Porous carbon derived from an inorganic silicon carbide (SiC) precursor, termed SiC-derived carbon, is an attractive material for electrochemical energy storage applications, including electrodes for electrical double layer capacitors (EDLCs). The objective of this thesis is to investigate the effects that the carbide-derived carbon (CDC) microstructure and pore structure have on the energy and power characteristics of the EDLC electrodes. Conventional SiC CDC is produced from non-porous crystalline SiC powder at temperatures above 800 °C. Here we studied the performance of SiC CDCs produced by chlorination at 700-900 °C of an ordered mesoporous SiC precursor, which was synthesized via a 1000 °C pyrolysis of polycarbosilane infiltrated into an SBA-15 silica template having ordered mesopores. The SiC CDC was purified from chlorine impurities by annealing in ammonia. The surface area and pore size of the purified SiC CDC was characterized via N2 and CO2 sorption using density functional theory (DFT) and Brunnauer, Emmet, and Teller (BET) theory. The specific capacitance, power and energy densities were characterized via electrochemical measurements of the SiC CDC electrodes in 1 M tetraethylammonium tetrafluoroborate (TEABF4) acetonitrile solution. The SiC CDC exhibited a specific surface area (SSA) in excess of 2400 m2/g and gravimetric capacitance values of up to ~ 150 F/g, among the highest ever reported for any electrodes in this electrolyte. The ordered mesopores allowed for fast ion transport within each particle, resulting in excellent capacity retention under high current rates and ultra-fast frequency response, thus allowing for extremely high power and energy densities. The best overall performance was achieved in SiC CDC samples chlorinated at the lowest temperature of 700 °C.

Energijos kaupiklių kūrimas pastaraisiais metais tapo svarbia mokslo aktualija. Elektrocheminiai kondensatoriai pasižymi didesne galia, ilgesniu savaiminio išsikrovimo laiku ir didesniu darbo ciklų skaičiumi negu tradicinės elektrocheminės baterijos. Didžiausia savitąja talpa pasižymi RuO2, tačiau jo praktinį taikymą riboja didelė kaina, todėl ieškoma pigesnių pakaitalų. Pastaraisiais metais aktyviai tiriamas kobalto oksidas. Svarbu yra surasti ekonomiškai efektyvius ir paprastus metodus, leidžiančius padidinti kobalto oksidų pseudotalpas. Darbe nustatytos elektrocheminio Co nusodinimo sąlygos, kurioms esant yra formuojama pluoštinė nanostruktūra, pasižyminti dideliu paviršiaus plotu. Anodinės poliarizacijos būdu buvo suformuoti kobalto hidro/oksido sluoksniai ir ištirta jų pseudotalpa ant skirtingų substratų: nanostruktūrinės, magnetroninės-plazminės dangos ir metalurginio Co. Pseudotalpinė kobalto hidro/oksidų elgsena buvo ištirta ciklinės voltamperometrijos metodu kartu su elektrocheminėmis kvarco kristalo svarstyklėmis (EKKM). Oksidiniai sluoksniai parodė grįžtamą redukcijos-reoksidacijos ir atitinkamą pseudotalpinę elgseną. Per kelis tūkstančius anodinių ciklų buvo parodyta, kad oksidinis sluoksnis išliko stabilus. Nustatyta, kad hidro/oksidų, suformuotų ant nanostruktūrinės Co dangos, talpa maždaug 5 kartus didesnė nei suformuotų ant metalurginio Co. Šiame darbe pasiūlytas naujas kobalto oksido formavimo būdas: sulfidinės Co(OH)2–CoS kompozicijos formavimas ir... [toliau žr. visą tekstą] / Design of high efficiency energy accumulators has recently become an important issue of science and technology. Electrochemical capacitors are distinguished for a greater power, a longer self-discharge time and a greater number of work cycles as compared to those of traditional electrochemical batteries. Effective supercapacitors are mainly produced of RuO2, which is distinguished from other metals for its nearly highest specific capacity, however its practical application is limited by its high price, therefore efforts are under way to find cheaper substitutes. Recently cobalt oxide has been intensively studied. It is important to find economically effective and simple methods which will make it possible to increase the pseudocapacities of cobalt oxides. The conditions of electrochemical cobalt deposition, under which the fiber Co