Dans le contexte d'un développement durable, les véhicules électriques apparaissent comme une solution incontournable dans le futur. Parmi les dernières évolutions sur les batteries, les systèmes constitués d'une électrode au lithium (technologie lithium métal) présente des performances remarquables en termes de densité d'énergie. L'inconvénient majeur de cette méthodologie est lié à la formation de dendrites lors de la recharge susceptibles d'occasionner des courts-circuits conduisant à l'explosion de la batterie. C'est dans cet axe que s'inscrit mon sujet de thèse dont l'objectif est de développer un électrolyte polymère solide présentant une conductivité ionique élevée (2.10-4 S.cm-1 à40°C) et une tenue mécanique suffisante (30 MPa) pour limiter les phénomènes de croissance dendritique. Pour cela, la polymérisation contrôlée par les nitroxydes (NMP) a été utilisée pour synthétiser des copolymères à blocs avec un bloc possédant des groupes d'oxyde d'éthylène –CH2-CH2-O- permettant la conduction des ions lithium et un bloc de polystyrène assurant la tenue mécanique de l'électrolyte final. Le bloc assurant la conduction ionique des architectures ainsi synthétisées sont constituées soit de POE sous forme linéaire soit de POE sous forme de peigne. / In the context of sustainable development, electric vehicles appear to be a major solution for the future. Among the lastest technologies, the Lithium Metal Polymer battery has presented very interesting performances in terms of energy density. The main drawback of this system is the formation of lithium dendrites during the refill of the battery that could cause short circuits leading to the explosion of the battery. The aim of my PhD is to develop a Solid Polymer Electrolyte showing a high ionic conductivity (2.10-4 S.cm-1 at 40°C) and a high mechanical strength (30 MPa) to prevent dendritic growth. For that purpose, Nitroxide Mediated Polymerization is used to synthesize block copolymers with a PEO moiety for ionic conduction –CH2-CH2-O- and polystyrene for mechanical strength. Different kind of architectures have been synthesized : block copolymer with linear PEO moiety or with grafted PEO moiety.

Au cours de cette thèse, un nouvel électrolyte polymère gel pour la réalisation de microbatteries au lithium a été développé. Le gel a été préparé par « confinement » d’une phase de N-propyl-N-méthylpyrrolidinium bis(fluorosulfonyl)imide (P13FSI) et de LiTFSI dans un réseau semi-interpénétré (sRip) de polymère (PVdFHFP/ réseau de POE). L’électrolyte gel a tout d’abord été optimisé et étudié en termes de propriétés physicochimiques et de transport ionique en fonction de sa composition. Ensuite, des batteries Li/LiNi1/3Mn1/3Co1/3O2 ont été assemblées en utilisant l’électrolyte sRip. Les performances ont par ailleurs été comparées aux systèmes de références utilisant l’électrolyte à base de POE ou de PVdF-HFP. Outre ses propriétés améliorées par rapport au PVdF-HFP et au réseau de POE (propriétés mécaniques, confinement), l’électrolyte sRip est compatible avec le procédé de dépôt de l’électrode négative en lithium par évaporation sous vide. L’électrolyte sRip optimisé a donc été utilisé pour fabriquer une nouvelle génération de microbatteries en s’affranchissant de l’électrolyte céramique, le LiPON, afin d’abaisser la résistance interne. Les microbatteries Li/sRip gel/LiCoO2 délivrent une capacité nominale stable de 850 μAh à C sur 100 cycles à 25°C. / In this thesis, a new polymer gel electrolyte was prepared and optimized for Li based microbatteries. The gel consisted of an ionic liquid based phase (P13FSI/LiTFSI) confined in a semi-interpenetrating polymers (sIPN) network (PVdF-HFP/crosslinked PEO). sIPN electrolytes were prepared and optimized according to the PVdFHFP/ crosslinked PEO ratio and the liquid phase fraction. Furthermore, the sIPN electrolyte was used as an electrolyte in Li/LiNi1/3Mn1/3Co1/3O2 battery. The performances of the battery (specific capacity, efficiency, cyclability) were determined and compared to batteries using a crosslinked PEO or PVdF-HFP based gel. Such a thin and stable sIPN electrolyte film enabled the preparation of Li based microbatteries using thermal evaporation deposition of lithium directly conducted on the sIPN electrolyte film. This assembly (Li/sIPN) was therefore used to prepare a LiCoO2/sIPN gel/Li quasi solid-state microbattery. This microbattery showed a stable nominal capacity of 850 μAh for over 100 cycles of charge and discharge under 1 C rate at 25°C.

Fast ionic conductors, i.e. materials in which charge transport mainly occurs through the motion of ions, are an important class of materials with immense scope for industrial applications. There are different classes of fast ionic conductors e.g. polymer electrolytes, glasses, oxide ion conductors etc. and they find applications such as solid electrolytes in batteries, in fuel cells and in electro active sensors. There are mixed conducting materials as well which have both ions and electrons as conducting species that are used as electrode materials. Specifically, polymer electrolytes 1−3 have been in use in lithium polymer batteries, which have much more advantages compared to other secondary batteries. Polymer electrolyte membranes have been in use in direct methanol fuel cells (DMFC). The membranes act as proton conductors and allow the protons produced from the fuel (methanol) to pass through. Oxide ion conductors are used in high temperature solid oxide fuel cells (SOFC) and they conduct via oxygen ion vacancies. Fuel cells are rapidly replacing the internal combustion engines, because they are more energy efficient and environment friendly. The present thesis is concerned with the preparation, characterization and conductivity studies on the following fast ionic conductors: (MPEG)xLiClO4, (MPEG)xLiCF3SO3 where (MPEG) is methoxy poly(ethylene glycol), the hydrotalcite [Mg0.66Al0.33(OH)2][(CO3)0.17.mH2O] and the nanocomposite SPE, (PEG)46 LiClO4 with dispersed nanoparticles of hydrotalcite. We also present our investigations of spin probe electron spin resonance (SPESR) as a possible technique to determine the glass transition temperature (Tg) of polymer electrolytes where the conventional technique of Tg determination, namely, differential scanning calorimetry, (DSC), is not useful due to the high crystallinity of the polymers. In the following we summarize the main contents of the thesis. In Chapter 1 we provide a brief introduction to the phenomenon of fast ionic conduction. A description of the different experimental techniques used as well as the relevant theories is also given in this chapter. In most solid polymer electrolytes (SPE), the usability is limited by the low value of the ionic conductivity. A number of different routes to enhance the electrical, thermal and mechanical properties of these materials is presently under investigation. One such route to enhance the ionic conductivity in polymer electrolytes is by irradiating the polymer electrolyte with gamma rays, electron beam, ion beams etc. In Chapter 2, we describe our work on the effect of electron beam (e-beam) irradiation on the solid polymer electrolytes (MPEG)xLiClO4 and (MPEG)xLiCF3SO3. The polymer used is methoxy poly(ethylene glycol) or poly(ethylene glycol) methyl ether with a molecular weight 2000. Salts used are LiClO4 and LiCF3SO3. ’x’ in the subscript is a measure of the salt concentration; it is the ratio of the number of ether oxygens in the polymer chain to that of the Li+ ion. ’x’ values chosen are 100, 46, 30 and 16. Nearly one order of magnitude increase in the conductivity is observed for samples (MPEG)100LiClO4 and (MPEG)16LiCF3SO3 on irradiation. It was found that the increase in the net ionic conductivity is a function of both the irradiation dose and the salt concentration. The enhanced ionic conductivity remains constant for ∼ 100 hrs, which signifies a possible near permanent change