Advances in 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 are taken as input, with chromaticity coordinates being generated as output. Colour stimulus measurement example calculation results are firstly presented for aqueous solutions of the dyes, Erythrosin B (red, x, y = 0.608, 0.365), Acid Green 25 (x, y = 0.086, 0.298) and Remaxol Brilliant Blue R (x, y = 0.153, 0.045), and then for tracking electrochromic in situ colour stimulus changes in the methyl viologen and di-n-heptyl viologen systems. The quantification of colour during each viologen dication to cationradical 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. Ruthenium purple (RP) films on transmissive tin-doped indium oxide (ITO)/glass substrates have been synthesised by a novel electrochemical coagulation technique. Using the CIE system of colorimetry, the colour stimulus of the electrochromic RP films and the changes that take place on reversibly switching to the colourless form have been calculated from in situ visible spectra recorded under electrochemical control. (Continues...)

Beyond the Butler–Volmer equation. Curved Tafel slopes from steady-state current–voltage curves

We report the discovery and analysis of curved Tafel slopes from the electrochemical reduction of hexamminecobalt(III) under steady-state conditions. In order to confirm the existence of the curvature, random assemblies of carbon microelectrodes (RAM™ electrodes) were employed to obtain experimental data over more than three orders of magnitude, without significant double layer charging currents and without ohmic distortion. Since the rate-determining step in the reduction reaction is electron transfer, and no ligand substitution reactions occur on the timescale of experiments, the curvature of the Tafel plot is attributed to the dependence of the symmetry factor on electrode potential

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

Synthesis, characterisation and in situ colorimetry of electrochromic Ruthenium purple thin films

Ruthenium purple (RP) films on transmissive tin-doped indium oxide (ITO)/glass substrates have been synthesised by an electrochemical coagulation technique using an aqueous RP colloidal suspension prepared from separate very dilute aqueous solutions of iron(III) chloride and potassium hexacyanoruthenate(II), with dilute potassium chloride as supporting electrolyte solution. To aid stability of the RP films, ruthenium(III) chloride was added to the RP colloidal suspension. Using the CIE (Commission Internationale de l’Eclairage) system of colorimetry, the colour stimulus of the electrochromic RP films and the changes that take place on reversibly switching to the colourless form have been calculated from in situ visible spectra recorded under electrochemical control. On electrochemical reduction, the intensely absorbing bright purple mixed-valence iron(III) hexacyanoruthenate(II) chromophore is reduced to the colourless iron(II) hexacyanoruthenate(II) form. Sharp and reversible changes in the hue and saturation occur, as shown by the track of the CIE 1931 xy chromaticity coordinates. Concurrently, as the purple chromophore is bleached, a large increase in the relative luminance of the electrochromic film is observed. For the purple state, the CIELAB 1976 colour space coordinates were L* = 64, a* = 27 and b* = −36, with a complementary wavelength (λc) calculated as 555 nm, in excellent agreement with the absorption maximum (λmax) of 550 nm for the intervalence charge-transfer (IVCT) band

Synthesis, electrochromism and display-device application of electroactive ruthenium purple films prepared by 'Directed Assembly' and electrochemical precipitation techniques

Electrochemical reduction of electroactive solid films of Prussian blue (iron(III) hexacyanoferrate(II), PB), produces iron(II) hexacyanoferrate(II), which appears transparent as a thin film. Oxidation of PB yields iron(III) hexacyanoferrate(III), via intermediate Prussian green. The blue-transparent (anodically-colouring) transition in PB at one electrode has often been partnered with cathodically-colouring electrochromic materials at a second electrode, in ‘complementary’ electrochromic devices (ECD’s), where both films are coloured simultaneously. Electrochemical reduction of thin-film ruthenium purple (iron(III) hexacyanoruthenate(II), RP), produces the transparent iron(II) hexacyanoruthenate(II) redox state, but no oxidized form is available. By contrast with prototype PB-based ECD’s, systems based on RP have rarely been reported, likely due to the electrosynthetic challenge in preparing stable thin films. Thin-film PB is readily available through electrochemical reduction of solutions containing iron(III) and hexacyanoferrate(III) ions. Salts of the analogous hexacyanoruthenate(III) ion are not commercially available, and although preparation by chemical or electrochemical oxidation of hexacyanoruthenate(II) is possible, the resulting solution is unstable. Here we describe the ‘directed assembly’ and electrochemical precipitation of thin-film RP on ITO-glass substrates. In both techniques, the hexacyanoruthenate(II) redox state is used directly. For ‘directed assembly’, the synthesis involves exposure of ITO-glass to solutions containing, alternately, adsorbable iron(III) cations and hexacyanoruthenate(II) anions, leading to well-defined multilayer structures. In the electrochemical precipitation technique, a precisely formulated iron(III) hexacyanoruthenate(II) sol is used, film formation again relying on electrostatics. The purple-transparent (anodically-colouring) transition in RP directly contrasts with the transparent-purple (cathodically-colouring) transition in di-n-alkyl viologens and we have constructed transmissive ECD’s using these materials. Although both the viologen dication and viologen radical cation redox states are water soluble, fast colour-switching in displays is demonstrated through the use of a thin-layer device construction, with capillary-filling of electrolyte solution. In addition to spectroelectrochemical measurements of the materials and devices, we report the quantitative description of colour and relative transmissivity of the RP/viologen displays as sensed by the human eye using our Microsoft® Excel® spreadsheet for the accurate calculation of CIE (Commission Internationale de l’Eclairage) 1931 xy colour coordinates and luminance data from visible region absorption spectra

