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Electrochromism in WO3 thin films. II. LiClO4-dioxolane-water electrolytes
Solid State lonics 6 (1982) 267-273 North-Holland Publishing Company
ELECTROCHROMISM IN WO 3 THIN FILMS. II. LiCIO4-DIOXOLANE-WATER ELECTROLYTES O. BOHNKE, C. BOHNKE, G. ROBERT Laboratoire d'Electrochimle des Solides ERA 810, Faeult( des Sciences et des Techniques, 25030 Besanfon Cedex, France
and B. CARQUILLE Laboratoire d'Optique LA 214, Facult6 des Sciences et des Techniques, 25030 Besan¢on Cedex, France
Received 22 December 1981 The performances of electrochromic cells with evaporated amorphous WO 3 thin film as electrochromic material in (2.5 M) LiClO4-dioxolane-water electrolytes are presented. A comparison with the results previously obtained in propylene carbonate instead of dioxolane solvent is carried out. The influence of the following parameters has been studied : the thickness of the film, the water content in the electrolyte, the potential applied to the electrochromic electrode during coloration and bleaching processes.
i. INTRODUCTION In a previous publication (i) the characteristics of electrochromic WO_ thin films in (i M) LiClO4-propylene carbonate-water mixture electrolytes were reported. The experimental results have clearly shown that water content in the electrolyte was of considerable importance for coloration and bleaching kinetics of evaporated WO_ thin films. In our search for other organic solvents, dloxolane was selected for several reasons. First, it has been used as solvent in rechargeable lithium-TiSp cells because of its high kinetic stability %owards alkali metals (2) moreover the problems encountered in batteries are very similar to those encountered in display devices with lithium electrolytes. Second, dioxolane is able to make chemical stable mixtures with water. Finally, since its physical properties are strongly different from those of propylene carbonate (PC), the properties of water dioxolane mixtures will be different from those of water-PC mixtures. Under these conditions, the characteristics of the electrochromic electrode in (2.5M) LiClO.-dioxolane-water mixture 4 electrolytes are presented in this paper. A comparison will be carried out between electrochrom~sm in PC and in dioxolane. We show that the influence of water contained in the electrolyte as previously observed in PC solvent on the electrochromism phenomenon is also observed in dioxolane solvent. In other words this particular behavior of WO 3 thin films is not related to the nature of the solvent. However, we shall show that erasing process is faster in H_O-dioxolane mixtures than in H_OPC mixtures. This point could be attributed2to the different properties of these two solvents.
0 3 7 8 - 5 9 6 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 02.75 © 1982 North-Holland
2. EXPERIMENTAL WO_ thin films are prepared as described elsewhere (i). The thickness varies from 2400 ~' to 6600 ~. Solutions of 2.5 M LiCIO. in dioxolane-water mixtures are used as e~ectrolytes. The solution containing 34 ppm of water was provided by "Salt Leclanch~" Company. From that solution, hydrated electrolytes are prepared with water content varying from 0.5 % to i0 % in weight. A P t plate is used as a counter electrode in a classical three electrodes potentiostatic cell. The reference electrode is Li in anhydrous (2.5 M) LiClOa-dioxolane electrolyte. The schematic diagram-of the cell as well as the optical and electrical measurement techniques used were previously described (1,3).
3. RESULTS AND DISCUSSION 3.1. Cyclic voltammetric study Typical voltammograms at indium-tin oxide (ITO)-WOR electrode in 2.5 M LiClO.-dioxolanewater erectrolytes ar~ shown in ~ig.l. The scan rate is 50 mV.s -I. C u r v e Q i s relative to an anhydrous electrolyte while c u r v e s O ~ a n d caracterize hydrated solutions. As previously mentioned (i) we shall assume that coloration of the WO 3 thin films is due to a simultaneous+ double +injection +of ~lectrons and + cations M (e.g. H , Li , Na , K ...) into WO 3 to form a blue bronze according to the follo ~ wing reversible reaction : xM + + xe-+ WO 3 transparent
~---- MxWO 3 blue
268
O. Bohnke et al. / Electrochromism in WO 3 thin films
curves of Fig. 2 are directly obtained from computations presented in a previous paper (3). It can be seen that the higher the potential, the faster the electrochromic reactions.