in the polymer electrolyte system due to irradiation. The samples were also characterized using DSC and Fourier transform infrared spectroscopy (FTIR). DSC results could be correlated with conductivity findings, giving low Tg values for samples having high conductivity. It was also found that there is a small increase in the crystalline fraction of the samples on irradiation, which agrees with earlier reports on samples irradiated with low dosage. FTIR results are suggestive of decreased cross linking as the reason for increased ionic conductivity. However, this aspect needs a further confirmatory look before the findings can be termed conclusive. In Chapter 3, we describe the studies we have carried out on Li -doped hydrotalcite. We report the details of preparation and characterization of hydrotalcite as well as NMR and ionic conductivity measurements on both doped (with Li+ ions) and undoped hydrotalcite. Hydrotalcite was prepared by co-precipitation method and the composition of hydrotalcite was chosen as [Mg0.66Al0.33(OH)2][(CO3)0.17.mH2O]. Samples were prepared with salt (LiClO4) concentration 5 %, 10 %, 15 %, 20 % and 25 %. It was found that the highest ionic conductivity occurs for the sample with 20 % doping. 7Li NMR plots for all the samples clearly show an overlap of a Gaussian and a Lorentzian lineshape. The Gaussian line is because of the presence of a less mobile fraction of the 7Li+ ions and the Lorentzian line is because of the presence of a more mobile fraction of 7Li+ ions. The highest ionic conductivity was found for the salt concentration 20 % and from the room temperature 7Li NMR studies we found that for this particular concentration, the mobile fraction of the 7Li ion is also maximum. Without the salt doping, the conductivity of the sample was too small to be measured. Temperature variation of both 1H and 7Li NMR was also done, to compare the ionic conductivities from NMR. Another method to obtain enhanced properties in polymer electrolytes is by forming ’nanocomposite’ polymer electrolytes. Nanocomposites are formed by dispersing nanoparticles of certain materials in the polymer electrolyte matrix. Till now, nanoparticles used are mostly oxides of metals, e.g. Al2O3, TiO2, MgO, SiO2 etc and clays like montmorillonite, liponite, hydrotalcite etc. Chapter 4 describes the preparation and characterization of the nanocomposite polymer electrolyte (PEG)46LiClO4 formed with hydrotalcite nanoparticles. The polymer used is PEG, poly(ethylene glycol) of molecular weight 2000, and salt used is LiClO4. The salt concentration is selected so as to give the highest ionic conductivity for the solid polymer electrolyte. Hydrotalcite belongs to a class of materials called LDH, layered double hydroxides. The composition selected is [Mg0.66Al0.33(OH)2][(CO3)0.17 .mH2O], since this is the most stable composition. These materials are easy to prepare in the nano size and are being used in a number of applications. These are characterized by the presence of layers of positively charged double hydroxides separated by layers of anions and water molecules. The water molecules give stability to the structure. Nanoparticles of hydrotalcite were prepared in the laboratory itself. XRD data of hydrotalcite confirm the crystal structure. TEM data show the particle size to be ∼ 50 nm. The polymer electrolyte (PEG)46LiClO4 was doped with these nanoparticles and the doping levels are 1.8 %, 2.1 %, 2.7 %, 3.6 % and 4.5 % by weight. Impedance spectroscopy was used to find the ionic conductivity. We have found that the sample with a doping of 3.6 % by weight gives the highest ionic conductivity and the increase in ionic conductivity is nearly one order of magnitude. DSC was used for thermal characterization of these nanocomposites. The glass transition temperatures, Tg , found from DSC measurements corroborates the ionic conductivity data, giving the lowest Tg for the sample with highest conductivity. Temperature variation of the ionic conductivity shows Arrhenius behavior. 7Li NMR was done on the pristine SPE (PEG)46LiClO4 and the nanocomposite of (PEG)46LiClO4 with 3.6 % filler. The ionic conductivity was also estimated from the temperature variation of 7Li NMR line widths. Studies on the DSC endotherms of the nanocomposites give the fractional crystallinity of the samples. From these studies it can be concluded that the variation in ionic conductivity can be attributed to the change in fractional crystallinity; the nanocomposite polymer electrolyte having highest ionic conductivity, i.e. the NCPE with filler concentration of 3.6 % also has the lowest fractional crystallinity. Additionally, a possible increase in the segmental motion inferred from a reduction in the glass transition temperature coupled with a lowering of the activation energy may also contribute to the increased ionic conductivity in the nanocomposite polymer electrolyte. Glass transition temperature Tg has a very important role in studying the dynamics of polymer electrolytes. In Chapter 5, we explore the possibility of using spin probe electron spin resonance (SPESR) as a tool to study the glass transition temperature of polymer electrolytes. When the temperature of the polymer is increased across the glass transition, the viscosity of the sample decreases. This corresponds to a transition from a slow tumbling regime with τc = 10−6 s to a fast tumbling regime with τc = 10−9 s where τc is the correlation time for the probe dynamics. Spin probe ESR can be used to probe this transition in polymers. We have used 4-hydroxy tempo (TEMPOL) as the spin probe which is dispersed in the nanocomposite polymer electrolyte based on (PEG)46LiClO4 and hydrotalcite. Below and across the glass transition, this nitroxide probe exhibits a powder pattern showing both Zeeman (g) and hyperfine (hf) interaction anisotropy. When the frequency of the dynamics increases such that the jump frequency f is of the same order of magnitude as the anisotropy of the hf interaction, i.e., ∼ 108 Hz, the anisotropy of the interactions averages out and a spectrum of reduced splitting and increased symmetry in the line shape is observed. This splitting corresponds to the nonvanishing isotropic value of the hyperfine tensor and is observed at a temperature higher than but correlated with Tg. The crossover from the anisotropic to isotropic spectrum is reflected in a sharp reduction in the separation between the two outermost components of the ESR spectrum, which corresponds to twice the value of the z-principal component of the nitrogen hyperfine tensor, 2Azz, from ∼75 G to ∼ 35 G. In our study, we have varied the concentration of the nano-fillers. The Tg for all the samples were estimated from the measurement of T50G and the known correlation between 4 T50G and Tg, where T50G is the temperature at which the extrema separation (2Azz) of the ESR spectra becomes 50 Gauss. The values obtained from this method are compared with the values found from DSC done on the same samples. Within experimental error, these two techniques give reasonably close values. Tg’s were also estimated by a cross over in the correlation time (τc) vs temperature plot. The τc values were calculated using a spectral simulation program. We conclude that spin probe ESR can be an alternative to the DSC technique for polymers with high fraction of crystallinity, for which DSC often does not give any glass transition signature. In Appendix I, ionic conductivity studies on quenched and gamma irradiated polymer electrolytes (PEG)46LiClO4 and (MPEG)16LiClO4 is done. It is observed that, (i) the samples quenched to 77 K after melting show enhancement of ionic conductivity by a factor of 3 & 4; (ii) on irradiation, the ionic conductivity decreases for a dose of 5 kGy and subsequently, keeps on increasing for higher doses of 10 kGy and 15 kGy. In Appendix II, the BASIC language program (eq-res.bas) used for impedance data analysis is given.

