The electrocatalytic nitrogen reduction reaction (NRR) is of considerable interest due to its potential for less energy intensive and environmentally friendly ammonia production which is critical for agricultural and clean energy applications. However, the selectivity of NRR compared to the hydrogen evolution reaction (HER) often poses challenges for various catalysts, including Earth-abundant transition metal oxynitrides like Ti, V, and Co. In this work, a comparative analysis of the selectivity of these three metal oxynitrides was conducted, each having different metal oxophilicities. A combination of electrochemical, surface characterizations and density functional theory (DFT) calculations were employed to directly assess NRR and HER activities under the same reaction conditions. Results show that cobalt oxynitrides exhibit NRR activity at pH 10, involving the electrochemical reduction of both lattice-bound nitrogen and dissolved N2, although more HER activity was observed. In contrast, vanadium oxynitride films displayed HER inactivity at pH 7 and 10 but demonstrated NRR activity at pH 7, while titanium oxynitrides were active at pH 3.2 but inactive under neutral and basic pH conditions. These comprehensive studies highlight substantial variations in HER and NRR selectivity based on transition metal oxophilicity/azaphilicity, indicating distinct mechanisms governing NRR and HER mechanisms.

[ES] La presente tesis doctoral se centra en la investigación y desarrollo de catalizadores avanzados con aplicaciones en la industria química. Desde la síntesis de nanopartículas de Rh2P hasta la exploración de catalizadores de cobalto dopados con heteroátomos, se ha buscado no sólo la eficiencia catalítica, sino también minimizar el impacto de estos procesos, permitiendo la producción de productos químicos esenciales con un menor consumo de recursos. Se ha investigado el proceso de síntesis de nanopartículas de Rh2P soportadas por impregnación húmeda seguida de pirólisis para su uso en reacciones de hidroformilación. Para ello, se han descrito y comparado dos métodos que se diferencian en los precursores empleados: uno a partir de un único precursor de fósforo y rodio y el otro a partir de dos precursores separados. Posteriormente, se ha realizado una caracterización de los materiales que ha permitido relacionar la forma de incorporar el fósforo con la composición de las partículas, determinando que el contenido de óxido de rodio es mayor cuando se emplean dos precursores. Para analizar la contribución del fósforo en el sistema se han preparado nanopartículas de rodio metálico soportadas para estudiar la influencia del fósforo en la composición y el tamaño de partícula. Se han analizado otros aspectos como la carga metálica, la temperatura de pirólisis y el uso de diferentes soportes empleando la microscopía electrónica; y se ha estudiado su capacidad de disociación de hidrógeno mediante ensayos de intercambio isotópico. Tras la caracterización de los materiales, éstos se han empleado como electrocatalizadores en la reacción de evolución del hidrógeno, en la que se ha estudiado su aplicabilidad a pH ácido, básico y neutro con bajas cargas de metal. Se ha demostrado que las especies de Rh2P son más activas que las de Rh metálico en la producción de hidrógeno y que el método de síntesis influye en la actividad catalítica. Para completar los ensayos experimentales, se han realizado cálculos DFT que han permitido comprobar por un lado la influencia de los defectos en la red de Rh2P y por otro, si las capas de carbono son efectivas en la protección de las nanopartículas. Se han aplicado los materiales de Rh y Rh2P en la hidroformilación de olefinas donde se ha estudiado la actividad catalítica, la influencia del fósforo y la estabilidad de los catalizadores. Se ha demostrado que el material Rh2P-1@C, obtenido a partir de un solo precursor de rodio y fósforo, es un sustituto viable del catalizador homogéneo RhCl(PPh3)3 ya que las energías de activación calculadas son muy cercanas entre sí. Se ha estudiado la influencia de algunos parámetros en la estabilidad del catalizador, como la temperatura de pirólisis a la que se sintetiza el material, demostrando que existe una relación inversa entre la temperatura de pirólisis y la cantidad de metal lixiviado durante la reacción. Otro factor estudiado es el del soporte, donde además del carbón se han empleado óxidos inorgánicos (Al2O3, CeO2, La2O3, ZnO), poniendo de manifiesto la superioridad del carbón como soporte no sólo en términos de actividad catalítica, sino de estabilidad del material. El catalizador Rh2P-1@C ha sido satisfactoriamente probado en reacciones de hidroformilación de etileno en un reactor de lecho fijo y en la reacción de hidroaminometilación. Como alternativa a los materiales de rodio, se