exceed 300 F g -1 in aqueous electrolytes [9].

In 2015, Li et al. prepared nitrogen-doped activated carbons (NACs) (Figure 1.8), utilizing agricultural waste (corncob) as a precursor [64]. Doping with nitrogen helps to increase the specific capacitance of AC materials effectively. The obtained NACs had a narrow micro-to meso-pore distribution and showed a high specific surface area of up to 2859 m 2 g -1 with a moderate N content of up to 4 wt%. The NACs delivered a high specific capacity of up to 129 mA h g -1 (185 F g -1 ) in an organic electrolyte at a current density of 0.4 A g -1 , as well as an excellent rate capability and much improved cycling stability.

Carbon nanotubes (CNTs)

Since the first report that showed a promising high power density (8 kW kg -1 ), there has been increasing interest in CNTs as supercapacitor electrodes [94] By fabricating electrodes from SWNTs with high carbon purity (99.98%), exclusive SWNT selectivity (> 99%), negligible carbonaceous impurity (< 2% amorphous carbon), and near-ideal specific surface area (1300 m 2 g -1 ), they achieved device operation at a higher voltage (4 V), while maintaining durable full charge-discharge cyclability, with an energy density of 94 Wh kg -1 (47 Wh L -1 ) and a power density of 210 kW kg -1 (105 kW L -1 ) far exceeding those of AC both gravimetrically (33 Wh kg - 1 , 60 kW kg -1 ) and volumetrically (19.8 Wh L -1 , 36 kW L -1 ).

Graphene

Graphene is a new type of two-dimensional planar carbon nanomaterial composed of carbon atoms with hexagonal honeycomb lattice [99,100]. Generally, graphene could be divided into single-layer graphene, double-layer graphene and multi-layer graphene, according to the number of the graphene layers. In single-layer graphene, carbon atoms are bonded to surrounding carbon atoms through sp 2 hybridization, with a C-C bond length of ~ 0.142 nm [101,102]. The three σ bonds in the crystal lattice are connected to each other to form a structure similar to a benzene ring. Therefore, the graphene structure is very stable, and the atoms are very flexible. The π bond pair in the direction perpendicular to the crystal plane promotes the conduction of charge in graphene, and the electron mobility at room temperature is as high as 1.5×10 4 cm 2 /(V s), conferring an excellent electrical conductivity. Furthermore, graphene also has excellent thermal conductivity and mechanical properties. These excellent physical, mechanical and chemical properties, especially the high conductivity of graphene, as well as the high specific surface area and rapid ion migration channel, have made graphene-based materials widely investigated in the field of supercapacitors [103][104][105].

Perfect graphene has an ideal two-dimensional structure with a theoretical specific surface area of up to 2.6 × 10 2 m 2 g -1 ; the theoretical specific capacitance of single-layer graphene is ~21 μF cm -2 , and the corresponding mass specific capacitance is ~550 F g - 1 [99,106]. At present, there have been many studies on the application of graphene in supercapacitors, and the specific capacitance of graphene supercapacitors reported is about 200 F g -1 [40,106].

In 2008, Ruoff et al. reported the application of graphene in supercapacitors for the first time (Figure 1.10) [107]. The as-obtained chemically modified graphene (CMG)

was made of 1-atom thick sheets of carbon, and their performance was demonstrated in a supercapacitor cell (Figure 1.10d). The measured conductivity of these CMG materials (∼2 × 10 2 S/m) closely approaches that of pristine graphite, the surface area of the CMG agglomerate is 705 m 2 g -1 , and calculated specific capacitances were 135 and 99 F g -1 in aqueous and organic electrolytes, respectively. Furthermore, in 2011, Ruoff et al. synthesized a porous microwave exfoliated graphene oxide (MEGO) and thermally exfoliated graphene oxide (TEGO) using a simple activation with KOH to achieve specific surface area values up to 3100 m 2 g -1 , exhibiting a specific capacitance of 166 F g -1 at a current density of 5.7 A g -1 [108].

Transition metal oxides/hydroxides (TMOs/TMHOs)

Compared with carbon-based materials, TMOs/TMHOs possess higher specific capacity due to the multiple oxidation states [109,[START_REF] Wang | Cyanometallic framework-derived hierarchical Co3O4-NiO/graphene foam as high-performance binder-free electrodes for supercapacitors[END_REF]. Early investigations on electrode materials have concentrated on noble TMOs, such as RuO2 and IrO2. RuO2

with three oxidation states within 1.4 V displays a large specific capacitance of 2000 F g -1 [START_REF] Wu | Anchoring Hydrous RuO2 on Graphene Sheets for High-Performance Electrochemical Capacitors[END_REF][START_REF] Jiang | Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage[END_REF]. However, the exceedingly high cost and environmentally poisonous nature of TMOs limit their practical applications. Hence, cheap and high-performance TMOs/TMHOs, such as MnO2, NiO, Co3O4, CuO, Fe3O4, Ni(OH)2 and FeOOH, have been extensively studied as electrode materials for SCs [12,[START_REF] Zhang | Engineering hierarchical Diatom@CuO@MnO2 hybrid for high performance supercapacitor[END_REF][START_REF] Wen | Morphology-controlled MnO2-graphene oxide-diatomaceous earth 3-dimensional (3D) composites for high-performance supercapacitors[END_REF][START_REF] An | Hierarchical architectures of Co3O4 ultrafine nanowires grown on Co3O4 nanowires with fascinating electrochemical performance[END_REF][START_REF] Moosavifard | Designing 3D highly ordered nanoporous CuO electrodes for high-performance asymmetric supercapacitors[END_REF][START_REF] Wang | Fe3O4 nanoparticles grown on graphene as advanced electrode materials for supercapacitors[END_REF][START_REF] Fan | Fe3O4@Carbon Nanosheets for All-Solid-State Supercapacitor Electrodes[END_REF]. Here, we will discuss some commonly used transition metal oxide electrode materials only.

Manganese dioxide (MnO2)

Manganese is the 12 th most abundant element in the Earth's crust, and MnO2 has drawn extensive interest as electrode for supercapacitors due to its low cost, high natural abundance, high theoretical specific capacitance (~1370 F g -1 ), rich polymorphisms (Figure 1.11), and environmental benignity [START_REF] Zhu | Structural Directed Growth of Ultrathin Parallel Birnessite on beta-MnO2 for High-Performance Asymmetric Supercapacitors[END_REF][START_REF] He | Freestanding Three-Dimensional Graphene/MnO2 Composite Networks As Ultralight and Flexible Supercapacitor Electrodes[END_REF][START_REF] Davoglio | Synthesis and characterization of α-MnO2 nanoneedles for electrochemical supercapacitors[END_REF][START_REF] Eustache | High Areal Energy 3D-Interdigitated Micro-Supercapacitors in Aqueous and Ionic Liquid Electrolytes[END_REF].

The energy storage process of MnO2 is mainly achieved by the III/IV oxidation state change at or near the surface. The reaction mechanism equation is as follows (Equation

1.7):

MnO 2 + C + + e -↔ MnOOC (Eq. 1.7)

where C represents electrolyte cations such as H + , K + , Na + and Li + . In 1999, Lee and Goodenough first applied amorphous MnO2•nH2O as an electrode for supercapacitors and tested the performance in a mild KCl aqueous electrolyte. They obtained a specific capacitance of about 200 F g -1 at 2 mA cm 2 [START_REF] Lee | Supercapacitor Behavior with KCl Electrolyte[END_REF] Additionally, to address the conductivity problem of MnO2-based electrodes, an effective approach consists of combining MnO2 with highly conductive materials such as porous metal substrates [START_REF] Xiao | Amorphous MnO2 supported on 3D-Ni nanodendrites for large areal capacitance supercapacitors[END_REF], conducting polymers [START_REF] Lv | Carbon fabric-aligned carbon nanotube/MnO2/conducting polymers ternary composite electrodes with high utilization and mass loading of MnO2 for super-capacitors[END_REF][START_REF] Bahloul | Polypyrrole-covered MnO2 as electrode material for supercapacitor[END_REF] and carbon-based materials [START_REF] Cheng | Graphene and nanostructured MnO2 composite electrodes for supercapacitors[END_REF][START_REF] Fan | Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated Carbon Nanofiber Electrodes with High Power and Energy Density[END_REF][START_REF] Brousse | Long-term cycling behavior of asymmetric activated carbon/MnO2 aqueous electrochemical supercapacitor[END_REF].

Cobalt oxides (Co3O4)

Due to the ultrahigh theoretical specific capacitance (3560 F g -1 ) [START_REF] Wang | Facilely prepared oxidized carbon Fiber@Co3O4@RGO as negative electrode for a novel asymmetric supercapacitor with high areal energy and power density[END_REF] and a variety of structures and morphologies (nanoparticles [START_REF] Liao | All-Solid-State Symmetric Supercapacitor Based on Co3O4 Nanoparticles on Vertically Aligned Graphene[END_REF], nanowires [START_REF] An | Hierarchical architectures of Co3O4 ultrafine nanowires grown on Co3O4 nanowires with fascinating electrochemical performance[END_REF][START_REF] Wang | Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes[END_REF], nanorods [START_REF] Meng | Co3O4 Nanorods with Self-assembled Nanoparticles in Queue for Supercapacitor[END_REF], nanosheets [START_REF] Qiu | Ultrathin mesoporous Co3O4 nanosheets on Ni foam for high-performance supercapacitors[END_REF] and porous nanostructures [START_REF] Deori | Morphology controlled synthesis of nanoporous Co3O4 nanostructures and their charge storage characteristics in supercapacitors[END_REF]), Co3O4 has been diffusely investigated as a candidate for supercapacitor electrodes and it is considered to be a potential candidate for state-of-the-art RuO2 [START_REF] Wang | Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes[END_REF]. The reason behind choosing Co3O4 as a promising candidate for pseudocapacitors over most promising hydrous RuO2 is its low cost, and can provide a variety of oxidation states for efficient redox charge transfer [START_REF] Wu | Anchoring Hydrous RuO2 on Graphene Sheets for High-Performance Electrochemical Capacitors[END_REF][START_REF] Feng | Sub-3 nm Co3O4 Nanofilms with Enhanced Supercapacitor Properties[END_REF].

Usually, the supercapacitive reactions of Co3O4 can be described by Equations 1.8 and 1.9: (599 F g -1 at 2 A g -1 and 439 F g -1 at 40 A g -1 ) as well as excellent cycling life, making them suitable for high-rate supercapacitor application.

Co 3 O 4 + OH -+ H 2 O ↔ 3CoOOH + e -(

Nickel hydroxide (Ni(OH)2)

Among various pseudocapacitive materials, Ni(OH)2 is cost-effective, stable in alkaline electrolytes and is available in various morphologies with high theoretical specific capacitance (2082 F g -1 ), making it an attractive candidate for high performance supercapacitors [START_REF] Xiong | Threedimensional ultrathin Ni(OH)2 nanosheets grown on nickel foam for high-performance supercapacitors[END_REF][START_REF] Wen | Ultrafine Ni(OH)2 nanoneedles on N-doped 3D rivet graphene film for high-performance asymmetric supercapacitor[END_REF][START_REF] Yan | Fabrication and electrochemical performances of hierarchical porous Ni(OH)2 nanoflakes anchored on graphene sheets[END_REF]. Typically, it has a hexagonal layered structure with two polymorphs, α-and β-Ni(OH)2 [START_REF] Bode | Zur kenntnis der nickelhydroxidelektrode-I.Über das nickel (II)-hydroxidhydrat[END_REF]. H2O [START_REF] Hall | Nickel hydroxides and related materials: a review of their structures, synthesis and properties[END_REF].

The α-form is isostructural with hydrotalcite (HT)-like compounds and consists of a stacking of positively charged Ni(OH)2-x layers, with intercalated anions (e.g., carbonate, nitrate, etc.) and water molecules in the interlayer space (~7 Å) to restore charge neutrality (Figure 1.13), whereas the β-form possesses a brucitelike (Mg(OH)2) structure and does not contain any intercalated species with an interlayer distance of ~4.6 Å (Figure 1.14) [START_REF] Hall | Nickel hydroxides and related materials: a review of their structures, synthesis and properties[END_REF][START_REF] Jeevanandam | Synthesis of Nanosized α-Nickel Hydroxide by a Sonochemical Method[END_REF]. It is generally accepted that α-nickel hydroxide will exhibit superior electrochemical properties compared to the β-form. However, the αnickel hydroxide is a metastable phase and is difficult to synthesize, because it changes rapidly to the β-form during synthesis or upon storage in a strong alkaline solution [START_REF] Jeevanandam | Synthesis of Nanosized α-Nickel Hydroxide by a Sonochemical Method[END_REF]. ) and trivalent (Fe 3+ , Al 3+ , Co 3+ , V 3+ , Cr 3+ ) metal cations, respectively, A n-represents the interlayer anion (CO3 2-, NO3 -, OH -), x = M 3+ /(M 2+ + M 3+ ) is the surface charge and is generally in the range of 0.2~0.33 [4,[START_REF] Yu | Preparation of Two Dimensional Layered Double Hydroxide Nanosheets and Their Applications[END_REF][START_REF] Ma | Fabrication of Cu(OH)2@NiFe-layered double hydroxide catalyst array for electrochemical water splitting[END_REF][START_REF] Wu | Electrostatic-Induced Assembly of Graphene-Encapsulated Carbon@Nickel-Aluminum Layered Double Hydroxide Core-Shell Spheres Hybrid Structure for High-Energy and High-Power-Density Asymmetric Supercapacitor[END_REF]. Each hydroxyl group in the LDH layers is oriented toward the interlayer region and may be hydrogen bonded to the interlayer anions and water molecules. Besides, interlayer bonding in LDHs is relatively weak, exhibiting excellent expanding properties.

LDHs have evoked great attention on the basis of their well-defined and high redox activity, wide tunability of the types of metal cations and environmentally friendly nature. They have been employed in several processes such as oxygen evolution reaction [START_REF] Fan | Nickel-vanadium monolayer double hydroxide for efficient electrochemical water oxidation[END_REF][START_REF] Zheng | Threedimensional NiCu layered double hydroxide nanosheets array on carbon cloth for enhanced oxygen evolution[END_REF], hydrogen evolution reaction [START_REF] Zhao | Stabilizing Silicon Photocathodes by Solution-Deposited Ni-Fe Layered Double Hydroxide for Efficient Hydrogen Evolution in Alkaline Media[END_REF], corrosion protection [START_REF] Zhang | Growth Behavior of MgAl-Layered Double Hydroxide Films by Conversion of Anodic Films on Magnesium Alloy AZ31 and Their Corrosion Protection[END_REF], hot coal gas desulfurization [START_REF] Wu | High-Sulfur Capacity and Regenerable Zn-Based Sorbents Derived from Layered Double Hydroxide for Hot Coal Gas Desulfurization[END_REF], supercapacitors [16,[START_REF] Luo | Construction of NiCo2O4@NiFe LDHs Core/Shell Nanowires Array on Carbon Cloth for Flexible, High-Performance Pseudocapacitor Electrodes[END_REF][START_REF] Yu | Synthesis of NiMn-LDH Nanosheet@Ni3S2 Nanorod Hybrid Structures for Supercapacitor Electrode Materials with Ultrahigh Specific Capacitance[END_REF] and so on.

Synthesis approach of electrode materials

The method of synthesis of electrode materials plays an important role in controlling the structures and properties of the materials. Some synthesis methods are described here briefly:

Chemical vapor deposition (CVD)

CVD is a well-established vacuum deposition technique used to produce high quality, high-performance, solid materials. In typical CVD, the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

Many electrode materials such as silicon (dioxide [START_REF] Teshima | Room-temperature deposition of high-purity silicon oxide films by RF plasma-enhanced CVD[END_REF], carbide [START_REF] Stoldt | A low-temperature CVD process for silicon carbide MEMS[END_REF], nitride [START_REF] Kshirsagar | Deposition and characterization of low temperature silicon nitride films deposited by inductively coupled plasma CVD[END_REF] and oxynitride [START_REF] Rangarajan | Low-temperature deposition of high-quality silicon oxynitride films for CMOS-integrated optics[END_REF]), carbon (nanotubes [START_REF] Qian | Synthesis and characterisation of carbon nanotubes grown on silica fibres by injection CVD[END_REF], diamond [START_REF] Kromka | Linear antenna microwave plasma CVD deposition of diamond films over large areas[END_REF] and graphene [START_REF] Wang | A platform for large-scale graphene electronics--CVD growth of single-layer graphene on CVD-grown hexagonal boron nitride[END_REF]), fluorocarbons [START_REF] Wang | Fluorocarbon thin film with superhydrophobic property prepared by pyrolysis of hexafluoropropylene oxide[END_REF] and titanium nitride [170] are widely used CVD method.

Electropolymerization/Electrodeposition

Electrodeposition is a well-known conventional surface modification method to produce in situ metallic coatings by the action of an electric current on a conductive material immersed in a solution containing a salt of the metal to be deposited.

