| Supercapacitors,also known as electrochemical capacitors,are a new type of electrochemical energy storage device with high power density and long working lifespan,showing promising application prospects in many fields.However,the energy density of supercapacitors still lags far behind commercial energy storage devices such as lithium-ion batteries.Hence,improving the capacitive performance of supercapacitors without sacrificing the instinctive advantages is the key to promoting the further practical application of supercapacitors.Electrode materials are the core components of supercapacitors,which play a vital role in the improvement of energy storage performance.Transition metal oxides,as a class of Faraday pseudocapacitive materials with abundant natural reserves,have higher theoretical capacitances than traditional electric double layer carbon materials,and have received extensive attention from researchers.However,transition metal oxides have problems such as poor electrical conductivity,low utilization of electrode materials,and unsatisfied cycle stability,and their electrochemical energy storage performance has not reached people’s expectations.Aiming at the above problems,this paper improves the electrochemical energy storage performance of various transition metal oxides by rationally designing and regulating the micro-nano structure of electrode materials,which provides a new route for the research of pseudocapacitive energy storage materials.The main findings of the paper are as follows:1.Using a flexible and controllable electrochemical method,graphite foils were exfoliated into interconnected layered sheet-like structures,and then high mass loading(15.4 mg cm-2)molybdenum oxides material were electrochemically grown on the three-dimensional graphite foil substrate by potentiostatic techniques.The surface of the exfoliated graphite foil has a large number of oxygen-containing functional groups,which provide active sites for the subsequent growth of molybdenum oxide,and promote the uniform growth of the high mass loading molybdenum oxide material on its surface.The"oxygen bridge"formed between the graphite foil substrate and molybdenum oxide promotes the rapid transfer of electrons,significantly reduces the charge transfer resistance of the active material,realizing the rapid charge transfer of the molybdenum oxide material with high mass loading.The area specific capacitance of the as-prepared Mo O3-x/FEG electrode reaches a high value of 4.34 F cm-2 at current density of 1 m A cm-2.In addition,the high mass loading Mo O3-x/FEG electrode still has a capacitance retention rate of 67.8%after a 20-fold increase in current density,showing excellent rate performance.Mo O3-x/FEG//Q-FEG asymmetric supercapacitors assembled with Mo O3-x/FEG as the negative electrode and a functionalized partially exfoliated graphite foil（Q-FEG）of charge matching as the positive electrode deliveries a high energy density of 2.2 m Wh cm-3 at the power density of 3.6 m W cm-3,showing excellent energy storage performance.2.Using a simple one-step hydrothermal method,oxygen-deficient tungsten oxide nanowires were grown in situ on carbon fiber substrates.The growth mechanism of tungsten oxide and the effects of different hydrothermal reagents on its morphology and oxygen defect construction were explored.It was found that ammonium acetate played a role in regulating the nucleation rate during the growth of tungsten oxide,while hydrogen peroxide reagents adjustable oxygen vacancy concentration.Afterwards,in view of the poor stability of tungsten oxide material,an acidic-neutral mixed electrolyte was designed and constructed,while the composition ratio was optimized.The electrolyte significantly reduced the lower cutoff potential of the tungsten oxide electrode.In the enviroment of optimized mixed electrolyte,the tungsten oxide electrode achieves the optimal p H effect and ion adsorption mechanism,avoiding chemical dissolution and stress deformation during long-term charge and discharge processes.Therefore,after 250,000 cycles of charge and discharge,the specific capacitance of the tungsten oxide electrode has no decay,showing a record-breaking working lifespan.Additionally,the WO3-x-N14-H8 electrode with a high mass loading of 8.19 mg cm-2 also showed a high specific capacitance of 4.28F cm-2(or 522.74 F g-1)at a current density of 5 m A cm-2.Finally,a practical WO3-x-N14-H8//PANI/L-FEG aqueous pouch supercapacitor with high safety was fabricated by industrial-level encapsulation technology.In the simulated destructive safety test,the pouch supercapacitor maintained a relatively stable capacitance output,showing excellent safety.At a power density of 175.5 W kg-1,the gravimetric energy density is as high as50.35 Wh kg-1,which is close to those of some commercial battery products.We use three series-connected pouch supercapacitors as the power source to effectively charge a smartphone or tablet,exhibiting the strong practicability.3.Using an effective electrochemical technique,the relative high mass loading manganese oxide nanowires(6.32 mg cm-2)were electrochemically grown on carbon fiber substrates.The growth rate of manganese oxide at different temperatures was explored,and the optimal mass loading conditions were obtained by adjusting the electrochemical parameters.The growth mechanism and electrochemical performance of manganese oxide under different growth temperature conditions were compared.At 60°C,the growth rate and nucleation rate of manganese oxide nanowires reach the perfect balance,which effectively solves the problem of agglomeration under low-temperature growth conditions.Under this condition,the manganese oxide grown formed a nanowire structure with high porosity,which further improved the utilization of active materials.At a current density of1 m A cm-2,the specific capacitance of the Mn Ox-C30-60 electrode reaches 204.1 F g-1,which remains 61.1%of the initial specific capacitance when the current density is increased by 20 times.The 2 V asymmetric supercapacitor assembled with the Mn Ox-C30-60 electrode as the positive electrode also maintained 66.9%of its initial specific capacitance after the current density was increased by 20-fold,showing excellent rate performance. |
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