Definitions contributed to the “Electrochemical dictionary” (edited by Allen J. Bard, György Inzelt, and Fritz Scholz).

The Electrochemical Dictionary is intended to provide authoritative definitions of terms used in electrochemistry and related fields. This document contains definitions of some diverse terms used in electrochemistry and related fields, namely, Electrochromism, electrochromic devices, Indium tin oxide (ITO) optically transparent electrode, Porphyrin, Tetraalkyl ammonium salts, Tungsten oxides

Harnessing solar energy with Grätzel cells

Project provides links to GCSE science and A-level chemistry specifications. Grätzel cells - TiO2 coated with light-sensitive dye molecules, immersed in organic electrolyte - mimic photosynthesis. Chemists from the Universities of Loughborough and Bristol have teamed up to take a researchbased project into local schools. The project, which involves using Grätzel solar cells to convert sunlight into electricity, gives students the opportunity to be involved in cutting-edge, green chemistry research

Characterisation and humidity-sensing properties of aluminium (oxy)-hydroxide films prepared by cathodically induced precipitation

Cathodically-induced precipitated aluminium (oxy)-hydroxide films, prepared using hydrogen-sorbing palladium electrodes, have been characterised and their humidity-sensing electrical properties studied. Infrared (IR) spectroscopy demonstrated that the films are hygroscopic, and this property allows their use as the active component in a humidity sensor. Films formed as bridging precipitates between twin palladium electrodes were tested for electrical parameter response to relative humidity changes. A linear correlation with impedance is obtained on application of a 100 mV applied ac signal amplitude at 1 kHz. The linearity of response is comparable to commercial sensors. Cathodically-induced precipitate films contain 24 wt% aluminium and 10 wt% sulfate, the balance being oxygen and hydrogen. As-deposited films are amorphous, but change to a crystalline form after heat treatment at high temperature. XRD patterns taken as films are heated reveal transformation into γ- Al2O3, followed by production of α-Al2O3 at temperatures greater than 1100°C. An electrochemical quartz crystal nanobalance (EQCN) study demonstrated that dissolution of the precipitate is possible at positive potentials, cycling to which allows film compaction

AC impedance characteristics of solid-state planar electrochemical carbon monoxide sensors with Nafion ® as solid polymer electrolyte

The AC impedance characteristics of planar solid-state electrochemical carbon monoxide (CO) sensors with Nafion® as the solid polymer electrolyte are reported. For a re-cast Nafion® sensor design, Nyquist plots indicated very high impedances (107–108 Ω), which slightly decreased with an increase in the working electrode surface area (0.25–0.50 cm2). Only one enlarged semi-circle was observed, relating to an equivalent circuit with a geometrical capacitance (Cg) in parallel with a bulk resistance (Rb). In contrast, a sensor design using a commercial membrane produced a system with a much lower impedance (<103 Ω), with Nyquist plots showing a flattened semi-circle at high frequencies (due to Cg and Rb in parallel), followed by the start of a large semi-circle (due to the double-layer capacitance, Cdl, in parallel with the charge-transfer resistance, Rct) over a lower frequency range. On addition of CO, the second circular arc reduced in size, indicating that the interfacial impedance of the electrochemical system had decreased

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

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

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

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* 5 73, a*5226 and b*5233, 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* 5 94, a* 5 2 and b* 5 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 lmax as reference point, coloration efficiencies, g 5 DA(lmax)/Q, were calculated as 143 and 150 cm2 C21 respectively for the PB/PW and PW/PB electrochromic transitions

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

An in situ colorimetric measurement study of electrochromism in the di-n-heptyl viologen system

An in situ colorimetric method, based on the CIE (Commission Internationale de l'Eclairage) system of colorimetry, has been applied to the study of the electrochromic N,N´-bis(n-heptyl)-4,4´-bipyridylium (‘di-n-heptyl viologen’) system in aqueous solution on transmissive ITO/glass substrates. On electrochemical reduction of the di-n-heptyl viologen di-cation, the purple di-n-heptyl viologen radical cation salt deposits as a film and the changes in hue and saturation have been tracked using CIE 1931 xy chromaticity coordinates. The CIELAB 1976 colour space coordinates of the purple di-n-heptyl viologen radical cation salt were L* = 76, a* = 33 and b* = –20, with a complementary wavelength of 548 nm. A sharp decrease in luminance was found on formation of the di-n-heptyl viologen radical cation salt. Colour coordinates for the reverse (oxidation) direction plots show hysteresis, implying that specific choice of colour values depends on both the potential applied and from which direction the potential is changed