Clarifying the chemical state of additives in membranes for polymer electrolyte fuel cells by X-ray absorption fine structure

• X-ray absorption fine structure technique was applied to fuel cell membranes.
• The chemical states of Ce and Mn in the membrane were directly observed.
• The valence state of Ce was observed to be 3+ before and after the degradation test.
• Mn with low and high valence states (2+ and 3+) existed after the degradation test.
• The reaction rate of Ce/Mn with hydroxyl/hydroperoxyl radicals are discussed.

Cerium and manganese compounds are used in the membrane for polymer electrolyte fuel cells (PEFCs) as radical scavengers to mitigate chemical degradation of the membrane. The chemical states of cerium and manganese in the membrane were investigated using a fluorescence X-ray absorption fine structure (XAFS) technique. Membrane electrode assemblies (MEAs) were subjected to open circuit voltage (OCV) condition, under which hydroxyl radicals attack the membrane; a shift in absorption energy in X-ray absorption near edge structure (XANES) spectra was compared between Ce- and Mn-containing membranes before and after OCV testing. In the case of the Ce-containing MEA, there was no significant difference in XANES spectra before and after OCV testing, whereas in the case of the Mn-containing MEA, there was an obvious shift in XANES absorption energy after OCV testing, indicating that Mn atoms with higher valence state than 2+ exist in the membrane after OCV testing. This can be attributed to the difference in the rate of reduction; the reaction of Ce4+ with ·OOH is much faster than that of Mn3+ with ·OOH, leaving some of the Mn atoms with higher valence state. It was confirmed that cerium and manganese redox couples reduced the attack from radicals, mitigating membrane degradation.

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