Deconvolution of Four Transmission-Line-Model Impedances in Ni-YSZ/YSZ/LSM Solid Oxide Cells and Mechanistic Insights

• 4 polarizations of cermet supported Ni/YSZ/LSM full cells are distinguished by EIS.
• Electrochemical and gas concentration for two electrodes alternate in the spectra.
• All 4 responses are described by the ideal form of transmission line (TL) models.
• Capacitance, not the resistance, mainly determines different electrode kinetics.
• More mechanistic insights can be found from full-force parametric analysis.

Four impedance components with the two respectively from the cathode and anode have been successfully deconvoluted for the temperature range between 800°C and 600°C of cermet-supported (H2,H2O)Ni-YSZ/YSZ/LSM-YSZ(O2,N2) solid oxide fuel cells with high quality 5 μm thin YSZ electrolyte layers with two different pore former contents of 10 and 20wt%. A Bisquert transmission line model in an ideal three-parameter equation was successfully applied for the electrochemical reactions for both Ni-YSZ and LSM-YSZ electrodes. Non-faradaic chemical reaction mechanism provided the hydrogen dissociation surface reaction energy of 0.58 eV with adsorption capacitance activated with 0.4∼0.5 eV in Ni-YSZ electrodes directly by impedance spectroscopy. Much higher adsorption capacitance of LSM-YSZ electrode explains the lower frequency response and may explain strongly activated kinetic parameters. The surface diffusivity is effectively enhanced by the YSZ network in the composite electrodes. Gas concentration impedance of Ni-YSZ cermets described by ideal Gerischer model is largely responsible for the performance under light current load condition at high temperature and the higher porosity led to lower resistance but to higher capacitance. Gas concentration polarization may also appear diffusion-reaction co-limited and thinner LSM-YSZ electrode layer exhibits the higher capacitance than the thicker cermet support by two orders of magnitude.

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