Magister Scientiae - MSc / Fuel cells convert chemical energy from a fuel into electricity through chemical reaction with oxygen. This possesses some challenges like slow oxygen reduction reaction (ORR), overpotential, and methanol fuel cross over in a direct methanol fuel cell (DMFC). These challenges cause inefficiency and use of higher amounts of the expensive platinum catalyst.Several binary catalysts with better ORR activity have been reported. In this study we investigate the best catalyst with better ORR and MOR performances and lower over-potentials for PEMFC and DMFC applications by comparing the in-house catalysts (10%Pt/C, 20%Pt/C,30%Pt15%Ru/C, 40%Pt20%Ru/C, 30%PtCo/C, 20%Pt20%Cu/C and 20%PtSn/C) with the commercial platinum based catalysts (10%Pt/C, 20%Pt/C, 20%Pt10%Ru/C, 20%PtCo/C,20%PtCu/C and 20%PtSn/C) using the cyclic voltammetry and the rotating disk electrode to determine their oxygen reduction reaction and methanol tolerance. HRTEM and XRD techniques were used to determine their particle size, arrangement and the atomic composition. It was observed that the 20%Pt/C in-house catalyst gave the best ORR activity and higher methanol oxidation current peaks compared to others catalysts followed by 20%Pt10%Ru/C commercial catalyst. The 20%PtCo/C commercial, 30%PtCo/C in-house and 20%PtSn/C in-house catalysts were found to be the most methanol tolerant catalysts making them the best catalysts for ORR in DMFC. It was observed that the ORR activity of 20%PtCo/C commercial and 30%PtCo/C inhouse catalysts were enhanced when heat treated at 350 0C. From XRD and HRTEM studies, the particle sizes were between 2.72nm to 5.02nm with little agglomeration but after the heat treatment, the particles were nicely dispersed on the carbon support.

This thesis describes the fabrication of an induction heating apparatus and its use to directly heat small platinum and gold electrodes in electrolyte solution. The heating characteristics of the electrodes were studied via the entropic shift of redox potential with temperature and change in Faradaic current. Temperature pulse voltammetry (TPV) and cyclic voltammetry were used for temperature calibration under various heating conditions. The maximum temperature reached at a 0.25 mm diam platinum electrode surface in solution was 84 degrees C. At heated electrodes an increase in current was found to be due to convection and diffusion. TPV was performed with inductively heated gold (0.5 mm diam) and platinum electrodes, which gave complete current-potential-temperature information. Induction heated Pt electrodes were employed to investigate the kinetics and mass transfer process of oxygen reduction reaction (ORR) in acidic and alkaline media.

Neste trabalho foi estudada a atividade eletrocatalítica dos eletrocatalisadores nanoestruturados de Ru, Ir, Hf ou La suportados em carbono Printex 6L frente à Reação de Redução de Oxigênio (RRO) em meio ácido. Inicialmente analisou-se a influência do Método de Impregnação e dos Precursores Poliméricos (MPP), também conhecido como Pechini para os eletrocatalisadores RuO2/C e IrO2/C. Ficou evidente neste estudo, que os materiais obtidos pelo MPP apresentaram uma maior eficiência de corrente para a eletrogeração de H2O2 quando comparado ao método da Impregnação. Na etapa seguinte, os eletrocatalisadores HfO2/C e LaONO3/C foram preparados apenas pelo MPP. As propriedades estruturais, morfológicas e de superfície foram investigadas por meio das técnicas de caracterização DRX, FRX, MET, XPS e TG. De acordo com os dados de DRX e MET, verificou-se que o método de incorporação do metal no carbono Printex 6L favoreceu a formação dos óxidos nanoestruturados. Ademais, foi verificado que os eletrocatalisadores obtidos pelo método de Pechini apresentam menores tamanho de cristalitos (1 a 5 nm), melhor distribuição dos óxidos sobre a matriz de carbono (menos aglomerados) e menores tamanhos de partículas. O comportamento eletroquímico dos eletrocatalisadores foi avaliado através das voltametrias lineares (curvas de polarização) obtidas pelo eletrodo de disco anel rotatório (RRDE). Os resultados obtidos pelas voltametrias lineares, cálculos de eficiência de corrente de H2O2 (H2O2 %), número total de elétrons trocados (nt) e de Koutecký-Levich mostraram que a incorporação dos eletrocatalisadores (Ru e Ir) no carbono Printex 6L obtidos por ambos os métodos de síntese influenciaram negativamente na eletrogeração de H2O2. Neste caso, os eletrocatalisadores de Ru e Ir apresentaram uma tendência ao mecanismo via 4 elétrons, ou seja, geração de H2O como produto final da RRO. Os resultados mostraram ainda que os eletrocatalisadores contendo maiores teores de Hf, apresentaram maiores eficiência de corrente para H2O2 quando comparado ao carbono Printex 6L, uma vez que o catalisador contendo 15 % de Hf apresentou valores de eficiência de corrente de H2O2 próximos a 80 % e número de elétrons de 2,4. Além disso, foi observado também um deslocamento no potencial de aproximadamente 200 mV para valores mais positivos, o que significa um menor consumo energético em termos de eletrogeração de H2O2. Para os eletrocatalisadores à base de La, a amostra contendo 7% apresentou uma melhor eficiência de corrente de H2O2, com valores próximos a 87% e número de elétrons de 2,3, além de um deslocamento do potencial de aproximadamente 250 mV para valores mais positivos. Pode-se inferir então, que os eletrocatalisadores de Hf e La obtidos pelo método de Pechini são promissores para utilização em Eletrodos de Difusão Gasosa (EDG) visando a eletrogeração in situ de H2O2, visto que apresentam uma tendência a mecanismo via 2 elétrons. / In the present work, the electrocatalytic activity of nanostructured electrocatalysts based on Ru, Ir, Hf or La supported in Printex L6 front of Oxygen Reduction Reaction (ORR) in an acid medium were studied. Initially, the influence of impregnation methods and polymeric precursors (MPP), also known as Pechini for the electrocatalysts RuO2/C and IrO2/C, was analyzed. It was evidenced in this study, that the materials obtained by MPP presented bigger efficiency for H2O2 electrogeneration when compared to the impregnation method. In the following stage, the electrocatalysts HfO2/C and LaONO3/C were prepared only by MPP. The structural properties and surface morphology were investigated by means of the characterization techniques DRX, FRX, TEM, XPS and TG. According to the XRD and TEM data, it was found that the method of metal incorporation in Printex 6L carbon promoted the formation of nanostructured oxides. Moreover, it was verified that the electrocatalysts obtained by Pechini\'s method presented smaller crystallite size (1 to 5 nm), better distribution of the oxides on the carbon matrix (fewer clusters) and smaller particle sizes. The electrochemical behavior of the electrocatalysts were evaluated by linear voltammetry (polarization curves) obtained by rotating ring-disk electrode (RRDE). The results obtained by linear voltammetry, calculations of current efficiency of H2O2 (H2O2 %), total number of exchanged electrons (nt) and of Koutecký-Levich showed that the incorporation of the electrocatalysts (Ru and Ir) in Printex 6L carbon obtained by both methods of synthesis influenced negatively on the electrogeneration of H2O2. In that case, the Ru and Ir electrocatalysts showed a tendency to a 4 electrons mechanism, that is, generation of H2O as final product of the ORR. The results also showed that the electrocatalysts containing higher Hf content, presented higher current efficiency for H2O2 when compared to carbon Printex L6, since that the catalyst containing 15% of Hf presented values of current efficiency for H2O2 around 80% and number of electrons of 2.4. Furthermore, a potential displacement for positive values of approximately 200 mV was also observed which means lower energy consumption in terms of H2O2 electrogeneration. For the La based electrocatalysts, the sample containing 7% showed better current efficiency for H2O2, with values near 87% and number of electrons of 2.3, besides a potential displacement of approximately 250 mV for more values positive. It can be inferred that the Hf and La electrocatalysts obtained by Pechini\'s method are promising for use in Gas Diffusion Electrodes (GDE) aiming in situ electrogeneration of H2O2, since they exhibit a tendency to a mechanism via 2 electrons.

