Kinetic Stabilization of Ordered Intermetallic Phases as Fuel Cell Anode Materials

The influence of fuel molecules on the stability of the ordered intermetallic PtBi and PtPb phases has been extensively studied by synchrotron-based in situ X-ray grazing incidence diffraction under active electrochemical control. Cycling the potential to increasingly positive values resulted in little change to the surface composition and crystalline structure when specific fuel molecules (such as formic acid for PtBi and formic acid or methanol for PtPb) were oxidized at the intermetallic electrode surface. This was demonstrated by the absence of diffraction peaks due to Pt domains that would be generated by the leaching out of the less noble metal. This phenomenon has been rationalized as a competition process between the oxidation of fuel molecules at the electrode surface and corrosion and damage of the surface due to the electrochemical treatment. For example, PtBi electrodes, which exhibit excellent catalytic activity toward the oxidation of formic acid, could be kinetically stabilized to such a corrosion/degradation process in the presence of formic acid even at relatively positive potentials. An analogous effect was observed for PtPb in the presence of methanol as fuel. In the absence of fuel molecules (formic acid for PtBi and formic acid and/or methanol for PtPb), various surface layers were generated by different electrochemical pretreatments in the presence of only a supporting electrolyte. Crystalline oxidized bismuth species (such as Bi2O3) with an ∼50 nm domain size were formed on the PtBi electrode surface by holding the potential at +1.00 V or beyond for at least 30 min. On the other hand, platinum nanopaticles with an ∼5 nm crystalline domain size were formed when cycling the potential to higher values. In the case of PtPb, the only detected corrosion product was PbSO4, whose diffraction peaks were utilized to qualitatively analyze the lead leaching-out and dissolution processes. No crystalline lead oxide species were detected after electrochemical pretreatment in the supporting electrolyte by holding the potential at +1.00 V or beyond.