2D-Like Stacked-Arrangement of Poly(p-Phenylene) Ionomers: Toward Durable Hydrocarbon-Based Membranes for Water Electrolysis

The heteroatom-free ionomer backbone is attracting attention for proton-conducting ionomers due to its chemical durability under the operating conditions of proton exchange membrane water electrolysis (PEMWE). In this context, the p -phenylene backbone has been widely studied as a suitable framework for proton exchange membranes (PEMs), yet its molecular arrangement in bulk membranes and the influence of ion exchange capacity (IEC) on overall membrane properties remain inadequately understood, particularly regarding trade-offs among ion conductivity, gas permeability, and mechanical and chemical stability. In this study, sulfonated poly( p -phenylene)-based ionomers (SPPs) with IECs ranging from 1.6 to 2.6 mequiv/g are synthesized via a Ni(0)-catalyzed coupling reaction. X-ray scattering analysis reveals a unique 2D-like stacking of ionomer chains in the thickness direction, which improves in-plane dimensional stability and modulus. The heteroatom-free backbone also provides superior oxidative stability compared to hydrocarbon ionomers containing ether or sulfone groups. Although the stacked configuration limits proton conduction through the membrane thickness, SPPs with IEC values above 2.2 mequiv/g demonstrates PEMWE performance comparable to that of commercial Nafion membranes. Under a constant current density of 2 A/cm 2, SPP-based membranes show a voltage degradation rate of 120 μV/h, approximately half that of a more flexible hydrocarbon PEM with an IEC of 1.9 mequiv/g (250 μV/h). These results underscore the potential of rigid, stacked SPP architectures as durable and high-performing alternatives to conventional PEM materials for PEMWE applications.