Novel Sulfonated and Phosphonated Ionomers and Ionomer (blend) Membranes for Electrochemical Applications

Abstract

The heart of a HT-PEM fuel cell is represented by the proton conductive membrane. Nowadays PFSA-based materials e.g. Nafion are state of the art in large scale energy applications. However, the PFSAs are relatively expensive, and give rise to concerns regarding toxic intermediates in the production process, and the questionability of their recyclability. This leads to a strong need for development of cost effective alternatives. Such alternatives have to exihibit customized properties, like reduced gas crossover, high ionic conductivity, mechanical strength, thermal stability and durability.In this work, we present novel cation-exchange ionomers, which are suitable as proton exchange membranes in electrochemical applications such as fuel cells, electrolysers, and redox-flow batteries. For our approach we developed novel fully and partially aromatic polymers with customized mechanical behavior, using two different synthetic pathways, the first being modification of the aromatic building blocks to more flexible ones and/or introduction of flexible side chains into the monomers or polymers.With these two approaches it was possible to synthesize ionomer materials which are less brittle than our earlier developed sulfonated[1] or phosphonated[2] poly(pentafluorostyrene) ionomers. Our development lead to ionomers which are suitable in pure form or as blend in proton exchange membranes with promising proton conductivity, mechanical and thermal stability.The novel ionomers and ionomer membranes were characterized in terms of ex-situ characterization including impedance spectroscopy, IR spectroscopy, NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis. Moreover, preliminary in-situ tests of the ionomer membranes in low and high-temperature fuel cells have been performed, yielding promising results. Furthermore the synthesized membranes were characterized in-situ in redox-flow batteries, yielding promising results. [1] Highly phosphonated polypentafluorostyrene. V. Atanasov, J. Kerres, Macromolecules 2011, 44, 6416–6423; DOI: 10.1021/ma2011574 [2] Sulfonated poly(pentafluorostyrene): synthesis & characterization. V. Atanasov, M. Bürger, S. Lyonnard, G. Gebel, J. Kerres, Solid State Ionics, 2013, 252, 75-83; DOI: 10.1016/j.ssi.2013.06.010