Preparation And Supercapacitor Performance Of Vanadium-based Materials And Transition Metal Phosphide/Carbon Composites

The performances of supercapacitors is highly dependent on electrode materials.Currently,many researchers have made great efforts to develop new electrode materials,which are helpful to obtain electrode materials with the optimal electrochemical performance.Vanadium-based materials and transition metal phosphides can be used as supercapacitor electrode materials to provide ideal pseudocapacitive properties due to their variable valence and high electronic conductivity.In recent years,these materials have made some progress for supercapacitor application,but their poor cycling stability,low specific capacitance,low energy,and too complicated and expensive synthesis methods limit their applications.Therefore,it is urgent to control the syntheses of micro/nano-structure materials,explore simple methods to prepare new type composite materials and develop asymmetric supercapacitors to improve the performance of supercapacitors.Nickle foam has drawn significant attention as a substrate or support or template for constructing 3D electrodes.This method not only eliminates the complicated process of electrode preparation,but also avoids the?dead capacitance?caused by the addition of binder and conductive,and improves the capacitance performance of electrode material.Meanwhile,the unique structure and large specific surface area of Ni foam are utilized to make the ion transfer and electron transport more efficient and fast,thereby improving the electrochemical properties of the electrode material.In addition,the chemical modification method of doping hetero-elements into metal compounds can also improve the electrochemical properties of the original materials.The development of metal compound/carbon composites is an important way to improve the conductivity and stability of initial materials.Coordination polymers?CPs?and metal-organic frameworks?MOFs?are constructed by metal ions or clusters and organic ligands.By thermal decomposition,they can be used as precursors or templates to construct different types of metal compound/carbon composite materials.Based on the above situation,A series of novel vanadium-based electrode materials and transition metal compound electrode materials with excellent performances were designed and synthesized by simple hydrothermal technique and calcination method.The composition,structure and morphology of these materials were verified by various characterizations,and their performances in supercapacitors were characterized by various electrochemical methods.The advantages and disadvantages of capacitance performances of these materials are analyzed in detail and the optimal synthesis condition is obtained.The main work and research contents are as follows:?1?Ni²⁺-doped?NH₄?₂V₃O₈ nanoflakes are successfully in situ grown on Ni foam through a facile one-pot hydrothermal technique in a NH₄VO₃ aqueous solution.As binder-and conductivity agent-free electrode in supercapacitor,it manifests a large specific capacitance of 465.5 F g^-1 at a current density of 0.2 A g^-1 and a superior rate capability of 317.5 F g^-1 at 10 A g^-1,which is beneficial from its three-dimensional?3D?porous architecture cross-linked by the ultrathin Ni²⁺-doped?NH₄?₂V₃O₈ nanoflakes on Ni foam.Meanwhile,the Ni²⁺-doped?NH₄?₂V₃O₈@Ni foam//Activated carbon?AC?asymmetric supercapacitor can deliver a maximum energy density of 20.1 Wh kg^-1 at a power density of 752.0 W kg^-1.Significantly,the Ni²⁺-doped?NH₄?₂V₃O₈@Ni foam electrode possesses reversible electrochromic behavior,and it shows obvious visible light-driven photoresponse with much higher specific capacitance(645.3 F g^-1 at 0.5 A g^-1)under illumination,which is probably associated with the semiconducting characteristics of the spin-polarized?NH₄?₂V₃O₈ and the quantum confinement effect of the nanoflakes.?2?The NiCl?OH?nanosheets and V-doped NiCl?OH?nanoplates assembled by nanosheet arrays supported on nickel foam have been successfully synthesized by a facile one-pot hydrothermal reaction.The dependences of the microstructure and electrochemical performance on hydrothermal reaction time and reactant concentration were investigated,and the optimal synthesis condition is 12 h with C_Ni²⁺=0.15 mM.As the template agent,vanadium dopant can alter the morphology of NiCl?OH?nanosheet arrays and form V-doped NiCl?OH?nanoplate arrays,then leading to a higher specific surface area and lower charge transfer resistance.The cost-effective,binder-free and optimized V-doped NiCl?OH?delivers high specific capacitances of 2753.3 and 1470.0F g^-1 at the current densities of 1 and 30 mA cm^-2,respectively.Meanwhile,an asymmetric supercapacitor is constructed with the optimized V-doped NiCl?OH?sample as the positive electrode and activated carbon as the negative electrode.The fabricated device can exhibit a high energy density of 45.5 Wh kg^-1 at the power density of 799.5W kg^-11 in the voltage window of 0-1.6 V.The efficient electrochemical performance of V-doped NiCl?OH?nanoplate arrays makes it a favorable candidate for supercapacitor electrode material.?3?NiV₂O₆/C composite was successfully prepared via a solid-state conversion process by low temperature annealing coordination polymer[Ni?phen?H₂O][V₂O₆]?phen=1,10-phenanthroline?.As supercapacitor electrode,it exhibits a specific capacitance of 745.6 F g^-1 at 0.5 A g^-1 with a good rate capability of 520.0 F g^-1?69.7%retention?at 10 A g^-1?20-folds?.The NiV₂O₆/C composite exhibits good cycling performance,the capacitance retentions at 6000^th and 12000^th charging/discharging cycles are 81.5%and 77.8%relative to the original capacitance,respectively.In order to analyze the cause of capacitance attenuation,the morphological and compositional changes of the NiV₂O₆/C composite electrode after 12000 GCD cycles are carefully elaborated through a series of characterizations.?4?A series of Ni_mP_n/C,Ni₂P/C,Co₂P/C and Ni_xCo_2-xP/C nanohybrids with different Ni/Co molar ratios were synthesized by in-situ calcination/phosphorization of MOF precursors.Taking advantage of the bimetallic synergism and carbon anchoring effect,the as-prepared NiCoP/C sample delivers the most prominent specific capacities of 775.7 C g^-1 at 1 A g^-1,and 582.4 C g^-1 at 20 A g^-1?20-fold?with a superior rate capability of 75.1%retention,much superior to those of the corresponding Ni_mP_n/C,Ni₂P/C,Co₂P/C,other Ni_xCo_2-xP/C samples and various reported metal phosphide nanostructures or nanocomposites.Furthermore,the constructed NiCoP/C//activated carbon asymmetric supercapacitor can exhibit a high energy density of 47.6 Wh kg^-1 at a power density of 798.9 W kg^-1,which was superior to those previously reported of Ni/Co phosphide nanomaterials.