Construction of Polymer Electrolyte Based on Soybean Protein Isolate and Hydroxyethyl Cellulose for a Flexible Solid-State Supercapacitor

Supercapacitors are a very active research topic. However, liquid electrolytes present several drawbacks on security and packaging. Herein, a gel polymer electrolyte was prepared based on crosslinked renewable and environmentally friendly soybean protein isolate (SPI) and hydroxyethyl cellulose (HEC) with 1.0 mol L−1 Li2SO4. Highly hydrophilic SPI and HEC guaranteed a high ionic conductivity of 8.40 × 10−3 S cm−1. The fabricated solid-state supercapacitor with prepared gel polymer electrolyte exhibited a good electrochemical performance, that is, a high single electrode gravimetric capacitance of 91.79 F g−1 and an energy density of 7.17 W h kg−1 at a current density of 5.0 A g−1. The fabricated supercapacitor exhibited a flexible performance under bending condition superior to liquid supercapacitor and similar electrochemical performance at various bending angles. In addition, it was proved by an almost 100% cycling retention and a coulombic efficiency over 5000 charge–discharge cycles. For comparison, supercapacitors assembled with commercial aqueous PP/PE separator, pure SPI membrane, and crosslinked SPI membrane were also characterized. The obtained gel polymer electrolyte based on crosslinked SPI and HEC may be useful for the design of advanced polymer electrolytes for energy devices.