Ion-Independent CO2 Reduction with Boron-Doped Diamond Electrodes for Sustained Formic Acid Production and In Situ Concentration

While the electrochemical reduction of CO2 to formic acid has been intensively investigated, conventional two-compartment electrolysis systems inherently rely on continuous ion migration to stabilize the electrolyte. Once supporting ions are depleted, production efficiency deteriorates, resulting in a fundamental limitation for long-term operation. Herein, we report a three-compartment CO2 electrolysis system that overcomes this intrinsic constraint and enables sustained formic acid production with reduced dependence on electrolyte-derived ion migration. By introducing a concentrate compartment between the anode and cathode compartments and employing boron-doped diamond (BDD) electrodes, stable electrolysis was achieved without external ion replenishment or continuous ionic flux. A Faradaic efficiency of 60–70% was maintained for more than 96 h under constant-current conditions, demonstrating the robustness of the system during prolonged operation. In addition to stable production, the proposed configuration simultaneously enabled in situ concentration of the product, yielding formic acid at approximately twice the concentration of that in the catholyte. Detailed analyses of ion transport behavior and pH evolution revealed a transition from charge compensation dominated by electrolyte-derived ion migration to that dominated by proton and hydrogen carbonate ion transport, without compromising production efficiency. These results demonstrate that sustained formic acid generation is feasible even after the completion of major ion transfer. The present three-compartment design provides a new strategy for decoupling CO2 reduction from electrolyte-derived ion migration, offering a scalable and energy-efficient approach for long-term carbon resource utilization.