PNNL

Abstract

Concentration polarization is important because it determines the maximum power output of a solid oxide fuel cell (SOFC) at high fuel utilization. Anodic concentration polarization occurs when the demand for reactants exceeds the capacity of the porous ceramic anode to supply them by gas diffusion mechanisms. Many models simulate this behavior by assuming an anomolous high value for the tortuosity (eg, t=17), a measure of the bulk diffusional resistance for a porous ceramic. However, recent experiments at several laboratories, including results reported herein, have provided strong evidence that typical sintered powder ceramics (30-50% porosity) have much lower tortuosities (t=2.5-3), indicating that the bulk diffusional resistance is too small to be responsible for concentration polarization. We find evidence that concentration polarization originates in the immediate vicinity of the reactive sites near the anode/electrolyte interface, at the triple phase boundaries (TPBs) between the Ni catalyst particles, the gas, and the oxygen conducting YSZ ceramic. A model is proposed to describe how concentration polarization is controlled by two localized phenomena: competetive adsorption of reactants in areas adjacent to the reactive TPB sites, followed by relatively slow surface diffusion to the reactive sites. The model parameters (adsorption activation energy and surface diffusion coefficients) were determined by fitting to well-characterized SOFC voltage-current performance data, and are in good agreement with data from the literature. Results suggest that future SOFC design improvements should focus on optimization of the reactive area, adsorption, and surface diffusion at the anode/electrolyte interface, rather than on anode thicknesses or bulk porosities.