Le développement de catalyseurs de la réaction de dégagement de l’oxygène (OER) pour les électrolyseurs à membrane échangeuse de protons (PEM) dépend de la compréhension du mécanisme de cette réaction. Cette thèse est consacrée à l'application de la spectroscopie d’émission de photoélectrons induits par rayons X (XPS) et de la spectroscopie de structure près du front d'absorption de rayons X (NEXAFS) operando sous une pression proche de l'ambiante (NAP) dans le but d’étudier les mécanismes de la réaction d’oxydation de l’eau sur des anodes à base d’iridium et de ruthénium et leurs dégradation dans les conditions de la réaction. Cette thèse montre les mécanismes différents de la réaction OER pour les anodes à base d’Ir et de Ru impliquant respectivement des transitions anioniques (formation d’espèce OI- électrophile) ou cationiques (formation des espèces de Ru avec l’état d'oxydation supérieur à IV) quelle que soit la nature (thermique ou électrochimique) des oxydes. / Development of oxygen evolution reaction (OER) catalysts for proton exchange membrane water electrolysis technology depends on the understanding of the OER mechanism. This thesis is devoted to the application of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and near edge X-ray absorption fine structure (NEXAFS) techniques for operando investigation of the Ir, Ru - based anodes. For Ru-based systems, we observe the potential-induced irreversible transition of Ru (IV) from an anhydrous to a hydrated form, while the former is stabilized in the presence of Ir. Regarding single Ir-based anodes, the analysis of O K edge spectra reveals formation of electrophilic oxygen OI- as an OER intermediate. Higher stability of Ir catalysts supported on antimony-doped tin oxide (ATO) is related to their lower oxidation. This work demonstrates different OER mechanisms on Ir, Ru-based anodes involving anion and cation red-ox chemistry, correspondingly, regardless the oxide nature.

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)

Synopsis of thesis entitled “Electrochemical and Photoelectrochemical Investigations of Co, Mn and Ir-based Catalysts for Water Splitting” by Ahamed Irshad M (SR No: 02-01-02-10-11-11-1-08823) under the supervision of Prof. N. Munichandraiah, Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore (India), for the Ph.D. degree of the Institute under the Faculty of Science. Hydrogen is considered as the fuel for future owing to its high gravimetric energy density and eco-friendly use. In addition, H2 is an important feedstock in Haber process for ammonia synthesis and petroleum refining. Although, it is the most abundant element in the universe, elemental hydrogen is not available in large quantities on the planet. Consequently, H2 must be produced from its various chemical compounds available on earth. Currently, H2 is produced in large scale from methane by a process called steam-methane reforming (SMR). This process releases huge amount of CO2 into atmosphere as the by-product causing serious environmental issues. The development of alternate clean methods to generate H2 is a key challenge for the realization of hydrogen economy. Production of H2 gas by water splitting using electricity or sunlight is known. Low cost, high natural abundance and carbon neutrality make water as the best source of hydrogen. Thermodynamically, splitting of H2O needs 237 kJ mol-1 of energy, which corresponds to 1.23 V according to the equation, ΔG = -nFE. However, commercial electrolyzers usually operate between 1.8 to 2.1 V, due to the need of large overvoltage. The high overvoltage and subsequent energy losses are mainly associated with the sluggish kinetics of oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode. The overvoltage can be considerably reduced using suitable catalysts. Hence, the design and development of stable, robust and highly active catalysts for OER and HER are essential to make water splitting efficient and economical. Attempts in the direction of preparing several novel OER and HER catalysts, physicochemical characterizations and their electrochemical or photoelectrochemical activity are described in the thesis. A comprehensive review of the literature on various types of catalysts, thermodynamics, kinetics and mechanisms of catalysis are provided in the Chapter 1 of the thesis. Chapter 2 furnishes a brief description on various experimental techniques and procedures adopted at different stages of the present studies. Chapter 3 explains the results of the studies on kinetics of deposition and stability of Nocera’s Co-phosphate (Co-Pi) catalyst using electrochemical quartz crystal microbalance (EQCM). The in-situ mass measurements during CV experiments on Au electrode confirm the deposition of Co-Pi at potential above 0.87 V vs. Ag/AgCl, 3 M KCl (Fig.1a and b). The catalyst is found to deposit via a nucleus mediated process at a rate of 1.8 ng s-1 from 0.5 mM Co2+ in 0.1 M neural phosphate solution at 1.0 V. Further studies on the potential and electrolyte dependent stability of the Co-Pi suggest that the catalyst undergoes severe corrosion at high overpotential and in non-buffer electrolytes. Current/ Fig.1 (a) Cyclic voltammograms and (b) mass variations vs. potential of Au-coated quartz crystal in 0.1 M potassium phosphate buffer solution (pH 7.0) containing 0.5 mM Co(NO3)2 Chapter 4 deals with the electrochemical deposition of a novel OER catalyst, namely, Co-acetate (Co-Ac) from a neutral acetate electrolyte containing Co2+ ions. Use of acetate solution instead of phosphate avoids the solubility limitations and helps to get thick layer of the catalyst in a short time from concentrated Co2+ solutions. In addition, the Co-Ac is found to be catalytically superior to Co-Pi (Fig. 2a). It is also observed that the Co-Ac catalyst undergoes ion exchange with electrolyte species during electrolysis in phosphate buffer solution, which results in the formation of a hybrid Co-Ac-Pi catalyst (Fig. 2b). The presence of both acetate and phosphate ions in the catalyst and their synergistic catalytic effect enhance the OER activity. Fig.2. (a) Linear sweep voltammograms of Co-Ac in (i) phosphate and (ii) acetate electrolytes, and that of Co-Pi in (iii) acetate and (iv) phosphate electrolytes. (b) SEM image showing the formation of two layers of the catalysts after electrolysis in phosphate solution. In Chapter 5, high OER activity of an electrodeposited amorphous Ir-phosphate (Ir-Pi) is investigated. The catalyst is prepared by the anodic polarization of a carbon paper electrode in neutral phosphate solution containing Ir3+ ions (Fig. 3). The Ir-Pi film deposited on the electrode has Ir and P in an approximate ratio of 1:2 with Ir in an oxidation state higher than +4. Phosphate ions play a major role for both the electrochemical deposition process and its catalytic activity towards OER. The Ir-Pi catalyst is superior to similarly deposited IrO2 and Co-Pi catalysts both in terms of onset potential and current density at any potential in the OER region. Tafel measurements and pH dependence studies identify the formation of a high energy intermediate during oxygen evolution. Fig.3. (a) Cyclic voltammograms during the Ir-Pi deposition and (b) SEM image of Ir-Pi on C. Chapter 6 is on the preparation of a composite of Mn-phosphate (MnOx-Pi) and reduced graphene oxide (rGO) and its utilization as an OER catalyst. The