Research Article Cite This: ACS Catal. 2018, 8, 35−42
pubs.acs.org/acscatalysis
Core−Shell Nanostructured Cobalt−Platinum Electrocatalysts with Enhanced Durability Lei Wang,† Wenpei Gao,‡ Zhenyu Liu,§ Zhenhua Zeng,∥ Yifan Liu,† Michael Giroux,† Miaofang Chi,⊥ Guofeng Wang,§ Jeffrey Greeley,∥ Xiaoqing Pan,‡,# and Chao Wang*,† †
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States Department of Chemical Engineering and Materials Science, University of California, Irvine, California 92697, United States § Department of Mechanical Engineering & Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ∥ Davison School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States ⊥ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States # Department of Physics and Astronomy, University of California, Irvine, California 92697, United States
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S Supporting Information *
ABSTRACT: Pt-bimetallic alloys involving 3d transition metals (Co, Ni, etc.) are promising electrocatalysts for the oxygen reduction reaction (ORR). Despite the enhanced catalytic activity versus Pt, the electrocatalytic performance of Pt-bimetallic catalysts is however limited by the lack of long-term durability, primarily due to the leaching of the non-noble element under harsh electrochemical conditions. Our study shows that the core−shell nanostructure comprising a Pt shell and a cobalt core (denoted as Co@Pt) could overcome this limitation, demonstrating ∼10 times improvement in catalytic activity versus commercial Pt catalysts and no more than 13% of loss after 30000 potential cycles. The evolutions of nanoscale and surface structures over the course of extensive potential cycling were followed by combining electron microscopic elemental mapping and electrochemical studies of CO stripping. Atomistic simulations and density functional theory calculations suggest that the core−shell nanostructure could protect the non-noble cobalt from leaching under the “electrochemical annealing” conditions while maintaining the beneficial mechanisms of bimetallic systems for catalytic activity enhancement. KEYWORDS: cobalt, platinum, core−shell nanoparticles, electrocatalysts, oxygen reduction reaction, fuel cells
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INTRODUCTION A fuel cell generates electricity from the hydrogen oxidation reaction at the anode and the oxygen reduction reaction (ORR) at the cathode. With hydrogen generated from water electrolysis using renewable electricity (e.g., solar and wind power), fuel cells are promising as an alternative energy technology for stationary and mobile applications.1 The need for large amounts of platinum (Pt) as catalysts for both of the electrochemical reactions, however, has limited the commercial viability of fuel cells.2,3 To improve the kinetics of these reactions, especially the cathodic reduction of oxygen, and thereby reduce the use of noble metals, a common approach is to alloy Pt with another transition metal (Co, Ni, etc.).2−12 Although significant progress has been made in improving the catalytic activity, the Pt-alloy catalysts are still challenged by their limited durability, primarily caused by leaching of the nonnoble elements under the harsh electrochemical conditions relevant to fuel cell operations.13−15 In comparison to homogeneous alloys, the core−shell nanostructure is potentially more advantageous for electrocatalytic applications.16−21 It could reduce the content of noble metal in the catalysts if a non-noble metal is used as the core, while a complete noble-metal shell can protect the inner non© 2017 American Chemical Society
noble metal from corrosion. Such structures may also possess the beneficial mechanisms of bimetallic catalysts (e.g., strain effect8) for surface property modification and catalytic enhancement. We have recently developed the synthesis of Co@Pt nanoparticles in organic solutions using seed-mediated growth and demonstrated their high catalytic activity for the ORR.22 These core−shell nanostructures are distinct from those previously reported Pt-bimetallic (e.g., Pt−Co, Ni, Cu) catalysts with a Pt-skin surface produced by thermally induced surface segregation20,23 or a Pt-rich shell generated from dealloying (via electrochemical or acid treatments).6,8 In the latter cases, the core is always an alloy phase comprising Pt and the 3d metal, and the amount of non-noble metal that can be preserved in such nanostructures under electrochemical environments has been found to be limited (e.g.,
