Design of a Catalytic Layer with Hierarchical Proton Transport Structure: The Role of Nafion Nanofiber

The chemical composition and architecture of catalytic layers significantly affect the performance of membrane electrode assemblies in proton exchange membrane fuel cells. In this work, a novel catalytic layer with hierarchical proton transport pathways has been designed by simultaneously employing Nafion nanofibers and Nafion ionomers. A H ₂ /O ₂ fuel cell based on the hierarchical catalytic layer shows an increase of 32.3% on power output in comparison with the conventional fuel cell at 70 °C and 100% relative humidity. That is attributed to unique roles of Nafion nanofibers in the hierarchical catalytic layer. First, the addition of Nafion nanofibers significantly increases the proton conductivity of the catalytic layer up to 8.3 × 10 ^-1 S cm ^-1 at 70 °C and 100% relative humidity, which is 15.7 times higher than that of the catalytic layer where the Nafion nanofibers are replaced by the same content of Nafion ionomers. That is confirmed by the accumulated proton transportation of a single Nafion nanofiber in the Nafion-nanofiber-based catalytic layer by a simulated model. Second, the porosity of the catalytic layer is increased due to the introduction of a Nafion nanofiber, leading to enhancement of mass transfer. Third, the Pt/C nanoparticles are homogeneously anchored on the surface of the Nafion nanofibers, which improves the electrochemical surface area and the utilization of the Pt catalyst.