Recently, a research team in the Institute of Solid State Physics (ISSP), Hefei Institutes of Physical Science, Chinese Academy of Sciences, has made a series of progress in fabrication and applications of metal-organic frameworks derived carbon-based energy nanomaterials.
Metal-organic frameworks (MOFs) and their derived carbon-based nanomaterials have exhibited extensive applications in gas capture and separation, catalysis and energy storage and conversion by virtue of their large specific surface area, controllable compositions and porous structures. Compared with the synthesized nanomaterials by traditional methods, MOFs are optimal precursor materials that contain carbon and metal elements simultaneously in one intact crystalline framework and their composition can be easily adjusted by substituting the metals or organic linkers based on different application demands.
To date, many studies have reported that MOFs can be transformed into valuable products such as porous carbon, metal/alloy, and metal oxides by direct pyrolysis under an inert atmosphere, and the obtained materials usually equip with high electrical conductivity and superior catalysis activities for varieties of applications.
On the basis of the above discussion, researchers in ISSP utilized two kinds of organic linkers containing S and N elements respectively in the presence of Co2+ and Ni2+ sources to controllably fabricate Co and/or Ni MOFs nanostructures (nanorods, nanofibers, nanosheets and 3D bulk) under room temperature and hydrothermal conditions.
And then, as precursor materials, the as-synthesized Co and/or Ni MOFs were further treated by high temperature pyrolysis at N2 atmosphere to obtain a series of carbon-based nanomaterials with high surface area and porous structure for electrocatalysis and supercapacitor applications.
Firstly, Co-MOFs nanostructures including nanorods, nanofibers, nanosheets and 3D bulk have been successfully fabricated by a ¡°pillar-layer¡± assembly method utilizing thiophene-2,5-dicarboxylate (Tdc) and 4,4¨@-bipyridine (Bpy) as organic ligands (Figure 1).
The resulting Co-MOFs as precursors were pyrolytically converted into Co/CoxSy@S,N-doped porous carbon materials, as electrocatalysts exhibiting superior bifunctional electrocatalytic activities of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) owing to their high surface area, porous structure and rich Co-related and S, N co-doped active sites. Further, the obtained carbon-based material from Co-MOFs nanosheets also exhibited superior performance as supercapacitor electrode material (Figure 2).
To further improve the capacitor performance, Co/Ni-MOFs nanosheets were first fabricated utilizing the above mentioned strategy, then pyrolytically transformed into CoNi alloy nanoparticles on S, N-codoped porous carbon composite, as shown in Figure 3.
The results demonstrated that the obtained 2D Co/Ni-MOFs derived carbon material supercapacitor electrode displays high conductivity and excellent capacitor storage ability, together with outstanding recycle performance, which can be ascribed to the higher number of electrochemical active sites and faster electron transmission provided by the CoNi nanoparticles during electrochemical measurements; also, the high surface area and porous structure of the composite are favorable for the exposure of the electrochemical active sites and redox-related mass transport.
In addition, researchers in ISSP have also achieved the controllable production of Co-MOFs crystal at macro-scale by using 1,3,5-benzenetricarboxylic acid (H3BTC) linkers in the presence of Co2+, triethylamine (TEA) and nonanoic acid by a facile solvothermal reaction. And then metallic Co nanoparticles embedded into N-doped porous carbon layers (Co@NPC) was obtained by a simple pyrolysis process. As electrocatalyst, the Co@NPC exhibits bifunctional electrocatalytic activities toward ORR and OER in alkaline media, which are key reactions in some renewable energy technologies such as fuel cells and rechargeable metal air batteries, as shown in Figure 4.
The findings in these works would be valuable for designing and developing high performance electrode materials for energy conversion and storage in further practical applications.
The results were published in Journal of Materials Chemistry A (J. Mater. Chem. A 5, 9873-9881 (2017)), Inorganic Chemistry Frontiers (Inorg. Chem. Front. 4, 491-498 (2017)), and ACS Applied Materials & Interfaces (ACS Appl. Mater. Interfaces 9, 34269-34278 (2017)).
These works were financially supported by the National Natural Science Foundation of China (Grant Nos. 51672277, 51372248 and 51432009), the CAS Pioneer Hundred Talents Program, and the CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Science, China.
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Figure 1. Synthesis of Co-MOFs with different structures under different conditions (Image by TONG Mingyu)
Figure 2. (a) Morphology and structure characterization of Co-MOFs; (b) Electrochemical tests for oxygen catalytic reaction and its stability; (c) Electrochemical test for energy storage and its stability (Image by TONG Mingyu)
Figure 3. (a) Morphology and composition characterization of Co/Ni MOFs; (b) Electrochemical tests for energy storage properties and its stability (Image by TONG Mingyu)
Prof. ZHANG Haimin, Ph.D Principal Investigator
Centre for Environmental and Energy Nanomaterials
Institute of Solid State Physics (ISSP), Hefei Institutes of Physical Science, Chinese Academy of Sciences
Hefei, Anhui 230000, China