The effect of water content on the electrochemical impedance response and microstructure of Ni-CGO anodes for solid oxide fuel cells

Using electrochemical impedance spectroscopy, the high frequency (HF) and low frequency (LF) impedance of nickel–gadolinium-doped ceria (Ni-CGO) symmetrical cells (Ni-CGO/YSZ/Ni-CGO) in dry and moist atmospheres were studied. The HF component of the impedance response in moist H2 varied with operating temperature, while the LF component varied with the H2 content in the fuel. The EIS response in dry fuel behaved differently. The LF impedance in dry fuel (97% H2, 3% N2) showed a large increase, and varied significantly with both temperature and H2 content in the fuel, while the HF component varied with temperature in a similar manner to that observed in moist fuel. This suggests that the HF impedance in both moist and dry fuel is associated with the charge transfer resistance. The LF impedance in the case of moist fuel can be reasonably attributed to mass transport effects, while that in the case of dry fuel cannot be attributed to the same mass transport process. The estimated time constant of the LF component was considerably larger in dry atmospheres, compared to that in moist conditions. Scanning electron microscopy (SEM) images showed crack formation in the anode cermets exposed to dry atmospheres, which were not evident in cermets exposed to moist conditions.