Mesoscale Model for Ostwald Ripening of Catalyst Nanoparticles

Abstract

A mesoscale model is proposed to characterize coarsening of platinum-based catalyst particles with the aim of understanding cathode degradation and power loss in proton exchange membrane fuel cells (PEMFC). The microstructure of a cathode catalyst layer is more complex than the ones typically described by Ostwald-ripening models, such as the Lifshitz and Slyozov and Wagner (LSW) theory or diffuse interface models. Our model captures the dissolution kinetics of the catalyst material, and the diffusivity of the ions in solution. The network structure of the model embeds the tortuosity of the microstructure and the effect of water content on Pt-ion mobility. Detailed diffusional interactions among catalyst particles are explicitly described with the aim of providing statistically averaged properties of coarsening ensembles. Through numerical tests, the scaling of coarsening kinetics is predicted as function of voltage. The effect of voltage cycling, and initial particle size distribution are also probed. The power-law exponents for the mean square radius vs time are affected by the reduced dimensionality of inter-particle diffusion, constrained by the carbon support and the surface wetting. This theoretical framework can be used to understand how material design influences the degradation pathways that are responsible for platinum surface area loss.