The synthesis of 2,4,6,8-Tetramethylcyclotetrasiloxane (D4H), and Poly(methylhydrosiloxane) (PMHS) average molecular weight 1700-3200 g/mol, were functionalized with different repeat units of methoxy polyethylene glycol (PEG) (n = 8,12,17). These compounds act as polymer electrolytes with a backbone of siloxane and they were prepared via hydro-silylation reaction to be functionalized with different molecular weights of Ally-PEG. The compounds were confirmed by FT-IR, 1H-NMR and 13C NMR spectroscopy. A hydro-silylation reaction between the functionalized AllyPEG of different molecular weights produced four compounds with a low glass transition temperature that could improve comb like polymer electrolytes conductivity by reducing crystalline phase of PEO. Another way to increase the percentage of the amorphous phase of PEO is to blend it with other polymers. The blending method is considered to be an important method to improve the ionic conductivities and dimensional stability of polymer electrolytes. The main advantages of the blend systems are the simplicity of preparation and the ease to control the physical properties. A high molecular weight of poly 2- vinyl pyridine (Mw=200,000) was added to improve the dimensional stability. Differential scanning calorimetry (DSC) thermal analysis shows that all the blend systems will exhibit an increase in the glass transition temperature by increasing the salt content. The other novel synthesis of polymer electrolytes are triglyme borane and borosilicate. They were synthesized via hydro-boration. These compounds were characterized and confirmed by FT-IR, 1H-NMR 13C NMR spectroscopy. The ionic conductivity of both systems, pure and blend, of different compositions were determined at four temperatures i.e. 25°C, 40°C, 55°C and 70°C. A maximum ionic conductivity value of the siloxane blend is 9.1x10-4 S cm-1 and the pure triglyme borane is 2.14x10-3 S cm-1 at ambient temperature. The ratios of ethylene oxide to lithium salt of siloxane blend and pure triglyme borane were 10:1 and 35:1 respectively. These ratios were the highest conductivity obtained in all the electrolyte systems. The ionic conductivity increases with increasing temperature and salt content to reach optimum concentration. This behavior results in ionic transport, which is supported by the segmental motion of the polymer matrix host.

Les batteries lithium-air et Li-S sont des techniques prometteuses pour un stockage efficace d’énergie électrochimique. Les principaux défis sont de développer un électrolyte solide à haute conductivité ionique et des cathodes efficaces. Dans ce travail, des aérogels de carbone conducteurs avec une double porosité ont été synthétisés en utilisant la méthode de sol-gel. Ils ont été utilisés comme cathode dans des batteries lithium-air. Ces cathodes peuvent fournir deux types de canaux pour le stockage de produits de décharge, facilitant la diffusion gaz-liquide et réduisant ainsi le risque de colmatage. Presque 100 cycles été obtenus avec une capacité de 0,4 mAh et une densité de courant de 0,1 mA/cm². Pour le développement d'électrolyte solide stable et conducteur, les sulfures, en particulier Li4SnS4 et son dérivé Li10SnP2S12 ont été particulièrement étudiés. Ces composés ont été synthétisés en utilisant une technique en deux étapes comprenant la mécanosynthèse et un traitement thermique à température relativement basse qui a été optimisé afin d'améliorer la conductivité ionique. La meilleure conductivité obtenue est de 8,27×10-4 S / cm à 25°C et ces électrolytes présentent une grande stabilité électrochimique sur une large gamme de voltage de 0,5 à 7V. Les couches minces ont également été déposées en utilisant la technique de pulvérisation cathodique, avec en général une conductivité ionique améliorée. La performance des batteries Li-S assemblées avec ces électrolytes massifs doit être améliorée, en particulier en améliorant la conductivité ionique de l'électrolyte. / Lithium-air and Li-S batteries are promising techniques for high power density storage. The main challenges are to develop solid electrolyte with high ionic conductivity and highly efficient catalyzed cathode. In this work, highly conductive carbon aerogels with dual-pore structure have been synthesized by using sol-gel method, and have been used as air cathode in Lithium-air batteries. This dual- pore structure can provide two types of channels for storing discharge products and for gas-liquid diffusion, thus reducing the risk of clogging. Nearly 100 cycles with a capacity of 0.4mAh at a current density of 0.1 mA cm-2 have been obtained. For developing stable and highly conductive solid electrolyte, sulfides, especially Li4SnS4 and its phosphorous derivative Li10SnP2S12 have been particularly investigated. These compounds have been synthesized by using a two-step technique including ball milling and a relatively low temperature heat treatment. The heat treatment has been carefully optimized in order to enhance the ionic conductivity. The best-obtained conductivity is 8.27×10-4 S/cm at 25°C and the electrolytes show high electrochemical stability over a wide working range of 0.5 – 7V. Thin films have also been deposited by using the sputtering technique, with generally improved ionic conductivity. The performance of the Li-S batteries assembled with these bulk electrolytes is still to be improved, particularly by improving the ionic conductivity of the electrolyte.