han desarrollado catalizadores de cobalto dopados con heteroátomos (fósforo y nitrógeno). Éstos se han empleado en la hidroformilación de olefinas, donde se ha evaluado la influencia del dopaje en la actividad catalítica y en su estabilidad. Se ha comprobado la efectividad del recubrimiento del carbón dopado con nitrógeno al aumentar su estabilidad en comparación con las nanopartículas de cobalto sin recubrir y se ha demostrado que las nanopartículas de Co2P, al contrario que las Rh2P, no son más activas que las de Co metálico. / [CA] Aquesta tesis doctoral es centra en la investigació i la síntesis de nous catalitzadors amb aplicacions en reaccions químics d'interès industrial. Durant el transcurs de la síntesis de les nanopartícules fins a l'estudi de nous catalitzadors, s'ha cercat no únicament l'eficiència catalítica dels processos sinó la disminució de l'impacte dels mateixos. S'ha investigat el procés de síntesis de nanopartícules de Rh2P suportades a través del mètode d'impregnació humida seguida d'un procés de piròlisis, per al posterior ús d'aquests materials en reaccions d'hidroformil·lació. Per tal fi, s'han descrit i comparat dos mètodes de síntesis que es diferencien en els precursors utilitzats: un a partir d'un únic precursor com a font de fòsfor i rodi, i l'altre a partir dels dos precursors - de rodi i fòsfor - per separats. Posteriorment, s'ha portat a terme una caracterització completa de cadascun dels materials preparats que a permès relacionar la forma com s'incorpora el fòsfor el material en funció de la composició de les partícules. Amb l'objectiu d'analitzar la contribució de les espècies de fòsfor en el sistema s'han preparat un material basat en nanopartícules de rodi metàl·lic suportades. S'han analitzat, mitjançant la microscòpia electrònic, com influeixen en aquest paràmetre altres aspectes, com ara: la càrrega metàl·lica, la temperatura de piròlisis i l'ús de diferents suports. Tanmateix, s'ha estudiat la capacitat dissociativa de l'hidrogen per part d'aquests materials mitjançant assajos d'intercanvi isotòpic. Després de la caracterització dels materials, aquests s'han utilitzat com a electrocatalitzadors en la reacció d'evolució de l'hidrogen en la què s'ha estudiat la utilització d'aquests materials, utilitzant una baixa càrrega metàl·lica, en diferents medis (pH àcid, bàsic i neutre). Els treballs han demostrat que les espècies de Rh2P són més actives que les de Rh metàl·lic i que el mètode de síntesis també influeix en l'activitat catalítica. Per a completar la part experimental, s'han realitzat càlculs DFT que han permès comprovar tan la influència dels defectes de la xarxa de Rh2P com la influència de les capes de carboni en quant a la protecció de les pròpies nanopartícules. Tot seguit, els materials de Rh y Rh2P s'han aplicat en la reacció d'hidroformil·lació d'olefines on s'ha estudiat l'activitat catalítica, la influència de les espècies de fòsfor i l'estabilitat dels catalitzadors utilitzats. S'ha demostrat que el material de Rh2P-1@C, obtingut a partir d'un sol precursor de rodi i fòsfor, pot convertir-se en un possible candidat per a substituir el catalitzador homogeni RhCl(PPh3)3. També, s'ha estudiat la influència d'alguns paràmetres relacionats amb l'estabilitat del catalitzador, com ara la temperatura de piròlisis i la quantitat de metall lixiviat durant la reacció. Una altre factor que ha estat objecte d'estudi és el suport, on a més a més del carboni s'han utilitzat òxids inorgànics - Al2O3, CeO2, La2O3 i ZnO - posant de manifest la superioritat del carboni com a suport; no només en termes d'activitat catalítica sinó d'estabilitat del propi material. A més a més, el catalitzador Rh2P-1@C s'ha provat satisfactòriament en reaccions d'hidroformil·lació de l'etilè en un reactor de tipus llit fix, i en la reacció d'hidroaminometil·lació. Com a alternativa als materials de rodi, s'han sintetitzat catalitzadores de cobalt dopats amb heteroàtoms (fòsfor i nitrogen). Aquests materials, s'han utilitzat en la reacció d'hidroformil·lació d'olefines, on s'ha estudiat la influència del dopatge en l'activitat catalítica i en la seva estabilitat. S'ha comprovat que l'efectivitat del recobriment del carbó dopat amb nitrogen augmenta l'estabilitat del catalitzador en front de les nanopartícules de cobalt sense recobriment, i s'ha aconseguit demostrar que les nanopartícules de Co2P, a diferència de les de Rh2P, no resulten ser més actives que les de Co metàl·lic. / [EN] The present doctoral thesis focuses on the research and development of advanced catalysts with applications in the chemical industry. From the synthesis of Rh2P nanoparticles to the exploration of cobalt catalysts doped with heteroatoms, the primary goal has been to enhance not only the catalytic efficiency but also to minimize the environmental