In-Situ Spectroscopic Studies of Electrochromic Hydrated Nickel Oxide Films

In this investigation, in-situ spectroscopic studies of anodically deposited electrochromic hydrated nickel oxide electrodes were performed by visible/near-infrared spectroscopy and Fourier-Transform Infrared (FTIR) spectroscopy. All measurements were taken while the nickel oxide thin films were switching between the bleached and colored states, where the electrodes were not removed from the electrochemical cell. Optical transmittance measurements of the nickel oxide film relative to tin oxide coated glass varied during coloring from the integrated solar spectral transmittance, T_s=101%-54%, and average near-infrared transmittance, T_nir = 101%-83%. The photopic transmission was T_p = 101 - 31%. Transmittance measurements versus time were also performed at selected wavelength values, ranging from 375 to 1100 nm. Also different scan rates (10-100 mV/s) were investigated at each of these wavelengths, where optimum switching rates could be determined. All changes in optical density were achieved by continuously cycling between a potential range of -500 to +800 mV. Coloration occurs at a faster rate than bleaching of the films at every switching rate selected. Also, maximum and minimum transmission measurements at 420 nm do not to correspond to the cathodic and anodic peak current densities. Instead these transmission measurements correspond to the regions past the peak current densities. From these optical experiments, plots of transmission (%) versus voltage (mV) and transmission (%) versus total extracted charge (mC) were obtained. For FTIR spectroscopic experiments, chemical identification of the 10-20 nm thick films showed that the films exhibit different bonding environments for both the colored and bleached states. There exist surface hydroxyl groups associated with nickel oxide in the region of 3600-3800 cm^-1 wave numbers. Fundamental water vibrations are also found at 3200-3500 cm^-1 and at 1600-1700 cm^-1 wave numbers. The nickel oxygen vibration region is at 400-525 cm^-1 for both states. The comparison of bleached and colored states exhibits distinctive molecular vibrational states, which correspond to Ni(OH)₂ and NiOOH respectively.