Le dihydrogène (H2) se présente comme le futur vecteur énergétique pour une économie basée sur des ressources propres et respectueuses de l'environnement. Il est le combustible idéal de la pile à combustible régénérative constituée de deux entités : un électrolyseur pour sa production, et une pile à combustible pour sa conversion directe en énergie électrique. Ce système présente l'avantage d'être compact et autonome. Cependant, l'amélioration de l'activité catalytique des matériaux, leur stabilité et l'élimination de métaux nobles dans leur composition sont nécessaires. Des catalyseurs bifonctionnels à base de métaux de transition associés au graphène ont alors été synthétisés. L'interaction oxyde-graphène a été étudiée sur un catalyseur Co3O4/NRGO. À faible teneur en cobalt, l'interaction entre les atomes de cobalt de l'oxyde et les atomes d'azote greffés sur les plans de graphène a été observée par voltammétrie cyclique. Cette interaction est responsable d'une diminution de la taille des nanoparticules de cobaltite et de l'activité de celles-ci vis-à-vis de la réaction de réduction du dioxygène (RRO). La substitution du cobalt par le nickel dans des structures de type spinelle (NiCo2O4/RGO) obtenu par voie solvothermale, a permis d'améliorer les performances électrocatalytiques vis-à-vis de la RRO et de la RDO. Ce matériau et un autre de type Fe-N-C préparé en collaboration avec un laboratoire de l'Université Technique de Berlin ont servi de cathode dans des études préliminaires réalisées en configuration pile à combustible alcaline à membrane échangeuse d'anion (SAFC). / Hydrogen, as an environmentally friendly future energy vector, is a non-toxic and convenient molecule for regenerative fuel cell, which connects two different technologies: an electrolyzer for H2 production, and a fuel cell for its direct conversion to electric energy. This kind of system possesses many advantages, such as lightness, compactness and more autonomy. However, improvement of activity and durability of electrode materials free from noble metals in their composition is needed. Thereby, bifunctional catalysts composed of transition metals deposited onto graphene-based materials were synthesized. The interaction between the metal atom of the oxide and the graphene doped heteroatom in the Co3O4/NRGO catalyst was investigated physicochemically. With a low cobalt loading, the interaction between cobalt and nitrogen was characterized by cyclic voltammetry, which revealed that it was responsible for decreasing the oxide nanoparticle size, as well as increasing the material activity towards the oxygen reduction reaction (ORR). The substitution of Co by Ni in the spinel structure (NiCo2O4/RGO) obtained by solvothermal synthesis, allowed the enhancement of the electrocatalytic performances towards the ORR and OER. Moreover, this catalyst as well as another material prepared in collaborative program with a lab from Technical University of Berlin were used as cathode in preliminary studies undertaken on solid alkaline fuel cell (SAFC).

Les oxydes de manganèse présentent un grand intérêt en raison de leur activité catalytique pour l'ORR (la réaction de réduction de l’oxygène) en milieu alcalin et peuvent être utilisés comme matériaux sans métaux nobles pour la cathode dans les piles à combustible. La présente thèse est consacrée à l’étude de l’activité d'oxydes de manganèse pour l’ORR. Il a été montré que Mn2O3 avec structure bixbyite a une meilleure activité catalytique vers l'ORR en milieu alcalin que les autres oxydes de manganèse étudiés. L'activité spécifique de Mn2O3 est seulement 4 fois inférieure à celle de Pt à une surtension de 0.3 V (ERH). Le lien entre la structure des oxydes de Mn et l'activité ORR est identifié: l'activité spécifique augmente exponentiellement avec le potentiel du couple redox Mn(III)/Mn(IV) de surface. Pour assurer l'activité électrocatalytique élevée de Mn2O3, il est nécessaire d'ajouter du carbone à la composition d'électrode, ainsi que de garder un potentiel supérieur à 0.7 V (ERH). / Manganese oxides are of great interest due to their catalytic activity towards the ORR (the oxygen reduction reaction) in alkaline media and can be used as noble metal-free materials for the cathode in liquid and polymer electrolyte alkaline fuel cells. The present thesis is devoted to the investigation of the ORR activity of manganese oxides. It was shown that Mn2O3 with bixbyite structure has a better catalytic activity toward the ORR in alkaline media than other investigated manganese oxide, the surface activity of Mn2O3 is only 4 times lower than that of Pt at an overvoltage of 0.3 V (RHE). The link between the structure of Mn oxides and the ORR activity is found: the specific ORR activity exponentially increases with the potential of the surface Mn(III)/Mn(IV) red-ox couple. To ensure the high electrocatalytic activity of Mn2O3, it is necessary to add carbon to the electrode composition, as well as to keep potential above 0.7 V (RHE).