impact of these processes. In a time where the transition to clean energy sources and environmentally friendly processes is of utmost importance, advances in catalysis and material synthesis play a crucial role in the quest for more sustainable solutions in the chemical industry. The synthesis of supported Rh2P nanoparticles by wet impregnation followed by pyrolysis has been investigated. Two methods, differing only in the precursors employed, have been described and compared: one using a single precursor of phosphorus and rhodium, and the other using two separate precursors. Material characterization has revealed the relationship between the method of phosphorus incorporation and particle composition, demonstrating that the rhodium oxide content is higher when two separate precursors are used. Additionally, supported metallic rhodium nanoparticles were prepared to study the influence of phosphorus on particle size and composition. In this chapter, other factors have also been analyzed such as metal loading, pyrolysis temperature, and the use of different supports, employing electron microscopy. Finally, the hydrogen dissociation capacity of the catalysts has been compared through isotopic exchange assays, and the presence of carbon patches around the nanoparticles has been studied using Raman spectroscopy. Following material characterization, these materials have been employed as electrocatalysts in the hydrogen evolution reaction, where their applicability at acidic, basic and neutral pH at one of the lowest metal loadings described has been studied. It has been demonstrated that Rh2P species are more active than metallic Rh species in hydrogen production, and the synthesis method influences catalytic activity. To complement experimental assays, DFT calculations have been performed, allowing for an examination of the influence of defects in the Rh2P lattice and an assessment of the effectiveness of carbon layers in protecting the nanoparticles. Rh and Rh2P materials have been applied in the hydroformylation of olefins, where catalytic activity, the influence of phosphorus, and catalyst stability have been investigated. It has been shown that the material Rh2P-1@C, obtained from a single precursor of rhodium and phosphorus, is a viable substitute for the homogeneous catalyst RhCl(PPh3)3, as their apparent activation energies are very close. The influence of parameters affecting catalyst stability, such as the pyrolysis temperature at which the material is synthesized, has been studied, demonstrating an inverse relationship between pyrolysis temperature and the amount of leached metal during the reaction. Another factor studied is that of the support, where, in addition to carbon, inorganic oxides (Al2O3, CeO2, La2O3, ZnO) have been employed, highlighting the superiority of carbon as support not only in terms of catalytic activity but also material stability. At the end of this chapter, the Rh2P-1@C catalyst has been applied in the hydroformylation of ethylene in a fixed-bed reactor and in the hydroaminomethylation reaction. As an alternative to rhodium materials, cobalt catalysts doped with heteroatoms (phosphorus and nitrogen) have been developed. These catalysts have been employed in the hydroformylation of olefins, where the influence of doping on the catalytic activity and leaching has been evaluated. The effectiveness of nitrogen-doped carbon coating in suppressing leaching has been demonstrated compared to undoped cobalt nanoparticles, and it has been shown that Co2P nanoparticles, unlike Rh2P, are not more active than metallic Co nanoparticles. / Galdeano Ruano, CP. (2024). Catalizadores heterogéneos de rodio y cobalto para la hidroformilación de olefinas y la reacción de evolución del hidrógeno [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/203121

Différents systèmes polyoxométallates@M-colorants ont été réalisés dans cette thèse pour électrochimique dégagement d'hydrogène catalytique et génération photocourant.• Des films hybrides, basés sur des interactions électrostatiques entre une porphyrine tetracationique et des nanoparticules stabilisées par des POMs du type POM@Pt sur ITO, ont été formés par la méthode dite couche par couche et ont été utilisés pour la génération de H2 ou de photocourant. • Pour améliorer le transfert de charge entre les nanoparticules POM@M et le substrat, la réduction de l'oxyde de graphène a été réalisée pour former des systèmes hybrides rGO/POM@Pt. Le dégagement d'hydrogène a été mesuré.• Les copolymères polycationiques bis-porphyrine ont également été obtenus par électropolymérisation avec des espaceurs bis-pyridinium. Par réaction de métathèse, l’incorporation avec divers POM de type Keggin ou des nanoparticules du type POM@Ag ont ensuite été realise. Leurs performances photovoltaïques ont ensuite été étudiées.