Microstructures and Proton Networks of Ionomer Film on the Surface of Platinum Single Atom Catalyst in Polymer Electrolyte Membrane Fuel Cells

Pt single atom catalyst (SAC) is a promising catalyst material with ultrahigh Pt utilization efficiency and excellent electrochemical activity for PEMFCs. However, the performance of fuel cells with Pt SAC is still insufficient for practical applications, which is mainly resulting from the significant increasing proton and oxygen transport resistance in the catalyst layer. In this study, the microstructures and transport properties of ionomer film on the surface of Pt SACs are investigated by classic molecular dynamics (MD) simulations. It is found that the hydrated ionomer film can be divided into three regions including dense layer region, middle region, and top region. Hydrophilic components in dense layer region increase with higher Pt single atom loading, due to the rise of water affinity from the Pt-ionomer interface. Water clusters are mainly confined to the middle region. Besides, with the increase of Pt single atom loading, the interactions between hydrophilic and hydrophilic species are weaker. Side chains locate below or above the continuous water clusters show upward or downward configurations, heading to the water surface. An appropriate loading of Pt SACs is essential for promoting the connectivity of hydrophilic clusters as well as the accessibility of protons to Pt SACs.