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Electrochimica Acta 52 (2007) 6911–6915

Electrochromic effects from a simple commercial polymer membrane

Frank Davis, Karen A. Law, Kerry Bridge, Seamus P.J. Higson ∗Cranfield Health, Cranfield University, Silsoe MK45 4DT, UK

Received 26 February 2007; received in revised form 27 April 2007; accepted 2 May 2007Available online 13 May 2007

bstract

A simple commercial polyester polymer membrane has been found to exhibit an intense electrochromic effect. Most polymers which undergo

lectrochromic effects contain either transition metals or extensive conjugated systems. We have found that a simple commercial polyester membranehen coated with gold and polarised to −4 V (versus Ag) in aprotic organic solvents displays an electrochromic colour change from a colourless

o an intense red state.2007 Elsevier Ltd. All rights reserved.

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eywords: Polyester; Electrochromic; Cyclic voltammetry; Ionic liquid; Organ

. Introduction

An electrochromic material is one where a reversible colourhange takes place upon reduction (gain of electrons) or oxida-ion (loss of electrons), on passage of electrical current uponhe application of an appropriate electrode polarising poten-ial [1]. A large number of chemical moieties have been foundo exhibit this effect. Recent reviews [2,3] describe many ofhese materials, however the polymers utilised are often complex

aterials which are not commercially available and may displayimited stability to oxygen and water. Materials that displaylectrochromic effects include inorganic films such as tung-ten oxide [2] and many transition metal complexes, whethern solution or incorporated within solid films [4–6]. Polymerlms have also been formulated to display this effect, usuallyither by incorporation of redox-active species such as violo-ens within the film [7,8] or by use of conducting polymersuch as polythiophene [9], polypyrrole [10,11] or polyaniline12–14]. For example a recent publication [15] describes theormation of red, green and blue electrochromic polymers basedn conducting polythiophenes which show a high stability and

eversibility (10,000 double potential cycles). Electrochromicolymers have a wide range of potential applications, includingsmart windows”, reusable labels and display devices [2,3].

∗ Corresponding author. Tel.: +44 1525 863453.E-mail address: s.p.j.higson@cranfield.ac.uk (S.P.J. Higson).

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013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.05.010

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We have recently discovered an entirely new class of highisibility electrochromic materials that change colour via anlectronic effect that has previously not been recognised. Aarticular advantage of these electrochromic materials and theffects they exhibit is that they consume very little power duringhe colour change and consume virtually no power thereafter to

aintain the colour. Whilst working on electrodepositing poly-er layers onto simple commercial membranes, we have found

hat polarising the membranes at cathodic potentials causes aolour change from a colourless state to an intense red colour.ur initial experiments involved depositing polydivinyl benzeneembranes onto porous supports for utilisation within electro-

hemical glucose and lactate sensors [16–18]. It was noticedhilst electrodepositing these films onto polyester membranes,

hat the membranes changed colour [17,18]. No colour changeccurred when alumina membranes were used as supports [16].e found that the presence of the monomer was not necessary

o the process and that electrochromism still occurred in simplerganic electrolyte solutions. Polyesters have been utilised pre-iously in electrochromic devices, however they have not beenhe active constituent of the device but rather a flexible supportor electrochromic layers of materials such as tungsten and nio-ium oxides [19]. This is, to our knowledge, the first time such aransition has been observed for a simple polyester membrane.

Commercial polyester membranes were evaporation-coatedith gold on one side to make them electrically conducting. Theyere then placed into an organic solvent containing an organic

lectrolyte and polarised up to −4 V versus a silver reference

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lectrode. A variety of polymer membranes, solvents and metaloatings were utilised.

. Experimental

All chemicals were obtained from Sigma–Aldrich. Ace-onitrile and DMF were of HPLC grade. The electrochromicolutions were obtained by dissolving tetrabutyl ammo-ium perchlorate (0.2 M) in either DMF or acetonitrile. Theholine/glycerol and choline/ethylene glycol ionic liquids wereormulated as reported [6]. Electrolube® silver conductive paintElectrolube® Ltd., Berkshire, UK) was purchased from Maplinslectronics, Manchester, UK. Silver wire (0.5 mm diameter) andlatinum gauze were purchased from Blundells (London, UK).

