The electrocatalytic splitting of water is recognized to be the most sustainable and clean technology for the production of hydrogen (H₂). Unfortunately, the efficiency is seriously restricted by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. In contrast to the OER, the electrooxidation of organic compounds (EOO) is more thermodynamically and kinetically favorable. Thus, the coupling of the EOO and hydrogen evolution reaction (HER) has emerged as an alternative route, as it can greatly improve the catalytic efficiency for the production of H₂. Simultaneously, value-added organic compounds can be generated on the anode through electrooxidation upgrading. In this Minireview, we highlight the latest progress and milestones in coupling the EOO with the HER. Emphasis is focused on the design of the anode catalyst, understanding the reaction mechanism, and the construction of the electrolyzer. Moreover, challenges and prospects are offered relating to the future development of this emerging technology.

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).

This thesis was conducted as cotutelle-de-thèse between the Université de Montréal and the Universität Würzburg (Germany). Cette thèse a été réalisée en cotutelle de thèse entre l'Université de Montréal et l'Universität Würzburg (Allemagne). / Cette thèse étudie la conception de différentes ligands pour les complexes de Ru(II) et leur activité comme photosensibilisateur (PS) dans l'évolution photocatalytique de l'hydrogène. Le système catalytique contient généralement un catalyseur, un donneur d'électron sacrificiel (SED) et un PS, qui doit présenter une forte absorption et luminescence et un comportement redox réversible. Les substituants pyridine attracteurs d'électrons sur le récepteur d'ions métalliques terpyridine entraînent une augmentation de la durée de vie de l'état excité et du rendement quantique (Φ = 74*10-5; τ = 3.8 ns) et permettent au complexe III-C1 de présenter une activité en tant que PS. Bien que la fréquence (TOFmax) et le nombre de cycle catalytique (TON) soient relativement faibles (TOFmax = 57 mmolH2 molPS-1 min 1; TON(44 h) = 134 mmolH2 molPS-1), le système catalytique a une longue durée de vie, ne perdant que 20% de son activité au cours de 12 jours. De manière intéressante, la conception hétérolytique dans III-C1 s'avère être bénéfique pour la performance en tant que PS, malgré des propriétés photophysiques et électrochimiques comparables à celles du complexe homoleptique IV-C2 (TOFmax = 35 mmolH2 molPS-1 min-1; TON(24 h) = 14 mmolH2 molPS-1). L'extinction réductive de la PS excitée par le SED est identifiée comme l'étape limitant la vitesse dans les deux cas. Par conséquent, les ligands sont modifiés pour être plus accepteurs d'électrons, soit par N-méthylation des substituants pyridine périphériques, soit par introduction d'un cycle pyrimidine dans le récepteur d'ion métallique, ce qui conduit à une augmentation des durées de vie des états excités (τ = 9–40 ns) et des rendements quantiques de luminescence (Φ = 40–400*10-5). Cependant, le caractère plus accepteur d'électrons des ligands entraîne également des potentiels de réduction décalés anodiquement, ce qui conduit à un manque de force motrice pour le transfert d'électrons du PS réduit au catalyseur. Ainsi, cette étape de transfert d'électrons s'avère être un facteur limitant de la performance globale du PS. Alors que des TOFmax plus élevés dans les expériences d'évolution de l'hydrogène sont observés pour les PS contenant le motif pyrimidine (TOFmax = 300–715 mmolH2 molPS-1 min-1), la longévité de ces systèmes est réduite avec des temps de demi-vie de 2–6 h. L'expansion des ligands contenant le motif pyrimidine en complexes dinucléaires conduit à une absorptivité plus forte (ε = 100–135*103 L mol-1 cm-1), une luminescence accrue (τ = 90–125 ns, Φ = 210–350*10-5) et peut également entraîner un TOFmax plus élevé si la force motrice est suffisante pour le transfert