Abstract
Operando vibrational spectroscopy of the membrane electrode assembly (MEA) of Pt catalyzed fuel cell cathodes elucidates potential dependent adsorptive properties of ionomers upon Pt surfaces. These include direct evidence of sulfonate group adsorption at fuel cell potentials of 700 mV. 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. C1 and C3V modes correlate to the sulfonic acid and sulfonate forms respectively. The gradual transition of the MEA spectra from C1 to C3V 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. Operando vibrational spectroscopy coupled with polarization modulated IR spectroscopy of Nafion coated Pt also confirms adsorption of the Nafion -CF3 group upon Pt. At the anode side of the operando spectroscopy cell, Stark tuning of CO stretching modes are observed and discussed. The absence of Stark tuning is evidence of metal based materials as non-catalytic surfaces.
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