O aumento no consumo energético e a crescente preocupação ambiental frente à emissão de gases poluentes criam um apelo mundial favorável para pesquisas de novas tecnologias não poluentes de fontes de energia. Baterias recarregáveis de lítio-ar em solventes não aquosos possuem uma alta densidade de energia teórica (5200 Wh kg-1), o que as tornam promissoras para aplicação em dispositivos estacionários e em veículos elétricos. Entretanto, muitos problemas relacionados ao cátodo necessitam ser contornados para permitir a aplicação desta tecnologia, por exemplo, a baixa reversibilidade das reações, baixa potência e instabilidades dos materiais empregados nos eletrodos e dos solventes eletrolíticos. Assim, neste trabalho um modelo cinético foi empregado para os dados experimentais de espectroscopia de impedância eletroquímica, para a obtenção das constantes cinéticas das etapas elementares do mecanismo da reação de redução de oxigênio (RRO), o que permitiu investigar a influência de parâmetros como o tipo e tamanho de partícula do eletrocatalisador, o papel do solvente utilizado na RRO e compreender melhor as reações ocorridas no cátodo dessa bateria. A investigação inicial se deu com a utilização de sistemas menos complexos como uma folha de platina ou eletrodo de carbono vítreo como eletrodos de trabalho em 1,2-dimetoxietano (DME)/perclorato de lítio (LiClO4). A seguir, sistemas complexos com a presença de nanopartículas de carbono favoreceu o processo de adsorção das moléculas de oxigênio e aumentou ligeiramente (uma ordem de magnitude) a etapa de formação de superóxido de lítio (etapa determinante de reação) quando comparada com os eletrodos de platina e carbono vítreo, atribuída à presença dos grupos laterais mediando à transferência eletrônica para as moléculas de oxigênio. No entanto, foi observada uma rápida passivação da superfície eletrocatalítica através da formação de filmes finos de Li2O2 e Li2CO3 aumentando o sobrepotencial da bateria durante a carga (diferença de potencial entre a carga e descarga > 1 V). Adicionalmente, a incorporação das nanopartículas de platina (Ptnp), ao invés da folha de platina, resultou no aumento da constante cinética da etapa determinante da reação em duas ordens de magnitude, o qual pode ser atribuído a uma mudança das propriedades eletrônicas na banda d metálica em função do tamanho nanométrico das partículas, e estas modificações contribuíram para uma melhor eficiência energética quando comparado ao sistema sem a presença de eletrocatalisador. Entretanto, as Ptnp se mostraram não específicas para a RRO, catalisando as reações de degradação do solvente eletrolítico e diminuindo rapidamente a eficiência energética do dispositivo prático, devido ao acúmulo de material no eletrodo. O emprego de líquido iônico como solvente eletrolítico, ao invés de DME, promoveu uma maior estabilização do intermediário superóxido formado na primeira etapa de transferência eletrônica, devido à interação com os cátions do líquido iônico em solução, o qual resultou em um valor de constante cinética da formação do superóxido de três ordens de magnitude maior que o obtido com o mesmo eletrodo de carbono vítreo em DME, além de diminuir as reações de degradação do solvente. Estes fatores podem contribuir para uma maior potência e ciclabilidade da bateria de lítio-ar operando com líquidos iônicos. / The increasing in energetic consumption and environmental concerning toward rising in the emission of pollutant gases create a favorable scenario to develop non-pollutant technologies and more efficient energy storages. Rechargeable non-aqueous lithium-air batteries possess high theoretical energy density (5200 Wh kg-1), characterizing as a promising system to stationary and electric vehicles applications. However, many issues on the cathode electrode should be addressed to enable this technology, for example, low reversibility of the reactions, low rate-capability and instabilities issues from cathode materials and electrolytic solvents. Here, a kinetic model was employed for modulate the