Boosting electrochemical CO₂ reduction to CO by regulating pressure in zero-gap electrolyzer

The electrochemical reduction reaction of CO₂ presents a promising strategy for both CO₂ utilization and renewable energy storage. However, for this process to be economically viable, it must achieve high energy efficiency, high product selectivity, and suppression of the hydrogen evolution reaction (HER) at low cell voltages and industrially relevant current densities. Thus, this paper introduces a high-pressure zero-gap membrane electrode assembly electrolyzer that uses pristine silver nanoparticles (<150 nm) as the cathode catalyst for CO₂-to-CO conversion. Operating at elevated CO₂ pressures of up to 1.5 MPa and in a highly alkaline environment (2 M KOH) considerably enhanced CO selectivity and energy efficiency by reducing ohmic losses and improving reaction kinetics. At an optimized pressure of 1.5 MPa, a high current density of -350 mA cm-² was sustained at an applied cell voltage of -3.2 V (-3.0 V, IR-compensated), achieving over 70 % CO Faradaic efficiency and 32 % CO energy efficiency. High-pressure operation also suppressed HER by increasing the local CO₂ concentration at the catalyst surface, thereby improving CO selectivity. Additionally, salt precipitation mechanisms and their effect on catalyst deactivation were discussed.