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Fig.l Cyclic voltammetric curves of ITO-WO_ o d (4000 A) electrode in 2.5M LiClO4-dioxolane electrol~tes containing 9ifferent amounts of water. Scan rate 50 mV.s- .
In Fig. 1 the cathodic current is associated with the WO 3 coloration and therefore with an insertion process into WO 3 while anodic current is associated with the WO_~ bleaching and with a deinsertion process from ~03. As observed in PC-water mixtures, no chan ge in the shape of the cathodic current curve is observed as water content increases in the solution. On the other hand, the form of the anodic current curve changes considerably and a large peak appears as water content increases. Moreover, a shouldering near + 2 V versus Li is observed on the anodic curves. As an interpretation of such a phenomenon we could suggest a double ionic injection of Li + and H + ions into WO^ during coloration. The fact that no inflexion ~ is observed on the cathodic side could be attributed to the high concentration of Li + ions in the electrolyte and/ or to the relatively high surface area of the electrode (~icmL). Experiments with less concentrated solutions are being carried out.
3.2. time
Variation
of
the optical
density versus
The variations of the optical density of WOq as a function of time is represented in Fi~. 2 durin E both the coloration and the bleachin E processes. The parameter used in this experiment is the potential applied to the electrode according to t h e voltammograms of FIE.1. Coloration is performed with applied potential varying from + 1.5 V to - 4 V versus Li reference electrode, and up to an optical density of WO 3 film of 0.46. Bleaching is performed with a potential varying from + 3.5 V to + 5.5. V versus Li. The bleached samples were previously colored up to an optical density of 0.30. The experimental
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Fig.2 Variation of o the optical density of W O _ thin films (4000 A) as a function of time during coloration and bleaching processes. V is the potential applied to the electrochromic electrode.
3.3. Coloration characteristics 3.3.1. Influence of the thickness, of the water content in the electrolyte and of the applied potential to ~he electrode on the optical density of the th~n film. Fi E . 3 shows the variation of the optical density versus the injected charge per square centimeter durin E coloration of WO_. The thickness of the film is the parameter. As observed in PC-water solvent, there is a saturation phenomenqn with a 2400 thick film. However for 5000 A and 6600 ~ thick films the variation is linear. The above results have been obtained with 0.5 % water in the electrolyte. The same behavior is observed with 1 % , 5 % and lO % water contents. The charge needed to2have an Optical density of 0.30 is 6-8 mC/cm depending on the film thickness. The optical colgration efficiency may be evaluated to 45 cm~/C. Fi E . 4 represents the variation of the optical density versus time. The overpotential applied to the electrochromic electrode as well as the water content of the electrolyte are used as the parameters. The overpotential n is defi ned as the difference between the applied po-
O. Bohnke et al. / Electrochromism in WO3 thin films
269
the water content, the faster the coloration kinetics. The variations of (O.D) are linear w i t h time. For exam21e, a variation of (O.D) of 0.30 for a d000 A thick film is obtained in 260 ms in 5 % water-dioxolane solvent instead of 640 ms in 0.5 % water-dioxolane solvent. I-
|
3.3.2. Optimization electrochromic cell.
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THICKN.: A ~
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(} iN , . C / a . ' Fig.3 Variation of the optical density of WO 3 electrode as a function of injected charge during coloration. Influence of the thickness.
