POROUS ACTIVATED CARBON AND NICKEL HYDROXIDE AS ELECTRODES FOR SYMMETRIC AND HYBRID SUPERCAPACITOR DEVICES

Abstract

Supercapacitors offer high energy density without compromising the high-power density, and they are considered as an efficient energy storage option. During the last decade, there has been a tremendous surge in understanding the mechanism of charge storage in different types of materials for developing them electrodes for supercapacitor devices. It is learnt that the type of materials plays very important role in the performance of a supercapacitor. In view of this, we attempted to contribute on developing different types of materials as supercapacitor electrodes. Broadly, we present here two types of materials, i.e., activated carbon derived from biomasses as EDLCs and some layered Ni(OH)2 as battery type supercapacitors. The thesis is divided into identifying the research objectives by analyzing literature, strategies for synthesis and characterization of electrode materials, and electrochemical studies of the electrode materials along with, including evaluation of device performances. Two batches of porous activated carbon were prepared from the wastes of Eucalyptus wood and rice straw by pyrolysis followed by thermal treatment with KOH. These batches are referred to as PACE and ACRS, respectively. The rice straw (ACRS) and Eucalyptus wood (PACE) biomasses were selected on the basis of their contrasting composition in terms of carbon and oxygen containing constituents, e.g., cellulose, hemicellulose and lignin. Therefore, these studies highlighted the role of chemical functionalities and composition based on different biomass precursors in addition to porous nature of activated carbon. The salient feature of the strategy in synthesis of these two batches of materials was to retain the oxygen functionalities in the activated carbon, and hence the temperature was kept at optimal minimum during thermal activation. Unlike commonly reported thermal heating at 800 oC or above for carbonization, we present here a systematic study showing optimum specific capacitances by PACE and ACRS heated at less than or equal to 700 oC. These batches exhibited porous nature, evident from transmission electron microscopy and BET surface area studies. The compositional analysis by X-ray photoelectron spectroscopy and FT-IR revealed sufficient oxygen functionalities which imparted hydrophilic character to the electrode for better interaction with electrolyte during charging-discharging process. The cyclic voltammetry (CV) and galvanostatic charging-discharging (GCD) measurements demonstrated capacitive type mechanism. The maximum capacitance was obtained for PACE-600 (284 F g−1) and ACRS-700 (365 F g−1), recorded at 0.5 A g−1. The capacitance retention of PACE-600 and ACRS-700 studied for 8,000 and more cycles of charging-discharging cycles recorded at 10.0A g−1, is promising for real applications. The fabrication of high-voltage symmetric supercapacitor (SSC) in 2M NaCl is presented for the best performing batches of PACE and ACRS. In addition, we present device performance study in non-aqueous electrolyte using ACRS electrode. For this, tetraalkylammonium based ionic liquid ([Bu4N+][NO3 −]) as non-aqueous electrolyte was synthesized and characterized. Next we have discussed about the supercapacitor studies of layered Ni(OH)2 synthesized by a simple thermal decomposition at 180 oC via refluxing in the presence of different aliphatic diols, e.g., 1,2-ethanediol, 1,4-butanediol and 1,6-hexanediol as solvent and also as intercalating agent. The respective batches are named as [Ni(OH)2]ED, [Ni(OH)2]BD and [Ni(OH)2]HD. In this work, the role of intercalation of different aliphatic diols on the structural and supercapacitor properties of layered Ni(OH)2 are studied. The CV and GCD measurements demonstrated battery type behavior. The aqueous and quasi-solid-state SSC type charge storage device using the best performing [Ni(OH)2]HD has also been constructed.