Hydroxide exchange membranes (HEMs) can enable platinum-group-metal-free HEM fuel cells (HEMFCs) and HEM electrolyzers (HEMELs) that are potentially economically viable. These membranes are also useful in other electrochemical devices like solar hydrogen electrodialysis cells. The basic structure of a HEM is an organic cation tethered to a polymer backbone. We have systematically studied the cation, the tether and the backbone in the past decade, and developed HEMs with quaternary phosphonium,1
and stabilized imidazolium4
that have shown high chemical stability, hydroxide conductivity and reasonably low water uptake. In this presentation, I review our past work and present our most recent work on a new class of poly(aryl piperidinium) (PAP) HEMs. The PAP polymers are designed by combining cyclic quaternary ammonium cations and ether-bond-free backbones, which together provide unprecedented chemical stability, mechanical robustness and ionic conductivity. The rigid and hydrophobic aryl backbone enables achievement of both high ion exchange capacities and low swelling ratios, leading to high hydroxide conductivities and mechanical robustness. I will also discuss our work on the development of a HEM based on molecular sieving crystalline porous polymers (organic zeolites).
1. S. Gu, R. Cai, T. Luo, Z. W. Chen, M. W. Sun, Y. Liu, G. H. He and Y. S. Yan, A Soluble and Highly Conductive Ionomer for High-Performance Hydroxide Exchange Membrane Fuel Cells, Angew. Chem.-Int. Edit., 2009, 48, 6499.
2. B. Z. Zhang, S. Gu, J. H. Wang, Y. Liu, A. M. Herring and Y. S. Yan, Tertiary sulfonium as a cationic functional group for hydroxide exchange membranes, RSC Advances, 2012, 2, 12683.
3. S. Gu, J. Wang, R. B. Kaspar, Q. Fang, B. Zhang, E. Bryan Coughlin and Y. Yan, Permethyl Cobaltocenium (Cp*2Co+) as an Ultra-Stable Cation for Polymer Hydroxide-Exchange Membranes, Sci Rep, 2015, 5, 11668.
4. J. H. Wang, S. Gu, R. B. Kaspar, B. Z. Zhang and Y. S. Yan, Stabilizing the Imidazolium Cation in Hydroxide-Exchange Membranes for Fuel Cells, Chemsuschem, 2013, 6, 2079.