nanostructure possessing a great surface area is formed, have been determined in the work. Cobalt hydro/oxide layers were formed by the anodic polarization method and their pseudocapacity was studied on various substrates: nanostructured, magnetron-sputtered coating and metallurgical cobalt. The pseudocapacitor behaviour of cobalt hydro/oxides was studied by using the cyclic voltammetry method along with electrochemical quartz crystal mocrobalance (EKKM). Oxide layers showed oxidation-reduction and corresponding pseudo-capacitor behaviour.It has been shown that the oxide layer is stable and it withstands a few thousand polarization cycles. It has... [to full text]

Design of high efficiency energy accumulators has recently become an important issue of science and technology. Electrochemical capacitors are distinguished for a greater power, a longer self-discharge time and a greater number of work cycles as compared to those of traditional electrochemical batteries. Effective supercapacitors are mainly produced of RuO2, which is distinguished from other metals for its nearly highest specific capacity, however its practical application is limited by its high price, therefore efforts are under way to find cheaper substitutes. Recently cobalt oxide has been intensively studied. It is important to find economically effective and simple methods which will make it possible to increase the pseudocapacities of cobalt oxides. The conditions of electrochemical cobalt deposition, under which the fiber Co nanostructure possessing a great surface area is formed, have been determined in the work. Cobalt hydro/oxide layers were formed by the anodic polarization method and their pseudocapacity was studied on various substrates: nanostructured, magnetron-sputtered coating and metallurgical cobalt. The pseudocapacitor behaviour of cobalt hydro/oxides was studied by using the cyclic voltammetry method along with electrochemical quartz crystal mocrobalance (EKKM). Oxide layers showed oxidation-reduction and corresponding pseudo-capacitor behaviour.It has been shown that the oxide layer is stable and it withstands a few thousand polarization cycles. It has... [to full text] / Energijos kaupiklių kūrimas pastaraisiais metais tapo svarbia mokslo aktualija. Elektrocheminiai kondensatoriai pasižymi didesne galia, ilgesniu savaiminio išsikrovimo laiku ir didesniu darbo ciklų skaičiumi negu tradicinės elektrocheminės baterijos. Didžiausia savitąja talpa pasižymi RuO2, tačiau jo praktinį taikymą riboja didelė kaina, todėl ieškoma pigesnių pakaitalų. Pastaraisiais metais aktyviai tiriamas kobalto oksidas. Svarbu yra surasti ekonomiškai efektyvius ir paprastus metodus, leidžiančius padidinti kobalto oksidų pseudotalpas. Darbe nustatytos elektrocheminio Co nusodinimo sąlygos, kurioms esant yra formuojama pluoštinė nanostruktūra, pasižyminti dideliu paviršiaus plotu. Anodinės poliarizacijos būdu buvo suformuoti kobalto hidro/oksido sluoksniai ir ištirta jų pseudotalpa ant skirtingų substratų: nanostruktūrinės, magnetroninės-plazminės dangos ir metalurginio Co. Pseudotalpinė kobalto hidro/oksidų elgsena buvo ištirta ciklinės voltamperometrijos metodu kartu su elektrocheminėmis kvarco kristalo svarstyklėmis (EKKM). Oksidiniai sluoksniai parodė grįžtamą redukcijos-reoksidacijos ir atitinkamą pseudotalpinę elgseną. Per kelis tūkstančius anodinių ciklų buvo parodyta, kad oksidinis sluoksnis išliko stabilus. Nustatyta, kad hidro/oksidų, suformuotų ant nanostruktūrinės Co dangos, talpa maždaug 5 kartus didesnė nei suformuotų ant metalurginio Co. Šiame darbe pasiūlytas naujas kobalto oksido formavimo būdas: sulfidinės Co(OH)2–CoS kompozicijos formavimas ir... [toliau žr. visą tekstą]

