Preparation Of Nickel/Tungsten Oxide Film And Their Application In Electrochromic Device

Electrochromism can autonomously and reversibly change their optical properites based on redox reaction as response to external voltage.Electrochromic technology holds merits with low comsumption,intelligent controllability and large optical modulation,which has a wide range of application in the field of smart windows,textile,anti-counterfeiting,and display device.WO₃and NiO,as classic electrochromic materials,both have advantages of good optical modulation and low cost,but there are still some bottlenecks hindering the promotion of industrialization.Sepcifically,the films based on all-solution method have poor cycling stability.Moreover,electrochromic technology with single function hardly satisfies the increasing demands for customers.To address these issues,this work balances problems between low cost and high cycling stability through focusing on constructing WO₃and NiO respective interface structure.Furthermore,the deep understanding of behind mechanism of electrochemical kinetics has been revealed by density functional theory（DFT）calculations.The specific research contents are as follows:（1）The preparation of NiO/Nicounter electrode is based on all-solution method.Theoretically,the discovery of conductive ionic bond between NiO and Niis firstly verified by the electron localization function（ELF）and the density of states（DOS）.Such a conductive ionic bond stabilizes the NiO lattice in the process of ion intercalation/deintercalation.Furthermore,NiO/Nicounter electrode shows an ultralong cycling life（87.0%retention of the original optical contrast at 550 nm after10000 cycles）and high visible transparency in the bleached state（90.1%）,which effectively solves issues of poor cycling stability and an aesthetically unpleasant yellowish color of NiO.（2）By taking the overall considerations in morphology,composition/crystal structure and crystal defect engineering,this work reports a study on 3D hierarchical structure consisting of ultrafine W₁₇O₄₇nanowires（diameters:3-5 nm）purposely knotted together by NaWO₃nanoknots.Interestingly,the W₁₇O₄₇/（NaWO₃-knots）electrode thus-made exhibits a large optical modulation of 75%at 633 nm and an outstanding cycling stability（7000 cycles without significant degradation）.Theoretcially,density functional theory（DFT）simulations reveal the formation of a robust W₁₇O₄₇/（NaWO₃-knots）interface structure,because of the high energy for the formation of oxygen vacancies at their 3D interfaces.The demonstration of a large-area electrochromic device（30.5 cm×26 cm）assembled with a W₁₇O₄₇/（NaWO₃-knots）cathode and a NiO/Nianode realizes the balance between low cost and high electrochemical performance.（3）A novel dual-functional electrochromic device（DED）possessing both energy-saving and energy-storage functionalities is demonstrated.This research employs Zn²⁺as intercalation ions,which demonstrates the well theoretical capacity(820 m Ah g^-1),cost-effectiveness and high safety.However,the rather strong electrostatic interactions of Zn²⁺ion with the host lattice often cause sluggish Zn²⁺migration kinetics.Encouraglingly,W₁₇O₄₇/（NaWO₃-knots）electrode exhibits outstanding cycling stability（4000 cycles without significant degradation）,a largeΔT of 83.2%at 633 nm,and a high specific capacitance of 353.8 F g^-1at 0.5 A g^-1,which is attributed to the robust interface structure and abundant oxygen vacancies.As a proof-of-concept demonstration for their application potentials,the excellent energy-saving effect of the prototype device is evaluated by a model house,which shows that the temperature difference between two inner compartments（with/without the Zn-based DED）is significant（18.3°C）.Simultaneously,the two series-connected Zn-based DEDs is shown to power 4 light-emitting diodes（LEDs,2 V）for more than 35 min,realizing the integration of energy-saving and energy-storing functionalities.（4）To broaden the application potential of electrochromism in the filed of wearable electronics,the W₁₇O₄₇/（NaWO₃-knots）@PEDOT:PSS cathode was reported for flexible dual-functional electrochromic and electrochemical device（FDED）.The hierarchical W₁₇O₄₇/（NaWO₃-knots）@PEDOT:PSS cathode thus-made simultaneously exhibits a large optical modulation（79.7%at 633 nm）,an ultra-long cycling life（76%of original optical modulation retained after 12400 cycles）,and a high areal capacitance(55.1 m F cm^-2at 0.1 m A cm^-2).Density Functional Theory（DFT）calculations demonstrate that the much improved dual-functional performance is correlated to the raised electronic conductivity and ion adsorption at the W₁₇O₄₇/（NaWO₃nano-knots）interface,together with the ion adsorption of PEDOT:PSS in the 3D-knotted architecture.