At present, electrodeposition is emerging as an accepted versatile technique involving simple processing conditions for the preparation of nanomaterials with no toxic chemicals in the process. Generally, conductive polymers such as polyaniline (PANI), poly (3, 4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), etc. are synthesized by this approach [START_REF] He | In-situ electropolymerization of porous conducting polyaniline fibrous network for solid-state supercapacitor[END_REF][START_REF] Cong | Flexible graphene-polyaniline composite paper for high-performance supercapacitor[END_REF][START_REF] Nabilah Azman | Effect of electropolymerization potential on the preparation of PEDOT/graphene oxide hybrid material for supercapacitor application[END_REF][START_REF] Shu | A Free-standing Graphene-Polypyrrole Hybrid Paper via Electropolymerization with an Enhanced Areal Capacitance[END_REF].

Hydrothermal/solvothermal methods

Hydrothermal method can be defined as a method of synthesis of single crystals that depends on the solubility of minerals in hot water under high pressure. The crystal growth is performed in an apparatus consisting of a steel pressure vessel called an autoclave, in which a precursor is supplied along with water. The difference between hydrothermal and solvothermal methods is that in the hydrothermal method the solution is aqueous (water), while in solvothermal method the solution is non-aqueous (solvent other than water) under relatively higher temperatures. It is universally acknowledged that hydrothermal method is a facile low-temperature sintering process and is simple to implement and scale up, which is extensively utilized for synthesis of electrode materials [START_REF] Tian | Controllable growth of NiSe nanorod arrays via one-pot hydrothermal method for high areal-capacitance supercapacitors[END_REF][START_REF] Mary | Hydrothermal synthesis of Mn-doped ZnCo2O4 electrode material for high-performance supercapacitor[END_REF][START_REF] Mohammadi | Engineering rGO-CNT wrapped Co3S4 nanocomposites for highperformance asymmetric supercapacitors[END_REF].

Co-precipitation method

Co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed and is a facile and convenient approach to prepare large-scale powder samples. Typically, co-precipitation reactions involve the simultaneous formation of nucleation, growth, coarsening and/or agglomeration processes. For precipitation to occur, the concentration of a solute should be greater than the solubility limit, and the temperature should be high enough to quickly separate into a precipitate. As a consequence, due to the rapid precipitation rate, it is difficult to adjust the morphology of the prepared sample.

Electrochemical measurements for electrode materials 1.5.1 Supercapacitors metrics of electrode materials

-Capacitance (C, F) quantifies the amount of charge stored within a specific potential window for a known weight (or area or volume) of a capacitive electrode. High specific capacitance is the key requirement for high charge storage or high energy density electrode.

-Capacity is the charge-storage amount for a battery-like electrode based on a Faradaic reaction, in Coulombs (C), or mAh.

-Rate capability is the charge storage/energy density capacity at high applied current density compared to the value at low applied current density. High rate capability value is an important requirement for high-rate applications and for fast charge of supercapacitors.

-Energy density (E) is the amount of energy that can be stored per mass (Wh kg -1 ), per area (Wh cm -2 ) or per volume (Wh cm -3 ).

-Power density (P) is the charge/discharge rate, which describes how fast the same amount of energy can be absorbed or delivered for a given time, which is directly related to rate capability.

-Cycling stability is charge capacitance/capacity degradation for a given number of fully charge-discharge cycles. High cycling stability is an important factor for the practical application of supercapacitors.

These metrics are totally determined by the properties of electrode/electrolyte interface. The surface morphology, architecture, porosity, conductivity, ionic diffusion and redox activity of electrode materials play a significant role on the performance of a supercapacitor.

Electrochemical measurements for electrode materials

Typically, electrochemical measurements are conducted in a classical threeelectrode configuration or two-electrode system. To perform an investigation on the performance of active materials, the three-electrode system is commonly utilized, including the active materials as the working electrode, a reference electrode and a counter electrode. For asymmetric supercapacitor devices, two-electrode system is

Cyclic voltammetry (CV)

CV is one of the most commonly used methods to study the electrochemical performance of electrode materials. The principle is to record the corresponding current change by controlling the change of the electrode potential at a constant rate.

The resulting I-V curve can provide useful information on the electrode reaction process, such as the reversibility of the electrode reaction, and the performance of hydrogen evolution and oxygen evolution, which is an effective and basic electrochemical technique to identify the capacitive behavior of SC electrode materials, which is introduced in detail with galvanostatic charge-discharge (GCD) together.

Galvanostatic Charge-Discharge (GCD)

GCD is an important method to study the electrochemical performance of electrode materials. A consecutive charging/discharging of the working electrode is performed at a constant current density with or without a dwelling period (a time period between charging and discharging while the peak voltage V0 remains constant). From the GCD curves, we can obtain the charge and discharge time and potential window.

Consequently, the specific capacitance/capacity (C) of the active material can be calculated using the charge-discharge curve according to Equation 1.11, and the charge-discharge test at different current densities can be used to study the rate performance of electrode materials.

C (capacitance)=

I ∆t m ∆V (Eq. 1.11.1)

C (capacity)= I ∆t m (Eq. 1.11.2)
where I is the applied current, Δt is the discharge time, m is the mass loading of SC electrodes and ΔV is the potential window.

Additionally, the cycling stability of electrode materials in this thesis was performed by the charge and discharge tests at different cycles.

For an asymmetric supercapacitor, the energy density (E) and power density (P) are important indicators to study their practical application value. According to the calculated specific capacitance/capacity, the E and P of the device can be further calculated. The formula used to determine respectively E and P are:

E = 1 2 C ∆V 2 = Q ∆V 2 (Eq. 1.12) P = E ∆t (Eq. 1.13)
Importantly, the CV and GCD (Figure 1.17

i (V) = k 1 v + k 2 v 1/2 (Eq. 1.14)
where k1v is the surface-controlled current and k2v 1/2 is the diffusion-controlled current.

The coefficients k1 and k2 can be obtained through the linear fitting of voltammetric currents at each potential. It leads to the calculation of k1v and k2v 1/2 , which, therefore, allows for the separation of capacitive and diffusion currents. In addition to GCD, CV also provides a method of calculating the capacitance/capacity. The integral area of the CV curve is proportional to the capacitance/capacity contribution of the material according to Equation 1.15, so it can be used to roughly compare the specific capacitance of electrode materials.

C (capacitance) = 1 2mv∆V ∫ I(V) dV V 2 V 1 (Eq. 1.15.1) C (capacity) = 1 2mv ∫ I(V) dV V 2 V 1 (Eq. 1.15.2)
where m is the mass loading of SC electrodes, v is the scan rate, V1 and V2 are respectively the low-and high-end potentials, and I (A) is the current.

Electrochemical Impedance Spectroscopy (EIS)

EIS can be used to analyze the charge transfer and material transport of electrode materials at the electrode/electrolyte interface and the electrode reaction kinetics.

Objectives and outline of the thesis

The primary purpose of this thesis is to explore new possibilities and synthesize high-performance electrode materials via low-cost facile method for supercapacitors.

New composites will be explored to take advantage of different materials. Therefore, this thesis includes four chapters describing the various materials proposed and their characterization and measurements. More detailed information will be introduced in each chapter.

In LDHs@CS/NF) was obtained by a two-step hydrothermal process. Furthermore, a hybrid supercapacitor was assembled using NiMnCr LDHs@CS/NF as the positive electrode and FeOOH coated on NF (FeOOH/NF) as the negative electrode. The hybrid supercapacitor was applied to power a home-designed windmill device and to light up a yellow LED and a red LED in parallel, indicating its potential application as an energy storage device.

In Chapter 3, we fabricated Ni(OH)2@CuO@Cu foam binder-free electrodes by a two-step process at room temperature with various deposition times (30, 50, 90, 150 and 200s). Among all the samples, Ni(OH)2@CuO@Cu-150 exhibited the largest areal capacity of 7063 mC cm -2 at 20 mA cm -2 which was chosen as the positive electrode in a hybrid supercapacitor. Combing with N-doped reduced graphene oxide on nickel foam (N-rGO/NF) as the negative electrode, a hybrid supercapacitor was assembled and displayed good flexibility, cycling stability and high areal energy density of 130.4 μWh cm -2 at a power density of 1.6 mW cm -2 .

In conclusion, all of the findings in this thesis imply the promising potential There are many approaches to prepare LDHs for various applications, and some representative examples are described below.

Synthesis methods and applications of LDHs

There are several common processing techniques that have been developed to synthesize LDHs such as hydrothermal/solvothermal, electrodeposition, coprecipitation and so on.

Hydrothermal/solvothermal synthesis

Hydrothermal and solvothermal techniques are the most widely used methods to obtain well-crystallized transition metal oxides, inorganic-organic hybrid materials and other ternary systems [6]. During the hydrothermal process, the reactants are heated in pressurized aqueous media to obtain LDHs with improved crystalline structure, whereas the reactants are heated in non-aqueous (solvent other than water) under relatively higher temperatures during the solvothermal synthesis.

Herein, we summarize some common LDHs-based hybrids using hydrothermal/solvothermal synthesis processes and the corresponding applications (Table 2.1).

Electrodeposition

Electrodeposition is a time-saving and convenient method to synthesize LDHs.

Typically, the electrodeposition experiments are normally performed in a threeelectrode electrochemical configuration in an electrolytic cell to control and monitor the deposition experiments. The electrolytic cell is composed of electrolyte, working electrode (substrate), counter and reference electrodes. It is assumed that the electrode/electrolyte interface is inert in this cell, and redox reaction in the cell is driven by external voltage/current, which is controlled by a potentiostat.

Generally, electrodeposition was developed using nitrate or sulfate solutions containing appropriate metal ions. The deposition can be achieved by reducing nitrate or sulfate ions to generate hydroxide ions that elevate the local pH on the working electrode, resulting in the precipitation of the corresponding LDHs:

NO 3 -+ H 2 O + 2e -→ NO 2 -+ 2OH - SO 4 2-+ H 2 O + 2e -→ SO 3 2-+ 2OH -
Herein, we summarize some common LDHs-based hybrids prepared using electrodeposition process and the corresponding applications (Table 2.2). CuO@CoFe LDH -1 V vs. SCE, 0-200 s micro-supercapacitor [19] ZnO@CoNi-LDH -1 V vs. Ag/AgCl, 30-300 s Photoelectrochemical water splitting [20] Co/Al-LDH films -0.9 V vs. SCE, 5-60 s Supercapacitors [21] Au@ErGO@ECoNi LDH -1 V vs. Ag/AgCl, 120 s Supercapacitors [22] NiFeAl-LDHs -1 V vs. SCE, 300 s Oxygen evolution reaction [23] Zn/Al LDH film -0.9 V, 40-120 s Sensors [24] ZnCo-LDH -1 V vs. Ag/AgCl, 50-180 s Oxygen evolution reaction [25]

Co-precipitation method

Co-precipitation is the most common method for the preparation of LDHs, in which inorganic salts are placed in an alkaline medium at constant or increasing pH. The morphology and particle size distribution of the materials can be tuned by controlling the evaporation rate or varying the pH. NiCo2O4@NiFe LDHs 464 C g -1 / 1 A g -1 79% (1000 cycles) 2 M KOH [40] Ni-Zn-Fe LDH 653.5 C g -1 / 5 mV s -1 95% (1000 cycles) 6 M KOH [41] rGO@NiMn-LDH@NF 932.8 C g -1 / 1 A g -1 95% (5000 cycles) 3 M KOH [42] NiCo LDH/NF 1341 C g -1 / 3 A g -1 / 1 M KOH [7] rGO@ZIF-67@NiCoAl-

LDHs 2291.6 F g -1 / 1 A g -1 92% (2000 cycles) 1 M KOH [43]
NiO@NiMn-LDH/NF 468.5 C g -1 / 0. Ni-foam@Cu-Al LDH/g-C3N4 415.9 C g -1 at 0.4 A g -1 92.71% (5000 cycles)

M KOH [63]

CuAl-LDH/CoWO4 337.5 C g -1 at 5 A g -1 95.1% (5000 cycles) 2 M LiOH [64] In this thesis, we focused on the preparation of Ni-based LDHs electrode materials.

First of all, NiFe LDHs deposited on reduced graphene oxide (rGO) supported by nickel foam (NF) skeleton (NiFe LDHs/rGO/NF) was prepared by electrodeposition method.

In the second part of this chapter, we describe the synthesis of NiAl LDHs, NiFe LDHs and NiCr LDHs on carbon spheres coated on NF skeleton by hydrothermal method. In the third section of this chapter, a ternary NiMnCr LDHs-carbon spheres modified Ni foam (NiMnCr LDHs@CS/NF) nanocomposite was prepared using a hydrothermal process. All the synthesized materials were characterized using various techniques to establish their morphology, structure and chemical composition. As positive electrode candidates for hybrid supercapacitors, their electrochemical performances were further investigated.

NiFe Layered Double Hydroxides

Herein, we prepared new 3D composite materials by combining NiFe LDHs and nickel foam (NiFe LDHs/NF) via a facile and eco-friendly electrochemical method.

Fe 3+ was employed in the LDH composites, because compared to most metals such as cobalt and aluminum, iron is much cheaper and more abundant. In order to further improve the conductivity of the composite, NF was electrochemically coated with reduced graphene oxide (rGO) prior to NiFe LDHs electrodeposition to produce NiFe LDHs/rGO/NF composite material. The electrochemical properties of the obtained materials were investigated using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) curves and impedance spectroscopy measurements. Moreover, using the as-prepared battery-like electrode as a positive electrode, a flexible hybrid supercapacitor device was constructed with mesoporous carbon (MC) coated on NF as a negative electrode. All the details will be discussed in the following sections.

Experimental section

Preparation of graphene oxide (GO)

Graphene oxide (GO) was synthesized by the improved Hummers' method [65].

Typically, to a mixture of graphite flakes (1 g) and KMnO4 (6 g) was added a 9:1 (volume) mixture of concentrated H2SO4/H3PO4 (40:13.3 mL) and allowed to react at about 50 °C under continuous stirring for 12 h. The reaction was cooled to room temperature, followed by adding ice (~100 mL) and 1 mL of 30% H2O2. Then, the mixture was centrifuged at 4500 rpm for 40 min, washed successively with 200 mL of 30% HCl, 200 mL of water, and 200 mL of ethanol. Finally, the obtained precipitate was vacuum-dried at room temperature.

Preparation of

Preparation of reduced graphene oxide coated nickel foam (rGO/NF)

To improve the conductivity of the electrode, reduced graphene oxide (rGO) was first coated on NF by the electrodeposition method. Typically, 0.1 M LiClO4 aqueous solution containing 1.5 mg mL -1 of graphene oxide (GO) was used as the electrolyte for graphene electrodeposition to obtain rGO/NF. The deposition of reduced graphene oxide (rGO) was carried out by a potentiostatic method at a constant potential of -1.2

V vs. SCE for 600 s. Then, the resulting black sample was washed with water for several times.

Preparation of NiFe LDHs/rGO/NF

A two-step electrodeposition process was used to synthesize NiFe LDHs/rGO/NF nanocomposite. First, rGO/NF was prepared using the same method mentioned above, followed by NiFe LDH electrodeposition using Ni(NO3)2•6H2O and Fe(NO3)3•9H2O at a molar ratio of 1:4 under constant potential of -1.2 V vs. SCE for 10 s to produce NiFe LDHs/rGO/NF, represented by S7. Additionally, S8 and S9 were also prepared using different deposition times of 20 s and 30 s, respectively.

Fabrication of an asymmetric supercapacitor

A hybrid supercapacitor device was assembled using the as-prepared NiFe LDHs/rGO/NF (S7, 1 cm×1 cm) as the positive electrode and mesoporous carbon (MC)/NF as the negative electrode, with the separator consisting of a filter paper soaked into 2 M KOH solution for 30 min.

To acquire the optimal performance of the hybrid supercapacitor, a charge balance Q+ (charges stored in the positive electrode) = Q-(charges stored in the negative electrode) between the two electrodes is conducted according to Equation 1.6. The optimal mass ratio is ~0.04, but it is difficult to control the accurate mass of MC when mixing and pressing MC powder with auxiliary materials such as carbon black or binder onto the Ni foam. Therefore, the mass loadings of NiFe LDHs/rGO/NF and MC on Ni foam are 0.8 and 10.1 mg, respectively, which corresponds to a mass ratio of 0.08 in our work.

Electrochemical characterization was performed in a 2-electrode system.