The polymer electrolyte membrane fuel cell (PEMFC) converts the chemical energy of hydrogen and oxygen (air) into usable electrical energy. At the cathode (the positive electrode), a considerable amount of platinum is generally required to catalyse the sluggish oxygen reduction reaction (ORR). This has implications regarding the cost in high-power applications, and for making a broad commercialisation of the PEMFC technology possible, it would be desirable to lower the amount of Pt used to catalyse the ORR. In this thesis a number of techniques are described that have been developed in order to investigate catalytic activity at the cathode of the PEMFC. These methodologies resemble traditional three-electrode research in liquid electrolytes, including cyclic voltammetry in inert gas, but with the advantage of performing the experiments in the true PEMFC environment. From the porous electrode studies it was seen that it is possible to reach mass activities close to 0.2 gPt/kW at potentials above 0.65 V at 60 ◦C, but that the mass activities may become considerably lower when raising the temperature to 80 ◦C and changing the measurement methodology regarding potential cycling limits and electrode manufacturing. The model electrode studies rendered some interesting results regarding the ORR at the Pt/Nafion interface. Using a novel measurement setup for measuring on catalysed planar glassy carbon disks, it was seen that humidity has a considerable effect on the ORR kinetics of Pt. The Tafel slopes become steeper and the activity decreases when the humidity level of the inlet gases decreases. Since no change in the the electrochemical area of the Pt/Nafion interface could be seen, these kinetic phenomena were ascribed to a lowered Pt oxide coverage at the lower humidity level, in combination with a lower proton activity. Using bi-layered nm-thick model electrodes deposited directly on Nafion membranes, the behaviour of TiO2 and other metal oxides in combination with Pt in the PEMFC environment was investigated. Kinetically, no intrinsic effect could be seen for the model electrodes when adding a metal oxide, but compared to porous electrodes, the surface (specific) activity of a 3 nm film of Pt deposited on Nafion seems to be higher than for a porous electrode using ∼4 nm Pt grains deposited on a carbon support. Comparing the cyclic voltammograms in N2, this higher activity could be ascribed to less Pt oxide formation, possibly due to a particle size effect. For these bi-layered films it was also seen that TiO2 may operate as a proton-conducting electrolyte in the PEMFC. / I polymerelektrolytbränslecellen (PEMFC) omvandlas den kemiska energin hos vätgas och syrgas (luft) direkt till användbar elektrisk energi. På katoden (den positiva elektroden) krävs betydande mängder platina för att katalysera den tröga syrereduktionsreaktionen (ORR). Detta inverkar på kostnaden för högeffektsapplikationer, och för att göra en bred kommersialisering av PEMFC-teknologin möjlig skulle det vara önskvärt att minska den Pt-mängd som används för att katalysera ORR. I denna avhandling beskrivs ett antal tekniker som utvecklats för att undersöka katalytisk aktivitet på katoden i PEMFC. Metodiken liknar traditionella treelektrodexperiment i vätskeformig elektrolyt, med cyklisk voltammetri i inert gas, men med fördelen att försöken utförs i den riktiga PEMFC-miljön. I försök med porösa elektroder visades att det är möjligt att nå massaktiviteter nära 0.2 gPt/kW för potentialer över 0.65 V vid 60 ◦C, men massaktiviteterna kan bli betydligt lägre om temperaturen höjs till 80 ◦C, och om potentialsvepgränser och elektrodentillverkningsmetod ändras. Försök med modellelektroder resulterade i intressanta resultat rörande ORR i gränsskiktet Pt/Nafion. Genom att använda en ny metodik för att mäta på katalyserade plana elektroder av vitröst kol (glassy carbon), var det möjligt att se att gasernas fuktighet har en betydande inverkan på ORR-kinetiken hos Pt. Tafellutningarna blir brantare och aktiviteten minskar när inloppsgasernas fuktighetsgrad minskar. Eftersom den elektrokemiska arean hos Pt/Nafion-gränsskiktet inte ändrades, ansågs dessa kinetiska effekter bero på en lägre täckningsgrad av Ptoxider vid lägre fuktigheter, i kombination med lägre protonaktivitet. Genom att använda Nafionmembran belagda med nm-tjocka tvåskiktsmodellelektroder undersöktes hur Pt i kombination med TiO2 och andra metalloxider verkar i PEMFC-miljön. Kinetiskt sett hade tillsatsen av metalloxider ingen inre påverkan på aktiviteten, men vid jämförelse med porösa elektroder tycks den specifika ytaktiviteten vara högre hos en 3 nm film av Pt på Nafion än för en porös elektrod baserad på ∼4 nm Pt-korn belagda på ett kolbärarmaterial. Jämför man de cykliska voltammogrammen i N2, kan den högre aktiviteten tillskrivas en lägre grad av Pt-oxidbildning, vilket i sin tur kan bero på en storlekseffekt hos Pt-partiklarna. Försöken med dessa tvåskiktselektroder visade också att TiO2 kan verka som protonledande elektrolyt i PEMFC. / QC 20100706

In recent years, electrochemical capacitors have emerged as devices with the potential to enable major advances in electrical energy storage. Electrochemical capacitors (ECs) are akin to conventional capacitors but employ higher surface-area electrodes and thinner dielectrics to achieve larger capacitances. This helps ECs to attain energy densities greater than those of conventional capacitors and power densities greater than those of batteries. Akin to conventional capacitors, ECs also have high cycle-lives and can be charged and discharged rapidly. But ECs are yet to match the energy densities of mid to high-end batteries and fuel cells. On the basis of mechanism involved in the charge-storage process, ECs are classified as electrical double-layer capacitors (EDLCs) or pseudocapacitors. Charge storage in EDLCs and pseudocapacitors is brought about by non-faradaic and faradaic processes, respectively. Faradaic process, such as an oxidation-reduction reaction, involves the transfer of charge between electrode and electrolyte. By contrast, a non-faradaic process does not use a chemical mechanism and charges are distributed on surfaces by physical processes that do not involve any chemical reaction. ECs employ both aqueous and non-aqueous electrolytes in either liquid or solid form, the latter providing the advantages of freedom from leakage of any liquid component, compactness, reliability and large operating potential-window. In the literature, polymer electrolytes are the most widely studied solid electrolytes. Complexation of functional-groups of certain polymers with cations results in the formation of polymer-cation complexes commonly referred to as solid-polymer electrolytes (SPEs). Mixing a