Liyuan Ren,Longyan Duan,Qiang Weng,Pei Chen,Yafei Hou,Aiai Gao,Xinbing Chen,Zhongwei An
A series of 1-methyl-1H-benzimidazole-based compounds, 2-(4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-1-methyl- 1H-1,3-benzimidazole derivatives (nPPMx-M) with terminal hydrogen, methyl and nitro moieties (coded as nPPMH-M, nPPMM-M and nPPMN-M, respectively), were prepared and their structures were characterised. The compounds display enantiotropic smectic mesophases for hydrogen and methyl terminated compounds (nPPMH-M and nPPMM-M), and enantiotropic nematic mesophases for nitro terminated compounds (nPPMN-M) with short alkoxy chain below than 10 carbon atoms, where the mesophase ranges are 24–72°C and 74–104°C on heating and cooling processes for nPPMH-M, 90–119°C and 110–135°C for nPPMM-M, and 102–129°C and 113–207°C for nPPMN-M, respectively. It is noted that the compounds nPPMx-M exhibit much lower melting points and much wider mesophase range both in heating and cooling than non-1-methyl substituted analogs, which are ascribed to the disruption of hydrogen bonding among the molecules caused by methyl substitution at 1-position of benzimidazole. Meanwhile, among the compounds nPPMx-M, much wider mesophase ranges are obtained for nPPMM-M and nPPMN-M, indicating a much high mesophase stability for the compounds bearing terminal moiety (CH3 and NO2).
In order to investigate the effect of side-chain position and polarity on the performance of anion exchange membrane (AEM), we design and fabricate a series of AEMs with alkyl extender (i.e. the cation's “tail”), alkyl additional side-chain (i.e. the cation's “neighbor”), alkoxy extender, alkoxy additional side-chain, and side-chain-free structure, which are named MImPSf-EC (EC = alkyl extender), MImPSf-AC (AC = alkyl additional side-chain), MImPSf-EO (EO = alkoxy extender), MImPSf-AO (AO = alkoxy additional side-chain) and MImPSf (side-chain-free), respectively. Our results indicate that both additional side-chain and alkoxy chain can promote micro-phase separation, and improve water uptake and conductivity of AEMs, which follow the order of MImPSf-AO > MImPSf-EO > MImPSf-AC > MImPSf > MImPSf-EC at similar ion exchange capacity (IEC). The AEM containing additional alkoxy chain, i.e. MImPSf-AO-150% exhibits the highest water uptake (77.6%) and hydroxide conductivity (23.7 mS/cm) at 30 °C. Compared with others, the alkoxy-containing AEM gives rise to higher H 2 /O 2 fuel cell peak power density (404 mW/cm 2 for MImPSf-AO and 383 mW/cm 2 for MImPSf-EO) and longer durability (137 min for MImPSf-AO and 133 min for MImPSf-EO at 100 mA/cm 2 ) at similar IEC due to its good hydrogen bond network and high stability of alkoxyl chain (both experimentally and theoretically confirmed). Our work demonstrates the advantage of alkoxy chain in AEMs as a partial replacement of degraded cationic groups so that the integrity of ion transport channel can be maintained, and fuel cell durability can be improved.
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