Electrochemical Impedance Spectroscopy Data from Solid Oxide Cells Undergoing Co-Electrolysis: The Influence of Rig Inductance

Abstract

High temperature solid oxide cells (SOCs) can be used for co-electrolysis of carbon dioxide and steam to produce carbon monoxide and hydrogen: key reactants for the production of synthetic fuels. During co-electrolysis, two electrochemical reactions occur at the fuel electrode, in addition to the reverse water gas shift reaction (rWGSR). It is currently not well understood which processes relating to these reactions are rate-limiting. Here we propose a method that enables rate-limiting processes to be identified using electrochemical impedance spectroscopy (EIS). SOCs were operated at 850oC under varied fuel concentrations and current densities. Inductance effects were accounted for and the resulting EIS data was de-convoluted using the Distribution of Relaxation Times (DRT) method, giving the characteristic frequencies of rate-limiting processes occurring during cell operation. The resistances and capacitances associated with these processes were quantified using the impedance and electric modulus formalisms, respectively. DRT results in combination with the analysis of differences in spectra (ADIS) associated with the EIS data, plotted in both impedance and electric modulus formalisms, were used to justify the number of processes associated with the cell polarization resistance and to develop a meaningful equivalent circuit to model cell performance.