CO 2 electrolyzer designed to operate on dilute CO 2 feed and low stoichiometric ratio would alleviate the separation costs for CO 2 purification and electrolyzer exit gas processing, respectively. The effect of CO 2 concentration, CO 2 flow rate, and CO 2 pressure on current density and faradaic efficiency of a solid polymer electrolyte membrane CO 2 electrolyzer was quantified. An approach for estimating voltage breakdown into activation overpotential for CO 2 reduction reaction as well as oxygen evolution reaction, ohmic losses, and concentration overpotential is introduced. No enhancement in current density (∼160 mA cm −2) was observed above stoichiometry ratio of 4 whereas reducing the stoichiometric ratio to 2.7 still yielded a current density of ∼100 mA cm −2. Dilution of CO 2 in the feed from 100 mol% to 30 mol%, at ∼90kPa of cell pressure, resulted in a monotonically decreasing current density. A square root dependency on CO2 partial pressure was observed under these conditions. Operation with pure CO 2 at different total pressure yielded only a minor increase in current density indicating some form of saturation-limited behavior. Long-term potentiostatic operation over 85 h revealed continuous drop in current density and a corresponding increase in electrode resistance, observed in electrochemical impedance response.