Low-temperature electrochemical CO₂ reduction has demonstrated high selectivity for CO when devices are operated with pure CO₂ streams. However, there is currently a dearth of knowledge for systems operating below 30% CO₂, a regime interesting for coupling electrochemical devices with CO₂ point sources. Here we examine the influence of ionomer chemistry and cell operating conditions on the CO selectivity at low CO₂ concentrations. Utilizing advanced electrochemical diagnostics, values for cathode catalyst layer ionic resistance and electrocatalyst capacitance as a function of relative humidity (RH) were extracted and correlated with selectivity and catalyst utilization. Staying above 20% CO₂ concentration with at least a 50% cathode RH resulted in >95% CO/H₂ selectivity regardless of the ionomer chemistry. At 10% CO₂, however, >95% CO/H₂ selectivity was only obtained at 95% RH under scenarios where the resulting electrode morphology enabled high catalyst utilization.

This study investigates strategies to improve the quality of electrochemical impedance spectroscopy (EIS) measurements using reference electrodes (RE) in proton exchange membrane water electrolyzers (PEMWE). We demonstrate that adding a low impedance wire in parallel to the RE significantly enhances signal accuracy, especially at high frequencies. Additionally, we identify electrical pad heaters as a source of measurement noise. EIS measurements fulfilling Kramers–Kronig validity criteria were only achieved in their absence. These insights advance the diagnostic capabilities of REs in water electrolyzers and support more reliable, spatially resolved analysis of electrochemical losses within the cell.