Mixed Conduction in Ceramic Hydrogen/Steam Electrodes by Hebb-Wagner Polarization in the Frequency Domain

The unique mixed conduction aspect has much to be explored for ceramic fuel/steam electrodes suggested for higher chemical stability than cermets in SOFC/SOEC applications. Typical symmetric cell configuration of the thin ceramic electrodes on the electrolytes was shown to be a versatile configuration of the electron‐blocking Hebb–Wagner polarization cell for the characterization of the mass and charge transport in mixed conducting oxides by impedance spectroscopy. The Jamnik–Maier transmission line model for the mixed conductors was generalized by replacing the capacitors with constant‐phase elements to describe the extremely non‐ideal impedance behavior of porous but negligibly catalytic La2Ti2O7 layers at various humidity levels up to 90% in hydrogen at 850 °C. Modeling allowed the deconvolution of the parameter set for the evaluation of chemical diffusivity and non‐stoichiometry as well as partial ionic and electronic conductivity of La2Ti2O7. Electrostatic double layer capacitance was found to be orders of magnitude higher than the theoretical despite the correct oxygen activity dependence, which appears generic and thus requires a theoretical clarification. Overlapping interfacial and chemical capacitance as well as the strongly non‐ideal behavior of the latter are mainly responsible for the less clearly distinguished diffusion‐limited feature of the Hebb–Wagner polarization cell.