UV-visible and IR spectroelectrochemical studies of FeVO4 sol-gel films for electrochromic applications

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 400degreesC 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 (DeltaT(v)is = 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.