Poly-γ-glutamic Acid Hydrogels as Electrolyte for Poly(3,4-ethylenedioxythiophene)-Based Supercapacitors

Biosynthetic poly-gamma-glutamic acid (gamma-PGA) has been used to produce hydrogels using cystamine as cross-linker and 1-[3-(dimethylamino)-propyl]-3-ethylcarbodiimide methiodide (EDC methiodide) as condensing agent. Eight different hydrogels with different properties were formulated by varying both the molecular weight of gamma-PGA and the gamma-PGA/EDC/cystamine ratio and subsequently characterized. The most appropriate gamma-PGA hydrogel was selected to perform as solid electrolytic medium in organic electrochemical supercapacitors (OESCs) using poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes based on their mechanical behavior (consistency and robustness to hold the PEDOT electrodes), morphology, and influence on the electrochemical response of the organic electrode (i.e., specific capacitance and both maximum energy and power density values). Hence, PEDOT/gamma-PGA energy storage devices fabricated using the most adequate hydrogel formulation displayed a supercapacitor response of 168 F/g and a capacitance retention of 81%. Moreover, after evaluating the maximum energy and power densities (Ragone plot), cyclability, long-term stability, leakage current, and self-discharging response of PEDOT/gamma-PGA OESC devices, results allow us to highlight the merits and great potential of gamma-PGA hydrogels as sustainable ion-conductive electrolytes for environmentally friendly energy storage technologies.