Materials for studies related to nanoscience and nanotechnology have gained tremendous attention owing to their unique physical, chemical and electronic properties. Among various anisotropic nanostructures, one dimensional (1D) materials have received immense interest in numerous fields ranging from catalysis to electronics. Imparting multi-functionality to nanostructures is one of the major areas of research in materials science. In this direction, use of nanosized materials in energy systems such as fuel cells has been the subject of focus to achieve improved performance. Tuning the morphology of nanostructures, alloying of catalysts, dispersing catalytic particles onto various supports (carbon nanotubes, carbon nanofibers, graphene, etc.) are some of the ways to address issues related to electrochemical energy systems. It is worth mentioning that highly stable and corrosion resistant electrodes are mandatory as electrochemical cells operate under aggressive environments. Additionally, carbon, which is often used as a support for catalysts, is prone to corrosion and is subsequently implicated in reduced performance due to poor adherence of catalyst particles and loss in electrochemically active area. Hence, there is a quest for the development of stable and durable electrocatalysts / supports for various studies including fuel cells. The present thesis is structured in exploring the multi-functional aspects of titanium carbide (TiC), an early transition metal carbide. TiC, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrical and electrochemical studies. The current study explores the versatility of TiC in bulk as well as nanostructured forms, in electrical and electrochemical studies towards sensing, electrocatalytic reactions and active supports. 1D TiC nanowires (TiC-NW) are prepared by simple solvothermal method without use of any template and are characterized using various physico-chemical techniques. The TiC-NW comprise of 1D nanostructures with several µm length and 40 ± 15 nm diameter (figure 1). Electrical properties of individual TiC-NW are probed by fabricating devices using focused ion beam deposition (FIB) technique. The results depict the metallic nature of TiC-NW (figure 2). Figure 1. (a) SEM, (b) TEM and (c) HRTEM images of TiC-NW prepared by solvothermal method. Figure 2. (a) SEM image and (b) I-V characteristics of TiC-NW - based device as a function of temperature. The contact pads are made of Pt. Subsequently, oxidized TiC nanowires are prepared by thermal annealing of TiC-NW, leading to carbon - doped TiO2 nanowires (C-TiO2-NW) (figure 3). Photodetectors are fabricated with isolated C-TiO2-NW and the device is found to respond to visible light (figure 3) radiation with very good responsivity (20.5 A/W) and external quantum efficiency (2.7 X 104). The characteristics are quite comparable with several reported visible light photodetectors based on chalcogenide semiconductors. Figure 3. (a) HRTEM, (b) EDAX, (c) Scanning TEM-DF images of C-TiO2-NW along with (d) Ti (e) O and (f) C mapping. (g) Current – voltage curves of single C-TiO2-NW recorded in dark (black) and in presence of visible light radiation (red) of intensity 57.7 mW/cm2 at 25oC. Inset of (g) shows the SEM image of the device (top) and schematic illustration of fabricated photodetector (bottom). The next chapter deals with the electrochemical performance of TiC demonstrated for studies involving oxygen reduction and borohydride oxidation reactions. Electrochemical oxygen reduction reaction (ORR) reveal that TiC-NW possess high activity for ORR and involves four electron process while it is a two electron reduction for bulk TiC particles (figure 4). The data has been substantiated by density functional theory (DFT) calculations that reveal different modes of adsorption of oxygen on bulk and nanowire morphologies. Stable performance is observed for several hundreds of cycles that confirm the robustness of TiC. The study also demonstrates excellent selectivity of TiC for ORR in presence of methanol and thus cross-over issue can be effectively addressed in direct methanol fuel cells. In the chapter on borohydride oxidation, bare TiC electrode is explored as a catalyst for the oxidation of borohydride. One of the major issues in direct borohydride fuel cells (DBFC) is the hydrolysis of borohydride that happens on almost all electrode materials leading to low efficiency. The present study reveals that TiC is a very good catalyst for borohydride oxidation with little or no hydrolysis of borohydride [figure 5 (a)] under the experimental conditions studied. Further, shape dependant activity of TiC has been studied and fuel cell performance is followed [figure 5 (b)]. Polarization data suggests that the performance of TiC is quite stable under fuel cell experimental conditions. Figure 4. (a) Linear sweep voltammograms for ORR recorded using (i) bulk TiC particles and (ii) TiC-NW in O2-saturated 0.5 M KOH at 1000 rpm. Scan rate used is 0.005 Vs-1. (b) Variation of number of electrons with DC bias. Black dots correspond to TiC bulk particles while red ones represent nanowires. Figure 5. (a) Cyclic voltammograms of borohydride oxidation on TiC coated GC electrode in 1 M NaOH containing 0.1 M NaBH4. Scan rate used is 0.05 Vs-1. (b) Fuel cell polarization data at 70oC for DBFC assembled with (i) bulk TiC particles and (ii) TiC-NW as anode catalysts and 40 wt% Pt/C as