Understanding and Engineering of Multiphase Transport Processes in Membrane Electrode Assembly of Proton-Exchange Membrane Fuel Cells with a Focus on the Cathode Catalyst Layer: A Review

The development of proton-exchange membrane fuel cell (PEMFC) technology will give rise to a great evolution toward a more sustainable and eco-friendly life by powering clean electric vehicles or stations. However, several vital problems still hinder the commercialization of PEMFCs. The foremost issue is the high cost compared to the contenders, deriving from both the high consumption of Pt catalysts and low service life under practical operations. In this review, deeper investigations on how to further lower the cost of PEMFCs show the importance of understanding and engineering the multiphase transport processes in the membrane electrode assembly (MEA). The definitions, rules, numerical simulations, and experimental validations of various mass transfer processes across the MEA are introduced to provide a holistic evaluation and enable optimization to improve the cell performance and reliability. In addition, because the sluggish oxygen reduction reaction in the cathode catalyst layer (CCL) requires most of the Pt catalysts, the oxygen/water-related multiphase transfer in CCLs is taken as a focus for detailed analysis. Several successful strategies, such as triple-phase boundary engineering, graded design, and novel ordered three-dimensional structure construction, are proven to be promising in greatly reducing the Pt consumption in the CCL and facilitating the microscopic multiphase transfer processes of the MEA. With optimized engineering of the electrode structure and configuration inside the MEA, the mass transfer resistances can be minimized to give the best operating conditions for electrochemical reactions to occur in the catalyst layers. Finally, the main challenges and some perspectives for developing advanced MEAs with lower cost in high-performance and reliable PEMFCs are provided.