Modeling the Effect of Underlying Cobalt on the Electrochemical Behavior of Pt-Skin / Pt100-x Cox(111) Single Crystal Electrodes
Authors: Tryk Donald A. , Kobayashi Shun , Inukai Junji , Uchida Hiroyuki 
Published: February 2020

Pt skin-covered Pt alloy nanoparticles are highly active catalysts for the oxygen reduction reaction (ORR),1 the hydrogen oxidation reaction (HOR),2 and the hydrogen evolution reaction (HER)3 in acid electrolyte. We have sought to understand the effect of the underlying alloying element, usually Fe, Co or Ni, using density functional theory (DFT). The ideal test for the calculations is an electrode that has been characterized at the atomic level, such as a single crystal with known near-surface composition. Recently, we found that a Pt-skin/Pt73Co27(111) electrode exhibited ORR activity that was much higher than that of Pt(111), by a factor of 27.4 Based on in situ surface X-ray scattering measurements, this electrode has a nearly pure Pt surface layer and a nearly pure Co second layer, with the third layer composition close to that of the bulk alloy.5 The objective of the present work is to understand the electrochemical behavior of this type of well-characterized electrode, starting with the cyclic voltammogram (CV) (Fig. 1). Similar to Pt3Ni(111),6 the H adsorption region is shifted negatively, and the OH adsorption region is shifted positively.
To model the behavior of the H adsorption region, we carried out DFT calculations on three-layer slab models using DMol3 (Biovia), with geometry optimization with semicore pseudopotentials and energy calculations with all-electron scalar relativistic accuracy.7 This approach has been found to provide realistic adsorption energies. The present work tests its ability to provide realistic trends in the Fermi energy E
Fas a function of H coverage alone, since it has been shown by Ishikawa and coworkers that the (111) surface with underpotentially deposited hydrogen HUPD is highly hydrophobic.7 This approach can also accurately model the vacuum experiment, in which the work function change is measured vs. H coverage.8

We also plot the integrated H adsorption charges taken directly from CV, which is the most straightforward way to compare with the calculations. We carried out calculations for a Pt(111)-6x6 3-layer model with sequentially added H atoms, up to unity coverage (36 atoms), with the further addition of 12 H atoms, corresponding to overpotentially deposited H (HOPD). This region is difficult to evaluate with CV, although it is observable spectroscopically.9 The coverage can be converted straightforwardly to a charge, but E
F must be empirically transformed to match the electrochemical potential. Interestingly, the calculated curve has a delayed onset, similar to that observed in vacuum.8 Calculations were carried out for a Pt/Pt3Co/Pt3Co model, in which the second and third layers both have the bulk composition (Fig. 1). The curve is shifted downward due to the effect of Co. Finally, calculations were carried out for a realistic Pt/Co/Pt3Co(111) model. Since these are extremely time-consuming, only the bare surface and the unity coverage cases have successfully completed thus far, but these show a further downward shift.
The present results show for the first time that the electrochemical behavior of a Pt-Co alloy (111) single crystal can be understood from first principles. Calculations are also in progress to model the OH adsorption region, as well as the catalytic reactions.
This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cell” (SPer-FC) project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
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Fig. 1. (Upper panel) CV for Pt(111) (black) and Pt-skin/Pt73Co27(111) (blue) electrodes in N2-saturated 0.1 M HClO4 at 50 mV s-1. (Lower panel) Integrated charges (solid lines) based on the CV, with calculated points (open symbols) for Pt/Pt3Co/Pt3Co(111) (red circles) and Pt/Co/Pt3Co (blue squares) models.

Figure 1