Vertically aligned multi walled carbon nanotube (MWCNT) arrays fabricated by xylene pyrolysis in anodized aluminum oxide (AAO) templates without the use of a catalyst, were integrated into a resistive sensor design. The steady state sensitivities as high as 5% and 10% for 100 ppm of NH3 and NO2 respectively at a flow rate of 750 sccm were observed. A study was undertaken to elucidate (i) the dependence of sensitivity on the thickness of amorphous carbon layers, (ii) the effect of UV light on gas desorption characteristics and (iii) the dependence of room temperature sensitivity on different NH3 and NO2 flow rates. An equivalent circuit model was developed to understand the operation and propose design changes for increased sensitivity. Multi Walled Carbon NanoTubes (MWCNTs) – Polymer composite based hybrid sensors were fabricated and integrated into a resistive sensor design for gas sensing applications. Thin films of MWCNTs were grown onto Si/SiO2 substrates via xylene pyrolysis using chemical vapor deposition technique. Polymers like PEDOT:PSS and Polyaniline (PANI) mixed with various solvents like DMSO, DMF, 2-Propanol and Ethylene Glycol were used to synthesize the composite films. These sensors exhibited excellent response and selectivity at room temperature when exposed to low concentrations (100ppm) of gases like NH3 and NO2. Effect of various solvents on the sensor response imparting selectivity to CNT – Polymer nanocomposites was investigated extensively. Sensitivities as high as 28% was observed for a MWCNT – PEDOT:PSS composite sensor when exposed to 100ppm of NH3 and -29.8% sensitivity for a MWCNT-PANI composite sensor to 100ppm of NO2. A novel nanostructured electrode design for Li based batteries and electrochemical capacitor applications was developed and tested. High density and highly aligned metal oxide nanowire arrays were fabricated via template assisted electrochemical deposition. Nickel and Molybdenum nanowires fabricated via cathodic deposition process were converted into respective oxides via thermal treatments and were evaluated as electrodes for batteries and capacitor applications via Cyclic Voltammetery (CV). Several chemical baths were formulated for the deposition of pristine molybdenum nanowires. Superior electrochemical performance of metal (Ni and Mo) oxide nanowires was observed in comparison to the previously reported nano-particle based electrodes.

Electric double layer capacitors (EDLCs) are an important class of electrical energy storage devices which store energy in the form of electric double layers. The charging mechanism is highly reversible physical adsorption of ions into the porous electrodes, which empower these devices to show a remarkable power performance (15kW/kg) and greater life expectancy (> 1 million cycles). However, they store a small amount of energy (5Wh/kg) when compared with batteries. Optimization of the performance of EDLCs based on porous activated carbons is highly challenging due to complex charging process prevailing in the Nano pores of electrodes. Molecular simulations provide information at the molecular scale which in turn can be used to develop insights that can explain experimental results and design improved EDLCs. The conventional approach to simulate EDLCs places both the electrodes and electrolyte region in a single simulation box. With present day computers, however, this one-box method limits us to system sizes of the order of nanometres whereas the size of a typical EDLC is at least of the order of micrometres. To overcome this system size limitation, a Gibbs-ensemble based Monte Carlo (MC) method was recently developed, where the electrodes are simulated in a separate simulation boxes and each box is subjected to periodic boundary conditions in all the three directions. This allows us to eliminate the electrode-electrolyte interface. The simulation of the bulk electrolyte is avoided through the use of the grand canonical ensemble. The electrode atoms in the electrode are maintained at an equal constant electric potential likewise the case in a pure conductor with the use of the constant voltage ensemble. In this thesis, the Gibbs-ensemble based MC simulations are performed for an EDLC consisting of porous electrodes. The simulations are performed with aqueous electrolytes of type MX and DX2 (where M=Na+, K+; D=Ca+2; X=Cl , F ) for a wide variety of operating conditions. The water is modelled as a continuum background with a dielectric constant value of 30. The electrodes are silicon carbide-derived carbon, whose microstructure generated from reverse MC technique, is used in the