J. MATER. CHEM., 1994, 4(8), 1289-1291 1289 Determination of the Potential Limits for W03 Colouration Peikang Shen and Alfred C. C. Tseung* Chemical Energy Research Centre, Department of Chemistry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ UK The determination of the maximum permissible degree of colouration for an electrochromic film in a particular device is technically important for long-term reversible modulation and safe operation. Chronopotentiometry combined with cyclic voltammetry have been used to examine the degree of colouration for WO, films on various conducting substrates and electrolytes. The results revealed that an overreduced WO, film with high amount of intercalated ions will lose its reversibility, lowering its electrochromic performance.WO, is the main optically functioning film in almost all electrochromic devices. It is able to change its optical proper- ties in a reversible and persistent way under a potential control.' W0,-based electrochromic devices are much better than those of the liquid-crystal-based displays in terms of colour and viewing properties. WO, films appear blue in colour during the electrochemical reduction in acid solution or alkali-metal ion-containing solution^.^^^ The reaction can be expressed as WO,+xM+ +xe- =M,W03 (M=H, Li, K,Na, ...) (1) The degree of colouration depends on the amount of proton or alkali-metal ion inserted in the film. It has been shown that the amount of ion (x) in the coloured film is a function of the electrode potentials a~plied.~ However, the limiting amount of inserted ion in the WO, film is crucially dependent on the electrode substrate and the electrolyte solution.This limit means that the WO, films can be used safely at the corresponding potential without any side-reactions taking place. Since the practical electrochromic devices rely on the long-term stability of the electrochromic film, it is important to control the appropriate operating conditions. This paper describes a simple and rapid method for measuring the limiting potential for WO, colouration and the corresponding value of x in MxW03. This method should provide a good guideline for the operational parameters for various WO, based electrochromics under world-wide development.Experimental WO, films were prepared on Au, Pt or indium-doped tin oxide coated (ITO) glass substrates. The Au and Pt substrates were cut into circular disks of 0.5 cm2 and were embedded in PTFE holders. They were then polished using sandpaper. All substrates were degreased with methanol in an ultrasonic bath for 5 min followed by a distilled water rinse. WO, films were prepared by constant potential deposition at -0.35 V (on Pt or Au) or -0.4 V (on ITO) for 30-45 min in a homogeneous solution of 50 mmol dm-, tungsten and 30% discharge the coloured film. Fig. l(u)shows a typical chrono- potentiogram of a WO, film on Pt substrate in 0.5 mol dmP3 H2S04at 20°C. The amount of intercalated ions in the film can be calculated by using the following relationship: x =izM/FW where i is the discharge current, z is the time used for complete oxidation of MxW03,M is the molecular weight of tungsten trioxide, F is the Faraday constant and W is the weight of tungsten trioxide determined by UV-VIS ~pectrophotometry.~ Since x is a function of conditioning potential, the amount of intercalated ions is different at different potentials, the oxi- dation charge should be varied. The procedure for examining the colouration behaviour of the WO, films was as follows: the W0,-coated electrodes were held at 1.0 V us.SCE for 60 s and then a negative constant current of 150 pA cm- was applied for varying times, dependent on the experimental conditions [Fig.1(b)]. Results and Discussion Chronopotentiometric measurements of a WO, film on Pt for colouration and bleaching are show in Fig. 1. As indicated in the figure, after complete oxidation of H,W03 the potential rapidly rises to the oxidation potential of Pt (ca 0.8 t') and then to oxygen evolution potential (1.45 V). In the reverse process, WO, is reduced after the reduction of Pt oxides. When the potential reached the potential where hydrogen is evolved, the main reaction is hydrogen evolution arid the potential remains constant. Under such a potential, the pro- tons are removed from the film by the formation of hydrogen PtO + H20+ Pt02+ 2H+ + 2e-Pt + H20+ PtO + 2H+ + 2e-1.6 72H20-+O2 + 4H+ + 46flw 0.8 -I 1PtO + 2H++ 2e-+ Pt + H,O v/v propan-2-01.The details have been reported el~ewhere.~>~ Electrochemical measurements were carried out on an EG&G PAR 273A potentiostat/galvanostat controlled by 270 Research Electrochemistry Software. A three-electrode cell with a Pt counter electrode and a saturated calomel electrode (SCE) reference electrode was used. Before the measurements, the solutions were sparged with nitrogen for 30min. All the chemicals were supplied by BDH and used as received. The intercalation of small ions into the WO, films was conducted by applying a fixed potential (conditioning poten- tial) for 60 s. A constant current of 150 p.A cm-2 was used to + xe-WO3 + +I+ + xe--+ HY03I -0.8 2H' + 2e--3 H2 I I._2 0.0 0.1 0.2 0.3 0.4 t/ks Fig.1 Chronopotentiograms of a WO, film on Pt in 0.5 ma1 dm-3 H,SO, solution at 20 "C: (a) under 150 PA cmP2 oxidation current after 60 s held at -0.25 V and (b) under -150pA cm-2 reduction current after 60 s held at 1.0 V 1290 J. MATER. CHEM., 1994, VOL. 4 r 0.8 - 2--II-L-LJ 0.0 0.10 0.20 _ 0.30 tlks Fig.2 Chronopotentiograms of WO, films on (a) Au, (b) Pt and (c) IT0 in 0.5 mol dmP3 H2S04solution at 150 pA crn-', 20 "C gas and this limits the increase in the amount of inserted ion. It is worth noting that gas formation is dangerous for an electrochromic device since it will insulate the film from electrolyte in