UV-Visible and IR Spectroelectrochemical Studies of FeVO₄ Sol-Gel Films for Electrochromic Applications

Abstract

The sol-gel synthesis route, in combination with dip-coating deposition, was used for the preparation of FeVO₄ films. TEM measurements of Fe/V (1 : 1)-oxide films heated at 400°C reveal that the films consist of a triclinic FeVO₄-I and an orthorhombic FeVO₄-II phases with a grain size of up to 50 nm. The electrochromic properties of the films were tested in 1 M LiClO₄/propylene carbonate (PC) using various electrochemical techniques and in-situ UV-visible spectroelectrochemical measurements. The best compromise between the charge capacity per film thickness (Qd⁻¹ = −0.14 mC cm⁻² nm⁻¹), electrochemical stability (>1000 cycles) and optical modulation (ΔT_vis = 0.15) was achieved in the potential range of 4.80 to 1.80 V vs. Li, which suggests that FeVO₄ films can be used as counter-electrodes in electrochromic devices.

Extensive IR-spectroscopy studies of FeVO₄ films in charged/discharged states revealed the following spectra changes: (i) small charging (−0.01 mC cm⁻² nm⁻¹) leads to a variation in the intensity of all the vibrational bands without shifting their frequencies, (ii) higher chargings bring about the intensity and frequency changes of bridging V—O···Fe and V···O···Fe stretchings showing that vanadium, and probably also iron, are involved in the insertion/extraction processes, (iii) below 500 cm⁻¹ broad absorption appears due to the Li⁺—O modes, which also remained in the IR spectra of discharged (bleached) states revealing the irreversible lithiation, and (iv) charging to −0.30 and −0.50 mC cm⁻² nm⁻¹ leads to the amorphisation of the film structure.