experimental impedance data in order to obtain the rate constants of elementary steps from oxygen reduction reaction (ORR), which allows the investigation of the role of some parameters such as, type and grain size of electrocatalysts, and the solvent influence. The initial investigation were with less complexes systems of platinum bulk or glassy carbon as the working electrode in 1,2-dimethoxyethane (DME)/lithium perchlorate (LiClO4). Based on that, the role of carbon nanoparticles in the ORR was an increasing the oxygen adsorption process, and by slightly increasing (one order of magnitude) the superoxide formation (rate determining step) as when compared with platinum and glassy carbon electrodes due to the presence of side groups acting as mediators to the electron transfer. Nonetheless, a fast surface passivation was observed in function of Li2O2 and Li2CO3 thin films formations, and these films increase the battery overpotential during the charge process (potential difference between charge/discharge >1V). In addition, dispersed platinum nanoparticles (Ptnp) resulted in an increase of two orders of magnitude on the rate constant of the rate determining step when compared to platinum bulk. This can be explained due to changes in electronic properties of metallic d-bands in function of nanometric size. These changes contributed to enhance the energetic efficiency of the practical device when compared to the non-catalyzed system. However, the Ptnp were non-specific toward the ORR catalyzing the electrolyte degradation reactions, and decreasing the energy efficiency faster than the non-catalyzed system. The ionic liquid rather than DME promoted better stabilization process for intermediary superoxide due to interaction between cations present in solution, resulting in an outstanding enhancement of the rate constant for rate determining step (three orders of magnitude) when compared to the same working electrode in DME. In addition, decrease the electrolyte degradation reaction. These factors can improve a higher rate-capability and cycle life of the practical lithium-air batteries.

Les systèmes électrochimiques lithium-air sont des concepts naissants mais exhibent des performances théoriques intéressantes qui laissent espérer une rupture technologique dans le domaine des batteries pour véhicule électrique. La possibilité d'atteindre une densité d'énergie supérieure à 500 Wh kg-1 est effectivement en ligne de mire. A contrario de la technologie lithium-air anhydre, les systèmes lithium-air aqueux n'ont, jusqu'à présent, fait l'objet d'aucune étude approfondie. Ce travail concerne donc le développement d'un système lithium-air aqueux, à trois électrodes, et vise également à améliorer nos connaissances fondamentales dans le domaine. La présente étude se focalise sur le compartiment positif de la cellule, dans lequel les réactions de l'oxygène sont mises en jeu. Dans un premier temps, une électrode spécifiquement dédiée à la réaction de dégagement d'oxygène a été élaborée à partir d'un acier 316L. L'étude de son comportement a révélé une bonne propension à catalyser la réaction de dégagement d'oxygène ainsi qu'une bonne stabilité sur 3 000 heures de fonctionnement. Néanmoins, d'importants problèmes de catalyse ont pu être observés et attribués à la présence des ions lithium dans l'électrolyte de la batterie. Les ions Li+ bloquent les transitions électrochimiques des sites actifs à l'origine des propriétés d'électrocatalyse. Le comportement d'une électrode à air, composée de carbone et d'oxydes de manganèse, a par la suite été caractérisé dans ce milieu. L'étude révèle deux phénomènes importants réduisant la performance de l'électrode et dont l'origine a également été attribuée aux ions lithium : un blocage des transitions (MnIII/MnIV), et une stabilisation des groupements oxygénés à la surface du carbone. Pour finir, il a été proposé d'optimiser le système électrolytique en limitant l'activité des ions Li+ en solution et ainsi d'améliorer le rendement en potentiel de charge/décharge de la batterie.

