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.

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

El conocimiento de las características interfaciales es de suma importancia para poder desarrollar materiales que sean capaces de dar lugar a reacciones electrocatalíticas eficientes. Por esta razón, en esta tesis se muestran diferentes estudios interfaciales sobre superficies monocristalinas de platino y cobre en diferentes electrolitos. Además se estudian las características de la interfase electrodoldisolución con superficies de platino modificadas con adátomos de hierro, cobalto y níquel. Para ello, se han empleado las técnicas de voltametría cíclica, espectroscopía infrarroja con transformada de Fourier, desplazamiento de carga con CO y salto de temperatura inducido por láser. Los resultados muestran cómo varía el campo eléctrico interfacial disminuye al aumentar el recubrimiento de hierro y níquel en la superficie de platino. Este efecto tiene un gran impacto en la reacción de evolución de hidrógeno, ya que la mejora electrocatalítica de esta reacción está relacionada con la energía de reorganización de las moléculas de agua, la cual, depende de la fortaleza del campo eléctrico interfacial. Los estudios realizados en medio alcalino para las diferentes superficies de cobre y platino sin modificar muestran una correlación entre el potencial de máxima entropía y las funciones de trabajo para cada una de las diferentes orientaciones atómicas superficiales. Por otro lado, debido a la aplicabilidad de las nanopartículas en los sistemas reales de conversión de energía, se realizaron experimentos de sincrotrón empleando la técnica de Bragg coherent difraction imaging con el objetivo de estudiar el deterioro de las nanopartículas en condiciones operando.

Monolayer two-dimensional (2D) materials have been regarded as a hot topic in the fields of condensed matter physics, materials science, and chemistry due to their unique physical, chemical, and electronic properties. However, the research on the preparation method and properties understanding of the 2D monolayer are inadequate. In this dissertation, taking 2D nickel-iron layered double hydroxides (NiFe LDHs) and molybdenum disulfide (MoS2) as examples, the practicability of the direct synthesis of NiFe LDHs monolayer and the thermal enhancement catalytic performance of 2D MoS2 monolayer (MoS2 ML) are discussed. First, a one-pot synthetic strategy (bottom-up method) is presented to synthesize 2D NiFe-based LDHs monolayers, including NiFe, Co-, Ru-, doped, and Au-modified NiFe LDHs. The prerequisite and universality of this strategy are investigated and confirmed. The features of LDHs are characterized by advanced technologies. The obtained LDH bulks own a large interlayer spacing up to 8.2 Å, which can be facilely exfoliated into monolayers in water by hand-shaking within 10 s. As a result, the as-prepared NiFe-based LDH monolayers display a good electrocatalytic oxygen evolution reaction (OER) performance. This facile strategy paves the way for designing easily exfoliated LDHs for highly active catalysts and energy conversion devices based on other monolayer LDHs. Second, with gold-modified tape, 2D MoS2 ML is exfoliated from the bulk crystal through a micromechanical exfoliation method (top-down strategy). The thermal effects of MoS2 ML are confirmed by Raman and photoluminescence (PL) spectra. Moreover, an on-chip MoS2 ML hydrogen evolution reaction (HER) reactor is designed and fabricated. The thermal effects generate efficient electron transfer in the MoS2 ML and at the electrolyte-catalyst (MoS2 ML) interface, leading to an enhanced HER performance. Compared to the results obtained at room temperature, the MoS2 ML shows a direct thermal enhanced HER performance at higher temperatures. In summary, the findings and understandings, the direct synthesis and direct thermal enhancement catalytic performance, of 2D monolayers offer a guideline for synthesizing and catalyst application of other 2D monolayers.

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.

