Systems for large-scale electrical energy-storage with low environmental impact deriving from pyrolysis of biomass waste

The increasing energy demands, and environmental concerns associated with fossil fuel consumption have accelerated the transition towards renewable energy sources such as solar, hydro and wind. But the intermittent nature of these renewable energy sources desires the development of more reliable energy storage devices. In recent years, electrochemical energy storage devices such as batteries, fuel cells and supercapacitors have been explored tremendously due to their pivotal role in addressing these challenges. Among these, supercapacitors have gained appreciable attention due to their high specific power, longer cycle life and stability. Supercapacitors have been significantly utilized in applications requiring quick energy delivery and energy storage, for example backup power supplies, hybrid vehicles and load balancing in power grids. Additionally, supercapacitors shine as sustainable and cost-effective energy storage devices, owing to their reliance on green materials and efficient, straightforward methods for preparing electrode and electrolyte materials. Among various electrode materials, biomass-derived carbons have been extensively used as highly efficient electrodes for supercapacitors due to the adjustable parameters such as specific surface area, pore size distributions. In this thesis, the activated carbon electrodes are prepared by synthesizing different biomass wastes such as asparagus and melon waste. The asparagus waste-based carbon powders were prepared by chemical/physical activation in the presence of an inert atmosphere. In this case, the raw asparagus waste was impregnated with a widely known chemical activating agent ZnCl2 in different ratios followed by a carbonization at 800 °C in the presence of Ar gas. The optimized activated carbon, prepared by impregnating asparagus waste with ZnCl2 at a 1:2 ratio, exhibits the highest BET surface area and an advantageous pore size distribution, resulting in superior performance. On the other hand, a comparative analysis was performed on the activated carbons, pre-treated differently before carbonization, derived from melon waste. In this work, the raw melon waste was pre-treated namely with hydrothermal and ethanol soaking before chemical/physical activation. For the comparison, the carbon powder was also prepared via simple chemical/physical activation as before. Among these three, the hydrothermally pre-treated activated carbon before chemical/physical activation exhibits the highest specific surface area and suitable pore size distribution for high performance supercapacitors. Electrolyte plays a vital role in ion transport and directly affects the performance of the supercapacitors. An ideal electrolyte must have high ionic conductivity, wide potential stability window, wide range of thermal stability, and environmental friendliness, while being cost-effective. Among various electrolytes, gel polymer electrolytes (GPEs) have emerged as a promising alternative due to their high ionic conductivity, flexible nature, and excellent thermal and mechanical stability. In this thesis, the prepared biomass derived carbon electrodes were electrochemically tested with GPEs for high energy density supercapacitors. The asparagus waste derived carbon electrodes were assessed with the well-known aqueous electrolyte i.e., 7 M KOH and ionic liquid based GPE, and it has been found that the GPE based supercapacitor exhibits almost 4 times increase in energy density which proves the advantage of GPE instead of aqueous electrolytes. Hence, the melon-based carbon powders were tested with ionic liquid based GPE. In this thesis, two kind of GPEs were prepared based on different ionic liquids immobilized in synthetic polymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP). But these prepared GPEs still have concerns about their cost and sustainability apart from their high ionic conductivity and wide electrochemical potential window. Hence, the GPE based on the natural bio-polymer pectin and ionic salt i.e., lithium chloride (LiCl) was prepared as a cost effective and sustainable gel electrolyte for biomass derived carbon electrodes-based supercapacitors. Due to limited ion transport mechanism and naturally rigid nature of pectin it has lower ionic conductivity as compared to synthetic polymers. So, to overcome these challenges an appropriate amount redox additive i.e., potassium iodide (KI) was introduced in the gel electrolyte film which substantially enhances the performance of the gel electrolyte film. The fabricated supercapacitor (based on melon waste derived carbon electrodes) with this redox additive gel electrolyte film shows an enormous increase in specific capacitance and energy density almost 4 to 5 times as compared to the gel electrolyte film with no redox additive. Hence, at last a more “greener and sustainable” high performance supercapacitor was successfully fabricated with biodegradable polymer pectin based gel electrolyte film and biomass derived carbon electrodes.