Core-Shell Nanocomposites Based on Gold Nanoparticle@Zinc- Iron-Embedded Porous Carbons Derived from Metal-Organic Frameworks as Efficient Dual Catalysts for Oxygen Reduction and Hydrogen Evolution Reactions Jia Lu, † Weijia Zhou,* ,† Likai Wang, † Jin Jia, † Yunting Ke, † Linjing Yang, † Kai Zhou, † Xiaojun Liu, † Zhenghua Tang, † Ligui Li, † and Shaowei Chen* ,†,‡ † New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong 510006, China ‡ Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California 95064, United States * S Supporting Information ABSTRACT: Core-shell nanocomposites based on Au nanoparticle@zinc-iron- embedded porous carbons (Au@Zn-Fe-C) derived from metal-organic frame- works were prepared as bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). A single Au nanoparticle of 50-100 nm in diameter was encapsulated within a porous carbon shell embedded with Zn-Fe compounds. The resulting Au@Zn-Fe-C hybrids exhibited apparent catalytic activity for ORR in 0.1 M KOH (with an onset potential of +0.94 V vs RHE, excellent stability and methanol tolerance) and for HER as well, which was evidenced by a low onset potential of -0.08 V vs RHE and a stable current density of 10 mA cm -2 at only -0.123 V vs RHE in 0.5 M H 2 SO 4 . The encapsulated Au nanoparticles played an important role in determining the electrocatalytic activity for ORR and HER by promoting electron transfer to the zinc-iron-embedded porous carbon layer, and the electrocatalytic activity was found to vary with both the loading of the gold nanoparticle cores and the thickness of the metal-carbon shells. The experimental results suggested that metal-embedded porous carbons derived from metal-organic frameworks might be viable alternative catalysts for both ORR and HER. KEYWORDS: core-shell structure, metal-organic frameworks, oxygen reduction reaction, hydrogen evolution reaction, electron transfer ■ INTRODUCTION Development of renewable and green energy sources has been attracting extensive research interest lately. 1 Of these method- ologies, oxygen reduction reaction (ORR) has been found to play key roles in fuel cells and zinc-air batteries, while hydrogen evolution reaction (HER) is important in the production of clean and sustainable energy. Hence, the development of highly efficient electrocatalysts for both ORR and HER is crucial. Until now, platinum-based materials have been used extensively as the electrocatalysts of choice, because of their excellent catalytic activity toward ORR 2 and HER. 3, 4 However, the low abundance and high price of platinum greatly limit their commercial applications. 5,6 Thus, development of cost-effective electrocatalysts with extraordinary ORR and HER activity is urgently needed. In fact, extensive research efforts have been devoted to the development of alternative materials that are low cost and naturally abundant that may eventually replace platinum-based catalysts. Toward this end, a variety of materials, including carbon nanostructures, 5,7-14 transition metals 6,15-18 and their oxides, 19-21 carbides, 22 and disul- fides 23-25 have been prepared and examined as electrocatalysts for ORR or HER. Nevertheless, it remains a key challenge to synthesize low-cost, highly active and stable catalysts for both ORR and HER. Metal-organic frameworks (MOFs)materials made of metal centers and functional organic ligands linked by coordination chemistry 26 have been widely used in various fields, including gas storage/separation, 27-29 catalysis, 30-32 sensing, 33,34 and drug delivery. 35,36 MOFs have also been used as precursors and templates for the preparation of metal nanoparticles encapsulated in porous carbons with controlled morphologies, high surface area, and uniform porosity. 37-41 For instance, Hong et al. 40 synthesized nitrogen-doped porous carbon nanopolyhedra by simple carbonization of a zeolitic imidazolate framework (ZIF), which showed a remarkable performance for ORR, with an onset potential of -0.017 V vs Ag/AgCl, excellent methanol tolerance, and robust stability. In another study, Wu et al. 42 prepared porous molybdenum Received: October 14, 2015 Revised: December 16, 2015 Published: January 4, 2016 Research Article pubs.acs.org/acscatalysis © 2016 American Chemical Society 1045 DOI: 10.1021/acscatal.5b02302 ACS Catal. 2016, 6, 1045-1053