Neste trabalho foi estudada a atividade eletrocatalítica dos eletrocatalisadores nanoestruturados de Ru, Ir, Hf ou La suportados em carbono Printex 6L frente à Reação de Redução de Oxigênio (RRO) em meio ácido. Inicialmente analisou-se a influência do Método de Impregnação e dos Precursores Poliméricos (MPP), também conhecido como Pechini para os eletrocatalisadores RuO2/C e IrO2/C. Ficou evidente neste estudo, que os materiais obtidos pelo MPP apresentaram uma maior eficiência de corrente para a eletrogeração de H2O2 quando comparado ao método da Impregnação. Na etapa seguinte, os eletrocatalisadores HfO2/C e LaONO3/C foram preparados apenas pelo MPP. As propriedades estruturais, morfológicas e de superfície foram investigadas por meio das técnicas de caracterização DRX, FRX, MET, XPS e TG. De acordo com os dados de DRX e MET, verificou-se que o método de incorporação do metal no carbono Printex 6L favoreceu a formação dos óxidos nanoestruturados. Ademais, foi verificado que os eletrocatalisadores obtidos pelo método de Pechini apresentam menores tamanho de cristalitos (1 a 5 nm), melhor distribuição dos óxidos sobre a matriz de carbono (menos aglomerados) e menores tamanhos de partículas. O comportamento eletroquímico dos eletrocatalisadores foi avaliado através das voltametrias lineares (curvas de polarização) obtidas pelo eletrodo de disco anel rotatório (RRDE). Os resultados obtidos pelas voltametrias lineares, cálculos de eficiência de corrente de H2O2 (H2O2 %), número total de elétrons trocados (nt) e de Koutecký-Levich mostraram que a incorporação dos eletrocatalisadores (Ru e Ir) no carbono Printex 6L obtidos por ambos os métodos de síntese influenciaram negativamente na eletrogeração de H2O2. Neste caso, os eletrocatalisadores de Ru e Ir apresentaram uma tendência ao mecanismo via 4 elétrons, ou seja, geração de H2O como produto final da RRO. Os resultados mostraram ainda que os eletrocatalisadores contendo maiores teores de Hf, apresentaram maiores eficiência de corrente para H2O2 quando comparado ao carbono Printex 6L, uma vez que o catalisador contendo 15 % de Hf apresentou valores de eficiência de corrente de H2O2 próximos a 80 % e número de elétrons de 2,4. Além disso, foi observado também um deslocamento no potencial de aproximadamente 200 mV para valores mais positivos, o que significa um menor consumo energético em termos de eletrogeração de H2O2. Para os eletrocatalisadores à base de La, a amostra contendo 7% apresentou uma melhor eficiência de corrente de H2O2, com valores próximos a 87% e número de elétrons de 2,3, além de um deslocamento do potencial de aproximadamente 250 mV para valores mais positivos. Pode-se inferir então, que os eletrocatalisadores de Hf e La obtidos pelo método de Pechini são promissores para utilização em Eletrodos de Difusão Gasosa (EDG) visando a eletrogeração in situ de H2O2, visto que apresentam uma tendência a mecanismo via 2 elétrons. / In the present work, the electrocatalytic activity of nanostructured electrocatalysts based on Ru, Ir, Hf or La supported in Printex L6 front of Oxygen Reduction Reaction (ORR) in an acid medium were studied. Initially, the influence of impregnation methods and polymeric precursors (MPP), also known as Pechini for the electrocatalysts RuO2/C and IrO2/C, was analyzed. It was evidenced in this study, that the materials obtained by MPP presented bigger efficiency for H2O2 electrogeneration when compared to the impregnation method. In the following stage, the electrocatalysts HfO2/C and LaONO3/C were prepared only by MPP. The structural properties and surface morphology were investigated by means of the characterization techniques DRX, FRX, TEM, XPS and TG. According to the XRD and TEM data, it was found that the method of metal incorporation in Printex 6L carbon promoted the formation of nanostructured oxides. Moreover, it was verified that the electrocatalysts obtained by Pechini\'s method presented smaller crystallite size (1 to 5 nm), better distribution of the oxides on the carbon matrix (fewer clusters) and smaller particle sizes. The electrochemical behavior of the electrocatalysts were evaluated by linear voltammetry (polarization curves) obtained by rotating ring-disk electrode (RRDE). The results obtained by linear voltammetry, calculations of current efficiency of H2O2 (H2O2 %), total number of exchanged electrons (nt) and of Koutecký-Levich showed that the incorporation of the electrocatalysts (Ru and Ir) in Printex 6L carbon obtained by both methods of synthesis influenced negatively on the electrogeneration of H2O2. In that case, the Ru and Ir electrocatalysts showed a tendency to a 4 electrons mechanism, that is, generation of H2O as final product of the ORR. The results also showed that the electrocatalysts containing higher Hf content, presented higher current efficiency for H2O2 when compared to carbon Printex L6, since that the catalyst containing 15% of Hf presented values of current efficiency for H2O2 around 80% and number of electrons of 2.4. Furthermore, a potential displacement for positive values of approximately 200 mV was also observed which means lower energy consumption in terms of H2O2 electrogeneration. For the La based electrocatalysts, the sample containing 7% showed better current efficiency for H2O2, with values near 87% and number of electrons of 2.3, besides a potential displacement of approximately 250 mV for more values positive. It can be inferred that the Hf and La electrocatalysts obtained by Pechini\'s method are promising for use in Gas Diffusion Electrodes (GDE) aiming in situ electrogeneration of H2O2, since they exhibit a tendency to a mechanism via 2 electrons.

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).

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)

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.

