Operando Raman micro-spectroscopy of the membrane electrode assembly (MEA) of a fully operating hydrogen/oxygen Nafion electrolyte fuel cell is described. Coarse depth profiling of the fuel cell system enabled appropriate positioning of the microspectroscopy laser focal point for MEA catalytic layer spectroscopy. An increase in the ionomer state-of-hydration, from oxygen reduction at the cathode, transitions ion exchange sites from the sulfonic acid to the dissociated sulfonate form. Visualization of density functional theory calculated normal mode eigenvector animations enabled assignments of Nafion side-chain vibrational bands in terms of the exchange site local symmetry: C 1 and C 3V modes correlate to the sulfonic acid and sulfonate forms respectively. The gradual transition of the MEA spectra from C 1 to C 3V modes, from the fuel cell open circuit voltage to the short circuit current respectively, demonstrate the utility of vibrational group mode assignments in terms of exchange site local symmetry.
The dehydration of SPEEK-[H] (protonated sulfonated poly(ether ether ketone)) converts 3-fold symmetric (C 3V) hydrated sulfonate sites to sulfonic acid sites with no local symmetry (C 1). Like Nafion-[H], SPEEK-[H] C 3V and C 1 environments afford IR group modes (C 3V,HF (1087 cm-1); C 3V,LF (1026 cm-1) and C 1,HF (1362 cm-1); C 1,LF (904 cm-1)) due to the mechanical coupling of vibrational internal coordinates of an ether link with those of the sulfonate or sulfonic acid exchange site. C 3V and C 1 bands are inversely correlated during membrane hydration/dehydration. Hydrated SPEEK-[M] (M: Cu 2+ , Ni 2+ , Cd 2+ , Pb 2+ , Sr 2+ or Ba 2+) exhibits SPEEK-[H] C 3V bands because cation waters of hydration preclude binding to the hydrated exchange site. When cation hydration spheres are thermally stripped at high vacuum, SPEEK-[M] C 3V bands supplant SPEEK-[H] C 3V bands, inducing SPEEK cross-linking. Incomplete cross-linking is evidenced by low intensities of SPEEK-[M] C 1,HF bands. The 1362 cm-1 band intensities are inversely correlated with cation hydration enthalpies.
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