cathode. Anolyte is 2.1 M NaBH4 in 2.5 M NaOH, and catholyte is 2.2 M H2O2 in 1.5 M H2SO4. Anode loading is 1.5 mg cm-2 and cathode loading is 2 mg cm-2. The corrosion resistance nature of TiC lends itself amenable to be used as an active support for catalytic particles (Pt and Pd) for small molecules oxidation reactions. In the present study, electro-oxidation of methanol, ethanol and formic acid have been studied. As shown in figure 6 (a), the performance of Pd loaded TiC (Pd-TiC) is found to be higher than that of Pd loaded carbon (Pd-C) suggesting the active role of TiC. The catalytic activities of TiC-based supports are further improved by tuning their morphologies. Figure 6 (c) reveals that the activities are higher in case of Pd-TiC-NW than that of Pd-TiC. Figure 6. (a) Cyclic voltammograms of Pd-TiC and Pd-C for ethanol oxidation, (b) T EM image of Pd-TiC-NW and (c) voltammograms of Pd-TiC-NW in N2-saturated 1 M ethanol in 1 M KOH medium, scan rate used is 0.05 Vs-1. The next aspect explored, is based on the preparation of C-TiO2 and its use as a substrate for surface enhanced Raman spectroscopy (SERS). Carbon doped titanium dioxide is prepared by thermal annealing of TiC. It is observed that the amount of dopant (carbon content) is dependent on the experimental conditions used. SERS studies using 4¬mercaptobenzoic acid (4-MBA) as the analyte, indicates that C-TiO2 [figure 7 (a)] enhances Raman signals based on chemical interactions between the analyte and the substrate. Raman signal intensities can be tuned with the amount of carbon content in C¬TiO2. Enhancement factors are calculated to be (7.7 ± 1.2) x 103 (for 4-MBA) and (1.7 ± 1.2) x 103 (for 4-nitrothiophenol). The SERS substrates are found to be surface renewable using visible light, a simple strategy to re-use the substrate [figure 7 (b)]. The regeneration of SERS substrates is based on self cleaning action of TiO2 that produces highly reactive oxygen containing radicals known to degrade the molecules adsorbed on TiO2. Thus, the versatility of TiC has been demonstrated with various studies. In addition to using TiC-based materials, nanoparticles of Rh, Ir and Rh-Ir alloy structures have also been used for borohydride oxidation reaction. This is explained in the last section. In Appendix-I, preliminary studies on the preparation of TiC-polyaniline (PANI) composites using liquid-liquid interfacial polymerization is explained. Raman spectroscopy results suggest that the presence of TiC-NW makes PANI to assume preferential orientation in the polaronic (conducting) form. Appendix-II discusses the role of TiC-NW as a fluorescence quencher for CdS semiconductor nanoparticles.

In recent years, there has been an increasing interest on renewable energy sources as substitute to fossil fuels. Among various processes of energy generation, electrochemical methods such as storage and conversion systems, electrolysis of water (production of H2 and O2), fuel cells, batteries, supercapacitors and solar cells have received great attention. The core of these energy technologies is a series of electrochemical processes, which directly depend on the nature of ‘electro catalyst’. The design and preparation of an electro catalyst is based on new concepts such as controlled surface roughness, atomic topographic profiles, defined catalytic sites, atomic rearrangements, and phase transitions during electrochemical reactions. Good electro catalysts should possess low over potential, high exchange current density, high stability, low cost and high abundance. The most fundamental reactions in the area of electrochemistry are hydrogen evolution (HER) and oxygen reduction (ORR) reactions. They are important in different energy systems such as fuel cells and batteries. Platinum has been a favoured electro catalyst due to its high activity, favourable density of states at Fermi level and chemical inertness. The low abundance, however, limits its large scale applications. Alternate materials with high catalytic activities are always required. In this particular direction, metal chalcogenides such as sulphides and selenides have attracted attention in recent years. The present thesis describes the synthesis of different phases of palladium and nickel chalcogenides and their applicability in various electrochemical reactions, both in aqueous and organic media. First part includes the synthesis of highly crystalline palladium selenide phases namely Pd17Se15, Pd7Se4 and Pd4Se by employing facile single source molecular precursor method. Pure palladium selenide phases are prepared by thrombolysis of highly processable intermediate complexes formed from metal and selenium precursors. Continuous films of different dimensions on various substrates (glass, ITO, FTO etc.) could be prepared (figure 1). This is one of the requirements for processing any new material. Thickness of the films could be altered by changing the volume of precursor complex coated on the substrate. All the phases are found to be metallic in nature with resistivity values in the range of 30 to 180 µΩ.cm. Figure 1. (a) Scanning electron micrograph and (b) photographic image of Pd17Se15 prepared on different substrates glass (1), Si (2), fluorine doped tin oxide (FTO) (3) and DSSC solar cell fabricated using FTO coated Pd17Se15 as the counter electrode (4). Other components of DSSC are given in the experimental section. All the palladium selenides phases