composite is prepared by the simultaneous electrochemical reduction of KMnO4 and graphene oxide (GO) in a phosphate solution (pH 7.0). Various analytical techniques such as TEM, XPS, Raman spectroscopy, etc. confirm the formation of a composite (Fig. 4) and electrochemical studies indicate the favourable role of rGO towards OER. Under identical conditions, MnOx-Pi-rGO gives 6.2 mA cm-2 at 2.05 V vs. RHE whereas it is only 2.9 mA cm-2 for MnOx-Pi alone. However, the catalyst is not very stable during OER which is ascribed to slow oxidation of Mn3+ in the catalyst. Fig.4. (a) Raman spectrum and (b) TEM image of MnOx-Pi-rGO. In Chapter 7, an amorphous Ni-Co-S film is prepared by a potentiodynamic deposition method using thiourea as the sulphur source. The electrodeposit is used as a catalyst for the HER in neutral phosphate solution. The composition of the catalyst and the HER activity are tuned by varying the ratio of concentrations of Ni2+ and Co2+. The bimetallic Ni-Co-S catalyst exhibits better HER activity than both Ni-S and Co-S (Fig. 5a). Under optimized deposition conditions, Ni-Co-S requires just 150 mV for the onset of HER and 10 mA cm-2 is obtained for 280 mV overpotential. The Ni-Co-S shows two different Tafel slopes, indicating two different potential dependent HER mechanisms (Fig. 5b). Presence of two different catalytic sites which contribute selectively in different potential regions is proposed. Fig.5. (a) Linear sweep voltammograms of HER at 1 mV s-1 in 1 M phosphate solutions (pH 7.4) using (i) Ni-S, (ii) Co-S and (c) Ni-Co-S. (b) Tafel plot of Ni-Co-S showing two Tafel slopes. Photoelectrochemical OER using ZnO photoanode and Co-acetate (Co-Ac) cocatalyst is studied in Chapter 8 of the thesis. Randomly oriented crystalline ZnO nanorods are prepared by the electrochemical deposition of Zn(OH)2 followed by heat treatment at 350 ºC in air. Co-Ac is then photochemically deposited onto ZnO nanorods by UV illumination in the presence of neutral acetate buffer solution containing Co2+ ions. The hybrid Co-Ac-ZnO shows higher photoactivity in comparison with bare ZnO towards PEC water oxidation (Fig. 6). Co-Ac acts as a cocatalyst and reduces the charge carrier recombination at the electrode/electrolyte interface. Fig.6. (a) Linear sweep voltammograms of ZnO under (i) dark and (ii) light conditions, and that of Co-Ac-ZnO in (iii) dark and (iv) light in 0.1 M phosphate (pH 7.0) electrolyte. Chapter 9 deals with PEC water oxidation using α-Fe2O3 photoanode and Ir-phosphate (Ir-Pi) cocatalyst. α-Fe2O3 is prepared by direct heating of Fe film in air which in turn is deposited by the electrochemical reduction of Fe2+. Thickness of the film as well as calcination temperature is carefully optimized. In order to further enhance the OER kinetics, Ir-Pi is electrochemically deposited onto α-Fe2O3. Under optimized conditions, Ir-Pi deposited α-Fe2O3 shows around 3 times higher photocurrent than that of bare α-Fe2O3 at 1.23 V vs. RHE (Fig. 7). Ir-Pi acts as a cocatalyst for OER and reduces the photogenerated charge carrier recombination. Fig.7. Photocurrent variation of α-Fe2O3 electrode at 1.23 V vs. RHE for (i) front and (ii) back side illuminations, against Ir-Pi deposition time. The thesis ends with a short summary and future prospectus of studies described in the thesis. The research work presented in the thesis is carried out by the candidate as the part of Ph.D. program. Some of the results have already been published in the literature and some manuscripts are under preparation. A list of publications is included at the end of the thesis. It is anticipated that the studies reported in the thesis will constitute a worthwhile contribution.

