Reducing Swelling in Heteropoly Acid Proton Exchange Membranes

With the increasing demand for new technologies that utilize renewable energies, the Herring Research Group at the Colorado School of Mines has begun exploring the use of heteropoly acids (HPAs) in proton exchange membranes for hydrogen fuel cells.

HPAs are a category of acidic molecules based on a central hetero atom surrounded by a cluster of metallic atoms and oxygen atoms. Since these spherical molecules carry a large negative charge, they behave as super acids in the H+ form. As such, these molecules show much potential for being conductive of protons, which, if incorporated into a polymer membrane that was electrically insulting, could serve as excellent membrane material for fuel cells.

The Herring Research Group has developed a polymeric membrane material based on a 1,1-difluoroethylene and hexafluoro-propylene backbone with a phenyl phosphonic acid side chain. The polymer is solvated, and HPA in the K+ form is added. The solution is then cast into a thin-film and allowed to dry overnight. The film is then heated to allow the HPA to react with the polymer, resulting in a cross-linked membrane. In order to make the membrane conductive of protons, the membrane is then soaked in acid to allow the HPA to ion exchange from the K+ form to the H+ form. Another heating step allows the HPA to react further with the polymer backbone. Finally, the film is soaked in water overnight to remove any residual acid.

Changes in the heating steps lead to drastically different film performance; in order to investigate these differences, multiple films were cast and processed at different conditions, varying temperature of heating, pressure of heating, and heating time for both heating steps. The main response variable under investigation was the swelling of the film in liquid water. Once optimized, these films will be incorporated into hydrogen fuel cells where they will undergo cyclical hydration and dehydration under high stress. If the membranes swell too much when hydrated, they will crack and the cell will no longer be usable. For this reason, swelling is to be minimized. Parallel to this goal, in order for the membrane to remain economically viable proton conductivity must be comparable to industrial standards.

Taking into account both of these properties, processed films underwent liquid water swelling tests to measure weight-percent swelling and electrochemical impedance spectroscopy to characterize proton conductivity. Results indicated that processing the material at high temperatures and longer times resulted in minimum weight percent swelling of 40%. Meanwhile, conductivity testing demonstrated that conditions of the first heating step were mostly inconsequential, and that higher temperatures gave more conductive films. Finally, varying the pressure of the heating steps indicated that processing the films at atmospheric yielded phenomenally better films.