Review of ionomers in catalyst layers of proton exchange membrane (PEM) modules: key parameters, characterization and manipulation methods

ABSTRACT

Ion-conducting polymers (Ionomers) are critical in catalyst layers (CLs) of proton exchange membrane (PEM) fuel cells and water/vapor-fed electrolyzers, serving essential roles in proton conduction, electronic insulation, and water absorption. This review summarized the physical, chemical and economic parameters of CL ionomers across various PEM modules, focusing on long- and short-side-chain perfluorosulfonic acid (PFSA) ionomers. It also outlines recent efforts in PEM air dehumidification. Characterization methods for ionomer morphology (e.g. rods, swollen clusters, networks) and mass transfer resistance (e.g. ionomer/gas interfacial, catalyst/ionomer interfacial and intra-ionomer) were summarized. Optimal ionomer distribution at electron-proton-gas interfaces is crucial, as proton transfer requiring a thick and connected distribution and gas transport requiring a thin and dispersed distribution. Adjusting ionomer equivalent weight and ratio to catalyst can enhance distribution. Besides, CL ionomer design should emphasize product escape for water electrolyzers and reactant diffusion for vapor ones. Integration of multidimensional and multifunctional techniques, such as visual characterizations of Transmission Electron Microscopy and Nanoscale X-ray computed tomography and numerical simulations of Molecular Dynamics and lattice Boltzmann methods, is recommended for accurate assessment of ionomer behavior. This comprehensive review provides insights into ionomers in CLs, highlighting characterization methods and manipulation strategies, offering guidance for next-generation PEM modules.

KEYWORDS:

• Proton exchange membrane
• catalyst layer
• ionomer
• mass transfer
• material characterization
• numerical simulation

Acknowledgements

The project is supported by National Natural Science Foundation of China (52122605, 51936005) and Guangzhou Science and Technology Plan Project (202201010112). It is also supported by the Fundamental Research Funds for the Central Universities (2023ZYGXZR027).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Research interest

coupled heat and mass transfer within/across membrane, PEM water/vapor-fed electrolyzers, air dehumidification, multi-phase flow, numerical simulation

Highlights

Key physical, chemical and economical parameters of various ionomers were summarized.

Methods for characterizing ionomer morphology and mass transfer resistance were compared

Ionomer distribution at electron-proton-gas interfaces is significant but difficult to represent in 2D imaging.

Experiments and numerical simulations should be combined to evaluate ionomer-related mass transfer in CLs.

Water and vapor electrolyzers have significantly different concerns of ionomers due to different feedstocks.

Selecting suitable equivalent weight and adjusting the ionomer/catalyst ratio can improve the distribution.

Additional information

Funding

The work was supported by National Natural Science Foundation of China [52122605, 51936005], Guangzhou Science and Technology Plan Project [202201010112] and the Fundamental Research Funds for the Central Universities [2023ZYGXZR027].