Preparation And Electrochemical Properties Of Novel Nanostructured Electrode Materials For Supercapacitors By Atomic Layer Deposition

With the rapid development of global economy,the traditional fossil energy has been consumed,and it is necessary to develop efficient energy conversion and storage equipment.Supercapacitors have attracted much attention due to their high power density,long cycle life,wide operation temperature range,safety and stability,but low energy density has become the bottleneck of their wide applications.Therefore,new supercapacitor electrode materials are being developed to improve their energy density,triggering the study on nanostructured electrode materials.Compared to bulk materials,nanomaterials have higher specific surface area with more energy storage,and the small nanoparticles can effectively alleviate expansion and contraction of electrode materials during the charge and discharge process and prevent from the pulverization effect,which is beneficial to improve the cycle stability.Atomic layer deposition(ALD)is a new deposition technique based on self-limiting and self-saturation chemical adsorption reaction on the growth surface using gaseous precursor.Owing to its excellent three-dimensional conformality and large area uniformity,ALD can deposit solid coating layers on a variety of complex surfaces with precise and controllable thickness,so it has a great prospect in the field of supercapacitors.In this thesis,we focused on the fabrication and electrochemical properties of novel nanostructured electrode materials for supercapacitors.Ni/Ni3S2 core-shell structure electrode material was prepared by simple annealing method.Its electrochemical performance and mechanism was investigated.The ZnO@ZnS/nickel foam(NF)and carbon nanotubes(CNTs)@Co3O4/carbon cloth(CC)nanocomposite structures were fabricated by ALD technique.The composition,microstructure,morphology,and the electrochemical properties of the nanocomposite structures were studied.Finally,the three dimensional porous carbon electrode materials were derived from molecular layer deposition(MLD)technique and its application in supercapacitors was explored preliminarily.The main achievements are listed below:1.Ni/Ni3S2 core-shell structure electrode material was prepared by annealing nickel foam in the hydrogen sulfide atmosphere.It is found that the areal capacitance of electrode material increases in the activation process constantly from 64.4 mF/cm2 to 5137.4 mF/cm2 at 100 mA/cm2 with greatly enhanced electrochemical performance.There are two main reasons responsible for this improvement.One hand,the Ni3S2 shell on the surface is transformed into NiO with higher capacity.On the other hand,the relatively smooth surface has become porous nanostructures with much larger specific surface area.The latter one is the dominant factor.Moreover,the electrode material shows good cycling stability without obvious decay in areal capacitance during 80000 charge and discharge cycles.In addition,the material exhibits excellent rate performance due to its good conductivity and large specific surface area.2.ZnO@ZnS/NF and CNTs@Co3O4/CC nanocomposite structures were successfully prepared by coating the three dimensional conductive substrates with active materials using ALD.After 300-cycle ZnS was deposited on the surface of ZnO nanowires/NF by ALD,ZnS is uniformly coated on the surface of ZnO nanowires to form core-shell structure.ZnO@ZnS/NF nanocomposite electrode has excellent rate performance.The capacitance retention rate is 86.8%when the current density is raised to 10 times.After 20000 cycles,the capacity of electrode material has no attenuation with excellent cycling stability.The better electrochemical performance is mainly attributed to the good conductivity and the large specific surface area of 3D substrates.Meanwhile,we also used plasma-enhanced ALD(PEALD)to deposit 800/1600/2400 cycles C03O4 on CNTs/CC.Co3O4 nanoparticles uniformly adhere to CNTs,and the size of nanoparticles increases with cycle numbers.After Co3O4 deposition,the areal capacitance of CNTs@Co3O4/CC increases obviously,about 10 times of CNTs/CC,and 3.3 times of Co3O4/CC.Furthermore,the nanocomposite electrode has good rate performance with the capacitance retention rate of 65.7%at the 8 times current density.After 50000 cycles,it has achieved remarkable cycling stability without capacity degradation.In addition,the areal capacitance of the electrode material increases with ALD cycle numbers.3.Three dimensional porous carbon materials were successfully prepared by using nickel foam as template and inorganic-organic hybrid material as precursor derived from MLD for the first time.The results show that Al-based fumaric acid hybrid films coated on nickel foam uniformly by MLD transform into a mixture of amorphous C and Al2O3 after 700? anneal in hydrogen.Finally the 3D porous carbon materials are obtained by removing nickel foam substrate and Al2O3 using chemical etching.These 3D porous carbon materials are superlight with a much low areal density of 0.07 mg/cm2 based on nickel foam template.At present,the electrochemical performance of 3D porous carbon materials still needs futher improvment.