d'électrons vers le catalyseur (1500 mmolH2 molPS-1 min-1). En comparant des complexes avec des forces motrices similaires, une luminescence plus forte se traduit par un TOFmax plus élevé. Outre les considérations thermodynamiques, les effets cinétiques et l'efficacité du transfert d'électrons sont supposés avoir un impact sur l'activité observée dans l'évolution de l'hydrogène. En résumé, ce travail montre que la conception ciblée de ligands peut faire du groupe précédemment négligé des complexes de Ru(II) avec des ligands tridentés des candidats attrayants pour une utilisation comme PS dans l'évolution photocatalytique de l'hydrogène. / This thesis investigates different ligand designs for Ru(II) complexes and the activity of the complexes as photosensitizer (PS) in photocatalytic hydrogen evolution. The catalytic system typically contains a catalyst, a sacrificial electron donor (SED) and a PS, which needs to exhibit strong absorption and luminescence, as well as reversible redox behavior. Electron-withdrawing pyridine substituents on the terpyridine metal ion receptor result in an increase of excited-state lifetime and quantum yield (Φ = 74*10-5; τ = 3.8 ns) and lead to complex III-C1 exhibiting activity as PS. While the turn-over frequency (TOFmax) and turn-over number (TON) are relatively low (TOFmax = 57 mmolH2 molPS-1 min-1; TON(44 h) = 134 mmolH2 molPS-1), the catalytic system is long-lived, losing only 20% of its activity over the course of 12 days. Interestingly, the heteroleptic design in III-C1 proves to be beneficial for the performance as PS, despite III-C1 having comparable photophysical and electrochemical properties as the homoleptic complex IV-C2 (TOFmax = 35 mmolH2 molPS-1 min-1; TON(24 h) = 14 mmolH2 molPS-1). Reductive quenching of the excited PS by the SED is identified as rate-limiting step in both cases. Hence, the ligands are designed to be more electron-accepting either via N-methylation of the peripheral pyridine substituents or introduction of a pyrimidine ring in the metal ion receptor, leading to increased excited-state lifetimes (τ = 9–40 ns) and luminescence quantum yields (Φ = 40–400*10-5). However, the more electron-accepting character of the ligands also results in anodically shifted reduction potentials, leading to a lack of driving force for the electron transfer from the reduced PS to the catalyst. Hence, this electron transfer step is found to be a limiting factor to the overall performance of the PS. While higher TOFmax in hydrogen evolution experiments are observed for pyrimidine-containing PS (TOFmax = 300–715 mmolH2 molPS-1 min-1), the longevity for these systems is reduced with half-life times of 2–6 h. Expansion of the pyrimidine-containing ligands to dinuclear complexes yields a stronger absorptivity (ε = 100–135*103 L mol-1 cm-1), increased luminescence (τ = 90–125 ns, Φ = 210–350*10-5) and can also result in higher TOFmax given sufficient driving force for electron transfer to the catalyst (TOFmax = 1500 mmolH2 molPS-1 min-1). When comparing complexes with similar driving forces, stronger luminescence is reflected in a higher TOFmax. Besides thermodynamic considerations, kinetic effects and electron transfer efficiency are assumed to impact the observed activity in hydrogen evolution. In summary, this work shows that targeted ligand design can make the previously disregarded group of Ru(II) complexes with tridentate ligands attractive candidates for use as PS in photocatalytic hydrogen evolution. / In dieser Arbeit werden verschiedene Liganden für Ru(II)-Komplexe und die Aktivität der Komplexe als Photosensibilisatoren (PS) in der photokatalytischen Wasserstoffentwicklung untersucht. Das katalytische System besteht typischerweise aus einem Katalysator, einem Opferelektronendonator (SED) und einem PS, welcher eine starke Absorption und Lumineszenz sowie ein reversibles Redoxverhalten aufweisen sollte. Elektronenziehende Pyridin-Substituenten am