The pressure on clean and sustainable energy supplies is increasing. In this regard energy conversion by electrochemical processes plays a major role, for both fuel cell reactions and electrolysis reactions. The sulphur dioxide oxidation reaction (SOR) is a common reaction found in the Hybrid Sulphur Cycle (HyS) and the HyS is a way to produce large-scale hydrogen (H2). The problem with the use of the HyS and fuel cells is the cost involved as large amounts of Pt are required for effective operation. The aim of the study was to determine whether there was an alternative catalyst which was more efficient and cost-effective than Pt. The oxygen reduction reaction (ORR), the ethanol oxidation reaction (EOR) and SOR were studied by means of different electrochemical techniques (cyclovoltammetry (CV), linear polarization (LP) and rotating disk electrode (RDE)) on polycrystalline platinum (Pt) and palladium (Pd). The SRR and EOR are common reactions occurring at the cathode and anode, respectively, in fuel cells and these reactions have been investigated extensively. The reason for studying the reactions was as a preparation for the SOR. This study compared polycrystalline Pt and Pd for the different reactions, with the main focus on the SOR as Pd is considerably cheaper than Pt, and for the SOR polycrystalline Pd has by no means been investigated intensively. Polycrystalline Pt and Pd were compared by different electrochemical techniques and analyses. The Koutecky-Levich and Levich analyses were used to (i) calculate the number of e- involved in the relevant reaction, (ii) to determine whether the reaction was mass transfer controlled at high overpotentials and (iii) whether the reaction mechanism changed with potential. Next the kinetic current density ( k) was calculated from Koutecky-Levich analyses, which was further used for Tafel slope analyses. If it was not possible to carry out the analyses, the activation energy (Ea) was used to determine the electrocatalytic activity of the catalyst. The electrocatalytic activity was also determined by comparing onset potentials (Es), peak potentials (Ep) and limited/maximum current density ( b/ p) of each catalyst. This study was only a preliminary study for the SOR and therefore, further studies are certainly required. It seemed Pd shows better electrocatalytic activity than Pt for the SRR in an alkaline electrolyte because of similar Es, but Pd produced a higher cathodic current density. Pt showed a lower Es than Pd for the SRR in an acid electrolyte, but Pd delivered a higher cathodic current density. This, therefore, means that the SRR in an acid electrolyte is kinetically more favourable on Pd than on Pt. For the EOR better electrocatalytic activity was obtained with Pd than with Pt in an alkaline electrolyte due to higher current densities at lower potentials and Pd showed lower Ea values than Pt in the potential range normally used for fuel cells. Pd was inactive for EOR in an acid electrolyte, while a reaction occurred on Pt. A possible reason for this observation may be due to the H2 absorbing strongly on Pd thus blocking the active positions on the electrode surfaces, preventing further reaction. Pd showed higher electrocatalytic activity for the SOR due to lower Es and higher current densities at low potentials. From the RDE studies it was established that the SRR in an alkaline electrolyte on polycrystalline Pt and Pd was mass transfer controlled at low potentials (high overpotentials), but the SRR in an acid electrolyte was only mass transfer controlled on Pt. The SOR was not mass transfer controlled on polycrystalline Pt and Pd at high potentials (high overpotentials). These assumptions were confirmed by Levich analysis. Using Koutecky-Levich analysis, it was determined that the reaction mechanism on polycrystalline Pt and Pd changed with potential for SRR in an alkaline electrolyte and the SOR. For the SRR in an acid electrolyte the reaction mechanism remained constant with changes in potential on polycrystalline Pd, but the reaction mechanism on polycrystalline Pt changed with potential. These assumptions were confirmed by the number of e-, calculated using Koutecky-Levich analyses. Levich and Koutecky-Levich analyses were not performed for EOR as an increase in rotation speed did not produce an increase in current density. Tafel slope analyses were conducted by making use of overpotentials and k, where possible. As in the case of ethanol, it was not possible to execute Koutecky-Levich analyses and, therefore, it was not possible to perform Tafel slope analyses using k. Tafel slope analyses for the EOR was therefore performed with normal current densities at 0 rotations per minute (rpm). The reaction mechanisms on Pt and Pd for the SRR in alkaline and acidic electrolytes differed due to different Tafel slopes. Pt and Pd displayed similar Tafel slopes for the EOR in alkaline electrolyte, thus suggesting that the reaction mechanisms on Pt and Pd were the same. For the SOR it seemed that the reaction mechanism on Pt and Pd were similar because of similar Tafel slopes. This was only a preliminary and comparative study for polycrystalline Pt and Pd, and the reaction mechanism was not further studied by means of spectroscopic techniques. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

