Here, we present a complete investigation of poly(2,6-dimethyl-1,4-phenylene) (PPO) AEMs with quaternary ammonium (QA) groups placed at different locations within the chemical structure of the polymer, e.g. a standard benzyltrimethyl ammonium control (BTMA), side-chain-type (SCQA), long side-chain-type (LSCQA), comb-shaped (CQA), or side-chain type/comb-shaped (SCCQA) AEMs have been designed and studied. These copolymers possess a similar composition but considerably different molecular architectures, the nature of which significantly alters their properties and device performance. Hydroxide conductivity was improved for the SCQA, LSCQA and comb-shaped membranes possessing a C-18 alkyl terminal pendant compared to that of BTMA and comb-shaped samples with a short alkyl chain. In chemical stability experiments under 10 M NaOH and 80 °C for 200 hours, LSCQA and SCCQA samples with a C-18 alkyl terminal pendant architecture showed less decrease in conductivity (∼10%) than the BTMA, SCQA and SCCQA polymers having short alkyl chains, which lost more than 50% conductivity after alkaline stability testing. Significant degradation was observed for the unstable PPO AEM samples by either S_N2 substitution or Hoffmann elimination according to ¹H NMR analysis. Interestingly, the fuel cell device performance provided counterintuitive data that showed that longer side chains with excellent alkaline stability were not superior in device function assessment. Specifically, the highly alkaline stable long side-chain-type LSCQA-30 membrane showed significant degradation in a fuel cell device with an operating lifetime of 3.3 h at 100 mA cm⁻². Analysis of the aged membrane showed S_N2 substitution and elimination of trimethylamine to be the dominant degradation mechanisms. In contrast, the BTMA-30 membranes with poor alkaline stability showed good durability in a working device without obvious degradation after 8.3 h operation as confirmed by ¹H NMR spectra. This new finding that contrasts membrane alkaline stability and device operation stability is extremely important and gives us directions for new polymer designs for high performance devices. Additionally, this study paves the way for coupled ex situ chemical stability and in-device degradation studies, which are sorely needed in this field.

To produce a stable anion exchange membrane (AEM) for deployable electrochemical devices with a long lifespan, we here present the synthesis and properties of a series of piperidinium-functionalized poly(2,6-dimethyl phenylene oxide)s with different locations of piperidinium groups along the polymer backbones. A distinct phase separated morphology was observed for long side-chain-type AEMs (LSCPi) as confirmed by AFM analysis, which in turn enabled its higher hydroxide conductivity over side-chain-type (SCPi) and standard benzylmethyl piperidinium AEMs (BPi). A hydroxide conductivity of 29.0 mS cm⁻¹ at 20 °C was achieved for the LSCPi membrane with an IEC value of 1.57 meq. g⁻¹. This level of conductivity was lower than that of the corresponding QA-based AEMs (LSCQA) (38.7 mS cm⁻¹ at 20 °C), probably as a result of its low IEC accompanied by low water uptake. The LSCPi membrane displayed excellent alkaline stability with 98% retention in conductivity after 560 h of testing in 1 M NaOH at 80 °C, and no obvious degradation was detected by NMR analysis of the aged sample. To demonstrate the feasibility of piperidinium-functionalized AEMs, both SCPi and LSCPi membranes were fabricated into a membrane electrode assembly for the H₂/O₂ alkaline fuel cell and AEM water electrolyzer applications. The highly conductive LSCPi membrane showed good cell performance with a peak power density of 116 mW cm⁻² at 60 °C in alkaline fuel cells and 300 mA cm⁻² at 1.80 V at 50 °C in AEM water electrolysis working with pure water. Although a gradual drop in performance was observed for both the alkaline fuel cell and water electrolyser durability testing at a constant current during the test of 8.7 h and 35 h respectively, the high durability of AEMs having piperidinium cations was verified by post-mortem analysis of aged AEMs by NMR spectroscopy. The current findings provided fundamental insights into the durability of AEMs under ex situ and in situ operating conditions and demonstrated that the piperidinium-functionalized AEM appears to be a promising material for durable AEM-based devices.

A series of novel quaternized polypropylene (PP) membranes with ‘side-chain-type’ architecture was prepared by heterogeneous Ziegler–Natta catalyst mediated polymerization and subsequent quaternization. Tough and flexible anion exchange membranes were prepared by melt-pressing of bromoalkyl-functionalized PP (PP-CH₂Br) at 160 °C, followed by post-functionalization with trimethylamine (TMA) or N,N-dimethyl-1-hexadecylamine (DMHDA) and ion exchange. By simple incorporation of a thermally crosslinkable styrenic diene monomer during polymerization, crosslinkable PP-AEMs were also prepared at 220 °C. PP-AEM properties such as ion exchange capacity, thermal stability, water and methanol uptake, methanol permeability, hydroxide conductivity and alkaline stability of uncrosslinked and crosslinked membranes were investigated. Hydroxide conductivities of above 14 mS cm⁻¹ were achieved at room temperature. The crosslinked membranes maintained their high hydroxide conductivities in spite of their extremely low water uptake (up to 56.5 mS cm⁻¹ at 80 °C, water uptake = 21.1 wt%). The unusually low water uptake and good hydroxide conductivity may be attributed to the “side-chain-type” structures of pendent cation groups, which probably facilitate ion transport. The membranes retained more than 85% of their high hydroxide conductivity in 5 M or 10 M NaOH aqueous solution at 80 °C for 700 h, suggesting their excellent alkaline stability. It is assumed that the long alkyl spacer in the ‘side-chain-type’ of 9 carbon atoms between the polymer backbone and cation groups reduces the nucleophilic attack of water or hydroxide at the cationic centre. Thus, PP-based AEMs with long “side-chain-type” cations appear to be very promising candidates with good stability for use in anion exchange membrane fuel cells (AEMFCs).