Design and diagnosis of high-performance CO₂-to-CO electrolyzer cells

This work reports the design and diagnostic analysis of a pH-neutral CO₂-to-CO zero-gap electrolyzer cell incorporating a nickel–nitrogen-doped carbon catalyst. The cell yields ~100% CO faradaic efficiency at applied current densities of up to 250 mA cm⁻² at low cell voltage and 40% total energy efficiency. It features a low stoichiometric CO₂ excess, λ_stoich, of 1.2 that yields a molar CO concentration of ~70%_vol in the electrolyzer exit stream at 40% single-pass CO₂ conversion, with over 100 h stability. Here we introduce the experimental carbon crossover coefficient (CCC) as a tool for electrolyzer cell diagnostics. The CCC describes the ratio between noncatalytic acid–base CO₂ consumption and catalytically generated alkalinity, thereby offering insight into the nature of the prevalent ionic transport and transport mechanisms of undesired CO₂ losses. We demonstrate the diagnostic value of the CCC in transport-based cell failure during oscillatory cell flooding between salt precipitation and salt redissolution. The present dynamic cell diagnostics provide practical guidelines toward improved CO₂ electrolyzer designs.