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To optimize the coloration time of an electrochromic cell, we studied the influence of different parameters on the coloration in order to obtain an optical density of 0.30. Such a value Elves a sufficient contrast for a visualisation application point of view. Fig. 5,6 and 7 show the variation of coloration time for A(O.D)=0.30 versus the overpotential. Water content in the electrolyte is used as a p~rameter. These figu[es are rela tire to 2400 A, 4000 ~ and 5000 A thick films respectively. First, we can see that the higher the overpotential, the faster the response time. Second, for each set of curves and then for each film thickness there is an optimum in the water content in the electrolyte. For 2400 ~ thick film, the coloration time decreases from 0.5 % to 5 ~ ; however, for i0 % water content, the time increases dramatically. Moreover, for 0.5 % water it seems that the film saturation has been reached. For a 4000 ~ thick film, the same phenomenon is observed but the increase of the coloration time with i0 % water ~s much less important. Finally, for a 5000 A thick film, the coloration time decreases from 0.5 % to i0 % water con tent. This result is visualized in Fi E . 8 where coloration time is recorded as a function of % water content in the electrolyte at a gi-
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FIE.4 Variation of the optical density of WO 3 electrode (4000 ~) as a function of time. Influence of the overpotential and •L i C l O . 4- d i o xolane electrolyte hydration (solld shapes : 0,5 % H20 ; hollow shapes : 5 % H20)
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tential to the elm~trode (V) and the equili• l=u . brlum potential (E ) of the elgc~rochromlc electrode in the electrolyte. E x=u is commonly equal to + 3.5 V versus Li in the above considered electrolyte. As with PC-water solvents, the hiEher the overpotential and the higher
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Fig.5 Coloration time for A(O.D)=0.30 as a function of the overpotential applied to the WO_ electrode (2400 ~). Influence of
Li~lO4-dioxol~e electrolyte hydration.
O, Bohnke et al. / Electrochromism in WO3 thin films
270
stoichiometry of the film which may be different from a thickness to the other or to the water content in the film which may influence the electrochromic coloration time. The influence of the thickness parameter is shown in Fig. 9. In 5 % water, for example, the shortest coloration time is obtained with a 4000 thick film. This behavior has also been obser-
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yen overpotential (n =-3V) for different film thicknesses. The maximum of each curve is function of both the water content and the thickness of the film. The thicker the film, the higher the % water content to reach the maximum and the shorter the coloration time. Such a behavior may be attributed either to the
' -i ' -i ' -i OVERPOTENTIAL IN VOLTS
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Fig.9 Coloration time for A(O.D)=0.30 as a function of the overpotential a p p l i e d to the WO_ electrode. Influence of WO_ thickness. (hydration=5 % water in LiClO4-dioxolane electrolyte)
O. Bohnke et aL / Electrochromism in WO3 thin jWms
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water and dioxolane are 78.5 and 6.9 respectively at 25°C. So the water-dioxolane mixtures will have strongly different dissociation properties from those of the pure Solvents. On the other hand, the dielectric constants of water and PC (¢_^=64.4 at 25°C) are very high • . F~ and very slmllar. I n that later case, the water-PC mixtures will have dissociation properties very similar to those of the pure solvents. More over, dioxolane is a stronger solvating solvent than PC. Finally it has also a strong basic character and then water is more acid in this solvent than in PC. Since H + ions might play an important role in the electrochromic process through a double ionic insertion phenomenon into WO_, this property may be of considerable imporfance on the rate of coloration. 3.4. Bleaching characteristics 3.4.1. Optimization electrochromic cell.
Fig. lO Coloration time for a function of water content lene carbonate electrolyte. overpotential applied to WO 3
W
A(O.D)=0.30 as in LiClO4-propyInfluence of the (4000 ~).
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To optimize the bleaching rate, we studied the decoloration of a film colored up to an optical density of 0.30. The reversibility of the coloration process is observed only when water is present in the solvent as in the case of PC (1). As for coloration, a lot of parameters influence the bleaching kinetics. The higher the applied potential, the thinner the film and the higher the water content, the shorter the bleaching time. Fig. 12, 13 and 14 show the variation of the bleaching time as a function of the applied potential to the electrode with % water content as a parameter for X, and thick film .respectively. We observe the same behavior as for the coloration process in regard to thickness and water content parameters. For each thick-
2400
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Fig.ll Coloration time for A(O.D)=0.30 as a function of water content in LiClO4-dioxolane electrolyte. Influence of the overpotential applied to WO 3 (4000 ~).
