Roger J. Mortimer

Electrochromic organic and polymeric materials for display applications

An electrochromic material is one where a reversible color change takes place upon reduction (gain of electrons) or oxidation (loss of electrons), on passage of electrical current after the application of an appropriate electrode potential. In this review, the general field of electrochromism is introduced, with coverage of the types, applications, and chemical classes of electrochromic materials and the experimental methods that are used in their study. The main classes of electrochromic organic and polymeric materials are then surveyed, with descriptions of representative examples based on the transition metal coordination complexes, viologen systems, and conducting polymers. Examples of the application of such organic and polymeric electrochromic materials in electrochromic displays are given.

Electrochromic Systems and the Prospects for Devices

Many inorganic and organic materials exhibit redox states with distinct electronic (UV-vis) absorption bands. When the switching of redox states generates new or different visible region bands, the material is electrochromic. Electrochromic materials are currently attracting much interest in academia and industry for both their fascinating spectroelectrochemical properties and their commercial applications. In this review some of the most important examples from the major classes of electrochromic materials are highlighted. Examples of their use in both prototype and commercial electrochromic devices are illustrated including car mirrors, windows and sun-roofs of cars, windows of buildings, displays (see Figure), printing, and frozen-food monitoring.

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.

Electrochemical polychromicity in iron hexacyanoferrate films, and a new film form of ferric ferricyanide

Abstract A detailed voltammetric and coulometric study is reported of both the conditions for electrodeposition from ferric ferricyanide solution, of Prussian Blue film on Pt, and of potential sweeps of the deposited film in KCl solution alone. These latter show the colours white (or clear), blue, green and a new golden yellow state at +0.88 to +1.00 V. The key factor in the electrodeposition of such a polychromic film is potentiostasis, in the range +0.40 V to +0.50 V vs. SSCE. Coulometry establihes the new state as being ferric ferricyanide in quite pure form. The key role of K+ as mobile cation in the electrochromic switching processes is demonstrated by establishing the ineffectiveness of other cations.

Voltammetry at carbon nanofiber electrodes

Carbon nanofibers with diameters in the range 10–500 nm have been evaluated as novel electrode materials for electrochemical applications. Compared with other forms of nanostructured carbons, such as aerogels or activated charcoal, carbon nanofibers exhibit low BET surface areas, 50 vs. , because their surfaces are not readily penetrated by gaseous nitrogen. But somewhat surprisingly, they exhibit higher electrochemical capacitances (ca. 60 vs. ) because the spaces between the fibers are readily penetrated by electrolyte solution. As a result, capacitive currents tend to mask voltammetric currents during cyclic voltammetry. The situation is quite different when the spaces between carbon nanofibers are impregnated by an inert dielectric material, such as high-melting paraffin wax. Then the carbon nanofibers form a high-density composite electrode with good conductivity and low capacitance. Indeed, well-defined voltammetric responses are readily observed for the reduction of Ru(NH3)63+ in aqueous solution, even in the absence of supporting electrolyte. Metal deposition and anodic stripping processes can also be observed for the reduction of Pb2+ in aqueous nitric acid. This suggests that carbon nanofibers represent a new class of material suitable for electroanalytical applications.

Quantification of colour stimuli through the calculation of CIE chromaticity coordinates and luminance data for application to in situ colorimetry studies of electrochromic materials

The development of a Microsoft® Excel® spreadsheet is described, for the accurate calculation of CIE (Commission Internationale de l’Eclairage) 1931 xy chromaticity coordinates and luminance data from visible region absorption spectra recorded in transmission mode. Using firmly established CIE principles, absorbance-wavelength data from visible spectra recorded using a Hewlett Packard 8452A diode array spectrophotometer are taken as input, with chromaticity coordinates being generated as output. The colorimetric transformations described are well known to colour scientists, with the methodology and background now being made accessible to the electrochromic materials community. Colour stimulus measurement example calculation results are firstly presented for aqueous solutions of the dyes, Erythrosin B (red), Acid Green 25 and Remaxol Brilliant Blue R, and then for tracking electrochromic in situ colour stimulus changes in the methyl viologen and n-heptyl viologen systems. The quantification of colour during each viologen dication to cation radical reduction process, and each reverse (oxidation) process, showed that subtle changes in both hue and luminance could be detected, with evidence of colour contributions from both the cation radical and the cation radical dimer.

Five Color Electrochromicity Using Prussian Blue and Nafion/Methyl Viologen Layered Films

The results reported here demonstrate that such polychromicity can be further extended to include a purple color state. The method involves application of an outer layer of the perfluorinated polysulfonate cation exchange polymer Nafion to a Prussian blue film. The methyl viologen dication (colorless)/radical cation (purple) system is then incorporated by ion-exchange into the Nafion

Influence of the film thickness and morphology on the colorimetric properties of spray-coated electrochromic disubstituted 3,4-propylenedioxythiophene polymers.

