Highly Flexible <scp>OPBI</scp> Membrane as Solid‐State Electrolyte for Organic‐Polymer‐Based Supercapacitor Applications

ABSTRACT In this work, we design poly(4,4′‐diphenylether‐5,5′‐bibenzimidazole) (OPBI) as a promising electrolyte membrane for high‐performance flexible supercapacitors. The solid‐polymer electrolyte membrane is designed by immersing the OPBI membrane in phosphoric acid (H 3 PO 4 ) and it was paired with polyaniline (PANI)‐treated multiwall carbon nanotubes (TMWCNTs) or poly(3,4‐ethylenedioxythiophene)‐poly (styrene sulfonate) (PEDOT: PSS) with TMWCNTs as the active electrode materials. Specifically, the PANI/TMWCNT‐based supercapacitor, integrated with the OPBI membrane electrolyte (OPBIME), demonstrated a high specific capacitance (Cs) of 519 F/g at a current density of 2.2 A/g. In contrast, a conventional cellulose‐PVA gel electrolyte yielded a reduced specific capacitance of 405 F/g under the same conditions. Further testing with PEDOT: PSS/TMWCNT electrodes and OPBIME resulted in a specific capacitance of 14 F/g at a current density of 0.5 A/g, along with excellent capacitance retention of 92% after 2000 cycles. Overall, these findings highlighted the dual role of OPBI membranes as both an electrolyte and a separator, significantly improving the performance of flexible supercapacitors. Their high ionic conductivity, electrochemical stability, and flexibility enable superior specific capacitance and long‐term cycling stability, positioning them as a promising electrolyte for next‐generation energy storage applications.