This thesis primarily focuses on the electrochemical properties of the H₂/H⁺ redox couple, at various metallic electrodes in room temperature ionic liquids. Initially, a comprehensive overview of room temperature ionic liquids, RTILs, compared to conventional organic solvents is presented which identifies their favourable properties and applications, followed by a second chapter describing the basic theory of electrochemistry. A third chapter presents the general experimental reagents, instruments and measurements used in this thesis. The results presented in this thesis are summarized in six further chapters and shown as follows. (1) Hydrogenolysis, hydrogen loaded palladium electrodes by electrolysis of H[NTf₂] in a RTIL [C₂mim][NTf₂]. (2) Palladium nanoparticle-modified carbon nanotubes for electrochemical hydrogenolysis in RTILs. (3) Electrochemistry of hydrogen in the RTIL [C₂mim][NTf₂]: dissolved hydrogen lubricates diffusional transport. (4) The hydrogen evolution reaction in a room temperature ionic liquid: mechanism and electrocatalyst trends. (5) The formal potentials and electrode kinetics of the proton_hydrogen couple in various room temperature ionic liquids. (6) The electroreduction of benzoic acid: voltammetric observation of adsorbed hydrogen at a Platinum microelectrode in room temperature ionic liquids. The first two studies show electrochemically formed adsorbed H atoms at a metallic Pt or Pd surface can be used for clean, efficient, safe electrochemical hydrogenolysis of organic compounds in RTIL media. The next study shows the physicochemical changes of RTIL properties, arising from dissolved hydrogen gas. The last three studies looked at the electrochemical properties of H₂/H⁺ redox couple at various metallic electrodes over a range of RTILs vs a stable Ag/Ag⁺ reference couple, using H[NTf₂] and benzoic acid as proton sources. The kinetic and thermodynamic mechanisms of some reactions or processes are the same in RTILs as in conventional organic or aqueous solvents, but other remarkably different behaviours are presented. Most importantly significant constants are seen for platinum, gold and molybdenum electrodes in term of the mechanism of proton reduction to form hydrogen.

This work reports on the cobalt pyridinoporphyrazinate (CoTmtppa) as a platinum-group metal-free catalyst for hydrogen evolution and oxidation reactions with the possibility of use in hydrogen energy and hydrogen potentiometric sensing. A different interaction of CoTmtppa with various electrode substrates, highly oriented pyrolytic graphite (HOPG) and annealed gold (Au(111)), affects its electrocatalytic behaviour in hydrogen reactions. The formation of a hydride-type complex with the bonding of hydrogen atoms to cobalt centre is supposed to be the rate-determining step. In the case of hydrogen evolution, the maximum catalytic activity of mediator was reached at pH = 11,0, when the HOPG/CoTmtppa showed overpotential decrease by 300 mV and an almost 60-fold increase of current densities compared to HOPG. The electrocatalytic activity of Au(111)/CoTmtppa resulted in a further decrease of overpotential by 175 mV in comparison with HOPG/Co(I)Tmtppa. The electrochemical oxidation of hydrogen was found to depend on hydrogen source which was electrochemically generated on-site or molecular hydrogen supplied from an external source. In the case of electrochemically generated hydrogen, the maximum activity of HOPG/CoTmtppa was reached at pH = 2.1 and an additional it was observed 50 % increase in current...

