Electrochemical Characterization of Nickel/Gadolinia Doped Ceria Fuel Electrodes under H 2 /H 2 O/CO/CO 2 -Atmospheres

Abstract

Modelling of the co-electrolysis process requires understanding of the underlying reaction pathways under H 2 /H 2 O/CO/CO 2 -atmospheres. These include the electrochemical steam reduction/hydrogen oxidation, the electrochemical CO 2 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 CO 2 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/CO 2 can be present in H 2 /H 2 O/CO/CO 2 -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 H 2 /H 2 O conversion is dominant for y CO + y C O 2 ≤ 50% and electrochemical CO/CO 2 -conversion onsets from y CO + y C O 2  ≥ 60%.