Preparation Of Tungsten Oxides As Negative Electrode Materials And Their Supercapacitor Performance

Due to its fast charge-discharge rate and long life cycle,etc.,supercapacitor（SC）has broad application prospects in the fields of new energy vehicles and convenient electronic equipment.To date,it has attracted a lot of research and development interests of researchers and enterprises.However,the low energy density in some certain degree limits its wide range of practical applications.The stduies show that the improving of the specific capacitance of supercapacitors can be increased by optimizing the morphology of electrode materials and adjusting the appropriate crystal structure.In this paper,nano-tungsten oxide（WO₃）electrode active materials were prepared by a class of simple hydrothermal routines.The effects of hydrothermal synthesis conditions on particle size,crystal structure and morphology were investigated systematically.The capacitive properties of electrode materials were further studied.The main research contents and results are summarized as follows:（1）The monoclinic tungsten oxide（m-WO₃）nanoplates were successfully synthesized by hydrothermal method using H₂C₂O₄ as the structure-directing agent,HCl to adjust the pH value,and Na₂WO₄·2H₂O as the tungsten source.The results show that with the increase of oxalic acid additive,the average thickness of nanosheets decreases,and the specific capacitance at 1 mV s^-1 increases gradually.We also improved the hydrothermal synthesis process by adding a step of washing the precursors,the nanosheets in the thickness of 32 nm were obtained;further studies show that these samples can reach a specific capacitance of454.5 F g^-1 at a scan rate of 1 mV s^-1 and also show full capacitance retention（¹00%）even after 3,000 cycles,indicating good cycle stability.（2）The hexagonal tungsten oxide（np-WO₃）nanoplates were successfully prepared by hydrothermal synthesis using L-（+）tartaric acid as a structure-directing agent.The h-WO₃nanosheets exhibits a specific capacitance of 242.4 F g^-1 at a scan rate of 2 mV s^-1 and the capacity retention rate of 3,000 cycles was 102%.（3）The self-assembled pancake-like hexagonal tungsten oxide with ordered mesoporous（nc-WO₃）could be synthesized via a facile one-pot hydrothermal method without any template and additional capping agent by adjusting pH,Na₂WO₄ concentration and hydrothermal synthesis time.The novel nc-WO₃ in our experiments can reach a high specific capacitance of 605.5 F g^-1 at a current density of 0.37 A g^-1;also show full capacitance retention（up to 110.2%）even after 4,000 cycles and lower intrinsic resistance is 0.45Ω.Finally,the energy storage mechanism of the material is analyzed.When the scanning rates are less than 5 mV s^-1,the contribution of the capacitance is mainly the proton insertion type capacitor.When the scanning rates are greater than 5 mV s^-1,the contribution of the capacitance is mainly the hybrid of proton insertion type and solid-state diffusion.The capacitance for nc-WO₃ from the most accessible area is account for up to 18.1%of total capacitance.The excellent electrochemical properties are attributed to the following reasons:the hexagonal lattice channel facilitates the insertion of protons into the interior of the material lattice;in addition,the specific surface area and electroactive sites of the electrode material are inproved,thereby improving the utilization of the electrode active material and electrochemical activity;the mesoporous structure with quite good mechanical stability can alleviate or accommodate the volume expansion caused by the insertion and de-insertion of H⁺,and further enhance the cycle life of electrode materials.In this paper,the self-assembled pancake-like hexagonal tungsten oxide with ordered mesoporous is successfully prepared by a simple hydrothermal synthesis method,which has excellent electrochemical properties and provides an opportunity for the application of transition metal tungsten oxide in supercapacitors.