Voltage Loss Diagnosis in CO₂ Electrolyzers Using Five-Electrode Technique

CO2 electrolysis is a promising carbon utilization technology. Currently, energetic efficiency still requires a significant improvement for commercialization. To rationally design a more efficient CO2 electrolyzer, diagnostic tools are necessary to pinpoint the source of voltage losses across the full cell at work. Here we develop a five-electrode technique to probe voltage drops at the cathode, anode, membrane, and their interfaces in a typical zero-gap cell. We show that the cathode/membrane ionic interface is the major source of overpotential, contributing 720 mV voltage loss at 600 mA cm–2. This loss can be mitigated by coating the catalyst directly onto the membrane to lower ionic resistances, reducing this voltage loss to 80 mV at the same current density. The improved design enables us to achieve a full cell performance of 3.55 V and >95% CO Faradaic efficiency at 800 mA cm–2, representing the highest performance for CO2 electrolysis with a dilute bicarbonate electrolyte. The insights provided by the five-electrode technique may guide the rational design of future membrane-based electrochemical cells.