Global warming and depletion in fossil fuels have forced the society to search for alternate, clean sustainable energy sources. An obvious solution to the aforesaid problem lies in electrochemical energy storage systems like fuel cells and batteries. The desirable properties attributed to these devices like quick response, long life cycle, high round trip efficiency, clean source, low maintenance etc. have made them very attractive as energy storage devices. Compared to many advanced battery chemistries like nickel-metal hydride and lithium - ion batteries, metal-air batteries show several advantages like high energy density, ease of operation etc. The notable characteristics of metal - air batteries are the open structure with oxygen gas accessed from ambient air in the cathode compartment. These batteries rely on oxygen reduction and oxygen evolution reactions during discharging and charging processes. The efficiency of these systems is determined by the kinetics of oxygen reduction reaction. Platinum is the most preferred catalyst for many electrochemical reactions. However, high cost and stability issues restrict the use of Pt and hence there is quest for the development of stable, durable and active electrocatalysts for various redox reactions. The present thesis is directed towards exploring the electrocatalytic aspects of titanium carbonitride. TiCN, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrochemical studies. The work function as well as the bonding in the material can be tuned by varying the composition of carbon and nitrogen in the crystal lattice. The current study explores the versatility of TiCN as electrocatalyst in aqueous and non-aqueous media. One dimensional TiC0.7N0.3 nanowires are prepared by simple one step solvothermal method without use of any template and are characterized using various physicochemical techniques. The 1D nanostructures are of several µm size length and 40 ± 15 nm diameter (figure 1). Orientation followed by attachment of the primary particles results in the growth along a particular plane (figure 2). (a) (b) (c) Figure 1. (a) SEM images of TiC0.7N0.3 nanowires (b) TEM image and (c) High resolution TEM image showing the lattice fringes. (a) (b) (d) Figure 2. Bright field TEM images obtained at different time scales of reaction. (a) 0 h; (b) 12 h; (c) 72 h and (d) 144 h. The next aspect of the thesis discusses the electrochemical performance of TiC0.7N0.3 especially for oxygen reduction. Electrochemical oxygen reduction reaction (ORR) reveals that the nanowires possess high activity for ORR and involves four electron process leading to water as the product. The catalyst effectively converts oxygen to water with an efficiency of 85%. A comparison of the activity of different (C/N) compositions of TiCN is shown in figure 3. The composition TiC0.7N0.3 shows the maximum activity for the reaction. The catalyst is also very selective for ORR in presence of methanol and thus cross-over issue in fuel cells can be effectively addressed. Density functional theory (DFT) calculations also lead to the same composition as the best for electrocatalysis, supporting the experimental observations. Figure 3. Linear sweep voltammetric curves observed for different compositions of titanium carbonitride towards ORR. The next chapter deals with the use of TiC0.7N0.3 as air cathode for aqueous metal - air batteries. The batteries show remarkable performance in the gel- and in liquid- based electrolytes for zinc - air and magnesium - air batteries. A partial potassium salt of polyacrylic acid (PAAK) is used as the polymer to form a gel electrolyte. The cell is found to perform very well even at very high current densities in the gel electrolyte (figures 4 and 5). Figure 4 Photographs of different components of the gel - based zinc - air battery. (a) (b) Figure 5. a) Discharge curves at different current densities of 5, 20, 50 and 100 mA/cm2 for zinc-air system with TiC0.7N0.3 cathode b) Charge – discharge cycles at 50 mA/cm2 for the three electrode configuration with TiC0.7N0.3 nanowire for ORR and IrO2 for OER and Zn electrode (2h. cycle period). Similarly, the catalytic activity of TiC0.7N0.3 has also been explored in non-aqueous electrolyte. The material acts as a bifunctional catalyst for oxygen in non- aqueous medium as well. It shows a stable performance for more than 100 cycles with high