Role of surface iron in enhanced activity for the oxygen reduction reaction on a Pd3Fe(111) single-crystal alloy.

Applications of polymer electrolyte membrane fuel cells (PEM-FCs) require a continued search for an optimal catalyst with attributes of high activity, durability, low cost, and resistance to negative effects of impurities in the fuel. Platinum-based bimetallic catalyst systems are widely utilized in PEM-FCs because of their good performance in both the cathode and the anode. Alloying with inexpensive metals can reduce the loading of platinum and lower the cost of the fuel cell, and in some cases the catalytic activity is maintained or even becomes higher. Surprisingly, some non-platinumbased alloy catalysts (e.g., palladium–iron and palladium– cobalt) appear to have even better performance than platinum-based alloy catalysts. Recently, higher electrocatalytic activity and stability were observed when metals or alloys were used as supports for a platinum or palladium monolayer. Development of improved cathode catalysts would be aided by a fundamental understanding of the oxygen reduction reaction (ORR) mechanism. However, the ORR system is complex and ultimately requires considering fully the effects of water, solvent ions, changing electrical potentials, and a detailed description of the composition and structure of all the chemical phases present at the electrode surface. Thus, progress has been limited in elucidating the origin of the enhanced performance of various bimetallic catalysts. An important factor that can be isolated is the surface composition and structure of the alloy responsible for the ORR kinetics. Herein, we report studies on a bimetallic alloy single crystal, Pd3Fe(111), that combine surface analytical techniques including low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM), electrochemical analysis, and quantum chemistry calculations to investigate the origin of the enhanced ORR properties of Pd–Fe alloys at a molecular level. We find excellent ORR performance for the Pd3Fe(111) crystal after heating to high temperatures in ultrahigh vacuum (UHV) and establish a strong correlation of this performance to the presence of surface Fe atoms. The lower surface energy of Pd causes significant Pd segregation to the topmost surface layer of Pd–Fe alloys. Pd3Fe(111) surfaces with different structures can be prepared by heating at different temperatures in UHV, and two surface structures are shown by STM images in Figure 1. These two surfaces were studied for catalyzing the oxygen reduction reaction.