Synthesis Of Ni-Co Based Hydroxides And Sulfides Based On ZIF And Study On Their Electrochemical Properties

The widespread and continuous growth of fossil fuels such as crude oil,coal and natural gas consumes limited natural energy sources and exacerbates environmental problems such as air pollution and excessive greenhouse gas emissions.Therefore,researchers urgently need to explore and develop efficient and sustainable energy conversion and storage technologies to improve energy efficiency.The zeolite imidazole framework（ZIF）material is ideal for supercapacitor electrodes due to its highly controllable structure and porous morphology,and the fact that most ZIF central atoms are usually transition metal ions.In this paper,we focus on the conversion of ZIF-Co/Niinto metal hydroxides by adding metal salts only,and synthesize metal hydroxides and sulfides with unique structure and high specific surface area.（1）In the first part,the transformation relationship between ZIF and its layered double hydroxide（LDH）was investigated by adjusting the ratios of metal ions and organic ligands,which led to the development of a novel strategy for the synthesis of LDH.The optimal molar ratios for the synthesis of Co-LDH and Ni-LDH were obtained for ZIF-Co and ZIF-Ni,and applied to supercapacitors.At a molar ratio of metal ions to Hmim of 4:1,both materials exhibit the highest electrochemical capacities of 263.8 and 520.8 C g^-1 at 1 A g^-1,good multiplicative discharge performance,and superior to those at other ratios.（2）In the second part,a novel layered flower-like nanosheet NiCo₂S₄ composed of nanoparticles was prepared based on the first series.Firstly,the molar ratio of metal ions to organic ligands in the first series was followed to directly synthesize NiCo-LDH.Subsequently,NiCo-LDH was further sulfurized with thiourea as the sulfur source to obtain the nanoparticles composed of the unique layered flower-like NiCo₂S₄nanosheet structure.It exhibits an ultra-high specific capacity of 968 C g^-1 at 1 A g^-1current density,good cycling stability,and the specific capacity can be maintained at83%after 3000 charge/discharge cycles,which is superior to Ni-LDH,Co-LDH and pure NiCo-LDH nanosheet structures.（3）In the third part,we developed a simple two-step strategy to grow Ni-Co-S nanosheet arrays（NSA）on nickel foam（NF）substrates with strong adhesion and use them as binder-free positive electrodes in supercapacitors.Firstly,the NiCo-LDH arrays were hydrothermally grown on NF and then converted to Ni-Co-S@NF by a suitable vulcanization time.As expected,Ni-Co-S@NF sample shows a high specific capacity of 1286 C g^-1.In addition,the hybrid supercapacitor device Ni-Co-S@NF//activated carbon（AC）assembled with the prepared materials also exhibit high energy density(74.0 Wh kg^-1)and high power density(850.0 W kg^-1);the cycling stability can reach 87.2%after 5000 charge/discharge cycles.