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i ved in PC solvent (i). However, the influence of water seems to be more important in PC than in dioxolane as shown in Fig. i0 and Fig. ii, for comparison. This might be due to the different solvent'properties of dioxolane and PC and therefore of dioxolane-watsr and PC-water mixtures. Indeed, the dielectric constants of
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APPEItO POTI[NTIAL VS. ti (V) Fig.12 Erasing time for A(O.D)=0.30 as a function of t h e potential applied to W O ~ e l e c trode (2400 ~). Influence of the "electrolyte hydration.
O. Bohnke et al.
272
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Electrochromism i . WO3 thin films
N,O 3.4.2. Comparative study of the bleaching rate in PC, dioxolane, and H2SO 4 solutions
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Fig. 16 shows the result of a comparative study. The recorded bleaching times are relative to a colored film of 4000 ~ thickness and previously colored up to an optical density of
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Fig.13 Erasing time for A(O.D)=0.30 as a function of the ° potential applied to W O s electrode (4000 A). Influence of the elec%rolyte hydration.
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WATER CONTENT
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• II% Fig.15 Erasing time for A(O.D)=0.30 as a function of water content in LiClO.-dioxolane 4 electrolyte. Influence of WO~ thickness. (V is the potential applied to ~he electrochromic electrode during erasing).
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8
L~lO, r*+su - DC0(+
+.. +4 +5 +1 APPLIED POTENTIALVS. Li (VJ Fig.14 Erasing time for A(O.D)=0.30 as a function of t h e p o t e n t i a l applied to WO n electrode (5000 ~). Influence of the electrolyte hydration.
ness, a maximum is observed in the decolo ration curves, as shown in Fig. 15. Decoloration is then performed at a potential of +5.5V versus Li which is the maximum potential to avoid any secondary oxidation reaction. As for coloration, the fastest bleaching time is obtained with 5 % water content in the electrolyte for both 2400 ~ and 4009 ~ thick films and with i0 % water f ~ a 5000 A thick film.
i I ) )
-
S
._
Hi SO4
@
m
WATER CONTENT Fig.16 Erasing tion of water lene carbonate electrolytes. also mentioned
time for A(O.D)=0.30 as a funccontent in (0) IM LiClO4-propyand (@) 2.5M LiClO.-dfoxolane Erasing time in H2~O 4 (N) is (---).
O. Bohnke et al. /Eiectrochromism in WO3 thin films
0.46. The applied voltage for bleaching is identical in the three experiments. We can observe that the times are shorter in dioxolane than in PC and even shorter than in sulfuric acid. The double ionic injection of Li + and H + ions into WO_ seems to enhance the kinetics of bleaching of~the thin film. First, the Li + ion injection from an anhydrous electrolyte (either dioxolane with 34 ppm of water or PC with 50 ppm of water) is irreversible. Second the H + ion injection from H SO4 electrolyte • 2 yields a poorer reversibllity o f the process than in a mixture of solvents. 3.5. Performances of an electrochromic cell in (2.5. M) LiCIO 4 dioxolane-water electrolytes The best experimental results obtained are the following : for a 4000 A thick film, with 5 % water content in the electrolyt e , coloration time is performed in 260 ms for an optical density variation of 0.30. Th~ corresponding injected charge is 7.5 mC/cm-. Bleachin~ time is performed in 500 ms. For a 5000 A thick film, in lO % water content, coloration time of 240 ms is obtained, t~e corresponding injected charge is 6.5 mC/cm~ and the blea ching time is 450 ms. These results are obtained by stopping the electrical pulse as soon as an optical density of 0.30 is reached. However, these coloration times can be decreased since we observe that coloration continues a short time after the removal of the electrical pulse. Therefore with a 4000 ~ thick film and with the same experimental conditions as mentioned above, we can achieve an optical density of 0.31 in 490 ms with a 200 ms pulse. T~e correspondin~ injected charge is 6.5 mC/cm-. For a 5000 A thick film in i0 % water, we can reach an optical density of 0.34 in 270 ms with a 200 ms pulse. The injected charge is then 6 mC/cm 2. This observation can lead either to decrease the consumption of energy or to achieve a better contrast.