CycloporeTM 0.2 �m polyester (polyethylene terephthalate)embranes, CycloporeTM 0.4 �m polycarbonate membranes,

.2 �m polyethersulphone membranes, 0.2 �m PTFE mem-ranes, 0.2 �m nylon membranes, 0.45 �m polypropyleneembranes, 0.2 �m polyamide membranes and AnoporeTM

.1 �m alumina membranes were purchased from Whatmannternational, Maidstone, UK. 0.4 �m polyester filter mem-ranes were also purchased from GE Osmonics (MN, USA).

elinex polyester film (0.1 mm thick) was purchased from HiFi

ndustrial Films (Herts, UK).The membranes were coated using an Edwards E480 evapora-

or. Films consisting of a single metal were 50 nm in thickness. In

Fig. 1. Experimental set-up for electrochromism studies.

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he case of gold–chromium films, 5 nm of chromium was evap-rated, immediately followed by 50 nm gold. Multicore wiresere attached to the coated membranes via the application of

Electrolube®’ silver conductive paint. The wire/electrode jointas then insulated with a coating of epoxy resin, which also

erved to enhance the mechanical strength of the electrode.The electrochromic measurements were performed in a glass

ell of volume 200 ml (Fig. 1). The metal-coated membrane wassed as the working electrode, silver wire as the reference andlatinum gauze for the counter electrode. The electrodes wereositioned in a removable glass stage, which served to maintainconstant distance between the electrodes. The connections to

he electrodes were able to enter the cell through three stopperednd sealed entry holes in the lid.

A Sycopel PCI 100 MK3 potentiostat computer interfaceas used in conjunction with a ‘Ministat Potentiostat’, (H.B.hompson and Associates, Newcastle upon Tyne, UK) for alllectrochemical measurements. The potential of the polymerlectrode was cycled from 0 to 4 V (versus Ag) and back at aate of 50 mV s−1. Colour co-ordinates of the resultant film werebtained using a CR-400 Chroma Meter (Konica Minolta). Theisible spectrum of the polyester was measured in reflectancethe presence of the gold film prevents measurement of trans-ission spectra) utilising a DH-2000 light source combined withUSB2000 Miniature Fibre Optic Spectrometer (Ocean Optics,L, USA).

. Results and discussion

.1. Electrochromism of polyester membranes

The cyclic voltammagram of the system is shown in Fig. 2.

s can be seen there is a clear transition occurring in the regionf −2 V. Repetitively sweeping the potential back and forth forp to 20 cycles did not affect the voltammagram.

ig. 2. Cyclic voltammagram of a gold coated polyester membrane from 0 to4 V (vs. Ag wire) in 0.2 M tetrabutyl ammonium perchlorate/DMF.

F. Davis et al. / Electrochimica Acta 52 (2007) 6911–6915 6913

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Fig. 3. Reversible electrochromic colour

Fig. 3 shows the colour change that occurs as a potential ispplied to the polyester membrane which is suspended in a 0.2 Molution of tetrabutyl ammonium perchlorate in DMF. As theotential reaches −2 V versus Ag, a colour change is observedo begin (stage 1). As the potential becomes more negative thisolour change spreads over the surface (stage 2) and at −4 Versus Ag, the membrane assembly has become completely redstage 3). The same effect could be obtained by simply apply-ng a −4 V polarisation to the membrane and led to completeolour change in <5 s. After being polarised at −4 V for approx-mately 20 s, current flow dropped to zero, indicating no furtherransitions take place. Setting the potential to 0 V, removing theonnection (open circuit) or removing the polymer film from theolution caused rapid reversal (<5 s) to the original state. Thisrocess could be repeated for at least 20 cycles, however delam-nation of the gold film was found to eventually occur and at thistage the colour change ceased to occur.

Fig. 4 shows the spectrum of the “red” polyester membrane.s can be seen there are two major adsorption peaks centred at

pproximately 420 and 580 nm. An unpolarised membrane wassed as the background spectrum. Below 300 nm, the polyestern both cases adsorbed so highly that no spectrum could be

btained, i.e. all of the radiation was adsorbed. Measurementf the colour co-ordinates of the resultant red membrane (Fig. 3,tage 3) gave L = 39.1, C = 38.6, h = 14.2◦—which appears to theuman eye as a claret/scarlet red.

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Fig. 4. Adsorption spectrum of the red polyes

ge from a colourless to a deep red state.

.2. Variation of the metal coating

Due to the delamination of the gold film, other metals werenvestigated. Firstly, in an attempt to stabilise the gold film, ahin layer of chromium was evaporated onto the surface of the

embrane and then gold was evaporated on top of this. Thisad no effect on the observed electrochromic effect, the intenseed colouration still being observed. The gold film was howevertabilised and this allows the electrochromic colour changes toe reversibly cycled for at least 200 cycles.

