Bimetal–Organic Framework Self-Adjusted Synthesis of Support-Free Nonprecious Electrocatalysts for Efficient Oxygen Reduction

Abstract

The development of low-cost catalysts with oxygen reduction reaction (ORR) activity superior to that of Pt for fuel cells is highly desirable but remains challenging. Herein, we report a bimetalorganic framework (bi-MOF) self-adjusted synthesis of support-free porous Co-N-C nanopolyhedron electrocatalysts by pyrolysis of Zn/Co bi-MOF without any post-treatments. The presence of initial Zn forms a spatial isolation of Co which suppresses its sintering during pyrolysis and the Zn evaporation also promotes the surface area of the resultant catalysts. The composition, morphology, and hence ORR activity of Co-N-C could be tuned by the Zn/Co ratio. The optimal Co-N-C exhibited remarkable ORR activity with a half-wave potential of 0.871 V vs RHE (30 mV more positive than that of commercial 20 wt% Pt/C) and a kinetic current density of 39.3 mA cm -2 at 0.80 V vs RHE (3.1 times that of Pt/C) in 0.1 M KOH, plus excellent stability and methanol tolerance. It also demonstrated comparable ORR activity and much higher stability than those of Pt/C in acidic and neutral electrolytes. Various characterization techniques including X-ray absorption spectroscopy revealed that the superior activity and strong stability of Co-N-C originated from the intense

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

“…By means of annealing under a controlled temperature and atmosphere, the organic units can be transformed into heteroatom-doped carbon materials, while metal-containing units are simultaneously converted into the corresponding transition-metal-based NPs supported on the as-formed carbon substrate. [24,38,41,42,44,53] More importantly, the advancement of chemical modification allows the precise manipulation of the component for the resultant composites. [41] From the view of composition, MOF-derived composites could offer strong synergistic effects between the carbon framework and metal component, which is highly preferable in enhancing electrocatalytic performance.…”

“…[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.…”

“…Shortly after, Sun's group also reported self-supported porous cobaltnitrogencarbon (CoNC) composite by the facile carbonization of self-adjusted Zn and Co bimetal-organic frameworks (bi-MOF). [38] Note that BMZIFs and bi-MOF are materials of similar concept but with different designations. As illustrated in Figure 1a, using both Zn 2+ and Co 2+ as metal centers and 2-methylimidazole (MeIM) as organic linkers, a series of Zn x Co 1−x (MeIM) 2 were prepared as precursor MOFs, wherein x (x = 0−1) represents the molar ratio of Zn 2+ in the initial reactants.…”

“…Generally, it has been widely reported that all N configurations, except for N-oxidize, improve electrocatalytic properties. [53] Specifically, pyridinic N is able to improve the capacity for accepting protons owing to its charge neutralization effect; [39] pyrrolic and pyridinic N can reduce the energy barrier of O 2 adsorption, thus facilitating the first rate-limiting step in the ORR by increasing electron transfer; [38] and graphitic N has shown to provide more valence electrons for increasing the conductivity and catalytic activity. [39] Metal-coordinated N usually withdraws electrons from the metal center and thus lower its electron density, which is beneficial for reducing the redox potential of the metal center by optimizing the bond strength between the metal center and intermediate.…”

“…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.…”