Oxygen Transport at Three-Phase Interfaces of Pt Single-Atom Catalysts in Proton Exchange Membrane Fuel Cells

High Pt-loading catalysts constitute a critical bottleneck hindering the large-scale development of fuel cells. Pt single-atom catalysts (Pt SACs) demonstrate ultrahigh atomic utilization and catalytic activity, while their performance remains unsatisfactory under the harsh reaction conditions in catalyst layers (CLs), particularly due to mass transport constraints. Herein, molecular dynamics simulations are employed to thoroughly investigate the nanoscale oxygen transport mechanisms at Pt SACs-ionomer-gas three-phase interfaces in CLs. The results show the dense ionomer layer (<1 nm) on carbon surface contributes dominantly to the overall oxygen resistance, within which Pt SACs primarily localize beneath voids between PFSA backbones and interfacial regions between backbones and water clusters. The water clusters in dense layer increases as hydration level (λ) increases from 3 to 11, then declines with further hydration. Oxygen tends to diffuse through the hydrophilic-hydrophobic interface regions around Pt SACs at tilted molecular orientations. The associated oxygen transport resistance exhibits a nonmonotonic dependence on hydration level, reaching a minimum at λ = 11, which provides the most favorable channels for oxygen approaching Pt SACs. Traditional oxygen permeation theory developed for bulk PFSA is only applicable to low to medium hydration levels in Pt SAC-based CLs, not to the full λ-range encountered in practice. This study provides insights into oxygen transport at three-phase interfaces of Pt SACs in CLs, offering valuable guidance for the design of CLs with ultralow Pt loadings.