Benzoate Acid-Dependent Lattice Dimension of Co-MOFs and MOF-Derived CoS@CNTs with Tunable Pore Diameters for Supercapacitors.

Herein three novel cobalt metal-organic frameworks (Co-MOFs) with similar ingredients, [Co(bib)(o-bdc)] (1), [Co(bib)(m-bdc)] (2), and {Co(bib)(p-bdc)(HO)} (3), have been synthesized from the reaction of cobalt nitrate with 1,4-bis(imidazol-1-yl)benzene (bib) and structure-related aromatic acids (1,2-benzenedicarboxylic acid = o-bdc, 1,3-benzenedicarboxylic acid = m-bdc, and 1,4-benzenedicarboxylic acid = p-bdc) by the solvothermal method. It is aimed to perform systematic research on the relationship among the conformation of benzoate acid, lattice dimension of Co-MOF, and pore diameter of MOF-derived carbon composite. Through the precursor strategy, Co-MOFs 1-3 have been utilized to synthesize porous cobalt@carbon nanotube composites (Co@CNTs). After the in situ gas-sulfurization, secondary composites CoS@CNTs were successfully obtained, which kept similar morphologies of corresponding Co@CNTs without destroying previous highly dispersed structures. Co-MOFs and two series of composites (Co@CNTs and CoS@CNTs) have been well characterized. Topology and Brunauer-Emmett-Teller analyses elucidate that the bdc ion could control the pore diameters of MOF-derived carbon composites by adjusting the lattice dimension of Co-MOFs. The systematic studies on electrochemical properties demonstrate that (p)-CoS@CNT possesses hierarchical morphology, moderate specific surface area, proper pore diameter distribution, and high graphitization, which lead to remarkable specific capacitances (839 F g at 5 mV s and 825 F g at 0.5 A g) in 2 M potassium hydroxide solution. In addition, the (p)-CoS@CNT electrode exhibits good electrochemical stability and still retains 82.9% of initial specific capacitance at the current density of 1 A g after 5000 cycles.