Hyperbranched poly(benzimidazole-co-benzene) with honeycomb structure as a membrane for high-temperature proton-exchange membrane fuel cells

Hyperbranched poly(benzimidazole-co-benzene) (PBIB) with a honeycomb structure is synthesized by condensation polymerization of trimesic acid (TMA) and 3,3'-diaminobenzidine (DAB) for use as a membrane high-temperature proton-exchange membrane fuel cells (HT-PEMFCs). The hyperbranched honeycomb structure of polybenzimidazole (PBI) has been introduced to impart higher mechanical strength to doped PBI membranes. The stress at break of the phosphoric acid doped PBIB (DPBIB) membrane (29 +/- 3 MPa) is comparable with that of Nafion (28 +/- 2 MPa) and much superior to doped PBI membranes. The DPBIB membrane exhibits lower proton conductivity than Nafion 115. On the other hand, the proton conductivity of Nafion 115 is enhanced with increase in relative humidity, whereas humidity has only a moderate effect on the proton conductivity of the DPBIB membrane. Consequently, the Nafion 115 membrane in a fuel cell cannot operate in the absence of humidity, whereas the DPBIB membrane can perform well. The power output of the DPBIB membrane in a fuel cell is superior under humid conditions than under dry conditions. The maximum power output from the DPBIB and Nafion 115 membranes is comparable under humid conditions. It is concluded that the DPBIB membrane, but not Nafion, is suitable for application in HT-PEMFCs. (C) 2009 Elsevier B.V. All rights reserved.