polymer with an alkali metal salt dissolved in an organic solvent result in the formation of a polymer gel electrolyte. Organic solvents with low molecular-weights, such as ethylene carbonate and propylene carbonate, employed in polymer gel electrolytes are commonly referred to as plasticizers. When water is used as a plasticizer, the polymer electrolyte is called a polymer hydrogel electrolyte. Part I of the thesis is directed to studies pertaining to Polymer Hydrogel Electrolytes for Electrochemical Capacitors and comprises four sections. After a brief survey of literature on polymer hydrogel electrolytes employed in ECs in Section I.1, Section I.2 of Part I describes the studies on electrochemical capacitors employing cross-linked poly (vinyl alcohol) hydrogel membrane electrolytes with varying perchloric acid dopant concentration. Acidic poly (vinyl alcohol) hydrogel membrane electrolytes (PHMEs) with different perchloric acid concentrations are prepared by cross-linking poly (vinyl alcohol) with glutaraldehyde in the presence of a protonic acid acting as a catalyst under ambient conditions. PHMEs are characterized by scanning electron microscopy and temperature-modulated differential scanning calorimetry in conjunction with relevant electrochemical techniques. An optimised electrochemical capacitor assembled employing PHME in conjunction with black pearl carbon (BPC) electrodes yields a maximum specific capacitance value of about 96 F g-1, phase angle value of about 79o and a discharge capacitance value of about 88 F g-1. Section I.3 of Part I describes the studies on cross-linked poly (vinyl alcohol)/ploy (acrylic acid) blend hydrogel electrolytes for electrochemical capacitors. Acidic poly (vinyl alcohol)/poly (acrylic acid) blend hydrogel electrolytes (BHEs) have been prepared by cross-linking poly (vinyl alcohol)/poly (acrylic acid) blend with glutaraldehyde in presence of perchloric acid. These acidic BHEs have been treated suitably to realize alkaline and neutral BHEs. Thermal characteristics and glass-transition behavior of BHEs have been followed by differential scanning calorimetry. Ionic conduction in acidic BHEs has been found to take place by Grötthus-type mechanism while polymer segmental motion mechanism is predominantly responsible for ion motion in alkaline and neutral BHEs. Ionic conductivity of BHEs has been found to range between 10-3 and 10-2 S cm-1 at 298 K. Electrochemical capacitors assembled with acidic PVA hydrogel electrolyte yield a maximum specific capacitance of about 60 and 1000 F g-1 with BPC and RuOx.xH2O/C electrodes, respectively. Section I.4 of Part I describes the studies on gelatin hydrogel electrolytes and their application to electrochemical capacitors. Gelatin hydrogel electrolytes (GHEs) with varying NaCl concentrations have been prepared by cross-linking an aqueous solution of gelatin with aqueous glutaraldehyde under ambient conditions, and characterized by scanning electron microscopy, temperature-modulated differential scanning calorimetry, cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic chronopotentiometry. Glass transition temperatures for GHEs range between 340 and 377 K depending on the dopant concentration. Ionic conductivity behavior of GHEs is studied with varying concentrations of gelatin, glutaraldehyde and NaCl, and conductivity values are found to vary between 10-3 and 10-1 S cm-1 under ambient conditions. GHEs have a potential window of about 1 V with BPC electrodes. The ionic conductivity of pristine and 0.25 N NaCl-doped GHEs follows Arrhenius behavior with activation energy values of 1.9×10-4 and 1.8×10-4 eV, respectively. Electrochemical capacitors employing GHEs in conjunction with black pearl carbon electrodes are assembled and studied. Optimal values for capacitance, phase angle, and relaxation time constant of about 81 F g-1, 75o, and 0.03 s are obtained for 3 M NaCl-doped GHE, respectively. EC with pristine GHE exhibits continuous cycle life for about 4.3 h as against 4.7 h for the electrochemical capacitor with 3 M NaCl-doped GHE. Unlike electrochemical capacitors, fuel cells do not store the charge internally but instead use a continuous supply of fuel from an external storage tank. Thus, fuel cells have the potential to solve the most challenging problem associated with the electrochemical capacitors, namely their limited energy-density. A fuel cell is an electrochemical power source with advantages of both the combustion engine and the battery. Like a combustion engine, a fuel cell will run as long as it is provided with fuel; and like a battery, fuel cells convert chemical energy directly to electrical energy. As an electrochemical power source, fuel cells are not subjected to the Carnot limitations of combustion (heat) engines. A fuel cell operates quietly and efficiently and, when hydrogen is used as a fuel, it generates only power and potable water. Thus, a fuel cell is a so called ‘zero-emission engine’. In the past, several fuel cell concepts have been tested in various laboratories but the systems that are being potentially considered for commercial developments are: (i) Alkaline Fuel Cells (AFCs), (ii) Phosphoric Acid Fuel Cells (PAFCs), (iii) Polymer Electrolyte Fuel Cells (PEFCs), (iv) Solid-Polymer-Electrolyte-Direct Methanol Fuel Cells (SPE-DMFCs), (v) Molten Carbonate Fuel Cells (MCFCs) and (vi) Solid Oxide Fuel Cells (SOFCs). Among the aforesaid systems, PEFCs that employ hydrogen as fuel are considered attractive power systems for quick start-up and ambient-temperature operations. Ironically, however, hydrogen as fuel is not available freely in the nature. Accordingly, it has to be generated from a readily available hydrogen carrying fuel such as natural gas, which needs to be reformed. But, such a process leads to generation of hydrogen with some content of carbon monoxide, which even at minuscule level is detrimental to the fuel cell performance. Pure hydrogen can be generated through water electrolysis but hydrogen thus generated needs to be stored as compressed / liquefied gas, which is cost-intensive. Therefore, certain hydrogen carrying organic fuels such as methanol, ethanol, propanol, ethylene glycol, and diethyl ether have been considered for fuelling PEFCs directly. Among these, methanol with a hydrogen content of about 13 wt. % (specific energy = 6.1 kWh kg-1) is the most attractive organic liquid. PEFCs using methanol directly as fuel are referred to as SPE-DMFCs. But SPE-DMFCs suffer from methanol crossover across the polymer electrolyte membrane, which affects the cathode performance and hence the cell performance during its operation. SPE-DMFCs also have inherent limitations of low open-circuit-potential and low electrochemical-activity. An obvious solution