Results and discussion

The procedures for the preparation of NiFe LDHs/NF and NiFe LDHs/rGO/NF are schematically illustrated in In NiFe LDHs/NF, the Ni2p high resolution XPS spectrum (12.9 at%) shows two peaks located at binding energies of 855. Raman spectroscopy is a valuable tool for analysis of carbon-based materials. 2.8, confirming the good performance for all the samples. Among all the samples, NiFe LDH 1:4 (S5) displays the larger specific capacity values, which were determined to be 500, 480, 460 and 420 C g -1 at the current densities of 10, 15, 20 and 30 A g -1 , respectively. The electrochemical impedance spectra are shown in Figure 2.8e. And it can be clearly seen that NiFe LDHs/rGO/NF (S7) has a smaller Rs (1.07 ohm cm -2 ) (a combination of the contact resistance of the active materials with the substrate and the test cell to the apparatus, the ionic resistance of the electrolyte, the intrinsic resistance of electrodes) and also a lower Rct (13.48 ohm cm -2 ). It might be that rGO provides easy electron-transfer access and more Fe 3+ ions enhanced the electrical conductivity of Ni(OH)2 [71]. Finally, the prepared electrodes are subjected to an extended chargedischarge cycling at 15 A g -1 and the result is depicted in To completely evaluate the electrochemical performance of the as-fabricated hybrid supercapacitor, the Ragone plot, according to Equations 1.12 and 1.13, is generated in

NiAl, NiCr and NiFe Layered Double Hydroxides

Herein, we prepared NiFe, NiCr and NiAl LDHs through a one-step hydrothermal technique and studied the influence of the Fe, Cr and Al in Ni-based LDHs on their electrochemical properties. The same LDHs were further coated on carbon spheres (CS) supported on NF using a facile two-step hydrothermal process to produce NiAl LDHs@CS/NF, NiFe LDHs@CS/NF and NiCr LDHs@CS/NF composites.

Furthermore, a hybrid supercapacitor was assembled using N-doped reduced graphene oxide (N-rGO) as the negative electrode and Ni-based LDHs as the positive electrode.

Experimental section

Synthesis of nitrogen-doped reduced graphene oxide (N-rGO)

N-rGO was prepared through graphene oxide (GO) hydrothermal reduction in presence of hydrazine monohydrate (NH2NH2 . H2O). Typically, 100 mg of GO were dispersed in 50 mL of Milli-Q water and sonicated for 2 h to form a GO suspension (2 mg mL -1 ), followed by the addition of 0.5 M hydrazine monohydrate. The resulting mixture was poured into a 100 mL Teflon-lined autoclave and heated at 160 o C for 3 h.

The solution was cooled to room temperature, and the resulting N-rGO was aged in Milli-Q water for 72 h by changing the water every 24 h. Finally, the product was collected by the freezing-dry method for 72 h.

Synthesis of carbon spheres on nickel foam (CS/NF)

Carbon spheres coated on nickel foam (CS/NF) nanocomposite was prepared via a simple one-step hydrothermal method [87]. In a typical procedure, 6 g of glucose were dissolved in Milli-Q (MQ) water ( 35 LDHs@CS/NF, respectively.

Assembly of a hybrid supercapacitor cell and device

A hybrid supercapacitor cell was assembled in 2M KOH aqueous solution. The NiAl LDHs@CS/NF sample was applied as the positive electrode. The negative electrode (N-rGO/NF) consisted of N-rGO and polyvinylidene difluoride (PVDF) in a mass proportion of 8:2 coated on NF.

To acquire the optimal performance of the hybrid supercapacitor, a charge balance Q+ (charges stored in the positive electrode) = Q-(charges stored in the negative electrode) between the two electrodes is necessary so that the area balancing follows the Equation 1.6. The optimal area ratio is ~0.2. Taking the real area into consideration, the areas of the positive and negative electrodes were chosen as 1 cm×1 cm and 1.5 cm×1.5 cm, respectively, and the real area ratio is ~0.4 in this work.

To test the mechanical properties and flexibility, a hybrid supercapacitor device was further fabricated using NiAl LDHs@CS/NF (1 cm×1 cm) as the positive electrode, N-rGO/NF (1.5 cm×1.5 cm) as the negative electrode, and a filter paper (immersed into 2 M KOH aqueous solution overnight) as the separator.

Results and discussion

The overall capacity of a hybrid supercapacitor is related to the capacity of the positive and negative electrodes, according to Equation 2.4:

1 C T = 1 C p + 1 C n (Eq. 2.4)
where CT is the total capacity of the hybrid supercapacitor, Cp is the capacity of the positive electrode and Cn is the capacity of the negative electrode. As previously stated, the N-rGO was meant to be applied as a negative electrode of a hybrid supercapacitor, so its electrochemical performance was further investigated in a classical 3-electrode cell containing using 2 M KOH aqueous electrolyte. The XRD pattern of NiAl LDHs presents typical (003), ( 006), ( 012), ( 015), ( 118), ( 110), ( 113) and ( 116 110), ( 101) and ( 110) planes (JCPDS No. 14-0117). Due to the existence of the NF skeleton, the peaks of LDHs exhibit relatively weak intensity so that the magnified XRD pattern in the range of 10-44 o was provided in Figure 2.13b for more details.

However, for NiFe LDHs@CS/NF and NiCr LDHs@CS/NF samples, no obvious peaks of LDHs were observed owing to the poor crystallinity (Figure 2 From Figure 2.16, we can see that a carbon spheres (CS, ~400 nm in diameter) layer with a smooth surface was formed on the NF, suggesting the good polymerization of glucose during the hydrothermal process. After the growth of NiFe LDHs, all the CSs exhibit pompom-like morphology. Notably, the big difference of the morphology between NiFe LDHs/NF and NiFe LDHs@CS/NF might be ascribed to the existence of CS, which leads to differences in crystal growth direction and rate. LDHs@CS/NF, where two peaks at 855.7 and 873.4 eV ascribed respectively to Ni2p3/2 and Ni2p1/2 along with two satellite peaks at 861.1 and 879.6 eV were observed, suggesting that Ni is in +2 oxidation state [89]. For NiFe LDHs@CS/NF, there is also Fe2p element in the spectrum (8.4 at%). The core level spectrum of Fe2p is depicted in respectively to Fe2p3/2 and Fe2p1/2, and two satellite peaks at 716.5 and 732.9 eV, indicating the Fe 3+ oxidation state in NiFe LDH nanosheets [90].

For NiCr LDHs@CS/NF, there is also Ni2p element (11.6 at%) and the highresolution spectrum in Figure 2.18b1 also comprised two peaks located at 856.1 and 873.7 eV due respectively to Ni2p3/2 and Ni2p1/2, and two satellite peaks at 861.5 and 879.8 eV, proving the existence of Ni 2+ as well. At the same time, Cr2p element accounts for 5.9 at%, and its core level spectrum can be deconvoluted with two components at 587.3 and 577.7 eV assigned respectively to Cr2p1/2 and Cr2p3/2, indicating that Cr is trivalent (Figure 2.18b2) [4]. LDHs/NF at 73.9 eV, ascribed to the presence of Al 3+ ions, which partly overlapped with Ni 3p peak at 68.7 eV [91]. For NiAl LDHs@CS/NF, the proportion of Al2p is 35.3 at% and its high-resolution spectrum (Figure 2.18c2) can be also seen at 73.6 eV NiAl LDHs@CS/NF and proves the existence of CS. However, the difference of carbon content is relatively low, because of the thicker NiAl LDH (thicker than the sampling depth of XPS). This was also the reason why the recorded carbon content was lower than that of NiFe LDHs@CS/NF and NiCr LDHs@CS/NF. andc) with an areal capacity of 74.2 mC cm -2 at 1 mA cm - 2 , which is slightly higher than that of Ni(OH)2/NF (70.6 mC cm -2 at 1 mA cm -2 ).

Upon incorporation of Fe, Cr and Al elements, the Ni-based LDHs composites showed improved performance (Figure 2.20b). Among all the samples, NiAl LDHs@CS/NF possessed the longest discharge time, signifying the largest areal capacity. According to Equation 1.11.2, the areal capacity values of NiAl LDHs@CS/NF were determined as 1042.2, 870.2, 787.3, 722.9 and 669 mC cm -2 at respectively 1, 2, 3, 4 and 5 mA cm -2 , indicating good rate performance. About 65-80% of areal capacity was maintained even at 5 mA cm -2 (Figure 2. 20c andd). The areal capacity values of all composites are summarized in Table 2.12, where one could observe that the NiAl LDHs/NF exhibited the best performance with areal capacity values of 851.9, 771.4, 714.6, 665.3 and 618.6 mC cm -2 at respectively 1, 2, 3, 4 and 5 mA cm -2 . andd). This might mainly be due to the larger electrochemical effective surface area.

Consequently, the real electrochemical effective surface area (A) of all electrodes was determined by chronocoulometry experiments using 0.1 mM K3[Fe(CN)6] as a probe molecule in 0.1 M KCl (Figure 2.20f), and the charge produced by the redox reaction at the electrodes using the Anson's Equation 2.5:

Q(t) = 2nFAcD 1 2 ⁄ t 1 2 ⁄ π 1 2 ⁄ + Q dl + Q ads (Eq. 2.5)
where c is the substrate concentration (mol cm -3 ), n is the number of electron transferred, F is the Faraday constant (9.6485×10 5 C mol -1 ), A is the effective area of the electrode (cm 2 ), D stands for the diffusion coefficient (7.6×10 -6 cm 2 s -1 ), Qads is the Faradaic charge, Qdl is the double layer charge, other symbols have their ordinary significance.

From the plot of Q vs. t 1/2 (Figure 2.20f), NiAl LDHs@CS/NF displayed the largest electrochemical effective surface area of 182.8 cm 2 , consistent with the above results.

Notably, NiAl LDHs/NF holds a smaller electrochemical effective surface area (121.1 cm 2 ) than that of NiCr LDHs@CS/NF (159.5 cm 2 ) but larger areal capacity, which may be due to its better conductivity with smaller Rs but larger Rct. As a result, the capacity of the electrode is not only related to conductivity, but also to the real electrochemical effective surface area. Both of them encompass a great effect on the capacitance of the supercapacitors.

A hybrid supercapacitor cell was assembled in which the NiAl LDHs@CS/NF (1 cm×1 cm) was applied as the positive electrode and N-rGO/NF (1.5 cm×1.5 cm) acted as the negative electrode. The CV plots of N-rGO/NF and NiAl LDHs@CS/NF, performed in KOH (2 M) at 100 mV s -1 in a classical 2-electrode configuration cell, are presented in Figure 2.22a. Even though N-rGO/NF showed smaller CV area than that of NiAl LDHs@CS/NF, it exhibited a relatively larger operating potential window (-1 to 0.4 V), which is expected to greatly enhance the energy density of the hybrid supercapacitor.

In order to find a suitable working voltage window of the hybrid supercapacitor cell, the CV plots were performed at 100 mV s -1 in different cell voltage windows of 0-1.1, 0-1.2, 0-1.3, 0-1.4, 0-1.5, 0-1.6, 0-1.7 and 0-1.8 V. As can be seen in Figure 2.22b, the cell voltage window could be extended up to 1.8 V but with the existence of a polarization phenomenon. To protect the electrodes, the cell voltage window was limited to 0-1.6 V. Based on the results, the CV plots were acquired at various scan rates between 0 and 1.6 V (Figure 2.22c). Furthermore, the stability of the hybrid supercapacitor cell plays an important role to evaluate its performance. Therefore, the hybrid supercapacitor was submitted to an extended charge-discharge cycling at 1 mA cm -2 . After 5,000 cycles, ~82% of the original capacity was retained, which slightly decreased to ~72% after 10,000 cycles

(Figure 2.22h).
To highlight the potential of the as-fabricated supercapacitor cell for real applications, a home-designed windmill device was assembled successfully (see the V with almost the same discharge time. Additionally, the two as-fabricated hybrid supercapacitor cells in series were applied to supply electricity to a red LED (1.8-2 V), a green LED (1.8-2 V), a yellow LED (1.8-2 V) and a blue LED (3-3.2 V) in parallel.

schematic diagram in
The green, yellow and red LEDs were lightened for 37, 195, and 459 s, respectively, while the blue one was not because of the bigger operating voltage (Figure 2.23b1-b5).

All of these findings demonstrate a promising practical energy storage application of the as-fabricated hybrid supercapacitor cells. Finally, a flexible hybrid supercapacitor device was assembled in which NiAl revealed that the cell voltage window could reach up to 1.8 V even though a polarization phenomenon was obvious when the cell voltage exceeds 1.7 V. In order to reduce the damage to the electrode and ensure stable operation of the device, the operating cell voltage window was chosen as 0-1.6V. Figure 2.24c displays the CV curves of the hybrid supercapacitor obtained at various scan rates (10 -100 mV s -1 ) between 0 and 1.6 V. The full device exhibited a capacitive-like characteristic with obvious deviation from ideal capacitive behavior although NiAl LDHs@CS/NF electrode performed as a battery electrode [93]. At the same time, as displayed in the Nyquist plot (Figure 2.24d),

we can observe that the whole system reveals a smaller internal resistance (Rs=0.20 ohm cm -2 ) and charge-transfer resistance (Rct=0.84 ohm cm -2 ) than that of the hybrid supercapacitor cell. A possible reason might be that the two integrated electrodes shorten the distance of electron transfer and ion diffusion. Consequently, the hybrid supercapacitor device exhibited a longer discharge time than that of the hybrid supercapacitor cell, signifying a larger areal capacity (Figure 2.24e and f).

According properties and providing promising potential for their application in wearable energy devices. Furthermore, the hybrid supercapacitor stability after 5,000 chargingdischarging cycles was assessed (Figure 2.24i); the areal capacity remained ~75% and ~67% of its original value respectively after 5,000 and 10,000 cycles, suggesting a good reversibility.

Fabrication of a hybrid supercapacitor

To assemble a hybrid supercapacitor device, FeOOH/NF electrode was used as the negative electrode. The FeOOH/NF electrode consisted of FeOOH, carbon black and polyvinylidene difluoride (PVDF) in a mass ratio of 7:2:1, which was pasted on the nickel foam; the final mass of FeOOH was 3.57 mg.

Using as-prepared NiMnCr LDHs@CS/NF (1 cm×1 cm) as the positive electrode and FeOOH/NF as the negative electrode, a hybrid supercapacitor device was assembled with the separator consisting of a filter paper soaked into KOH (2 M) solution for 30 min.

To acquire the optimal performance of the hybrid supercapacitor, a charge balance Q+ (charges stored in the positive electrode) = Q-(charges stored in the negative electrode) between the two electrodes is necessary so that the mass balancing follows the Equation 1.6. The optimal mass ratio is ~0.13, but it is difficult to control the accurate mass of FeOOH when mixing and pressing FeOOH powder with auxiliary materials such as carbon black or binder onto the Ni foam. Therefore, the mass loadings of NiMnCr LDHs@CS/NF and FeOOH on Ni foam are 1.165 and 3.57 mg, respectively, which corresponds to a mass ratio of 0.33 in our work.

Electrochemical characterization was performed in a two-electrode system at room temperature.

Results and discussion

The fabrication process of the NiMnCr LDHs@CS/NF electrode is illustrated in and NH4F precursors. The chemical process can be described using Equations (

NH4F + H2O → NH4 + + OH -+ HF (Eq. 2.6) CO(NH2)2 + 3H2O → 2NH4 + + 2OH -+ CO2 (Eq. 2.7)

Ni 2+ + Mn 2+ + Cr 3+ + OH -→ NiMnCr LDHs (Eq. 2.8)

The reactions, under the hydrothermal conditions, mainly involve progressive The Ni2p high-resolution spectrum (3.5 at%) of NiMnCr LDHs@CS/NF composite (Figure 2.33b) comprises two peaks due to Ni2p3/2 and Ni2p1/2 located respectively at 854.8 and 872.5 eV along with two satellite peaks [37]. The presence of the satellite peaks is a good indication that the nickel in the LDH is in its +2 oxidation state. that of NF (24.4 C g -1 ), CS/NF (88 C g -1 ) and NiMnCr LDHs/NF (174.2 C g -1 ). To establish the performance of NiMnCr LDHs@CS/NF composite, galvanostatic chargedischarge (GCD) curves were measured at various current densities (Figure 2.36d). NiMnCr LDHs@CS/NF composite exhibited a more stable morphology compared to NiMnCr LDHs/NF owing to the presence of CSs. Furthermore, by analyzing the elements composition changes (Table 2.18), one clearly sees that NiMnCr LDHs/NF experienced a significant loss as compared to NiMnCr LDHs@CS/NF, which is consistent with the cycling performance results in The NiMnCr LDHs@CS/NF//FeOOH/NF hybrid supercapacitor possesses a great cell voltage window, which could be extended to 1.8 V (Figure 2.43a). Interestingly, the CV curves display a similar shape, suggesting an excellent capacitive performance.

Obviously, there exists a polarization phenomenon when the cell voltage was higher than 1.7 V, so the cell voltage window was chosen as 0-1.7 V to reduce the damage to the electrode. Figure 2.43b depicts the CV curves of the hybrid supercapacitor in the cell voltage window between 0 and +1.7 V acquired at different scan rates. Upon increasing the scan rate, one sees a shift of the redox peak potential; this may be ascribed to increased internal resistance at higher scan rates. which shows a good rate performance (60% of initial areal capacity was maintained at a current density of 10 A g -1 and 36% of areal capacity was retained even at a current density of 20 A g -1 ).

Additionally, the hybrid supercapacitor was characterized by electrochemical impedance spectroscopy (EIS) in the 0.01 Hz and 100 kHz frequency range; the equivalent circuit for impedance analysis was illustrated in the inset of

Conclusion

Firstly, NiFe LDHs/rGO/NF composites were synthesized using a two-step electrodeposition method, exhibiting improved performance with a specific capacity of 585 C g -1 at a current density of 5 A g -1 . Additionally, the capacity retention of 64.7% after 2,000 cycles was recorded, suggesting a reasonable stability of the material.

