Hierarchical Structure Regulation For Environment-friendly Cobalt-based MOFs-derived Multiphase Selenides And Research On Energy Storage Characteristics

As a safe/stable green energy storage device,supercapacitors（SCs）complement the market gap between batteries and traditional capacitors,and play an indispensable role in the new energy industry and high-tech emerging industries.The environmentally friendly and mature preparation process,low-cost,high power density,high-speed charge and discharge,long cycle life,and other advantages make SCs have great potential for development.However,restricted energy density is a major impede to the improvement of SCs.According to the formula E=1/2 C V²,electrode capacitance（C）and voltage window（V）are the determining factors of capacitor energy density.In conventional SCs,the potential of electrode capacitance development far exceeds the voltage window.Therefore,the development and optimization of capacitance factors of electrodes is an effective strategy to develop high-capacitance/stable electrodes and SCs.Hybrid-supercapacitors（HSCs）assembled by a battery-type cathode（high capacity）and a capacitive anode（high stability）exhibit several times higher energy density than conventional pseudocapacitors（PCs）and electrical double-layer capacitors（EDLCs）,showing great advantages in the application of SCs.Developing novel and efficient strategies to improve the capacity and stability of battery-type electrodes remains a major challengeRecently,non-toxic and environment-friendly transition metal selenides（TMSes）are highly respected in the green energy storage field because of their strong metal bonds and electrical conductivity.Compared with other high-performance chalcogens（O,S）,the conductivity of Se can reach 1×10^-3 S m^-1,which is well above 5×10^-28 S m^-1 of S.The above facts provide a solid basis for the high theoretical specific capacity of TMSes.However,the powder state,which is easy to accumulate and is not conducive to rapid charge transfer,the single phase,and the conventional composite synthesis method all pose certain challenges to the further development of TMSes.Environmentally friendly metal-organic frameworks（MOFs）with a special structure and active composition are introduced,and a further optimized multiphase selenide hierarchical structure is obtained.To systematically research materials changes from single phase to multiphase active sites,single metal to numerous metalcore with the change of the capacity and stability.Besides,Multistage structures derived from single-stage structures play an important role in the rational allocation and fixation of active sites.Finally,the cathode material with the optimal classification structure,active site combination,performance,and stability is selected.To further study the energy storage behavior of the electrode,the pseudocapacitance contribution of the electrode is calculated and analyzed.To verify the practical value of the prepared electrode,the optimal cathode and the high-efficient anode（activated carbon:AC）are used to assemble HSCs.Moreover,important parameters:energy/power density,and long-term cycle stability are systematically tested.In addition,the homemade LED series（in parallel）can be lit by the charged HSCs（two in series reaching a voltage of 3.2V）.The research content of specific work is as follows:1.Construction of environment-friendly multi-metal hierarchical electrode Ni_xCo_1-xSe₂ and research on energy storage characteristicsThe reasonable structure design is a significant preparation strategy for high-performance electrodes to disperse and immobilize abundant active sites that can bind electrolytes rapidly.The two-dimensional（2D）ZIF-L（zeolite imidazole organic framework,one of MOFs）is fixed on Co（OH）F/CC（carbon cloth）and then reconstructed with Lewis acid at room temperature.The precursor Ni Co-LDH@CNFs@Co（OH）F/CC（CNFs:ZIF-L-derived carbon-nitrogen frameworks）with numerous channels is successfully prepared.The performance of the hierarchical multiphase active electrode Nix Co1-x Se2@CNFs@Co O prepared by high-temperature gradient selenization is studied systematically.The concentration of Ni ions（Lewis acid）and the reconstruction time directly determined the size and stability of ZIF-L derivatives and the proportion,quantity,and distribution of Ni-Co bimetals（（Ni,Co）Se2and Co Se2 phases）in the electrodes.The gradient selenization process effectively optimized the charge environment of the electrode and increased the amount of more hydroxy-adsorbed active substances（multiphase selenides（Ni,Co）Se2 and Co Se2）.CNFs with high conductance/specific surface area made the active substances uniformly dispersed,which promoted the high performance and stability of the electrode.The capacity of the optimal hierarchical multiphase electrode Nix Co1-x Se2@CNFs@Co O/CC can reach 207.8 m Ah g^-1 at 1 A g^-1.The optimal sample and AC are selected as the cathode and anode,and 6 M KOH is used as the electrolyte to assemble the high-performance HSC.The maximum energy density is 45.0 Wh kg^-1(800 W kg^-1),and the stability is 98.0%（12,000 cycles）.2.Construction of environment-friendly unitary-metal hierarchical multiphase electrode M-Co Se₂ and research on energy storage characteristicsA multichannel high-efficiency electrode system is constructed by inducing and immobilizing ZIF-67 nanoboxes on Co Al-LDH nanoflowers by a simple in-situ self-deposition method at room temperature.The formation/transformation process of active sites（multiphase-Co Se2）and the influence of charge transfer channel changes on ion diffusion ability and redox activity in the process of temperature gradient（300-500 ℃）selenium modification were systematically studied.High-temperature selenide has made active sites evenly distributed and fixed on the box,and fully exposed to the electrolyte.With