are shown to be catalytically active towards electrochemical reactions such as HER and ORR. It is observed that the activities of the phases depend on the stoichiometric ratio of palladium to selenium. Higher the palladium content in the phase, higher is the catalytic activity observed. Therefore, the activities of the chalcogenides can be easily tuned by varying the ratio of metal to chalcogen. Tafel slopes of 50–60 mV/decade are observed for all three phases towards HER indicating that Volmer- Heyrovsky mechanism is operative. The exchange current densities are in the range of 2.3 x 10-4 A cm-2 to 6.6 x 10-6 A cm-2 (figure 2a). Figure 2. (a) Linear sweep voltammograms of Pd17Se15, Pd7Se4 and Pd4Se in 0.5 M H2SO4 (HER) and (b) 0.1 M KOH (ORR) at a scan rate of 2 mVs-1. These phases are found to be highly robust and stable under different pH conditions. Stability of the phases is confirmed by characterizing the catalysts post-HER process, using various techniques such as XPS, XRD and SEM. High activities observed for Pd4Se is explained based on electrochemically active surface area values determined from under potential deposition studies and also based on DFT calculations. Computational studies reveal the presence of different charge distribution on palladium in all the three phases which is likely to be another reason for varied activities. Palladium selenides are also explored as catalysts towards ORR in alkaline medium. Kinetic parameters and reaction mechanism are determined using RDE studies. All the three phases are found to be active and Pd4Se shows the highest activity, following a direct 4 electron reduction pathway (figure 2b). Other two phases follow 2 electron pathway terminating at hydrogen peroxide stage. Catalytic activity of Pd17Se15 is further improved by Nano structuring of the material and by synthesizing the material on active supports such as rGO, acetylene black and today carbon. ORR plays an important role in metal-air batteries. The palladium chalcogenides are used as electrodes in metal-air batteries. Specific energy density observed in the case of Mg-air primary batteries is higher for Pd4Se than the other two phases (figure 3a). Figure 3. (a) Discharge curves of Mg-O2 battery with different phases of palladium selenides as cathodes. Constant current density of 0.5 mA cm-2 is used for discharge. (b) Characteristic J–V curves of DSSCs with Pd17Se15, Pd7Se4 and Pt as counter electrodes. Versatility of these phases is further studied towards redox reaction in non-aqueous medium (I3-/I-). This reaction plays a crucial role in the regeneration of the dye in dye-sensitized solar cells (DSSC). Palladium selenide phases prepared on FTO plates are employed as counter electrodes in DSSC. The solar light conversion efficiencies are found to be 7.45 and 6.8% for Pd17Se15 and Pd7Se4 respectively and are comparable to that of platinum (figure 3b). The reason for high activities may be attributed to high electronic conductivity and low work function of the phases. The following chapter deals with the synthesis of palladium sulphide phases (Pd4S and Pd16S7) using both hydrothermal and single source precursor methods. Electro catalytic activities of the phases are shown towards HER and ORR and Pd4S exhibits better catalytic activities than that of Pd16S7 phase. Direct electrochemistry of cytochrome c is achieved on Pd4S with ∆E of ~64 mV (figure 4a). Electrochemical oxidation of ethanol, ethylene glycol (EG) and glycerol are also studied on the Pd4S phase and the activity is found to follow the order, glycerol > ethylene glycol > ethanol (figure 4b). Figure 4. (a) Cyclic voltammograms of Pd4S in (1) 0.1 M phosphate buffer solution (pH 7.0) and (2) in presence of 0.2 mM cytochrome c at a scan rate of 50 mVs-1 and (b) Voltammograms of Pd4S in presence of different alcohols (ethanol, EG and glycerol) in 1 M KOH solution at sweep rate of 50 mVs-1. Concentration of alcohols used is 0.1 M. The effect of dimensionality on the electro catalytic activity of nickel selenide phases forms part of the next chapter. Nickel selenide (NiSe) nanostructures possessing different morphologies of wires, spheres and hexagons are synthesized by varying the selenium precursors namely, selenourea, selenium dioxide (SeO2) and potassium selenocyanate (KSeCN), respectively using hydrothermal method. The different selenium precursors result in morphologies that are probably dictated by the by-products as well as relative rates of amorphous selenium formation and dissolution. The three different morphologies are used as catalysts for HER, ORR and glucose oxidation reactions. The wire morphology is found to be better than that of spheres and hexagons for all the reactions. Among the reactions studied, NiSe is found to be good for HER and glucose oxidation while ORR seems to terminate at the peroxide stage. In alkaline medium, nickel forms hydroxides and oxy-hydroxides and these oxyhydroxides are catalytically active towards the oxidation of glucose. Therefore, nickel selenides are employed as highly selective non-enzymatic glucose sensors and detection limit of 5 µM is observed. Electrical measurements on a single nanowire and a hexagon morphology of NiSe are carried out on devices fabricated by focused ion beam (FIB) technique (figure 5). The semiconducting nature of NiSe is revealed in the I-v measurements. The band gap of the material is found to be 1.9 eV and hence