Understanding Limitations in Electrochemical Conversion to CO at Low CO 2 Concentrations.

Low-temperature electrochemical CO 2 reduction has demonstrated high selectivity for CO when devices are operated with pure CO 2 streams. However, there is currently a dearth of knowledge for systems operating below 30% CO 2 , a regime interesting for coupling electrochemical devices with CO 2 point sources. Here we examine the influence of ionomer chemistry and cell operating conditions on the CO selectivity at low CO 2 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% CO 2 concentration with at least a 50% cathode RH resulted in >95% CO/H 2 selectivity regardless of the ionomer chemistry. At 10% CO 2 , however, >95% CO/H 2 selectivity was only obtained at 95% RH under scenarios where the resulting electrode morphology enabled high catalyst utilization.