Electron spin resonance study of chemical reactions and crossover processes in a fuel cell: Effect of membrane thickness

Abstract

Spin trapping ESR experiments were performed in a fuel cell inserted in the resonator of the ESR spectrometer, and allow visualization of crossover processes and separate detection of chemical reactions at anode and cathode. The membrane-electrode assembly (MEA) consisted of the perfluorinated 3M membranes and Pt as catalyst; the detection of unstable intermediates was possible with 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) as the spin trap. The major goal was to determine the effect of membrane thickness on the type and intensity of the spin adducts; MEAs with membrane thicknesses of 25 μm, 50 μm and 100 μm were studied. DMPO/H and DMPO/OOH adducts were detected in most experiments and their presence is explained by the generation of hydrogen atoms, H·, at the catalyst, and reaction with crossover O2. A strong effect of membrane thickness was detected: For a 25 μm thickness the relative intensities of DMPO/H and DMPO/OOH are similar at both anode and cathode. For 100 μm thickness the relative intensity of DMPO/H at the cathode decreased significantly, due to decreased H2 crossover. At the anode DMPO/H was the only adduct detected; the absence of DMPO/OOH is assigned to negligible O2 crossover. The results provide evidence for the involvement of hydrogen atoms in the membrane degradation. Enhancement of crossover rates in thinner membranes is expected to reduce membrane durability.