Membrane-free two-step water electrolysis enabled by a stable organic redox mediator

Electrochemical decoupled water splitting utilizes a solid-state redox mediator (SSRM) to reversibly store proton-electron pairs, thereby separating hydrogen and oxygen evolution without the use of a membrane and promoting the conversion of sustainable energy sources into hydrogen. However, the widespread application of this approach is currently limited by issues such as the instability and discontinuity of SSRM. Herein, a novel and membrane-free system is designed for decoupled hydrogen production in an acidic medium, based on the use of a highly reversible SSRM on a sustainable quinone-based organic polymer (P-ACD). Due to intramolecular hydrogen bonds, the P-ACD electrode with a low P-ACD loading (1-2 mg cm^-2) exhibits a high discharge specific capability (215.32 mAh g^-1 at 0.2 A g^-1), ultrafast chargeability (86.90 mAh g^-1 at 100 A g^-1, just 3.12 s), and exceptional cyclability over 30 000 cycles. More importantly, two-step water electrolysis under high P-ACD loading achieves a remarkable coulombic efficiency of nearly 100% even after 600 consecutive cycles, whilst concurrently exhibiting favorable cycle stability for hydrogen production that surpasses 184 h. Consequently, this system provides a new avenue for achieving operational flexibility and membrane-free hydrogen production.