Moreover, a flexible hybrid supercapacitor was assembled using NiFe LDHs/rGO/NF as positive electrode and mesoporous carbon coated on NF as negative electrode; the device displayed an energy density of 17.71 Wh kg -1 at a power density of 348.49 W kg -1 , suggesting a promising potential in supercapacitor applications.

Secondly, NiFe LDHs, NiCr LDHs and NiAl LDHs on nickel foam (NF) were synthesized by a simple hydrothermal process. With the assistance of carbon spheres (CS), NiFe LDHs@CS/NF, NiCr LDHs@CS/NF and NiAl LDHs@CS/NF were further obtained. Among all the samples, NiAl LDHs@CS/NF exhibited the best electrochemical performance as a binder-free electrode and the largest areal capacity of 1042.2 mC cm -2 at 1 mA cm -2 . Consequently, NiAl LDHs@CS/NF//N-rGO/NF hybrid supercapacitor cell in KOH solution and flexible NiAl LDHs@CS/NF//N-rGO/NF hybrid supercapacitor devices were successfully assembled, and further applied for windmill device operation, lasting for 21 and 32 s, respectively. Moreover, two NiAl LDHs@CS/NF//N-rGO/NF hybrid supercapacitor cells connected in series were able to supply electricity to LEDs in parallel and the red one was lighted up for the longest time of ~459 s. The flexible NiAl LDHs@CS/NF//N-rGO/NF hybrid supercapacitor device displayed favorable flexibility, mechanical properties and stability; two devices in series could light a yellow LED (199 s), a red LED (542 s) and a green LED (37 s) in parallel simultaneously.

Finally, NiMnCr LDHs@CS/NF composites were prepared by the hydrothermal method, and successfully applied as efficient electrode material for supercapacitors.

NiMnCr LDHs@CS/NF features a better rate performance and a specific capacity of 569 C g -1 at a current density of 3 A g -1 . Furthermore, NiMnCr LDHs@CS/NF electrode revealed good reversibility and stability. The electrode retained ~76% of the initial specific capacity after 10,000 discharge cycles. Furthermore, a hybrid supercapacitor was prepared using NiMnCr LDHs@CS/NF as the positive electrode and FeOOH coated on NF as the negative electrode. The device exhibited an energy density of 48

Wh kg -1 at a power density of 402.7 W kg -1 with good stability. A home-designed windmill device was successfully driven for 25 s by the as-fabricated supercapacitor device and two as-fabricated hybrid supercapacitor cells in series were used to light up a yellow LED and a red LED for ~111 s and ~343 s, respectively.

All the findings suggest that the developed electrodes hold a promising potential for energy storage applications, especially for hybrid supercapacitor devices.

of deposition time on the electrochemical properties of the resulting Ni(OH)2@CuO@Cu foam electrode materials. Among them, Ni(OH)2@CuO@Cu-150 composite exhibited the largest areal capacity of ~7063 mC cm -2 at 20 mA cm -2 . Hence, Ni(OH)2@CuO@Cu-150 was investigated as a positive electrode to assemble a hybrid supercapacitor device, while N-doped reduced graphene oxide on nickel foam (N-rGO/NF) was applied as a negative electrode. The device displayed a large areal energy density of 130.4 μWh cm -2 at a power density of 1.6 mW cm -2 . For practical applications, two hybrid supercapacitor devices were assembled in series to successfully lighten up a red LED for 12 min 39 s. In addition, a three-digit digital display was powered for 1 min 28 s using three devices in series. Finally, a sunlight-powered integration system based on the hybrid supercapacitor device, solar cells, and a homemade windmill device was constructed. The hybrid supercapacitor was charged by the solar cell, which further powered the windmill device and ensured its continuous operation for 59 s, indicating the successful practical application of the hybrid supercapacitors based on Ni(OH)2@CuO@Cu foam composites as energy storage devices.

Experimental section

Synthesis of CuO@Cu foam

CuO@Cu foam was prepared by chemical oxidation at room temperature (RT). Cu foam (13 cm 2 ) was washed sequentially before use with HCl (2M), acetone and MQ water under sonication for 10 min each. Then, an aqueous solution (40 mL) of 8 mM (NH4)2S2O8 and 60 mM NaOH was prepared to serve as the alkaline oxidative etchant solution. The clean Cu foam was placed in the above solution for 20 min to produce Cu(OH)2@Cu foam with a blue color. Furthermore, the Cu(OH)2@Cu sample was annealed at 190 o C for 3 h to produce CuO@Cu foam.

Synthesis of Ni(OH)2@CuO@Cu foam

Ni(OH)2@CuO@Cu foam was synthesized through electrochemical deposition by applying CuO@Cu foam, platinum foil and Ag/AgCl respectively as the working, counter, and reference electrodes. The electrolyte was Ni(NO3)2•6H2O (0.3 M, 40 mL). The potentiostatic deposition was carried out at -1 V vs. Ag/AgCl for 30, 50, 90, 150 and 200 s to afford respectively Ni(OH)2@CuO@Cu-30, Ni(OH)2@CuO@Cu-50, Ni(OH)2@CuO@Cu-90, Ni(OH)2@CuO@Cu-150 and Ni(OH)2@CuO@Cu-200. The resulting Ni(OH)2@CuO@Cu samples were rinsed with MQ water, and dried in ambient air.

Assembly of a hybrid supercapacitor device

A hybrid supercapacitor was assembled using Ni(OH)2@CuO@Cu-150 (positive electrode), N-rGO (negative electrode) and a filter paper (separator), immersed into 2 M KOH aqueous solution overnight. The current collector (nickel foam) was coated with N-rGO and polyvinylidene difluoride (PVDF) in a mass proportion of 8:2.

Construction of a sunlight-powered energy storage system

A sunlight-powered energy storage system was constructed by combining the as-prepared hybrid supercapacitor device with two solar cells (SC-3012-2A) obtained from abandoned solarpowered toys. Furthermore, a home-designed windmill device including an engine (1.5-9 V) and a windmill was prepared, and was powered by the hybrid supercapacitor device.

Results and discussion

The main target of the present work was to prepare high performance positive electrode for hybrid supercapacitors. Ni-based electrodes display high theoretical capacitance values. Therefore, we investigated Ni(OH)2 modified CuO@Cu foam [Ni(OH)2@CuO@Cu] as a binder-free electrode for supercapacitors. As illustrated in Figure 3.1, Cu foam with gold color (Figure 3.2a)

was partially oxidized at ambient temperature to produce Cu(OH)2@Cu sample with a blue-grey At the same time, the EDS spectra in Figure 3.9 and elemental composition (Table 3.1) also demonstrated differences between CuO@Cu foam, Ni(OH)2@CuO@Cu-30 and Ni(OH)2@CuO@Cu-50, revealing 1.2 and 4.3 at.% of Ni element in Ni(OH)2@CuO@Cu-30 and Ni(OH)2@CuO@Cu-50, respectively, confirming Ni(OH)2 successful deposition on CuO@Cu foam. GCD plots of all the samples were carried out to assess their performance. As seen in Figure 3.12c, the Ni(OH)2@CuO@Cu-150 composite exhibited the longest discharge time, signifying the largest areal capacity compared with other samples, in accordance with the results of CV curves in Figure 3.12a. Furthermore, the GCD profiles of the electrodes at various current densities were acquired and the corresponding coulombic efficiencies are depicted in Table 3.3. All Ni(OH)2@CuO@Cu foam composites revealed higher coulombic efficiencies than CuO@Cu foam. and Table 3.4). One sees that the areal capacity is deposition time dependent. At the initial stage, the areal capacity increases with increasing the deposition time, but saturates when the deposition times exceeds 200 s. This is might be attributed to partial removal of the active materials on Cu foam (Figure 3.7b). Moreover, compared with CuO@Cu foam, all Ni(OH)2@CuO@Cu composites exhibited improved rate performance (Figure 3.12e). Electrochemical impedance spectroscopy (EIS) analysis in the frequency range of 100 kHz to 0.01 Hz was recorded to gain further insight into the electrochemical processes at different time constants (Figure 3.12f). For all the Ni(OH)2@CuO@Cu composites, the Nyquist plots revealed a very small depressed semicircle (high-to-medium frequency region) related to the electrochemical reaction process, and a straight line (low-frequency region) arising from ion diffusion in the electrode material. Among all Ni(OH)2@CuO@Cu composites, Ni(OH)2@CuO@Cu-150 displayed the smallest Rct (0.34 ohm cm -2 ) (Table 3.5) and the biggest slope for the straight line, suggesting favorable access for the charge transfer and ion diffusion. However, for CuO@Cu foam, there is no ion diffusion process in the 100 kHz to 0.01 Hz frequency range. Two charge transfer processes were observed instead, which take place at the CuO/electrolyte and Cu foam/CuO interfaces. Using the equivalent circuit in Figure 3.13b, Rs = 0.94 ohm cm -2 , Rct1 = 3.22 ohm cm -2 and Rct2=22.05 ohm cm -2 were determined. These values are larger than those recorded for Ni(OH)2@CuO@Cu foam composites (Table 3.5). Furthermore, to investigate the kinetics during the electrochemical reaction, the relationship between log(i) and log(v) for cathodic and anodic peak currents according to the CV curves of Ni(OH)2@CuO@Cu-90 (Figure 3.14a) and Ni(OH)2@CuO@Cu-150 (Figure 3.12b) was studied to determine the capacitive contribution: i = av b , where i stands for the current, v represents the scan rate. When the variable b=1, the electrochemical reaction is controlled by a capacitive process, while when the variable b=0.5, the electrochemical reaction is derived from a semi-infinite diffusion process (Faradaic mode). Relationship between logarithm peak current and logarithm scan rate of Ni(OH)2@CuO@Cu-90.

(c) Capacitive contribution ratio of Ni(OH)2@CuO@Cu-90 at different scan rates.

As displayed in Figure 3.14b, the b-values of Ni(OH)2@CuO@Cu-90 were calculated as 0.56 and 0.60 from the anode and cathode peak currents, respectively, and for Ni(OH)2@CuO@Cu-150 (Figure 3.15a), the b-values were found to be as 0.51 and 0.55 at different scan rates, signifying the contribution of both capacitive and Faradaic battery behaviors in the charge storage process.

The whole electrochemical process can be represented by the equation i = k1v + k2v 1/2 , and from the relationship of v 1/2 vs. 1/v 1/2 , we can determine the slope (k1) and the y-axis intercept (k2). From Figure 3.15b, we can see that the capacitive contribution ratio for Ni(OH)2@CuO@Cu-150 sample at 5 mV s -1 is about 24.1%, and further improves with the increasing the scan rate to attain 41.6% at 25 mV s -1 . For the Ni(OH)2@CuO@Cu-90 composite (Figure 3.14b), the capacitive contribution ratio at 2 mV s -1 is ~14.5%, and reaches 45.8% at 30 mV s -1 . All the results revealed the complex charge storage mechanism based on the combination of capacitive process and diffusion controlled faradaic characteristic with the latter contributing more on the performance.

capacities of the hybrid supercapacitor device were calculated and displayed in Figure 3.16e. Areal capacities of 587, 387, 318, 274, 209, 186 and 162 mC cm -2 were obtained respectively at 2, 3, 4, 5, 8, 10 and 15 mA cm -2 . Accordingly, the Ragone plot related to energy density and power density, calculated by Equations 1.12 and 1.13, is depicted in Figure 3.16f, achieving the largest areal energy density of 130.4 μWh cm -2 at a power density of 1.6 mW cm -2 . In comparison, this value is higher than that achieved by NiCo2S4//C (121 μWh cm -2 at 0.8 mW cm -2 ) [12], Mn3O4//Ni(OH)2

(12 μWh cm -2 at 0.04 mW cm -2 ) [13], Ni-Co LDH//ketjenblack (14.4 μWh cm -2 at 312.5 μW cm -2 )

[14], Ni(OH)2-AC-based hybrid capacitor (33.6 μWh cm -2 at 0.46 mW cm -2 ) [15], YP-80F//Ni1Co1-OH (125.6 μWh cm -2 at 62.16 mW cm -2 ) [16] and RGO@CoNi-LDH//RGO@AC (8.89 μWh cm -2 at 0.525 mW cm -2 ) [17] hybrid supercapacitor devices.

EIS analysis was further performed in the 100 kHz to 0.01 Hz frequency range to highlight the electrochemical processes of the hybrid supercapacitor device (Figure 3.16g), showing small internal resistance (Rs=0.166 ohm cm -2 ) and charge-transfer resistance (Rct=0.438 ohm cm -2 ).

Additionally, the mechanical characteristics and flexibility of the device were assessed by bending at various angles, as illustrated in the schematic diagram in the middle of Figure 3.16h.

We can see that there was no obvious shape change of the CV plots at various angles 0 o , 30 o , 60 o , 90 o and 150 o , signifying the good mechanical stability and enhanced flexibility of the hybrid supercapacitor device. In addition, the stability was tested for 10,000 charging-discharging cycles at 2 mA cm -2 . The device retained about 92% and 72% of the original capacity after 2,000 and 10,000 cycles, respectively, suggesting a good cycling stability. To explore the potential of the device for practical applications, two as-prepared hybrid Furthermore, taking benefit of the cleanest and earth abundant solar energy to power a hybrid supercapacitor device represents an appealing method. Therefore, solar cells were used to supply

Conclusion

In conclusion, Ni(OH)2@CuO@Cu foam composites were prepared by electrochemical deposition at different times (30, 50, 90, 150 and 200 s) of Ni(OH)2 on CuO@Cu foam. Among all the samples, Ni(OH)2@CuO@Cu-150 achieved the highest performance with an areal capacity of 7063.2 mC cm -2 at 20 mA cm -2 . Therefore, this composite was further investigated as a positive electrode to assemble a hybrid supercapacitor device. As an energy storage device, the asassembled hybrid supercapacitor attained a high areal energy density of 130.4 μWh cm -2 at a power density of 1.6 mW cm -2 . Two hybrid supercapacitor devices in series were able to power a red LED for 12 min 39 s and three devices in series successfully supplied electricity for a three-digit digital display for 1 min 28 s. Finally, an integrated system combining solar cells with the hybrid supercapacitor device for operating a homemade windmill device was constructed. After charging by solar cells, the hybrid supercapacitor device stored energy and further powered a homemade windmill device for 59 s. All of the findings suggested that Ni(OH)2@CuO@Cu foam composites have potential for hybrid supercapacitor devices application, and the combination of solar cells with the supercapacitor device described a promising future for sunlight-powered energy storage device applications.

foam (N-rGO/NF) as the negative electrode, a hybrid supercapacitor was assembled. It displayed good flexibility, cycling stability and high areal energy density of 130.4 μWh.cm -2 at a power density of 1.6 mW.cm -2 . Two hybrid supercapacitor devices were connected in series to successfully lighten up a red LED for 12min 39s, while three devices assembled in series were able to successfully power a three-digit digital display for 1min 28s. Interestingly, the hybrid supercapacitor device, charged by solar cells, further supplied a homemade windmill device for 59s, achieving sunlight-powered integration system.

Even though the high energy density of the hybrid supercapacitor devices were achieved and were successfully applied for practical applications, they are still far from real commercialization.

There is plenty of room to explore new electrode composites for supercapacitors. At the same time, in this thesis, we only investigated the electrochemical performance of the as-prepared composites in aqueous electrolytes, which limited the working potential window, further reducing the energy density. In future works, it will be of interest to investigate low-cost high-performance electrode materials, and explore their electrochemical performance in different electrolytes such as organic and ionic liquids electrolytes with the aim to construct hybrid supercapacitor devices with higher energy and power densities, which can potentially replace batteries for practical applications.

is to use monochromatic X-rays incident on the surface of the material to obtain X-ray diffraction in different directions and intensities. According to its diffraction direction and intensity, different diffraction patterns are obtained, depending on different phase components and crystal structure.

In this thesis, the crystal structure of the synthesized nanomaterials was investigated by powder X-ray diffraction analysis. XRD patterns were recorded using a Bruker D8 Advance equipped with 1D LynxeEye detector at a scanning rate of 3° min -1 and 2θ value ranging from 10 to 80° using Cu Kα (α= 1.54056Å) as the X-ray source and operates at a generator voltage of 40 kV and a current of 40 mA.

S1.2.4 X-ray photoelectron spectroscopy (XPS)

XPS can be used to analyze the elemental composition, content and chemical valence of the material surface. By fitting the individual high-resolution spectra of a certain element to the peak, the valence bond form and the content of different valence bonds can be analyzed.

In this thesis, XPS experiments were performed on a PHI 5000 VersaProbe-Scanning ESCA Microprobe (ULVAC-PHI, Japan/USA) instrument at a base pressure below 5×10 -9 mbar.