• Enfin des films hybrides PEDOT dopés avec des nanoparticules du type POM@M ont également été fabriqués. Les performances photovoltaïques ont été examinés montrant une forte amélioration sous illumination dans le domaine du visible. L’ensemble de ces matériaux hybrides ont montré des propriétés intéressantes pour des applications photovoltaïques et la conversion d'énergie. / Polyoxometalates@M NPs-dyes molecular hybrid systems were realized in this thesis for electrochemical catalytic hydrogen evolution and photocurrent generation. • First, hybrid films, based on electrostatic interactions between the tetracationic porphyrin and POMs@Pt NPs composites on ITO slides, were formed by the so called Layer-by-Layer method for HER and photocurrent generation.• To improve the charge transfer between POMs@M NPs and the substrate, reduced graphene oxide was introduced to form rGO/POMs@Pt NPs hybrid systems. Hydrogen evolution was measured after dropping this composites onto the surface of glassy carbon electrodes.• Polycationic bis-porphyrin copolymers have been also obtained by an electropolymerization leading to the formation of new bis-porphyrin copolymers with pyridinium as spacers. Incorporation with various Keggin type POMs or POMs@Ag was then achieved, their photovoltaic performances were also studied.• POMs@M NPs doped PEDOT hybrids films have been also fabricated. The photovoltaic performances has been examined showing particularly strong enhancement under visible light. In conclusion, these polyoxometalates based hybrids materials have shown interesting properties for photovoltaic application and energy conversion.

Le présent travail a pour objectif d’étudier la synergie entre la déformation plastique et les processus d’adsorption et d’absorption de l’hydrogène en surface et en subsurface du nickel monocristallin. La Réaction d’Evolution de l’Hydrogène (REH) et l’absorption de l’hydrogène en subsurface (RAH)partagent le plus souvent l’intermédiaire commun : l’hydrogène adsorbé (Hads). Le chemin réactionnel de la REH sur des surfaces de nickel (100) en milieu acide sulfurique peut être présenté par un mécanisme de Volmer-Heyrovsky. Les paramètres cinétiques élémentaires correspondants comme les coefficients de symétrie, les enthalpies d'activation, le nombre de sites actifs, ont été simulés via un modèle thermocinétique en utilisant les données expérimentales. Ces paramètres peuvent être affectés par la déformation plastique. Cette dernière modifie la densité et la distribution des dislocations stockées affectant la rugosité de surface à l'échelle atomique et engendrant des sites actifs supplémentaires d'adsorption. En revanche, l’émergence de ligne de glissement à la surface conduit à un phénomène de désactivation associé la formation de plan plus compact (111). L’entrée d’atomes d’hydrogène associée à l’étape de transfert surface-Subsurface peut être mesurée à l’aide d’une méthode potentiostatique de type pulsé. Cette dernière a permis de caractériser la diffusion et le piégeage de l'hydrogène en subsurface. Deux zones peuvent être distinguées, l’une proche de la surface (subsurface) et l’autre au coeur de l'échantillon. Dans ce cas, le coefficient de diffusion associé à la subsurface semble être beaucoup plus élevé que celui obtenu au coeur du métal. En revanche,l’application d’une contrainte mécanique conduit à une augmentation de la densité de pièges. Cette dernière, développée au voisinage de la surface : « subsurface », est plus faible que celle à coeur du matériau, ce qui suggère un effet adoucissant en subsurface. / The present work has for objective to study the synergy between the plastic strain and the processes of adsorption and absorption of hydrogen on the surface and the subsurface of nickel single crystal.Hydrogen Evolution Reaction (HER) and Hydrogen Absorption in subsurface (HAR) share mostly the common intermediate: the adsorbed hydrogen (Hads). The HER pathway on nickel (100) single crystal surfaces in sulphuric acid medium can be related by a Volmer-Heyrovsky mechanism. The corresponding elementary kinetic parameters as symmetry coefficients, activation enthalpies, number of active sites, have been identified via a thermokinetic model using experimental data. These parameters can be affected by defects associted with plastic strain. Irreversible plastic strain modifies the density and the distribution of storage dislocations affecting the surface roughness at atomic scale and generating additional active adsorption sites. Further more, surface emergence of mobile dislocations induces the formation of slip bands, which modify the surface roughness and the electronic state of the surface and increases the (111) surface density. The entry of hydrogen atoms associated to the transfer step surface-Subsurface can be measured using a potentiostatic double-Steptechnique (pulse method). This last allowed to characterizing the diffusion and trapping of hydrogen in the subsurface. Two domains can be distinguished, that of the subsurface and that of the bulk of the sample. In this case, the diffusion coefficient near the surface (subsurface) seems to be much higher than that obtained in the bulk of the metal. On the other hand, the application of mechanical stressleads to an increase of traps density. This last, developed near the surface: « subsurface », is lower than that at the bulk of material, which suggest a softening effect in the subsurface.