O aumento no consumo energético e a crescente preocupação ambiental frente à emissão de gases poluentes criam um apelo mundial favorável para pesquisas de novas tecnologias não poluentes de fontes de energia. Baterias recarregáveis de lítio-ar em solventes não aquosos possuem uma alta densidade de energia teórica (5200 Wh kg-1), o que as tornam promissoras para aplicação em dispositivos estacionários e em veículos elétricos. Entretanto, muitos problemas relacionados ao cátodo necessitam ser contornados para permitir a aplicação desta tecnologia, por exemplo, a baixa reversibilidade das reações, baixa potência e instabilidades dos materiais empregados nos eletrodos e dos solventes eletrolíticos. Assim, neste trabalho um modelo cinético foi empregado para os dados experimentais de espectroscopia de impedância eletroquímica, para a obtenção das constantes cinéticas das etapas elementares do mecanismo da reação de redução de oxigênio (RRO), o que permitiu investigar a influência de parâmetros como o tipo e tamanho de partícula do eletrocatalisador, o papel do solvente utilizado na RRO e compreender melhor as reações ocorridas no cátodo dessa bateria. A investigação inicial se deu com a utilização de sistemas menos complexos como uma folha de platina ou eletrodo de carbono vítreo como eletrodos de trabalho em 1,2-dimetoxietano (DME)/perclorato de lítio (LiClO4). A seguir, sistemas complexos com a presença de nanopartículas de carbono favoreceu o processo de adsorção das moléculas de oxigênio e aumentou ligeiramente (uma ordem de magnitude) a etapa de formação de superóxido de lítio (etapa determinante de reação) quando comparada com os eletrodos de platina e carbono vítreo, atribuída à presença dos grupos laterais mediando à transferência eletrônica para as moléculas de oxigênio. No entanto, foi observada uma rápida passivação da superfície eletrocatalítica através da formação de filmes finos de Li2O2 e Li2CO3 aumentando o sobrepotencial da bateria durante a carga (diferença de potencial entre a carga e descarga > 1 V). Adicionalmente, a incorporação das nanopartículas de platina (Ptnp), ao invés da folha de platina, resultou no aumento da constante cinética da etapa determinante da reação em duas ordens de magnitude, o qual pode ser atribuído a uma mudança das propriedades eletrônicas na banda d metálica em função do tamanho nanométrico das partículas, e estas modificações contribuíram para uma melhor eficiência energética quando comparado ao sistema sem a presença de eletrocatalisador. Entretanto, as Ptnp se mostraram não específicas para a RRO, catalisando as reações de degradação do solvente eletrolítico e diminuindo rapidamente a eficiência energética do dispositivo prático, devido ao acúmulo de material no eletrodo. O emprego de líquido iônico como solvente eletrolítico, ao invés de DME, promoveu uma maior estabilização do intermediário superóxido formado na primeira etapa de transferência eletrônica, devido à interação com os cátions do líquido iônico em solução, o qual resultou em um valor de constante cinética da formação do superóxido de três ordens de magnitude maior que o obtido com o mesmo eletrodo de carbono vítreo em DME, além de diminuir as reações de degradação do solvente. Estes fatores podem contribuir para uma maior potência e ciclabilidade da bateria de lítio-ar operando com líquidos iônicos. / The increasing in energetic consumption and environmental concerning toward rising in the emission of pollutant gases create a favorable scenario to develop non-pollutant technologies and more efficient energy storages. Rechargeable non-aqueous lithium-air batteries possess high theoretical energy density (5200 Wh kg-1), characterizing as a promising system to stationary and electric vehicles applications. However, many issues on the cathode electrode should be addressed to enable this technology, for example, low reversibility of the reactions, low rate-capability and instabilities issues from cathode materials and electrolytic solvents. Here, a kinetic model was employed for modulate the experimental impedance data in order to obtain the rate constants of elementary steps from oxygen reduction reaction (ORR), which allows the investigation of the role of some parameters such as, type and grain size of electrocatalysts, and the solvent influence. The initial investigation were with less complexes systems of platinum bulk or glassy carbon as the working electrode in 1,2-dimethoxyethane (DME)/lithium perchlorate (LiClO4). Based on that, the role of carbon nanoparticles in the ORR was an increasing the oxygen adsorption process, and by slightly increasing (one order of magnitude) the superoxide formation (rate determining step) as when compared with platinum and glassy carbon electrodes due to the presence of side groups acting as mediators to the electron transfer. Nonetheless, a fast surface passivation was observed in function of Li2O2 and Li2CO3 thin films formations, and these films increase the battery overpotential during the charge process (potential difference between charge/discharge >1V). In addition, dispersed platinum nanoparticles (Ptnp) resulted in an increase of two orders of magnitude on the rate constant of the rate determining step when compared to platinum bulk. This can be explained due to changes in electronic properties of metallic d-bands in function of nanometric size. These changes contributed to enhance the energetic efficiency of the practical device when compared to the non-catalyzed system. However, the Ptnp were non-specific toward the ORR catalyzing the electrolyte degradation reactions, and decreasing the energy efficiency faster than the non-catalyzed system. The ionic liquid rather than DME promoted better stabilization process for intermediary superoxide due to interaction between cations present in solution, resulting in an outstanding enhancement of the rate constant for rate determining step (three orders of magnitude) when compared to the same working electrode in DME. In addition, decrease the electrolyte degradation reaction. These factors can improve a higher rate-capability and cycle life of the practical lithium-air batteries.

