Integrating nanostructured Pt-based electrocatalysts in proton exchange membrane fuel cells

Abstract Platinum-based nanomaterials remain one of the most effective options as proton exchange membrane fuel cell (PEMFC) cathode electrocatalysts for enhancing the sluggish kinetics of the oxygen reduction reaction (ORR). Their morphology has been greatly improved throughout the last decade, shifting from 2 to 3 nm nanoparticles (NPs) supported on carbon blacks to complex shaped nanostructures (such as nanoframes, octahedra, etc.). These nanostructures take advantage of electronic and structural effects, such as the (i) strain-ligand effect achieved through alloying, (ii) preferential crystallite orientation, or (iii) positive use of the structural defects. Improvement factors in specific activity of up to 60 have been achieved compared to classic Pt NPs in liquid electrolyte, however, such tremendous enhancements do not translate to solid electrolyte, e.g. in PEMFCs. Here, we discuss the PEMFCs-induced limitations for these complex electrocatalysts mainly evolving around the ionomer, i.e. Nafion®, which (i) exhibits a heterogenous dispersion onto the support surface, (ii) has difficulty impregnating the nanostructure's inner pores (for nanoframes or porous-hollow nanoparticles), and (iii) electrostatically interacts with Pt, therefore displacing the nanoparticles depending upon the PEMFC operation potential. We suggest several options in overcoming these challenges, including (i) functionalizing the support surface with nitrogen moieties, increasing the density of anchoring sites, and thus facilitating the nanostructure dispersion and (ii) initially encapsulating the nanostructures with well-defined ionic liquids and eventually replacing the Nafion® in the catalytic layer.