Polarization mechanism of high temperature electrolysis in a Ni–YSZ/YSZ/LSM solid oxide cell by parametric impedance analysis

Abstract

Comprehensive modeling of the spectra of the state-of-the-art Ni–YSZ/YSZ/LSM solid oxide cells including the instrumental stray impedance allowed the systematic deconvolution of the four major polarization losses ranging over one order of impedance magnitude. The stray impedance can be successfully modeled as an inductor connected in parallel with a parasitic resistor whose resistance was shown proportional to the inductance. From the high frequency the ohmic losses, the ‘charge-transfer’ impedance of the Ni–YSZ electrode, the surface diffusion and reaction co-limited impedance of LSM electrode, and the gas phase transport impedance of Ni–YSZ electrode were successfully distinguished. The latter two were satisfactorily described by the ideal Gerischer impedance with two independent parameters, respectively. The gas-concentration impedance increases with electrolysis due to the gas density decrease with hydrogen production, while the LSM polarization decreases due to the increased oxygen activity. Compensation of the opposite polarization behavior of Ni–YSZ and LSM electrodes explains the apparently ohmic polarization over a wide electrolysis range until the upturn where exponentially increasing gas-concentration impedance of Ni–YSZ electrode prevails. Apparently being quite distinct from the fuel cell polarization behavior, the polarization of the high temperature electrolysis can be consistently explained by the chemical potential variations of the reactants and products, which is suggested to be general characteristic of the gas electrodes of solid oxide cells, co-limited by surface diffusion and reaction process. The finite-length Gerischer model constituted of series resistors, shunt resistors, and shunt capacitors, allows the evaluation of the surface diffusivity (ca. 2⋅10−4cm2s−1), reaction constant (ca. 103s−1), and the utilization length (ca. 5μm) among the LSM–YSZ composite functional layer of thickness of ca. 10μm. The strong decrease in LSM polarization with electrolysis at the humidity of 30% can be contributed by the increase in surface diffusivity, chemical capacitance, and the surface reaction constant in the decreasing order, while the adsorption capacitance increases is mainly responsible for the polarization decreases at higher humidity condition of 50%.