Green-synthesized PVA/Borax/MnO2-based flexible and self-healable all-in-one solid-state capacitor

This study presents a fully integrated, flexible, and self-healable All-in-One Solid-State Capacitor (SSC) fabricated through a simple, green two-stage gelation process using polyvinyl alcohol (PVA), manganese dioxide (MnO₂), and borax as the components, eliminating external electrolytes and separators. The symmetric device consists of PVA/MnO₂/borax composite electrodes and a PVA/borax solid electrolyte, assembled by water-assisted self-adhesion. Electrochemical characterization revealed Electric Double Layer Capacitor (EDLC)-dominated behavior with specific capacitances of 144.9 mF g⁻¹ (charge) and 103.8 mF g⁻¹ (discharge) at 10 mV s⁻¹, an operating voltage of 1.7 V, an energy density of 0.042 Wh kg⁻¹, and a power density of 0.88 W kg⁻¹. After 1000 charge–discharge cycles, the device retained 81% of its initial capacitance, with a Coulombic efficiency of up to 89%. Contrary to expectations, the pseudocapacitive contribution of MnO₂ was largely suppressed owing to the formation of secondary phases (MnB and Mn₂OBO₃) and restricted ion mobility in the solid matrix, shifting the charge storage mechanism toward the EDLC behavior. This green-synthesized AiO-SSC offers a sustainable, toxic-chemical-free alternative for flexible/wearable electronics, despite moderate energy density, and highlights the need for further optimization of MnO₂–borax interactions to recover pseudocapacitive performance.