Paul M. S. Monk

Electrochromism and electrochromic devices

Preface Symbols, abbreviations and acronyms 1. Introduction to electrochromism 2. A brief history of electrochromism 3. Electrochemical background 4. Optical effects and quantification of colour 5. Kinetics of electrochromic operation 6. Metal oxides 7. Electrochromism within metal co-ordination complexes 8. Electrochromism by intervalence charge - transfer coloration: metal hexacyanometallates 9. Miscellaneous inorganic electrochromes 10. Conjugated conducting polymers 11. The viologens 12. Miscellaneous organic electrochromes 13. Applications of electrochromic devices 14. Fundamentals of device construction 15. Photoelectrochromism 16. Device stability Index.

The effect of ferrocyanide on the performance of heptyl viologen-based electrochromic display devices

Abstract The write-erase efficiency of electrochromic cells incorporating heptyl viologen (HV) is decreased if di-reduced HV0 is formed: this is observed as a failure to electro-oxidise the colour of the display. This failure is caused by comproportionation of electrogenerated HV0 with HV2+ (from the solution bulk); comproportionation results in the generation of electrochemically inert radical-cation dimer HV22+. Addition of ferrocyanide to the solution as an electron mediator improves the write-erase efficiency of an HV-based device by promoting a different comproportionation mechanism that does not involve the radical-cation dimer.

Charge-transfer complexes of the viologens: effects of complexation and the rate of electron transfer to methyl viologen

Abstract The kinetics of reductive electron transfer to the methyl viologen dication (MV 2+ ) to form a radical cation (MV +. ) have been investigated in the presence of outer-sphere charge-transfer complexation. The solvent was water and the electrode substrate was platinum. It was found that the rate of electron transfer does not depend on the equilibrium constant of complexation, but rather on the rate of dissociation of the solution-phase complex. Accordingly, it has been suggested that the reaction of a CE reaction with at least a partial dissociation of the complex, to separate the acceptor and donor components—prior to the successful electron-transfer reaction.

The effect of doping electrochromic molybdenum oxide with other metal oxides: Correlation of optical and kinetic properties

Abstract The properties of electrochromic molybdenum oxide (as a thin film) has been adjusted by incorporating the oxides of Co, Cr, Fe, Ni, W or Zn; films were made by cathodic electrodeposition. The deposition solutions employed were prepared by dissolving powdered molybdenum metal in hydrogen peroxide solutions. Such films were robust and showed a good contrast ratio. Addition of other metal salts to the deposition solution allowed mixed-metal oxides to form. Further “fine tuning” of composition and hence properties is possible since the ratio of constituent atoms in solid films is a function of the potential employed for deposition. Some of the mixed-metal oxides evinced an extended electrochemical “potential window”, since the onset of gas formation is suppressed during reductive electrolysis. All films evince a relatively slow electrochromic transition. A correlation is shown between shifts in optical bands and the (logarithm of) chemical diffusion coefficient for the proton through the film which is taken to infer ion motion proceeds via a hopping mechanism.

Electrochemical behaviour of methyl viologen in a matrix of paper

Abstract Methyl viologen (MV) has been incorporated into paper as an aqueous solution and in the viscous electrolyte polyAMPS, and the availability of the MV for electron-transfer has been investigated by cyclic voltammetry at a stationary platinum disc electrode. In this study, which is the first rigorous study of an electrochrome in paper, the variables of choice were scan rate, relative humidity and electrolyte composition. The implications of the incorporation of MV into paper are discussed in terms of electrochromism. It is concluded that (i) electrochemical generation of colour in the paper is feasible; (ii) the rate of colour formation in paper is likely to be very slow; (iii) the MV 2+ , MV +· couple is fully electroactive in paper but (iv) the MV o redox state is best avoided since its immediate product, a spin-paired radical–cation dimer formed via comproportionation, is slightly electro-inactive. Dimer also complicates the electrochemical interpretations.

Electrochromic paper: utility of electrochromes incorporated in paper

Abstract Investigation of the electrochemical behaviour of the inorganic electrochromes tungsten oxide, molybdenum oxide, and Prussian blue (PB), each within mashed paper, shows that WO3 and MoO3 are ideal electrochromes to manufacture ‘electrochromic paper’, but that PB is not a viable electrochrome in this context. The organic electrochromes methyl viologen (MV) and benzyl viologen (BzV) are water-soluble, so were incorporated by pre-soaking but without mashing the paper. The electrochromic images are intense but impermanent. From the adsorptive behaviour of methyl viologen, it is shown that MV adheres via chemisorption, the extent being a function of the type of paper used.

Electro-deposition of films of electrochromic tungsten oxide containing additional metal oxides

Abstract Films of tungsten oxide have formed by electro-deposition from solutions containing other transition metals. The electrolysis solution was prepared using hydrogen peroxide to dissolve the metallic tungsten. One other metal salt was added to this solution using cobalt, chromium, iron, molybdenum, nickel, ruthenium or zinc. Such multiple metal oxide films differ from pure WO3 and exhibit different cyclic voltammograms, ac impedance behaviour and uv-visible spectra. The diffusion coefficients of intercalating ions do not vary greatly with film composition. The resistances to charge transfer, measured using complex ac impedance, are affected dramatically by film composition.

Spin pairing ('dimerisation') of the viologen radical cation : kinetics and equilibria

Abstract Viologen radical cations dimerise (spin pair) in aqueous solution, causing the colour of the viologen dye to alternate between blue (monomer) and dimer (red). Equilibrium constants K of viologen dimerisation have been determined using UV–vis spectroscopy for alkyl viologen radical-cation species in halide solutions. Equilibrium constants of dimerisation were obtained by deconvolution of optical spectra. Values of ΔG ° dim have a crude linear dependence on n , the length of the alkyl substituent chain, for n =2 to 8. Rate constants of spin-pairing k dim (i.e. dimerisation of the radical cation) have been determined. Bjerrum plots indicate that the rate-limiting reaction during dimerisation involves anions: it is postulated that the dimerisation reaction is two-step, with dimer formation occurring after the formation of an association pair comprising a radical cation and a counter anion. Recent comproportionation results are used to analyse the structure of the dimer and the role of anions during dimerisation. The possibility that dimer comprises anions placed between the planes of the two bipyridilium radicals is also discussed.

Kinetics of comproportionation of the bipyridilium salt p-cyanophenyl paraquat in propylene carbonate studied by rotating ring–disc electrodes

The kinetics of the comproportionation reaction CPQ2++ CPQ0→ 2CPQ+(CPQ is 1,1′-(p-cyanophenyl)-4,4′-bipyridilium) have been studied at a rotating ring–disc electrode (RRDE) in propylene carbonate solution. Some elaboration of the existing analysis is required to encompass comproportionation. The second-order rate constants for comproportionation of CPQ satisfy criteria established in the elaboration.

Toward the tailoring of electrochromic bands of metal–oxide mixtures

Abstract The optical absorption bands of an electrochromic mixture of oxides can be adjusted systematically by varying the ratio of constituent oxides. The oxides in this study were CoO, FeO MoO3, NiO and WO3. The way optical absorption maxima (at constant insertion coefficient x) vary with mole fraction of constituent oxides is best predicted in terms of the optical-shift parameter S via the frequency maximum of the optical band, in a manner akin to optical basicity Λ. S is followed by bands originating from optical transitions on specific ions and by bands due to charge-transfer and intervalence transitions between ions. It is also shown that computer speciation modelling is a good predictor of the compositions of metal–oxide mixtures prepared by electrodeposition from aqueous solutions.