Ultrafine Dual‐Phased Carbide Nanocrystals Confined in Porous Nitrogen‐Doped Carbon Dodecahedrons for Efficient Hydrogen Evolution Reaction

Abstract

compositions have been extensively investigated. Many approaches, including chemical vapor deposition, [12,13] pyrolysis of metal complexes, [14,15] and carburization of the mixed Mo/W-based compounds and carbon sources, have been developed. [16,17] However, these traditional methods usually suffer from the inevitable aggregation and/or uncontrollable particle sintering with excessive growth at high temperatures, which may lead to the loss of active sites. [6,16,18] To further improve the electrocatalytic performance, three main strategies have been developed as follows: i) nanostructuring to provide larger specific surface areas and expose more active sites; [8,14] ii) doping of heteroatoms to modify the electronic state of the metal elements; [12,19] iii) compositing with carbon-based materials to improve the conductivity and stability of carbides. [20][21][22] Nevertheless, construction of ultrafine dual-phase carbides within nitrogen-rich carbon matrix has rarely been reported until now. [8,23] Recently, metal-organic frameworks (MOFs) have been regarded as the ideal reactive precursors to establish welldefined nanocrystals due to their highly ordered porous structures and diversified metal nodes/organic ligands. [24][25][26] In this work, Zn-based zeolitic imidazolate framework (ZIF-8) substituted with MO 4 units (denoted as ZIF-8-MO 4 , M = Mo or W) dodecahedrons are obtained through an anion exchange reaction between the Zn(imidazolate) 4 2− units in ZIF-8 and MO 4 2− anions in inorganic salts. Their similar four-connected open frameworks ensure the reaction possible. [27] Benefiting from the internal long-range ordered Zn-O-M connectivity, high carbon/nitrogen content, low boiling point of Zn, and high porosity of ZIF-8, the in situ generated carbides are evenly dispersed within the PNCDs in the form of ultrafine nanocrystals. The ultrafine nanocrystals confined within this porous nanostructure results in the strong contact between the carbides and the conductive carbon support, which not only provides more stable active sites but also facilitates the electron transport in the HER process. [18,21,28] Besides, the abundant nitrogen dopants can function as electron acceptors to assist the carbon atoms in the metal lattice, which is immensely conducive to enhance the catalytic activity. [16] Benefiting from the advantages of ultrafine nanocrystals and porous nitrogen-doped Designing novel non-noble electrocatalysts with controlled structures and composition remains a great challenge for efficient hydrogen evolution reaction (HER). Herein, a rational synthesis of ultrafine carbide nanocrystals confined in porous nitrogen-doped carbon dodecahedrons (PNCDs) by annealing functional zeolitic imidazolate framework (ZIF-8) with molybdate or tungstate is reported. By controlling the substitution amount of MO 4 units (M = Mo or W) in the ZIF-8 framework, dual-phase carbide nanocrystals confined in PNCDs (denoted as MC-M 2 C/PNCDs) can be obtained, which exhibit superior activity toward the HER to the single-phased MC...

Metal–Organic Frameworks Based Electrocatalysts for the Oxygen Reduction Reaction

“…3) Various heteroatoms can be directly doped into the carbon matrix through the organic ligands, which can not only induce the charge redistribution in the carbon matrix, but also interact with other functional components, producing more active sites . 4) The metal atoms can evolve into ultrafine metal and metal compound NPs confined in the carbon network, preventing them from further agglomeration and sintering . On the basis of chemical composition, we divided these derivatives into the following three categories to summarize their recent progresses as electrocatalysts towards ORR.…”

“…[60] 4) Them etal atoms can evolve into ultrafine metal and metal compound NPs confined in the carbon network, preventing them from further agglomeration and sintering. [15,61] On the basis of chemical composition, we divided these derivatives into the following three categories to summarize their recent progresses as electrocatalysts towards ORR. Herein, we highlight recent works that have guiding significance in the synthesis strategy,m orphology/ structure control, component manipulation, and practical application in fuel cells and metal-air batteries.…”

“…As one new class of porous materials, metal–organic frameworks (MOFs) interconnected by the coordination bonds of metal ions/clusters and organic ligands possess high crystallinity and long‐range order . In view of their intriguing features of high specific surface area, adjustable pore sizes and topologies, well‐defined metal nodes, tunable chemical components, and surface functionality, MOF‐based materials have been proved highly competitive to other porous materials as electrocatalysts towards ORR . According to the chemical nature, MOF‐based ORR electrocatalysts are divided into three categories: pristine MOFs, MOF composites, and their derivatives (Figure ).…”

“…[12] In view of their intriguing features of high specific surface area, adjustable pore sizes and topologies,well-defined metal nodes,tunable chemical components,and surface functionality,M OF-based materials have been proved highly competitive to other porous materials as electrocatalysts towards ORR. [13][14][15] According to the chemical nature, MOF-based ORR electrocatalysts are divided into three categories:p ristine MOFs,M OF composites,a nd their derivatives ( Figure 2). Thec ombination of porous channels in MOFs with accessible sites possessing redox ability may enable ah igher utilization efficiencyo fa ctive sites than typical heterogeneous catalysts.H owever,i nsufficient electrical conductivity and low chemical stability seriously limit their electrocatalytic applications.…”

Hierarchical MoP Hollow Nanospheres Anchored on a N,P,S‐Doped Porous Carbon Matrix as Efficient Electrocatalysts for the Hydrogen Evolution Reaction

“…To tackle the aforementioned challenges, intensive efforts have been dedicated to designing various optimized Pt‐free electrocatalysts, such as transition metal sulfides, carbides, nitrides, and phosphides . In particular, among the various promising alternatives, molybdenum phosphide (MoP) exhibits a prominent advantage owing to their similar electronic structure to Pt‐group metals .…”