| Abstract | CO₂ electrolyzer designed to operate on dilute CO₂ feed and low stoichiometric ratio would alleviate the separation costs for CO₂ purification and electrolyzer exit gas processing, respectively. The effect of CO₂ concentration, CO₂ flow rate, and CO₂ pressure on current density and faradaic efficiency of a solid polymer electrolyte membrane CO₂ electrolyzer was quantified. An approach for estimating voltage breakdown into activation overpotential for CO₂ 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 CO₂ 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 CO₂ 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. |
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