Rational design of hollow N/Co-doped carbon spheres from bimetal-ZIFs for high-efficiency electrocatalysis
Graphical abstract
Introduction
The electrochemical oxygen reduction reaction (ORR) at the cathode is the rate-limiting process for the commercial applications of several emerging renewable energy storage and conversion technologies, including fuel cells and rechargeable metal-air batteries [1], [2], [3], [4]. Currently, Pt and its alloys are regarded as the best ORR electrocatalysts, but their poor durability to methanol and CO, scarcity and high cost greatly impede their large-scale commercialization [5], [6], [7], [8], [9], [10]. Recently, porous carbons doped with various non-precious transition metal and nitrogen (M-NC) have stimulated a great deal of interest especially for ORR application because of their low-cost, good resistance to corrosion and good electrical conductivity properties, which make them promising alternatives to Pt-based catalysts in energy-related technologies [11], [12], [13], [14], [15], [16]. It has been demonstrated that both M-N moieties and the nitrogen dopants within the carbon matrix are of great importance for the improvement of their ORR performance [14], [17], [18], [19], [20]. Moreover, the high specific surface areas, and the favorable porosities and structures are also vital for the accessibility of the active sites and thus the ORR activity of M-NC electrocatalysts [21], [5], [22], [23]. Clearly, it would be advantageous to design and fabricate M-NC catalysts with uniform doping of nitrogen and even distribution of transition metal, high surface areas and appropriate structures to achieve superior activity and durability for ORR.
In this regard, metal-organic frameworks (MOFs) constructed by transition-metal clusters and N-involved ligands have been considered as ideal precursors to prepare various M-NC through simple thermal decomposition under controlled atmospheres [24], [25], [26], [27], [28]. So far a number of MOFs have been regarded as promising templates to yield highly active carbon-based ORR electrocatalysts with different structures and compositions. For example, ZIF-67, a typical subclass of ZIF (zeolite imidazolate framework), is usually served as templates to prepare highly active ORR catalysts due to its abundant Co-Nx active sites and highly graphitized carbon [29], [30], [31]. However, ZIF-67 derived Co-NC can neither provide high surface area nor high nitrogen content [32], [33], which greatly lowered their ORR electrocatalytic activities. On the other hand, ZIF-8, which is isostructural to ZIF-67, has been demonstrated to be able to produce porous carbons with high surface areas and nitrogen dopings, whereas it cannot offer high degree of graphitized carbons and critical M-N active sites [34], [35], [36]. Fortunately, these problems could be partly resolved by synthesizing bimetallic ZIFs (BMZIFs) and further using them as templates to prepare Co-NC-based ORR electrocatalysts [37], [38], [39], [40], which inherited both of the individual properties of carbon-based hybrids independently from ZIF-8 and ZIF-67. Besides the chemical properties, considerable efforts have also been devoted to endowing MOF-derived electrocatalysts with novel morphologies and hierarchical structures to better expose the catalytic active sites and facilitate the mass transfer [41], [42], [43]. Among various strategies, constructing hollow nanostructures has attracted fast-growing interest due to their fascinating properties such as lower densities, higher surface-to-volume ratios and sufficient exposed active sites [44], [45], [46], which are highly beneficial especially for the ORR catalyst. Therefore, we believe that it should be remarkably desirable to endow the BMZIFs-derived Co-NC electrocatalysts with hollow architecture, which will combine their merits and therefore further improve their ORR performances.
Bearing these in mind, here we developed, for the first time, an elaborate bottom-up strategy to fabricate highly porous and hollow N-doped carbon spheres embedded with tiny Co NPs by one-step pyrolysis of a core-shell type composite with carboxylate-terminated polystyrene template as core and BMZIFs as shell. Firstly, a series of bimetallic MOFs with various starting Zn contents were successfully synthesized in order to design the optimal combination of ZIF-8 and ZIF-67. In the subsequent pyrolysis process, the following crucial changes occurred: (1) Zn species in BMZIFs can both spatially separate Co species to impede the aggregation of Co NPs and be evaporated to afford extra pores; (2) PS core can also be decomposed and then disappeared to afford the hollow structure; (3) 2-methylimidazole (2-MeIm) contained shell was polymerized to hollow N-doped porous carbon spheres, where Co2+ ions of BMZIFs were in-situ reduced to highly dispersed Co NPs. Consequently, the optimal Co-HNCS-0.2 displayed high surface area, good graphitic nature, enhanced mass transport, abundant active sites, and thus remarkable ORR performance in terms of high activity, excellent stability and methanol tolerance capability. Such electrocatalytic performance of our Co-HNCS-0.2 was superior to most of the previously reported Co-based catalyst and even outperformed the commercial Pt/C (20 wt%).
Section snippets
Chemicals
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%, Shanghai Macklin Biochemical Co., Ltd), Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%, Shanghai Macklin Biochemical Co., Ltd), 2-Methylimidazole (C4H6N2, 98%, Aladdin Industrial Corporation), Styrene (C8H8, 98%, Tianjin Damao Chemical reagent Factory), Methyl methacrylate (C5H8O2, 98%, Tianjin Damao Chemical reagent Factory), Acrylic acid (C3H4O2, 98%, Tianjin Damao Chemical reagent Factory), Ammonium bicarbonate (NH4HCO3, 98%, Sinopharm Chemical
Results and discussion
The Co-HNCS-x materials were prepared by one-step pyrolysis of a series of core-shell PS@BMZIFs precursors, as schematically depicted in Fig. 1. Firstly, by the solvothermal reaction of a methanol solution containing Co(NO3)2·6H2O, Zn(NO3)2·6H2O, 2-methylimidazole and carboxylic acid-terminated polystyrene microspheres (PS), a uniform Co-Zn-BMZIFs shell could be successfully crystallized on the surface of PS due to their isoreticular structures as [M(MeIm)2]n (M = Zn or Co) and their similar unit
Conclusions
In summary, a PS@Zn1-xCox-ZIF core-shell structure have been successfully designed and fabricated as precursors to prepare hollow structured N/Co-doped carbon spheres without any post treatment. During the pyrolysis synthesis, the Zn ions in BMZIFs could both spatially separate Co species to impede the aggregation of Co NPs and be evaporated to afford extra pores, while the polystyrene core would also be decomposed and then disappeared to produce the unique hollow structure. Benefiting from its
ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China (21322606, 21436005, 21576095, and 21606087), the State Key Laboratory of Pulp and Paper Engineering (2017ZD04), Fundamental Research Funds for the Central Universities (2017MS069, 2017PY004), and Guangdong Natural Science Foundation (2014A030310445, 2016A050502004 and 2013B090500027) for financial support.
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