simulations. The results from these simulations help us understand the charge storage mechanism, the effect of size and valence of ions on the performance of nonporous carbon based EDLCs when the hydration effects are indignant. The thesis first demonstrates the presence of finite size effects in the simulations performed with the one-box method for KCl electrolyte. The capacitance (ratio of the charged stored on the positive electrode to the voltage applied) values obtained for KCl electrolyte with the one-box method are significantly higher than the corresponding values obtained from the Gibbs-ensemble method. This shows the presence of finite size effects in the one-box method simulations and justices the use of the Gibbs-ensemble based method in our simulations. The fundamental characteristics of aqueous electrolytes in the EDLC are analyzed with the simulation results for KCl electrolyte. In agreement with experiments and modern mean held theory, the capacitance monotonically decreases with voltage (bell-shaped curve) due to overcrowding of ions near the electrode surface. The charge storage mechanism in both the electrodes is mainly a combination of countering (ions oppositely charged to that of the electrode) adsorption and ion exchange, where coins (ions identically charged to that of the electrode) are replaced with countering. However, at higher voltages, the mechanism is predominantly counter ion adsorption because of the scarcity of coins in the electrodes. The mechanism is preferentially more ion exchange for the positive electrode because of its relatively bulky countering, Cl . The shifting of mechanism towards counter ion adsorption at higher voltages and preferential ion exchange process for the positive electrode are in qualitative agreement with the recent experimental results. The constraint of equal electric potential on all the electrode atoms of the amorphous electrode in the simulations resulted in a non-uniform average charge distribution on the electrodes. It shows that the Gibbs-ensemble simulation approach can account for the polarization effects which arises due to a complex topology of the electrodes. In agreement with earlier experiments and simulation studies, the local structure analyses of the electrodes shows that the highly conned ions store charge more efficiently. On the application of voltage difference between the electrodes, the electrolyte ions move towards higher degree of con ned regions of the electrodes indicating the charging process involves local rearrangement and rescuing of electrolyte ions. The thesis also discusses the effect of temperature and bulk concentration on the performance of EDLCs. The Gibbs-ensemble based simulations are performed for the EDLC with varying temperature and bulk concentration for the KCl electrolyte independently. In agreement with the Guo -Chapman theory and experiments, the capacitance decreases with the temperature and increases with the bulk concentration. This is because the concentration of countering in the electrodes decreases with an increase in the temperature but increases with an increase in the bulk concentration. Lastly, the effect of ion size and valency on the performance of EDLCs is analyzed. The capacitance monotonically decreases with voltage (bell-shaped curve) for all the electrolytes, except for NaF, where a maximum is observed at a non-zero finite voltage (camel-shaped curve). The capacitances of NaCl and NaF are greater than that for KCl and KF, respectively. This is because the smaller Na+ ions have more accessibility to narrow con ned regions, where the charge storage efficiency is high. As expected, the capacitance for CaCl2 and CaF2 are highest among their monovalent counterparts, NaCl and KCl; NaF and KF, respectively. This is attributed to the relatively smaller double layer thickness of the bivalent Ca+2 ions. Interestingly, at higher voltages, the capacitance for the bivalent electrolytes approaches the capacitance for the monovalent electrolytes because the concentration of Ca+2 ions in the negative electrode increases sluggishly with voltage due to a strong electrostatic repulsion between Ca+2 ions.