A galvanic cell converts chemical energy into electrical energy. Devices that carry out these conversions are called batteries. In batteries, generally the chemical components are contained within the device itself. If the reactants are supplied from an external source as they are consumed, the device is called a fuel cell. A fuel cell converts chemical energy into electrical energy as long as the chemicals are supplied from external reserves. The working principle of a metal-air battery involves the principles of both batteries and fuel cells. The anode of a metal-air cell is stored inside the cell, whereas O2 for the air-electrode is supplied from either atmosphere or a tank. There are several metal-air batteries available academically, which include Zn-air, Alair, Fe-air, Mg-air, Ca-air, Li-air and Na-air batteries. So far, only Zn-air battery is successfully commercialized. Li-air battery is attractive compared to other metal-air batteries because of its high theoretical energy density (11140 Wh kg-1). The energy density of Li-air battery is 3 - 5 times greater than state-of-art Li-ion battery. Li-air (or Li-O2) battery comprises Li-metal as the anode and a porous cathode. The cathode and the anode are separated by a suitable separator soaked in an organic electrolyte. Atmospheric air can enter the battery through the porous cathode. Out of the mixture of gases present in the air, only O2 is electrochemically active. For optimization purpose, most of researchers use pure O2 gas instead of air. Li-air battery is not commercialized till now because of several issues associated with it. The issues include: (i) sluggish kinetics of O2 electrode reaction, (ii) decomposition of the electrolyte during charge-discharge cycling, (iii) formation of Li dendrites, (iv) contamination by moisture, etc. Among these scientific and technical problems related to Li-O2 cell system, studies on rechargeable O2 electrode with fast kinetics of oxygen reduction reaction (ORR) during the cell discharge and oxygen evolution reaction (OER) during charge in non-aqueous electrolytes are important. In non-aqueous electrolytes, the 1-electron reduction of O2 to form superoxide (O2 -) is known to occur as the first step. (ii) Subsequently, superoxide undergoes reduction to peroxide (O2 2-) and then to oxide (O2-). The kinetics of ORR is slow in non-aqueous electrolytes. Furthermore, the reaction needs to be reversible for rechargeable Li-air batteries. In order to realize fast kinetics, a suitable catalyst is essential. The catalyst should be bifunctional for both of ORR and OER in rechargeable battery applications. Noble metal particles have been rarely investigated as catalysts for O2 electrode of Li-O2 cells. Graphene has two-dimensional planar structure with sp2 bonded carbon atoms. It has become an important electrode material owing to its high electronic conductivity and large surface area. It has been investigated for applications such as supercapacitors, Li-ion batteries, and fuel cells. Catalyst nanoparticles prepared and anchored to graphene sheets are expected to sustain discrete existence without undergoing agglomeration and therefore they possess a high catalytic stability for long term experiments as well as applications. In this context, it is intended to explore the catalytic activity of noble metal nanoparticles dispersed on reduced graphene oxide (RGO) for O2 electrode of Li-O2 cells. While a majority of the investigations reported in the thesis involves noble metal and alloy particles dispersed on RGO sheets, results on polypyrrole-RGO composite and also magnesium cobalt silicate for Li-O2 system are included. A chapter on electrochemical impedance analysis of LiMn2O4, a cathode material of Li-ion batteries, is also presented in the thesis. Introduction on electrochemical energy storage systems, in particular on Li-O2 system is provided in the 1st Chapter of the thesis. Synthesis of Ag nanoparticles anchored to RGO and catalytic activity are presented in the 2nd Chapter. Ag-RGO is prepared by insitu reduction of Ag+ ions and graphene oxide in aqueous phase by ethylene glycol as the reducing agent. The product is characterized by powder XRD, UV-VIS, IR, Raman, AFM, XPS, SEM