Structure Design And Performance Regulation Of Nickel-based Electrochromic Materials

With the continued development of information technology,electrochromism is rapidly developing concerning implementation in smart windows,electronic displays,switchable glasses,and rearview mirrors.For example,electrochromic smart windows can manage light and heat radiation entering the room by controlling their optical properties,effectively improving the living comfort of modern buildings and reducing energy consumption.The smart window can also be used as a conventional energy storage device to monitor the level of stored energy through visual changes.Therefore,designing high-performance electrochromic materials is essential to promote the practical application of smart windows.Especially,the ultra-large optical modulation and neutral opaque color in the electrochromic materials are extraordinarily desirable for smart windows due to their role in protecting personal privacy and avoiding light pollution in buildings to the greatest extent.As an anode electrochromic material,nickel oxide possesses a transmissive-to-black color tunability,a wide range of visible light as well as good photothermal stability.Nevertheless,it is still a significant challenge for NiO film to achieve stable ultra-large optical modulation,which is also an issue faced by other transition metal oxides.In recent years,great efforts have been put into designing a variety of micro-/nano-structures with various dimensionalities,including nanoparticle,nanosheet,as well as dandelion flower-type NiO films.However,the singleness of reported NiO nanostructures determines that their bleaching and coloration states in electrochromism cannot be synchronously optimized,which results in difficult breakthroughs in satisfied optical modulation.Additionally,excessive stress caused by large volume expansion during electrochromic process can also lead to cyclic instability of nickel-based material structures.In this regard,it is of great importance to establish a smart strategy of micro and nano structural design,wherein ultra-large optical modulation is guaranteed without sacrificing cycling stability.and energy storage performanceTo solve the above problems,one type of layer-stacked NiO nanowire/nanosheet homostructure was fabricated.On the one hand,the effective combination of nanowires and nanosheet structures with unique photoelectric properties provides enough optical tunability for the electrochromic characteristics,greatly promotes the charge transfer at the contact interface and inside the electrode material during the electrochemical process,and achieves a substantial increase in the optical modulation range.On the other hand,due to the low intrinsic electron conduction rate caused by the wide band gap of nickel-based materials,Ni Co-LDH was prepared by introducing the second metal element into nickel-based materials,and the band structure of nickel-based materials was regulated to improve the electrochromic properties of materials.The synergistic effect of bimetal and the layered structure of LDH were used to improve the electrochemical activity and charge storage performance,so as to improve its color-changing speed,cycling stability,and energy storage performance.（1）Preparation and electrochromic properties of NiO nanowires/nanosheets:The novel layer stacked NiO nanowires/nanosheets homostructure films were prepared on fluorine-doped tin oxide（FTO）transparent conductive substrate by a simple hydrothermal method.This unique structure guarantees the uninterrupted transfer of electrons and rapid ion diffusion,which greatly improves the electrochromic characteristics.Secondly,the optical modulation of the NiO film can be effectively adjusted by controlling the thickness of the nanowire layer.The prepared NiO film has an excellent performance in electrochromic and energy storage.The film has 93.4%ultra-high optical modulation at 550 nm,colored time is12.2 s,bleached time is 9.5 s,and coloration efficiency can reach 72.1 cm² C^-1.The film also has excellent optical memory performance（the transmittance only attenuates by 36%after 24h of the open circuit）.At a current density of 0.1 m A cm^-2,the volume capacitance of 866.7m Ah cm^-3 remains 91%of the initial optical modulation after 1000 cycles.Based on this,we construct a laboratory prototype device（photovoltaic electrochromic multifunctional smart window）,in which titanium dioxide（Ti O₂）film is used as the counter electrode of NiO film to realize the charge matching of the device.The introduction of solar cells enables the device to have the function of self-power supply and energy storage under solar radiation.（2）Development and construction of NiCo-LDH electrochromic nanomaterials and smart devices:Ni-Co layered double hydroxide（Ni Co-LDH）nanosheets were synthesized on a transparent conductive substrate by electrodeposition.The bimetallic thin layer in Ni Co-LDH together with the interlayer anions formed an open-layered structure,which maintained 80.5%of the initial optical modulation at 2000 cycles.The homogeneous distribution of the bimetallic elements becomes the active center of the reaction,giving the material a high electron conduction rate.The anions between the support layers provide a convenient ion transport tunnel and expose the bimetallic active site substantially,achieving a fast conversion rate,coloring time of 2.3 s,and bleaching time of 5.1 s.The material also has wide-band optical modulation（63.6%at 472 nm,32.8%at 1000 nm）,high coloration efficiency(56.7 cm² C^-1),excellent specific capacity(913.3 m Ah cm^-3 at 0.1 m A cm^-2 current density).In addition,to maintain a fast conversion rate,we have also successfully prepared a large-size smart window of 100 cm² using a carbon paper frame as an efficient ion storage layer for the Ni Co-LDH electrochromic layer and a semi-solid KOH/PVA as the electrolyte,achieving the bleached and colored process of 14.3 and 10.9 s.This smart window enables dynamic modulation of natural light and visualization of energy storage,which is essential for the development of a highly efficient Ni Co-LDH electrochromic layer.This will provide theoretical guidance and a scientific basis for the development of efficient nickel-based electrochromic materials and devices.