Membrane-free CO₂ electrolyzer design for economically efficient formic acid electro-synthesis

Reducing the electrical energy consumption for formic acid electro-synthesis is indispensable for advancing its industrial implementation. In a conventional CO₂ electrolyzer, most input electrical energy is consumed by the unprofitable anodic oxygen evolution reaction (OER) and ohmic drop. Electrolyzer engineering provides a promising platform to boost electrical energy utilization efficiency beyond catalyst optimization. Herein, we demonstrate a membrane-free CO₂ electrolyzer design that pairs electrochemical CO₂ reduction (CO₂R) with an all-liquid-phase anodic reaction, enabling dual production of formate at both electrodes with significantly reduced cell voltage. The optimized design exhibits the lowest electrical energy consumption (< 310 kJ mol^-1_formate) at cell voltages below 2.7 V across a current density range of 0.05-0.4 A cm^-2. This cell also maintains stable operation at 2.25 V for 313 h with a < 20 % increase in electrical energy consumption. Systematic techno-economic analysis (TEA) evaluates the economic viability of this design for formic acid electro-synthesis, revealing a potential roadmap towards low-cost formic acid production. This strategy provides guidelines for CO₂R electrolyzer engineering toward energy-efficient, economically viable production of valuable chemicals.