the single nanowire and hexagon are shown to act as visible light photodetector. Figure 5. SEM images of (a) single NiSe nanowire and (b) single NiSe hexagon with Pt contacts fabricated by FIB technique. Figure 6. Cyclic voltammograms of NiSe nanowires in 0.5 M aqueous NaOH in the (i) absence and (ii) the presence of 0.5 mM glucose, at a scan rate of 20 mVs-1 and (b) Galvanostatic discharge performance of Ni3Se2 with different morphologies (A, B and C represent Ni3Se2 prepared from SeO2, selenourea and KSeCN respectively). The next chapter includes the synthesis of different morphologies of Ni3Se2 using three different selenium precursors (SeO2, KSeCN and selenourea) and the study of their activities towards electrochemical reactions such as HER and glucose oxidation (figure 6a). Electrical measurements demonstrated the metallic behaviour of the material. These are also shown to be efficient electrode materials in energy storage devices such as supercapacitors with high specific capacitance of 2200 F/g (figure 6b). The studies are summarized in the last chapter with scope for further work. The appendixes show preliminary studies on electrooxidation of glycerol and propanol on Pd supported on TiN, synthesis of other selenides of Ni, Cu, Ag and Ti, and electro synthesis of metal-organic frameworks. (For figures pl refer the abstract pdf file)

<p dir="ltr">Catalyzing a green hydrogen economy can accelerate progress towards achieving the goal of a sustainable energy map with net-zero carbon emissions by rapid strides. An environmentally benign electrochemical energy conversion system is the Polymer Electrolyte Fuel Cell (PEFC) which uses hydrogen as a fuel to produce electricity and is notably used in a variety of markets such as industries, commercial setups, and across the transportation sector, and is gaining prominence for use in heavy-duty vehicles such as buses and trucks. Despite its potential, the commercialization of PEFCs needs to address several challenges which are manifested in the form of mass transport limitations and deleterious mechanisms at the interfacial scale under severe operating conditions. Achieving a robust electrochemical performance in this context is predicated on desired interactions at the triple-phase boundary of the electrochemical engine of the PEFC – the porous cathode catalyst layer (CCL) where the principal oxygen reduction reaction (ORR) takes place. The liquid water produced as a byproduct of the ORR helps minimize membrane dehydration; however, excess water renders the reaction sites inactive causing reactant starvation. In addition, the oxidation of the carbonaceous support in the electrode and loss of valuable electrochemically active surface area (ECSA) pose major barriers that need to be overcome to ameliorate the life expectancy of the PEFC.</p><p dir="ltr">In this thesis, the multimodal physicochemical interactions occurring inside the catalyst layer are investigated through a synergistic blend of visualization and computational techniques. The spatiotemporal dynamics of capillary force-driven liquid transport that ensues concentration polarization thereby affecting the desired response will be probed in detail. The drop in efficacy of the ORR due to competing catalyst aging mechanisms and the impact of degradation stressors on chemical potential-induced instability will be examined. The reaction-transport-mechanics interplay in core-shell nanoparticles, a robust class of electrocatalysts that promises better mass activity compared to the single metal counterparts is further highlighted. Finally, the influence of electrode microstructural attributes on the electrochemical performance of the reverse mode of fuel cell operation, i.e., Proton Exchange Membrane Water Electrolyzers (PEMWEs) is investigated through a mesoscale lens.</p>

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Der steigende weltweite Energiebedarf treibt die Entwicklung sauberer Energiequellen voran, die dazu beitragen werden, den Verbrauch fossiler Brennstoffe zu reduzieren. Brennstoffzellen und Metall-Luft-Batterien sind vielversprechende Alternativen, um traditionelle fossile Energie zu ersetzen und durch die Reduzierung von O2 an der Kathode grünen Strom zu erzeugen. Aufgrund der langsamen Reaktionsraten der Sauerstoffreduktionsreaktion (ORR) ist hierfür jedoch elektrokatalytisches Material mit geringen Kosten und hoher Effizienz erforderlich. In den letzten Jahrzehnten wurde eine Vielzahl von Materialien als Nicht-Pt-Katalysatoren getestet, von metallfreien Katalysatoren bis hin zu Katalysatoren auf Übergangsmetallbasis. Aufgrund des mangelnden Verständnisses des Reaktionsmechanismus und der Wechselwirkung zwischen Elektrolyt und Elektrokatalysator befinden sich neue Designs stickstoffhaltiger Katalysatoren auf Kohlenstoffbasis jedoch noch in der Entwicklungsphase. Zu diesem Zweck wurden verschiedene (in situ) spektroskopische und elektrochemische Techniken eingesetzt, um die Wechselwirkung zwischen N-dotiertem Kohlenstoff und Elektrolyten sowie die katalytischen Mechanismen zu verstehen. Darüber hinaus weisen die neu entwickelten Katalysatoren für die ORR eine überlegene elektrokatalytische Leistung auf, die in dieser Dissertation ausführlich diskutiert wird. Die Struktur-Leistungs-Beziehung unserer ORR-N-dotierten Kohlenstoffkatalysatoren wurde gründlich untersucht. Diese Forschung zeigt, wie die Kombination fortschrittlicher