Terpyridin-Metallionenrezeptor resultieren in einer Erhöhung der Lebensdauer des angeregten Zustands sowie der Quantenausbeute (Φ = 74*10-5; τ = 3.8 ns), was dazu führt, dass Komplex III-C1 als PS aktiv ist. Während die Wechselzahl (TOFmax) und der Umsatz (TON) relativ niedrig sind (TOFmax = 57 mmolH2 molPS-1 min-1; TON(44 h) = 134 mmolH2 molPS 1), ist das katalytische System langlebig und verliert im Laufe von 12 Tagen nur 20% seiner Aktivität. Das heteroleptische Design in III-C1 erweist sich als vorteilhaft für die Leistung als PS, obwohl III-C1 vergleichbare photophysikalische und elektrochemische Eigenschaften besitzt wie der homoleptische Komplex IV-C2 (TOFmax = 35 mmolH2 molPS-1 min-1; TON(24 h) = 14 mmolH2 molPS-1). In beiden Fällen erweist sich das reduktive Lumineszenzlöschen des angeregten PS durch den SED als geschwindigkeitsbestimmender Schritt. Daher werden die Liganden entweder durch N-Methylierung der peripheren Pyridin-Substituenten oder durch Einführung eines Pyrimidinrings in den Metallionenrezeptor elektronenziehender gestaltet, was zu erhöhten Lebensdauern des angeregten Zustands (τ = 9–40 ns) und Lumineszenzquantenausbeuten (Φ = 40–400*10-5) führt. Der stärker elektronenziehende Charakter der Liganden führt allerdings auch zu anodisch verschobenen Reduktionspotentialen, wodurch die treibende Kraft für den Elektronentransfer vom reduzierten PS zum Katalysator reduziert wird. Daher erweist sich dieser Elektronentransferschritt als ein limitierender Faktor für die Gesamtleistung des PS. Während höhere TOFmax in Wasserstoffproduktionsexperimenten für Pyrimidin-haltige PS beobachtet werden (TOFmax = 300–715 mmolH2 molPS-1 min-1), ist die Langlebigkeit für diese Systeme mit Halbwertszeiten von 2–6 h deutlich reduziert. Die Erweiterung der Pyrimidin-haltigen Liganden zu zweikernigen Komplexen führt zu einem stärkeren Absorptionsvermögen (ε = 100–135*103 L mol-1 cm-1), erhöhter Lumineszenz (τ = 90–125 ns, Φ = 210–350*10-5) und kann bei ausreichender treibender Kraft für den Elektronentransfer zum Katalysator auch zu einer höheren TOFmax führen (TOFmax = 1500 mmolH2 molPS-1 min-1). Beim Vergleich von Komplexen mit ähnlichen treibenden Kräften spiegelt sich die stärkere Lumineszenz in einem höheren TOFmax wider. Es wird angenommen, dass neben thermodynamischen Faktoren auch kinetische Effekte und die Effizienz des Elektronentransfers die beobachtete Aktivität bei der Wasserstoffentwicklung beeinflussen. Zusammenfassend zeigt diese Arbeit, dass gezieltes Ligandendesign die bisher vernachlässigte Gruppe der Ru(II)-Komplexe mit tridentaten Liganden zu attraktiven Kandidaten für den Einsatz als PS in der photokatalytischen Wasserstoffentwicklung machen kann.

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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)

The aminoacid sequence of papain (EC 3.4.22.2) consists of 212 aminoacids. It has only one free sulfhydryl group, which is located in the active site of the protein. Some organometallic complexes could be bonded only to this free -SH group due to their structure. The artificial metalloproteins synthesised by this way may have different electrochemical properties. In this work, we have studied the electrochemical properties of papain and its derivatives. We compared the ability of papain and its three artificial derivatives to catalyse the hydrogen evolution by the chronopotenciometry. The work was completed by the study of the electrochemical properties of the organometallic complexes of ruthenium, which were used for the artificial metalloprotein preparation. The electrochemical properties of the compounds were never studied before. The process of the hydrogen evolution catalysed by the proteins is held in the adsorbed state of the catalyst. Due to this fact we have also studied the adsorption properties of papain on the substrates with different level of hydrofobicity. (In Czech)

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.