Monochromatic AlKɑ radiation was used and the X-ray beam, focused to a diameter of 100 μm, was scanned on a 250 × 250 µm surface, at an operating power of 25 W (15 kV). Photoelectron survey spectra were acquired using a hemispherical analyzer at pass energy of 117.4 eV with a 0.4 eV energy step. Core-level spectra were recorded at pass energy of 23.5 eV with a 0.1 eV energy step. All spectra were obtained with 90° between X-ray source and analyzer and with the use of low energy electrons and low energy argon ions for charge neutralization. After subtraction of the linear-type background, the core-level spectra were decomposed into their components with mixed Gaussian-Lorentzian (30:70) shape lines using the CasaXPS software. Quantification calculations were performed using sensitivity factors supplied by PHI.

  displayed good flexibility, cycling stability and high areal energy density of 130.4 μWh RÉSUMÉ Les supercondensateurs, en tant que dispositifs de stockage d'énergie, ont attiré une grande attention dans notre vie quotidienne pour combler l'écart entre les batteries et les condensateurs. Par conséquent, la préparation d'électrodes en matériaux composites à hautes performances pour les supercondensateurs joue un rôle vital dans leurs futurs développements technologiques. Dans ce contexte, les hydroxydes doubles lamellaires (HDL) et Ni(OH)2 ont été reconnus comme des électrodes prometteuses pour les supercondensateurs en raison de leurs réactions redox rapides et de leur comportement de type batterie. Le chapitre 1 donne un bref aperçu historique ainsi que les principes et le mécanisme de stockage d'énergie, les matériaux d'électrode des supercondensateurs, et les méthodes de caractérisation correspondantes. Le chapitre 2, après un bref rappel sur la synthèse et l'utilisation de matériaux à base d'HDL comme électrodes de supercondensateurs, on décrit la préparation d'HDL à base de Ni comme électrodes pour les supercondensateurs. Premièrement, des HDLs à base de NiFe sur mousse de Ni (NF) recouverte d'oxyde de graphène réduit (LDF NiFe/rGO/NF) ont été préparées par une méthode de dépôt électrochimique.

  Hydroxydes doubles lamellaires, Hydroxyde de nickel, procédé hydrothermal, dépôt électrochimique, supercondensateur, dispositifs de stockage d'énergie ACKNOWLEDGEMENTS This research work was performed in the Nanobiointerfaces group at the Institute of Electronics, Microelectronics and Nanotechnology (IEMN). I would like to express my deepest gratitude to all those people who have helped me during my doctoral study. First of all, I would like to give my most sincere thanks to my supervisors, Dr. Rabah Boukherroub and Prof. Sabine Szunerits. I highly appreciate your guidance, encouragement, continued advice and support in my Ph.D. study and research in past three years and thanks a lot for your patience to revise the manuscript of this dissertation. Thank you very much for leading me to the right direction and helping me acquiring precious research experience such as scientific thinking, laboratory techniques and publications writing. I also want to thank the jury members: Prof. Saïd Sadki, Prof. Sorin Melinte, Prof. Christine Taviot-Gueho and Dr. Christophe Lethien for their efforts devoted to my Ph.D. work. Thanks for your precious time and opinions on my work. Also, I would like to say thank you to Prof. Pascal Roussel and Prof. Ahmed Addad. I truly appreciate your help on the XRD and TEM measurements, which play vital roles on my work.

  Zhang, Yuanyuan Miao, Jing Zhang, Léa Rosselle, Anna Voronova, Vladyslav Mishyn, Madjid Ifires, Adel Khane...It's my pleasure to work with all of you! I also want to thank my friends here, Ningkui Wang, Ce Zheng, Siyuan Wang, Hong Pan, Shuie Sun, Wenting Zhu, Yue Shi, Lei Shi, Longfei Liao…for their friendship. I would like to say thank you to my best friends Guoxi Luo, Mengyu Yang and Xueyan Wang in China for their big support. Specially, I want to thank the China Scholarship Council (CSC) for supporting me to study in France.

Finally, I would

  like to thank my parents, brother and sister. Thank you very much for your unrequited dedication for no return. Thank you very much for supporting me to pursue my dream.

  ............................................................................ 3.4 Conclusion ................................................................................................ 3.5 References ................................................................................................ Conclusion and Perspectives...................................................................................... S1 APPENDIX ........................................................................................................

  4 X-ray photoelectron spectroscopy (XPS) ....................................... S1.2.5 Raman spectroscopy ....................................................................... PUBLICATIONS .......................................................................................................

Figure 1 . 1 :

 11 Figure 1.1: Ragone plot illustrating the performances of specific power vs. specific

Figure 1 . 3 :

 13 Figure 1.3: Historic timeline for the development of supercapacitors.

( 3 .

 3 4~4.0 V), owing to the greater decomposition voltage of the non-aqueous electrolyte as compared to aqueous electrolyte. Consequently, they can accommodate higher charge densities and provide greater specific energy storage since the storable energy increases with the square of the voltage attainable on charge. In order to increase the specific capacitance of SC, in 1971, Trasatti et al. discovered a new type of electrochemical capacitor based on RuO2 film which involved the Faraday process, called pseudocapacitor [20]. The discovery of pseudocapacitors opened a new approach to enhance the charge storage capacity of electrochemical capacitors. During 1975~1980, B. E. Conway has extensively studied RuO2 pseudocapacitors. In 1989, the U.S. Department of Energy (DOE) supported a long-term research, aiming to develop supercapacitors with high energy density for electric drivetrains, which is a part of its Electric and Hybrid Vehicle Program. After that, Maxwell Technologies. Inc., the world's leading supercapacitor manufacturing company, signed a contract with the DOE to develop high-performance supercapacitors, further working with batteries or fuel cells in electric or hybrid vehicles to collect braking energy and release electrical energy to accelerate driving. Since then, supercapacitor companies from around the world, such as Nesscap (Korea), ELTON (Russia), Nippon Chemicon (Japan) and CAP-XX (Australia), have been developing and delivering different types of supercapacitors such as EDLCs, pseudocapacitors, and asymmetric supercapacitors for different applications. Since 2000, with the growing demand for high-power, high-reliability and safe energy storage equipment, the research related to supercapacitors has continued to increase significantly. At the same time, the rapid development of nanoscience and advanced characterization technologies promoted the development of EDLC and pseudocapacitor systems, which obviously need further research based on the charge storage mechanism.

A

  supercapacitor (SC) consists of two electrodes, electrically isolated by a separator in the electrolyte, as is illustrated in Figure 1.4. The electrode material is a carrier for the SC to store charges, so the choice of electrode materials directly determines the performance of the supercapacitor. It is necessary to investigate the working mechanism of different electrode materials.

Figure 1 . 4 :

 14 Figure 1.4: Schematic diagram of a supercapacitor structure [21].

Figure 1 . 5 :

 15 Figure 1.5: Classification of supercapacitors.

2 )

 2 higher energy density and capacitance than EDLCs. The pseudocapacitance (C) is acquired by the derivative of charge acceptance (ΔQ) and changing potential (ΔV) Transition metal oxides/hydroxides and conductive polymers are greatly applied as pseudocapacitive electrode materials [51-53]. Based on various materials, the faradaic mechanisms can typically be classified into three types, as shown in Figure 1.7 [54]: (a) underpotential deposition, (b) redox pseudocapacitance, and (c) intercalation pseudocapacitance.

Figure 1 . 7 :

 17 Figure 1.7: Different types of reversible redox mechanisms that give rise to

Figure 1 . 8 :

 18 Figure 1.8: (a) SEM image of NAC-400, TEM images of (b) NAC-0, (c) NAC-400,

Figure 1 . 9 :

 19 Figure 1.9: (a, b, c, d) Electrode fabrication and characterization [98]. (a) SWNT

Figure 1 . 10 :

 110 Figure 1.10: (a) SEM image of CMG particle surface, (b) TEM image showing

Figure 1 . 11 :

 111 Figure 1.11: Crystal structures of MnO2 polymorphs (Mn: magenta and O: red) [123].

Figure 1 . 12 :

 112 Figure 1.12: SEM images of vacant AAO template (a), as-prepared Co3O4 nanotubes

Figure 1 . 13 :

 113 Figure 1.13: The idealized crystal structure of α-Ni(OH)2 represented by (a) unit cell

Figure 1 . 14 : 1 . 3 . 2 . 4 1 . 15 ,

 1141324115 Figure 1.14: The crystal structure of β-Ni(OH)2 represented by (a) unit cell projection

Figure 1 . 15 :Figure 1 . 16 ,

 115116 Figure 1.15: The idealized structure of carbonate-intercalated LDHs with different

Figure 1 . 16 :

 116 Figure 1.16: SEM micrographs of Co1-xNix LDHs thin films: A1 (1.0:0.0), A2

  practical and widely employed to investigate the electrochemical performance. Both of them contain cyclic voltammetry (CV),galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) tests, which are the most commonly used techniques to evaluate the electrochemical properties of SCs. The above test modes can measure three basic parameters: voltage, current and time, and then can derive other performance indicators, such as operating voltage, specific/areal capacitance/capacity, equivalent series resistance, and subsequent energy density and power density.

)

  provide convenient means to categorize the mode of charge storage for an electrode material. For EDLCs and redox pseudocapacitors, the CV curves typically exhibit a rectangular shape, which usually show a potential-independent current (capacitive feature) (Figure 1.17a, b). The corresponding GCD curves show a linear relationship between V and t (Figure 1.17c). However, the CV curves of batteries ordinarily show prominent and widely separated redox peaks involved in charge storage (Figure 1.17g, h), and V versus t for the discharge is profoundly nonlinear and characterized by plateaus of nearly constant potential, corresponding to the potentials at which the Faradaic reduction or oxidation of the metal centers is occurring (Figure 1.17i). An intermediate behavior between these two extremes signifies the presence of pseudocapacitance (Figure 1.17d-f).

Figure 1 . 17 :

 117 Figure 1.17: (a, b, d, e, g, h) Schematic cyclic voltammograms and (c, f, i) the

Figure 1 . 18 :

 118 Figure 1.18: Capacitive (red) and diffusion-controlled (grey) contributions to charge

Figure 1 .Figure 1 . 20 :

 1120 Figure 1.20 displays a typical EIS and the corresponding equivalent circuit [180].

chapter 1 ,

 1 a brief historic overview, basic background, principles and mechanism of energy storage, electrode materials of supercapacitors and the corresponding characterization methods were discussed, revealing a deep insight into the foundation of supercapacitors. In Chapter 2, NiFe LDHs on Ni foam coated with reduced graphene oxide (NiFe LDHs/rGO/NF) was prepared by electrochemical deposition method. The effect of different Ni/Fe ratio on the performance of the composites was explored. Combining with mesoporous carbon (MC) coated on NF as the negative electrode, a hybrid supercapacitor device was constructed and fully characterized. Moreover, NiAl layered double hydroxides coated on carbon spheres (CS) supported by Ni foam (NiAl LDHs@CS/NF) nanocomposite was synthesized via a facile hydrothermal method. The performance of the prepared materials as binder-free electrodes in supercapacitors was assessed. Moreover, a hybrid supercapacitor device comprising NiAl LDHs@CS/NF as the positive electrode and N-doped reduced graphene/NF as the negative electrode was successfully fabricated. Furthermore, taking advantage of plentiful oxidation states of Cr, composite of NiMnCr LDHs-carbon spheres modified Ni foam (NiMnCr

  A m-represents the interlayer anion (CO3 2-, NO3 -, OH -), x = M 3+ /(M 2+ + M 3+ ) is the surface charge and is generally in the range of 0.2~0.33, and n is the number of crystal water molecules[1, 2].

Figure 2 . 1 :

 21 Figure 2.1: Schematic illustration of LDH structures [3].

  NiFe LDHs/NF NiFe LDHs were electrochemically deposited onto the surface of nickel foam (NF) using different molar ratios: 5:0, 4:1, 3:2, 1:1, 2:3, 1:4, 0:5 of Ni(NO3)2•6H2O and Fe(NO3)3•9H2O; the overall molar concentration was 50 mM. Nickel foam (NF) acts as the working electrode; the counter and the reference electrodes are platinum foil and saturated calomel electrode (SCE), respectively. Typically, the potentiostatic deposition was performed at -1.2V vs. SCE for 10 s to form NiFe LDHs/NF. Different NiFe LDHs/NF electrodes were successfully prepared and samples with different Ni(NO3)2•6H2O and Fe(NO3)3•9H2O molar ratios of 5:0, 4:1, 3:2, 1:1, 2:3, 1:4, 0:5 were named as S0, S1, S2, S3, S4, S5, S6, respectively. All the synthesized samples were washed with water and ethanol, and further dried at 60 o C for 6 h.

Figure 2 . 1 .Figure 2 . 1 :

 2121 Figure 2.1: Synthesis of (a) NiFe LDHs/NF and (b) NiFe LDHs/rGO/NF using

Figure 2 . 2 : 3 ( 2

 2232 Figure 2.2: SEM images of the samples deposited using Ni(NO3)2•6H2O and

  3 and 872.9 eV due to Ni2p3/2 and Ni2p1/2 of Ni(OH)2, respectively, along with two satellite peaks (Figure2.3b), while the Ni2p in NiFe LDHs/rGO/NF (9.4 at%) is deconvoluted in two bands at 856.6 and 874.4 eV with a slight shift resulting from the interaction between NiFe LDHs and rGO[68, 69]. The high-resolution Fe2p spectrum reflects that Fe species are mostly in their Fe 3+ oxidation state (Figure2.3c and 2.4c), with two prominent peaks at 711.5 (Fe2p3/2) and 724.8 eV (Fe2p1/2) along with two satellite peaks (716.3 and 732.9 eV) for NiFe LDHs/NF (4.0 at%), and 712.5 eV (Fe2p3/2) and 725.2 eV (Fe2p1/2) along with two satellite peaks (718.6 and 733.9 eV) for NiFe LDHs/rGO/NF (3.2 at%)[70][71][72].

Figure 2 . 3 :

 23 Figure 2.3: XPS survey spectrum (a) and high-resolution spectra of (b) Ni2p, (c) Fe2p,

Figure 2 . 4 :

 24 Figure 2.4: XPS analysis of NiFe LDH/rGO/NF: (a) Survey spectrum, and high

Figure 2 . 5

 25 Figure 2.5 depicts the Raman spectrum of rGO/NF composite. It comprises two peaks

Figure 2 . 5 :

 25 Figure 2.5: Raman spectra of (a) rGO/NF and (b) NiFe LDH/rGO/NF.

Figure 2 . 1 .

 21 Figure 2.6a depicts the CV curves of NiFe LDHs/NF in the potential range from 0

Figure 2 . 6 :

 26 Figure 2.6: Electrochemical characteristics of the electrodes upon immersion in 2 M

  supercapacitors. In order to improve the performance of the electrodes, NiFe LDHs/rGO/NF (S7) was prepared using a two-step electrodeposition according to the process in Figure 2.1b. At the same time, the electrochemical properties of the samples were assessed using cyclic voltammetry. Compared with NiFe LDH 1:4 (S5), NiFe LDHs/rGO/NF 20 s (S8) and NiFe LDHs/rGO/NF 30 s (S9), NiFe LDHs/rGO/NF (S7) possessed larger currents density with obviously a pair of characteristic redox peaks in the CV, as shown in Figure 2.7 and Figure 2.8a.

Figure 2 . 7 :

 27 Figure 2.7: CV curves of NiFe LDHs/rGO/NF 10 s (S7), NiFe LDHs/rGO/NF 20 s

Figure 2 . 8 :

 28 Figure 2.8: Electrochemical performance of the electrodes in 2 M KOH aqueous

Figure 2 .

 2 8f. After 2,000 cycles, 47% and 64.7% of the original capacitance are retained for NiFe LDH/NF and NiFe LDHs/rGO/NF electrodes, respectively.

Figure 2 .Figure 2 .Figure 2 .

 222 Figure 2.9a displays the CV curve of mesoporous carbon (MC) measured at a scan

Figure 2 .