Variation of the colorimetric properties as a function of the film thickness and morphology has been investigated for two spray-coated electrochromic disubstituted 3,4-propylenedioxythiophene polymers. Changes in the luminance, hue, and saturation have been tracked using CIE 1931 Lxy chromaticity coordinates, with CIELAB 1976 color space coordinates, L*, a*, and b*, being used to quantify the colors. For (precycled) neutral PProDOT-(Hx)(2) films, with an increase in the thickness, L* is seen to decrease, with a* and b* coordinates moving in positive and negative directions, respectively, with quantification of the pink/purple (magenta) color as the summation of red and blue. For all thicknesses, L* is comparable, pre- and postcycling, with a* decreasing (less red) and b* becoming more negative (more blue) and the film now appearing as purple in the neutral state. Color coordinates for the reverse (reduction) direction exhibited hysteresis in comparison with the initial oxidation, with the specific choice of perceived color values depending not only on the film thickness but also on both the potential applied and from which direction the potential is changed. Neutral PProDOT-(2-MeBu)(2) films appear blue/purple to the eye both as-deposited and after potential cycling to the transparent oxidized state. For the neutral, colored state, with an increase in the thickness, L* is seen to decrease, with a* and b* coordinates moving in positive and negative directions, respectively. For PProDOT-(2-MeBu)(2) films, the a* coordinates are lower positive values and the b* coordinates are higher negative values, thus quantifying the high dominance of the blue color in the blue/purple films compared to the pink/purple PProDOT-(Hx)(2) films. As for the PProDOT-(Hx)(2) films, the tracks of the color coordinates show that the specific choice of perceived color values depends on the film thickness. Unlike the PProDOT-(Hx)(2) films, hysteresis is absent in the oxidation/reduction track of the x-y coordinates for the PProDOT-(2-MeBu)(2) films, although slight hysteresis is present in the luminance. Characterization of the film morphologies through atomic force microscopy reveals a much rougher, higher surface area morphology for the PProDOT-(2-MeBu)(2) films versus the PProDOT-(Hx)(2) films. The branched repeat unit in the PProDOT-(2-MeBu)(2) films provides a structure that allows ions to ingress/egress more effectively, thus removing hysteresis from the optical response.

Electrochemical responses of bilayer electrodes with Prussian blue as the ‘inner’ layer and electroactive cation-incorporated Nafion® as the ‘outer’ layer

Abstract Bilayer electrodes with Prussian blue as the ‘inner’ layer and Nafion® as the ‘outer’ layer allow design of multiple redox centre systems by electrostatic binding of electroactive cations into the outer layer. Whilst the overall electrochemical response is a composite of the Prussian blue and electroactive cation redox waves, the presence of the Prussian blue is proposed to impose a slight morphology change in the Nafion® compared with when the inner layer is absent. Thus the electrochemical response for the electrostatic binding of the hydrophilic [ Ru(NH 3 ) 6 ] 3+ 2+ redox couple is identical in both cases. In contrast, uptake of the hydrophobic [ Os(bpy) 3 ] 3+ 2+ redox couple takes place at a slower rate. For the 1,1′-dimethyl-4,4′-bipyridilium cation/radical cation ( MV 2+ MV +· ) system, known to be quasi-reversible in Nafion®, radical cation re-oxidation in the presence of the inner layer occurs over a wide potential range, with complete oxidation only occurring on mediation with electrogenerated Prussian blue.

Electrochemical and spectroscopie studies of pyridin intervention in the electrooxidation of pyrrole

Abstract The role of pyridins in inhibiting the electropolymerisation of pyrrole and N-methylpyrrole is examined. Pyrrole in the presence of excess pyridin yields either thin insulative films or else coloured soluble species depending on the pyridin/pyrrole ratio, the nature of the pyridin and the kinetic regime of the reaction; thin film formation predominates at equimolar ratios. Ten major inferences drawn from the electrochemistry, reflectance and transmittance IR and XPS studies of the films, and the molecular properties of the pyridins, allow some reasonably well-founded mechanistic conclusions to be drawn. The pyridin acts to remove the proton from the nitrogen atom of the first-formed pyrrole radical cation before dimerisation occurs. Following the major possible secondary processes for film formation, XPS and IR show the composition of the final thin-film surface to depend upon several factors including pyridin structure; such conclusions regarding termination point to addition of deprotonated radical to polymer radical as the dominant propagation step. Brief comparative studies of N-methylpyrrole underlined the particular role of the pyrrole N-proton, and indicated other differences including lower sensitivity to the presence of pyridin and failure to form insulative film.