Concentration-gradient strategy for controllable cell voltage in electrocatalytic reduction of CO₂ to high-concentration formate

In CO₂ electroreduction systems using neutral or alkaline electrolytes with cation exchange membranes, the rising cell voltage during operation increases energy consumption, limits formate accumulation, and compromises long-term stability. In this work, an H-type cell was employed as a model to systematically investigate both neutral KHCO₃ and alkaline KOH electrolyte systems. It was elucidated that K⁺, acting as the primary charge carriers, continuously migrates from the anode compartment to the cathode compartment. This migration results in the development of a concentration gradient and an associated increase in diffusion potential, ultimately leading to a continuous rise in the overall cell voltage. Based on this understanding, a “concentration-gradient” strategy was proposed, in which the concentration of the anolyte is increased to alleviate the conflict between formate accumulation and rising cell voltage. This strategy effectively limits cell voltage fluctuations within ±0.5 V and enables continuous and stable operation for up to 30 h, approximately 3 to 4 times longer than durations previously reported for conventional 1 mol·L⁻¹ KHCO₃ systems, significantly extending the single-run operation time. The formate yield reached 310 μmol·cm⁻²·h⁻¹·mA⁻¹, representing an increase of 200% compared to the average level. In addition, similar results were obtained using NaHCO₃ as the electrolyte, demonstrating the broad applicability of this strategy.