Gas concentration impedance of solid oxide fuel cell anodes I. Stagnation point flow geometry

This paper series presents detailed modeling and simulation studies of the nature of the impedance of solid oxide fuel cell (SOFC) anodes caused by gas-phase transport processes. The present part treats bulk gas-phase transport in button-cell experiments where gases are supplied from an outlet perpendicular to the electrode surface. This geometry is described using a stagnation point flow model based on the Navier-Stokes conservation equations, and the electrochemistry at the anode is modeled as global hydrogen oxidation using Butler-Volmer kinetics. These physical models account for the full nonlinear spatiotemporal coupling of gas phase diffusion, convection, and electrochemical kinetics. Polarization curves and impedance spectra are calculated through transient numerical simulation. Simulations are performed for various flow-field parameters, electrochemical rate laws, reference electrode placements, electrode polarization, gas phase compositions (H-2/H2O/N-2), and temperatures. A strong and diverse influence of gas transport on impedance is predicted, including various shapes in the Nyquist plot and capacitive or inductive behavior. The study shows that in the bulk gas phase, diffusion and convection are closely coupled and cannot be separated in their impedance response. We propose description of these effects using the general term gas concentration impedance. (c) 2006 The Electrochemical Society.