MAS and PFG NMR studies of proton exchanges membranes for electrolytic water splitting

Proton exchange membranes (PEMs) are a critical component in acidic water electrolyzers. They serve the purpose of separating cathode and anode compartments and thus preventing a loss in faradaic efficiency, while simultaneously providing a high proton conductivity for charge compensation. In order to minimize the ohmic losses during electrolysis, the electrode material is often coated directly onto the PEM in order to create a membrane-electrode-assembly (MEA) [1].While high ionic conductivity and impermeability for electrolysis products are crucial properties of a PEM, long-term stability and resistance to chemical and structural alteration are a decisive factor for their industrial application. It is assumed that losses in the stability of the polymer occur due to the formation and attack of radicals on the polymer backbone and the side chains. In particular, the polar side chains are responsible for the formation of hydrophilic channels inside the polymer which promote the proton transport of the material. The Fenton reaction, where iron cations react with peroxide formed during electrolysis, is discussed to be a main mechanism of radical formation [2].Nuclear magnetic resonance (NMR) spectroscopy has been established as a powerful tool for the investigation of polymers and especially ionomers. In this study, magic angle spinning (MAS) and pulsed field gradient (PFG) NMR techniques has been applied for the investigation of the Fenton reaction in short-side-chained PEM materials. The PEMs were treated in a Fenton reagent, i.e. an aqueous solution of Fe(ii) and H2O2. In addition, the PEMs were treated in an aqueous iron(ii) solution as a reference experiment. The effect of Fenton degradation on the chemical structure and chain mobility was studied using MAS NMR experiments, and changes in proton transport properties were assessed by PFG NMR. It was shown that radicals preferentially attack the side chains of the PEM polymer, which decreases the overall mobility of the polar groups and thus affects proton mobility.We thankfully acknowledge funding and support by the Bundesministerium für Bildung und Forschung (BMBF). [1] S. Shiva Kumar et al., Materials Science for Energy Technologies, 2. 442-454, 2019.[2] L. Ghassemzadeh et al., Journal of Power Sources, 196, 2490-2497, 2011