 2 Figure 2.9h and Table 2.10. The results demonstrate that the hybrid supercapacitor

Figure 2 . 9 :

 29 Figure 2.9: (a) CV curve of mesoporous carbon measured at a scan rate of 100 mV s -1 .

2 . 3 . 1 . 3 2 . 3 . 1 . 4

 23132314 mL) under continuous stirring to form a homogeneous solution. Then, a piece of nickel foam (3 cm×2 cm×0.5 cm) was placed in the homogeneous solution and transferred into a 50 mL Teflon-lined stainless-steel autoclave for hydrothermal reaction at 190 o C for 4 h. After being cooled naturally to room temperature, the nickel foam decorated with carbon spheres (CS/NF) was washed with water and ethanol for several times, and dried at 60 o C for 24 h. Preparation of NiCr LDHs/NF, NiFe LDHs/NF and NiAl LDHs/NF Typically, a homogeneous solution of Ni(NO3)26H2O (87 mg), Cr(NO3)39H2O (120 mg), and CO(NH2)2 (180 mg) in MQ water (35 mL) was prepared through magnetic stirring. NF was dipped into the solution, and the mixture was transferred into an autoclave (50 mL) and heated for 12 h at 120 o C. The resulting NiCr LDHs/NF was rinsed copiously with water, ethanol, and dried in an oven for 24 h at 60 o C. Al(NO3)3•9H2O (113 mg) and Fe(NO3)3•9H2O (121 mg) were used to replace Cr(NO3)39H2O (120 mg) for the synthesis of NiAl LDHs/NF and NiFe LDHs/NF, respectively. Preparation of NiCr LDHs@CS/NF, NiFe LDHs@CS/NF and NiAl LDHs@CS/NF Typically, a mixture of Ni(NO3)24H2O (87 mg), Cr(NO3)39H2O (120 mg) and CO(NH2)2 (180 mg) in 30 mL MQ water was stirred till formation of a homogeneous solution. The CS/NF was dipped in the above solution, and the mixture was transferred into an autoclave (50 mL) and heated for 12 h at 120 o C. The resulting NiCr LDHs@CS/NF was rinsed copiously with water, ethanol, and dried for 24 h at 60 o C. Al(NO3)3•9H2O (113 mg) and Fe(NO3)3•9H2O (121 mg) were used to replace Cr(NO3)39H2O (120 mg) for the synthesis of NiFe LDHs@CS/NF and NiAl

  Herein, N-doped reduced graphene oxide coated nickel foam (N-rGO/NF) was first synthesized through hydrothermal reduction of GO in presence of hydrazine monohydrate reducing and doping agent, and applied as a negative electrode with the aim to widen the potential window of a hybrid supercapacitor. The morphology of N-rGO was assessed by scanning electron microscopy (SEM), Figure2.10a. A porous skeleton consisting of overlapped N-rGO nanosheets could be observed; this morphology is favorable for electrolyte diffusion.

Figure 2 . 1 .Figure 2 . 10 :

 21210 Figure 2.10: (a) SEM image, and (b) Raman spectrum of N-rGO.

Figure 2 . 11 :

 211 Figure 2.11: XPS analysis of N-rGO: (a) Survey spectrum and core level spectra of

Figure 2 . 2 .

 22 Figure 2.12a depicts the galvanostatic charge-discharge (GCD) plots from -1 to

Figure 2 . 12 :

 212 Figure 2.12: The electrochemical behavior of N-rGO in 2 M KOH aqueous solution.

Figure 2 . 13 :

 213 Figure 2.13: (a) XRD patterns of NiAl LDHs/NF and NiAl LDHs@CS/NF, (b)

Figure 2 . 14 :

 214 Figure 2.14: (a) XRD patterns of NiFe LDHs/NF, NiFe LDHs@CS/NF, NiCr

Figure 2 .

 2 15a, b and g, h display the SEM images of NiFe LDHs/NF and NiFe LDHs@CS/NF. Interestingly, NiFe LDHs/NF consisted of wool-like nanowires intertwined with each other (Figure 2.15a, b), whereas NiFe LDHs@CS/NF was composed of abundant nanosheets evenly attached to the surface of the carbon spheres (Figure 2.15g, h).

Figure 2 . 15 :

 215 Figure 2.15: SEM images of (a, b) NiFe LDHs/NF, (c, d) NiCr LDHs/NF and (e, f)

Figure 2 . 16 :Figure 2 . 17 :

 216217 Figure 2.16: SEM images of CS/NF at different magnifications.

Figure 2 .

 2 Figure 2.17b and Figure 2.18c1 present the high -resolution spectrum of Ni2p in

Figure 2 .

 2 Figure 2.18a1 displays the core level spectrum of Ni2p (13.7 at%) in NiFe

Figure 2 .

 2 Figure 2.18a2, showing two main peaks at 711.4 and 725.1 eV which correspond

Figure 2 .

 2 Figure 2.17c depicts the high resolution spectrum of Al2p (37.8 at%) in NiAl

Figure 2 . 18 :

 218 Figure 2.18: XPS analysis: (a) Survey and core level spectra of (a1) Ni2p and (a2)

Figure 2 . 19 :

 219 Figure 2.19: High resolution spectra of the C1s of (a) NiFe LDHs@CS/NF, (b) NiCr

Figure 2 .

 2 Figure 2.17d, 2.19a,b, c display the core level spectra of the C1s of NiAl LDHs/NF,

Figure 2 .f

 2 17e, 2.19d, e and exhibit the O1s high-resolution spectra of NiAl LDHs/NF, NiFe LDHs@CS/NF, NiCr LDHs@CS/NF and NiAl LDHs@CS/NF. All of them comprise two peaks ascribed to M-O-M (~531.2 eV) and M-OH (~531.9 eV) bonds[92]. All the results indicate the successful formation of LDHs.The electrochemical behavior of all samples was evaluated in a classical 3-electrode cell in KOH (2 M) aqueous solution in which the as-obtained samples (1 cm×1 cm) were applied as the working electrode, a platinum foil and Hg/HgO electrodes acted as the counter and reference electrodes, respectively. From Figure 2.20a, we can see obvious redox active peaks in all the CV plots in the 0 ~ 0.7 V potential range, corresponding to the redox process shown in Equation 2.1. Ni(OH)2 + OH-⇋ NiOOH + H2O + e - In comparison with the CV plots of Ni(OH)2/NF and Ni(OH)2@CS/NF (Figure 2.21a), the redox peaks separation became more important after LDH formation, which could be due to the synergistic effects of Ni with Fe, Cr, and Al elements. Even with the assistance of carbon spheres (CS), Ni(OH)2@CS/NF displays poor electrochemical performance (Figure 2.21b

Figure 2 . 20 :

 220 Figure 2.20: The electrochemical characteristics of the electrodes measured in a

  frequency range. (f) Q-t 1/2 plot in 0.1 mM K3[Fe(CN)6]/0.1 M KCl aqueous solution.

Figure 2 . 21 :

 221 Figure 2.21: The electrochemical performance of Ni(OH)2/NF and Ni(OH)2@CS/NF

Figure 2 . 22 :

 222 Figure 2.22: (a) CV curves of N-rGO/NF and NiAl LDHs@CS/NF performed at 100

and Equation 1 . 11 . 2 , 2 (

 11122 Rs (the intersection of the plot with the X coordinate axis) and Rct (the semicircle) of all the electrodes, obtained from the Nyquist plots, were 0.38 and 4.30 ohm cm -2 , respectively. From the GCD plots recorded at various current densities (Figure2.22e) the areal capacity of the hybrid supercapacitor cell was calculated as 137.2, 123.1, 111.4, 100.4, 91.7 and 85.8 mC cm -2 respectively at 0.5, 1, 2, 3, 4 and 5 mA cm -2 (Figure 2.22f). Consequently, according to Equations 1.12 and 1.13, the highest energy density achieved was 31 μWh cm -2 at a power density of 0.41 mW cm -Figure 2.22g).

Figure 2 .

 2 23a); it consists of an engine (1.5-9 V), a windmill and one as-fabricated supercapacitor cell. From Figure 2.23a1-4, we can see that the windmill rotated quickly, and with the consumption of energy, it stopped completely after 21 s. Furthermore, when two hybrid supercapacitor cells were connected in parallel (insert of Figure 2.23b), the charge-discharge cell voltage window can reach up to 3.2

Figure 2 . 23 :

 223 Figure 2.23: (a) Schematic of the hybrid supercapacitor cell operating a homemade

  /NF (1 cm×1 cm) was applied as a positive electrode, (1.5 cm×1.5 cm) as a negative electrode, and one piece of filter paper immersed into KOH (2 M) aqueous solution for 30 min as a separator (Figure 2.24a). The electrochemical behavior of the device was assessed in a 2-electrode system, and the CV plots acquired at various cell voltage windows (scan rate = 100 mV s -1 ) are displayed in Figure 2.24b. The results

Furthermore, to test

  the flexibility and mechanical properties of the device and meet wearable requirements, the hybrid supercapacitor device in the middle of Figure 2.24e was bent at different angles (150, 90 and 60 o ); the CV curves, acquired at different angles, are almost overlapping together, indicating excellent flexibility and mechanical

Figure 2 . 24 :

 224 Figure 2.24: (a) Schematic of the hybrid supercapacitor. CV plots measured at (b)

Figure 2 . 25 : 2 V to supply 3 Figure 2 . 26 :

 22523226 Figure 2.25: (a) Schematic diagram of the hybrid supercapacitor device for operating

Figure 2 .

 2 Figure 2.27a. Carbon spheres (CS) were first decorated on NF by a simple

  hydrolysis of NH4F and decomposition of CO(NH2)2, forming the NiMnCr LDHs on the NF substrate. It is noteworthy to mention that Ni 2+ required for NiMnCr LDH formation originates from the Ni foam dissolution under our experimental conditions. Indeed, during the hydrothermal process, NH4F dissolves nickel oxides on the Ni foam to release Ni 2+ in the solution. As illustrated in Figure 2.27b, the existence of carbon spheres (CSs) layer on the NF surface could act as the bridge between NiMnCr LDHs and NF, which could decrease the length of charge transfer and facilitate the charge transfer, further promoting the electrochemical kinetics. For comparison, a control sample consisting of NiMnCr LDHs/NF was prepared under the same experimental conditions, but in the absence of CS.

Figure 2 . 27 :

 227 Figure 2.27: Schematic illustrations of (a) the synthesis of NiMnCr LDHs@CS/NF

Figure 2 .

 2 Figure 2.28a displays the XRD patterns of as-obtained materials. The patterns

Figure 2 .

 2 Figure 2.28:(a) XRD patterns of NiMnCr LDHs/NF and NiMnCr LDHs@CS/NF,

Figure 2 . 29 :

 229 Figure 2.29: XRD pattern of FeOOH nanorods.

Figure 2 . 30 :

 230 Figure 2.30: SEM images of (a, b) CS/NF, (c, d) NiMnCr LDHs/NF and (e, f)

Figure 2 . 31 :

 231 Figure 2.31: (a) Cross-section SEM images of NiMnCr LDHs@CS/NF, (b) the

Figure 2 . 32 :

 232 Figure 2.32: (a) TEM and (b) high-resolution TEM images of NiMnCr

Figure 2 . 34 )

 234 photoelectron spectroscopy (XPS) was acquired to evaluate the elemental composition of NiMnCr LDHs@CS/NF (Figure2.33) and NiMnCr LDHs/NF (composites. The wide scan XPS spectrum revealed that the electrode material consists of Ni2p, Mn2p, Cr2p, C1s and O1s elements, in agreement with the elemental composition of the material. The corresponding elemental compositions are illustrated in Table2.15.

Figure 2 .

 2 Figure 2.33e displays the C1s core level spectrum, which can be curve-fitted with two

Figure 2 . 33 :Figure 2 . 34 :

 233234 Figure 2.33: XPS analysis of NiMnCr LDHs@CS/NF: (a) Survey spectrum, and core

Figure 2 . 35 :Furthermore, Figure 2 .Figure 2 . 36

 2352236 Figure 2.35: CV curves of (a) NiCr LDHs/NF, NiMn LDHs/NF, NiMnCr LDHs/NF

Figure 2 .

 2 Figure 2.36c indicates that in the presence of CS, NiMnCr LDHs@CS/NF

Figure 2 .

 2 Figure 2.36e and Table 2.16 summarize the specific capacity values of the various

Figure 2 . 37 :

 237 Figure 2.37: Cycling performance of NiMnCr LDHs@CS/NF and NiMnCr LDHs/NF

Figure 2 .

 2 Figure 2.38 depicts the SEM images and the corresponding EDS elemental

Figure 2 . 38 :

 238 Figure 2.38: SEM images of (a) NiMnCr LDHs/NF and (b) NiMnCr LDHs@CS/NF

Figure 2 . 37 .

 237 The existence of CS could help release the deformation and stress during the charging/discharging cycles to immobilize the NiMnCr LDHs layer, further improving the mechanical stability[103].

Figure 2 . 39 :

 239 Figure 2.39: XPS analysis of NiMnCr LDHs/NF after 10,000 cycles test: (a) Survey

Figure 2 . 40 :

 240 Figure 2.40: XPS analysis of NiMnCr LDHs@CS/NF after 10,000 cycles test: (a)

Finally, a hybridFigure 2 . 41 :

 241 Figure 2.41: SEM images of FeOOH nanorods.

Figure 2 .

 2 42 depicts the CV curves of FeOOH/NF electrode between -1 V and -0.5 V at various scan rates, which implies its potential use as a negative electrode. Using Equation 1.11.2, the specific capacity of FeOOH/NF was calculated to be 126, 88.8, 72.8, 59.9, 51.6 and 49.1 C g -1 respectively at 1, 2, 3, 5, 10 and 15 A g -1 , as illustrated in Figure 2.42b and c, which proves that FeOOH could be a promising negative material.

Figure 2 . 42 :

 242 Figure 2.42: The electrochemical performance of the FeOOH electrode measured in 2

Figure 2 .

 2 Figure 2.43c displays the GCD curves of the hybrid supercapacitor device at

Figure 2 . 43e (

 243e Rct, Rs, Cdl, Wo, Cl and Rl are respectively the charge-transfer resistance, internal resistance, double-layer capacitance, Warburg element, limit capacitance and limit resistance). From the Nyquist plot, one clearly sees a semicircle at the high-frequency range associated with the charge transfer resistance, and a straight line at the low-supercapacitor. (g) Cycling performance of NiMnCr LDHs@CS/NF//FeOOH hybrid supercapacitor at a constant current density of 5 A g -1 .

Figure 2 .

 2 43g depicts the charging/discharging cycles at 5 A g -1 . The hybrid supercapacitor retained 83% of its initial specific capacity after 10,000 cycles, signifying a good stability of the energy storage device.To highlight the practical use of the as-fabricated supercapacitor device, it was applied for operating a home-designed windmill device (Figure2.44a and c). The device comprises an engine (1.5-9 V), windmill and one as-fabricated supercapacitor device. The windmill can be rotated for about 25 s using the home-made device upon charging the supercapacitor at 1.7 V (Figure2.44b). Additionally, as is shown in Figure2.44d, two as-fabricated hybrid supercapacitor cells (SC) in series whose working cell voltage window can reach 3.4 V (Figure2.44b) were used to supply electricity to a red LED and a yellow LED in parallel; the red LED was lighted for ~343 s and the yellow one for ~111 s at the same time.

Figure 2 . 44 :

 244 Figure 2.44: (a) Schematic diagram of the packed hybrid supercapacitor for windmill

Figure 3 . 1 :Figure 3 . 2 :

 3132 Figure 3.1: Illustration of the synthesis of Ni(OH)2@CuO@Cu foam electrode.

Figure 3 . 3 :

 33 Figure 3.3: CV curves of Cu(OH)2@Cu and CuO@Cu measured at a scan rate of 20 mV s -1 .

Figure 3 . 4 :

 34 Figure 3.4: (a) XRD patterns of the prepared samples, (b) magnified patterns in the 30°-42 o and 55°-70° 2 ranges.

Figure 3 .

 3 5a and b displays the SEM images of CuO@Cu foam where 1dimensinal (1D) CuO nanowires decorated with nanoflowers were observed. This structure was expected to be beneficial for electron and ion transfer.

Figure 3 . 5 :

 35 Figure 3.5: SEM images of (a,b) CuO@Cu foam, (c) Ni(OH)2@CuO@Cu-30, (d)

Figure 3 . 6 : 5 μmFigure 3 .

 3653 Figure 3.6: EDS elemental mapping images of CuO@Cu foam.5 μm

Figure 3 . 12 :

 312 Figure 3.12: The electrochemical behavior of the different electrodes measured in KOH (2 M) aqueous solution. (a) CV plots acquired at 20 mV s -1 . (b) CV curves of Ni(OH)2@CuO@Cu-150 recorded at different scan rates. (c) GCD plots of the different electrodes acquired at 20 mA cm -2 .

Figure 3 .

 3 Figure 3.12d displays the areal capacity values of Ni(OH)2@CuO@Cu-150, determined using

Figure 3 .

 3 13a illustrates the corresponding equivalent circuit, where Rs is the internal resistance, Rct represents the charge-transfer resistance related to the depressed semicircle in the curves, Q is the pseudocapacitance, Wo is the ion transport resistance.

Figure 3 . 13 :

 313 Figure 3.13: The equivalent circuit of (a) Ni(OH)2@CuO@Cu foam and (b) CuO@Cu composites.

Figure 3 . 14 :

 314 Figure 3.14: (a) CV curves of Ni(OH)2@CuO@Cu-90 measured at different scan rates. (b)

Figure 3 . 16 :

 316 Figure 3.16: (a) Schematic of the packed hybrid supercapacitor device. CV plots measured at 100 mV s -1 at (b) different cell voltage windows, and (c) different scan rates between 0 and 1.6 V. (d)

aFigure 3 . 17 .

 317 Figure 3.17. (a) Schematic diagram of two hybrid supercapacitor devices in series for lighting of

Figure 3 . 18 .

 318 Figure 3.18. Schematic diagram and equivalent circuit of integrated system combining solar

TABLE OF CONTENTS

 OF ABSTRACT ................................................................................................................... CHAPTER 3 .............................................................................................................. 3.1 Introduction .............................................................................................. 3.2 Experiment section ................................................................................... 3.2.1 Synthesis of CuO@Cu foam .......................................................... 3.2.2 Synthesis of Ni(OH)2@CuO@Cu foam ........................................ 3.2.3 Assembly of a hybrid supercapacitor device ................................. 3.2.4 Construction of a sunlight-powered energy storage system .......... 3.3 Results and discussion ..

