Bromide‐mediated membrane‐free <scp> CO ₂ </scp> electrolyzer design for economically efficient carbon monoxide production

Abstract Electrocatalytic CO 2 reduction reaction (ECO 2 RR) faces limitations in conventional reactors due to high energy consumption, inefficient mass transport, and poor durability caused by ion‐exchange membranes. Herein, we developed a membrane‐free flow electrolyzer that dynamically decouples cathodic CO 2 ‐to‐CO reduction from the anodic oxygen evolution reaction via a regenerative bromide cycle (Br − /BrO 3 − ). Operating at 216 mA cm −2 , the system achieves a record‐low cell voltage of 1.85 V, maintains over 99% CO Faradaic efficiency, attains 32.9% single‐pass CO 2 conversion, and reaches 71.9% energy efficiency (EE), with negligible degradation over 100 h of continuous operation. Operando spectroscopy reveals that local pH stabilization at 7.2 optimizes *CO/*H adsorption, thereby enhancing CO selectivity and EE. Techno‐economic analysis shows a 60% lower power cost (111.8 vs. 277.79 $ ton −1 CO) vs. conventional systems. This work provides new insights into addressing persistent challenges in ECO 2 RR, including mass transfer limitations, carbonate formation, and anodic reaction mismatch.