reversibility for ORR and OER (figure 6). Li-O2 battery fabricated with a non-aqueous gel- based electrolyte yields very good output. (a) (b) (c) Figure 6. Galvanostatic charge –discharge cycles. (a) at 1 mA/cm2 (b) specific capacity as a function of no. of cycles (c) photographs of PAN-based gel polymer electrolyte. Another reaction of interest in non –aqueous medium is I-/I3-. redox couple. TiC0.7N0.3 nanowires show small peak to peak separation, low charge transfer resistance and hence high activity. The catalyst is used as a counter electrode in dye sensitized a solar cell that shows efficiencies similar to that of Pt, state of the art catalyst (figure 7). (a) (b) (c) Figure 7 (a) Cyclic voltammograms for I-/I3 - redox species on TiC0.7N0.3 nanowires (red), TiC0.7N0.3 particle (black) and Pt (blue). (b) Photocurrent density - voltage characteristics for DSSCs with different counter electrodes. TiC0.7N0.3 nanowire (black), TiC0.7N0.3 particle (blue), Pt (red). (c) Photograph of a sample cell. (a) (b) (c) (d) Figure 8 a) Comparison ORR activity for (i) NiTiO3(black), (ii) N-rGO (red), (iii) NiTiO3 – N-rGO (green) and (iv) Pt/C (blue) (b) Linear sweep voltammograms for OER observed on NiTiO3 – N-rGO composite (black), NiTiO3 (brown), N-rGO (blue), glassy carbon (red) in 0.5 M KOH. (c) Galvanostatic discharge curves of NiTiO3 – N-rGO as air electrode (d) Charge – discharge cycle at 5 mA/cm2 for the rechargeable battery with 10 min. cycle period. The last part of the thesis discusses about a ceramic oxide, nickel titanate. The electrocatalytic studies of the material towards ORR and OER reveal that the catalyst shows remarkable performance as a bifunctional electrode. A gel - based zinc - air battery fabricated with nickel titanate – reduced graphene oxide composite shows exceptional performance of 1000 charge-discharge cycles in the rechargeable mode (figure 8). Of course, the primary battery configuration works very well too The thesis contains seven chapters on the aspects mentioned above with summary and future perspectives given as the last chapter. An appendix based on TiN nanotubes and supercapacitor studies is given at the end.

In dieser Dissertation wird der Einfluss unterschiedlicher Sauerstoff-Koordinationsgeometrien um ein zentrales Kobaltatom evaluiert. Genauer werden Oxide, die tetraedrisch und oktaedrisch koordiniertes Kobalt enthalten, synthetisiert und charakterisiert. Zudem wird ihre Aktivität in Hinblick auf die Sauerstoffentwicklungsreaktion (OER) unter alkalischen Bedingungen untersucht. Die elektrochemischen Analysen zeigen dabei, dass Materialien, die Kobalt in tetraedrischer Sauerstoffkoordination enthalten, die besseren Katalysatorvorläufer für die OER sind. Weiterhin kann demonstriert werden, dass das Herauslösen von inaktiven Metallen aus einer Struktur die Aktivität erhöht. Darauf basierend wird das neue Material Zn0.35Co0.65O vorgeschlagen. Es kristallisiert in der Wurtzitstruktur und enthält nur tetraedrisch koordinierte Atome. In alkalischen Lösungen wandelt sich die Wurtzitstruktur über die Zwischenstufe Co(OH)2 zum gamma-Co(O)OH um, und nahezu alles Zink wird aus der Struktur herausgelöst. Dadurch wird ein Material mit einer großen elektrochemisch aktiven Oberfläche gewonnen, das unterkoordinierte CoO(6-x) Oktaeder als aktive Zentren für die OER enthält. Hierdurch wird eine herausragende katalytische Leistung erreicht. Um weitere Einblicke in die OER zu generieren, wird Diffuse Reflexions UV/Vis (DRUV) Spektroskopie verwendet. Es werden neuartige Durchflusszellendesigns vorgeschlagen, die es erlauben, DRUV Spektren während der Katalyse aufzunehmen, d.h. operando. Durch diesen spektroelektrochemischen Ansatz werden Veränderungen der Katalysatoren während der OER beobachtet. So kann die Phasenumwandlung von Zn0.35Co0.65O erfolgreich verfolgt werden. Ebenso kann gezeigt werden, dass CoAl2O4 und Co2SnO4 nur an ihrer Oberfläche katalytische Aktivität aufweisen. Durch den Vergleich mit ex situ Analysen werden eindeutige Struktur-Eigenschaftsbeziehungen vorgeschlagen und tiefere Einsichten in die katalytisch aktiven Strukturmotive erhalten. / In this PhD thesis, the influence of different coordination geometries of oxygen atoms around a central cobalt atom is evaluated. Specifically, oxides containing tetrahedral and octahedral coordinated cobalt are synthesized, characterized and their activity towards