  ... S1.1 Chemicals ..................................................................................................

S1.2 Characterization techniques of electrode materials ................................... S1.2.1 Scanning Electron Microscopy (SEM) ........................................... S1.2.2 Transmission Electron Microscopy (TEM) .................................... S1.2.3 X-ray diffraction (XRD) ................................................................. S1.2.

Table 2 . 1 .

 21 Representative LDH-based materials with hydrothermal/solvothermal synthesis methods and their applications.

	Materials	Synthesis method	Applications	Ref.
		Hydrothermal
	Ni-Co LDH		Supercapacitors	[7]
		(180 o C, 24 h)
		Hydrothermal
	NiFe-LDH nanolates		Oxygen evolution reaction	[8]
		(120 o C, 12 h)
		Hydrothermal
	rGO/NiAl-LDH		Adsorbents (uranium (VI))	[9]
		(120 o C, 24 h)
		Hydrothermal	Supercapacitors/ Hydrogen
	NiCo-LDH/Co9S8 hybrid			[10]
		(120 o C, 2 h)	evolution reaction
		Hydrothermal
	NiV LDH		Supercapacitors	[11]
		(120 o C, 12 h)
		Hydrothermal
	CoAl-LDH/NF		Supercapacitors	[12]
		(120 o C, 6 h)
		Hydrothermal
	Mg-Al LDH		Adsorbents (methyl orange)	[13]
		(200 o C, 20 h)
		Hydrothermal
	Fe-Cu-LDH/biochar		Catalysts	[14]
		(90 o C, 24 h)
		Hydrothermal
	Zn/Al LDHs		/	[15]
		(75 o C, 24 h)
	NiCoAl LDH/CNT	Solvothermal	NiZn battery	[16]

Table 2 . 2 .

 22 Representative LDH-based materials obtained through electrodeposition method and their applications.

	Materials	Conditions	Applications	Ref.

Table 2 .

 2

	3 summarizes some common LDHs-

2.1.1.4 Other methods

There are some other common methods to prepare LDHs such as templating methods and urea synthesis. For example, Liu et al. used a metal-organic framework as a sacrificial templating precursor to derive CoMn LDHs from different manganese solutions [36]. Jaiswal et al. synthesized CoBi layered double hydroxides (LDHs) via urea hydrolysis method, in which the use of urea instead of NaOH as a precipitating agent is obviously advantageous because during the precipitation reaction, lower degree of super-saturation is achieved. Hydrolysis of urea proceeds in two steps: the flexibility of incorporating mixed valence transition metal ions into the LDH structure in different compositions, LDHs have drawn extensive research attention in the field of supercapacitors. As electrode materials, LDHs with a lamellar structure provide a large specific surface area, a high number of active sites, and a shortened diffusion distance that is beneficial to ions to transfer from the electrolyte to the surface of the active material quickly due to the ultrathin thickness of the nanosheets. Besides, tunable metal cation compositions in the LDH host layers, such as Ni 2+ , Co 2+ , or Mn 2+ enable multiple redox states to greatly improve the capacitive performance. From the view of composition of LDHs, we divided them into Ni-based LDHs, Co-based LDHs and others.

Table 2 . 4 .

 24 Representative Ni-based LDHs materials and their electrochemical supercapacitor performance.

	Materials	Specific capacity	Cycle stability	Electrolyte Ref.
	Ni-V LDH/carbon cloth	1226 C g -1 / 1 A g -1	75% (2000 cycles)	2 M KOH	[37]
	Ni/Al LDH/graphene	286 C cm -3 / 1 mA cm -2	88.1% (1000 cycles)	6 M KOH	[38]
	NiMn-LDH/Ni foam (NF)	680 C g -1 / 2.5 A g -1	92.8% (3000 cycles)	1 M KOH	[39]

Table 2 .

 2 4, we can see that as battery-like pseudocapacitive electrodes, Nibased LDHs exhibit a great specific capacity. For example, Liu et al. prepared 2D

	5 A g -1	91% (1000 cycles)	3 M KOH	[44]

hierarchical NiO/NiMn-LDH nanosheet arrays for supercapacitors with a high specific capacitance of 468.5 C g -1 at 0.5 A g -1

[44]

. Bandyopadhyay et al. synthesized graphitic carbon nitride modified graphene/Ni-Al LDH (fgC3N4-rGO/NiAl LDH) for supercapacitors, generating a specific capacity value of 595 C g -1 at 1 A g -1

[47]

. All of these results reveal the potential application of Ni-based LDHs electrode materials.

2.1.2.2 Co-based LDHs

Besides Ni-based LDHs, Co-based LDHs also received extensive attention as electrode candidates for supercapacitors due to plentiful oxidation states of Co. The reactions for Co-based LDHs in alkaline electrolyte can be illustrated by Equations 2.2 Some representative Co-LDHs-based hybrids and their electrochemical supercapacitor performance are summarized in Table 2.5.

Table 2 . 5 .

 25 Representative Co-based LDHs materials and their electrochemical supercapacitor performance. There are also many LDHs consisting of other transition metal ions besides Ni and Co ions, which are investigated as electrodes for supercapacitors. Some representative examples and their electrochemical supercapacitor performance are summarized in

	Materials	Specific capacity	Cycle stability	Electrolyte	Ref.
	GO@CoAl LDH	604 C g -1 at 1 A g -1	95% (5000 cycles)	M KOH	[49]
	CoCr-LDHs	182.5 C g -1 at 0.5 A g -1	80% (1000 cycles)	M KOH	[50]
	CoMn LDH/NF	915.85 C g -1 at 1 A g -1	93.2% (3000 cycles)	M LiOH	[51]
	CoAlNi LDH	669.8 C g -1 at 1 A g -1	82.2% (3000 cycles)	M KOH	[52]
			67.7% (10000
	Co3O4@CoNi-LDH/NF	1338.45 C g -1 at 0.5 A g -1		M KOH	[53]
			cycles)
	CoFe-LDH/multi-layer
		353 C g -1 at 1 A g -1	36.8% (5000 cycles)	M KOH	[54]
	graphene
		582.07 mC cm -2 at 2 mA
	CoMn-LDH		77% (5000 cycles)	M KOH	[36]
		cm -2
	Co II Co III -CO3 LDH	150.75 C g -1 at 1 A g -1	85% (300 cycles)	0.1 M KOH [55]
	NiCo2O4@Co-Fe LDH/NF	623 C g -1 at 1 A g -1	/	M KOH	[56]
	MnCo LDH	229.95 C g -1 at 2 A g -1	91.6% (2000 cycles)	M KOH	[57]
			90.4% (10000
	Co-LDH/graphene	76.4 F cm -3 at 2 A g -1		M KOH	[58]
			cycles)
	Zn-Co LDH	270 mA h g -1 at 1 A g -1	97% (1000 cycles)	M KOH	[59]
	Zn1Co2-LDH@rGO/NF	1071 C g -1 at 1 A g -1	90.9% (6000 cycles)	M KOH	[60]
	Mg-Co-Al-LDH	247.85 C g -1 at 1 A g -1	86.32% (10000	M KOH	[61]

Table 2 .6.Table 2 . 6 .

 226 Representative LDHs electrodes without Ni and Co ions and their electrochemical supercapacitor performance.

	Materials	Spercific Capacity	Cycle stability	Electrolyte	Ref.
	RGO@MgAl LDH	1067.2 C g -1 at 1 A g -1	87% (10000 cycles)	1 M KOH	[62]

Table 2 .7.Table 2 . 7 :

 227 Elemental composition of the samples.

	Elements
	(at%)	Ni2p	Fe2p	C1s	O1s
	Composites
	NiFe LDHs/NF (S5)	12.9	4.0	39.1	44
	NiFe LDHs/rGO/NF (S7)	9.4	3.2	47.1	40.3

Table 2 .8. Specific

 2

	(a)	(b)		(c)
	(d)	(e)
	Current density
	(A g -1 )	5	10	15	20	30
	Samples
	Ni(OH)2 (S0)	280	260	240	220	210
	NiFe LDH 4:1 (S1)	430	400	360	320	270
	NiFe LDH 3:2 (S2)	220	180	150	140	120
	NiFe LDH 1:1 (S3)	250	244	240	230	210

capacity (C g -1 ) of the electrodes.

ZSimDemo3.30d, where Rs is the internal resistance of an electrode material, electrolyte resistance and contact resistance between the active material and substrate, Rct is the charge-transfer resistance, Cdl is the double-layer capacitance, Wo is the Warburg element. All the values are summarized in Table

2

.9, in which all samples show small Rs and Rct, indicating a good conductivity.

Table 2 . 9

 29 Resistance values (ohm cm -2 ) of the electrodes.

	Samples	Rs	Rct
	Ni(OH)2 (S0)	1.03	25.46
	NiFe LDH 4:1 (S1)	1.02	24.68
	NiFe LDH 3:2 (S2)	1.50	31.25
	NiFe LDH 1:1 (S3)	1.50	15.97
	NiFe LDH 2:3 (S4)	1.10	25.09
	NiFe LDH 1:4 (S5)	1.91	24.23
	Fe(OH)3 (S6)	1.04	24.98

Table 2 .10. The

 2

	results demonstrate that the hybrid supercapacitor
	device can reach an energy density of 17.71 Wh kg -1 at a power density of 348.49 W
	kg -1 . This energy density value is greater than those recorded for CoMn-LDH//AC
	hybrid supercapacitor [80], PPy/rGO//NCs hybrid supercapacitor [81], MnO2//P-GA
	asymmetric supercapacitor [82], MnO2//AC asymmetric supercapacitor [83], Ni-Co
	oxide//AC hybrid supercapacitor [84], Cu2O/CuO/Co3O4//AG hybrid supercapacitor
	[85] and MnFe2O4//LiMn2O4 hybrid supercapacitor [86].

Table 2 .

 2 10 Comparison of energy densities (Wh kg -1 ) and power densities (W kg -1 ) of different supercapacitor devices.

		Energy density	Power density
	Device	(Wh kg -1 )	(W kg -1 )
	CoMn-LDH//AC	5.9	250
	PPy/rGO//NCs	15.8	140
	MnO2//P-GA	13.9	13300
	MnO2//AC	10.4	14700
	Ni-Co oxide//AC	12	95.2
	Cu2O/CuO/Co3O4//AG	12	162
	MnFe2O4//LiMn2O4	5.5	1800
	This work	17.71	348.49

Furthermore, the mechanical properties and flexibility of the hybrid supercapacitor device were assessed (Figure

2

.9i).

There is no obvious change in the CV curves at different bending angles, indicating a satisfactory flexible performance of the hybrid supercapacitor.

  ) planes for LDHs, appearing at 11.7 o , 23.6 o , 35.1 o , 39.7 o , 47.3 o , 61.2 o , 62.6 o and 66.5

o , respectively (JCPDS No. 15-0087) [45]. Moreover, there are four small peaks of Ni(OH)2 located at 19.3 o , 33.1 o , 38.5 o and 59.4 o ascribed to the (001), (

Table 2 . 11 .

 211 Elemental composition of the different samples.

	Elements (at%)
		Ni2p	Fe2p/Cr2p/Al2p	C1s	O1s
	Composites
	NiFe LDHs@CS/NF	13.7	8.4	27.4	50.5
	NiCr LDHs@CS/NF	8.5	31.1	22.4	38.0
	NiAl LDHs/NF	8.2	37.8	14.5	39.4
	NiAl LDHs@CS/NF	9.7	35.3	16.4	38.6

Table 2 .

 2 12 The areal capacity (mC cm -2 ) of as-obtained LDH composites.

	Current density
	(mA cm -2 )	1	2	3	4	5
	Composites
	NiFe LDHs/NF	478.6	442.3	406.4	370.6	333.6
	NiFe LDHs@CS/NF	705.8	685.7	649.8	610.1	576.6
	NiCr LDHs/NF	592.6	527.3	482	446.4	417
	NiCr LDHs@CS/NF	814.9	703	646.9	609.6	575.4
	NiAl LDHs/NF	851.9	771.4	714.6	665.3	618.6
	NiAl LDHs@CS/NF	1042.2	870.2	787.3	722.9	669
	Notably, with the incorporation of carbon spheres, the areal capacity of NiFe
	LDHs@CS/NF, NiCr LDHs@CS/NF and NiAl LDHs@CS/NF were improved by 48%,

37% and 22% compared to NiFe LDHs/NF, NiCr LDHs/NF and NiAl LDHs/NF, respectively. All the samples presented excellent coulombic efficiencies (see Table

Table 2 .

 2 13 Coulombic efficiencies of as-prepared electrode materials.

	Current density
	(mA cm -2 )	1	2	3	4	5
	Composites
	NiFe LDHs/NF	94.9%	96.3%	96.2% 95.9% 95.2%
	NiFe LDHs@CS/NF	71.1%	87.2%	91.1% 92.3% 93.2%
	NiCr LDHs/NF	86.6%	92.8%	94.3% 94.8% 94.6%
	NiCr LDHs@CS/NF	78.6%	86.4%	90.5% 92.3% 93.0%
	NiAl LDHs/NF	95.1%	96.6%	97.1% 97.1% 96.8%
	NiAl LDHs@CS/NF	81.0%	89.5%	92.4% 93.5% 94.0%

To further analyze the ion diffusion (low-frequency region) and charge transfer resistance (high-to-medium frequency region) of the samples, EIS was measured as illustrated in Figure

2

.20e. Rs (the intersection of the plot with the X coordinate axis) and Rct (the semicircle) of all the electrodes were obtained from the Nyquist plots (

Table 2 .14). Table 2.14 Resistance

 2 values of as-prepared electrode materials.Rs values of 1.28, 1.30, 0.11, 0.94, 0.72 and 1.44 ohm cm -2 were determined for NiFe LDHs/NF, NiFe LDHs@CS/NF, NiCr LDHs /NF, NiCr LDHs@CS/NF, NiAl LDHs/NF and NiAl LDHs@CS/NF, respectively, and the corresponding Rct were 10.42, 20.78, 38.78, 14.18, 2.44 and 22.40 ohm cm -2 . Compared to NiFe LDHs@CS/NF and NiCr LDHs@CS/NF, NiAl LDHs@CS/NF possessed the smallest Rs but larger Rct, even though it exhibited the largest areal capacity (Figure2.20b

	Composites	Intrinsic resistance (Rs ) / ohm cm -2	Charge transfer resistance (Rct) / ohm cm -2
	NiFe LDHs/NF	1.28	10.42
	NiFe LDHs@CS/NF	1.30	20.78
	NiCr LDHs/NF	1.08	38.78
	NiCr LDHs@CS/NF	0.94	14.18
	NiAl LDHs/NF	0.72	2.44
	NiAl LDHs@CS/NF	1.44	22.40

  According to Equations 1.12 and 1.13, the hybrid supercapacitor device attained the largest areal energy density of 43 μWh cm-2

	at a power density of 0.805 mW cm -2 (Figure 2.24g). This value is higher than that
	achieved by Mg-Al LDH//rGO (24.07 μWh cm -2 at 81.2 μW cm -2 ) [62], Ni-Co
	LDH//ketjenblack (14.4 μWh cm -2 at 312.5 μW cm -2 ) [94], Co0.85Se//AC (35.65 μWh
	cm -2 at 210 μW cm -2 ) [95] hybrid supercapacitors, HCF fiber (27.1 μWh cm -2 at 66.5
	μW cm -2 ) [96], rGO/V2O5-rGO asymmetric flexible device (8.1 μWh cm -2 at 4.17 mW
	cm -2 ) [97], Cu@Ni/porous Ni/MnCo2O4 symmetric supercapacitor (12.8 μWh cm -2 at
	110 μW cm -2 ) [98] and MnO2/CNT//PI/CNT asymmetric supercapacitor (36.4 μWh
	cm -2 at 0.78 mW cm -2 ) [99].

to Equation 1.11.2, the areal capacity of the hybrid supercapacitor device was calculated as 191.3, 166.1, 153.7, 149.8 and 145.2 mC cm -2 at 1, 2, 3, 4 and 5 mA cm -2 respectively (Figure 2.24f).

Table 2 . 15 :

 215 Elemental composition of the samples.

	Elements
	(at%)	Ni2p	Mn2p	Cr2p	C1s	O1s
	Composites
	NiMnCr LDHs/NF	4.6	6.0	2.4	43.3	43.7
	NiMnCr LDHs@CS/NF	3.5	7.6	5.6	42.4	40.9
	Figure 2.33c depicts the Mn2p core level spectrum (7.6 at%) of NiMnCr
	LDHs@CS/NF composite, which can be fitted with two components at 652.4 and 641.1
	eV assigned respectively to Mn2p1/2 and Mn2p3/2 peaks, indicating the presence of Mn 3+
	[101, 102]. It is easy to oxidize Mn 2+ to Mn 3+ due to the low redox potential under
	alkaline conditions. The Cr2p core level spectrum (2.4 at%) of NiMnCr LDHs@CS/NF
	composite can be deconvoluted in two bands at 586.1 and 576.4 eV ascribed
	respectively to Cr2p1/2 and Cr2p3/2, suggesting that Cr is trivalent [4] (Figure 2.33d).