to the aforesaid problems is to explore other promising hydrogen carrying fuels such as sodium borohydride, which has a hydrogen content of about 11 wt. %. Such fuel cells are called direct borohydride fuel cells (DBFCs). Part II of the thesis includes studies on direct borohydride fuel cells and comprises three sections. After a brief introduction to DBFCs in section II.1, Section II.2 describes studies on an alkaline direct borohydride fuel cell with hydrogen peroxide as oxidant. A peak power density of about 150 mW cm-2 at a cell voltage of 540 mV could be achieved from the optimized DBFC operating at 70oC. Section II.3 describes studies on poly (vinyl alcohol) hydrogel membrane as electrolyte for direct borohydride fuel cells. This DBFC employs a poly (vinyl alcohol) hydrogel membrane as electrolyte, an AB5 Misch metal alloy as anode, and a gold-plated stainless steel mesh as cathode in conjunction with aqueous alkaline solution of sodium borohydride as fuel and aqueous acidified solution of hydrogen peroxide as oxidant. The performance of the PHME-based DBFC in respect of peak power outputs, ex-situ cross-over of oxidant, fuel, anolyte and catholyte across the membrane electrolytes, utilization efficiencies of fuel and oxidant as also cell performance durability under ambient conditions are compared with a similar DBFC employing a Nafion®-117 membrane electrolyte (NME). Peak power densities of about 30 and 40 mW cm-2 are observed for the DBFCs with PHME and NME, respectively. The PHME and NME-based DBFCs exhibit cell potentials of about 1.2 and 1.4 V, respectively, at a load current density of 10 mA cm-2 for 100 h. Publications of Nurul Alam Choudhury 1. Gelatin hydrogel electrolytes and their application to electrochemical supercapacitors, N. A. Choudhury, S. Sampath, and A. K. Shukla, J. Electrochem. Soc., 155 (2008) A74. 2. Cross-linked polymer hydrogel electrolytes for electrochemical capacitors, N. A. Choudhury, A. K. Shukla, S. Sampath, and S. Pitchumani, J. Electrochem. Soc., 153 (2006) A614. 3. Hydrogel-polymer electrolytes for electrochemical capacitors: an overview, N. A. Choudhury, S. Sampath, and A. K. Shukla, Energy and Environmental Science (In Press). 4. Cross-linked poly (vinyl alcohol) hydrogel membrane electrolytes with varying perchloric acid dopant concentration and their application to electrochemical capacitors, N. A. Choudhury, S. Sampath, and A. K. Shukla, J. Chem. Sc. (Submitted) 5. An alkaline direct borohydride fuel cell with hydrogen peroxide as oxidant, N. A. Choudhury, R. K. Raman, S. Sampath, and A. K. Shukla, J. Power Sources, 143 (2005) 1. 6. Poly (vinyl alcohol) hydrogel membrane as electrolyte for direct borohydride fuel cells, N. A. Choudhury, S. K. Prashant, S. Pitchumani, P. Sridhar, and A. K. Shukla, J. Chem. Sc. (Submitted). 7. A phenyl-sulfonic acid anchored carbon-supported platinum catalyst for polymer electrolyte fuel cell electrodes, G. Selvarani, A. K. Sahu, N. A. Choudhury, P. Sridhar, S. Pitchumani, and A. K. Shukla, Electrochim. Acta, 52 (2007) 4871. 8. A high-output voltage direct borohydride fuel cell, R. K. Raman, N. A. Choudhury, and A. K. Shukla, Electrochem. Solid-State Lett., 7 (2004) A 488. 9. Carbon-supported Pt-Fe alloy as a methanol-resistant oxygen-reduction catalyst for direct methanol fuel cells, A. K. Shukla, R. K. Raman, N. A. Choudhury, K. R. Priolkar, P. R. Sarode, S. Emura, and R. Kumashiro, J. Electroanal. Chem., 563 (2004) 181.

Οι κυψέλες καυσίμου είναι ηλεκτροχημικές διατάξεις οι οποίες επιτρέπουν την απευθείας μετατροπή της ελεύθερης χημικής ενέργειας ενός καυσίμου σε ηλεκτρική. Οι κυψέλες καυσίμου πολυμερικής μεμβράνης (ΡΕΜ) αποτελούν μία υποσχόμενη τεχνολογία που βρίσκεται κοντά στο στάδιο της εμπορευματοποίησης. Το κυριότερο καύσιμο που χρησιμοποιείται στις κυψέλες καυσίμου είναι το υδρογόνο, το οποίο παράγεται συνήθως από αναμόρφωση υδρογονανθράκων ή αλκοολών. Το μονοξείδιο του άνθρακα που παράγεται επίσης κατά την διαδικασία της αναμόρφωσης αποτελεί ένα σημαντικό άλυτο πρόβλημα στις κυψέλες ΡΕΜ, καθώς η ρόφησή του στην άνοδο της κυψέλης προκαλεί την υποβάθμιση της λειτουργίας της. Το φαινόμενο της ηλεκτροχημικής ενίσχυσης συνίσταται στην μη-φαρανταϊκή τροποποίηση της ενεργότητας ενός καταλύτη που βρίσκεται σε επαφή με έναν στερεό ηλεκτρολύτη, ως αποτέλεσμα της μετακίνησης προωθητικών ειδών από τον ηλεκτρολύτη προς την καταλυτική διεπιφάνεια μετάλλου/αερίου, που προκαλείται από την επιβολή ρεύματος ή δυναμικού μεταξύ του καταλύτη και ενός ηλεκτροδίου αναφοράς. Στην παρούσα διατριβή μελετήθηκε η ηλεκτροχημική ενίσχυση της οξείδωσης μίγματος αναμόρφωσης μεθανόλης από ανόδους πλατίνας και χρυσού μίας κυψέλης ΡΕΜ. Αποδείχθηκε πως η ηλεκτροχημική ενίσχυση επηρεάζεται σημαντικά από το διαχεόμενο διαμέσου της πολυμερικής μεμβράνης οξυγόνο, όπως επίσης και από τις συνθήκες λειτουργίας της κυψέλης καυσίμου. Επίσης μελετήθηκε η ηλεκτρική απόδοση ανόδων πλατίνας και χρυσού παρουσία CO. Προσδιορίστηκαν οι τιμές της ενθαλπίας ρόφησης του CO στα ηλεκτρόδια πλατίνας και χρυσού, καθώς και οι τιμές της ενέργειας ενεργοποίησης της απομάκρυνσης του CO από το κάθε ηλεκτρόδιο. Επίσης μελετήθηκε η επίδραση της θερμοκρασίας στο φαινόμενο της πολλαπλότητας μονίμων καταστάσεων κατά την λειτουργία κυψελών ΡΕΜ. Παρατηρήθηκε εξασθένηση του φαινομένου με την αύξηση της θερμοκρασίας, σε συμφωνία με τις προβλέψεις του μοντέλου γ. / Fuel cells are electrochemical devices which convert chemical energy of a fuel directly to electricity. Polymer electrolyte membrane (PEM) fuel cells are close to commercialization. The most common fuel used is hydrogen, which is usually produced via hydrocarbons or alcohol reforming. However, during this process, carbon monoxide is formed as well, adsorbs strongly on the anode of the cell and thus impairs significantly its performance. The electrochemical promotion effect is a phenomenon where application of constant current or potential between a catalyst supported on a solid electrolyte and a reference electrode, leads to non-Faradaic changes in catalytic activity. In this thesis, it was studied the electrochemical promotion of oxidation of a methanol reformate mixture on platinum and gold anodes of a PEM fuel cell. It was found that electrochemical promotion is influenced by oxygen crossover through the polymer membrane and also by the cell operating conditions. Moreover, the electrical efficiency of platinum and gold anodes in presence of CO was studied and the values of the heat of CO adsorption on each anode and the activation energies of CO removal were estimated. Finally, the effect of temperature on the phenomenon of steady-state multiplicities was studied. It was found that increasing the temperature, the phenomenon of multiplicities is suppressed in agreement with the gama model.

