PNNL

Abstract

This paper demonstrates how detailed surface science data bases can lead to improved results in practical applications. The case investigated concerns how surface diffusivities affect concentration polarization on metal electrocatalyst particles. The application is to solid oxide fuel cells (SOFCs). A common assumption for SOFCs is that the hydrogen-oxygen reaction that produces the electrical current is strictly localized at the triple phase boundary (TPB) between the metal catalyst particle, the zirconia support, and the gas atmosphere. Detailed treatment of oxygen spillover indicates that the reactive area simply spreads over the catalyst surface as needed to support the current, leading to TPB widths of several hundred Angstroms. Lower adspecies surface diffusivities (due to catalyst crystallography), lower reactant partial pressures (due to electrode design), higher temperatures, and higher current demands, generally shift the peak turnover number (TON) for H2O generation away from the TPB in practical SOFCs with cermet anodes. The diffusivity-coverage relationship (repulsive, neutral, or attractive adspecies interactions) affects the location of the TON peak on the catalyst surface in a non-monotonic manner, indicating that detailed surface science data are needed for decisive determination of the source of anodic concentration polarization in SOFCs. The most detailed surface diffusion model investigated in this work indicates that the catalytic process is limited by oxygen surface diffusion on the metal particle.