Spektroskopietechniken, einschließlich In-situ-Spektroskopie und elektrochemischer Charakterisierung, ein tieferes Verständnis der Katalysator-/Elektrolyt-Wechselwirkung, des katalytischen Mechanismus und der optimierten elektrokatalytischen Leistung stickstoffhaltiger Kohlenstoffmaterialien, ORR-Katalysatoren, insbesondere nanoporöser N-dotierter Kohlenstoff, fördern kann Eisen-Stickstoff-codotierte Kohlenstoffmaterialien. / Increasing global energy demand drives the development of clean energy sources that will help reduce the consumption of fossil fuels. Fuel cells and metal-air batteries are promising alternatives to replace traditional fossil energy to generate green electricity by reducing O2 at the cathode. However, due to sluggish reaction rates of oxygen reduction reaction (ORR), this requires electrocatalytic material with low cost and high efficiency. Over the last few decades, a variety of materials have been tested as non-Pt catalysts, from metal-free catalysts to transition metal-based catalysts. However, due to the lack of understanding of the reaction mechanism and the interaction between electrolyte and electrocatalysts, new designs nitrogen-containing carbon-based catalysts are still under the development stage. To this aim, a variety of (in situ) spectroscopic and electrochemical techniques to understand N-doped carbon electrocatalysts/electrolyte interaction and catalytic mechanisms have been employed. Moreover, the newly-designed catalysts for ORR demonstrate superior electrocatalytic performance which are discussed in detail in this dissertation. The structure-performance relationship for our ORR N-doped carbon catalysts has been thoroughly investigated. This research highlights how the combination of advanced spectroscopy techniques including in situ spectroscopy and electrochemical characterization may promote a deeper understanding of catalyst/electrolyte interaction, catalytic mechanism and optimized electrocatalytic performance of nitrogen-containing carbon materials ORR catalysts, especially nanoporous N-doped carbon and iron-nitrogen-co-doped carbon materials.

Single-atom catalysts (SACs), as promising alternatives to Pt-based catalysts, suffer from the limited choice of center metals and low single-atom loading. Here, we report a pentacoordinated Zr-based SAC with nontrivial axial O ligands (denoted O−Zr−N−C) for oxygen reduction reaction (ORR). The O ligand downshifts the d-band center of Zr and confers Zr sites with stable local structure and proper adsorption capability for intermediates. Consequently, the ORR performance of O−Zr−N−C prominently surpasses that of commercial Pt/C, achieving a half-wave potential of 0.91 V vs. reversible hydrogen electrode and outstanding durability (92 % current retention after 130-hour operation). Moreover, the Zr site shows good resistance towards aggregation, enabling the synthesis of Zr-based SAC with high loading (9.1 wt%). With the high-loading catalyst, the zinc-air battery (ZAB) delivers a record-high power density of 324 mW cm−2 among those of SAC-based ZABs.

Les électrocatalyseurs conventionnels utilisés dans les piles à combustibles à membrane échangeuse de protons (PEMFC) sont composés de nanoparticules de platine supportées sur des noirs de carbone de forte surface spécifique. A la cathode de la PEMFC, siège de la réaction de réduction de l’oxygène (ORR), le potentiel électrochimique peut atteindre des valeurs élevées - notamment lors de phases arrêt-démarrage - engendrant des dégradations irréversibles du support carboné. Une solution « matériaux » consiste à remplacer ce dernier par des supports à base d’oxydes métalliques. Ceux-ci doivent être résistants à la corrosion électrochimique, conducteurs électroniques et posséder une structure poreuse et nano-architecturée (permettant le transport des réactifs et produits et une distribution homogène de l’ionomère et des nanoparticules de platine). Dans ce travail, nous avons donc élaboré et caractérisé des électrocatalyseurs à base de nanoparticules de platine (Pt) déposées sur du dioxyde d’étain (SnO₂) et de titane (TiO₂) texturés (morphologies aérogel, nanofibres ou « loosetubes ») et conducteurs électroniques (dopés au niobium Nb ou à l’antimoine Sb). Le support permettant d’atteindre les meilleures propriétés électrocatalytiques est un aérogel de SnO₂ dopé à l’antimoine, noté ATO. En particulier, l’électrocatalyseur Pt/ATO présente une activité spécifique vis-à-vis de l’ORR supérieure à celle d’un électrocatalyseur Pt/carbone Vulcan® synthétisé dans les mêmes conditions, suggérant des interactions bénéfiques entre les nanoparticules de Pt et le support oxyde métallique (Strong Metal Support Interactions, SMSI).Des tests de durabilité simulant le fonctionnement d’une PEMFC en conditions automobile ont été effectués en électrolyte liquide à 80 °C sur ces deux électrocatalyseurs : cyclage entre 0,60 et 1,00 V vs l’électrode réversible à hydrogène (RHE) ou entre 1,00 et 1,50 V vs RHE. Le catalyseur Pt/ATO présente une durabilité accrue par rapport au catalyseur Pt/carbone Vulcan® de référence. Cependant, de nouveaux mécanismes de dégradation ont été mis en évidence dans cette étude : tout d’abord, l’élément dopant Sb est progressivement dissout au cours du vieillissement électrochimique, ce qui implique une perte de conductivité électronique. Cette perte est en partie liée à des incursions à bas potentiel, notamment