Random and Triblock Copolymers with Phosphonium Cations for Alkaline Fuel Cells

Comparing with well-studied proton exchange membrane fuel cells (PEMFCs), a strongly competitive candidate of alkaline fuel cell (AFC) by using anion exchange membrane (AEM) as solid electrolyte has been joined attention^1,2 Researchers worldwide are searching for new AEM electrolytes met with both long operation life and sufficient ionic conductivity in order to realize good AFC performance.^3,4

Block copolymers are expected with highly ordered morphology, which will have interesting influence on the water uptake as well as conductivity performance. In our study, we compare tris(2,4,6-trimethoxyphenyl) phosphonium functionalized AEMs with both triblock (Figure 1. (a)) and random (Figure 1. (b)) backbone structures. The reason investigating on the bulky phosphonium cation is that attaching electron-donating functional groups (2,4,6-trimethoxyphenyl) has been tested beneficial for improvement of the chemical stability based on our previous study.⁵Besides, based on exploration on water uptakes, morphologies, conductivities and transport properties, Insights on block and random copolymer structures impact on fuel cell performance are developed. A further understanding of correlations between hydration number/morphology and conductivity performance are investigated. In this study, water uptake and hydration number are measured through dynamic vapor sorption (DVS). Conductivities under different relative humidity are performed by electrochemical impedance spectroscopy (EIS). Water transportation is measured by pulsed field gradient NMR (PFGNMR) and anion transportation is calculated via Mitra equation based on conductivity result. Morphologies are obtained from small angle X-ray scattering (SAXS) and atomic force microscopy (AFM).

Reference

[1] J.R. Varcoe, P. Atanassov, D.R. Dekel, A.M. Herring, M.A. Hickner, P.A. Kohl, A.R. Kucernak, W.E. Mustain, K. Nijmeijer, K. Scott, Anion-exchange membranes in electrochemical energy systems, Energy & Environmental Science 7 (2014) 3135-3191.

[2] G. McLean, T. Niet, S. Prince-Richard, N. Djilali, An assessment of alkaline fuel cell technology, Int. J. Hydrogen Energy 27 (2002) 507-526.

[3] V. Neagu, I. Bunia, I. Plesca, Ionic polymers - VI. Chemical stability of strong base anion exchangers in aggressive media, Polym. Degrad. Stab. 70 (2000) 463-468.

[4] A.A. Zagorodni, D.L. Kotova, V.F. Selemenev, Infrared spectroscopy of ion exchange resins: chemical deterioration of the resins, React. Funct. Polym. 53 (2002) 157-171.

[5] Y. Liu, J. Wang, Y. Yang, T.M. Brenner, S. Seifert, Y. Yan, M.W. Liberatore, A.M. Herring, Anion Transport in a Chemically Stable, Sterically Bulky Α-C Modified Imidazolium Functionalized Anion Exchange Membrane. J. Phys. Chem. C 118 (2014) 15136-15145.