the OER under alkaline conditions is evaluated. The electrochemical analyses reveal, that materials containing cobalt in tetrahedral oxygen coordination are better precatalysts for the OER. Furthermore, it is demonstrated that leaching of inactive metals from a structure increases the activity as well. Based on the previous mentioned, the new material Zn0.35Co0.65O is proposed. It crystallizes in the wurtzite structure and contains solely tetrahedrally coordinated atoms. In alkaline solutions, it transforms from wurtzite structure via a hydroxide to gamma-Co(O)OH, and nearly all Zn is leached from the structure. By this, a material with a large electrochemically active surface area is generated, that contains under-coordinated CoO(6-x) octahedra as active centers for the OER. Thus, outstanding catalytic performance is achieved. To generate further insights into the OER, diffuse reflectance ultraviolet visible (DRUV) spectroscopy is facilitated. Novel flow-cell designs are proposed, that allow to record DRUV spectra of catalysts under working conditions, i.e. operando. By this spectroelectrochemical approach, changes the catalysts undergo during the OER are observed. The phase transitions of Zn0.35Co0.65O are successfully followed, and it can be further shown, that CoAl2O4 and Co2SnO4 are only active at their surface. By comparison to ex situ analyses, clear structure-activity correlations are proposed, and deeper insights in the catalytically active structural motifs are obtained.

Tesis por compendio / [ES] El efecto invernadero, causado por el consumo masivo de combustibles fósiles en los últimos 100 años, ha exacerbado la tendencia del calentamiento global y también ha tenido un impacto severo en la salud humana y el medio ambiente ecológico. Las tecnologías de generación de energía renovable son cruciales para abordar el cambio climático y lograr la sostenibilidad energética, pero estas tecnologías también enfrentan algunos problemas y desafíos comunes, como la intermitencia y la imprevisibilidad, la integración y estabilidad de la red, problemas de almacenamiento de energía, etc. Por lo tanto, investigar dispositivos eficientes de almacenamiento de energía electroquímica y desarrollar tecnologías avanzadas de conversión de energía eléctrica renovable en energía química se vuelve particularmente crucial. El supercondensador es un nuevo tipo de dispositivo de almacenamiento de energía electroquímica que permite el rápido ciclo de descarga y carga. Su mayor densidad de potencia, capacidades de carga y descarga rápidas, amplio rango de temperatura de operación y seguridad permiten aplicaciones generalizadas en fuentes de alimentación de respaldo industriales, vehículos eléctricos, el sector militar y otros campos. Por lo tanto, en la tesis doctoral actual, se han utilizado materiales de óxido de metal y fosfuro de metal como electrodos de supercondensador para el estudio de los mecanismos de almacenamiento de energía y explorar el potencial para aplicaciones prácticas. Por otro lado, convertir la energía eléctrica sostenible en energía química para almacenamiento y utilización también es una forma efectiva, en la que la energía eléctrica se utiliza para impulsar reacciones químicas no espontáneas. La tesis doctoral actual desarrolló un catalizador de fosfuro de metal utilizando un método de síntesis verde y libre de contaminación y lo aplicó a reacciones electrocatalíticas de división de agua. Los resultados experimentales muestran que el material puede operar de manera estable durante mucho tiempo a alta densidad de corriente. Además, la conversión electrocatalítica de nitrato a amoníaco se considera una ruta eficiente para el tratamiento de aguas residuales de nitrato y la producción de combustible de amoníaco rico en hidrógeno. Preparamos con éxito un catalizador de carburo bimetálico utilizando un método sonoquímico, que muestra excelentes propiedades de reducción de nitrato de baja sobretensión y alta eficiencia de rendimiento de amoníaco. / [CA] L'efecte hivernacle, causat pel consum massiu de combustibles fòssils en els últims 100 anys, ha exacerbà la tendència del escalfament global i també ha tingut un impacte sever en la salut humana i el medi ambient ecològic. Les tecnologies de generació d'energia renovable són crucials per a abordar el canvi climàtic i aconseguir la sostenibilitat energètica, però aquestes