A thin film of silver could also be utilised and was foundo give a strong colouration, identical to that observed with theold and gold-chromium films, however repeated cycling didventually cause delamination of the metal coating from theembrane surface. Chromium films were also utilised, however,

n this instance the electrochromic effect was only transient andhe metal films were found to discolour rapidly, possibly indicat-ng a corrosion effect. Aluminium coatings were found to giveimilar results to chromium.

.3. Variation of solvent

Acetonitrile, which has a good ability to solvate organic elec-rolytes, was utilised in place of DMF. When gold–chromiumoated polyester membrane were polarised as before, colourhanges identical to those in DMF were observed. As a possi-

ter film, measured in reflectance mode.

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le substitute for an organic solvent/organic electrolyte mix, twoonic liquids [20] based on choline chloride combined with eitherthylene glycol or glycerol were investigated instead, althoughn this case no electrochromic effect was observed. This coulde due to low conductivity of the liquid as a result of its highiscosity.

.4. Variation of membrane

Polyester membranes from GE Osmonics were also goldoated and tested and showed identical electrochromic effects tohe Whatman membranes. However, when a non-porous, thickerheet of Melinex polyester was utilised, no electrochromic effectould be observed. This indicates that there must be good pen-tration of the solvent and electrolyte into the membrane forhe electrochromic effect to occur. Probably the highly resistiveature of the Melinex sheet inhibits any electrochromism. Thisay explain why the electrochromic effect has not previously

een observed for polyester since although polyester substratesave been utilised as supports in many previous electrochromismtudies, they have been materials of the Melinex type rather thanorous membranes.

As an alternative to the polyester membrane, other mate-ials were used. These included glass microscope slides,lumina membranes and membranes based on polyamide,olypropylene, polycarbonate, polyethersulphone, polytetraflu-roethylene and nylon. These were all coated with gold andolarised at potentials between 0 and −7 V versus Ag wire. Inll cases, no electrochromic effect could be observed.

.5. Variation of electrolyte

Several electrolytes were tested as alternatives to tetrabutylmmonium chlorate. Replacement of the chlorate with the cor-esponding tetrabutyl ammonium bromide had no effect, withstrong red colour still being generated. No electrochromismas observed when 0.2 m lithium chloride or perchlorate (inMF) or sodium iodide (in acetonitrile and DMF) were used.

t is thought that at the high potentials used the electrolyte iseacting rather than the polymer membrane. This is confirmedn the case of NaI by the rapid appearance of a yellow/brownolour in the solution attributed to iodine formation.

.6. Mechanism of the transition

We have described within this paper a strong electrochromicffect that occurs as a polyester membrane is polarised attrongly negative potentials. There are several possible explana-ions for this behaviour, although some of these can be ruled out.t first we thought that perhaps the solvent was being decom-osed electrochemically to form a coloured product; howeverhe use of two different solvents, DMF and acetonitrile allowedn identical electrochromic effect to be seen. A colouration of

he metal film can also be ruled out since both gold (with andithout chromium) and silver gave similar results. The elec-

rochromic effect was however only observed when polyesterembranes were used, with other membranes of similar physi-

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ig. 5. Structure of (a) polyethylene terephthalate, (b) a possible radical anion,c) a possible diradical dianion, (d) resonance structure of the dianion.

al form—but composed of different polymers, giving no effect.e therefore conclude that the polymer itself must be taking an

ctive part in this phenomenon.The cathodic potential at which the electrochromic transi-

ion occurs indicates that electrons are being injected into theolymer structure. Polyethylene terephthalate has the structurehown in Fig. 5a with the conjugated terephthalate unit beinghe most likely moiety to which electrons can be added. Weropose a mechanism where either one or two electrons can bedded to the terephthalate unit to give either a radical (Fig. 5b)r diradical (Fig. 5c) structure as shown. What is interesting ishat a resonance structure exists (Fig. 5d) where the unpairedlectrons combine to give a non-radical dianion which couldtabilise the structure.

. Conclusions

We have reported here a novel electrochromic system whichives a strong red colour based on a simple commercial polymerembrane. Although the current system is unlikely to be com-ercially viable, the fact that such simple materials undergo a

trong colour change indicates the possibility of a range of newaterials.Future investigations will involve chemical modification of

he membranes to see whether other colours can be attained. Alsohe potential required for the electrochromic effect is relatively

igh compared to many others in the literature [2,3], modifi-ation may reduce this voltage. Finally, attempts will be madeo replace the organic electrolyte with a less volatile and toxiclternative such as a suitable ionic liquid or a solid electrolyte.

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cknowledgement

This project was funded by the BBSRC as part of the Centreor BIOARRAY Innovation within the Consortium for Postge-omic Science.

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