Benzoate Acid-Dependent Lattice Dimension of Co-MOFs and MOF-Derived CoS₂@CNTs with Tunable Pore Diameters for Supercapacitors

Herein three novel cobalt metal-organic frameworks (Co-MOFs) with similar ingredients, [Co(bib)(o-bdc)]∞ (1), [Co₂(bib)₂(m-bdc)₂]∞ (2), and {[Co(bib)(p-bdc)(H₂O)](H₂O)0.5}∞ (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 CoS₂@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 CoS₂@CNTs) have been well characterized. Topology and Brunauer-Emmett-Teller analyses elucidate that the bdc²⁻ 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)-CoS₂@CNT possesses hierarchical morphology, moderate specific surface area, proper pore diameter distribution, and high graphitization, which lead to remarkable specific capacitances (839 F g⁻¹ at 5 mV s⁻¹ and 825 F g⁻¹ at 0.5 A g⁻¹) in 2 M potassium hydroxide solution. In addition, the (p)-CoS₂@CNT electrode exhibits good electrochemical stability and still retains 82.9% of initial specific capacitance at the current density of 1 A g⁻¹ after 5000 cycles.