[ES] Esta Tesis Doctoral se centra en la recuperación mediante electrodeposición del Sb presente en un efluente residual proveniente de la metalurgia del cobre. El efluente en el que se basa este estudio se produce en el proceso de regeneración con HCl concentrado de las resinas de intercambio iónico de una de las subetapas del electro-refinado del cobre. Se trata de un electrolito ácido que contiene diversas impurezas, entre las que destacan, Sb y Bi, que son considerados materias primas críticas por la Unión Europea, debido al riesgo de suministro que presentan y a su importancia en la economía. El objetivo de esta Tesis es la recuperación del Sb y la reutilización del HCl. Para alcanzar ambos propósitos se ha estudiado el empleo de la electrodeposición como operación de separación para el tratamiento del efluente. Este proceso se ha investigado mediante técnicas electroquímicas, en particular, voltametría cíclica y de barrido lineal. En primer lugar, se ha realizado el estudio con disoluciones sintéticas que emulan el efluente generado en las metalurgias chilenas. La caracterización electroquímica de la disolución de Sb en medio HCl ha evidenciado que la reducción del Sb(III) a Sb metálico está limitada por la transferencia de materia y que una concentración elevada de HCl favorece la recuperación del Sb. Los ensayos de electrodeposición han destacado la relevancia de las reacciones secundarias, la reacción de evolución del hidrógeno (HER) y la generación de cloro, y su influencia negativa sobre la deposición de Sb. La HER tiene lugar en el cátodo y conlleva la generación de burbujas de hidrógeno a elevadas densidades de corriente, que reducen la superficie útil del electrodo de trabajo e incluso causan el desprendimiento del depósito de Sb. Por su parte, la generación de cloro gas en el ánodo provoca la redisolución de los depósitos de Sb desprendidos a causa de la HER. Tras la realización de estos ensayos se ha concluido que se puede aumentar la tasa de recuperación de Sb si se tienen mayores concentraciones del elemento en la disolución o si se mejoran las condiciones hidrodinámicas. Se ha analizado también cómo influye la presencia de Bi en la disolución sintética durante la electrodeposición del Sb. El potencial de reducción de ambos elementos es similar, lo que implica que la recuperación individual de Sb sea compleja. No obstante, si se trabaja bajo condiciones en las que no se supere la densidad de corriente límite del sistema, se logra una elevada selectividad hacia el Sb. Si la concentración de Bi en la disolución es superior a la de Sb, se electrodepositan ambos elementos, pero se logra un elevado valor de eficiencia de corriente debido a que la influencia de la HER sobre el Bi no es tan fuerte. A continuación, se ha estudiado la aplicación de las técnicas previamente analizadas con un efluente real, facilitado por una empresa española. Se ha verificado que las conclusiones obtenidas con las disoluciones sintéticas son aplicables al efluente real. En particular, se ha confirmado que, al aumentar la densidad de corriente aplicada, la deposición de Sb se ve mermada debido a la relevancia del resto de reacciones: la reducción de los demás elementos y la evolución del medio. Los depósitos obtenidos contienen algunos de los elementos presentes en el efluente real: Sb, Bi, As y Cu, siendo mayoritario el primero. Como resultado, se ha concluido que es posible purificar el HCl mediante la electrodeposición de los elementos contenidos en el electrolito. Finalmente, se ha planteado la posibilidad de sustituir el HCl empleado como regenerante por un disolvente eutéctico profundo, debido a la alta solubilidad de metales y su procedencia de fuentes renovables. En particular, la investigación se ha llevado a cabo con la oxalina, la cual presenta una elevada capacidad de disolución para el Sb y una amplia ventana de potencial que beneficiaría la aplicación de la técnica de electrodeposición para la recuperación del Sb. / [CA] Esta Tesi Doctoral se centra en la recuperació mitjançant electrodeposició del Sb present en un efluent residual provinent de la metal·lúrgia del coure. L'efluent en què es basa este estudi es produïx en el procés de regeneració amb HCl concentrat de les resines d'intercanvi iònic d'una de les subetapes de l'electro-refinat del coure. Es tracta d'un electròlit àcid que conté diverses impureses, entre les quals destaquen, Sb i Bi, que són considerats matèries primeres crítiques per la Unió Europea, a causa del risc de subministrament que presenten i a la seua importància en l'economia. L'objectiu d'esta Tesi és la recuperació del Sb i la reutilització de l'HCl. Per assolir aquests dos propòsits s'ha estudiat l'ús de l'electrodeposició com a operació de separació per al tractament de l'efluent. Este procés s'ha investigat mitjançant tècniques electroquímiques, en particular, voltametria cíclica i de rastreig lineal. En primer lloc, s'ha realitzat l'estudi amb dissolucions sintètiques que emulen l'efluent generat a les metal·lúrgies xilenes. La caracterització electroquímica de la dissolució de Sb en medi HCl ha evidenciat que la reducció del Sb(III) a Sb metàl·lic està limitada per la transferència de matèria i que una concentració elevada de HCl afavorix la recuperació del Sb. Els assajos d'electrodeposició han destacat la rellevància de les reaccions secundàries, la reacció d'evolució de l'hidrogen (HER) i la generació de clor, i la seua influència negativa sobre la deposició de Sb. La HER té lloc al càtode i comporta la generació de bambolles d'hidrogen a elevades densitats de corrent, la qual cosa reduïx la superfície útil de l'elèctrode de treball i fins i tot causa el despreniment del depòsit de Sb. Per la seua banda, la generació de clor gas a l'ànode provoca la redissolució dels depòsits de Sb despresos a causa de la HER. Després de la realització d'estos assajos s'ha conclòs que es pot augmentar la taxa de recuperació de Sb si es tenen majors concentracions de l'element en la dissolució o si es milloren les condicions hidrodinàmiques. S'ha analitzat també com influïx la presència de Bi en la dissolució sintètica durant l'electrodeposició