Model Long Side-Chain PPO-Based Anion Exchange Ionomers: Properties and Alkaline Stability
Résumé
The stability of anion exchange membranes is paramount for the use in alkaline fuel cells. Long chain ionomers are supposed to be more alkaline-resistant with respect to short chain isomers. In this paper the synthesis, properties and stability of ionomers with a long side chain are investigated. Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) is chosen as backbone, due to its reported stability in alkaline conditions. The functional group is pentyl-ammonium with trimethylamine (TMA) or 1,4-diazabicyclo[2.2.2]octane (DABCO) as model amines. The synthesis is carried out via metalation reaction and is optimized as a function of temperature and time. The water uptake is relatively low, in accordance with the large hydrophobicity of the PPO backbone. The through-plane ionic conductivity is consistent with literature data; it amounts to 15.3 mS/cm at 80 °C for the TMA derivative. The mechanical properties are typical of ionomers below the glass transition temperature (for the TMA derivative at ambient humidity: Young Modulus = 1310 ± 30 MPa). The stability in alkaline conditions, studied by thermogravimetric analysis and measurements of ionic conductivity and ion exchange capacity, is higher than that of short-side chain ionomers with the same basic group. The decrease of ionic conductivity (57 vs 22% residual conductivity after 72 h in 2 M NaOH at 80 °C) and IEC is monitored showing that the degradation is fast in the first hours and may by described by second order kinetics. These results help in selecting high performance anion exchange membranes for electrochemical energy technologies.
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