Preparation Of Transition Metal Nanosheets Composites And Study Of Their Electrochemical Properties

The energy crisis and environmental pollution issues urgently require us to develop storage and conversion devices for sustainable renewable energy.Supercapacitors and electrocatalytic water splitting are both energy conversion technologies that are currently widely concerned and studied.Among them,the performance of electrode materials is the key to realizing efficient energy storage and conversion of supercapacitors and electrocatalytic water splitting devices.Transition metal oxides,hydroxides and their sulfides are considered to be ideal electrode materials due to their wide sources and good electrochemical reaction activity,but they are still suffering from problems such as fewer reactive sites,low conductivity,and low cycle life.Therefore,in this article,the starting point is to obtain transition metal composite electrodes with large electrochemically active area,high conductivity and stable structure.Four two-dimensional transition metal composite nanosheets are designed and prepared,and strategies such as multi-component recombination,metal element doping,anion defect oxygen vacancy adjustment or heterostructure construction have been adopt to enhance their electrochemical reaction activity and stability.Through the structural characterization and electrochemical performance analysis of these electrode materials,the synergistic optimization effect between the composition and the structure is explained,and it is clear that the constructed two-dimensional transition metal composite nanosheets have characteristics of large active surfaces,enhanced conductivity and stable structure,so that it has excellent electrochemical performance as an electrode material for supercapacitors and electrocatalytic water splitting,and has achieved the following research results:1.The CuS/MnS composite nanosheet clusters composed of hexagonal nanosheets with a size of about 100 nm were successfully prepared by the multi-metal component composite method,and used as the electrode material for supercapacitors.The addition of high-conductivity CuS induces the growth of MnS,which makes the two metal sulfides closely combine with each other,and realizes the synergistic optimization of the multi-metal components,which effectively overcomes the agglomeration of simple MnS or CuS,and forms stable three-dimensional nanosheet cluster structures composed of hexagonal nanosheets in different directions,thereby providing enhanced electron transport,efficient contact surfaces with the electrolyte and a stable cyclic structure.The synthesized CuS/MnS-3h electrode has a high specific capacitance of 1144 F g^-1 under 1 A g^-1,which is more than five times higher than that of CuS and MnS electrode under the same conditions.At the same time,the CuS/MnS-3h electrode exhibits good rate,showing a specific capacity of 567 F g^-1 at 50 A g^-1,and it still maintains 40.8%of the capacity at the increased current density of 100 A g^-1.The electrode was subjected to 10,000 charge and discharge cycles,and finally showed 85.9%of the initial capacity,which shows that it has good energy storage stability.2.The rMnCo₂O₄@rMnO₂ core-shell structured nanosheet arrays self-supporting supercapacitor electrodes with appropriate oxygen vacancies grown on Ni foam were successfully prepared by constructing a heterostructure and introducing anion defect oxygen vacancies using NaBH₄ solution reduced treatment at room temperature.And adjusting the content of oxygen vacancies in the material through controlling the different NaBH₄solution reduction treatment time,it was found that the synergistic effect of the core-shell nanosheet arrays and appropriate oxygen vacancies jointly improve the electrochemical performance.A certain amount of oxygen vacancy defects enhance the conductivity of the electrode,reduce the charge transfer resistance,and create more electrochemical surface active sites.At the same time,the core-shell structures on the in-situ grown nickel foam make it exhibit reliable structural stability.Compared with pure MnCo₂O₄ orMnCo₂O₄@MnO₂,the electrochemical performance of rMnCo₂O₄@rMnO₂-2h electrode has been significantly improved.The areal capacitance is 3.39 F cm^-2 at 3 mA cm^-2,it still retains 49.0%of the initial capacitance at 60 mA cm^-2,and shows 92.5%of the initial capacity after being subjected to 3000 cycling test under 15 mA cm^-2,indicating that it has a good rate and capacity retention rate.When the electrode applied to the hybrid capacitor device,the device also exhibits good electrochemical performance,which further shows that the rMnCo₂O₄@rMnO₂-2h electrode has excellent application potential.3.The copper-doped Ni₃S₂ nanosheet arrays in situ grown on nickel foam were successfully synthesized by doping different metal elements,and the composition of Ni_2-xCu_x-S was adjusted to study the effect of Cu doping amount on the OER performance of Ni₃S₂ in the electrocatalytic water splitting process.It was found that the cross-linked porous nanosheet structures with moderate Cu doping can generate more catalytically active sites,effectively reduce the contact resistance,promote charge transfer,thus obtaining an improved OER performance.The optimized Ni_1.9Cu_0.1-S(chemical composition is Ni_2.85Cu_0.15S₂)electrode shows a small overpotential(the overpotential?10 at 10 mA cm^-2 is259 mV),and a low Tafel slope(54.9 mV dec^-1),better than those of pristine Ni₃S₂ catalyst electrode.In addition,the Ni_1.9Cu_0.1-S electrode has good OER stability and here is almost no potential rise in long term chronopotentiometry test(the overpotential rises only by 1.9%after 12 hours).Therefore,Ni_1.9Cu_0.1-S is an OER electrocatalytic electrode material that has the potential to be used in the electrolysis of water for hydrogen production.4.The Cu-doped Ni₃S₂(Cu-Ni₃S₂)nanosheets were used as the core based on the determined amount of copper doping,and NiFe double hydroxide(NiFe-ldh)was used as the outer shell,the Cu-Ni₃S₂/NiFe-ldh with two-dimensional/two-dimensional heterostructure composite nanosheets were controlled and synthesized by electrodeposition,and then applied to bifunctional electrocatalytic electrode materials for water splitting to produce hydrogen.The Cu-Ni₃S₂ nanosheet arrays as a core support promote the uniform deposition of NiFe-ldh and improve the overall conductivity,while the NiFe-ldh composite increase more catalytically active sites.The composite electrode exerts the synergy between the multi-component and multi-layer structure,and shows excellent electrocatalytic performance,which is better than single Cu-Ni₃S₂and NiFe-ldh.The OER and HER overpotential of Cu-Ni₃S₂/NiFe-ldh is 193 mV and 105 mV at a current density of 10 mA cm^-2,respectively.It shows good catalytic stability with almost no increase in the overpotential in the stability test for 12 hours.When the Cu-Ni₃S₂/NiFe-ldh were used as bifunctional electrodes for overall water splitting,the voltage of the electrolytic cell under10 mA cm^-2 is 1.53 V,and it has good stability in the 12-hour electrolysis test.