The electrocatalytic water decomposition has been investigated in this thesis by means of its two half standard reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). These reactions occur in different locations in a typical electrochemical cell: the anode and the cathode, respectively. Motivated by the lack of understanding about the reaction mechanisms occurring at the anodes and cathodes, we have proposed first: novel representations of typical TiO2 surfaces, based on small cluster systems, which can be used for a quick and more detailed assessment of the OER activities at modified TiO2 surfaces, and secondly we investigated the HER in two sets of model surfaces which represent recently synthesized materials, based on Mo2C and graphene with promising activities toward the HER. We have employed Density Functional Theory (DFT) based methods within both localized and extended basis sets, as implemented in GAMESS and VASP packages, respectively, to examine the structural, electronic and vibrational properties of the proposed models. We propose new reaction mechanisms for the OER on a number of molecular representations of TiO2 electrodes. For each reaction pathway, the free energy profile is computed, at different biases, from the DFT energies, the entropic and the zero-point energy contributions. The mechanisms explored in this thesis are found to be energetically more feasible than alternative reaction pathways considered in previous theoretical works based on molecular representations of the TiO2 surfaces. The representation of the surface of specific, commonly occurring, titanium dioxide crystals (e.g., rutile and anatase) within the small cluster approximation is able to reproduce qualitatively the rutile (110) outperforming of the anatase (001) surface. We subsequently investigate the influence of doping TiO2 surfaces with transition metals (TMs) on the performance of TiO2 -based electrodes for the water splitting electrochemical reaction. Two cluster models of the TM-doped active sites which resemble both the TiO2 anatase (001) and rutile (110) surfaces, respectively, are considered for the evaluation of the water decomposition reaction when a Ti is replaced by a TM atom. A set of TMs spanning from Vanadium to Nickel is considered. The late TMs explored here: Fe, Co and Ni are found to reproduce the observed experimental trends for the overpotentials in TiO2-doped electrodes. In the case of Cr and Mn, the present study predicts an enhancement of the OER activity for the anatase-like clusters while a reduction of this activity is found for the rutile-like ones. The vanadium-doped structures do not show relevant influence in the OER activity compared to pure TiO2-based cluster models. The last part of this work is devoted to the theoretical study of the HER on recently found materials based on the synergistic combination of molybdenum carbide and graphene layers. We propose two major structural models to describe the HER mechanism within the framework of DFT: Mo2C-based clusters adsorbed on carbon nanosheets and the Mo2C (001) surface covered by pure and nitrogen-doped graphene layers. The former system evaluates the influence of Mo2C nanoparticles adsorbed on carbon nanosheets towards the HER. The second one is employed to gain insight about the high HER activity observed in molybdenum carbide anchored on nitrogen-doped porous carbon nanosheets (Mo2C@2D-NPC), recently synthesized. The H-adsorption free energy has been used as a principal descriptor to asses the HER activity at the proposed model active sites. It resembles the value for the best state of the art catalyst for the HER (i.e., platinum at carbon substrate Pt@C) in some of the proposed structural models. Furthermore, a pH-correction is added within a simplified model, to the H-adsorption free energy barrier in every proposed structure. The pH dependence of the H-adsorption free energy barriers allows the assessment of the HER at acidic and alkaline conditions simultaneously. An overall agreement with experimental results is found and further predictions, promoting the development of better HER catalysts, have been done.

