HEMs are typically composed of organic cations tethered to a polymer backbone, with the charge balanced by free OH− anions. The alkaline stability of both the organic cation and the polymer backbone limit the overall stability of the HEM.5 Recently, Marino and Kreuer reported that piperidinium exhibited a 21-fold increase in alkaline stability as compared to benzyl trimethyl ammonium, the benchmark ammonium.6 Meanwhile, a reliable and scalable synthetic route to tether piperidinium to a stable polymer backbone remains elusive. It is generally accepted that functionalized ether-bond-containing engineering plastics, e.g., poly(arylene ether) and poly(phenylene oxide), are susceptible to chemical degradation via ether-bond scission and the subsequent decomposition.7 Another key challenge in the development of HEMs is the tradeoff between ionic conductivity and mechanical strength.
In this work, we report a new family of poly(aryl piperidinium) (PAP) HEMs/HEIs in which alkaline stable piperidinium cations were tethered to an ether-bond-free, rigid and hydrophobic aryl backbone. PAP’s excellent properties originate from the combination of the stable piperidinium cation and the rigid ether-bond-free aryl backbone which enables an unprecedented combination of a high ion exchange capacity (IEC) and a low water uptake/swelling ratio. PAP HEMs have demonstrated excellent alkaline stability (e.g., no degradation in 1 M KOH for 2000 h at 100 °C), high OH− conductivity (e.g., 200 mS cm−1 at 95 °C in water), and high mechanical strength (140 MPa). PAP also has the selective solubility to make ionomer solutions and can be cast into membranes with thicknesses down to 5 µm. MEAs using PAP HEM and the corresponding ionomer showed a peak power density of 860 mW cm−2 at 95 °C with minimal optimization. Further, discharge at 90 °C revealed no voltage degradation at constant current density of 200 mA cm−2 for 60 h, in contrast to the rapid degradation of a commercial membrane/ionomer under identical conditions (within minutes).
(1) Yoshida, T.; Kojima, K. Electrochem Soc Inte 2015, 24, 45.
(2) Setzler, B. P.; Zhuang, Z. B.; Wittkopf, J. A.; Yan, Y. S. Nat Nanotechnol 2016, 11, 1020.
(3) Varcoe, J. R.; Slade, R. C. T. Fuel Cells 2005, 5, 187.
(4) Dekel, D. R. J Power Sources 2017.
(5) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; Xu, T. W.; Zhuang, L. Energ Environ Sci 2014, 7, 3135.
(6) Marino, M. G.; Kreuer, K. D. Chemsuschem 2015, 8, 513.
(7) Arges, C. G.; Ramani, V. P Natl Acad Sci USA 2013, 110, 2490.