Poly(aryl piperidinium) anion exchange membranes with cationic extender sidechain for fuel cells

Anion-exchange membrane fuel cells (AEMFCs) is a promising solution to decrease the cost of fuel cell, because it is adaptable to non-noble-metals catalysts and low-cost stack components. However, anion exchange membranes (AEMs), a crucial component of AEMFCs devices, with desired properties (high ions conductivity, excellent chemical stability, robust mechanical strength, etc.) are currently unavailable. In this paper, poly(biphenyl N-methylpiperidine) (PBP) was synthesized as the backbones of AEMs, which could provide good chemical stability and robust mechanical strength. Quaternary ammonium cations containing various types of substituent sidechains, including hydrophobic alkyl chain, hydrophilic PEG chain and multi-PEG chains, were introduced onto the PBP backbones, and resulting AEMs were named PBP-alkyl, PBP-PEG and PBP-TPEG, respectively. The relationship between AEMs structure and performance were studied, and molecular dynamics (MD) simulations were carried out to microcosmically reveal the mechanism of structure-activity relationship. All the PBP-based AEMs have acceptable alkaline stability and performance loss is less than 10% under 2 M NaOH at 80 °C for about 500 h. PBP-PEG has the highest ions conductivity of 97.3 mS cm-1 and lowest water uptake (WU), while PBP-TPEG has the lowest ions conductivity of 56.1 mS cm-1 and highest WU. For single cell evaluations at 80 °C under H2/O2 condition, AEMFC containing PBP-PEG membrane bursts out a highest peak power density of 373 mW cm-2, while PBP-alkyl and PBP-TPEG only achieve 133 and 93 mW cm-2, respectively. MD results show the water channel of PBP-PEG is denser and more continuous than other structures, and the proportion of completely hydrated transport is higher than other structures, which contributes to the increase of overall performance. On the other hand, DFT results show the low basicity and strong hydrophily of cation containing multi-PEG substituents leads to the poor performance of PBP-TPEG. Therefore, for the design of AEMs molecular structures with outstanding performances, hydrophilic PEG spacer chain is more effective than hydrophobic alkyl spacer chain, and PEG is more effective as a spacer chain than as an extender chain.