the cell and increase the hydrogen pressure inside the pores, which may cause rupture of the electroch- romic film.From the measured z value at a given condition potential, x can be calculated by using eqn. (2). The potential just before the hydrogen evolution is the limiting potential for a WO, film operating in aqueous acid solution. Therefore, the limit, xL,for WO, colouration can be determined from the value of zL. xLis the maximum amount of ions inserted in coloured film. This is a simple method but is very sig- nificant in determining the safe operating potential. For a practical electrochromic device, one can use this method to determine the operating potential for protecting the device from the damage due to hydrogen evolution. Fig. 2 compares the chronopotentiograms of the WO, films colouration on different substrates in 0.5 mol dm-, H2S04 solution.It is seen that the potential decreases monotonically on H intercalation (in a two-electrode electrochromic device, + the potential decrease on WO, coated electrode will result in a decrease in the electromotive force of the device). Monotonic potential variations indicate that the ion intercalation takes place without major structural rearrangements.6 The param- eters measured from these curves are summarised in Table 1. The results show that the limiting degree of colouration is strongly dependent on the substrate in aqueous media. WO, films have higher limiting degree of colouration on substrate that has higher overpotential for hydrogen evolution. In addition, the effect of pH on the degree of colouration was also examined.Changing the concentrations of the sulfuric acid solutions from 0.01 to 1mol drn-,, the limiting potentials shifted to more positive values that obey the Nernst equation. However, the limiting amount of inserted ions are unchanged. The maximum value of x in H,WO, on IT0 glass substrate at corresponding limiting potential is 0.157 in this study. In non-aqueous solutions, the colouration process of WO, film is more complex. Fig. 3 shows a response curve of a WO, film on Au under -150 pA cmV2 charging in 1mol dmP3 LiC10,-propylene carbonate (PC) solution at 20 "C. It is obvious that there are several transition points on the curve Table 1 Limiting potentials and limiting x values for WO, colouration on different substrates in 0.5 mol dm-, H,S04 solution at 20 "C limiting potential/ electrode type V us.SCE XL WO, on Pt -0.27 0.07 1 WO, on Au -0.28 0.075 W03 on IT0 -0.55 0.157 -3.0t Ii -uiI____L -I 0.0 1.o 2.0 3.0 tlks Fig. 3 Chronopotentiogram of a W03 film on Au in 1 mol dm-3 LiC104-PC solution at 150 pA cm--, 20 "C at about -1,- 1.5 and -2 V. The same behaviour has also been observed for WO, film on a Pt substrate. Since both Li+ ion and PC are stable at potentials less negative than -3 V us. NHE,7v8 there could not be any side-reactions during the charging process except the reduction of WO,. Fig. 4 shows the cyclic voltammograms of a WO, film on Au in 1rnol dmP3 LiC10,-PC at a scan rate of 50 mV SKI. There is a pair of quasi-reversible peaks between -1.0 and -0.6 V.When the potential sweeps towards potentials more negative than -1.2 V, the film loses its reversibility perma- nently. The broad cathodic peaks at -1.0 and -1.5V corre-spond to the transition points on the chronopotentiogram in Fig. 3. The values of x calculated using eqn. (2) are 0.33 at -1.0 V and 0.79 at -1.5 V. It has been shown that the X-ray photoelectron spectroscopy (XPS) applied to core levels can give information on the valence states of the tungsten ions in WO, film at different degrees of colouration. In highly dis- ordered tungsten trioxide, the amount of W6 transformed+ into W5 + determines the optical response. For an evaporated WO, film with different amounts of intercalated H+ , the deconvoluted XPS spectra showed that W6+ and W5+ co-existed in the film at x=O.O9; however, there are three valence states of W6+, W5+ and W4+ at x =0.42.9This means the tungsten ion could be reduced to lower valence states by the increased amount of intercalated ions.It is possible that some of the tungsten ions were reduced to W4+ at -1.5V with an increased amount of Li+ in the film. However, such lower-valence tungsten ions might be difficult to reoxidise (Fig. 4).Li+ ions may be unable to deintercalate after a high degree of intercalation. In fact, Li+ ion is found both in the intercalated and in the deintercalated state by using secondary- -2.0 0.0---2.0 --4.0 -2.0 -1.2 -0.4 0.4 EN vs. SCE Fig. 4 Cyclic voltammograms of a WO, film on Au in 1 rnol dmP3 LiC104-PC solution at 20 "C,scan rate, 50 mV s-': (a) -0.5-0.8 V, (b) -0.13-0.8 V and (c) -2.0-0.8 V J.MATER. CHEM., 1994, VOL. 4 ion mass spectrometry (SIMS)." It is clear that the limiting potential should be no more negative than -1.2 V for WO, colouration in 1mol dmW3 LiC10,-PC solution to maintain its reversible property. This gives a limiting xL of 0.46. Conclusion As shown above, the limiting degree of colouration for WO, films can be readily examined by chronopotentiometry com- bined with cyclic voltammetry. The electrochromism of W03 hinges on ion intercalation and deintercalation from an adjac- ent electrolyte and, of course, the electron insertion/extraction occurs jointly with the ionic movement.The limiting amount of intercalated ions is strongly dependent on the conducting substrate and the electrolyte. In aqueous solutions, the colouration process is limited by hydrogen evolution. In non-aqueous solutions, it is limited by overreduction of the WO,, resulting in an electrochemically irreversible film. References C. G. 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Zhang, S. A. Wessel, B. Heinrich and K. Colbow, Solar Energy Muter., 1990,20, 289. Paper 4/01957A; Received 31st March, 1994