Preparation And Electrochemical Performance Of Transitional Metal(Hydroxide) Oxide-based Asymmetric Supercapacitor Electrode Materials

Supercapacitors are attracting much recent attention because of their combined features of high-power performance,indefinite lifespan,and nontoxic nature.However,their intrinsically low energy density widens the gap to the outperforming rechargeable battery in many practical applications.To address this energy density issue,the design of aqueous hybrid supercapacitors（HSCs）allows us to combine faradic and capacitive mechanisms in one device.The battery-type electrode is normally composed of transition metal-based materials,which stores charge by solid-state faradic redox reactions.While the capacitor-type electrode consists of activated carbon（AC）materials,which stores charge via electrochemical double-layer（EDL）mechanism.Hence,the overall device performance was expected to be improved significantly in terms of capacity,operating potential window（OPW）,and therefore the specific energy compared to the symmetric counterparts.In this dissertation,we explored the strategies of how to improve the performance of electrode materials via constructing nanomaterials with different structures.Moreover,the structure-effect relationship between the physical and chemical properties of the electrode materials and the electrochemical properties was also investigated.Finally,based on the overall supercapacitor design,asymmetric supercapacitors with high energy density and high power density were successfully constructed.Firstly,Sandwich-like nitrogen-doped porous carbon/graphene nanoflakes（NPCFs）were prepared via a two-step approach with graphene sheets as the shape-directing agent and conductive matrix.The as-prepared NPCFs with low pore aspect ratio facilitate ultrafast ion transportation during the charge/discharge process.Besides,the large interior surface and heteroatom-containing functional groups act as“ion adsorption sites”and“faradic reaction sites”,respectively,which enhance the overall capacitance.NPCFs as a promising electrode material displays high specific capacitance(341 F g^–1),excellent rate capability(over 71%retention ratio at 50 A g^–1)and outstanding cycle stability（almost no capacitance loss after 2000 cycles）in 6 mol/L KOH aqueous electrolyte.Secondly,a novel sandwich-like composite with ultrathin Co Al-LDH nanoplates electrostatically assembled on both sides of 2D polypyrrole/graphene（PG）substrate has been successfully fabricated via a facile hydrothermal method.The PG not only serves as conductive and structural support to facilitate the electron transmission and prevent aggregation of Co Al-LDH nanoplates but also contributes to the total specific capacitance.Owing to the homogeneous dispersion of Co Al-LDH nanoplates and its intimate interaction with PG substrate,the resulting Co Al-LDH/PG nanocomposite material exhibits excellent capacitive performance,e.g.high specific capacitance(864F g^–1 at 1 A g^–1),excellent rate capability and outstanding cycling stability in a 6 mol/L KOH aqueous electrolyte.Besides,the assembled asymmetric supercapacitor delivers high energy density of 46.8 Wh kg^–1 at 1.2 k W kg^–1 and maintains 90.1%of its initial capacitance after 10000 cycles.Thirdly,we show that controllable electrochemical lithiation of the Co₃O₄ crystal overcomes its intrinsic drawbacks including poor conductivity and sluggish faradic reaction kinetics,resulting in efficiently boosted pseudocapacitive performance in aqueous supercapacitor.The experimental investigation and theoretical calculations reveal that lithiation weakens the coordination of the Co-O band and generates abundant vacancies(octahedral Co²⁺sites),which favourably optimizes the electrochemical activity of pristine Co₃O₄,resulting in a fast charge transfer and lower adsorption energy barrier of OH^-in the electrolyte.The lithiated Co₃O₄ exhibits approximately fourfold enhancement of specific capacity(260 m Ah g^-1)in comparison with the pristine Co₃O₄(66 m Ah g^-1),which is one of the best values reported in Co₃O₄ based electrode materials.Moreover,the lithiated Co₃O₄//N-doped activated carbon asymmetric device achieves high energy density of 76.7 Wh kg^-1 at 0.29 k W kg^-1 and exceptional power density of 18.7 k W kg^-1 at 46.9 Wh kg^-1.Finally,due to the energy storage mechanism difference in two electrodes of the asymmetric supercapacitor,the capacity imbalance issue often results in limited specific capability in the asymmetric supercapacitor.To fundamentally address this problem,here we propose a new concept of asymmetric electrolyte design in the HSCs where Ni Co layered double hydroxide（LDH）battery-type electrode operates in the 6mol/L KOH electrolyte while electrolyte-soluble redox couples are deliberately introduced to the carbon capacitive electrode.The redox couples contribute extra faradic capacity to the capacitive carbon electrode,resolving the capacity imbalance problem in the two electrodes with equal mass loading.By balancing the capacity of two electrodes,the working voltage of the positive electrode is fully exploited and the specific capacity shows threefold enhancement.Due to simultaneously improved working voltage and specific capacity,the redox couple balanced HSC delivers specific energy of 79.6 Wh kg^-1,which nearly 2.5 times the value in conventional mass balanced asymmetric supercapacitor and fourfold the one in pristine unbalanced asymmetric supercapacitor.