Identification of gas diffusion phenomena on highly active Ni–ceramic anodes using the DRT technique

Nickel–ceramic composites are conventional and basic materials for anodes of solid oxide fuel cells. Usually, nickel–cermets have insufficient electrochemical activity due to the fact that the hydrogen oxidation reaction is limited by the triple phase boundary (TPB) length. The electrochemical activity of nickel–cermets can be improved by increasing the TPB length or through the wet impregnation with ceria. A distinctive feature of the electrochemical impedance spectra of highly active nickel–ceramic anodes impregnated with ceria is the response from gas diffusion phenomena in the low frequency range. Under certain conditions, the resistance of gas diffusion phenomena cannot be determined by conventional approaches to the analysis of impedance spectra. A tremendous assistance in the analysis of impedance spectra over the past few years has become the method of distribution of relaxation times (DRT). This paper presents the results of studies of highly active nickel–ceramic electrodes made from NiO and Zr0.84Sc0.16O1.92 powders of various dispersion and impregnated with ceria. Using the DRT method was found that in the case of electrode with low porosity the hydrogen oxidation rate is limited by the rates of two parallel gas diffusion processes relaxing in almost the same frequency range.