Mechanism for Oxygen Reduction Reaction on Pt₃Ni Alloy Fuel Cell Cathode

Abstract

We use quantum mechanics, density functional theory at the PBE level, to predict the binding-site preferences and reaction barriers for all intermediates involved in the oxygen reduction reaction (ORR) on the low energy surface of Pt 3 Ni alloy. Here we calculate that the surface layer is Ni depleted (100% Pt) while the second layer is Ni enriched (50% Pt) as shown by experiment. Even though the top layer is pure Pt, we find that the sublayer Ni imposes strong preferences in binding sites for most intermediates, which in turn strongly influences the reaction barriers. This strong preference leads to a strong site dependence of the barriers. Considering water as the solvent, we predict that, at low coverage of O ad and OH ad , the barrier for the rate-determining step is 0.81 eV, whereas, at high coverage, this barrier decreases to 0.43 eV. It can be compared to a barrier of 0.50 eV for pure Pt, explaining the improved ORR rate for the Pt 3 Ni alloy. We report the results both for gas phase and for aqueous phase environments.

A comprehensive review of Pt electrocatalysts for the oxygen reduction reaction: Nanostructure, activity, mechanism and carbon support in PEM fuel cells

“…The sublayer Ni of Pt 3 Ni alloy, the "most optimal ORR electrocatalyst" ever reported, imposes strong preferences in binding sites for most intermediates, which in turn strongly inuence the reaction barriers. 51 Stephens et al 52 also conrmed that the presence of subsurface 3d metals can weaken the binding of a Pt surface to OH*. They concluded that only a slight weakening of DE OH* by $0.1 eV, relative to Pt (111), will lead to optimal activity for ORR, resulting in an 8-fold improvement in the oxygen reduction activity.…”

Elucidating challenges of reactions with correlated reactant and product binding energies on an example of oxygen reduction reaction

“…O hydration has a lower barrier than direct OH formation (O ad + H ad → OH ad ) for pure Pt, making it the more relevant reaction to study. In the case of Pt alloys when the surface is 100% Pt but the second layer is not, the O hydration barrier was lower than direct OH formation (0.34 eV vs. 0.57 eV for Pt 3 Ni ) 3 . Therefore, O hydration is the more relevant reaction to study for the case of Pt-M-Pt sandwiches due to the 100% Pt surface which has shown a preference for O hydration over direct OH formation.…”

“…Pure Pt appears to have a too low oxo-asymmetry index value and appears to be limited in process II as it was shown in our previous work. 3,4 Pt-Pd-Pt, Pt-Ir-Pt, and Pt-Rh-Pt need to be moved more to the upper left which also means improving process II of the ORR. Pt-Os-Pt needs to be moved more down which means process III, H 2 O formation, limits the ORR.…”

“…The onset potential for the correlated binding reaction, O + H + + e -→ OH can be calculated by defining the energy of H + by the definition of the standard hydrogen electrode reaction, H + + e -→ ½H 2 , as zero volts 3 . Previous studies have shown that this reaction 3 is the least exothermic of the four electron ORR reaction, making its energy difference the negative of the total onset potential:…”

“…Pt-Ru was found to be one of the better alloys in terms of metal leaching to the surface. When comparing the energy of Pt-skin versus Pt-bulk composition for the (111) surface when OH was adsorbed on the surface, Pt-skin was found to be favorable 35 for Pt 3 Ru, while it was shown to be unfavorable for Pt 3 Ni.…”