La thèse présentée traite principalement de l'influence de la composition et de l’état de surface d’électrodes à base de Ni sur la cinétique et le mécanisme des réactions d'oxydation/dégagement de l'hydrogène (HOR/HER) en milieu alcalin. En combinant les résultats de mesures électrochimiques avec une modélisation microcinétique, il a pu être montré que l'activité spécifique du Ni pour l’HOR/HER augmente jusqu'à 10 fois en présence à la fois d’oxydes de Ni et de Ni métallique à la surface de l'électrode. En outre, l'influence de l'addition d’un second métal aux électrocatalyseurs à base Ni sur leurs activités pour l’HOR/HER a été étudiée dans le cas des systèmes NiMo/C et NiCu/C. Dans les deux cas, une augmentation de l'activité spécifique a été observée par rapport à l'échantillon Ni/C de référence et a été attribuée à une diminution de l'énergie d'adsorption de l'hydrogène adsorbé sur Ni, espèce intermédiaire de l’HOR/HER. / The present thesis is mainly focused on the influence of the surface state of Ni electrodes on the kinetics and the mechanism of the hydrogen oxidation/evolution reactions (HOR/HER) in alkaline medium. By combining the results of electrochemical measurements with microkinetic modeling, it was shown that specific activity of Ni in the HOR/HER increases up to 10 times if along with metallic Ni, Ni oxide species are present on the electrode surface. In addition, the effect of the addition of a second metal to Ni electrocatalysts on their activity in the HOR/HER was investigated for NiMo/C and NiCu/C systems. In both cases an enhancement of specific activity was observed in comparison with the reference Ni/C sample, which was assigned to a decrease of the adsorption energy of the hydrogen intermediate on Ni participating in the HOR/HER.

The path towards mitigation of anthropogenic greenhouse gas emissions lies in the transition from conventional to sustainable energy resources. The Hydrogen Economy, a cyclic economy based on hydrogen as a fuel, is suggested as a tool in the necessary energy transition. Photocatalysis makes use of sunlight to promote thermodynamically non-favoured reactions such as water splitting, allowing for sustainable hydrogen production. Harvesting thermal energy along with photonic energy is an interesting concept to decrease the activation energy of water splitting (i.e. ΔG = + 237.2 kJ∙mol−1). This work aims to confront this hypothesis in a gas phase photo-thermal reactor designed specifically for this study. The photocatalyst chosen is graphitic carbon nitride (g-C3N4), an organic semiconductor possessing a narrow band gap (i.e. 2.7 eV) as well as a band structure which theoretically permits water splitting. The photocatalytic performance of Pt/g-C3N4 for hydrogen evolution was tuned by altering its synthetic temperature. Electron paramagnetic resonance was used to gain insight on the evolution of the photocatalyst activity with synthesis temperature. Then, gold nanoparticles were deposited on g-C3N4 surface. Localized surface plasmon resonance properties of gold nanoparticles are reported in the literature to be influenced by temperature. Therefore Au/g-C3N4 appeared as a promising candidate for photo-thermal water splitting. X-ray spectroscopy unveiled interesting observations on the gold oxidation state. Moreover, under specific reduction conditions, gold nanoparticles with a wide variety of shapes characterized by sharp edges were formed. Finally, the development of the photo-thermal reactor is presented. The design process and the implementation of this innovative reactor are discussed. The reactor was successfully utilized to probe photoreactions. Then, the highly energy-demanding photocatalytic water splitting was proven not to be activated by temperature in the photo-thermal apparatus.