The electrocatalytic water decomposition has been investigated in this thesis by means of its two half standard reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). These reactions occur in different locations in a typical electrochemical cell: the anode and the cathode, respectively. Motivated by the lack of understanding about the reaction mechanisms occurring at the anodes and cathodes, we have proposed first: novel representations of typical TiO2 surfaces, based on small cluster systems, which can be used for a quick and more detailed assessment of the OER activities at modified TiO2 surfaces, and secondly we investigated the HER in two sets of model surfaces which represent recently synthesized materials, based on Mo2C and graphene with promising activities toward the HER. We have employed Density Functional Theory (DFT) based methods within both localized and extended basis sets, as implemented in GAMESS and VASP packages, respectively, to examine the structural, electronic and vibrational properties of the proposed models. We propose new reaction mechanisms for the OER on a number of molecular representations of TiO2 electrodes. For each reaction pathway, the free energy profile is computed, at different biases, from the DFT energies, the entropic and the zero-point energy contributions. The mechanisms explored in this thesis are found to be energetically more feasible than alternative reaction pathways considered in previous theoretical works based on molecular representations of the TiO2 surfaces. The representation of the surface of specific, commonly occurring, titanium dioxide crystals (e.g., rutile and anatase) within the small cluster approximation is able to reproduce qualitatively the rutile (110) outperforming of the anatase (001) surface. We subsequently investigate the influence of doping TiO2 surfaces with transition metals (TMs) on the performance of TiO2 -based electrodes for the water splitting electrochemical reaction. Two cluster models of the TM-doped active sites which resemble both the TiO2 anatase (001) and rutile (110) surfaces, respectively, are considered for the evaluation of the water decomposition reaction when a Ti is replaced by a TM atom. A set of TMs spanning from Vanadium to Nickel is considered. The late TMs explored here: Fe, Co and Ni are found to reproduce the observed experimental trends for the overpotentials in TiO2-doped electrodes. In the case of Cr and Mn, the present study predicts an enhancement of the OER activity for the anatase-like clusters while a reduction of this activity is found for the rutile-like ones. The vanadium-doped structures do not show relevant influence in the OER activity compared to pure TiO2-based cluster models. The last part of this work is devoted to the theoretical study of the HER on recently found materials based on the synergistic combination of molybdenum carbide and graphene layers. We propose two major structural models to describe the HER mechanism within the framework of DFT: Mo2C-based clusters adsorbed on carbon nanosheets and the Mo2C (001) surface covered by pure and nitrogen-doped graphene layers. The former system evaluates the influence of Mo2C nanoparticles adsorbed on carbon nanosheets towards the HER. The second one is employed to gain insight about the high HER activity observed in molybdenum carbide anchored on nitrogen-doped porous carbon nanosheets (Mo2C@2D-NPC), recently synthesized. The H-adsorption free energy has been used as a principal descriptor to asses the HER activity at the proposed model active sites. It resembles the value for the best state of the art catalyst for the HER (i.e., platinum at carbon substrate Pt@C) in some of the proposed structural models. Furthermore, a pH-correction is added within a simplified model, to the H-adsorption free energy barrier in every proposed structure. The pH dependence of the H-adsorption free energy barriers allows the assessment of the HER at acidic and alkaline conditions simultaneously. An overall agreement with experimental results is found and further predictions, promoting the development of better HER catalysts, have been done.

No description available.

