Phosphoric acid-doped polybenzimidazole (PA-PBI) used in high-temperature proton exchange membranes (HT-PEMs) frequently suffers from a serious loss of mechanical strength because of the “plasticizing effect” of the dopant acid. Conventional cross-linking approaches generally enhance membrane stability. However, acid doping levels (ADLs) and consequently proton conductivity inevitably decrease. This is due to the formation of more compact molecular structures and a reduced amount of functional imidazole units, caused by their consumption in introducing the cross-linker. To resolve the common problems of current PA-PBI-based HT-PEMs, herein, a highly acidophilic imidazole-rich cross-linked network with superior “antiplasticizing” ability is constructed based on a novel multifunctional cross-linker. This unique bischloro/bibenzimidazole (“A2B2-type”) molecular structure has extremely high reactivity, including “self-reaction” among the cross-linkers and “inter-reaction” between the cross-linker and PBI molecules. The resulting imidazole-rich cross-linked membranes exhibit the desired combination of high ADLs, high conductivity, outstanding dimensional–mechanical stability, and excellent fuel cell performance. In comparison to a corresponding linear PBI membrane, one membrane with a high content of the cross-linker of 30% has a 100 wt % increased acid uptake, a doubling in proton conductivity at 200 °C, and a maximum power density of 533 mW·cm–2 at 160 °C without humidification.

The practical applications of phosphoric acid-doped polybenzimidazole (PA–PBI) as high-temperature proton exchange membranes (HT-PEMs) are mainly limited by their poor dimensional-mechanical stability at high acid doping levels (ADLs) and the leaching of PA from membranes during fuel cell operation. In this work, to overcome these issues, we fabricated novel cross-linked PBI networks with additional imidazole groups by employing a newly synthesized bibenzimidazole-containing dichloro compound as cross-linker and an arylether-type Ph-PBI as matrix. Ph-PBI featured by good solubility under high molecular weight offers satisfactory film-forming ability and mechanical strength using for the matrix. Importantly, the additional imidazole moieties in BIM-2Cl endow the cross-linked PBI membranes improved dimensional-mechanical stability with simultaneously enhanced ADLs and proton conductivity. Furthermore, superior acid retention capability is obtained by incorporating porous polyhydroxy SiO2 nanoparticles into these cross-linked networks. As a result, the SiO2/cross-linked PBI composite membranes are suitable to manufacture membrane electrode assemblies (MEAs), and an excellent H2/O2 cell performance with a peak power density of 497 mW cm–2 at 160 °C under anhydrous conditions can be achieved.

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Polymers of intrinsic microporosity are for the first time incorporated into PBIs to form some novel alloys for HT-PEMFC applications.