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Tuning the Properties of Poly(2,6-dimethyl-1,4-phenylene oxide) Anion Exchange Membranes and Their Performance in H2/O2 Fuel Cells


Here, we present a complete investigation of poly(2,6-dimethyl-1,4-phenylene)s (PPO) AEMs with the quaternary ammonium (QA) groups placed in 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 similar composition but considerably different molecular architecture, the nature of which significantly altered their properties and device performance. Hydroxide conductivity was significantly improved for the SCQA, LSCQA as well as 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 SN2 substitution or Hoffmann elimination according to 1H NMR analysis. Interestingly, the fuel cell device performance provided counterintuitive data that showed 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 200 min at 100 mA/cm2. Analysis of the aged membrane showed degradation by the dominant SN2 substitution and elimination of trimethylamine. In contrast, the BTMA-30 membranes with poor alkaline stability showed good durability in a working device without obvious degradation after 500 min operation as confirmed by 1H NMR spectra. This new finding that contrasts material alkaline stability and device 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.

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Publication details

The article was received on 29 Aug 2017, accepted on 22 Dec 2017 and first published on 02 Jan 2018

Article type: Paper
DOI: 10.1039/C7EE02468A
Citation: Energy Environ. Sci., 2018, Accepted Manuscript
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    Tuning the Properties of Poly(2,6-dimethyl-1,4-phenylene oxide) Anion Exchange Membranes and Their Performance in H2/O2 Fuel Cells

    L. Liu, X. Chu, J. Liao, Y. Huang, Y. Li, Z. Ge, M. A. Hickner and N. Li, Energy Environ. Sci., 2018, Accepted Manuscript , DOI: 10.1039/C7EE02468A

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