Gas concentration impedance of solid oxide fuel cell anodes

This series of papers presents details of numerical studies of the nature of the impedance of solid oxide fuel cell (SOFC) anodes caused by gas-phase transport processes. The present part treats channel geometries where gases are transported parallel to the electrode surface. Two cases are investigated: (i) channel flow by forced convection, a typical situation in planar stack segments; and (ii) channel diffusion without convective flow, a typical situation in laboratory-scale single-chamber experiments using symmetrical cells. Current/voltage curves and electrochemical impedance spectra are simulated based on the Navier-Stokes transport equations and nonlinear electrochemistry models. Both channel flow and channel diffusion cause a capacitive behavior in the form of an resistance-capacitive (RC)-type semicircle in the Nyquist diagram. Its resistance and relaxation frequency strongly depend on operation parameters (gas concentration, flow rate, temperature, electrochemical polarization) and geometry (channel length and cross-sectional area). The model predictions are in good quantitative agreement with four different experimental studies published in the literature. The simulation approach thus allows a physically based assignment of observed gas concentration impedance processes. (c) 2007 The Electrochemical Society.