Sulfonated PVDF-<i>co</i>-HFP Entangled Statistical Copolymeric Polyelectrolyte Separator with Improved Interfacial Adhesion for Efficient Water Electrolysis

Developing proton exchange membranes (PEMs) with high conductivity, stability, and strong interfacial adhesion between the membrane and catalyst layer is essential for efficient proton exchange membrane water electrolyzer (PEMWE) performance. In this work, a series of blend PEMs were fabricated by combining a partially sulfonated fluoropolymer, i.e., sulfonated poly(vinylidene fluoride-co-hexafluoropropylene (sPVDF-co-HFP) with a hydrocarbon-based statistical copolymer of poly(methyl methacrylate)-co-polystyrene sulfonic acid (PMMA-co-PSSA, prepared via free radical copolymerization of MMA and sodium-4-styrenesulfonate followed by acid reflux). The optimized blending ratio induced nanophase separation of hydrophilic and hydrophobic domains, improving both proton transport and dimensional stability. Strong hydrogen bonding and dipole interactions enhanced the electroactive β-phase of sPVDF-co-HFP, contributing to excellent electrochemical performance. MSP-73, with 70% w/w PMMA-co-PSSA, exhibited the highest proton conductivity (26.3 mS/cm at 80 °C), ion-exchange capacity (IEC, 1.1 meq/g), and low high-frequency resistance (HFR, 0.62 Ω cm2), achieving a current density of 453.1 mA/cm2 at 1.8 V (∼13% higher than Nafion 117), but with limited durability of 37 h. In contrast, MSP-64 (IEC = 1.04 meq/g), with a 60:40 (w/w) hydrocarbon-to-fluoropolymer ratio, delivered a balanced performance, achieving 420 mA/cm2 at 1.8 V with lower ohmic overpotentials, retaining 93.8% of its initial performance after 72 h of durability test, with only a modest HFR increase (0.69→0.8 Ω cm2). sPVDF-co-HFP acts as a protective shield for the PMMA-co-PSSA polymeric chain, safeguarding it from harsh oxidative attack and impeding premature creep failure.