Unprecedented dependence of the oxygen reduction activity on Co content at Pt Skin/Pt–Co(111) single crystal electrodes

Significance Statement

Platinum-based nanoparticles dispersed on carbon supports, which have been employed in polymer electrolyte fuel cells PEFCs, usually consist of low-index crystal facets such as (111) and (100). It has been reported that the (111) face prepared under an ultrahigh vacuum condition exhibited the highest enhancement of kinetically-controlled area-specific activity jk for the oxygen reduction reaction ORR among the low-index crystal faces of Pt-based single crystals alloyed with either nickel Ni, cobalt Co or copper Cu, by a factor of about ten when compared to pure Pt(111) electrodes.

However, the dependence of  jk on the alloy composition has not been demonstrated for well-defined alloy single crystals, due to the difficulty in preparing alloy single crystals with controlled composition.

Researchers at the University of Yamanashi evaluated for the first time the jk values for the ORR at well-defined Pt-skin/Pt100-xCox(111) single crystal electrodes as a function of cobalt content by use of the rotating disk electrode RDE technique. They found that the jk value reached a maximum at 25 atom%-Co (x = 25) as high as 3.0 mA cm-2 in air-saturated 0.1 M HClO4 solution at 0.9 V vs. RHE, which is ca. 25 times larger than that on pure Pt(111). The research was recently published in the journal, Electrochemistry Communications.

A low-energy ion scattering spectrum for a Pt-Co(111) surface indicated no Co signal around 280 eV after a heat treatment in hydrogen, demonstrating the formation of a platinum-skin layer on the Pt-Co(111) surface. A low-energy electron diffraction pattern exhibited a hexagonal (1 ´ 1) structure of the platinum-skin layer. An in situ scanning tunneling microscopy image showed atomically flat terraces of platinum-skin layer in nitrogen-purged 0.1 M HClO4.

Dramatic changes were observed in the cyclic voltammograms of Pt-skin/Pt100-xCox(111) single crystal electrodes with the variation of cobalt atom% in nitrogen-purged 0.1 M HClO4 solution:  the hydrogen underpotential deposition HUPD wave shifted to less positive potentials, while the hydrogen underpotential deposition charge QHUPD decreased nearly linearly with increasing x, which can be ascribed to the modified electronic structure of the platinum-skin layer by the underlying platinum-cobalt alloys.

The “butterfly-like” wave due to surface oxidation shifted to higher potentials. The authors have demonstrated using X-ray photoelectron spectroscopy with an electrochemical cell EC-XPS that the reversible butterfly wave with a spike peak at  pure Pt(111) electrode can be ascribed to OH formation with a honeycomb framework of bilayer water on the Pt(111) surfaces.

The authors then evaluated the surface oxidation charges Qox,0.9 up to 0.9 V, at which the jk values were evaluated, and it was discovered that the Qox,0.9 values were identical with that at pure Pt(111), even at the maximum activity point for 25 atom%-Co. These results indicate that the coverages of OH adsorbed on the platinum-skin layers at 0.9 V were nearly constant, particularly for x < 30. Hence, the enhanced ORR activity of the Pt-skin/Pt-Co(111) electrodes cannot be explained on the basis of the conventional view that alloying leads to decreased site-blocking by spectator oxygen-species due to surface oxidation, often described as OH poisoning.

This study showed that the enhancement factor of the ORR activity at platinum-skin layers prepared by heating in hydrogen was two times larger than those reported for platinum-based alloy (111) single crystal electrodes prepared in ultrahigh vacuum. 

  

Unprecedented dependence of the oxygen reduction activity on Co content at Pt Skin/Pt–Co(111) single crystal electrodes. Advances in Engineering

 

About the author

Mitsuru Wakisaka received a B. Eng. degree from Tohoku University, Sendai, Japan, in 1996, an M. Eng. degree from Tohoku University in 1999, and a Ph. D. degree from Tohoku University in 2002. He worked at the University of Illinois at Urbana-Champaign (2002-2003) and then at the Clean Energy Research Center, University of Yamanashi (2003-2008) as a Post-doc. He was a Research Associate (2008-2010) and then a Lecturer (2010-2011) at the Fuel Cell Nanomaterials Center, University of Yamanashi. He has been an Associate Professor since 2011.

His research interests are in reaction mechanisms relating to fuel cells and other electrochemical systems, including single-crystal electrodes, and surface analysis techniques such as scanning tunneling microscopy and X-ray photoelectron spectroscopy. 

