Converting soy protein isolate into biomass-based polymer electrolyte by grafting modification for high-performance supercapacitors.

Soy protein isolate (SPI) is one of the most abundant plant proteins in nature. Enormous studies have been carried out on SPI-based materials, however, the application for energy storage devices is still limited due to its fiddly film forming process and lack of electrochemical performance. Herein, we presented a SPI-based polymer electrolyte by grafting modification with the hydrophilic functional monomer acrylamide (AM). The optimized gel polymer electrolyte (GPE) exhibited an excellent ionic conductivity up to 5.10 mS cm-1, which was greatly higher than gel polymer electrolyte based on original pure SPI (1.84 mS cm-1). More significantly, supercapacitors constructed by grafting-modified SPI delivered a specific capacitance of 141.74 F g-1 at 1.0 A g-1 and exhibited high capacitance retention of 95% after 8000 charge-discharge cycles. Beyond that, redox-active polymer electrolyte based on grafting-modified SPI combining with KI enormously improved the energy density of supercapacitor up to 27.52 Wh kg-1 at a current density of 0.5 A g-1. This work provided a novel and facile strategy to obtain a high-performance SPI-based polymer electrolyte and laid an experimental foundation for the high-value application of SPI in the field of energy storage devices.

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.

A Crosslinked Soybean Protein Isolate Gel Polymer Electrolyte Based on Neutral Aqueous Electrolyte for a High-Energy-Density Supercapacitor.

A crosslinked membrane based on renewable, degradable and environmentally friendly soybean protein isolate was formed by solution casting method. A series of gel polymer electrolytes were prepared with the crosslinked membranes saturated with 1 mol·L-1 Li2SO4. The solid-state electric double-layer capacitors were fabricated with the prepared gel polymer electrolytes and activated carbon electrodes. The optimized solid-state supercapacitor delivered a single electrode specific capacitance of 115.17 F·g-1 at a current density of 1.0 A·g-1, which was higher than the supercapacitor assembled with the commercial separator in 1 mol·L-1 Li2SO4. The solid-state supercapacitor exhibited an outstanding cycling stability, indicating that the gel polymer electrolyte based on the crosslinked soybean protein isolate membrane could be a promising separator for a solid-state supercapacitor.