Non-toxic gel electrolytes and their use in printable supercapacitors

Abstract

Supercapacitors are devices where energy is stored to an electric double layer. The double layer formation takes place at the interface of an electronically conducting electrode and an electrolyte, which is an ionic conductor. High surface area materials such as activated carbon and carbon nanotubes are used to maximize the area of this interface which directly increases the capacitance of the device. In this work the goal was to design a non-toxic gel electrolyte that could be used in flexible printed supercapacitors. The main focus was in natural, biodegradable polymers from which hydrogels were fabricated with an aqueous salt solution to provide the ions required in the electrolyte. The ionic conductivity was compared to the liquid electrolyte traditionally used in the printable supercapacitors studied in the research group. In addition to high ionic conductivity of the gel, mechanical strength and elasticity were among the main requirements for the gel as it is expected to be able to substitute the traditionally used combination of a liquid electrolyte and a physical separator layer. Impedance spectroscopy was used to study the electrical properties of the gels. Starch and gelatin samples with varying sodium chloride concentrations were measured and a long term stability study was conducted on the best performing alternatives. Best results in terms of mechanical performance and electric properties were reached with a gelatin hydrogel manufactured with 2 M sodium chloride solution. High sodium chloride concentrations were verified to weaken the gelling capacity of the gelatin. Conductivities close to that of the 1 M sodium chloride reference solution were reached while still maintaining satisfying mechanical performance, but achieving higher values of ionic conductivity is challenging without compromising the mechanical properties.