del Sb. El potencial de reducció dels dos elements és similar, la qual cosa implica que la recuperació individual de Sb siga complexa. No obstant això, si es treballa sota condicions en què no se supere la densitat de corrent límit del sistema, s'aconseguix una elevada selectivitat cap al Sb. Si la concentració de Bi en la dissolució és superior a la de Sb, s'electrodepositen ambdós elements, però s'aconseguix un elevat valor d'eficiència de corrent pel fet que la influència de la HER sobre el Bi no és tan forta. A continuació, s'ha estudiat l'aplicació de les tècniques prèviament analitzades amb un efluent real, facilitat per una empresa espanyola. S'ha verificat que les conclusions obtingudes amb les dissolucions sintètiques són aplicables al efluent real. En particular, s'ha confirmat que, en augmentar la densitat de corrent aplicada, la deposició de Sb es veu minvada a causa de la rellevància de la resta de reaccions: la reducció dels altres elements i l'evolució del medi. Els depòsits obtinguts contenen alguns dels elements presents en el efluent real: Sb, Bi, As i Cu, sent majoritari el primer. Com a resultat, s'ha conclòs que és possible purificar l'HCl mitjançant l'electrodeposició dels elements continguts en l'electròlit. Finalment, s'ha plantejat la possibilitat de substituir l'HCl emprat com regenerant per un dissolvent eutèctic profund, a causa de l'alta solubilitat de metalls i la seua procedència de fonts renovables. En particular, la investigació s'ha dut a terme amb l'oxalina, la qual presenta una elevada capacitat de dissolució per al Sb i una àmplia finestra de potencial que beneficiaria l'aplicació de la tècnica d'electrodeposició per a la recuperació del Sb captat durant el procés de regeneració de les resines. / [EN] This Doctoral Thesis focuses on the recovery by electrodeposition of Sb present in an effluent from the copper metallurgy. The effluent on which this study is based is generated during the regeneration with concentrated hydrochloric acid of the ion exchange resins used in one of the sub-stages of copper electro-refining. It is an acid electrolyte containing several impurities, among them, Sb and Bi, which are considered critical raw materials by the European Union due to their supply risk and economic relevance. The aim of this Thesis is the recovery of Sb and reuse of HCl. To achieve both objectives, electrodeposition has been studied as a separation operation for the treatment of the effluent. This operation has been investigated through electrochemical techniques, primarily, cyclic and linear sweep voltammetry. First, the study has been conducted with synthetic solutions emulating the effluent generated in the Chilean metallurgy industries. The electrochemical characterization of the Sb solution in the HCl medium has shown that, mass-transfer limits the reduction of Sb(III) to metallic Sb and a high HCl concentration favors the recovery of Sb. From the electrodeposition tests the relevance and negative influence on Sb deposition of the secondary reactions, the hydrogen evolution reaction (HER) and the chlorine generation, has been proven. The HER occurs at the cathode and involves the generation of hydrogen bubbles at high current densities, which decrease the effective surface area of the working electrode and even cause the detachment of the Sb deposits. The chlorine generation at the anode leads to the redissolution of the Sb deposits detached by the HER. After conducting these experiments, it has been concluded that it is possible to increase the Sb recovery rate, if higher concentrations of this element are present in the solution or the hydrodynamic conditions of the electrodeposition process are improved. The influence of the presence of Bi in the synthetic solution on the Sb electrodeposition has also been investigated. The reduction potential of both elements is similar, implying that the individual recovery of Sb by electrodeposition is challenging. However, if the operating conditions do not imply exceeding the limiting current density of the system, a high electrodeposition selectivity towards Sb is achieved. If the concentration of Bi in the solution is higher than that of Sb, the simultaneous electrodeposition of both elements takes place, but, on the contrary, a high value of current efficiency is obtained because the influence of the HER on Bi reduction is not as significant as that on Sb deposition. The application of the previously analyzed techniques has been studied with a real effluent, that was provided by a Spanish company. It has been proven that the conclusions obtained with the synthetic solutions can be applied to a real effluent. It has been confirmed that, as the applied current density increases, the Sb deposition worsens due to the relevance of the other reactions that take place during the process: the reduction of the other elements and the evolution of the medium. The deposits obtained contain some elements present in the real effluent: Sb, Bi, As and Cu, the most significant being the first. From this study, it has been concluded that purifying the HCl by removing the elements contained in the electrolyte via electrodeposition is possible. Finally, the possibility of replacing the HCl used as a regenerant with a deep eutectic solvent has been considered based on their properties, among which are a high solubility of metals and their synthesis from renewable sources. In particular, the research has been carried out with oxaline, which presents a high dissolution capacity for Sb and a wide potential window that would benefit the application of the electrodeposition technique to recover Sb captured during the resins regeneration process. / Me gustaría agradecer la financiación a la Agencia Estatal de Investigación (AEI/10.13039/501100011033) (España) bajo el proyecto PCI2019-103535, gracias al cual he podido desarrollar la presente Tesis Doctoral, y a FEDER Una manera de hacer Europa / Hernández Pérez, L. (2024). Estudio electroquímico para la recuperación de antimonio de efluentes del electro-refinado de cobre por electrodeposición [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/203892