Controlled preparation of CoNi2S4 nanorods derived from MOF-74 nanoarrays involving an exchange reaction for high energy density supercapacitors

Binary transition metal sulfides are considered to be a promising material for supercapacitors, possessing richer electrochemically active sites and superior electrochemical performance. Metal-organic frameworks (MOFs) are often used as self-sacrificing templates in the preparation of metal sulfides. Usually, direct sulfidation of MOFs tends to cause collapse of the morphological structure and blockage of the ion transport channels, so that the morphology of the original MOF template can be well preserved by using pyrolysis followed by S2- ion exchange. In this paper, we first prepared NiCo-MOF-74 on nickel foam by an in situ transformation method from layered double hydroxides (LDHs) through a ligand exchange reaction. Then, CoNi2S4 was synthesized in two steps involving the pyrolysis of NiCo-MOF-74 and a subsequent S2- ion exchange reaction. Compared with direct sulfidation, this synthetic strategy can well maintain the rod-like morphology of MOF-74 arrays and prevent structural collapse. The surface of CoNi2S4 has a fine nanosheet structure, which exposes more active sites and shows a high specific capacitance of 7.50 F cm-2 at 2 mA cm-2 and an excellent Coulomb efficiency (96.32%). In addition, the hybrid supercapacitor assembled with activated carbon shows a high energy density of 0.64 mW h cm-2 at a power density of 1.64 mW cm-2 and a high capacitance retention of 88.39% after 5000 cycles. These results indicate that rod-shaped CoNi2S4 can be controllably prepared from MOF-74 involving an exchange reaction and has promising application in high-performance supercapacitors.