Το Η2 είναι το ελαφρύτερο και πλέον άφθονο στοιχείο στη φύση. Βρίσκεται παντού στη γη, στο νερό, στα ορυκτά καύσιμα και σε όλα τα έμβια όντα. Αν το Η2 αξιοποιηθεί κατάλληλα και χρησιμοποιηθεί για τροφοδοσία των κελιών καυσίμου, θα ελαχιστοποιηθεί η εξάρτηση του σύγχρονου πολιτισμού από τα ορυκτά καύσιμα, με συνεπακόλουθο τη μείωση των εκπομπών βλαβερών αερίων στην ατμόσφαιρα. Η χαμηλή θερμοκρασία λειτουργίας των κελιών καυσίμου πολυμερούς ηλεκτρολύτη (PEMFCs) προσφέρει πολλά πλεονεκτήματα και σε συνδυασμό με την υψηλή πυκνότητα ισχύος που αποδίδουν, τα καθιστά κύριους υποψήφιους για εφαρμογή στην αυτοκίνηση. Ωστόσο, η χαμηλή θερμοκρασία εγείρει και σημαντικά προβλήματα, όπως η χρήση ευγενών μετάλλων για την επιτάχυνση των αντιδράσεων και η ευαισθησία σε φαινόμενα δηλητηρίασης. Το κυριότερο δηλητήριο είναι το CO, βασικό παραπροϊόν των διεργασιών παραγωγής H2 από τους υδρογονάνθρακες, οι οποίοι προς το παρόν αποτελούν την κύρια πηγή του. Στην παρούσα διδακτορική διατριβή εξετάστηκαν τα φαινόμενα δηλητηρίασης από το CO της ανόδου του PEMFC. Καθώς το CO δεσμεύεται ισχυρότερα στην επιφάνεια του Pt από το καύσιμο Η2, η παρουσία του στην τροφοδοσία ακόμα και σε ίχνη απενεργοποιεί δραματικά τη λειτουργία της ανόδου. Έτσι, μελετήθηκαν διμεταλλικά και τριμεταλλικά καταλυτικά συστήματα, βασισμένα στο Pt, για την πιθανή αντιμετώπιση του προβλήματος, διαμέσου εξασθένισης του δεσμού Pt-CO ή ενίσχυσης της ηλεκτροχημικής οξείδωσής του από το Η2Ο, που είναι άφθονο στο περιβάλλον ενός PEMFC. Στο κεφάλαιο 1 περιγράφονται οι βιβλιογραφικές πληροφορίες για την τεχνολογία του Η2, όπως μέθοδοι παραγωγής του, καθαρισμού του και αποθήκευσης/μεταφοράς του. Στο κεφάλαιο 2 αναφέρονται οι βασικές αρχές λειτουργίας των κελιών καυσίμου, όσον αφορά στη θερμοδυναμική και στην κινητική, στα είδη τους και στις πιθανές εφαρμογές τους. Στο κεφάλαιο 3 γίνεται εκτενής περιγραφή των δομικών στοιχείων που απαρτίζουν ένα PEMFC, και βιβλιογραφική ανασκόπηση των καταλυτικών συστημάτων που έχουν μελετηθεί για τις βασικές αντιδράσεις. Στο κεφάλαιο 4 περιγράφονται συνοπτικά οι μέθοδοι χαρακτηρισμού και ανάλυσης καθώς και οι πειραματικές διατάξεις που χρησιμοποιήθηκαν. Στο κεφάλαιο 5 εξετάστηκε η επίδραση του υποστρώματος TiO2 στα χαρακτηριστικά του Pt, όσον αφορά την αλληλεπίδρασή του με το CO, σε διάταξη μονής κυψέλης καυσίμου. Παρουσιάστηκε αυξημένη ενεργότητα για την ηλεκτροοοξείδωση του CO και ασθενέστερη αλληλεπίδρασή του με την επιφάνεια του Pt, συντελώντας σε ενεργοποιημένη ρόφηση. Στο κεφάλαιο 6 με φασματοσκοπία υπερύθρου μελετήθηκαν τα χαρακτηριστικά της ρόφησης/εκρόφησης του CO σε μια σειρά καταλυτών Pt-Mo σε υπόστρωμα TiO2. Παρουσία των οξειδίων του Mo η θερμοκρασία εκρόφησης του CO ήταν σημαντικά μειωμένη σε σχέση με μονομεταλλικό Pt, υποδεικνύοντας ασθενέστερο δεσμό του CO με την καταλυτική επιφάνεια. Ωστόσο, παρουσία H2 ο δεσμός ισχυροποιείται, με αποτέλεσμα η εκρόφηση να πραγματοποιείται σε υψηλότερη θερμοκρασία. Αυτό εξηγήθηκε με βάση την ανταγωνιστική αντίδραση του H2 με τις οξειδικές ομάδες, τόσο του υποστρώματος TiO2, όσο και των οξειδίων του Mo. Στο κεφάλαιο 7 εξετάστηκε η οξείδωση του CO σε καταλύτη Pt4Mo/C, δεδομένου του αποσταθεροποιητικού ρόλου του Mo στα χαρακτηριστικά της αλληλεπίδρασης με το CO. Έτσι, αναγνωρίστηκε η ικανότητα των οξειδίων του Mo να διασπούν το Η2Ο σε δυναμικά που συμπίπτουν με τη λειτουργία της ανόδου ενός PEMFC, ενώ παρουσίασαν ενεργότητα για την οξείδωση του CO σε συνθήκες ανοιχτού κυκλώματος διαμέσου της αντίδρασης μετατόπισης με ατμό σε χαμηλή θερμοκρασία μέχρι και 60οC. Ωστόσο, η παραπάνω ιδιότητες δεν ήταν κατανεμημένες ομοιόμορφα στην καταλυτική επιφάνεια, παρά μόνο στη διεπιφάνεια Pt/MoOx, ενώ οι θέσεις μονομεταλλικού Pt παρουσίασαν έντονα φαινόμενα δηλητηρίασης. Επιπλέον, το Mo παρουσιάστηκε ευαίσθητο σε φαινόμενα διάλυσης στο όξινο υδατικό περιβάλλον του PEMFC για δυναμικά μεγαλύτερα από 0.2 V. Στο κεφάλαιο 8 μελετήθηκε η αλληλεπίδραση του CO με τριμεταλλικό καταλύτη Pt-Ru-Co σε σύγκριση με εμπορικό PtRu/C. Ο τριμεταλλικός καταλύτης παρουσιάστηκε ενεργότερος, με χαμηλότερη φαινόμενη ενέργεια ενεργοποίησης για την οξείδωση ροφημένου CO, εμφανίζοντας ισχυρότερη εξάρτηση από το εφαρμοζόμενο δυναμικό. / Hydrogen is the lighter and more abundant element in nature. It is everywhere in earth, water, fossil fuels and in all the living creatures. If H2 can be properly extracted and utilized as a fuel in fuel cells, the dependence of the global economy on fossil fuels will be minimized, resulting in significant attenuation of the greenhouse gases emissions in the atmosphere. The low operation temperature of the polymer electrolyte membrane fuel cells (PEMFCs) offers a lot of advantages. In combination with the high power density yielded by the PEMFCs renders them as the main candidates for application in automotive industry. However, the low temperature raises significant problems, such as the use of noble metals for the acceleration of the basic reactions and the susceptibility in poisoning phenomena. The basic poison is carbon monoxide (CO), one of the main side-products of H2 production from fossil fuels, which for the moment is the main source of H2. In this thesis, the poisoning phenomena of the PEMFCs anode electrocatalysts from CO were investigated. Since CO is bounded on the surface of Pt stronger than the H2 fuel, its presence in the fuel feed in ppm levels deactivates the anode electrocatalyst. In order to eliminate this problem, bimetallic and ternary catalytic systems, based on Pt, were studied with the aim to reduce the Pt-CO bond strength or to promote the electrocatalytic oxidation of CO by water, which is abundant in the PEMFC environment. In chapter 1 is reported the literature information about H2 