Tailoring Backbone Geometry Via Positional Isomerism in Poly(carbazole)-Based Proton Exchange Membranes with High Conductivity and Dimensional Stability for Water Electrolysis and Fuel Cells

Conventional structural designs of proton exchange membranes often fail to decouple ion transport efficiency from the adverse effects of dense polymer chain packing. Consequently, achieving a balance among high proton conductivity, favorable dimensional stability, and superior mechanical toughness remains a formidable challenge. In this study, we present a strategic approach to manipulate the segmental distribution of polycarbazole-based proton exchange membranes by exploiting the positional isomerism of phenyl substitution, thereby promoting the formation of free volume and microphase-separated morphologies. Two isomeric polymers, S-PEM-100 (featuring a rigid linear backbone) and T-PEM-100 (featuring a rigid twisted backbone), were synthesized via superacid-catalyzed polycondensation. Molecular dynamics simulations and CO2 adsorption–desorption measurements revealed that the twisted geometry of T-PEM-100 hinders dense interchain packing, leading to a substantially increased free volume, whereas S-PEM-100 retains compact π–π stacking. Small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), and transmission electron microscopy (TEM) confirmed that this enlarged free volume facilitates the self-assembly of sulfonic acid groups into large, well-connected hydrophilic domains. Consequently, T-PEM-100 achieved a high proton conductivity of 166.9 mS cm–1 at 80 °C and exhibited outstanding performance in both pure water electrolysis and H2/O2 fuel cells. This work provides fundamental insights into leveraging molecular geometry for the design of high-performance proton exchange membranes for sustainable energy applications.