and TEM studies. The SEM images show the layered morphology of graphene and TEM images confirm the presence of Ag nanoparticles of average diameter less than 5 nm anchored to RGO (Fig. 1a). Ag-RGO is investigated for ORR in alkaline (1 M KOH), neutral (1 M K2SO4) and non-aqueous 0.1 M tetrabutyl ammonium perchlorate in dimethyl sulphoxide (TBAP-DMSO) electrolytes. The ORR follows 4e- reduction in aqueous and 1e- reduction pathway in non-aqueous electrolytes. Li-O2 cells are assembled with Ag-RGO as (iii) Fig. 1. (a) TEM image of Ag-RGO and (b) charge-discharge voltage profiles of Li-O2 (Ag-RGO) cells. oxygen electrode catalyst in non-aqueous electrolyte (1 M LiPF6-DMSO) and subjected to charge-discharge cycling at several current densities. The discharge capacity values obtained are 11950 (11.29), 9340 (5.00), and 2780 mAh g-1 (2.47 mAh cm-2) when discharged at 0.2, 0.5, 0.8 mA cm-2, respectively (Fig. 1b). Powder XRD studies of discharged electrodes indicate the formation of Li2O2 and Li2O during the cell discharge. In addition to these studies, Na-O2 cells are also assembled with Ag-RGO in non-aqueous electrolyte. It is concluded that the chemistry Li-O2 and Na-O2 cells are similar except for the capacity values. Metal nanoparticles of Au, Pd and Ir are decorated on RGO sheets by reduction of metal ions on graphene oxide by NaBH4. Au-RGO, Pd-RGO and Ir-RGO are characterized by various physicochemical techniques. Particle size of metal nanoparticles ranges from 2 to Fig.2. Charge-discharge voltage profiles Li-O2(RGO) (i) and Li-O2(Au-RGO) (ii) cells at current density 0.3 mA cm-2. 0 2500 5000 7500 10000 12500 15000 10 nm on graphene sheets. All samples are studied for ORR in aqueous and non-aqueous electrolytes by cyclic voltammetry and rotating disk electrode experiments. Li-O2 cells are assembled in 1 M LiPF6-DMSO and discharge capacity values obtained are 3344, 8192 and 11449 mAh g-1 with Au-RGO, Pd-RGO and Ir-RGO, respectively, at 0.2 mA cm-2 current density. The results of these studies are described in Chapter 3. Synthesis and electrochemical activity of Pt-based alloy nanoparticles (Pt3Ni, Pt3Co and Pt3Fe) on RGO are presented in Chapter 4. The Pt3Ni alloy particles are prepared by simultaneous reduction of Pt4+, Ni2+ and graphene oxide by hydrazine in ethylene glycol medium. Pt3Co-RGO and Pt3Fe-RGO are also prepared similar to Pt3Ni-RGO. Formation of alloys is confirmed with XRD studies. O2 reduction reaction on Pt-alloys in non-aqueous electrolyte follows 1e- reduction to O2 -. RDE results show that Pt3Ni-RGO is a better catalyst than Pt for O2 reduction (Fig. 3). Li-O2 cells are assembled with all samples and subjected to Fig. 3. Linear sweep voltammograms of Pt3Ni-RGO, Pt3Co-RGO and Pt3Fe-RGO in 0.1 M TBAPDMSO with 1600 rpm at 10 mV s-1 scan rate. The area of GC electrode was 0.0314 cm2 with a catalyst mass of 200 μg. charge-discharge cycling at several current densities. The initial discharge capacity values obtained are 14128, 5000 and 10500 mAh g-1 with Pt3Ni-RGO, Pt3Co-RGO and Pt3Fe-RGO, respectively, as the air electrode catalysts. Polypyrrole (PPY) is an attractive conducting polymer with advantages such as high electronic conductivity and electrochemical stability. A combination of advantages of graphene and PPY composite are explained in the Chapter 5. PPY is grown on already synthesized RGO sheets by oxidative polymerization of pyrrole in an acidic PY composite is characterized by XRD and Raman spectroscopy studies. Li-O2 cells are assembled in non-aqueous electrolyte and subjected for charge-discharge cycling at different current densities. The discharge capacity value of Li-O2(PPY-RGO) cell is 3358 mAh g-1 Fig. 4. (a) Discharge-charge performance of Li-O2(PPY-RGO) cell with a current density of 0.2 mA cm-2 limiting to a capacity of 1000 mAh g-1 and (b) variation of cut-off voltages on cycling. (3.94 mAh cm-2) in the first cycle. Li-O2(PPY-RGO) cell delivers 3.7 times greater discharge capacity than Li-O2(RGO) cell. Cycling stability of Li-O2 (PPY-RGO) cell is investigated by charge-discharge cycling by limiting the capacity to 1000 mAh g-1, and the cell voltage at the end of discharge and at the end of charge are