Systematic studies on benzimidazole derivatives: Molecular structures and their hydrogen bond networks formation toward proton transfer efficiency

A series of benzimidazole derivatives with varied benzimidazole groups, i.e., mono- (B-1), di- (B-2 and B-3), and trifunctional (B-4), are focused as model compounds for a systematic study to understand the relationship between the hydrogen bond network including its consequent packing structure and the proton conductivity in an anhydrous system. The different number of benzimidazole units in a molecule initiates a different packing structure under the hydrogen bond network of which B-1 provides a perpendicular hydrogen bond network, B-2 and B-3 provide a parallel hydrogen bond network under a lamellar structure, and B-4 forms a helical hydrogen bond network under a columnar structure. The study reveals that the proton conductivity of heterocycles in anhydrous system and doped with polyphosphoric acid is significant when (i) more benzimidazole groups is exist in a single molecule to form hydrogen bond networks, (ii) each benzimidazole is closely packed to allow an effective proton transfer from an NH in one benzimidazole unit to another, and (iii) the packing structure forms a specific channel to favour efficient proton transfer such as columnar packing structure. The present work is a guideline to develop high efficient benzimidazole-based membrane for a high temperature polymer electrolyte membrane fuel cell (PEMFC). (C) 2011 Elsevier B.V. All rights reserved.