technology, such as H2 production and cleaning methods and the transport and storage infrastructure. In chapter 2, the basic thermodynamic and kinetic rules of fuel cells operation are referred together with the types of fuel cells and the possible applications. In chapter 3 the structural characteristics of the PEMFCs are outlined and the basic catalytic systems that have been studied for the fuel cell reactions are reviewed. The catalysts’ characterization methods, as well as the experimental procedures utilized in this thesis, are briefly described in chapter 4. In chapter 5 the effect of TiO2 support on the CO chemisorption’s and oxidative properties of Pt was investigated in a single PEMFC configuration. The activity of the CO electrooxidation reaction was enhanced and the Pt-CO bond was destabilized comparing to a commercial Pt/C catalyst. In chapter 6 the CO adsorption/desorption properties were studied by Infrared Spectroscopy, on a series of Pt-Mo catalysts supported on anatase TiO2. The presence of Mo oxides on the catalyst surface reduces significantly the CO desorption temperature in comparison to monometallic TiO2 supported Pt, suggesting the weak CO bonding on the catalytic surface. However, in the presence of H2, the Pt-CO bond strengthens, resulting in higher CO desorption temperature for all the catalysts tested. This was explained on the basis of competitive reaction of H2 with the oxidic surface species, originating from the TiO2 support and the surface Mo oxides. The CO electrooxidation activity of a Pt4Mo/C catalyst is described in chapter 7, considering the destabilizing effect of Mo on the Pt-CO bond. The surface Mo oxide species were able to dissociate H2O at potential values that coincide with the potential window of the PEMFC anode operation. This catalyst oxidized CO under open circuit conditions through the water gas shift reaction and at temperature as low as 60oC. However, the catalytic activity was not homogeneously distributed on the entire catalyst surface, but it was located at the Pt/MoOx interface, with the monometallic Pt sites to be strongly susceptible to CO poisoning. Furthermore, Mo was sensitive to dissolution phenomena in the hydrous acidic environment of the PEMFC for potentials higher than 0.2 V vs. rhe. Finally, in chapter 8 is described the interaction of CO with a ternary Pt-Ru-Co catalyst surface, in comparison to a commercial PtRu/C catalyst. The ternary catalyst was more active for the adsorbed CO electrooxidation, with a lower apparent activation energy than the bimetallic commercial one. The ternary catalyst exhibited zero reaction order with respect to CO partial pressure, while the PtRu/C showed negative reaction order due to competitive adsorption of CO and oxidic species for the same catalytic sites. The kinetic rate constant of the CO electrooxidation reaction for the ternary catalyst showed stronger dependence on the applied potential.

Le but de cette thèse était de développer de nouveaux électrolytes polymères pour une application batteries lithium métal polymère. Le premier volet concerne le développement des réseaux semi-interpénétrés à base de POE et d'un polycondensat. Ces types d'électrolytes ont permis de d'améliorer les propriétés mécaniques et les conductivités à haute et basse température. L'ajout de NCC comme renfort sur ces réseaux semi-interpénétrés a permis d'atteindre propriétés physico-chimiques intéressantes et des durées de vie élevées. De plus l'hydrogénation du polycondensat permettant de moduler sont taux de réticulation a permis d'obtenir un électrolyte (en présence du LiTFSI) présentant des conductivités de 1S.cm-1 à 90°C pour un rapport O/Li=20 et O/Li=30 avec une tenue mécanique de 0,5MPa jusqu'à 100°C. Dans le second volet une série de sels de lithium à anion organique a été synthétisée et caractérisée. Ces sels de lithium présentent des bonnes stabilités électrochimiques, thermiques et des conductivités cationiques parfois plus élevées que LITFSI en milieu polymère. Le dernier volet concerne la synthèse et la caractérisation physico-chimique des nouveaux ionomères perfluoré. Ces nouveaux ionomères à conduction cationique unipolaire sont obtenus à partir de monomères aromatiques porteurs de fonctions ioniques ayant une forte aptitude à la dissociation et des nombres de transport cationique proche de 1 à 70°C. / The aim of this thesis was to develop new polymer electrolytes for application of lithium metal polymer batteries. The first part concerns the development of semi-interpenetrating networks based on POE and a polycondensat. These types of electrolytes made it possible to improve the mechanical properties and conductivity at high and low temperatures. The addition of NCC as a reinforcement on the semi-interpenetrating network has led to interesting physicochemical properties and high cycle life for batteries.The partial hydrogenation of the polycondensat allowing the modulation of the reticulation ratio has allow to elaborate as an electrolyte (in the presence of LiTFSI) exhibiting 1S.cm-1 conductivities at 90 ° C for a ratio O/Li=20 and O/Li=30 with a mechanical strength of 0.5MPa to 100 ° C. In the second part a range of lithium with organic anion was synthesized and characterized. These lithium salts show good electrochemical and thermal stability, whereas ionics conductivities are sometimes higher than LiTFSI in polymer medium. The last part concerns the synthesis and physicochemical characterization of new perfluorinated ionomers. These new cationic ionomers with a unipolar conduction are obtained from aromatic monomers carriers ionic functional having a high ability to dissociation and cation transport numbers close to 1 at 70 ° C.

