Structural and transport properties of ultrathin perfluorosulfonic acid ionomer film in proton exchange membrane fuel cell catalyst layer: A review

Although the properties of perfluorosulfonic acid (PFSA) membranes (10–100 s μm thick) in proton exchange membrane fuel cells (PEMFCs) have been well studied for decades, the limited understanding of nano-sized PFSA ionomer within catalyst layer hampers the improvement of ultra-low platinum loading PEMFC. Recently, the vital role of PFSA ionomer in the local transfer of proton, oxygen, and water in fuel cell catalyst layer has been recognized, thus its properties have attracted more attention. In this review, a comprehensive comparison between PFSA bulk membrane and ultrathin PFSA ionomer film is summarized as morphology, physical properties (e.g., water uptake, surface wettability, diffusivity), and mechanical properties (e.g., crystallization, antiplasticization, glass transition point). Afterwards, the characteristics of ionomer in different dispersion media and the microstructure of PFSA ionomer in the formed catalyst layers are illuminated. The proton conduction and mass transport (i.e., oxygen and water) behaviors in the confined PFSA ionomer are focused and highlighted to reveal the ionomer's influence on fuel cell performance. Extrapolating from the insight into structural and transport properties of PFSA ionomer, we are optimistic that the performance of ultralow Pt loading PEMFCs could be greatly improved via modifying ionomer chemical structure and engineering ionomer/catalyst interaction.