Investigation and Optimisation of Operating Conditions for Low-Temperature CO₂Reduction to CO in a Forward-Bias Bipolar-Membrane Electrolyser

The most recent investigations of operating conditions in a forward-bias bipolar-membrane zero-gap electrolyser using a silver cathode catalyst for the reduction of CO₂to CO at low temperatures and near-ambient pressures are reported. First, the CO₂electrolyser performance was investigated as a function of cathode feed humidification and composition. The highest CO partial current density was 127 mA cm⁻², which was obtained at an iR-corrected cell voltage of 2.9 V, a cathode feed humidification of 50%RH, CO₂feed concentration of 90% and a CO Faradaic efficiency of 93%. The cells were tested continuously for 12 h at 3 V and 8 h at 3.4 V cell voltage to investigate system stability. While Faradaic efficiencies were maintained during the measurements at 3.0 V, a shift in selectivity was observed at 3.4 V, while a deterioration in current densities occurred in both cases. Using a specially designed electrochemical cell with an integrated reversible hydrogen reference electrode, it was found that the cathode catalyst is the main responsible for the observed loss in performance. It was furthermore determined via post-mortem SEM and EDX investigations that cathode deterioration is caused by catalyst agglomeration and surface poisoning.