Electrochromic behaviour of Ni-Al, Ni-Mg-Al and Ni-Co-Al layered double hydroxide thin films prepared by electrodeposition

Abstract

Because of the growing energy consumption nowadays, the development of sustainable and renewable resources is highly needed in research area. Electrochromic (EC) material has attracted the attention in energy saving world. It refers to the type of material showing reversible optical properties (absorption and transmittance, etc) change under the application of a small electric field. Ni-Al containing layered double hydroxides (LDHs) are popularly studied as the electrochromic layer and storage layer of the electrochromic device. With large surface area provided by LDH structure, the structural stability for electrochemical reaction is enhanced in the electrochromic device. From lower to higher potential (anodic scan) in cyclic voltammetry scans, the thin film reveals an immediate color change from transparent to dark brownish color due to the oxidation reaction of Ni(II) into Ni(III). In the cathodic scan, the dark brownish color on thin film slowly shifts to light color with higher transmittance, due to the reduction of Ni(III) back to Ni(II). However, for electrochromic behavior of Ni-Al LDH, the reversibility is poor upon continuous potential cycling, which is attributed to the low electronic conductivity of reduced Ni-Al LDH films. In this project, Ni-Al, Ni-Mg-Al and No-Co-Al LDHs were synthesized on FTO coated glass substrates by electrodeposition method. Characterizations including SEM, XRD, EDX and UV-visible spectroscopy were done. The thin films were uniformly electrodeposited on substrates with longer electrodeposition time 120s, 150s and 180s. For Ni-Mg-Al and Ni-Co-Al thin films after cyclic voltammetry tests, XRD peaks corresponding to (003) and (006) planes of Nickel Aluminum Carbonate Hydroxide Hydrate (Takovite) were observed, which represent layered double hydroxides structure with Ni2+, Al3+ and OH- in Brucite-like layer, and CO32- anions and H2O molecules in interlamellar space. From the electrochromic behavior results characterized by cyclic voltammetry (CV) scans and potentiostatic tests, the transmittance variation of Ni-Al LDHs was improved due to the addition of Mg(II) and Co(II). Compared with Ni-Al LDH electrodeposited with the same amount of time (150s), the bleached state transmittance of Ni-Mg-Al LDH thin films (Ni:Mg=2:1) was largely enhanced to 92.3% under 0.1V at wavelength 550 nm, and showed high stability of color shifting during 20 CV scans. The coloration was improved in Ni-Co-Al LDH (Ni:Co=1:2, 150s) due to the oxidization of Co(II), which achieved transmittance variation △T%=31.45%. Further studies should concentrate on the detailed mechanism of Mg(OH)2 and Co(OH)2 precipitation during electrodeposition process. What’s more, the alternative LDH synthesis method could be tried such as coprecipitation or hydrothermal method. And conductive transparent layer can be introduced to enhance the conductivity. The Polyaniline -LDH multilayered thin films, or Ag nanowires-LDH multilayer thin films can be tried to deposit on FTO glass substrates to enhance the electronic conductivity of LDH and improve reversibility.