Pseudo-capacitances And Devices Of Oxides Based On Multi-oxidation State Metals

The energy is affecting people’s lives all the time,but how to store the high grade energy that converted from the low grade energy and output continuously and steady has become the pressing problem in the current society.Among the electrochemical energy storage devices,electrochemical capacitors not only have high power densities but also have satisfactory energy densities.Electrode materials have played vital roles on the performance of electrochemical capacitors.Among the electrode materials,metal compounds can provide higher energy densities than conventional carbon materials and better electrochemical stability than conducting polymers,which is more suitable for commercial applications.On this basis,this paper designs and prepares several kinds of manganese oxides and vanadium oxide,and assembles the symmetric,the quasi symmetric and the asymmetric electrochemical capacitors.The material composition,morphology and microstructure were analyzed through a variety of characterization testing methods,and the comprehensive analyses of electrochemical performances have been carried out by using the electrochemical testing system.The main contents are as follows:1.Basis on the theory of electrode potential,the slippery hunk-like β-Bi₂O₃ electrode material has been designed and synthesized via the combination of a similar hydrothermal method and subsequent annealing treatment.In addition,the amorphous Bi₂O₃ nano particles have been synthesized by using different precursor for comparison.The results show that the β-Bi₂O₃ electrode possessing a wide potential window between-1.5 V to +1.5 V in neutral electrolyte manifests satisfying capacity of 871.2 C g^-1,outstanding rate capability of 74.4% and excellent cycling stability of 78.9% after 1000 cycles,which confirms this design and provides a feasible method for later design of electrode materials.2.NiMoO₄-modified MnO₂ nanosheets compound electrode material on nickel foam has been designed and synthesized using a combination of hydrothermal syntheses and subsequent thermal treatment.Based on the morphology,the surface modification of NiMoO₄ not only prevents the MnO₂ from dissolving in an alkaline electrolyte but also improves the capacity.This electrode manifests a satisfying capacitance of 2525 F g^-1,outstanding rate capability of 75.9%（higher than the pure MnO₂ and NiMoO₄ electrodes）and excellent cycling stability of 74.8% after 3000 cycles.It is due to the coupling effect and the possible semiconductor surface recombination effect between NiMoO₄ and MnO₂.3.Mn₃（PO₄）₂ ultrathin nanosheets electrode material has been designed and synthesized via the hydrothermal syntheses.After a series of tests and characterizations,Mn₃（PO₄）₂ can possess wide potential windows（-0.9 V to +0.7 V in neutral electrolyte and-0.5 V +0.6 V in alkaline electrolyte）,satisfying specific capacitances(203 F g^-1 in neutral electrolyte and 194 F g^-1 in alkaline electrolyte),outstanding rate capabilities（88% in neutral electrolyte and 85.1% in alkaline electrolyte）and excellent cycling stabilities（91.1% in neutral electrolyte and 88.9% in alkaline electrolyte）in different electrolyte systems.It is due to the layered crystal structure of Mn₃（PO₄）₂ that can offer many nano-paths and improves the diffusion of electrolyte ions,which can noticeably promote electrochemical performance.4.The different vanadium oxides have been designed and synthesized via thermal decomposition,and the morphology and electrochemical performance also have been researched systematically.Three pure phase products of V₂O₃,VO₂,V6O13 and their morphological evolution,mesoporous variation,improved electronic conductivity caused by an O vacancy mechanism and electrochemical measurements show that these materials all exhibit high specific capacitance(230 F g^-1 of V₂O₃,487 F g^-1 of VO₂,195 F g^-1 of V6O13)and rate capabilities（76.1% of V₂O₃,60.5% of VO₂,51% of V6O13）,and possess a potential window of 1 V to +1 V in an aqueous solution of Na₂SO₄.5.Based on the manganese and vanadium oxides developed above,the pseudocapacitors have been constructed respectively.The AC//NiMoO₄-modified MnO₂ asymmetric electrochemical capacitor can possess a capacitance of 135 F g^-1 and a potential window between 0 V to +1.6 V,showing a higher energy density of 48 Wh kg^-1 and stable power characteristic of 6400 W kg^-1.Both the Mn₃（PO₄）₂//Mn₃（PO₄）₂ symmetric electrochemical capacitor and the Mn₃（PO₄）₂//AC asymmetric electrochemical capacitor manifest excellent electrochemical stabilities and stable energy and power characteristics,but the specific capacitance of the former is higher than the latter.The VO₂//VO₂ symmetric electrochemical capacitor can provide high specific capacitance of 115.6 F g^-1,nice energy density of 41.1 Wh kg^-1 and stable power characteristic,and other devices of vanadium oxides also reveal high application values.It can offer a feasible method for the devices construction in the future and verify the design concept of our work.