La demande urgente d'innovations dans le domaine du stockage de l'énergie est liée au développement récent de la production d'énergie renouvelable ainsi qu'à la diversification des produits électroniques portables qui consomment de plus en plus d'énergie. Il existe plusieurs technologies pour le stockage et la conversion électrochimique de l'énergie, les plus notables étant les batteries aux ions lithium, les piles à combustible et les supercondensateurs. Ces systèmes sont utilisés de façon complémentaire des uns aux autres dans des applications différentes. Par exemple, les batteries sont plus facilement transportables que les piles à combustible et ont de bonne densité d'énergie alors que les supercondensateurs ont des densités de puissance plus élevés et une meilleure durée de vie. L'objectif principal de ces travaux est d'étudier les performances électrochimiques d'une nouvelle famille de matériaux bidimensionnel appelée MXène, en vue de proposer de nouvelles solutions pour le stockage de l'énergie. Pour y arriver, plusieurs directions ont été explorées. Dans un premier temps, la thèse se concentre sur les supercondensateurs dans des électrolytes aqueux et aux effets des groupes de surface. La seconde partie se concentre sur les systèmes de batterie et de capacités à ions sodium. Une cellule complète comportant une anode en carbone et une cathode de MXène a été développées. La dernière partie de la thèse présente l'étude des MXènes pour les supercondensateur en milieu organique. Une attention particulière est apportée à l'étude du mécanisme d'intercalation des ions entre les feuillets de MXène. Différentes techniques de caractérisations ont été utilisées, en particulier la voltampérométrie cyclique, le cyclage galvanostatique, la spectroscopie d'impédance, la microscopie électronique et la diffraction des rayons X. / An increase in energy and power densities is needed to match the growing energy storage demands linked with the development of renewable energy production and portable electronics. Several energy storage technologies exist including lithium ion batteries, sodium ion batteries, fuel cells and electrochemical capacitors. These systems are complementary to each other. For example, electrochemical capacitors (ECs) can deliver high power densities whereas batteries are used for high energy densities applications. The first objective of this work is to investigate the electrochemical performances of a new family of 2-D material called MXene and propose new solutions to tackle the energy storage concern. To achieve this goal, several directions have been explored. The first part of the research focuses on MXene behavior as electrode material for electrochemical capacitors in aqueous electrolytes. The next part starts with sodium-ion batteries, and a new hybrid system of sodium ion capacitor is proposed. The last part is the study of MXene electrodes for supercapacitors is organic electrolytes. The energy storage mechanisms are thoroughly investigated. Different characterization techniques were used in this work, such as cyclic voltammetry, galvanostatic charge-discharge, electrochemical impedance spectroscopy, scanning electron microscopy and X-ray diffraction.

No description available.

Metalorganic Chemical Vapour Deposition (MOCVD) is a versatile technique for the deposition of thin films of oxide materials as it offers advantages, such as deposition over large surface area, conformal coverage, selective area deposition, and a high degree of compositional control. The MOCVD process uses metalorganic (MO) complexes, such as β-diketonate and alkoxide-based complexes, as precursors. These complexes are stable and moderately volatile. Because of the direct bond between metal and oxygen, MO complexes are natural precursors for oxide coatings. As the process involves chemical reactions taking place on the substrate surface, growth of thin films by MOCVD depends on various parameters such as the chemical nature and concentration of precursors, reaction pressure, reaction temperature, and the nature of the substrate. Such a large parameter space of the CVD process, when combined with the dynamics (thermodynamics and fluid dynamics) and kinetics, makes it rather complex. This complexity allows one to make thin films of metastable phases, including amorphous materials. One of the important findings of the work is that MOCVD process is capable of making composite coatings of carbonaceous metal oxide. Manganese is multivalent and forms various stable oxides, such as MnO, Mn2O3, Mn3O4 and MnO2. There are various potential applications of manganese oxides. MnO2 is a very well studied material for its electrochemical applications in dry