Oxygen Reduction Reaction on Platinum-Terminated “Onion-structured” Alloy Catalysts
Using periodic, self-consistent density functional theory (GGA-PW91) calculations, a series of onion-structured metal alloys have been investigated for their catalytic performance towards the oxygen reduction reaction (ORR). The onion-structures consist of a varying number of atomic layers of one or two metals each, pseudomorphically deposited on top of one another to form the overall structure. All catalysts studied feature a Pt overlayer, and often consist of at least one Pd layer below the surface. Three distinct ORR mechanisms were analyzed on the close-packed facets of all the structures considered. These mechanisms include a direct route of O2 dissociation and two hydrogen-assisted routes of O–O bond-breaking in peroxyl (OOH) and in hydrogen peroxide (HOOH) intermediates. A thermochemical analysis of the elementary steps provides information on the operating potential, and thereby energy efficiency of each electrocatalyst. A Sabatier analysis of catalytic activity based on thermochemistry of proton/electron transfer steps and activation energy barrier for O–O bond-breaking steps leads to a “volcano” relation between the surfaces’ activity and the binding energy of O. Several of the onion-structured alloys studied here show promise for achieving energy efficiency higher than that of Pt, by being active at potentials higher than the operating potential of Pt. Furthermore, some have at least as good activity as pure Pt at that operating potential. Thus, a number of the onion-structured alloys studied here are promising as cathode electrocatalysts in proton exchange membrane fuel cells.
KeywordsDensity functional theory Oxygen reduction Operating potential Layered metal structures Sabatier analysis Activity
JH, JJ, KH, GP, and MM dedicate this paper to the landmark occasion of the 70th birthday of Dr. Radoslav R. Adzic. They all feel privileged to have had the opportunity to collaborate with him and be inspired by his influential ideas in the field of electrocatalysis, and wish him the very best on his birthday. Work at UW-Madison was supported by DOE-BES, Division of Chemical Sciences. JAH thanks Air Products & Chemicals, Inc. for a graduate fellowship. JJ thanks Drs. A. U. Nilekar and P. A. Ferrin for help at the initial phase of her work in this project. The computational work was performed in part using supercomputing resources from the following institutions: EMSL, a National scientific user facility at Pacific Northwest National Laboratory (PNNL); the Center for Nanoscale Materials at Argonne National Laboratory (ANL); the National Center for Computational Sciences at Oak Ridge National Laboratory (ORNL); and the National Energy Research Scientific Computing Center (NERSC). EMSL is sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL. CNM, NCCS, and ORNL are supported by the U.S. Department of Energy, Office of Science, under contracts DE-AC02-06CH11357, DEAC05-00OR22725, and DE-AC02-05CH11231, respectively.
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