A Study of the Catalytic Interface for O2 Electroreduction on Pt: The Interaction between Carbon Support Meso/Microstructure and Ionomer (Nafion) Distribution

The catalytic activity of PEM fuel cell electrodes is determined by the complex physicochemical interactions among the components of the electrocatalytic interface: precious metal catalyst, support, and ionomer. In the present study, the effect of the carbon support meso- and microporosity was investigated in relation with both the Nafion content and distribution in the catalyst layer. The ionomer load was between 0.09 and 1.1 mg cm(-2) in the catalyst layers prepared either by the Nation-coated or Nafion-mixed techniques, while the Pt load was kept constant at 0.1 mg cm(-2). Three supports were investigated: Vulcan XC-72R, Denka, and graphitized carbon (GC). Employing both the BET (Brunauer-Emmett-Teller) and the BJH (Barrett-Joyner-Halenda) surface measurement techniques, a complete characterization of the support and supported catalyst (Pt/C) pore volume distribution and surface area in the micro- and mesopore size ranges was carried out. It was found that Pt nanoparticles (mean diameter between 4.1-4.9 nm by XRD) reduced the micropore volume of the carbon supports. Therefore, the supports with high BJH mesoporous area (Vulcan XC-72R and GC) yielded Pt/C catalysts with the highest electrochemically active Pt area as well. For the Denka support, characterized by the lowest BJH area, the Nation-coated procedure gave about 7% larger electrochemically active area compared to the Nafion-mixed method. Regarding the oxygen electroreduction, the effective oxygen permeability in the catalyst layer, the intrinsic kinetic current density, and the area and mass-specific activities at 0.9 V-RHE were determined as a function of support type, Nafion incorporation method, and load.