ZnO@ZIF-8 Derivative-based Electrochromic Supercapacitors

Information age calls for the development of intelligent,energy-efficient and multi-functional electronic devices.Electrochromic technology is able to control the light with low energy consumption,applications can be found in the fields of intelligent privacy,aerospace thermal control and military camouflage.Incoperating electrochromic technology into supercapacitors prodives possibilities for the energy storage devices to be operated in an interactive mode.Their residual capacity can be monitored intuitively and dynamically by the color and patterns.In the case of electrochromic supercapacitors,the composition and microstructure of the electrode impact significantly on the performance of the devices.Ion/electron transport and intercalation/deintercalation during the electrochemical redox reactions of the electrochromic supercapacitors can be promoted by the highly porous structure.Nevertheless,unsatisfactory energy storage and severe performance degradation during long-term electrochemical cycling due to simple porous structures still remain.Herein,a kind of zeolite imidazolium ester skeleton coated zinc oxide（ZnO@ZIF-8）film was used as the precursor to prepare the derived high-performance layered nickel/cobalt bimetallic hydroxide coated zinc oxide（ZnO@Ni/Co-LDH）and titanium dioxide（Ti O₂）films,and the devices using them as the electrodes were also fabricated.The electrochromic and energy storage performance of the films and devices werestudied.The main research contents are as follows:（1）ZnO nanorod films were electrodeposited on the fluorine-doped tin oxide（FTO）transparent conductive substrates.ZnO nanotube films were then obtained by electrochemical etching,and were used as templates for the deposition of ZnO@ZIF-8 by hydrothermal.ZnO@ZIF-8 films with various morphologies and crystallinities were generated by adjusting the added 2-methylimidazole（Hmim）concentration.Results show that ZnO@ZIF-8 gradually exhibited a complete core-shell structure and high crystallinity with the increased Hmim concentration.With a concentration of 76.12 mmol·L^-1,ZnO@ZIF-8 films presented the highest crystallinity and a complete core-shell heterostructure.（2）Five ZnO@Ni/Co-LDH films with different Ni²⁺and Co²⁺molar ratios were prepared by hydrothermal using the ZnO@ZIF-8 films as templates,and their electrochromic and energy storage performance was studied.Results show that when the moles of Ni²⁺and Co²⁺were the same,ZnO@Ni/Co-LDH exhibited the largest optical modulation（56%@550 nm）,the highest specific capacity(507.2 C·g^-1,0.1 m A·cm^-2)and short coloration/bleaching switching time（0.74 s/2.72 s）.The electrochromic supercapacitor device using ZnO@Ni/Co-LDH films as the electrode was constructed,it can switch reversibly between light yellow and dark brown,displaying an excellent optical modulation（63%@660 nm）and energy storage capability(7.7μW·h·cm^-2,375μW·cm^-2).In addition,photovoltaic solar panels were also connected with the device to achieve energy collection,storage and optical modulation.（3）Ti O₂ films were prepared using the ZnO@ZIF-8 thin films as templates by impregnation and calcination,and the electrochromic and energy storage performance was investigated.Results indicate that the reaction process of the Ti O₂ films in the electrolyte of propylene carbonate lithium perchlorate（PC-Li Cl O₄）was controlled by the semi-infinite diffusion.Moreover,the Ti O₂ films exhibited excellent bi-functional performance,with a surface capacitance of 25.3 m C·cm^-2 and the optical modulation of 31.9%@800 nm.The optical modulation of the films increased to 46.6%after 1500 cycles.This is due to the long-term intercalation/deintercalation of Li⁺enriched the porous structure of Ti O₂,providing more active sites for Li⁺intercalation/deintercalation to improve the electrochromic performance of the films.