cells, lithium-ion batteries, and in supercapacitors. Hence, it becomes pertinent to explore the properties of thin films of manganese oxides prepared by MOCVD for various electrochemical and other applications. The thesis work is divided into two parts. Part 1 describes the synthesis of manganese complexes, their characterization, and their application to the CVD of coatings, especially those of carbonaceous MnO. Part 2 is devoted to a detailed study of electrochemical aspects of the carbonaceous MnO coatings, followed by a report on their unusual transport properties. Chapter 1 begins with a brief introduction to thin film deposition processes. In particular, the CVD process is described with reference to various parameters such as carrier gas flow, pressure, temperature and most importantly, the CVD precursor. The chapter ends with a description of the scope of the work undertaken for the present thesis. Chapter 2 deals with “Synthesis and Characterization of MO complexes”. It begins with a description of the classification of CVD precursors with the description of MO complexes such as β-diketonates, which are generally subliming crystalline solids. Manganese β-diketonate complexes are discussed in detail. Due to the multivalent nature of Mn, there are two possible complexes namely Mn(acac)2(H2O)2 and Mn(acac)3. These complexes have been synthesised and characterized (confirmed) by various techniques, such as elemental analysis (CHN), X-ray diffraction (XRD), FTIR spectroscopy, and mass spectroscopy. Thermal analysis of the complexes shows that they are suitable as MOCVD precursors. We have used Mn(acac)2(H2O)2 as a precursor in the present work. Metalorganic complexes, where metal ion is directly bonded with both nitrogen and oxygen, can be potential candidates for the precursor for oxynitrides coatings. We have therefore studied solid crystalline anthranilate complexes of various metal ions, such as Mn2+, Co2+, Cu2+ and Zn2+ and confirmed their formation. Thermal analysis shows that anthranilate complexes are fairly volatile below 250oC and decompose below 500oC. These complexes were pyrolysed in open air and in sealed tube at different temperatures, and the resulting powder product examined by XRD, SEM, EDAX and FTIR. This preliminary study shows that anthranilate complexes yield different oxides of Mn, Co and Cu under different pyrolysis conditions, with very interesting morphological features. Pyrolysis of Zn(aa)2 in a sealed tube leads to the formation of a nanocomposite of carbon and zinc oxide (wuerzite), rich in carbon, with potential for applications in catalysis. On the other hand, the pyrolysis of Zn(aa)2 in air at the same temperature leads to leads to crystalline, nanostructured zinc oxide (wuerzite). However, no attempt has been made to use these anthranilates as CVD precursors. Chapter 3 deals with “MOCVD of Manganese Oxides and their Characterization”. It begins with a brief review of various manganese oxides and their properties. This is followed by description of the CVD reactor used for the present work, together with the conditions employed for the deposition of MnOx films. Depositions have been carried out on different substrates such as SS-316, ceramic alumina and Si (111), while varying various deposition parameters, viz., substrate, reactor pressure, carrier gas (argon) flow rate, and the duration of deposition. Significantly, depositions are divided into two categories: one, carried out in argon ambient, in the absence of a supply of oxygen (or any other oxidant) and the second one, under oxygen flow, using argon as carrier gas. The films deposited in the absence of oxygen flow are thick, black in colour, and electrically conducting, indicating the presence of carbon. The growth rate follows a typical thermal pattern, with activation energy of ~ 1.7 eV. Detailed characterization by XRD, TEM/ED, Raman, FTIR and XPS (X-ray photoelectron spectroscopy) shows that these films are composed of MnO in a carbon-rich amorphous matrix. High-resolution SEM (fig. 1) reveals a fractal pattern of cauliflower morphology, comprising very fine particles (4 – 10 nm), characteristic of very large specific surface area of the film, which is confirmed by volumetric BET measurement (~2000 m2/g). We conclude that growth in argon ambient leads to a homogenous nanocomposite film of hydrated MnO in carbon-rich matrix. Thus, our study reveals that MOCVD is a novel one-step chemical method to produce homogenous composite thin films, wherein all components of the nanocomposite film emerge from the same chemical precursor. Carbon incorporation is generally avoided by empirical process design, as it is viewed as an impurity. The potential