The present thesis demonstrates and discusses polymeric ion and mixed ion-electron conductors for rechargeable batteries based on lithium viz. lithium-ion and lithium-sulphur batteries. The proposed polymer ion conductors in the thesis are discussed primarily as potential alternatives to conventional liquid and solid-crystalline electrolytes in lithium-ion batteries. These discussions are part of Chapters 2-4. On the other hand, the polymer based mixed ion-electron conductor is demonstrated as a novel electrode for lithium-Sulphur battery in Chapter 5. Possibility of application of polymer ion conductors is discussed in the context of Li-S battery in Chapter 6. A distinct correlation between the physical properties and electrochemical performance of the proposed conductors is highlighted in detail in this thesis. Systematic investigation of the ion transport mechanism in the polymeric ion conductors has been carried out using various spectroscopic techniques at different time and length scales. Such detailed investigations demonstrate the key structural and physical parameters for design of alternative polymer conductors for rechargeable batteries. Though the thesis discusses the various polymeric conductors in the context of lithium-based batteries, it is strongly felt that the design strategies are equally likely to be beneficial for different battery chemistries as well as for other electrochemical generation and storage devices. A brief discussion of the contents and highlights of the individual chapters are described below: The thesis comprises of six Chapters. Chapter 1 briefly reviews the important developments and materials of lithium-based batteries, with specific focus on Li-ion and Li-S batteries. It starts with discussions on different types of liquid, solid crystalline and solid-like electrolytes. Their materials characteristics, advantages and disadvantages are discussed in the context of secondary batteries such as lithium-ion and lithium-sulphur batteries. As prospective alternative electrolytes polymer based soft matter electrolytes are discussed in detail. Special emphasis is given to the recent developments in polymer electrolytes and their ion conduction mechanism, which are central themes to this thesis. The importance of investigation of charge transport, typically ion, on electrochemical processes is also briefly discussed in Chapter 1. A brief discussion about the characteristics, materials and non-trivialities of the electrochemical storage process in Li-S battery is also reviewed. Chapter 2A demonstrates a binary polymer physical network based gel (PN-x) electrolyte, comprising of an ionic liquid confined inside a binary polymer system for electrochemical devices such as secondary batteries. The synthesis, physical property and electrochemical performances are studied as a function of content of one of the polymers in this Chapter. A physical network of two polymers with different functional groups leads to multiple interesting consequences. The polymer physical network characteristics determine all physical properties including electrochemical property of the ionic liquid integrated PN based GPE. The conductivities of the proposed gel are nearly an order in magnitude higher than the unconfined ionic liquid electrolyte and displays good dimensional stability and electrochemical performance in a separator-free battery configuration. The ac-impedance spectroscopy, steady shear viscosity measurement, dynamic rheology are employed to study physical properties of the proposed gel polymer electrolyte. Chapter 2B discusses the detailed investigations of the ion transport mechanism of the gel polymer electrolyte, as discussed in Chapter 2A. Ion conduction mechanism is investigated in the light of ion diffusion and solvent dynamics of the entrapped ionic liquid inside the polymer. The studies reveal a heavy influence of network characteristics on the ion conduction mechanism. The influence of solvent dynamics on the ion transport is drastically altered by polymer physical network. Consequently, a drastic change in the ion mobility and nature of predominant charge carrier is observed in the polymer physical network based gel electrolyte. A clear transformation from dual ion conductivity to a predominantly anion conductivity is observed on going from single polymer to a dual polymer network. The spectroscopic tools such as pulsed field gradient nuclear magnetic resonance (PFG–NMR), Brillouin light scattering spectroscopy, ac-impedance spectroscopy, FT-Raman and FTIR spectroscopy were used to elucidate the ion transport mechanism in the Chapter. Chapter 3 demonstrates a simple design strategy of gel polymer electrolyte comprising of a lithium salt (lithium bis(trifluoromethanesulfonyl) imide, LiTFSI) solvated by two plastic crystalline solvents, one a solid (succinonitrile, abbreviated as SN) and another a (room temperature) ionic liquid (1-butyl-1-methyl-pyrrolidinium bis(trifluoromethane sulfonyl) imide, (abbreviated as IL) confined inside a linear network of poly(methyl methacrylate) (PMMA). The concentration of the IL component determines the physical properties of the unconfined electrolyte and when confined inside the polymer network in gel polymer electrolyte. Intrinsic dynamics of one plastic crystal influences the conduction mechanism of gel polymer electrolytes. The enhanced disordering in the plastic phase of succinonitrile by IL doping alters both the local ion environment and viscosity. The proposed plastic crystal electrolytes show predominantly anion conduction (tTFSI ≈ 0.5) however, lithium transference number (tLi ≈ 0.2) is nearly an order higher than the ionic liquid electrolyte (IL-LiTFSI) (tLi ≈ 0.02-0.06), discussed in Chapter 2. The gel polymer electrolyte displayed high mechanical compliability, stable Li-electrode | electrolyte interface, low rate of Al corrosion and stable cyclability. The promising electrochemical performance further justifies simple strategy of employing mixed physical state plasticizers to tune the physical properties of polymer electrolytes requisite for application in rechargeable batteries. Chapter 4A proposes a novel liquid dendrimer–based single ion conducting liquid electrolyte as potential alternative to conventional molecular liquid solvent–salt solutions and conventional solid polymer electrolytes for rechargeable batteries, sensors and actuators. The physical properties are investigated as a function of peripheral functionalities in the first generation poly(propyl ether imine) (G1-PETIM)–lithium salt complexes. The change in peripheral group simultaneously affects the effective physical properties viz. viscosity, ionic conductivity, ion diffusion coefficients, transference numbers and also the electrochemical response. The specific change from ester (–COOR) to cyano (–CN) terminated peripheral group resulted in a remarkable switch over from a high cation (tLi+ = 0.9 for –COOR) to a high anion (tPF6- = 0.8 for –CN) transference number. Chapter 4B presents an analysis of the frequency dependent ionic conductivity of single ion dendrimer conductors by using time temperature scaling principles (TTSPs) and dielectric modeling of the electrode polarization. The TTSP provides information on the salt dissociation and number density of mobile charges and hence provides direct insights into the ion conduction mechanism. Summerfield and Baranovskii–Cordes scaling laws, which are well known TTSPs, have been applied to analyze the ion conductivity. The electrode polarization, which quantifies the number density of mobile charges and ionic mobility, is studied using Macdonald-Coelho model of electrode polarization. The combination of these two theoretical investigations of the experimental data emanating from one technique i.e. ac– impedance spectroscopy, predicts independently the contributions of the effect of mobile ion charges and ionic mobility to ion conduction mechanism. In Chapter 5 focus shifts from polymer ion conductors to polymer mixed ion-electron conductor. The polymer mixed ion-electron conductor is demonstrated as a novel electrode material for Li-S battery. A simple strategy to overcome the challenges towards practical realization of a stable high performance Li–S battery is discussed. A soft mixed conducting polymeric network is utilized to configure sulphur nanoparticle. The soft matter network provides efficient and distinct pathways for lithium and electron conduction simultaneously. A lithiated polyethylene glycol (PEG) based surfactant tethered on ultra-small sulphur nanoparticles and wrapped up with polyaniline (PAni) (abbreviated as S-MIEC) is demonstrated here as an exceptional cathode for Li–S batteries. The S-MIEC is characterized by several methods: powder-X-ray diffraction (PXRD), thermo gravimetric analysis (TGA), fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), ac-impedance spectroscopy and dc current-voltage measurements are performed to evaluate conductivity of S-MIEC cathode. Electrochemical studies such as cyclic voltammetry, galvanostatic charge-discharge cycling, galvanostatic intermittent titration (GITT) are performed to demonstrate feasibility of S-MIEC in the Li–S battery performance. Chapter 6 provides a brief summary of the work carried out as part of this thesis and also demonstrates the future perspective of the present work. Potential of the polymer physical network based gel polymer electrolytes, which are discussed in Chapter 2A-B for lithium-ion batteries, are demonstrated in Li-S battery. The proposed polymer physical network confines higher order lithium polysulfides (typically Li2S8) dissolved in tetraethylene glycol dimethyl ether (TEGDME) based electrolyte (TEGDME-1M LiTFSI). The three dimensional polymer network is proposed to be formed by physical blending of the poly(acrylonitrile) (PAN) with the copolymer of AN and poly(ethylene glycol) methyl ether methacrylate (PEGMA), [ P(AN–co–PEGMA)]. We extend here the similar synthetic approaches as described in Chapter 2A. The approach proposed and demonstrated in this concluding Chapter is expected to mitigate some of the major issues of Li-S chemistry. The proposed Li2S8 confined gel electrolyte exhibits moderately high values of ionic conductivity, 2 × 10-3 Ω-1cm-1 and shows a stable capacity of 350 mAhg-1 over 30 days in a separator free Li-S battery.

This master’s thesis deals with supercapacitors and polymer gel electrolytes. The practical part deals with the preparation of samples of polymer gel electrolyte with addition of alkali salts by measuring their electrical conductivity and evaluation potential windows.