Table 2 . 16 :

 216 Specific capacity values (C g -1 ) of the various electrodes investigated in this work at different current densities. Rs, Cdl, and Wo stand respectively for the chargetransfer resistance, internal resistance, double-layer capacitance, and Warburg element).

	Current	Samples	NF	CS/NF	NiMnCr LDHs/NF	NiMnCr LDHs@CS/NF
	density (A g -1 )
	3		24.4	88	174.2	569
	5		24.2	82.3	148.8	494.2
	10	19.2	69.1	110.7	442.2
	15	17.6	61.1	92.9	427.9
	20	14.9	59.3	78.9	414.7
	The results clearly revealed that NiMnCr LDHs/NF and NiMnCr LDHs@CS/NF
	exhibit higher specific capacity values than NF and CS/NF with NiMnCr
	LDHs@CS/NF featuring a better rate performance and specific capacity of 569, 494.2,
	442.2, 427.9 and 414.7 C g -1 at 3, 5, 10, 15 and 20 A g -1 , respectively. The specific
	capacity value remained high (73% of the specific capacity recorded at 3 A g -1 ) even at
	20 A g -1 . At the same time, the IR drop of NiMnCr LDHs@CS/NF composite was lower
	(8 mV) than that of NiMnCr LDHs/NF (12 mV), indicating a relatively lower charge-

Table 2 .17), consistent

 2 with the low IR drop. The results indicate that the existence of CS could facilitate the charge transfer and promote the electrochemical kinetics, as illustrated in Figure 2.27b.

Table 2 .

 2 17 Resistance values of the various electrodes investigated in this work. NiMnCr LDHs@CS/NF electrode displays good stability after 10,000 cycles at 20 A g -1 ; the electrode maintained 76% of its initial value, which is better than 54% achieved by NiMnCr LDHs/NF under the same cycling conditions.

	Composites	Rs (Ohm cm -2 )	Rct (Ohm cm -2 )
	NF	0.703	622.7
	CS/NF	1.078	379.6
	NiMnCr LDHs/NF	0.911	18.28

Table 2 . 18 :

 218 Elemental composition of the samples after 10,000 cycles.

	Elements
	(at%)	Ni2p	Mn2p	Cr2p	C1s	O1s
	Composites
	NiMnCr LDHs/NF	0.3	2.7	0	69.3	27.7
	NiMnCr LDHs@CS/NF	5.2	6.1	0	52.1	36.5

Table 2 .19). The

 2

specific capacity values, deduced from the discharging curves in Figure 2.43c, are displayed in Figure 2.43d. The NiMnCr LDHs@CS/NF//FeOOH/NF hybrid supercapacitor achieved specific capacity values of 167.4, 154.4, 137.5, 127.5, 129.6, 104.6, 99.6, 74.7 and 59.8 C g -1 respectively at 1.5, 2, 3, 4, 5, 8, 10, 15 and 20 A g -1 ,

Table 2 .

 2

		19: Coulomb efficiency (%) of the hybrid supercapacitor device at various
				current densities (A g -1 ).
	1.5	2	3	4	5	8	10	15	20
	62%	79%	90%	91%	92%	93%	100%	100%	100%
	To achieve a full characterization of the electrochemical performance of the as-
	prepared hybrid supercapacitor, the Ragone plot is provided in Figure 2.43f. The hybrid
	supercapacitor device attained a great energy density of 48 Wh kg -1 at a power density
	of 402.7 W kg -1 , which is larger than that of CoMn-LDH/AC hybrid supercapacitor (4.4
	Wh kg -1 at 2500 W kg -1 ) [80], NiFe LDH/rGO/NF//MC/NF hybrid supercapacitor (9.8
	Wh kg -1 at 560 W kg -1 ) [104], MnFe2O4/LiMn2O4 hybrid supercapacitor (5.5 Wh kg -1
	at 1800 W kg -1 ) [105], Ni-Co oxide/AC hybrid supercapacitor (12 Wh kg -1 at 95.2 W
	kg -1 ) [106], MnO2@Co-Ni LDH (21.3 Wh Kg -1 at 4.8 kW kg -1 ) [107] Co3O4@NiMn-
	LDH/Ni (47.15 Wh kg -1 at 376 W kg -1 ) [108] and CoSx/Ni-Co LDH nanocages (35.8
	Wh kg -1 at 800 W kg -1 ) [109]. Even though the developed NiMnCr
	LDHs@CS/NF//FeOOH/NF hybrid supercapacitor achieved good performance, there
	is room for improvement by applying suitable negative materials.

Table 3 . 3

 33 Coulombic efficiencies of as-prepared electrode materials.

	Current density
	(mA cm -2 )	20	30	40	50	60	70	80
	Composites
	CuO@Cu	77.8% 78.7% 72.8%	67.6	60%	63.3% 56.8%
	Ni(OH)2@CuO@Cu-30	88.8% 90.6% 90.4% 89.4% 83.4% 85.9% 79.3%
	Ni(OH)2@CuO@Cu-50	87.4% 90.9% 91.2% 90.5% 90.4% 89.2% 87.8%
	Ni(OH)2@CuO@Cu-90	94%	95.8% 96.1% 95.6% 95.1% 94.7% 94.3%
	Ni(OH)2@CuO@Cu-150	91.5% 94.2% 96.2% 96.3% 96.1%	96%	95.6%
	Ni(OH)2@CuO@Cu-200	95.5% 96.8% 97.5% 96.4% 96.4% 94.9%	95%

Table 3 . 4 .

 34 The areal capacity (mC cm -2 ) of as-obtained composites at various current densities.

	Current density
	(mA cm -2 )	20	30	40	50	60	70	80
	Composites
	CuO@Cu	412.8	360	321.6	300	280.8	260.4	240
	Ni(OH)2@CuO@Cu-30	1792	1699	1632	1572	1519	1478	1430
	Ni(OH)2@CuO@Cu-50	2770	2664	2578	2502	2441	2360	2298
	Ni(OH)2@CuO@Cu-90	4092	3906	3758	3642	3521	2360	2298
	Ni(OH)2@CuO@Cu-150	7063	6606	6259	5970	5710	5443	5203
	Ni(OH)2@CuO@Cu-200	6737	6174	5688	5262	4889	4544	4205

Table 3 . 5 .

 35 Resistance values of as-prepared electrode materials.

	Composites	Intrinsic resistance (Rs ) / ohm cm -2	Charge transfer resistance (Rct) / ohm cm -2
	CuO@Cu	0.94	Rct1=3.22, Rct2=22.08
	Ni(OH)2@CuO@Cu-30	0.80	1.01
	Ni(OH)2@CuO@Cu-50	0.88	0.82
	Ni(OH)2@CuO@Cu-90	0.91	0.67
	Ni(OH)2@CuO@Cu-150	1.04	0.34
	Ni(OH)2@CuO@Cu-200	0.97	1.69

NiMnCr Layered Double Hydroxides

Even though, LDHs-based supercapacitors are described in the binary structure, there are many opportunities for designing efficient electrode materials for supercapacitors based on LDHs, given the plethora of combinations of M 2+ /M 3+ offered by these materials. Owing to its plentiful oxidation states (from +1 to +6), Cr was chosen as the third metal in LDHs with the aim to further improve the intrinsic activity and performance of LDHs electrodes in supercapacitors.

We exploit, in this work, the capacitive properties of NiMnCr LDHs electrodes for energy storage and their improvement through hybridization with carbon spheres (CS) supported on nickel foam (NF) collector.

Experimental section

Synthesis of NiMnCr LDHs@CS/NF

Typically, Mn(NO3)24H2O (0.45 mmol), Cr(NO3)39H2O (0.075 mmol), CO(NH2)2 (4.5 mmol) and NH4F (8 mmol) were placed in 30 mL of Milli-Q (MQ) water and stirred until formation of a homogeneous solution. The CS/NF, prepared using the same procedure as in 2.3.1.2, was introduced into the above solution, and then transferred into a 50 mL autoclave. The mixture was placed in an oven at 120 o C for 12 h. After being cooled to room temperature (RT), NiMnCr LDHs@CS/NF was separated by centrifugation, rinsed copiously with water and ethanol, and dried at 60 o C for 24 h. It is noteworthy to mention that the nickel foam acts as a support, but also as a Ni source for LDHs formation during the hydrothermal reaction.

The morphology of CS/NF, NiMnCr LDHs/NF and NiMnCr LDHs@CS/NF was assessed by scanning electron microscopy (SEM). [7]. Even though the electrode materials achieved a good performance, the capacity was far below the theoretical value. Therefore, there is enough room for alternative methods to improve the properties of Ni-based electrodes.

In this chapter, we synthesized Ni(OH)2 through electrochemical deposition on Cu foam as current collector due to its three-dimensional (3D) structure and good conductivity. Before .9: EDS spectra of CuO@Cu, Ni(OH)2@CuO@Cu-30, Ni(OH)2@CuO@Cu-50, Ni(OH)2@CuO@Cu-90 and Ni(OH)2@CuO@Cu-200. the existence of CuO [8]. There is a small apparent shift in the binding energy compared with Ni(OH)2@CuO@Cu-150, which can be assigned to the interaction between Ni(OH)2 and CuO. Cu2p and (f) O1s.

All samples obtained in this work were investigated by electrochemical measurements in aqueous KOH (2 M) electrolyte in a classical 3-electrode cell (Figure 3.12). Figure 3.12a depicts the CV curves of all the samples in the potential range of -0.2 to +0.9 V recorded at 20 mV s -1 . The presence of redox active peaks for Ni(OH)2@CuO@Cu foam composites is consistent with the redox process shown in Equation 2.1.

By varying the deposition time, Ni(OH)2@CuO@Cu foam composites exhibited different electrochemical performances. Among all samples, Ni(OH)2@CuO@Cu-150 attained the largest current density at 20 mV s -1 . The areal capacity values were determined by integrating the area under the CV curves using the following Equation 1.15.2, where m (the mass loading of SC electrodes) is replaced by A (cm -2 ) (the area of active material). The results are summarized in Table 3.2, which revealed that Ni(OH)2@CuO@Cu-150 possessed the largest areal capacity of 7742 mC cm -2 at 20 mV s -1 .

Table 3.2. The areal capacitance of as-obtained composites at a scan rate of 20 mV s -1 .

Composites Areal capacitance (mC cm -2 )

CuO@Cu 1683.4

Ni(OH)2@CuO@Cu-30 3430.9

Ni(OH)2@CuO@Cu-50

4495.5

Ni(OH)2@CuO@Cu-90 5711.9

Ni(OH)2@CuO@Cu-150

7742.2

Ni(OH)2@CuO@Cu-200 6820.9

Conclusion and Perspectives

As a link for energy-power difference between a traditional capacitor and fuel cells/batteries, supercapacitors (SCs) have drawn widespread interest owing to their advantages like enhanced power density, being almost maintenance-free, experiencing no memory effect, safe, short charge/discharge time, and low cost. However, the relatively low energy density hinders their wide application in daily life. Therefore, the research in the field of SCs is mainly oriented towards the development of novel high-performance electrode materials.

Among all the materials, Ni-based hydroxides have drawn great attention and interest in the field of supercapacitors. Ni(OH)2 with large theoretical specific capacitance (2082 F g -1 ) has been extensively investigated for supercapacitors. Besides, Ni-based layered double hydroxides (LDHs), due to their well-defined and high redox activity, wide tunability of the types of metal cations and environmentally friendly nature, were also greatly explored for supercapacitor applications.

However, the low electrical conductivity of hydroxides hinders their potential application. To overcome this problem, composites combining Ni-based hydroxides with carbon-based (reduced graphene oxide and carbon spheres) materials were prepared with the aim to enhance their electrical conductivity. Additionally, deposition on porous supports such as nickel foam (NF) or cupper foam (Cu foam) endows the composite with high surface area and certain flexibility.

In this thesis, NiFe LDHs/rGO/NF, NiAl LDHs@CS/NF and NiMnCr LDHs@CS/NF composites were first synthesized, and showed improved specific/areal capacity than that of the same composites without carbon-based materials. NiFe LDHs/rGO/NF achieved a specific capacity (585 C g -1 at 5 A g -1 ) higher than that of NiFe LDHs/NF (530 C g -1 at 5 A g -1 ). NiAl LDHs@CS/NF electrode material attained an areal capacity (1042.2 mC cm -2 at 1 mA cm -2 ) greater than that of NiAl LDHs/NF (851.9 mC cm -2 at 1 mA cm -2 ). NiMnCr LDHs@CS/NF composite exhibited a much larger specific capacity (569 C g -1 at 3 A g -1 ) than that of NiMnCr LDHs/NF (173.2 C g -1 at 3 A g -1 ).

To further improve the energy density, hybrid supercapacitor devices with wide working potential window were fabricated. A hybrid supercapacitor device was assembled based on asprepared NiFe LDHs/rGO/NF as the positive electrode and mesoporous carbon (MC)/NF as the negative electrode, exhibiting an energy density of 17.71 Wh kg -1 and a power density of 348.49

W kg -1 . Besides, a hybrid supercapacitor device comprising NiAl LDHs@CS/NF as the positive electrode and N-doped reduced graphene/NF as the negative electrode attained an energy density of 43 μWh cm -2 at a power density of 0.805 mW cm -2 and was applied successfully to operate a windmill device continuously for 32 s. Moreover, two flexible NiAl LDHs@CS/NF//N-rGO/NF hybrid supercapacitors, connected in series, were able to light up a green, a red and a yellow LED in parallel, lasting for 37 s, 542 s and 199 s, respectively. Finally, a hybrid supercapacitor was fabricated using NiMnCr LDHs@CS/NF as the positive electrode and FeOOH coated on NF (FeOOH/NF) as the negative electrode, reaching an energy density of 48 Wh kg -1 at a power density of 402.7 W kg -1 . For practical applications, the as-fabricated NiMnCr LDHs@CS/NF//FeOOH/NF supercapacitor device was successfully used to operate a homedesigned windmill device and two as-fabricated hybrid supercapacitor cells in series were used to light up a yellow LED and a red LED in parallel for ~111 s and ~343 s, respectively, indicating their potential application for flexible energy storage devices.

At the same time, Ni(OH)2@CuO@Cu foam binder-free electrodes were fabricated by a twostep process at room temperature with different deposition times. Ni(OH)2@CuO@Cu-150 exhibited large areal capacity of 7063 mC cm -2 at 20 mA cm -2 , and was therefore chosen as the positive electrode in a hybrid supercapacitor. Using N-doped reduced graphene oxide on nickel Graphene oxide (GO) was purchased from Graphitene (UK).

Nickel foam (NF) was purchased from Jiayisheng Company (China).

Carbon paper (CP) was purchased from Maipengchen Electronic Technology Co., Ltd. (China).

The water used throughout the experiments was purified with a Milli-Q system from Millipore Co.

(resistivity = 18 MΩ.cm).

S1.2 Characterization techniques of electrode materials

For the as-prepared electrode materials, it is necessary to assess their morphologies, and chemical compositions, which provide an evaluation method for supercapacitor applications.

Various characterization techniques are commonly used and are introduced as follows.

S1.2.1 Scanning Electron Microscopy (SEM)

SEM is the most commonly used method to observe the surface morphology of the material.

The principle is to collect the signals of different characteristic particles (mainly secondary electrons) generated by the high-energy electron beam irradiated on the sample surface, forming the different images to obtain the resulting topography information of the sample surface. In addition to secondary electrons, the irradiated area can also be excited to produce characteristic Xrays, and the content and type information of elements in the sample can be obtained based on the obtained X-ray information, which is the corresponding energy dispersive spectrometer (EDS) and element scanning analysis (mapping).

SEM images in this thesis were obtained using an electron microscope ULTRA 55 (Zeiss) equipped with a thermal field emission emitter and three different detectors (EsB detector with filter grid, high-efficiency In-lens SE detector, Everhart-Thornley secondary electron detector).

For EDS elemental mapping images, the k-line was used. For each point of images, a spectrum from 0 to 20 keV was recorded.

For the samples obtained on NF, SEM imaging was directly performed on the samples. For powder samples, they were first dissolved in 50 μL MQ water or ethanol, and then dropped on a clean silicon wafer, followed by drying in an oven at 80 o C to remove the solvent.

S1.2.2 Transmission Electron Microscopy (TEM)

TEM provides more detailed microscopic morphological observations on the prepared materials. The high-resolution tests on specific selection areas allow observing the lattice fringes of the materials, and the corresponding crystal plane spacing can further determine the composition of the materials. In addition, surface scan element analysis (mapping) of the material selection gives a precise image of the elements' distribution on the surface of the samples.

In this thesis, TEM analysis was performed on a TECNAI-G2 working at 200 kV, equipped with an EDS spectrometer. The samples on NF were scratched off and were dispersed on a thin carbon film of a 3 mm diameter copper grid to perform the measurement.

S1.2.3 X-ray diffraction (XRD)

XRD is widely used for phase and crystal structure analysis of materials. The working principle