Understanding Limitations in Electrochemical Conversion to CO at Low CO₂ Concentrations

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.