tecnologies també enfronten alguns problemes i desafiaments comuns, com la intermitència i la imprevisibilitat, la integració i estabilitat de la xarxa, problemes d'emmagatzematge d'energia, etc. Per tant, investigar dispositius eficients d'emmagatzematge d'energia electroquímica i desenvolupar tecnologies avançades de conversió d'energia elèctrica renovable en energia química es torna particularment crucial. El supercondensador és un nou tipus de dispositiu d'emmagatzematge d'energia electroquímica que permet el ràpid cicle de descàrrega i càrrega. La seua major densitat de potència, capacitats de càrrega i descàrrega ràpides, ampli rang de temperatura d'operació i seguretat permeten aplicacions generalitzades en fonts d'alimentació de reserva industrials, vehicles elèctrics, el sector militar i altres camps. Per tant, en la tesi doctoral actual, s'han utilitzat materials d'òxid de metall i fosfurs de metall com a electrodos de supercondensador per a l'estudi dels mecanismes d'emmagatzematge d'energia i explorar el potencial per a aplicacions pràctiques. D'altra banda, convertir l'energia elèctrica sostenible en energia química per a emmagatzematge i utilització també és una forma efectiva, en la qual l'energia elèctrica s'utilitza per a impulsar reaccions químiques no espontànies. La tesi doctoral actual va desenvolupar un catalitzador de fosfurs de metall utilitzant un mètode de síntesi verd i lliure de contaminació i l'aplicà a reaccions electrocatalítiques de divisió d'aigua. Els resultats experimentals mostren que el material pot operar de manera estable durant molt temps a alta densitat de corrent. A més, la conversió electrocatalítica de nitrats a amoníac es considera una ruta eficient per al tractament d'aigües residuals de nitrats i la producció de combustible d'amoníac ric en hidrogen. Vam preparar amb èxit un catalitzador de carburs bimetàlics utilitzant un mètode sonoquímic, que mostra excel·lents propietats de reducció de nitrats de baixa sobretensió i alta eficiència de rendiment d'amoníac. / [EN] The greenhouse effect, caused by the massive consumption of fossil fuels over the past 100 years, has exacerbated the trend of global climate warming and has also severe impact on human health and the ecological environment. Renewable energy power generation technologies are crucial for addressing climate change and achieving energy sustainability, but these technologies also face some common problems and challenges, such as intermittency and unpredictability, grid integration and stability, energy storage issues, etc. Therefore, researching efficient electrochemical energy storage devices and developing advanced energy technologies converting renewable electrical energy into chemical energy becomes particularly crucial. Supercapacitor is a novel type of electrochemical energy storage device that enables the rapid cycle of discharge and charge. Its greater power density, rapid charge-discharge capabilities, wider operating temperature range and safety allow widespread applications in industrial backup power supplies, electric vehicles, the military sector, and other fields. Therefore, in the current doctoral thesis, metal oxides and metal phosphide materials have been employed as supercapacitor electrodes for the study of energy storage mechanisms and explore the potential for practical applications. On the other hand, converting sustainable electrical energy into chemical energy for storage and utilization is also an effective way, in which electrical energy is employed to drive non-spontaneous chemical reactions. The current doctoral thesis developed a metal phosphide catalyst using a green and pollution-free synthesis method and applied it to electrocatalytic water-splitting reactions. Experimental results show that the material can operate stably for a long time at high current density. Furthermore, electrocatalytic conversion of nitrate to ammonia is considered an efficient route for nitrate wastewater treatment and production of hydrogen-rich ammonia fuel. We successfully prepared a bimetallic carbide catalyst using a sonochemical method, which exhibits excellent low overpotential nitrate reduction and high-efficiency ammonia yield properties. / Hu, J. (2024). Metal Oxides, Carbides and Phosphides for Supercapacitor and Electrocatalysis [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/207537 / Compendio