Polyphenylquinoxaline based hyperbranched/linear composite proton exchange membranes with high free volume and through-plane conductivity for high-temperature proton exchange membrane fuel cells

Homogeneous and transparent proton exchange membranes (PEMs) denoted as HBPPQ/PPQ-x are fabricated via in situ combination of hyperbranched polyphenylquinoxaline (HBPPQ) and linear polyphenylquinoxaline (PPQ). The rigid hyperbranched structure generates stable nanocavities, giving HBPPQ/PPQ-50 high fractional free volume (FFV) of 37%. Moderate main-chain basicity provides favorable phosphoric acid (PA) affinity, leading to the PA uptake of 334%. HBPPQ/PPQ-x membranes exhibit pronounced anisotropic swelling and proton conduction behavior. HBPPQ/PPQ-50 delivers the σ_⊥ of 76 mS cm⁻¹ at 180 °C, 3.1 times the in-plane conductivity (σ_//). It shows an open-circuit voltage of 1.004 V, higher than commercial PBI (0.977 V), and maximum power density of 1122 mW cm⁻², twice that of PBI at 180 °C without back pressure. No obvious voltage decay is observed during 200 h durability test at 160 °C and 0.2 A cm⁻². These results demonstrate that constructing high free volume hyperbranched/linear composite membranes is an effective strategy to simultaneously enhance PA uptake, proton conduction, and HT-PEMFC performance.