advantages of carbon incorporation are thus not examined and the composite nature of carbonaceous films not recognized in the literature. Carbonaceous nanocomposite film can be significant as an electrode in supercapacitors, as discussed in part 2 of the thesis. Chapter 3 describes films deposited under oxygen flow, which are no longer black and are highly resistive, indicating the absence of carbon in the film, as confirmed by Raman spectroscopy. XRD, FTIR and Raman spectroscopy reveal that the films obtained under oxygen flow are more crystalline than the ones obtained in the absence of oxygen flow, and that the films are generally nanocrystalline composites of two manganese oxides, such as MnO and Mn3O4. Given the context of the carbonaceous MnO films described above, chapter 4 begins with a review of electrochemical capacitors (also called supercapacitors or ultracapacitors), which are emerging as important energy storage devices. Until now, in the Mn-O system, hydrated MnO2 has been well-studied as an electrode material due to its low cost and environmental compatibility, but the low electrical conductivity of MnO2, together with irreversible redox reactions, reduces its performance. In electrochemical capacitor applications, metal-oxide/carbon composites are finding importance. Chapter 4 deals with “MnO/C Nanocomposite Coatings as Electrodes for Electrochemical Capacitor”. In this chapter, we have examined the novel EM, i.e., the hydrated MnO/C nanocomposite coating prepared by the MOCVD process on a conducting substrate (current collector) such as SS-316 as an electrode. Electrochemical measurements have been carried out for both the 3-electrode assembly (for basic aqueous electrolyte) and 2-electrode assembly (for gel polymer electrolyte) using cyclic voltammetry (CV), AC impedance and charge-discharge techniques. The studies lead to a maximum specific capacitance of 230 – 270 F/g at 1 mA/cm2 discharge current density for the MnO/C nanocomposite coating grown at 680oC. The Bode plot shows a maximum phase angle of around 74 – 82o, indicating capacitive behaviour. The MnO/C nanocomposite film shows a very small time constant (0.5 – 3 msec), which is good for high frequency applications. The pulse power figure of merit is found to be 650 – 2000 W/g. Capacitance determined for a large number of charge-discharge cycles (~20000), and at large current densities (50 mA/cm2) show promising results. The energy density (5 - 32 Wh/kg) and power density (2 – 4 kW/kg) estimated from charge-discharge data at 1 mA/cm2 shows the potential of the nanocomposite MnO/C as electrode for superior capacitor devices. Gel polymer electrolytes (GPE) offer the advantage of large electrochemical potential window due to its structural and chemical stability. Studies have been carried out to show that the MnO/C nanocomposite film is compatible with gel polymer electrolytes based on poly(methyl methacrylate) (PMMA) and poly(acrylonitrile) (PAN) with salts of magnesium triflate and magnesium perchlorate, respectively) and plasticizers (ethylene carbonate (EC) + propylene carbonate (PC)), in a 2-electrode assembly. Chapter 5 deals with “Magnetoconductance in MnO/C Nanocomposite Coatings on Alumina”. Amorphous systems, such as MnO/C composites wherein carbon is amorphous and MnO is nearly so, are highly symmetric condensed phases, which do not possess long range translational or orientational order. Disorder in the system creates Anderson localized states just above the valence band, which lead to reduced electrical conductivity. Amorphous systems show either a small negative magnetoresistance (~ 5%) or a small positive magnetoconductance (~ 7%) at very low temperatures (~ 10 K). As such, the transport properties of the MnO/C nanocomposite film have been investigated, and are reported in chapter 5. Transport and magnetotransport measurements have been made on the MnO/C nanocomposite film grown on alumina. It is found that the MnO/C nanocomposite coating exhibits a giant negative MR (22.3%) at a temperature as high as 100 K, which is unusual because pure MnO is anti-ferromagnetic and does not ordinarily show any magnetoresistance (MR), while amorphous carbon is known to show a small MR at very low temperatures (~7 K), due to weak-localization. The present results mean that a mechanism other than weak-localization plays a role in this nanocomposite material. Further study of this material is called for, which can perhaps lead to giant magnetoresistance (GMR) at room temperature in a metal-oxide/carbon nanocomposite. A summary of the work and an outlook for further research are given in the concluding chapter 6.

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.