The ever increasing demand for energy due to over consumption of non-renewable fossil fuels has emphasized the need for alternate, sustainable and efficient energy conversion and storage systems. In this direction, electrochemical energy conversion and storage systems involving various fundamental electrochemical redox processes such as hydrogen evolution (HER), oxygen reduction (ORR), oxygen evolution (OER), hydrogen oxidation (HOR) reactions and others become highly important. Electrocatalysts are often used to accelerate the kinetics of these reactions. Platinum (Pt), ruthenium oxide and iridium oxide (RuO2 and IrO2) are known to be the state of the art catalysts for several of these reactions due to favouarable density of states (DOS) near the Fermi level, binding energy with the reactant species, chemical inertness etc. Apart from HER, OER and ORR, chlorine evolution reaction (Cl-ER) is another industrially important reaction associated with water purification, disinfection, bleaching, chemical weapons and pharmaceuticals. Dimensionally stable anodes (RuO2/IrO2 mixed with TiO2 on Ti) are the most commonly used catalysts for this process. Issues related to surface poisoning, corrosion and cost of the catalysts, in addition to selectivity and specificity towards a particular reaction are various aspects to be addressed. For example, Pt is not very specific for ORR in presence of methanol in addition to high cost and corrosion in certain media. On the other hand, DSA can efficiently catalyze both OER and Cl-ER, and hence there is overlap of the two processes in the potential range available. There is an on going search for efficient, cost-effective, stable catalysts that possess high specificity for a particular redox reaction. Towards this goal, the present study explores certain layered (phospho)chalcogenides for catalyzing HER, ORR, OER and Cl-ER. The present thesis is structured in two parts, where the first part explores the multi-functional catalytic aspects of new classes of compounds based on layered transition metal mixed chalcogenides (MoS2(1-x)Se2x) and ternary phosphochalcogenides (FePS3, FePSe3 and MoPS). In addition, lithium insertion and desinsertion has been studied with the aim of using the layered materials for rechargeable batteries. The second part of the thesis explores organic electrode materials with active carbonyl groups such as rufigallol, polydihydroxyanthrachene succinic anhydride (PDASA) as battery electrodes. Additionally, covalently functionalized transition metal phthalocyanines with reduced graphene oxide are studied as counter electrodes in dye sensitized solar cells (DSSCs). MoS2(1-x)Se2x (x = 0 to 1) compositions are solid solutions of MoS2 and MoSe2 in different ratios. They crystallize in hexagonal structure with space group P63/mmc (D6h4) having Mo in trigonal prismatic coordination like the pristine counterparts. X-Ray diffraction studies reveal that Vegard’s law (figure 1a) is followed and hence complete miscibility of MoS2 and MoSe2 is established. MoS2(1-x)Se2x (x = 0 to 1) are layered in nature and the layers are held together by long range, weak van der Waal’s forces. This gives us the flexibility of exfoliation to produce corresponding few-layer materials (figure 1b). Figure 1. (a) Variation of lattice parameter corresponding to (002) reflection of MoS2(1-x)Se2x with different x values. (b) Scanning electron micrograph of few-layer MoS2(1-x)Se2x (x = 0.5). The electrocatalytic activity of the few-layer sulphoselenides have been studied towards HER in aqueous 0.5 M H2SO4 and towards Cl-ER in 3 M aqueous NaCl (pH = 3) solution. The mixed chalcogenides exhibit very good activities for both HER and Cl-ER as compared to the activity of their pristine counter parts (i.e. MoS2 and MoSe2) (figures 2a and 2b). Electrocatalytic activity on different compositions reveal that MoS1.0Se1.0 exhibits the maximum activity. Additionally, it has been observed that MoS1.0Se1.0 shows high specificity for Cl-ER with negligible interference of