Electrochemical Impedance Characteristics of a Low-Temperature Single Cell for CO₂/H₂O Co-Reduction to Produce Syngas (CO+H₂)

In this study, the electrochemical impedance characteristics of CO₂/H₂O co-reduction to produce CO/H₂ syngas were investigated in a low-temperature single cell. The effect of the operating conditions on the single-cell performance was evaluated at different feed concentrations and cell voltages, and the corresponding electrochemical impedance spectroscopy (EIS) data were collected and analyzed. The Nyquist plots exhibited two semicircles with separated characteristic frequencies of approximately 1 kHz and tens of Hz. The high-frequency semicircles, which depend only on the catholyte concentration, could be correlated to the charge transfer processes in competitive CO₂ reduction and hydrogen evolution reactions at the cathodes. The EIS characteristics of the CO₂/H₂O co-reduction single cell could be explained by the equivalent circuit sug-gested in this study. In this circuit, the cathodic mass transfer and anodic charge transfer processes are collectively rep-resented by a parallel combination of resistance and a constant phase element to show low-frequency semicircles. Through nonlinear fitting using the equivalent circuit, the parameters for each electrochemical element, such as polarization resis-tances for high-and low-frequency processes, could be quantified as functions of feed concentration and cell voltage.