Transition metal oxides graphene composites for supercapacitor electrodes

Abstract

Abstract According to World Energy Council projections, current global energy requirements are expected to double by the year 2050. At present, nearly 85% of the primary global energy supply is derived from fossil-based resources, including petroleum, coal, and natural gas. In view of the current environmental conditions, sustainable and green energy storage solutions are the need of the hour. Supercapacitors have emerged as a promising green energy storage solution with high power density. Supercapacitors also offer high cyclic stability with good capacitance retention. The low energy density of supercapacitors restricts their usage in most practical domains. In order to achieve high energy density along with high power density and high cyclic stability in wide working potential range an effective solution has been suggested by the assembling of hybrid supercapacitors (symmetric and asymmetric). Owing to the distinct potential ranges of the cathode and anode, the asymmetric arrangement requires the operating voltage of the device to be expanded. Graphene and nitrogen doped graphene based TMO composites offer synergistic advantages that include increased specific surface area, improved interfacial charge transport, elevated specific capacitance, tailored morphologies, and enhanced energy density. Collectively, these innovations have significantly boosted the electrochemical performance and practical scalability of Transition Metal Oxide@ graphene composites for high-efficiency energy storage systems. The entire work is presented in ten chapters which are as follows: Chapter 1 introduces briefly the ever increasing problem of global energy demands, supercapacitor as an electrochemical energy storage solution, its types and components. The importance and significant characteristics of nitrogen doped graphene and NiCo2O4 has been presented. The role of third metal incorporation candidates for ternary T-NiCo2O4, (T = Mo, V and Zn), their substitution sites based on crystal field stabilization theory and finally the significance of transition metal oxides @ graphene composites has been discussed. Chapter 2 describes the details of literature related to development of advanced materials for high-performance energy storage systems, particularly supercapacitors. This chapter presents a critical review of the literature pertaining to electrode materials, with a focus on nitrogen-doped graphene, binary transition metal oxides such as nickel cobalt oxide (NiCo2O4), and ternary metal oxides containing metals like molybdenum (Mo), zinc (Zn), and vanadium (V) as third metal in NiCo2O4. Chapter 3 presents the details of precursors and reagents which are used for synthesis of samples. The morphological, structural and physio-chemical characterizations used to study the physical and chemical properties of the synthesized samples are explained in detail. All the electrochemical techniques used to analyze the supercapacitive performance of the samples are explained with useful formulas. The method of preparation of active catalyst and device fabrication for electrochemical mearsurements is highlighted. Chapter 4 presents the results of systematic study for optimization of synthesis parameters for N doped graphene (NG) with appreciable range of N content. Microwave synthesis has been adopted to synthesize N-doped graphene in the present work. The samples were synthesized at different weight ratio of GO:urea. To understand the properties of synthesized samples, FESEM, XRD, RAMAN, N2 adsorption-desorption isotherm, XPS studies were carried out. The study also examines how different nitrogen bonding configurations, along with the ionic mobility and hydrodynamic sizes of the cations and anions in various aqueous electrolytes (0.5 M H2SO4; acidic, 0.5 M K2SO4; neutral and 0.5 M KOH; alkaline), influences the electrode's performance. The best optimized NG sample was chosen to make composites with transition metal oxides for further study and is presented in next chapters. Chapter 5 deals with the optimization of hydrothermal reaction parameters for synthesizing NiCo2O4 nanorods for high supercapacitive performance. Further, a systematic study has been put forth to highlight the effect of varying amount of NG sheets on the morphology and the resulting supercapacitive behavior of the composite nanoflowers formed from NiCo2O4 nanorods. To understand the properties of synthesized samples, FESEM, XRD, RAMAN, N2 adsorption-desorption isotherm and XPS studies were carried out. The samples are tested for supercapacitive performance using CV, GCD and PEIS. ECSA was calculated to determine the number of electrochemically active sites. Dunn’s method was used to determine the percentage contribution from redox controlled and double layer controlled capacitance. The study results in the composite with optimal NG sheets weight ratio having superior supercapacitive performance. This was fixed in the synthesis of ternary transition metal oxides in the further chapters. Chapter 6 presents hydrothermal synthesis approach employed to synthesize MoNiCoO and MoNiCoNG composites at varying synthesis temperatures, enabling a systematic investigation of their morphological and structural features. The NG sheets played a pivotal role in directing the growth of marigold-like nanoflowers composed of wavy, lamellar nanosheets, enhancing both structural integrity and electrochemical performance. To understand the properties of synthesized samples, FESEM, XRD, RAMAN, N2 adsorption-desorption isotherm, XPS studies were carried out. The samples are tested for supercapacitive performance using CV, GCD and PEIS. ECSA was calculated to determine the number of electrochemically active sites. Dunn’s method was used to determine the percentage contribution from redox controlled and double layer controlled capacitance. Chapter 7 discusses the effect of vanadium incorporation in nickel cobalt oxide at varying molar ratios systematically. The introduction of V induced lattice distortions due to the difference in ionic radii between Ni, Co, and V, initiating the formation of a V-NiCo2O4 and Co3V2O8 phase. Further the best synthesized oxide was attached with porous carbon framework: N-doped graphene (NG) and Carbon Black (CB) which significantly altered the structural evolution and electrochemical activity. To understand the properties of synthesized samples, FESEM, XRD, RAMAN, N2 adsorption-desorption isotherm, XPS studies were carried out. The samples are tested for supercapacitive performance using CV, GCD and PEIS. ECSA was calculated to determine the number of electrochemically active sites. Dunn’s method was used to determine the percentage contribution from redox controlled and double layer controlled capacitance. Chapter 8 presents a systematic study of Zn incorporated nickel cobalt oxide (Zn-NiCo2O4) composites integrated with NG synthesized via hydrothermal method The incorporation of NG sheets provided abundant nucleation sites and altered the reaction kinetics, leading to the formation of Zn-NiCo2O4@NG nanoflowers morphology. To understand the properties of synthesized samples, FESEM, XRD, RAMAN, N2 adsorption-desorption isotherm, XPS studies were carried out. The samples are tested for supercapacitive performance using CV, GCD and PEIS. ECSA was calculated to determine the number of electrochemically active sites. Dunn’s method was used to determine the percentage contribution from redox controlled and double layer controlled capacitance. Chapter 9 details the hybrid supercapacitor coin cell devices developed utilizing the best optimized samples identified from the preceding chapters. These devices were assembled in both symmetric and asymmetric configurations, and their electrochemical performance was systematically evaluated under various testing conditions. Further, two combination cells (MoNiCoNG//VNiCoCB and ZnNiCoNG//VNiCoCB) were also fabricated in order to check the supercapacitive performance parameters for enhanced voltage range leading to superior energy density. Chapter 10 presents the conclusions drawn from the work done. It further describes the future prospects that could lead more advanced and efficient next generation hybrid supercapacitors.