The pressure on clean and sustainable energy supplies is increasing. In this regard energy conversion by electrochemical processes plays a major role, for both fuel cell reactions and electrolysis reactions. The sulphur dioxide oxidation reaction (SOR) is a common reaction found in the Hybrid Sulphur Cycle (HyS) and the HyS is a way to produce large-scale hydrogen (H2). The problem with the use of the HyS and fuel cells is the cost involved as large amounts of Pt are required for effective operation. The aim of the study was to determine whether there was an alternative catalyst which was more efficient and cost-effective than Pt. The oxygen reduction reaction (ORR), the ethanol oxidation reaction (EOR) and SOR were studied by means of different electrochemical techniques (cyclovoltammetry (CV), linear polarization (LP) and rotating disk electrode (RDE)) on polycrystalline platinum (Pt) and palladium (Pd). The SRR and EOR are common reactions occurring at the cathode and anode, respectively, in fuel cells and these reactions have been investigated extensively. The reason for studying the reactions was as a preparation for the SOR. This study compared polycrystalline Pt and Pd for the different reactions, with the main focus on the SOR as Pd is considerably cheaper than Pt, and for the SOR polycrystalline Pd has by no means been investigated intensively. Polycrystalline Pt and Pd were compared by different electrochemical techniques and analyses. The Koutecky-Levich and Levich analyses were used to (i) calculate the number of e- involved in the relevant reaction, (ii) to determine whether the reaction was mass transfer controlled at high overpotentials and (iii) whether the reaction mechanism changed with potential. Next the kinetic current density ( k) was calculated from Koutecky-Levich analyses, which was further used for Tafel slope analyses. If it was not possible to carry out the analyses, the activation energy (Ea) was used to determine the electrocatalytic activity of the catalyst. The electrocatalytic activity was also determined by comparing onset potentials (Es), peak potentials (Ep) and limited/maximum current density ( b/ p) of each catalyst. This study was only a preliminary study for the SOR and therefore, further studies are certainly required. It seemed Pd shows better electrocatalytic activity than Pt for the SRR in an alkaline electrolyte because of similar Es, but Pd produced a higher cathodic current density. Pt showed a lower Es than Pd for the SRR in an acid electrolyte, but Pd delivered a higher cathodic current density. This, therefore, means that the SRR in an acid electrolyte is kinetically more favourable on Pd than on Pt. For the EOR better electrocatalytic activity was obtained with Pd than with Pt in an alkaline electrolyte due to higher current densities at lower potentials and Pd showed lower Ea values than Pt in the potential range normally used for fuel cells. Pd was inactive for EOR in an acid electrolyte, while a reaction occurred on Pt. A possible reason for this observation may be due to the H2 absorbing strongly on Pd thus blocking the active positions on the electrode surfaces, preventing further reaction. Pd showed higher electrocatalytic activity for the SOR due to lower Es and higher current densities at low potentials. From the RDE studies it was established that the SRR in an alkaline electrolyte on polycrystalline Pt and Pd was mass transfer controlled at low potentials (high overpotentials), but the SRR in an acid electrolyte was only mass transfer controlled on Pt. The SOR was not mass transfer controlled on polycrystalline Pt and Pd at high potentials (high overpotentials). These assumptions were confirmed by Levich analysis. Using Koutecky-Levich analysis, it was determined that the reaction mechanism on polycrystalline Pt and Pd changed with potential for SRR in an alkaline electrolyte and the SOR. For the SRR in an acid electrolyte the reaction mechanism remained constant with changes in potential on polycrystalline Pd, but the reaction mechanism on polycrystalline Pt changed with potential. These assumptions were confirmed by the number of e-, calculated using Koutecky-Levich analyses. Levich and Koutecky-Levich analyses were not performed for EOR as an increase in rotation speed did not produce an increase in current density. Tafel slope analyses were conducted by making use of overpotentials and k, where possible. As in the case of ethanol, it was not possible to execute Koutecky-Levich analyses and, therefore, it was not possible to perform Tafel slope analyses using k. Tafel slope analyses for the EOR was therefore performed with normal current densities at 0 rotations per minute (rpm). The reaction mechanisms on Pt and Pd for the SRR in alkaline and acidic electrolytes differed due to different Tafel slopes. Pt and Pd displayed similar Tafel slopes for the EOR in alkaline electrolyte, thus suggesting that the reaction mechanisms on Pt and Pd were the same. For the SOR it seemed that the reaction mechanism on Pt and Pd were similar because of similar Tafel slopes. This was only a preliminary and comparative study for polycrystalline Pt and Pd, and the reaction mechanism was not further studied by means of spectroscopic techniques. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014