Pinning-down polarization losses and electrode kinetics in cermet-supported LSM solid oxide cells in reversible operation

• Up to four Gerischer-like impedances for Ni–YSZ/YSZ/LSM–YSZ solid oxide cells
• Reversible operation at absolute humidity up to 90% at constant flow rate
• Humidity-dependent gas concentration Ni–YSZ impedance controlled by bias
• 3-Parameter TLM for surface diffusion–reaction kinetics of LSM–YSZ electrode
• Capacitance decrease with oxygen activity for enhanced electrode kinetics

The impedance behavior of the archetypal Ni–YSZ/YSZ/LSM–YSZ solid oxide cells was investigated in reversible operation at different humidity conditions up to 90%. The major electrode polarizations of the full cell at 850 °C are identified with the electrochemical polarization of the LSM–YSZ electrode modeled by three-parameter transmission line model and the gas concentration impedance of the Ni–YSZ electrode modeled by an ideal Gerischer element. The gas concentration polarization increases with the dehumidification by the electrolysis, which is thus reduced by the increased ambient humidity. Capacitance effects vary oppositely. According to the non-faradaic chemical reaction controlled mechanism for the electrochemical reaction suggested by Mizusaki, diffusion–reaction parameter for LSM electrode was estimated from the transmission line model analysis. Surface diffusivity is shown to enhance strongly with oxygen activity induced by electrolysis, which can be partly ascribed to the decreased surface adsorption capacitance as well as increased mobility. Surface reaction rate increase essentially reflects the decreased adsorption capacitance. The kinetics becomes more limited by the surface adsorption reactions but overall diffusion-limited, which is represented by the Gerischer-like response. Less pronounced performance deterioration in SOFC operation may be ascribed to the mixed conduction contribution.