In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue

Abstract

An in situ colorimetric method, based on the CIE (Commission Internationale de l'Eclairage) system of colorimetry, has been successfully applied to the study of electrochromism in electrochemically deposited films of Prussian blue (iron(III) hexacyanoferrate(II), PB) on transmissive ITO/glass substrates for the first time. On electrochemical reduction of PB to Prussian white (iron(II) hexacyanoferrate(II), PW), sharp and reversible changes in the hue and saturation occur, as shown by the track of the CIE 1931 xy chromaticity coordinates. For PB, the CIELAB 1976 colour space coordinates were L* = 73, a* = −26 and b* = −33, with a dominant wavelength calculated as 488 nm. Concurrently, as the intensely absorbing PB mixed-valence chromophore is ‘bleached’ to the transparent PW, a large increase in the relative luminance of the electrochromic film is observed. On oxidation of PB, the CIELAB 1976 colour space coordinates show the transition through intermediate green to the Prussian yellow (iron(III) hexacyanoferrate(III), PY) state (L* = 94, a* = 2 and b* = 18), with a steady increase in relative luminance. To reliably compare the power requirement of PB films with other electrochromic systems, composite coloration efficiencies (CCE's) have been calculated, using a tandem chronoabsorptometry/chronocoulometry method, as previously developed for organic polymer systems. Using 95% of the total transmittance change at λmax as reference point, coloration efficiencies, η = ΔA(λmax)/Q, were calculated as 143 and 150 cm2 C−1 respectively for the PB/PW and PW/PB electrochromic transitions.