The microporous layer in proton exchange membrane fuel cells, from transport mechanism to structural design

Hydrogen is a zero-carbon emission, high energy density, and sustainable sources of energy, while the proton exchange membrane (PEM) fuel cells have the potential to maximize their merits and achieve high energy conversion efficiency to electricity. However, in a real-world application, the ohmic resistance and transport resistance limit the power density of the fuel cells to achieve a desirable and saleable apparatus, which originates from the structure of each component. The microporous layer (MPL) is one of the key components in PEM fuel cells to provide uniform gas distribution and facilitate water transport. Herein, we discussed the theoretical and experimental developments in the field of MPL, with a focus on the fundamental mechanisms of mass transport in MPL and the design principles. The recent progress of rationalities of the structural design, the material selection, and the preparation methods of MPL that have played enabling roles in advancing this frontier are further reviewed. Finally, we provide an outlook for inspiration to MPL towards high performance, easy production, low costs, and environmentally friendly.