No / Here we report a hierarchical composite structure composed of few-layers molybdenum disulfide nanosheets supported by vertical graphene on conductive carbon cloth (MDNS/VG/CC) for high-performance electrochemical hydrogen evolution reaction (HER). In the fabrication, 3D vertical graphene is first prepared on carbon cloth by a micro-wave plasma enhanced chemical vapor deposition (MPCVD) and then few-layers MoS2 nanosheets are in-situ synthesized on the surface of the vertical graphene through a simple hydrothermal reaction. This integrated catalyst exhibits an excellent HER electrocatalytic activity including an onset potential of 50 mV, an overpotential at 10 mA cm(-2) (eta(10)) of 78 mV, a Tafel slop of 53 mV dec(-1), and an excellent cycling stability in acid solution. The excellent catalytic performance can be ascribed to the abundant active edges provided by the vertical MoS2 nanosheets, as well as the effective electron transport route provided by the graphene arrays on the conductive substrate. Moreover, the vertical graphene offers robust anchor sites for MoS2 nanosheets and appropriate intervals for electrolyte infiltration. This not only benefits hydrogen convection and release but also avoids the damaging or restacking of catalyst in electrochemical processes. / This work was financially supported by the National Natural Science Foundation of China (Grant nos. 61176007, 51372213, and 51402343).

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)

Le platine constitue le matériau de référence pour l’électroréduction de l’eau (HER) et l’électrooxydation du dihydrogène (HOR). Les propriétés physicochimiques de ce matériau d’électrode synthétisé sous forme Pt/C par la méthode BAE et les paramètres cinétiques des réactions électrochimiques HOR et HER à sa surface en milieu alcalin ont été déterminés. Le palladium et l’or ont aussi des activités intéressantes vis-à-vis de ces réactions. L’activité de ces métaux nobles supportés sur carbone et obtenus par la même méthode de synthèse a été étudiée dans les mêmes conditions pour établir une étude comparative.L’effet de Ni et Nb sur l’activité catalytique de Pt pour HOR et HER a aussi été étudié. Des catalyseurs de type PtxM1-x/C (M=Ni, Nb) ont été préparés et leurs propriétés physicochimiques ainsi que leurs activités électrocatalytiques étudiées. Bien que Ni et Nb ne catalysent pas ces réactions dans le domaine de potentiel scruté, leur présence induit des modifications du site catalytique et influence l’activité catalytique des différents matériaux d’électrode. Pour chaque catalyseur les paramètres cinétiques ont été déterminés et les résultats révèlent des catalyseurs bimétalliques très prometteurs. Les mesures de CO-stripping ont ensuite mis en évidence un effet électronique sur le platine favorable à l’oxydation du CO à bas potentiel sur les catalyseurs PtxNi1-x/C.Des résultats très encourageants ont montré que la modification du support de l’électrode (ajout de CeO2) a permis d’obtenir un catalyseur (Pt0,5Ni0,5/CeO2/C) possédant des activités catalytiques en HER et HOR similaires à celles de Pt/C tout en diminuant de 50% la quantité de Pt. / Platinum is the reference material for the electroreduction of water (HER) and the electrooxidation of hydrogen (HOR). The starting point of this work was the synthesis of Pt/C by the BAE method. The physicochemical properties of this material and the corresponding kinetic parameters for HOR and HER in alkaline medium were obtained. Palladium and gold also exhibit interesting activities towards these reactions. The activity of these metals supported on carbon and obtained by the same synthesis method was therefore studied in the same conditions so as to perform a comparative investigation.The effect of Ni and Nb on the catalytic activity of Pt for both reactions (HOR and HER) was also investigated. Thus, two sets of PtxM1-x/C (M = Ni, Nb) catalysts were prepared and their physicochemical properties and electrocatalytic activities studied in alkaline medium. Although Ni and Nb do not catalyze HOR nor HER in the scrutinized potential range, their presence can lead to changes in the catalytic site and consequently influence the electrocatalytic activity of the various materials towards the studied reactions. For each material, the kinetic parameters were determined for both HER and HOR and compared with those obtained for Pt/C. The results revealed very promising bimetallic catalysts. Moreover, the CO-stripping measurement highlighted an electronic effect on platinum favorable to the CO oxidation at low potential values for the PtxNi1-x/C materials.Finally, the modification of the electrode support by adding CeO2 resulted in a Pt0,5Ni0,5/CeO2/C catalyst exhibiting excellent catalytic activities towards HER and HOR while decreasing significantly (50%) the amount of Pt.