Electrochemical Characterization of Nickel/Gadolinia Doped Ceria Fuel Electrodes under H₂/H₂O/CO/CO₂-Atmospheres

Abstract

Modelling of the co-electrolysis process requires understanding of the underlying reaction pathways under H2/H2O/CO/CO2-atmospheres. These include the electrochemical steam reduction/hydrogen oxidation, the electrochemical CO2 reduction/CO oxidation and their coupling via the catalytic (reverse) water gas shift reaction ((R)WGS). The assumption of a very fast RWGS and therefore neglectable electrochemical CO2 conversion is commonly used to model the co-electrolysis process. In contrast, previous studies on Ni/GDC fuel electrodes suggest that the electrochemical conversion of CO / CO2 can be present in H2/H2O/CO/CO2-atmospheres. To deconvolute surface-related and non-surface-related processes in the impedance response we present results from a complex variation of operating parameters for process identification by the use of electrochemical impedance spectroscopy and the subsequent impedance analysis by the distribution of relaxation times. A physically meaningful equivalent circuit model, based on a single channel transmission line, is then derived. The model enables quantification of the surface reaction resistance under varied C/H-ratios. From a kinetic analysis it is shown that the electrochemical H2/H2O conversion is dominant for y_CO+y_(CO_2 )≤ 50% and electrochemical CO/CO2-conversion onsets from y_CO+y_(CO_2 )≥ 60%.