Electrochemical capacitors or supercapacitors or ultracapacitors are potential energy storage devices that could help bringing major advances in future energy storage applications. Unlike batteries that store energy in chemical reactants capable of generating charge, electrochemical capacitors store energy directly through charge separation. Most electrochemical capacitors rely on carbon-based structures utilizing electrical double-layer capacitance effect. By contrast, a pseudocapacitor relies on charge stored due to fast faradaic charge-transfer processes with surface atoms. A combination of faradaic and non-faradaic components would generate hybrid electrochemical capacitors or hybrid ultracapacitors that attain high capacitance for pulse power and sustained energy. This thesis comprises studies pertaining to design, development and applications R&D on substrate-integrated lead-carbon hybrid ultracapacitors. The thesis comprises ten chapters. Chapter 1 is a brief introduction on essentials of electrochemical capacitors explaining their operating principles, classification and applications. Chapter 2 describes studies on materials for electrical double-layer capacitors. Activated carbons are the most common materials for electrical double-layer capacitors. Various activated carbon samples are screened as suitable materials for electrical double-layer capacitor followed by their optimization under varying experimental conditions to form the negative plate in the substrate-integrated lead-carbon hybrid ultracapacitor. Chapter 3 deals with the studies on design and development of 2 V substrate-integrated lead-carbon hybrid ultracapacitors with flooded, absorbent-glass-mat and silica-gel sulfuric acid electrolyte configurations. Lead-carbon hybrid ultracapacitors comprise substrate-integrated lead dioxide sheets as positive plates and high surface-area-carbon-coated graphite-sheets as negative plates. Operating principle for 2 V lead-carbon hybrid ultracapacitors is explained and optimization of their operating conditions along with their electrochemical performance is studied. Chapter 4 is a study on the integration of 2 V substrate-integrated lead-carbon hybrid ultracapacitors to 12 V devices. 12 V substrate-integrated lead-carbon hybrid ultracapacitors with flooded, absorbent-glass-mat and silica gel sulfuric acid electrolyte are developed by connecting six 2 V cells in series. These hybrid ultracapacitors exhibit high power-density values and excellent cycle-life. The problem of uneven performance among the six 2 V cells in the 12 V hybrid ultracapacitors is addressed and resolved by applying voltage-management cell-balancing circuitry. Chapter 5 details the studies on kilo-Farad range 12 V substrate-integrated lead-carbon hybrid ultracapacitors. The hybrid ultracapacitors are performance tested through a standard protocol. Thermal runaway in these hybrid ultracapacitors at high load currents is studied by thermal imaging. Studies on performance comparison between 12 V lead-carbon hybrid ultracapacitors with substrate-integrated and conventional pasted-positive plates are presented in Chapter 6. For substrate-integrated-positive plate lead-carbon hybrid ultracapacitors, capacitance and energy-density values are lower but power-density values are higher than pasted-positive plate configuration due to their shorter response-time. Accordingly, internal resistance values are lower for substrate-integrated lead-carbon hybrid ultracapacitors. Both types of lead-carbon hybrid ultracapacitors exhibit similar faradaic efficiency and cycle-life in excess of 100,000 pulse charge/discharge cycles with only a nominal loss in their capacitance values. Chapter 7 is a study on the design and development of low-cost substrate-integrated lead-carbon hybrid ultracapacitors using poly-aniline organic metal. The hybrid ultracapacitor employs flexible exfoliated graphite sheets as negative plate current-collectors, which are coated with a thin layer of poly-aniline to provide good adhesivity to activated carbon layer and good substrate-conductivity. These ultracapacitors are estimated to cost about 4 US$/Wh as compared to 20-30 US$/Wh for presently available commercial ultracapacitors. In Chapter 8, an application R&D study on the suitability of a substrate-integrated lead-carbon hybrid ultracapacitor bank in powering medical gadgets is described. A practical application that provides 30 W power back-up to medical gadgets for use in grid-power-deficient rural areas is presented. Chapter 9 is another application R&D study in realizing a photovoltaic stand-alone lighting system using substrate-integrated lead-carbon hybrid ultracapacitors. At present, harnessing solar electricity generated through photovoltaic cells with lead-acid batteries remains the most compelling option. But lead-acid batteries have encountered problems in photovoltaic installations, mainly due to their premature failure. To circumvent this problem, substrate-integrated lead-carbon hybrid ultracapacitors are developed for solar energy storage for a lighting application. The last Chapter of the thesis comprises field studies on substrate-integrated lead-carbon hybrid ultracapacitors. In the study, hybrid ultracapacitors are installed for lighting applications for field tests. Grid-power chargers and mechanical dynamos are introduced as fast-charging tools for hybrid ultracapacitors. It is hoped that the studies presented in this thesis would constitute a worthwhile contribution to science and technology of electrochemical capacitors. Considering the technology need, availability, safety and cost, substrate-integrated lead-carbon hybrid ultracapacitors are set to play a seminal role in future energy storage and management.