Model of catalyst layers for proton exchange membrane fuel cells: Progress and perspective

Abstract

<p indent="0mm">The proton exchange membrane fuel cell (PEMFC) is one of the most high-efficient energy conversion devices with zero carbon emission, which is of great significance to achieve the strategic goal of “carbon emission peak and carbon neutrality”. The catalyst directly determines the cell performance and is the core in the large-scale commercial application of PEMFCs. Due to the complex multi-physics and multi-scale process in catalyst layers, it is necessary to establish an accurate numerical model to reveal the internal mechanisms of transport and reactions. The present work reviews the progress of modeling catalyst layers, and the models can be divided into five types: Interface model, homogeneous model, agglomerate model, pore-scale model and particle model. The modeling ideas are introduced, and their typical applications are discussed. The interface model assumes that the catalyst layer is infinitely thin, so it is simple to set up and fast to calculate. It can be used to study the reaction process in fuel cells except the catalyst layer. However, due to the excessively simple assumption, the simulation under high current densities disagrees with the experiment result, which limits the application of the model. The homogeneous model assumes uniform distribution of substances in the catalyst layer. This model completely simulates the reaction and mass transfer process in the cell and has the ability to guide the cell optimization. However, the homogeneity hypothesis does not conform to the image of the catalyst layer observed in the experiment, so the model is not suitable for the study of the catalyst layer with high precision. The agglomerate model assumes that the substances in the catalyst layer agglomerate into a specific geometric shape. There are more parameters about catalyst layers in the agglomerate model, so it can be used to optimize the structure of the catalyst layer. However, some parameters are difficult to be measured experimentally and can only be assumed, which makes the model less scientific. In addition, it also takes the homogeneity hypothesis. The pore-scale model reconstructs the complex structure of the catalyst layer. It has the highest accuracy in theory but lacks perfect reconstruction methods. Regular reconstruction, stochastic reconstruction and process-based reconstruction cannot reflect the real structure characteristics of catalyst layers. Image-based reconstructions and statistical function-based reconstructions are limited by the image resolution and experimental error. Therefore, the pore-scale model is not a perfect model for catalyst layers. The particle model can accurately simulate microcosmic processes, but lacks the ability to combine with macroscopic simulations. So, the catalyst layer model still needs to be further developed. It is suggested to develop advanced experiment methods to ensure the accuracy of catalyst layer model parameters and verify the prediction result of the model. In addition, more complete catalyst layer model with multi-scale and multi-physics should be further developed, incorporating microscopic model results into macroscopic models, thus to fully understand the influence of the microscopic process of catalyst layers on the cell performance. It is also suggested that more research on low platinum, non-platinum catalysts as well as cell performance degradation should be carried out and new models considering their different reaction process and degradation mechanisms should be developed in the future. Moreover, similarity analysis approach is suggested to be adopted for cell models, which can reveal output characteristics and save cell optimization time.