Controllable Synthesis And Capacitive Performances Of Nanostructured Tungsten Oxides Materials

As the population swelling and the pollution aggravating,the research of clean energy storage system has attracted great attention.Owing to the characteristic of fast charging/discharging and long cycle life,supercapacitors are widely used in the power type equipment.The development of high performance electrode materials is the key to improve the electrochemical performance of supercapacitors.A typical hybrid capacitor use LIB/NIB-type anode to provide high capacity via Li⁺/Na⁺insertion/desertion reaction and the capacitor-type cathode to deliver high power by fast ion adsorption/desorption process.Hybrid capacitors can achieve higher energy density owing to the wide voltage window and high capacity compared to supercapacitors.As the Li⁺/Na⁺insertion/desertion reaction at the battery-type anode is slower than the ions adsorption/desorption process at the capacitive cathode,it is crucial to prepare the new anode with fast reaction kinetics.Also,the achievement of the high capacity cathode is very important.With the abundant and high specific capacity in aqueous phase and organic phase electrolyte,WO₃ is as the focuse of research.To this,we prepared different kinds of WO₃ to improve the capacitance properties in supercapacitors and hybrid capacitors.The main research results and innovation points are as follows:?1?WO₃ mesoscopic microspheres composed of self-assembled nanofibers were prepared by facile hydrothermal reaction of tungsten acid potassium.The electrode exhibited high specific capacitance of 797 F g^-1 at the current density of 0.5 A g^-11 and showed excellent cycling stability without decay after 2000 cycles.An asymmetric aqueous supercapacitor is constructed using the WO₃ anode and activated carbon cathode,which achieved the maximum energy density of 97.6 Wh kg^-11 at 28 kW kg^-1.The WO₃mesoscopic microspheres provide large effective active area and high utilization ratio to improve the capacitive properties.?2?We have prepared hierarchical urchin-like WO₃/C microspheres by two-step hydrothermal treatment,with the thiner conductive carbon layers of 10 nm evenly coating on the nanorod units.The WO₃/C electrode showed 508 mAh g^-1 at 0.1 C after 160 cycles and achieved 152.7 mAh g^-1 at the high current density of 10 C,showing promising rate performance and cycle stability.The LIC comprising of WO₃/C anode and metal organic framework-nitrogen doped carbon?MOF-NC?cathode with high specific surface area boasts a large energy density of 160 Wh kg^-1 at 173.6 W kg^-1,accelerating the process of the commercialization of LICs.The hierarchical mesoporous WO₃/C microspheres assembled by radially oriented WO₃/C nanorods enable effective Li⁺conductivity,electronic conductivity and structural robustness to improve the electrochemical performance.?3?We have prepared the phosphorus-doped WO_3-x/N-doped carbon(P-WO_3-x/NC)heterogeneous nanowires,which are achieved via calcination of organic-inorganic hybrid WO₃/ethylenediamine?EDA?nanowires in Ar and subsequent thermal phosphorization.The P-WO_3-x/NC nanowires deliver a large capacity of 490 mAh g^-1 at 1 A g^-1 and a high rate capacity of 230 mAh g^-1 at 3.2 A g^-1.Moreover,the P-WO_3-x/NC nanowires exhibit excellellent cycle reversibility with 96.2%capacity retention over 2,000 cycles at 1 A g^-1.A LIC composed of the P-WO_3-x/NC anode with commercial activated carbon cathode shows an ultra-high energy density of 195.6 Wh kg^-1 at 597.2 W kg^-1 with 90.7%retention over 6000 cycles.In the P-WO_3-x/NC heterogeneous nanowires,the small P-WO_3-x-x nanocrystals offer short paths for Li⁺diffusion and the continuous NC matrix facilitates electron transport and buffers the volume change of P-WO_3-x during discharging/charging.The introduction of oxygen vacancies and P heteroatoms with smaller electronegativity result in enhanced electrochemical activity due to the larger degree of covalent bonding and sharing electrons.?4?WO₂/WS₂-rGO composites were prepared by sulfidation of the urchin-like WO₃ and the electrostatic attraction effect with GO.The urchin-like microspheres assembled with WO₂/WS₂ heterogeneous nanosheets and completely wrapped by rGO with high electronic conductivity and elasticity.The WO₂/WS₂-rGO electrode exhibits 100 mAh g^-1 at 25.6 A g^-1 and shows excellent cycling stability of 1000 cycles keeping 90%capacity retention at1 A g^-1,which is evaluated to be the promising anode material of high performance Na-ion capacitors.On the other hand,3D phosphorus-doped carbon framework?P-C?composed with nanoparticles demonstrates 80 mAh g^-1 at 0.1 A g^-1 and 40 mAh g^-1 at 10 A g^-1.Based on the perfect matching in electrochemical properties of WO₂/WS₂-rGO anode and P-C cathode,a high performance NIC is constructed,demonstrating 140 Wh kg^-1 at 200 W kg^-1along with 79%capacity retention over 6000 cycles at 5 A g^-1.We have achieved the WO₂/WS₂-rGO composites,which provide excellent rate properties and cycle stability during electrochemical process.Interestingly,ultrathin WO₂/WS₂ nanosheet units offer numerous active sites and 3D highly porous rGO network acts as the protective layer of the stable structure during sodiation/desodiation cycles.Also,metallic phase WO₂ ensures high electronic conductivity of every nanosheet unit,while rGO network acts as the fast outer electrons transport layer.