Biased Expanded Polytetrafluoroethylene Reinforced Composite Membranes with Naturally Formed 3D Surface Structures for High- Performance Proton Exchange Membrane Fuel Cells

As a crucial factor dictating the power performance of proton exchange membrane fuel cells, the interface combination between the proton exchange membrane and the catalyst layer should be considered seriously. Herein, we propose a facile interface optimization strategy in which an expanded polytetrafluoroethylene (ePTFE) skeleton layer is biasedly embedded on the surface of the perfluorosulfonic acid (PFSA) membrane bulk to form three-dimensional surface structures naturally. The experimental results show that the biased ePTFE-reinforced membranes (BRMs) demonstrate significantly enhanced surface roughness compared with conventional central ePTFE-reinforced membranes (CRMs), thus providing an enlarged cathode membrane/catalyst layer contact area. With optimized membrane/catalyst layer interface bonding, the fuel cells equipped with BRM1 and BRM2 (ionomer concentrations of 10 and 20 wt %, respectively) exhibit markedly reduced interfacial resistance, increased electrochemical surface area, and improved power performance compared with conventional CRMs. Furthermore, this biased reinforcing process does not weaken the mechanical reinforcing and dimension constraint function of ePTFE skeletons in PFSA membranes. Instead, the BRM membranes demonstrate the same level of mechanical properties, swelling rates, and hydrogen crossover as CRM membranes. After the wet/dry cycle test, BRM1 exhibits less mechanical degradation than CRMs due to enhanced interface bonding, reflected in lower hydrogen crossover and interfacial resistance increase.