The effect of –NH− on quaternized polybenzimidazole anion exchange membranes for alkaline fuel cells

To investigate the effect of –NH− in polybenzimidazole (PBI) on the properties of PBI anion exchange membranes (AEMs), naphthalene based polybenzimidazole (NPBI) was partially grafted with cationic side chains to afford NPBI-QAx membrane, and then reacted with bromobutane to achieve fully grafting polymer namely NPBI-QAx-By. Surprisingly, after attaching n-butyl on the benzimidazole groups in NPBI backbones completely, the fully grafting polymer NPBI-QA55-B45 showed much higher hydroxide conductivity, achieving 31.7 mS/cm at 20 °C. This value was tenfold compared with that of NPBI-QA55 (2.96 mS/cm at 20 °C) membrane having similar IEC value without N-substitute. It was assumed that ionic interaction occurred between positively charged fixed cations and negatively charged benzimidazolide rings when the NPBI-QAx membranes were immersed in 1 M NaOH. FT-IR and XPS analysis on NPBI-QAx membranes confirmed that ionic interaction between positively charged fixed cations and negatively charged benzimidazolide rings occurred when treated in 1 M NaOH, while no obvious ionic interaction was observed when immersion in 1 M NaHCO3. However, the ionic interaction resulted in excellent stability of the membrane (NPBI-QA55) with –NH− groups, e.g. no obvious chemical structure degradation after alkaline stability test at 80 °C in 5 M NaOH for 760 h, while the NPBI-QA55-B45 membrane suffered from chemical degradations via ring opening reaction of NPBI backbones and nucleophilic substitution of pendant QA cations as confirmed by 1H NMR. Moreover, improved conductivity resulted in excellent fuel cell performance for NPBI-QA55-B45 membrane and the peak power density was about 260.1 mW/cm2. However, no fuel cell performance was obtained for the NPBI-QA55 membrane due to the formation of ionic interaction.