A dual promotion of oxygen reduction on Pt in membrane electrode assemblies by hydroxyphenyl metal porphyrins

Abstract

The oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) is severely hindered by the strong adsorption of sulfonate groups in perfluorinated sulfonic acid (PFSA) ionomers on Pt, which not only blocks the catalytic sites, but also induces dense PFSA films resisting O₂ toward Pt. Herein, we address this issue by incorporating hydroxyphenyl metal porphyrins (MPOHs) as additives in cathodic catalyst layers (CCLs). Detailed physical and electrochemical characterization techniques and molecular dynamics simulations reveal that MPOHs enable ORR-favorable Pt/ionomer interfaces by a dual promotion mechanism, by forming hydrogen-bonding networks with hydronium and sulfonates to break the sulfonate–Pt interaction and enlarge the O₂-diffusive hydrophilic domains in ionomer films, thus enriching and carrying O₂ molecules through adsorption/desorption processes. Consequently, the fuel cells with MPOH-added CCLs exhibit marked reduction in the local oxygen transport resistance of ionomer films and notable improvements in the output power accordingly, with CuPOH, which is suggested by density functional theory calculations as an optimal O₂ carrier due to the near zero O₂ adsorption free energy, showing a more pronounced improvement. These results should inspire more molecule-level strategies to boost PEMFC efficiency through regulating Pt/ionomer interfaces and stimulate the development of electrode optimization-oriented materials in a broad range of application areas of various electrochemical energy technologies.