For the cathodic oxygen reduction reaction in proton exchange membrane fuel cell (PEMFC), several Pt-3d (where 3d = Ni, Co, Fe, Mn, Cr, V, Ti) bimetallic alloys have been identified with higher activity than pure Pt^1,2. The focus of this study is to quantify the adsorbated-induced segregation of the 3d admetal, a degradation mechanism for bimetallic catalysts, and to elucidate fundamental correlations of bimetallic systems in order to maximize the lifetime of these novel electrocatalysts.

A number of degradation methods for monometallic electrocatalysts have been identified for PEMFC, (i) Ostwald ripening, (ii) metal dissolution, (iii) electrocatalyst/membrane delamination and (iv) corrosion of the electrocatalyst support.³ For bimetallic electrocatalysts, an additional mode of degradation can occur due to the dependence of activity on the configuration of the admetal within the host metal in the first few atomic layers from the surface. There are three main types of admetal/host metal configurations near the surface: the subsurface Pt-3d-Pt, intermixed surface alloy, and surface 3d-Pt-Pt.⁴ It has been shown previously that the highest activity for Pt-3d bimetallic alloys occurs for those with the subsurface Pt-3d-Pt configuration, which are often referred to as having a “Pt skin” layer. Therefore, admetal segregation and dissolution from bimetallic electrocatalysts, or preferential metal dissolution, is an additional degradation mechanism that can occur.³

Using surface science methods in conjunction with density functional theory (DFT) predictions, the thermodynamics and kinetics of preferential metal segregation, which can lead to preferential metal dissolution, are quantified. It will be shown that the thermodynamic stability of bimetallic configurations can be correlated with the surface d-band center using DFT. The trend for the adsorbated-induced segregation is discussed for a few common reaction environments (atomically adsorbed O, H, C, N, S, and P) on Pt-3d-Pt(111) systems (where 3d=Ni, Co, Fe, Mn, Cr, V, or Ti). These predicted trends will be compared with experimental results of Pt-Ni and Pt-Co bimetallic systems studied using a number of techniques. In ultra-high vacuum, Auger electron spectroscopy was used to monitor the segregation of Ni or Co within a Pt(111) single crystal⁵ and a polycrystalline Pt foil⁶. The activation barrier of oxygen-induced segregation of Ni or Co will be presented and compared with the trends found by DFT. In order to show the effect of pressure, the oxygen-induced segregation of Ni from subsurface Pt-Ni-polycrystalline Pt is studied using in-situ x-ray absorption near edge structure (XANES) at atmospheric pressure. Finally, using the aforementioned fundamental results as guidance, the stability and dissolution of Ni from subsurface Pt-Ni-polycrystalline Pt exposed to 0.5 M H₂SO₄ solution with an applied potential between 0.0 to 1.0 V vs. NHE reference electrode will be discussed.

1. Stamenkovic, V.; Mun, B.S.; Mayrhofer, K.J.J.; Ross, P.N.; Markovic, N.M.; Rossmeisl, J.; Greeley, J.; Norskov, J.K., Angew. Chem. Int. Ed., 2006, 45, 2897-2901.

2. Stamenkovic, V.R.; Fowler, B.; Mun, B.S.; Wang, G.F.; Ross, P.N.; Lucas, C.A.; Markovic, N.M., Science, 2007, 315, 493-497.

3. Shao, Y.Y.; Yin, G.P.; Gau, Y.Z., J. Power Sources, 2007, 171, 558-566.

4. Chen, J.G.; Menning, C.A., Zellner, M.B., Surf. Sci. Rep., 2008, 63, 201-254.

5. Menning, C.A.; Hwu, H.H., Chen, J.G., J. Phys. Chem. B., 2006, 110, 15471-15477.

6. Menning, C.A.; Chen, J.G., J. Chem. Phys., 2008, 128, 164703.