Investigation On The Construction And Electrochromic Properties Of Conductive MOFs Electrocdes

MOFs,a kind of metal organic hybrid crystalline materials with controllable structure and functional design,which can be used as a potential candidate for electrochromic materials due to its designable redox sites,inherent porosity abundant nanopore size.However,currently reported electrochromic MOFs such as naphthalenediimide-based MOFs（~4.7×10-10 S m-1）and pyrenylbased MOFs（~10-11 S m-1）,exhibit poor bulk electron conductivity and slow electron transport to the redox center,resulting in poor redox reversibility,long response time,and poor cycle stability,despite their relatively faster ion diffusion.Triphenylene-based MOFs,one kind of conductive MOFs,which can provide an effective path for electron transport,possessing good electrochemical performance and redox activity due to unique π-d conjugated orbital,are widely used in electrochemical sensing and energy storage.As one of triphenylene-based MOFs,Ni3（HITP）2 has good electron conductivity and high redox activity,which can be a potential candidate for electrochromic materials.In this paper,we first prepared Ni3（HITP）2 films by hydrothermal method,but failed to grow a large area thin film.But a dark blue particle film appeared on the liquid surface of the hydrothermal method could be found and further characterized its phase by using X-ray diffractometer（XRD）.The results showed that the film on the liquid surface was Ni3（HITP）2 film.The Ni3（HITP）2 film was synthesized by the water-system gas-liquid interface method,and the influence of the reaction temperature,the dropping position and speed of ammonia water,the reaction time and other factors on the quality of film were also studied.Using scanning electron microscope（SEM）,transmission electron microscope（TEM）,XRD,Raman spectrometer,automatic specific surface and analyzer（BET）,X-ray photoelectron spectrometer（XPS）,the phase composition and microscopic morphology were carried out to characterize the film.The results showed that Ni3（HITP）2 films synthesized by the aqueous gasliquid interface method are composed of close-packed nanoparticles with a size of 50-100 nm.In order to study the effect of thickness on electrochromic properties,five Ni3（HITP）2 film electrodes with different thicknesses（205 nm,290 nm,362 nm,435 nm and 524 nm）were obtained by controlling the concentration of reactants.The electrochemical and electrochromic properties of the electrodes were tested by electrochemical workstation and UV-vis spectrophotometer.The results showed that the Ni3（HITP）2 electrodes changed from blue to light yellow（gradually became transparent）under-1.5 V voltage.As the thickness increases,the light modulation range linearly increased from 15.7% to 38.8%,and the response time increased from 4.3 s/5.4 s to 6.8/9.4 s,both of which were less than 10 s.The cycle stability could still be maintained at 90% after 100 cycles.The excellent response speed benefits from the high conductivity（73.4 S m-1）and fast ion transport（10-12 cm2 s-1）of Ni3（HITP）2 electrodes.In addition,the discoloration mechanism of Ni3（HITP）2electrodes were investigated by electron paramagnetic resonance（EPR）,and it was determined that the discoloration mechanism of Ni3（HITP）2 electrodes were based on the redox of the Ni node,that is,the conversion between Ni2+ and Ni+.The Ni3（HITP）2 electrodes constructed by the water-based gas-liquid interface method were used to assemble electrochromic displays and camera light modulators,and showing application potential in the display field and light modulator field.Anhydrous system-based gas-liquid interface of Ni3（HITP）2 electrodes were used to construct the Ni3（HITP）2 film to improve cycle stability by increasing crystallinity.By using the weaker proton-removing agent sodium acetate and the lower polar formamide,Ni3（HITP）2electrodes with higher crystallinity were obtained.The phase and morphology analysis were characterized by XRD,Raman spectroscopy,TEM,SEM,etc.The electrochemical workstation and UV-vis photometer（UV-vis）were used to test its color change performance.The results showed that Ni3（HITP）2 electrodes could quickly switch between dark blue and light yellow.With the increase of crystallinity,the light modulation range of the electrode was 28%,the response time was2.6/7.2 s,and the cycle stability can be maintained above 98% after 100 cycles due to the increase of crystallinity making Ni3（HITP）2 structure more compact and stable.