Electrochemical Characterization of Nickel/Gadolinia Doped Ceria Fuel Electrodes under H$_2$/H$_2$O/CO/CO$_2$ -Atmospheres

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. ... mehrFrom a kinetic analysis it is shown that the electrochemical H$_2$/H$_2$O conversion is dominant for $y_{CO}+y_{CO_2} ⩽ 50$% and electrochemical CO/CO$_2$-conversion onsets from $y_{CO}+y_{CO_2}⩾ 60$%.