The Alloying Effect on Cathode Catalyst Activity

Early reports on the use of Pt alloys as ORR catalysts for PEM fuel cells were published by a group of scientists at Texas A M University in the early 1990s [4-6]. They created a series of Pt alloys, including Pt-Ni, Pt-Co, Pt-Cr, Pt-Mn, and Pt-Fe, at high temperature (900 °C) under an inert atmosphere. These alloys can be expressed as Pt-M (where M is the non-noble metal alloying component). In the process of activity down-selection, a composition of 75(Pt) 25(M) was found to be 

Johnson Matthcy also carried out considerable work in the late 1990s on binary alloys such as Pt-Fe, Pt-Mn, Pt-Ni, Pt-Cr, Pt-Cu, and Pt-Ti [7]. For fitel cell catalysts, Pt alloys with a 50 50 Pt M ratio were heat-treated at various temperatures to improve activity. The catalyst (20 wt% Pt alloy on Vulcan carbon) was integrated into an MEA with 25 cm of active area for the fitel eell test. During fuel cell operation at 75 °C and 308/377 kPa pressure, H2 and O2 were used as the fuel and oxidant, respectively. It was found that a 20-40 mV performance 

Most recently, R alloy catalysts that have the so-called structure of a R skin, i.e., with a pure R topmost atomic layer on the surface of the alloys, were reported to be the most aetive catalysts towards the ORR [14—17]. Stamenkovic et al. [14] studied the surface properties of R-M (M = Co, Ni, Fe) polycrystalline alloys prepared by sputtering, aimealing, and leaching, respectively. They found two kinds of surface structure on the alloys, depending on the preparation procedure. The merely sputtered alloys could form R-skeleton outermost layers due to the dissolution of transition metal atoms in acid electrolyte, whereas the aimealed alloys had a R-skin topmost layer containing only R. The eatalytie activity towards the ORR on these two surfaces was much higher than that on a pure polycrystalline R surface, and the Pt-skin surface displayed the highest activity. In particular, the RsNi alloy with the (111) face was 10 times more active than the 

Pt(lll) surface, and 90 times more active than the current state-of-the-art Pt/C catalysts [17]. 

Based on the above review of the ORR activity of Pt alloys, it appears Pt-based alloys can improve specific activity and mass activity by several times relative to pure Pt catalyst in both an acid electrolyte and a real PEM fuel cell environment. In particular, the reported Pt3Ni(lll)-skin surface, which exhibits the most remarkable ORR catalytic activity that has ever been detected, looks very promising in terms of further activity improvement of Pt alloy catalysts for PEM fuel cell applications. The remaining challenge is how to create a fuel cell nanocatalyst with electronic and morphological properties that mimic the Pt3Ni(l 11) surface. 

---