Protonation Modification of Commercial OPBI as Binders Toward High-Performance High-Temperature PEM Fuel Cells

Polybenzimidazole (PBI) is a promising binder for catalyst-layer (CL) of high-temperature proton exchange membrane fuel cells (HT-PEMFCs), offering acid-doped proton conductivity and better membrane compatibility than PTFE. However, commercial PBI binders suffer from chain aggregation and nonuniform distribution, which block Pt active sites and compromise fuel cell performance. Herein, we demonstrate a facile formic-acid protonation strategy to regulate commercial OPBI binders without disrupting the polymer backbone. Protonation introduces positively charged benzimidazole sites, inducing interchain electrostatic repulsion that loosens chain packing, enlarges intermolecular spacing, and promotes uniform binder dispersion over catalyst surfaces. As a result, protonated OPBI exhibits a 382% increase in phosphoric acid uptake and a proton conductivity of 357 mS cm^-1. In CL, protonated OPBI reduces proton transport resistance by 63.7%, thereby promoting triple-phase boundary formation, as evidenced by nearly doubled ECSA, while preserving reactant-transport pathways. The resulting membrane electrode assembly delivers a peak power density of 912 mW cm^-2 and a Pt-mass-specific peak power density of 5.2 W mg_Pt ^-1 at an ultralow Pt loading of 0.075 mg cm^-2. This work highlights protonation-induced chain-packing regulation as an effective strategy for high-performance PBI binders in HT-PEMFCs.