About the author

Shun Kobayashi received a B. Eng. Degree in the Special Educational Program on Clean Energy from the University of Yamanashi, Kofu, Japan, in 2015. He is currently a Master’s Degree student in the Special Doctoral Program for Green Energy Conversion Science and Technology at the University of Yamanashi. His admission to the Ph. D. program (from 2017) of the same Program has been confirmed. He received an ISE (International Society of Electrochemistry) poster award at the International Fuel Cell Workshop 2015.

He is working on electrocatalysis for the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs) by the use of single-crystals with his supervisor, Prof. Hiroyuki Uchida. 

 

About the author

Donald A. Tryk received a B. S. in Chemistry from the University of Florida in 1969 and a Ph. D. in Chemistry from the University of New Mexico in 1980. He worked at Case Western Reserve University in what later became the Yeager Center for Electrochemical Sciences from 1980 to 1995 as a Senior Research Associate. He was a Research Associate (1995-1998) and then a Special Associate Professor (1998-2001) at the University of Tokyo, Department of Applied Chemistry. He was a Researcher and Visiting Professor at Tokyo Metropolitan University (2001-2003, 2005-2008) and a Researcher at the University of Puerto Rico (2003-2005).

In 2008, he joined the Fuel Cell Nanomaterials Center, University of Yamanashi as a Professor. His main interest is in the theory of fuel cells and other electrochemical systems, including density functional theoretical calculations. 

About the author

Masahiro Watanabe received a B. Eng. degree from Yamanashi University (presently the University of Yamanashi, since 2002), Kofu, Japan, in 1966, a M. Eng. degree from Yamanashi University in 1968, and a Ph. D. degree from the University of Tokyo, Tokyo, Japan, in 1976. He worked as a Research Associate (1968-1976), an Assistant Professor (1976-1977), and an Associate Professor (1977-1989) at Yamanashi University.

He was promoted to Professor in 1989, and acted as the Director of the Clean Energy Research Center (2001-2009), and the Director of the Fuel Cell Nanomaterials Center (2008-2015), both of which were newly constructed at the University of Yamanashi. He has been an Emeritus Professor since 2009.

His research interests include R & D relating to fuel cells (DMFCs, PEFCs, PAFCs, SOFCs) and hydrogen production/purification. He has promoted several national projects with his colleagues involving several companies and universities on these R & D subjects at these Centers. 

About the author

Akihiro Iiyama received a B. Eng. degree from the University of Tokyo, Tokyo, Japan, in 1980, an M. Eng. degree from the University of Tokyo in 1982, and a Ph. D. degree from the University of Tokyo in 1991. He worked at the Nissan Motor Co., Ltd. since 1982, and acted as the General Manager of the Fuel Cell Laboratory (2008-2010), General Manager of the EV System Laboratory (2010-2011), and an Expert Leader in the EV System Laboratory (2011-2015) at the Nissan Research Center.

Since 2015, he has been a Professor and Director of the Fuel Cell Nanomaterials Center, University of Yamanashi. His main research interests include mechanical engineering, heat and mass transfer, internal combustion engines (gasoline engine, diesel engine), and fuel cells (fuel cells for automobiles). 

 

About the author

Hiroyuki Uchida received a B. Eng. degree from Osaka University, Osaka, Japan, in 1977, an M. Eng. degree from Osaka University in 1979, and a Ph. D. degree from Kyoto University, Kyoto, Japan, in 1984. He worked as a Research Assistant at Tottori University (1979-1989), an Assistant Professor at Osaka University (1989-1993), and an Associate Professor at Yamanashi University (presently the University of Yamanashi) from 1993 to 2001.

He was promoted to Professor in 2001, and is currently working as a Professor and Director of the Clean Energy Research Center at the University of Yamanashi. His current research interests include the R & D of materials for PEFCs and SOFCs and hydrogen production.  

Journal Reference

M. Wakisaka1, S. Kobayashi2, S. Morishima2, Y. Hyuga2, D.A. Tryk1, M. Watanabe1, A. Iiyama1, H. Uchida1,3. Unprecedented Dependence of the Oxygen Reduction Activity on Co Content at Pt Skin/Pt–Co(111) Single Crystal Electrodes,  Electrochemistry Communications, Volume 67, 2016, Pages 47–50.

[expand title=”Show Affiliations”]
  1. Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae, Kofu 400-0021, Japan
  2. Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan
  3. Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan
[/expand]

 

 

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