Twenty years of operando IR, X-ray absorption, and Raman spectroscopy: Direct methanol and hydrogen fuel cells

Abstract

Mixed metal catalysts in polymer electrolyte fuel cell membrane electrode assemblies were characterized by operando infrared, X-ray absorption and Raman spectroscopy. Windowed direct methanol and hydrogen-air fuel cell assemblies allow for spectral acquisition with controlled potential, temperature and flowing reactant streams to the graphite flow fields. Johnson Matthey Pt and PtRu (1:1), in-house prepared PtRu (1:1 and 3:1), PtRuOs (65:25:10), PtNi (1:1), and a melamine-synthesized FeNC catalyst were studied. The near invariant core structure of phase segregated alloy catalysts over practical fuel cell potentials enable a simultaneous fit of metal component EXAFS to quantitatively characterize core structures: JM PtRu is a face centered cubic lattice with 50% of the Ru in an amorphous phase. Stark tuning of COads as a reaction intermediate and as a surface structure probe elucidates potential dependent co-adsorption of Nafion functional groups on Pt. Operando Raman spectroscopic tracking of membrane hydration at a fuel cathode is facilitated by symmetry-based group mode assignments of Nafion IR bands. Fully hydrated Nafion sulfonate groups have C3V local symmetry. Dehydrated sulfonic acid groups have C1 local symmetry. C3V and C1 IR bands coexist at intermediate states of membrane hydration.