In Situ Confinement Pyrolysis Transformation of ZIF‐8 to Nitrogen‐Enriched Meso‐Microporous Carbon Frameworks for Oxygen Reduction

Abstract

Metal organic framework (MOF)‐derived nitrogen‐enriched nanocarbons have been proposed as promising metal‐free electrocatalysts for oxygen reduction reaction. However, the characteristic microporous feature of MOF‐derived carbon determined by the MOF structure significantly hinders the mass transfer and exposure of active sites, resulting in unsatisfactory electrocatalytic performance. Here an in situ confinement pyrolysis strategy that can simply but efficiently transform monodisperse ZIF‐8 polyhedrons to nitrogen‐enriched meso‐microporous carbon (NEMC) frameworks is reported. Using this strategy, 3D NEMC frameworks, 1D NEMC fibers, and 2D NEMC on graphene (NEMC/G) can be successfully obtained. As a metal‐free elctrocatalyst, optimized NEMC/G can reach a comparable electrocatalytic activity with superior stability and methanol resistance to commercial 30 wt% Pt/C catalyst in 0.1 m KOH solution. Such enhanced performance can be ascribed to the stable and highly open network consisting of NEMC and G with fully exposed active sites, thereby leading to durable catalytic activity.

Design Strategies toward Advanced MOF‐Derived Electrocatalysts for Energy‐Conversion Reactions

“…[58,60] For instance, in order to fabricate N-doped MOF-derived materials, zeolitic imiazolate frameworks (ZIFs), a subclass of MOFs composed of tetrahedrally coordinated transition metal ions (e.g., Zn, Fe, Co, Cu) and imidazolate (IM) ligands, are frequently utilized as the precursor and/or template because of their high N content and other preferred properties, such as large surface area and uniform particle size. [24,33,34,36,[38][39][40][41]44,53,[64][65][66][67][68][69] Recently, You et al reported the conversion of ZIF-67 (Co(MIM) 2 , MIM = methylimidazole) to Co-and N-doped carbon (CoNC) via direct pyrolysis, producing a N-doped material with high N content. [39] Similarly, Chen and co-workers studied porous carbon materials derived from a series of bimetallic ZIFs (BMZIFs), which are based on ZIF-8 and ZIF-67 with varied Zn/Co ratio.…”

“…[40b, 41,67f ] As expected, heteroatom-doped carbon materials have emerged as a new class of advanced metalfree electrocatalysts, due to their extraordinary electrocatalytic performance, better defined reaction mechanisms, low cost, simple preparation, and easy scale-up. [33,[56][57][58][59][60][61][62][63] www.advancedsciencenews.com Among the single heteroatom-doped carbon materials, one of the most widely studied cases is the substitution of partial C (χ = 2.55) with more electronegative N (χ = 3.04). [33,56,58,60,63] In particular, MOF precursors with N-containing ligands can be directly converted into N-doped electrocatalysts after pyrolysis under inert atmosphere.…”

“…[33,[56][57][58][59][60][61][62][63] www.advancedsciencenews.com Among the single heteroatom-doped carbon materials, one of the most widely studied cases is the substitution of partial C (χ = 2.55) with more electronegative N (χ = 3.04). [33,56,58,60,63] In particular, MOF precursors with N-containing ligands can be directly converted into N-doped electrocatalysts after pyrolysis under inert atmosphere. [58,60] For instance, in order to fabricate N-doped MOF-derived materials, zeolitic imiazolate frameworks (ZIFs), a subclass of MOFs composed of tetrahedrally coordinated transition metal ions (e.g., Zn, Fe, Co, Cu) and imidazolate (IM) ligands, are frequently utilized as the precursor and/or template because of their high N content and other preferred properties, such as large surface area and uniform particle size.…”

“…[10] For this purpose, rational catalyst design lies in constructing novel electrocatalysts with favourable structures. [33,35,41,64,83,101] Structure engineering is a promising and indispensable strategy to develop improved catalysts in this respect, [102] and plays www.advancedsciencenews.com a critical role in integrating all components to make full use of their functionalities. Until now, a variety of nanostructures or architectures based on MOF-derived materials, such as edge-rich strucutures, [48] core-shell structures, [64,83] sandwich structures, [33] porous nanowire array structures, [35] and threedimensional hierarchical structures, [101] have been synthesized and widely investigated.…”

“…When used as electrode materials in energy-conversion reactions, MOF-derived materials have displayed appealing performance in electrochemical systems, and comprehensive studies indicate that MOF-derived materials have several advantageous characteristics for electrochemical energy generation and storage. These are: 1) the carbon matrix obtained from the organic framework is able to act as a highly conductive network, promoting fast electron transfer; [31][32][33][34][35][36] 2) various heteroatoms (e.g., N, O, S, and P) contained in the organic ligands can be directly doped into the carbon framework, which can not only provide more active sites, but also create a favourable microenvironment for the growth of functional nanoparticles (NPs, e.g., transition metal carbides, nitrides, and oxides); [24,[36][37][38][39][40][41] 3) the metal atoms in MOFs can form corresponding metals and/or compounds and homogeneously distribute in the carbon network, leading to the in situ synthesis of a carbon framework decorated with transition metals and/or transition metal compounds; [24,[41][42][43] 4) a well-defined morphology, size, and structure can be obtained by adjusting synthesis parameters; [42][43][44][45] 5) many of the features (e.g., large surface area and high porosity) inherited from the MOF precursors can be preserved in the resultant materials after post-treatments. [46][47][48] Unsurprisingly, owing to the advantages discussed above, these MOF-derived materials have stimulated much interest in energy fields, [49][50][51] and have been studied as electrode materials in batteries and supercapacitors.…”