Surface diffusion and concentration polarization on oxide-supported metal electrocatalyst particles

A common assumption for solid oxide fuel cells (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 analysis of oxygen spillover onto the catalyst surface indicates that the reactive area simply spreads over the 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), 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 care should be taken when applying research data to models of practical 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.