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, λstoich, 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. Optimizing CO2-to-CO electrolyzers is important for developing tandem electrolysis processes. Now an efficient precious metal-free CO2-to-CO electrolyzer cathode design allows operation under a low stoichiometric CO2 excess ratio that yields a molar CO concentration of 70% in the exit stream along with a diagnostic approach to its catalytic and mass transport characteristics.