Carbon Nanotubes Modulated Nickel-cobalt MOF-based Electrode Materials And Their Energy Storage Performances

The growing demand for energy storage puts forward higher and higher standards on electrochemical energy storage(EES)devices.To boost the energy storage capacity of EES devices,the reasonable design and construction of new electrode materials has become an important topic under the current new energy development strategy,and an important guarantee to promote the sustainable development of social economy.As a promising EES device,hybrid supercapacitors well combine the advantages of both battery and supercapacitor,which can realize the integration of high energy storage and high-power output.As the vital part of hybrid supercapacitor,battery-type electrode has a decisive influence on the energy storage performance of whole device.Therefore,the reasonable design of high-performance battery-type electrode materials plays a very important role in promoting the development of hybrid supercapacitors.Herein,carboxylated carbon nanotubes(C-CNTs)were employed to regulate the crystallization and growth process of Ni/Co-based MOFs,and different MOFs based hybrid structures were constructed to promote the electron transport and ion diffusion in the electrochemical reaction process.By optimizing the physicochemical structures and micro-morphology,the Ni/Co-based MOFs hybrid electrode materials with high specific capacity and excellent rate performances were developed to realize the assembly of high-performance hybrid supercapacitors.The roles of C-CNTs in inducing the microstructure transformation of Ni/Co-based MOFs were discussed in detail.The intrinsic relationship between the structural evolution of Ni-,Co MOFs based hybrid materials and their electrochemical performances was investigated,and the electrochemical reaction kinetics and charge storage mechanisms of different materials were analyzed in detail.Through controlling the growth of Ni-MOF by C-CNTs,the Ni-MOF nanosheets embedded with C-CNTs(Ni-MOF/C-CNTs)were synthesized by solvothermal method.After the electrostatic adsorption of Ni²⁺on C-CNTs,the nucleation center and skeleton were formed,which further guided the crystallization and growth of Ni-MOF.The proper amount of C-CNTs not only effectively regulated the lamellar stack structure of Ni-MOF,and assisted constructing a continuous ultra-thin hybrid nanosheets,thus promoting the diffusions of electrolyte ions and the exposure of metal active sites,but also fuctioned as the electron pathway to facilitate the charge transfer during the electrochemical reaction process.By optimizing the amount of C-CNTs,the as-synthesized Ni-MOF/C-CNTs40 exhibited the best charge storage capacity.At the current density of 1 A g^-1,its specific capacity is 680 C g^-1,and at 10A g^-1,its specific capacity can be retained 65%.In addition,the assembled Ni-MOF/C-CNTs40//AC hybrid supercapacitor demonstrated good energy storage performances at a work voltage of 1.7V,which deliveried the maximum energy density of 44.4 W h kg^-1 and the capacitance retention of 77%after 3000 charge-discharge cycles.To further improve the energy storage capacity and rate performance of Ni-MOF,a hybrid network composed of Ni-Co bimetallic MOF intercalated with C-CNTs(Ni₂Co-MOF/C-CNTs)was synthesized by solvothermal method.By selecting different transition metal ions and adjusting the molar ratio of Ni²⁺/Co²⁺,the effects of different ions on the microstructures and electrochemical performances of Ni-MOF were studied,and the optimal ratio of Ni²⁺and Co²⁺in Ni-Co bimetallic MOF was determined.Using C-CNTs as nucleation center and skeleton,Ni₂Co-MOF/C-CNTs hybrid network was constructed to realize the rapid transport of electrolyte ions and electrons.The synthesized Ni₂Co-MOF/C-CNTs50 exhibited the specific capacity of765 C g^-1 at 1 A g^-1,and the specific capacity could retain 85.4%at 10 A g^-1.The oxygen-enriched nitrogen doped porous carbon nanosheet(NOCN)was prepared by pyrolysis of citric acid and urea using basic magnesium carbonate as activation agent,which was used as capacitive-type materials.Among all samples,the NOCN900 has the optimal capacitive performance.At 0.5 A g^-1,it exhibited a specific capacitance of 232 F g^-1 and the capacitance retention of 71.5%at 20 A g^-1.By matching the positive and negative electrode,the assembled Ni₂Co-MOF/C-CNTs50//NOCN900hybrid supercapacitor showed well energy storage performances,which deliveried a wide work voltage of 1.6V and the maximum energy density of 38.2 Wh kg^-1.In addition,the capacitance retention of hybrid supercapacitor could reach 80.4%after5000 charge-discharge cycles,which demonstrated its well comprehensive performance.The C-CNTs induced crystal growth strategy was purposed to synthesize ZIF-67polyhedron connected with C-CNTs(ZIF-67/C-CNTs).Then,ZIF-67/C-CNTs was transformed into C-CNTs connected hollow Ni Co-LDH nanoarchitecture by using self-template ion etching and exchange method.The introduced C-CNTs have connected ZIF-67 particles and made the particle size of ZIF-67 significantly reduced,and thus increasing the specific surface area of Ni Co-LDH/C-CNTs.The formed porous nanosheets and hollow structures shorten the transport path of electrolyte ions,while C-CNTs provide a charge transfer channel between the external circuit and the internal surface of Ni Co-LDH.By comparison,it is found that the introduction of appropriate amount of C-CNTs and CNTs can improve the electrochemical performances of ZIF-67 derived Ni Co-LDH electrode materials to some extent.Among all samples,the Ni Co-LDH/C-CNTs16 has the best charge storage capability.It exhibited the specific capacity of 855 C g^-1 at 1 A g^-1 and the capacity retention of91.5%at 15 A g^-1.In addition,the assembled Ni Co-LDH/C-CNTs16//AC hybrid supercapacitor delivered the energy density of 49.9 Wh kg^-1 and 27.7 Wh kg^-1 at the power density of 895 W kg^-1 and 7612 W kg^-1,respectively,demonstrating its well energy storage performance.