Electrochromic devices based on surface-confined Prussian blue or Ruthenium purple and aqueous solution-phase di-n-heptyl viologen

Prototype electrochromic devices (ECDs) based on anodically-colouring thin-film Prussian blue (PB) or Ruthenium purple (RP) and cathodically-colouring aqueous solution-phase di-n-heptyl viologen are described. The initial (‘off’) state of each ECD is set with the PB (or RP) in the colourless reduced form and the di-n-heptyl viologen as the colourless di-cation. Switching the ECDs to the coloured state (‘on’), forms on oxidation the coloured mixed-valence PB (or RP), with simultaneous reduction of the di-n-heptyl viologen di-cation to form the purplish-red di-n-heptyl viologen radical cation dimer salt as a thin film. The overall perceived reversible colour changes were colourless to deep blue/purple for the PB/di-n-heptyl viologen ECDs and colourless to pinkish-purple for the RP/di-n-heptyl viologen ECDs. Using the Commission Internationale de l'Eclairage (CIE) system of colorimetry, the colour stimuli of the ECDs were calculated from in situ visible region spectra recorded under electrochemical control, the depth of colour being controlled by the di-n-heptyl viologen concentration. For the coloured states of the PB/di-n-heptyl viologen ECDs, the CIELAB 1976 colour space coordinates for a D55 illuminant were L⁎=60, a*=22 and b*=−47, and L*=39, a*=47 and b*=−55, respectively for 5 and 10 mmol dm−3 di-n-heptyl viologen solution concentrations. For the RP/di-n-heptyl viologen ECDs, the coordinates were L*=70, a*=31 and b*=−27, and L*=63, a*=44 and b*=−34, respectively for 5 and 10 mmol dm−3 di-n-heptyl viologen solution concentrations. L* quantifies the lightness, with +a*, −a*, +b* and −b* respectively giving the red, green, yellow and blue directions away from the achromatic point (0, 0)

In situ spectroelectrochemistry and colour measurement of a complementary electrochromic device based on surface-confined Prussian blue and aqueous solution-phase methyl viologen

The fabrication, in situ spectroelectrochemistry and colour measurement of hybrid electrochromic devices (ECDs) based on a surface-confined metal hexacyanometallate – Prussian blue (PB, containing the iron(III) hexacyanoferrate(II) chromophore) – and aqueous solution-phase methyl viologen (N,N´-dimethyl-4,4´-bipyridylium) are described. In the ECDs, the initial (‘off’) bleached state is set with PB in its reduced form and the methyl viologen as the di-cation. Switching to the coloured state (‘on’), forms the mixed-valence iron(III) hexacyanoferrate(II) chromophore on oxidation of iron(II) hexacyanoferrate(II), with simultaneous reduction of the methyl viologen dication to form a mixture of the radical cation monomer/dimer. Using the Commission Internationale de l'Eclairage (CIE) system of colorimetry, the colour stimulus of such ECDs and the changes that take place on reversibly switching between the colourless and coloured states have been calculated from in situ visible region spectra recorded under electrochemical control. The concentration of the solution-phase methyl viologen and its diffusion to the cathode controlled both the proportion of surface-confined (reduced) PB that is switched to the blue form and the overall ECD changes. For the ECDs’ ‘on’ states, the CIELAB 1976 color space coordinates for a D55 illuminant were L* = 60, a* = 3 and b* = −46, and L* = 49, a* = 9 and b* = −59, respectively for 5 and 10 mM methyl viologen solution concentrations. The low a* and high (negative) b* chromaticity coordinates quantified the overall ECD colour stimulus of the ‘on’ state as being deep blue, with a broad absorption across the visible spectral region. Combination of the methyl viologen system in the ECDs served to remove the green tint perceived in single film PB. CIELAB 1976 colour space coordinates showed that the ECDs were fully transparent and nearly colourless in the ‘off’ states, with L* = 100, a* = 1 and b* = 1. The changes in the transparency were 83.0% (5 mM methyl viologen) and 93.1% (10 mM methyl viologen) between the ‘off’ (bleached) and ‘on’ (coloured) states of the ECDs

