Molecular Structures at Nafion/Graphene Interfaces Investigated by Sum-Frequency Generation Spectroscopy

The interfacial structures of a Nafion/graphene composite, which is used to mimic the interfaces of the catalyst layer in proton-exchange membrane fuel cells (PEMFCs), under different humid environments, are investigated by sum-frequency generation (SFG) vibrational spectroscopy. The Nafion thin film seems to form a lamellar structure on the graphene surface where the sulfonate moieties of Nafion at the buried graphene surface probably orientate to the Nafion bulk, the same as that of the free Nafion/air interface. The SFG observation shows that the structure of the buried Nafion/graphene interface is distinct from that on a buried hydrophilic substrate surface such as fused quartz (SiO2) and calcium fluoride (CaF2). The structural differences in the buried Nafion/graphene interfaces significantly affect water adsorption behaviors at the interface. This new information promises to alleviate some of the significant problems in modeling the PEMFC system.