found constant at 2.75 and 4.10 V, respectively (Fig. 4 a, b). This study shows that PPY-RGO is stable in Li-O2 cells. Electrochemical impedance study shows that charge-transfer resistant is 500 Ω for freshly assembled Li- O2(PPY-RGO) cell and it decreases to 200 Ω after 1st discharge. Synthesis of magnesium cobalt silicate and its electrochemical activity are presented in Chapter 6. MgCoSiO4 is synthesized by mixed solvothermal approach and characterized by various physicochemical techniques. Cubic shaped MgCoSiO4 is investigated for oxygen evolution reaction (OER) activity in alkaline and neutral media. The current values at 0.95 versus SHE are 43, 0.18, 16 mA cm-2 on MgCoSiO4, bare carbon paper and Pt foil electrodes, respectively (Fig. 5), indicating that MgCoSiO4 is a good catalyst for OER. The onset potential for OER is 0.68 V versus SHE on MgCoSiO4 in 1 M KOH. OER activity on MgCoSiO4 is also studied in K2SO4 and phosphate buffer electrolytes. The results indicate good catalytic activity of MgCoSiO4 in neutral electrolytes also. The catalytic activity of Fig. 5. Cyclic voltammograms of bare carbon paper (i), Pt foil (ii), MgCoSiO4 coated carbon (iii) electrodes in 1 M KOH (sweep rate = 5 mV s-1, loading level = 1.15 mg, area = 0.5 cm-2). MgCoSiO4 towards ORR in aqueous and non-aqueous electrolytes is studied by RDE experiments. Li-O2 cells are assembled with bifunctional MgCoSiO4 catalyst in 1 M LiPF6- DMSO electrolyte and the discharge capacity values obtained are 7721 (8.27), 2510 (1.66) and 1053 mAh g-1 (0.92 mAh cm-2) when discharged at 0.3, 0.5 and 0.8 mA cm-2 current densities, respectively. Electrochemical impedance spectroscopy (EIS) measurements of LiMn2O4 electrode are carried out at different temperatures from -10 to 50 0C and in the potential range from 3.50 to 4.30 V, and the data are analysed in Chapter 7. In the EIS spectra recorded over the frequency range from 100 kHz to 0.01 Hz at different temperatures, there are two semicircles present in the Nyquist plot (Fig. 6a). But in 3.90 to 4.10 V versus Li/Li+(1M) potential range at low temperatures (-10 to 15 oC) range, another semicircle also appears (Fig. 6b). Impedance parameters such as solution resistant (Rs), charge-transfer resistance (Rct), doublelayer capacitance (Cdl), electronic resistance (Re) and Warburg impedance (WR), etc., are obtained by analysis of the EIS data. The variations of resistances with temperature are analysed by Arrhenius-like relationships and the apparent activation energies of the corresponding transport properties are evaluated. The values of activation energy for chargetransfer process are 0.37, 0.30 and 0.42 eV, at 3.50, 3.90 and 4.10 V versus Li/Li+(1M), respectively. The chemical diffusion coefficient of Li+ ions into LiMn2O4 calculated from EIS data. The values of diffusion coefficient calculated are in the range of 2.50 x 10-12 - 4.10 Fig. 6. Nyquist plot of impedance study of Li/LiMn2O4 cell at 3.50 V (a) and 3.90 V (b) at -10 0C. Details of the above studies are described in the thesis.

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.

The purpose of this diploma thesis is to describe the impact of compaction pressure on the electrochemical parameters of lithium-sulfur batteries. Theoretical part of this thesis contains briefly described terminology and general issues of batteries and their division. Every kind of battery is provided with a closer description of a specific battery type. A separate chapter is dedicated to lithium cells, mainly lithium-ion batteries. Considering various composition of lithium-ion batteries, this chapter deeply analyzes mostly used active materials of electrodes, used electrolytes and separators. Considering that the electrochemical principle of Li-S and Li-O batteries is different to Li-ion batteries, these accumulators of new generation are included in individual subhead. In the experimental part of this thesis are described methods used to measure electrochemical parameters of Li-S batteries. Next chapter contains description of preparing individual electrodes and their composition. Rest of the experimental part of my thesis is dedicated to the description of individual experiments and achieved results.