Quantum design of ionic liquids for extreme chemical inertness and a new theory of the glass transition

In many modern technologies (such as batteries and supercapacitors), there is a strong need for redox-stable ionic liquids. Experimentally, the stability of ionic liquids can be quantified by the voltage range over which electron tunneling does not occur, but so far, quantum theory has not been applied systematically to this problem. Here, we report the electrochemical reduction of a series of quaternary ammonium cations in the presence of bis(trifluoromethylsulfonyl)imide (TFSI) anions and use nonadiabatic electron transfer theory to explicate the results. We find that increasing the chain length of the alkyl groups confers improved chemical inertness at all accessible temperatures. Simultaneously, decreasing the symmetry of the quaternary ammonium cations lowers the melting points of the corresponding ionic liquids, in two cases yielding highly inert solvents at room temperature. These are called hexyltriethylammonium TFSI (HTE-TFSI) and butyltrimethylammonium TFSI (BTM-TFSI). Indeed, the latter are two of the most redox-stable solvents in the history of electrochemistry. To gain insight into their properties, very high precision electrical conductivity measurements have been carried out in the range +20 °C to +190 °C. In both cases, the data conform to the Vogel-Tammann-Fulcher (VTF) equation with “six nines” precision (R 2 > 0.999999). The critical temperature for the onset of conductivity coincides with the glass transition temperature T g. This is compelling evidence that ionic liquids are, in fact, softened glasses. Finally, by focusing on the previously unsuspected connection between the molecular degrees of freedom of ionic liquids and their bulk conductivities, we are able to propose a new theory of the glass transition. This should have utility far beyond ionic liquids, in areas as diverse as glassy metals and polymer science

Electrochromic and colorimetric properties of nickel(II) oxide thin films prepared by aerosol-assisted chemical vapor deposition

Aerosol-assisted chemical vapor deposition (AACVD) was used for the first time in the preparation of thin-film electrochromic nickel(II) oxide (NiO). The asdeposited films were cubic NiO, with an octahedral-like grain structure, and an optical band gap that decreased from 3.61 to 3.48 eV on increase in film thickness (in the range 500−1000 nm). On oxidative voltammetric cycling in aqueous KOH (0.1 mol dm−3) electrolyte, the morphology gradually changed to an open porous NiO structure. The electrochromic properties of the films were investigated as a function of film thickness, following 50, 100, and 500 conditioning oxidative voltammetric cycles in aqueous KOH (0.1 mol dm−3). Light modulation of the films increased with the number of conditioning cycles. The maximum coloration efficiency (CE) for the NiO (transmissive light green, the “bleached” state) to NiOOH (deep brown, the colored state) electrochromic process was found to be 56.3 cm2 C−1 (at 450 nm) for films prepared by AACVD for 15 min followed by 100 “bleached”-to-colored conditioning oxidative voltammetric cycles. Electrochromic response times were <10 s and generally longer for the coloration than the bleaching process. The films showed good stability when tested for up to 10 000 color/bleach cycles. Using the CIE (Commission Internationale de l’Eclairage) system of colorimetry the color stimuli of the electrochromic NiO films and the changes that take place on reversibly oxidatively switching to the NiOOH form were calculated from in situ visible spectra recorded under electrochemical control. Reversible changes in the hue and saturation occur on oxidation of the NiO (transmissive light green) form to the NiOOH (deep brown) form, as shown by the track of the CIE 1931 xy chromaticity coordinates. As the NiO film is oxidized, a sharp decrease in luminance was observed. CIELAB L*a*b* coordinates were also used to quantify the electrochromic color states. A combination of a low L* and positive a* and b* values quantified the perceived deep brown colored state

Beyond the Butler-Volmer equation. Curved Tafel slopes from steady-state current voltage curves [keynote lecture]

This lecture describes the first observation (with T.S. Varley) of a curved Tafel slope in a steady-state current voltage curve