Electrochemical Reactions, Chemical Ordering Effects, and Calculated Electronic Structure, for Pt 100-x M x (M = V, Zr)
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Electrochemical Reactions, Chemical Ordering Effects, and Calculated Electronic Structure, for Pt100-xMx (M = V, Zr) Thin-Film Surfaces in Acid Electrolytes Charles C. Hays1and Uichiro Mizutani2, 3 1 Department of Physics & Astronomy, Texas A&M University, College Station, TX 77843, U.S.A., 2Nagoya Industrial Science Research Institute, Chikusa-ku, Nagoya, Aichi Ken, Japan, and 3Crystalline Materials Science, Nagoya University, Nagoya, Aichi Ken, Japan. ABSTRACT Microstructural, chemical, and electrochemical property measurements results, for (111) crystallographically oriented Pt100-xMx (M = V, Zr; 3 < xv < 14; and 4 < xZr < 35, At.%) sputtered thin films are presented, with electronic structure calculations. These Pt-based alloys were prepared to investigate early transition metal (ETM), late-transition-metal (LTM) alloys as potential electrode materials in hydrogen-air polymer-electrolyte-membrane fuel cells (PEMFCs). The Pt100-xMx oxygen-reduction-reaction (ORR) currents peak for 8 < x < 10 atomic percent, so local chemical-short-range-order, may exist; as the peak in ORR activity is commensurate with the strong ordering in Pt8M (M = Ti, V, Zr). The hydrogen under potential deposition (Hupd) at Pt active area, and ORR reaction kinetics, on the alloyed surfaces are composition dependent, suggesting three possible effects: 1) charge transfer from V-(3d)3 [or Zr(4d)2] states, to the hole in the top of the Pt-(5d)9 band alters the electronic structure at the Fermi energy; 2) alloying Pt with the ETM elements introduces a bi-functional character to the electrode surface, and 3) or the presence of short range chemical order induces a Fermi energy shift. To confirm the 1st and 3rd hypotheses, the electronic structure of Pt8Ti, Pt8V, and Pt8Zr, were calculated using the WIEN2k program package. The electronic structure calculations for ordered Pt8M give strong confirmation of the hypotheses, as they reveal that the Pt8M Fermi energy lies within the Pt-5d anti-bonding band, and also falls into a pseudogap in between the Md bonding and anti-bonding bands. In addition, the Pt8M DOS calculations confirm the presence of a deep pseudogap formed across the Fermi energy for both the Pt-sp and M-sp electrons. These experimental and theoretical results motivate additional studies of the novel Pt8M phases. INTRODUCTION US DOE performance metrics for H2-Air PEMFCs require improved catalyst electrode materials in the following areas: 1) reduction of the catalyst platinum-group-metal (PGM) loading; 2) increased ORR reaction kinetics by a factor of 3, or more; and 3) improved catalyst durability.1 The results presented here, follows DOE-EERE work initiated at California Institute of Technology’s Jet Propulsion Laboratory (JPL), with teams at 3M Corp. and Argonne National Laboratory (ANL). To improve the catalyst properties, an understanding of the physical mechanisms governing the position of the Fermi energy, Ef, in an alloy composed of PGM and other transition metal moieties is required. The influence of the bulk catalyst’s Fermi ene
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