Integration of a Flexible, Stretchable, Environment-Benign, and Highly Conductive PVA/H3PO4 Hydrogel as a Quasi Solid-State Electrolyte in Reduced Graphene Oxide Supercapacitors

The emerging demands for portable and miniaturized energy storage devices stimulated the use of lightweight electrodes and solid-state electrolytes. Due to its high specific surface area and porosity, a binder-free, multilayer reduced graphene oxide (rGO) electrode is promising for fabricating supercapacitors. Concomitantly, gel polymer electrolytes (GPEs) are inherently safe, cost-effective, and easily integrated in these devices. However, drop casting of a viscous GPE on top of a thick electrode followed by drying leads to poor infiltration (partial electrode wetting) and prevents the full utilization of the electrode's electrochemical sites. Further, a sandwiched freestanding GPE film between electrodes may slide off and detach after repeated use due to its poor adhesion with the electrode. To address these issues, this work demonstrates integrating a poly(vinyl alcohol)/phosphoric acid (PVA/H3PO4) hydrogel in rGO-based solid-state supercapacitors (rSCs). Aqueous H3PO4 improves the electrode's wettability, and stickiness enhances the hydrogel's adhesion to the electrode, conducive to double-layer formation at the electrode/electrolyte interface. Moreover, the hydrogel has an excellent ionic conductivity (30 mS cm–1) and is flexible. This work investigates the electrochemical performance of rSCs fabricated using a freestanding PVA/H3PO4 hydrogel. It was observed that a hydrogel-based rSC achieved a high cell voltage (0.7 V), specific capacitance (187.4 mF cm–2), and energy and power density (9.7 mW h cm–2 and 0.35 mW cm–2, respectively), at a current density of 2 mA cm–2 depicting the hydrogel as a potential quasi solid-state electrolyte for rGO supercapacitors.