OER. Figure 2. Voltammetric data for (a) hydrogen evolution reaction (in 0.5 M aqueous H2SO4) and (b) chlorine evolution reaction (in 3 M aqueous NaCl solution, pH = 3) on MoS2(1-x)Se2x (x = 0, 0.5, 1). Figure 3. (a) XRD pattern of MoS2(1-x)Se2x (x = 0.5) electrode after a cycle of Li insersion and deinsersion (red) along with as-synthesized material (black) (b) Cycling behaviour of rGO supported (black) and pristine (red) MoS2(1-x)Se2x (x = 0.5) as electrode in rechargeable lithium-ion battery. The equiatomic MoS1.0Se1.0 has also been studied as an anode material for rechargeable lithium batteries. The cyclic voltammogram and characterization after charge-discharge cycle (figure 3a) indicate intercalation of Li with in the layers followed by conversion type formation of Li-S and Li-Se type compounds. The pristine material shows continuous capacity fading while the composites of sulphoselenides functionalized with conducting carbon supports such as rGO, MWCNT, super P carbon, toray carbon show marked improvement in capacity as well as cycling behavior. The rGO functionalized MoS1.0Se1.0 reveals ~1000 mAh/g of stable specific discharge capacity for 500 cycles (figure 3b). In the next two chapters, new class of transition metal-based layered materials FePS3 and FePSe3, containing both P and chalcogen (S and Se) is indroduced for electrocatalysis. FePS3 crystallizes in monoclinic symmetry with an indirect band gap of ~1.55 eV while FePSe3 possesses rhombohedral crystal structure with comparatively low band gap (~1.3 eV) as shown in figure 4a. The FePS3 and FePSe3 have been exfoliated as has been done for MoS1.0Se1.0 (liquid exfoliation method) using acetone as the solvent. Stable colloids with few-layer nanosheets having lamellar morphology and lateral sizes of ~100 to 200 nm are obtained. Electrical characterization indicates that they are semiconducting and the conductivity of the Se analogue is ~50 times higher than that of the S analogue (figure 4b). Figure 4. (a) Catholuminescence of FePX3 ( X = S and Se) reveals the band gap of the material. Band gap of the S analogue is 1.52 eV and that of the Se analogue is 1.33 eV (b) Resistivity of FePX3 ( X = S and Se) as a function of temperature. The tri-functional electrocatalytic activities on rGO-few layer FePX3 (X = S and Se) have been evaluated for HER over a wide pH range (0.5 M H2SO4, 0.5 M KOH, phosphate Figure 5. Catalytic activity of rGO-few-layer FePX3 (X = S, Se) towards HER in (a) aqueous 0.5 M H2SO4 and (b) 3.5 wt % NaCl solutions. (c) ORR activity of the catalysts in oxygen saturated 0.5 M KOH (d) OER behaviour on the catalysts in 0.5 M KOH at a rotation speed of 1600 rpm. buffer, pH 7 and 3.5 % NaCl), ORR and OER in alkaline media (0.5 M KOH). The studies clearly reveal that both rGO-FePS3 and rGO-FePSe3 exhibit excellent HER activity in acidic media (figure 5a) with high stability. The HER studies in 3.5 wt % aqueous NaCl solution (figure 5b) suggests that the catalysts are effective in evolving hydrogen from sea-water environment. Studies on ORR activity (figure 5c) indicate that the rGO composites of both S and Se analogues follow 4-electron pathways to produce water as the final product. They are also found to be highly methanol tolerant. In the case of OER (figure 5d), XPS characterization of the electrodes after the voltammetric studies reveals the presence of very thin layer of Fe2O3 (not detectable by XRD). All the three reactions (HER, ORR and OER) catalyzed by the Se analogue are better than the S analogue (figure 5). This could be due to the low band gap and high conductivity of FePSe3 as compared to FePS3. The over potential to achieve 10 mAcm-2 current density is ~108 mV for rGO-few-layer FePS3 catalyst where in the case of rGO-few layer FePSe3, it is ~97 mV (table 1). Table 1. Catalytic activities of rGO-few layer FePS3 and rGO-few layer FePSe3 towards HER, ORR and OER. Reaction studied rGO-FePS3 rGO-FePSe3 HER (η @ 10mAcm-2) ~108 mV ~97 mV ORR (peak potential) ~0.81 V ~0.87 V OER (η @ 10mAcm-2) ~470 