Measurement and adjustment of proton activity in solid polymer electrolytes

•Polymer electrolytes with modified pH properties are required for many applications.•Several membranes were prepared from modified Nafion or phosphonic acid polymers.•World first measurement of pH of modified solid polymer electrolytes is presented.•Cheap, simple and versatile technique can be easily adopted.

Technological progress in electrochemical energy conversion devices requires new solid polymer electrolyte membrane (PEM) materials with particular properties. The strongly acidic nature of Nafion® is not always desirable; for instance, CO2 electroreduction requires a low proton activity to avoid excess hydrogen evolution. This communication presents a novel measurement technique for determining the acidity of a PEM, using a pH-sensitive electrode half-cell attached to the membrane sample, connected to a reference electrode via a salt bridge. A dynamic hydrogen electrode on platinized‑platinum surface (DHE) was found to give repeatable results within 5% uncertainty. Several membranes based on Nafion® impregnated with various basic species were tested, as well as an anion-exchange membrane and a poly-phosphonic acid membrane. The technique is expected to have wider reaching applications in PEM fuel cell and redox flow battery development, as well as in the electrolysis and electrodialysis industries.

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