(485e) Dealloyed Pt100-x-YCoxNiy Electrocatalysts for Oxygen Reduction Reaction

In this work we have studied the synthesis, structure and elecrocatalytic behavior for the oxygen reduction reaction of a ternary alloy, Pt_100-x-yCo_xNi_y.  Electrodeposition of ternary alloy films was studied from chloride baths in which Pt is overpotentially deposited (OPD) while Co and Ni deposit by underpotential deposition (UPD) trapping at more positive potentials, shifting from UPD to mixed OPD-UPD at more negative potentials.  The Pt content displays a monotonic decrease as deposition potential decreases.  For an electrolyte containing equal parts Ni²⁺ and Co²⁺, the iron group (IG) content coincides with compositional trends expected from their redox potentials at more positive deposition potentials but transitions to anomalous codeposition of Co and Ni at more negative deposition potentials with substantially greater Co content in the deposit with respect to Ni.  

The potential dependent morphology exhibits a smooth progression from characteristic Pt- to Ni- and finally Co-rich deposits with decreasing potential.  Xray diffraction (XRD) spectra indicate the as-deposited films are solid solutions with an fcc crystallographic structure at more positive potentials that transition to a hexagonal phase for Co-rich deposits.  The films were activated by dealloying or selective removal of the IG metals, creating a variety of Pt-rich nanoporous morphologies, as shown by electron microscopy.  The compositions of the dealloyed remnants were close to a 3:1 Pt:IG stoichiometry as shown by energy dispersive X ray spectroscopy (EDS) and XRD peak positions.

The Pt_100-x-yCo_xNi_y  electrocatalysts were examined using a rotating disk electrode to find the kinetic activity and surface area of each composition.  The Pt mass loading was determined from EDS values and electrochemical quartz crystal microbalance.  IG-rich films displayed the highest specific activity and hydrogen underpotential deposition (H_UPD) surface area with the most active films exhibiting an iR corrected specific activity enhancement factor of 6.8 and 4.7 over a polycrystalline Pt rotating disk electrode at 0.90V and 0.95V RHE.  The combined improvements in specific activity and H_UPD lead to a drastic increase in mass activity for IG-rich films.