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.

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“…Oswald ripening causes a broadening of the size distribution, but in its simplest treatment it should not lead to a bimodal distribution. However, in real-world scenarios where local interactions among dissolving/coarsening particles cannot be neglected, bimodal particle distributions have been described . Due to a large number of unknown parameters, we cannot pinpoint how or why this is the case for these nanospheres.…”

“…However, in real-world scenarios where local interactions among dissolving/coarsening particles cannot be neglected, bimodal particle distributions have been described. 67 Due to a large number of unknown parameters, we cannot pinpoint how or why this is the case for these nanospheres.…”

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“…(b) Ostwald ripening, where larger particles grow at the expense of smaller ones [146,147], can be minimized by starting with uniformly sized nanoparticles and optimizing synthesis conditions [148][149][150].…”

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“…Following from our previous work, [1][2][3][4] the S2A framework aims to develop a template for multiscale modeling of reactive flow problems and to predict operando activity of catalyst materials. Future developments include predicting operando catalyst stability, transport, and transient phenomena related to the catalyst support structure.…”