durant les caractérisations électrochimiques. De plus, entre 5 000 et 10 000 cycles de vieillissement électrochimique (entre 0,60 et 1,00 V vs RHE ou entre 1,00 et 1,50 V vs RHE à 57 °C), le matériau support perd sa structure poreuse et forme un film amorphe peu conducteur. / Conventional electrocatalysts used in proton exchange membrane fuel cells (PEMFC) are composed of platinum nanoparticles supported on high specific surface area carbon blacks. At the cathode side of the PEMFC, where the oxygen reduction reaction (ORR) occurs, the electrochemical potential can reach high values - especially during startup-shutdown operating conditions - resulting in irreversible degradation of the carbon support. A “material” solution consists of replacing the carbon with supports based on metal oxides. The latter have to be resistant to electrochemical corrosion, be electronic conductor and have a porous and nano-architectural structure (for the transport of reagents and products and the homogeneous distribution of the ionomer and platinum nanoparticles).In this work, we have developed and characterized electrocatalysts composed of platinum (Pt) nanoparticles based on tin dioxide (SnO2) and titanium dioxide (TiO2) with optimized textural (aerogel, nanofibres or loosetubes morphologies) and electron-conduction properties (doped with niobium Nb or antimony Sb). The best electrocatalytic properties are reached for an antimony-doped SnO2 aerogel support, denoted ATO. The Pt/ATO electrocatalyst has especially a higher specific activity for the ORR than a Pt/carbon Vulcan® electrocatalyst, synthesized in the same conditions, suggesting beneficial interactions between the Pt nanoparticles and the metal oxide support (Strong Metal Support Interactions SMSI).Durability tests simulating automotive operating conditions of a PEMFC were carried out in liquid electrolyte at 57 °C on these two electrocatalysts by cycling between 0.60 and 1.00 V vs the reversible hydrogen electrode (RHE) or between 1.00 and 1.50 V vs RHE. The Pt/ATO electrocatalyst has an increased stability compared to the reference Pt/carbon Vulcan® electrocatalyst. However, new degradation mechanisms were highlighted in this study: first, the doping element (Sb) is progressively dissolved during electrochemical ageing, which implies a loss of electronic conductivity. This loss is partly due to incursions at low potential, including during electrochemical characterizations. Moreover, between 5,000 and 10,000 cycles of the accelerated stress tests (between 0.60 and 1.00 V vs RHE or between 1.00 and 1.50 V vs RHE at 57 °C), the support loses its porous structure and forms a poorly conductive amorphous film.

Research on new, environment-friendly, clean and efficient energy sources have contributed immensely to the development of new technologies for the generation and storage of electrical energy. Heterogeneous ‘electrocatalysis’ involves catalysis of redox reactions where the electrode material, termed as ‘electrocatalyst’ reduces the overpotential and maximizes the current for the processes occurring at the electrode/electrolyte interface. Efficient catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are of paramount importance for electrochemical energy generation and storage applications in water splitting, fuel cells and batteries. However, high cost of Pt catalysts that are commonly used for such applications restricts their commercial viability. In addition, there are issues related to poisoning of the surface under certain conditions. One particular case of direct methanol fuel cells involves problems of methanol tolerance as well. Hence, the on-going search in this direction, is to search for alternate catalysts that can match the performance of Pt. There is a quest for the development of stable and durable electrocatalysts/ supports for various electrochemical redox reactions particularly based on energy storage and conversion. The present thesis is structured in exploring the multi-functional aspects of ternary palladium phosphochalcogenides (PdPS and PdPSe) that possess layer-type structure with high crystallinity. They are semiconducting in nature and possess favorable electrochemical, electrical and optical properties. The chalcogenide compounds crystallize in orthorhombic symmetry with an indirect band gap close to 1.5 eV. The current study shows the versatility of ternary phosphochalcogenides in the bulk phase as well as in small sizes. The electrocatalytic activities of the chalcoenides are found to be dramatically improved by increasing the electrical conductivity by way of forming composites with reduced graphene oxide (rGO). The average crystallite size of the PdPS and PdPSe are 30 μm ±10 μm (figure 1). The composites are prepared by simple hydrothermal methods without use of any reducing agent and are characterized using various physico-chemical techniques. Figure 1. FESEM images of (a) PdPSe and (b) PdPS. In the present investigations, PdPS and its reduced graphene oxide composite (rGO-PdPS) are shown to be very efficient hydrogen evolution electrocatalysts (figure 2a). The bulk form of PdPS is found to be very active and the composite of PdPS with reduced graphene oxide improves the hydrogen evolution performance dramatically, even superior to state of the art, MoS2-based catalysts. Figure 2. (a) Linear sweep voltammograms of rGO, bulk PdPS, rGO-PdPS composite and 40 % Pt-C in 