the directional regulation of temperature,selenization time,selenium source quality,and other factors,the active substance Co Se2 with various phases can form the best material matching ratio,good coordination,and synergy in the process of charge transfer,which can successfully enhance the efficiency of energy storage.In addition,porous hollow nanoboxes rich in multiphase active sites form channels between selenized/oxidized LDH derivatives（with numerous channels between layers）,which make them cooperate in energy storage behavior and reduces the expansion phenomenon,which not only increases the charge transport efficiency but also effectively improves the capacity and stability of electrodes.The specific capacity of the best cathode M-Co Se2@CNFs@Co Al-LDO/CP can reach 343.6 m Ah g^-1 at 1 A g^-1,which is more than twice that of the monomer material and further than that of working 1.Under the condition of 6 M KOH as the electrolyte,the optimal cathode and anode AC are combined to form HSC（M-Co Se2@CNFs@Co Al-LDO/CP//AC）.The highest energy density is 50.8 Wh kg^-1,and after 10,000 cycles of testing,the capacity retention rate of the HSCs could still reach 96.8%,showing a considerable advantage.3.Construction of environment-friendly ternary-metal hierarchical multiphase electrode M-（Ni,Co,Mn）-Se and research on energy storage characteristicsHierarchical structure and multi-phase cooperation strategy have shown great strength in solving the problems of low conductivity and capacity of electrodes.However,the complex structure still makes the stability of the electrode insufficient.Therefore,it is feasible to construct a multi-channel graded electrode system rich in multiphase active sites through in situ synthesis strategy to further develop the performance and improve the stability of TMSes.In this work,1D ZIF-L is in-situ reconstructed using Lewis acid at room temperature to obtain the precursor with multichannel trimetallic layered double hydroxyl compound（Ni Co Mn-LDH@CNFs）.The fully dispersed metals with abundant valence lay a solid foundation for further selenization into high-performance multiphase selenides.The optimum structure and composition of the precursor are obtained by adjusting the Lewis acid metal proportion,metal concentration,and reaction time.At further high-temperature gradient selenization（180 ℃）,the hierarchical structure is accompanied by the gradual production of polyphase M-（Ni,Co,Mn）-Se@CNFs,reaching the optimum proportion,and excess（structural destruction）.The LDH layer spacing of the multi-channel is adjusted to reduce the charge transport distance,and the multiphase active sites are homogeneously distributed in the hierarchical structure and form a good synergistic effect.The optimal electrode demonstrated ultra-high capacity(334.7 m Ah g^-1,1 A g^-1)and an extremely high ratio of performance（83.0%）in the range of 1-10 A g^-1.All these highlight the superior stability/capacity performance of the multiphase electrode.The assembled HSCs（M-（Ni,Co,Mn）-Se@CNFs/CC//AC）displayed ultra-high energy density(91.6 Wh kg^-1)and outstanding stability（104.5%,16,000 cycles）,which is better than most of the currently HSCs using TMSes,demonstrates the superiority of the electrodes prepared by this in situ strategy,and the high stability,high capacity electrode plays a decisive role in the capacity and stability of HSCs.4.Phase/layer spacing regulation of environment-friendly hierarchical multiphase electrode M-（Ni,Co）-Se and research on energy storage characteristicsThe in-situ reconstruction strategy can make the electrolyte participate in the reaction process more efficiently,reduce the damage to the structure due to surface volume expansion and the attenuation of the active phase,and better maintain the cycle stability of the electrode and device.In this work,1D ZIF-L is reconstructed with Lewis acid at a high temperature under alkali regulation,a highly stable regular hexagonal Ni Co-LDH tightly combined with hollow CNFs is obtained,and the stable hierarchical system is constructed.After selenization with time and temperature gradients,the phase change of LDH derivatives with the layer spacing,the adsorption of electrolyte,the promotion of charge transport rate,and the change of performance are systematically studied.At 1 A g^-1,the optimal electrode M-（Ni,Co）-Se@CNFs/CC exhibits an extremely high specific capacity(378.9 m Ah g^-1).The assembled HSC（M-（Ni,Co）-Se@CNFs/CC//AC）shows an ultra-high energy density(96.6 Wh kg^-1).After 10,000cycles,the HSC remained stable at 133.6%,which is better than most similar efforts.The in-situ construction strategy of the multichannel multiphase electrode system is successfully realized.The study of the layer spacing and phase evolution process of LDH derivatives is helpful to further explore the formation of high-capacity and high-stability electrodes and the successful preparation of efficient HSC.In the works,the hierarchical structures are designed and the number and distribution of the active sites of green cobalt-based ZIFs derivatives were modified by the precise regulation of Lewis acid and other strategies.Then,the phase of the active sites of the electrode was precisely regulated by the high-temperature selenization and other strategies.The abundance of active sites on the surface,the ion adsorption,and redox activity increased obviously.Moreover,the internal resistance and volume expansion of the material is reduced by in-situ fabrication,and high redox activity is achieved while maintaining high stability.Furthermore,systematic regulation of the phase of the active site and ion transport distance is introduced and significantly improved charge transport,ion diffusion,and redox activity.The electrochemical properties of the electrode and HSCs were improved by surface control and diffusion control.