Cage-Confinement Pyrolysis Route to Ultrasmall Tungsten Carbide Nanoparticles for Efficient Electrocatalytic Hydrogen Evolution Yan-Tong Xu, † Xiaofen Xiao, § Zi-Ming Ye, † Shenlong Zhao, ⊥ Rongan Shen, ‡ Chun-Ting He,* ,† Jie-Peng Zhang,* ,† Yadong Li,* ,‡ and Xiao-Ming Chen † † MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, China § Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Key Laboratory of High Performance Polymer-Based Composites of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China ⊥ CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China * S Supporting Information ABSTRACT: The size-controlled synthesis of ultrasmall metal-based catalysts is of vital importance for chemical conversion technologies. Here, a cage-confinement pyrolysis strategy is presented for the synthesis of ultrasmall tungsten carbide nanoclusters/nanoparticles. An RHO type zeolitic metal azolate framework MAF-6, possessing large nanocages and small apertures, is selected to confine the metal source W(CO) 6 . High temperature pyrolysis gives tungsten carbide nanoclusters/nanopar- ticles with sizes ca. 2 nm, which can serve as an excellent electrocatalyst for the hydrogen evolution reaction. In 0.5 MH 2 SO 4 , it exhibits very low overpotential of 51 mV at 10 mA cm -2 and Tafel slope of 49 mV per decade, as well as the highest exchange current density of 2.4 mA cm -2 among all tungsten/molybdenum-based catalysts. More- over, it also shows excellent stability and antiaggregation behavior after long-term electrolytic process. O ver the past decade, molybdenum- and tungsten-based materials have been considered to be promising alternative of the costly and low reserves noble metals (in particular platinum) for application in energy storage and conversion, especially in the field of electrocatalytic hydrogen evolution reaction (HER), which is the most economical and sustainable method for hydrogen industry. 1 Actually, due to the unique electronic structures, Mo 2 C, 2 WC, 3 W 2 C, 4 MoP, 5 WN, 6 MoS 2 , 7 WSe 2 , 8 and so on, 9 have demonstrated their considerable potential as substitutes to Pt. Thereinto, it has been confirmed that the electronic density of states of WC closely resembles that of platinum than that of W. 10,11 Moreover, WC exhibits outstanding thermal/chemical stabil- ity, 3,12 making a valuable candidate in catalysis industry. However, the lack of efficient methods to produce size- controlled WC with high activity has prevented its widespread application. 4,13 To some extent, greatly reducing the particle sizes, especially preparation of nanoclusters or even single atoms, would be an effective way to obtain ultrahigh catalytic activities. 14 Unfortu- nately, uncontrollable particle sintering/agglomeration is usually hard to avoid, especially for metal carbides, 3,4 because ultrahigh thermolysis temperature (>1000 K) is necessary to overcome thermodynamic and kinetic barriers when carbon atoms incorporate into the metal lattice. In fact, traditional synthetic methods are very difficult to achieve metal carbide nanoparticles (NPs) with single-component/phase and uniform sizes smaller than 5 nm (Table S1). Despite nonsintered WC NPs have recently been successfully synthesized via a 6-step strategy, 3,9b,15 developing facile and large-scale synthesis methods of stable ultrasmall WC NPs is still a great challenge. Herein, we propose a cage-confinement pyrolysis (CCP) strategy to produce ultrasmall WC NPs (Scheme 1). Metal-organic frameworks (MOFs), possessing highly ordered porous structures, diversified metal/organic composi- tions and adjustable crystal morphologies, have been regarded as ideal reactive precursors for synthesizing well-defined nanocatalysts. 16 Under different thermal conditions, MOFs Received: January 10, 2017 Published: April 5, 2017 Scheme 1. Comparison of the Cage-Confinement and Non- Confinement Pyrolysis Methods for Synthesizing Nanocatalysts Communication pubs.acs.org/JACS © 2017 American Chemical Society 5285 DOI: 10.1021/jacs.7b00165 J. Am. Chem. Soc. 2017, 139, 5285-5288