Designing High-Performance Symmetric Supercapacitors from Waste Jute and Bio-electrolyte

Abstract

Electronic technologies, particularly cell phones to electronic cars, have been essential in our everyday needs. However, the consequences of the energy demand at a high rate to power up these devices severely affect our biodiversity.  We need to design highly efficient, clean, and renewable energy storage systems to address these issues. Supercapacitors are one option for clean and renewable energy devices, minimizing the gap between batteries and traditional capacitors. The potential demand for supercapacitors is growing in hybrid electrical devices [1]. Attempts have been made on enhancing the energy-storage capability of electric double layer (EDL) based supercapacitors while maintaining a high-power density. The biomass-derived carbonaceous materials are promising in performance compared to the conventional graphene-based materials (GO, rGO), single-wall carbon nanotube (SWCNT), or multiwall carbon nanotube (MWCNT), etc. for their specific morphological features [2]. Generally, the features include the large surface area, highly porous nanostructures with large pore volume [2]. Additionally, the biomass-derived carbonaceous material offers waste mitigation issues in environmentally-friendly forms. However, the synthesis of carbonaceous materials from biomass via chemical or physical carbonization for supercapacitor anode electrodes seems especially attractive but suffers from lower yield and cost issues. Besides, the carbonization process requires massive sophistication, especially the precursors' pre-treatments, carbonization temperatures, exposure time, activating agents, and gas flow during the pyrolysis process [3, 4]. Instead, the hydrothermal process to convert biomass precursors into carbonaceous material is expected to be feasible than the conventional carbonization process. The hydrothermal process usually deals with convenient reaction conditions like- lower temperature, lesser precursor treatments, and facile incorporation pathway to dope other materials into the biomass precursor [5].

Now, as all the biomass precursors are structurally and morphologically different, without performing proper electrochemical experiments, there are no other practical methods to predict the electrode material's performance prepared from such resources. As a result, the electrochemical performance of supercapacitors from various biomass materials has been studied, including rice husk, sugar cane bagasse, potato starch, bamboo, eggshell membranes, etc. [6–8]. Similarly, jute is an agricultural by-product, and the abundance of cellulose and hemicellulose contents makes jute a prospective source to obtain electrode material for supercapacitor applications [9]. In this work, waste jute fiber is hydrothermally treated into a stainless-steel autoclave at 220^oC for 24 h to get nano-carbon electrodes for supercapacitor application. The pre-treated jute fiber evolves reactive gases at this high temperature, and pressure in the closed tube allows the introduction of various porosity into the carbonaceous material. Camila Zequine group chemically activated (KOH as activating agent) jute fiber to fabricate a carbon-based supercapacitor. They have found a specific capacitance of 185 Fg^-1 at 500 mAg^-1 and at an energy density of 21 Whkg^-1 in a standard three-electrode system using 3M KOH electrolyte [8]. Most of the early works reported low specific capacitance, short cycle life, and lower energy density.

However, the fundamental trick to increasing the electrochemical potential window thus energy density is selecting appropriate electrolytes. Different electrolytes have been reported to design supercapacitors, including aqueous, organic, ionic liquid, and polymer-gel electrolytes. Though aqueous electrolytes are the most widely used, conventional aqueous electrolytes have limited potential windows and biocompatibility [10–12]. In this context, bio-electrolytes could open captivating prospects to encourage future generation modern electrolytes. Compared with conventional aqueous electrolytes, bio-electrolytes like sodium acetate show attractive benefits, such as higher energy density, biocompatibility, biodegradability, and lower cost [13]. They are living organism-generated biomolecules and also waste materials of various industries like medical and pharmaceuticals. It has compelled to bias research and development for harnessing clean energy and storage systems. Hence by limiting the O₂/H₂ evaluation, the sodium acetate electrolyte can broaden the thermodynamic stability of water. Generally, the acid equilibrium of water and the hydrogen bond between the hydrogen molecule to its nearest oxygen molecule in the water are the two significant factors responsible for water decomposition [10]. Since sodium acetate is very soluble in water, strong hydration of Na⁺ and C₂H₃O₂^- with water may lower the free water content attached to H⁺ to form H₃O⁺ ions. Moreover, lowering the hydrogen bond causes a strong O-H bond which results in a wide potential window [14]. To better understand the mechanism of how sodium acetate expands the potential window, one should consider the electrode-electrolyte interaction. The hydrothermally treated carbon contains hydrophilic surface groups (OH-, COO-, C-O-C, etc.), therefore, the sodium acetate molecules are absorbed on the carbon surface with their hydrophilic groups [9]. It results in an increase in the energy barrier of 1.3 eV for water molecules, where the energy barrier for Na⁺ of sodium acetate electrolyte to be absorbed in the carbon surface is lowered to 0.406 eV [10].  This lower electronic barrier for sodium acetate demonstrates that it is facile for Na⁺ to go to the carbon surface faster than water molecules. Thus, using aqueous sodium acetate electrolytes, the electrochemical window and specific capacitance of carbon-based supercapacitors are expected to be enhanced. Therefore, they have the potential to be used in supercapacitor devices as bio-electrolytes for ensuring a clean and sustainable environment. A Schematic illustration to the way of fabricating supercapacitor with waste jute carbon electrode with sodium acetate is shown in fig. 1.