0.5 M H2SO4 solution (pH 0.8). Scan rate used is 1 mV s-1. (b) Tafel plots for PdPS, rGO, rGO-PdPS and 40 wt% Pt-C in 0.5 M H2SO4 at 1 mVs-1 scan rate. The Tafel slope and the exchange current density values associated with hydrogen evolution reaction are 46 mV dec-1 and 1.4 x 10-4 A cm-2 respectively (figure 2b). The stability of the PdPS-based catalyst is found to be excellent retaining same current densities even after thousand cycles. Moreover, post-HER characterization reveals the durability of the material even after cycling for a long time. Preliminary spectroelectrochemical investigations are attempted to gain further insight in to the HER. Subsequently, the PdPS and its composite are explored as ORR catalysts in alkaline medium. The composite of PdPS with rGO is formed to enhance the catalytic activity of pure PdPS and the electron transfer kinetics is found to be very favorable. The kinetics of the oxygen reduction reactions are followed by RDE/RRDE measurements. It is experimentally verified that the composite eletrocatalyst is very stable, efficient and methanol tolerant in alkaline medium. The characteristics of the composite catalyst are comparable with widely used standard Pt-C for ORR (figure 3a). Moreover, ternary phophochalcogenide, PdPS, combined with rGO shows good catalytic activity towards OER and it affords a current density of 10 mA cm-2 at an overpotential of η = 570 mV (figure 3b). Figure 3. (a) Comparative voltammograms for rGO, bulk PdPS, rGO-PdPS and 40 % Pt-C in 1M KOH at 1600 rpm. The potential is swept at a rate of 5 mVs-1. (b) Linear sweep voltammograms of oxygen evolution reaction on rGO-PdPS, PdPS and 40 % Pt-C in 1 M KOH electrolyte. Scan rate 5 mV s-1. Apart from its tri-functional electrocatalytic behavior, PdPS and its rGO composite act as an anode material for Li-ion batteries showing high storage capacity of lithium (figure 4). The capacity fading of bulk PdPS is analyzed using XRD and SEM. The introduction of rGO, a well-known conducting matrix, improves the performance. Palladium phosphorous selenide (PdPSe) and its composite with rGO (rGO-PdPSe) are also explored as electrocatalysts for HER, ORR and OER. They show the tri¬functional electrocatalytic behavior as well. Figure 4. Discharge capacity as a function of number of cycles for PdPS, rGO rGO-PdPS electrode at current density of 35 mAg-1 in rechargeable lithium ion battery. The next chapter deals with single or few layer PdPS where layer-type PdPS is exfoliated by several methods such as ultra-sonication and solvent exfoliation. Various microscopic and spectroscopic techniques have been used to characterize the material. These sheets show significantly improved electrocatalytic activity towards ORR and HER with notably low onset potential and low Tafel slopes. The charge storage capacity also increases by an order from its bulk counterpart. The catalyst shows excellent stability for HER and good methanol tolerance behavior towards ORR is also observed. This opens up possibilities for applications of few-layer ternary phosphosulphides in energy conversion and storage. However, one should be cautious since the exfoliation results in a slightly different composition of the material. Different aspects of electrodeposition of gallium nanoparticles on exfoliated graphite surfaces from aqueous acidic solution forms part of the next study. The electrodeposited surface is characterized by various microscopic and spectroscopic techniques. The presence of surface plasmon peak in the visible region has led us to explore the use of Ga on EG for SERS studies. This preliminary work shows that the Raman signal of R6G is enhanced in the presence of Ga deposited on EG surface. The research work presented in the next part of the thesis deals with the preparation, physicochemical, spectroscopic characterization of room temperature molten electrolytes based on amides. Room temperature ternary molten electrolyte involving a combination of acetamide, urea and gallium nitrate salt is prepared and the molten eutectic is characterized. An electrochemical process is developed for depositing gallium nitride from the ternary molten electrolyte on Au electrode. Gallium ion is reduced at low potentials while nitrate ion is reduced to produce atomic nitrogen, forming gallium nitride under certain conditions. Au coated TEM grid is used for patterning gallium nitride (figure 5). The deposited gallium nitride is further annealed at high temperature to increase the crystalinity and improve the stoichiometry of gallium nitride. Figure 5. The FESEM image of patterned gallium nitride deposited on Au coated TEM grid. Elemental mapping of Ga and N from the same region is given. The last chapter explores the prepration and uses of textured GaN tubes synthesized from GaOOH rod-like morphology. The precursor material is prepared by simple hydrothermal technique, maintaining certain value for the pH of the solution. The thermal treatment under ammonia atmosphere leads to highly crystalline, single phase textured tube- like morphology. The as-prepared material is explored as photoanodes in photoelectrochemical water splitting, dye sensitized solar cells and active substrate for SERS. The appendix-I discusses the Na-ion storage capacity by rGO-PdPS composite whereas appendix-II deals with the synthesis of InN and FeN from ternary molten electrolyte. (For figures pl refer the abstract pdf file)