An Edible Quasi-Solid Electrolyte Derived from Agarose Gel for High-Performance, Flexible Supercapacitors

Abstract

As one of the energy storage devices (ESD), supercapacitors have attracted considerable attention with their remarkable advantages such as fast charge-discharge rate, long cycle life, and environmental friendliness in nature. To improve their performance in terms of higher energy and power densities, their unit components, such as an electrolyte and electrode materials, have to be improved in preference. In particular, electrolyte serves as an ion transport media between anode and cathode as well as being a key parameter for deformable attributes in cell configuration. Therefore, a focused study on the electrolyte geometry allows further pursuit of application, in accordance with current demand for transformable compact power sources. Consequently, development of suitable electrolyte should be oriented under the notion of combining high performance and shape adapting features, mild synthesis condition involving negligible vapor pressure and flame retardancy. Agarose gel is a soft, deformable waterborne gel, featuring high elasticity and eco-friendliness with low costs. Inside such agarose based hydrogels, the interconnected porous network filled with an aqueous medium provides an ion reservoir, hence facilitating the diffusion of charged molecules (DNA, proteins and ions) under an electric field. On the basis of this phenomenon, agarose-based hydrogels have been widely used in the fields of food production, biological research and various gel-based electronic devices. However, practical applications of agarose gels as an electrolyte material in supercapacitor configurations have not been reported. The dissertation referred to herein, contains physico-chemical characterization of supercapacitor assembled with agarose-NaCl hydrogel and MnO2 electrodes, with focusing on a finding optimal status of electrode, optimal synthetic conditions of electrolyte based on facile fabrication steps to exhibit desirable performance in flexible supercapacitor. The findings show that employing agarose-NaCl gel electrolyte within the assembly of supercapacitor has noticeable advantages: (1) the preparation of the gel electrolyte consists of relatively simple steps such as solution-mixing, heating and casting in moderate conditions, which are facile, non-toxic and cost competitive fabrication. (2) The shape of the gel, including size and thickness, can easily be tuned by using razor blade method and the number of glass blocks. Furthermore, with the aid of foresaid gadgets, the gel can be transferred onto most of substrates regardless of material types such as glass, stainless steel and plastic plates, which reveals the use of such gel is industrially feasible. (3) Flexible supercapacitor can be fabricated by introducing the polymeric current collector and our gel together. This assures the gelation-based full cell build-up of a promising method for the fabrication of enlarged flexible devices. On the basis of above-mentioned process, the supercapacitor incorporating NaCl - agarose gel electrolyte was assembled. The as-prepared agarose hydrogel consists of a 3-dimensionally interconnected agarose backbone and inter-particular submicropores filled with water and sodium, chloride ions. As a framework, the agarose matrix provides structural stability of the gel electrolyte. Furthermore, the developed porous networks with the water filler provide an efficient ion transport pathway between anode and cathode electrodes. In accordance with these properties, the quasi-solid gel facilitates the assembled supercapacitor to have a specific capacitance of 286.9 F g-1 under a MnO2 symmetric cell, and high rate capability that is 80 % of specific capacitance compared to a liquid electrolyte (NaCl aqueous solution) at a scan rate of 100 mV s-1. In addition, linking with high capacitive performance, the gel electrolyte features highly scalable, cost effective, safe and environment-friendly characteristics, which are attributed to the simple procedure and easily available components of the agarose gel. Hence, we concluded that the developed quasi-solid gel is promising material for use in various energy storage and delivery systems.