Design And Energy Storage Mechanism Of High Performance Aqueous Asymmetric Supercapacitors

Supercapacitors（SCs）have a wide application prospect in the field of energy storage because of their fast charge-discharge rate,high power characteristics,long service life,no memory effect,high safety,and low cost.However,the main bottleneck of SCs is that its low energy density is difficult to meet the needs of future multifunctional electronics.Therefore,it is crucial to enhance the energy density of SCs without sacrificing its high-power characteristics and excellent cycling performance.In view of this,building aqueous asymmetric supercapacitors（ASCs）has been considered as an effective strategy to extend working voltage,thus increasing energy density by governing separate potential windows of positive and negative electrode materials.In view of the current scientific problems and challenges in aqueous ASCs,such as narrow voltage window,low energy density and mismatching charge storage at positive and negative electrodes,this dissertation successfully studies high capacity pseudocapacitance materials,carbon nanotube current collectors,the matching of positive and negative electrodes,and charge storage mechanism for building high-performance aqueous ASCs.These works will provide novel avenues and guidance to further design and prepare electrode materials and aqueous ASCs with high-energy density,high-voltage window,and high stability performance.The details are as follows:1.Nanostructured birnessite with tunneled structures and nearly ideal capacitive behaviors are attractive as positive electrode material for aqueous ASCs.Rich oxygen defect modulated sodium-intercalated manganese oxides(Na_0.55Mn₂O_4-x·1.5H₂O,NMO_x)with intercalating Na⁺and water crystals are massively fabricated,which can effectively stabilize the layered structure and achieve rapid ion diffusion.In-situ Raman spectroscopy,electrochemical quartz crystal microbalance（EQCM）and ex-situ characterizations verify that the excellent electrochemical capability of NMO_xelectrode is controlled by interlayer cation intercalation and deintercalation,accompanied by the expansion/contraction of interlayer spacing.Subsequently,Oriented carbon nanotube microfilm excellent electrical conductivity and mechanical flexibility is continuously prepared as current collectors for aqueous ASCs.The as-assembled aqueous ASCs show a wide operating voltage of 2.4 V,a high energy density of 88.9 Wh kg^-1,and an excellent cycle performance（92.7%capacity retention after50 000 cycles）.2.The exploration of appropriate positive electrode materials with high oxygen evolution overpotential and negative electrode materials with high hydrogen evolution overpotential is vital to construct high-voltage and high energy density aqueous ASCs.Herein,novel hierarchical carbon coated Fe₃O₄ nanorods array and Mn O₂ nanowires array growth on carbon cloth are fabricated as the negative electrode（CC/CW/Fe₃O₄@C）and positive electrode（CC/CW/Mn O₂）,respectively.Benefitting from high surface area and short ion diffusion channels of the hierarchical nanostructured Fe₃O₄ nanorods and Mn O₂ nanowires,the prepared positive and negative electrodes possess similar charge storage capacity.The as-assembled aqueous ASC shows a wide operating voltage of 2.6 V and an ultra-high energy density of 91.1Wh kg^-1.3.Aqueous zinc-ion capacitors（ZICs）usually constitutes electric double-layer capacitance carbon positive electrode and battery-type Zn negative electrode,which theoretically combines the advantages of zinc-ion batteries and supercapacitors to offer high power density,high energy density and superb cycling stability.Nevertheless,the rate capability and energy density of aqueous ZICs are still limited due to its the sluggish diffusion of Zn²⁺and unsatisfactory carbon positive electrode.Nitrogen-doped mesoporous carbon nanospheres（NMCSs）with suitable pore size distribution and excellent storage capacity of Zn²⁺is synthesized by phenolic resin.The hierarchically porous structure offers high surface area and rapid diffusion channel for Zn²⁺.Moreover,nitrogen doped NMCS contributes abundant active sites,which can promote the chemical adsorption process for efficient Zn²⁺storage.As a result,the as-assembled aqueous ZICs exhibit high specific capacitance(157.8 m Ah g^-1),high energy density(126.2 Wh kg^-1),ultra-high power density(39.9 k W kg^-1),and excellent cyclic stability（96.2%capacity retention after 50 000 cycles）.4.The inaccessibility of the hydrated Zn²⁺(Zn（H₂O）₆²⁺)with a large hydration radius in micropores,resulting in a serious loss of the charge density and active site utilization,and energy storage capacity.Herein,phosphorus and nitrogen dual-doped hierarchical ordered porous carbon（PN-HOPC）is prepared by eliminating the micropore confinement effect and synchronously introducing multi-chemisorption sites.Density functional theory calculation shows that P and N doping markedly decrease the adsorption energy barrier of Zn²⁺,and effectively promote the reversible adsorption/desorption of Zn²⁺and proton.Consequently,the as-assembled aqueous ZICs exhibit a high specific capacitance(211.9 m Ah g^-1),a significant energy density(169.5Wh kg^-1),and ultra-long cycle life（99.3%capacity retention after 60 000 cycles）.In addition,in-situ and ex-situ characterizations verify that the charge storage mechanism of PN-HOPC electrode is attributable to the synergistic effect of the co-adsorption of Zn²⁺,H⁺and SO₄^2-as well as reversible chemisorption.