Last modified: 2019-10-08
Abstract
The increasing demand to develop a renewable energy has increased drastically due to limited resources and environmental problems of fossil fuel. However, the focus have been shifted to electrical energy to smooth the intermittency of the energy sources. The most widely used energy storage devices are batteries and capacitors. The latest research of battery has demonstrated better battery with higher energy density and longer life cycle but struggles with power densities. While the limited application of capacitors caused by the small amount of energy thus can be charged and discharged easily and deliver pulses of energy.
Supercapacitors, on the other hand, can induced higher power density by fast charging/discharging rate which resulting of higher power density and longer cycle life compared to batteries and fuel cells. Supercapacitor consists of electrodes (cathode and anode), electrolyte, and separator to separate the electrodes. To fulfill the demand of safer energy sources, the use of highly flammable organic liquid electrolytes and polymer separators must be replaced by solid electrolytes. Therefore, supercapacitors with solid electrolytes can be an alternative renewable energy.
Commercially, there are three types of electrolytes of supercapacitors: aqueous electrolytes, organic electrolytes, and ionic liquids. In this paper, there are selective review on solid electrolytes of supercapacitors including chitosan, polyacrylamide, poly(aryl ether ketone), and polyethylene glycol. Overall, this paper aims to provide comprehensive reviews on recent advances in potential solid electrolytes of supercapacitors and the remaining challenges.
Chitosan is a non-toxic and biodegradable biopolymer composed of glucosamine and N-acetyl glusamine units glucosidic bonds. Chitosan is chosen as electrolyte due to the presence nitrogen and oxygen atoms which enables ionic conduction from the added salt. Earlier studies have shown that chitosan exhibits high retention ionic liquids and high electrochemical stabilitiy. Solid-state electrolytes have three types including solid electrolyte, gel polymer electrolyte, and polyelectrolyte. However, solid electrolyte has low conductivity (10-8 and 10-7 S/cm). Therefore, Cao et. al. chose to synthesis a biopolymer-chitosan based supramolecular hydrogel solid-state electrolyte since gel polymer electrolyte has higher ionic conductivity in order of ̴ 10-3 S/cm at ambient temperature. Their work showed that chitosan based hydrogel solid-state electrolyte has high ionic conductivity with temperatur ranges from 313 K and 353 K increases from 1.6 x 10-3 S/cm to 4.6 x 10-3 S/cm and high cyclic stability.
Polyacrylamide is a synthetic polimer known for its high hidrophilicity formed by polymerization af acrylamide monomer by a catalyst with bifunctional cross linking agents. Polyacrylamide has considerably high ionic conductivity of 1.7 x 10-2 S/cm but still considered to be lower than commercialized PVA/H2SO4. Recent research from Li et. al. proposed simple approach method to synthesis ionic conducting polyacrylamide hydrogels as solid-state electrolyte. The results showed that polyarclamide as solid-state electrolyte of supercapacitor could exhibit high specific capacitance, long self-discharge time, and low leakage.
Poly(aryl ether ketone) is one of semicrytalline thermoplastics posseses excellent mechanical properties, good environmental resistance, thermo-oxidative stabilitiy, chemical and radiation resistance. While polyethylene glycol is a biocompatible poly(ether) which highly soluable in aqueous solutions and organic solvents. Na et. al. Suggested solid-state polymer electrolyte by combinining poly(aryl ether ketone)-polyethylene glycol copolymer as a polymer host and LiClO4 as an electrolyte salt aiming to provide alternative to make a flexible solid-state supercapacitor. The results showed that the solid-state supercapacitor has excellent electrochemical performance especially at high temperature, high spesific capactance, and high energy density.
The recent advances of solid-state electrolytes to produce a solid-state supercapacitors have showed that there are several methods and approachments which are needed in orded to come up with better solid-state supercapacitors as energy storage thus yet still needed to be improved and developed by tuning either the electrode mass ratios, increasing the ionic conductivity, energy density, and capacitance.