mV ~430 mV It is likely that there is a strong interaction between FePX3 (metal d-orbital) and rGO, as observed from the downward shift of Fe 2p peak in high resolution XPS studies. This interaction may extend the density of states of metal d-orbitals thereby improving the catalytic activities. The next chapter deals with molybdenum-based phosphosulphide compound (MoPS). Molybdenum-based phosphide catalysts have been explored recently as excellent catalysts for various electrochemical reactions such as HER. It is expected that the catalyst containing both S and P will show positive effects on catalytic activities due to the synergy between S and P. In the present study, P incorporated MoS2 is studied towards HER. The XRD pattern of the as-synthesized crystal suggests the presence of mixed phase of MoS2, MoP2 and MoP while the elemental mapping in microscopy indicates the ratio of Mo, P and S to be 1:1:1. The electrochemical HER in 0.5 M H2SO4 indicates that the activity is improved drastically as compared to bulk and few-layer MoS2. The next section explores the use of different organic electrode materials possessing active carbonyl groups for Li-storage studies. The advantage of the use of carbonyl-based compounds lies in the high reversible activity towards Li ion insersion and de-insersion. Rufigallol (figure 6a) exhibits very stable capacity of ~200 mAh/g (at C/20 rate) upto 500 Figure 6. (a) and (c) Schematic representation of rufigallol and poly-dihydroanthracene succinic anhydride (PDASA) respectively. (b) and (d) Cyclic behaviour of rufigallol (at C/20 rate) and PDASA (at 20 mAg-1 current rate) in Li-storage devices. (e) and (f) represent the coulombic efficiency of rufigallol (at C/20 rate) and PDASA (at 20 mAg-1 current rate) as a function of number of cycles. cycles along (figure 6b) and with very good rate capability. A triptycene-based mesoporous polymer, PDASA (figure 6c) is introduced and explored as efficient electrode material for Li-storage. PDASA exhibits very high capacity of ~1000 mAh/g at a current rate of 50 mA/g upto 1000 cycles (figure 6d). Even at very high current rates (3A/g) excellent cyclability is observed. The mechanistic details of lithium uptake and release are studied using various spectroscopic techniques. In both the cases the coulombic efficiency observed is ~80 to 90 % (figures 6e and f). Figure 7. (a) Digital photograph of the dye sensitized solar cell with rGO-Co-TAPc counter electrode. (b) Photoconversion efficiency of DSSCs with different counter electrodes as mentioned in the figure. (c) Photo conversion efficiency of Pt and rGO-Co-TAPc based DSSCs as function of storage time. (d) Schematic illustration of DSSC wherein the energy level of the counter electrodes and electrolyte are shown for different M-TAPcs. In a slightly different direction, metal phthalocyanine - rGO composites (rGO-M-TAPc; M = Co, Zn, Fe) have been explored as counter electrodes in DSSC. Figure 7a depicts the digital image of a DSSC constructed using rGO-Co-TAPc as the counter electrode. It has been observed that rGO-cobalt tetraamino phthalocyanine (rGO-Co-TAPc) counter electrode exhibits ~6.6 % of solar conversion efficiency (figure 7b) and is close to that of standard DSSC (Pt counter electrode) under identical experimental conditions and are highly stable (figure 7c). Other metal phthalocyanines show less efficiency and is analysed based on the relative positions of HOMO energy levels of the materials and the energy level of the redox system (I-/I3- system) as given in figure 7d. The thesis contains eight chapters on aspects discussed above along with summary and future perspectives given at the end. It is devided into various chapters in two sections, one comprising inorganic chalcogenide-based electrocatalysts and another comprising organic electrode materials. Appendix I discusses the Na-storage behaviour of MoS1.0Se1.0 and appendix II describes the Li-storage behaviour of rGO functionalized benzoquinone and diamino anthraquinone electrode materials.