4. CONCLUSION The experimental results have shown that in dioxolane solvent, the water content in the electrolyte is of considerable importance for both coloration and bleaching kinetics. In an electrolyte made of (2.5M) LiCIO. and 34 ppm of water i n dloxolane, coloration of WO_ thin 3 film is irreversible as previously observed in propylene carbonate solvent. As soon as water is added to the solvent, the process of coloration becomes reversible and the higher the water content, the faster the response time of the electrochromic cell. If for coloration, the same time is obtained in dioxolane as in propylene carbonate (240 ms), decoloration is faster in dioxolane than in PC (450 ms instead of 750 ms). This longer time i n P C is not caused by the Pt counter electrode since the use of a carbon counter electrode gives the same result.
273
5. ACKNOWLEDGMENTS Thanks are due to "SAFT-LECLANCHE" for providing the anhydrous LiClO.-dioxolane electrolyte. We are grateful to MiChel GUIGNARD, Laboratoire d'Optique BesanQon for technical assistance.
6. REFERENCES (i) O. Bohnke, C. Bohnke, B. Carquille, G. Robert (and references herein) (to be published). Solid State Ionics (2) L.P. Klemann, G.H. Newman, J. Electrochem Soc. 128 1 (1981) 13-18. (3) M. Guignard, B. Carquille, C. Bohnke, O. Bohnke (to be published). Displays Technology and Applications.
ELECTROCHROMISM IN WO 3 THIN FILMS. II. LiCIO4-DIOXOLANE-WATER ELECTROLYTES O. BOHNKE, C. BOHNKE, G. ROBERT Laboratoire d'Electrochimle des Solides ERA 810, Faeult( des Sciences et des Techniques, 25030 Besanfon Cedex, France
and B. CARQUILLE Laboratoire d'Optique LA 214, Facult6 des Sciences et des Techniques, 25030 Besan¢on Cedex, France
Received 22 December 1981 The performances of electrochromic cells with evaporated amorphous WO 3 thin film as electrochromic material in (2.5 M) LiClO4-dioxolane-water electrolytes are presented. A comparison with the results previously obtained in propylene carbonate instead of dioxolane solvent is carried out. The influence of the following parameters has been studied : the thickness of the film, the water content in the electrolyte, the potential applied to the electrochromic electrode during coloration and bleaching processes.
i. INTRODUCTION In a previous publication (i) the characteristics of electrochromic WO_ thin films in (i M) LiClO4-propylene carbonate-water mixture electrolytes were reported. The experimental results have clearly shown that water content in the electrolyte was of considerable importance for coloration and bleaching kinetics of evaporated WO_ thin films. In our search for other organic solvents, dloxolane was selected for several reasons. First, it has been used as solvent in rechargeable lithium-TiSp cells because of its high kinetic stability %owards alkali metals (2) moreover the problems encountered in batteries are very similar to those encountered in display devices with lithium electrolytes. Second, dioxolane is able to make chemical stable mixtures with water. Finally, since its physical properties are strongly different from those of propylene carbonate (PC), the properties of water dioxolane mixtures will be different from those of water-PC mixtures. Under these conditions, the characteristics of the electrochromic electrode in (2.5M) LiClO.-dioxolane-water mixture 4 electrolytes are presented in this paper. A comparison will be carried out between electrochrom~sm in PC and in dioxolane. We show that the influence of water contained in the electrolyte as previously observed in PC solvent on the electrochromism phenomenon is also observed in dioxolane solvent. In other words this particular behavior of WO 3 thin films is not related to the nature of the solvent. However, we shall show that erasing process is faster in H_O-dioxolane mixtures than in H_OPC mixtures. This point could be attributed2to the different properties of these two solvents.
0 3 7 8 - 5 9 6 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 02.75 © 1982 North-Holland
2. EXPERIMENTAL WO_ thin films are prepared as described elsewhere (i). The thickness varies from 2400 ~' to 6600 ~. Solutions of 2.5 M LiCIO. in dioxolane-water mixtures are used as e~ectrolytes. The solution containing 34 ppm of water was provided by "Salt Leclanch~" Company. From that solution, hydrated electrolytes are prepared with water content varying from 0.5 % to i0 % in weight. A P t plate is used as a counter electrode in a classical three electrodes potentiostatic cell. The reference electrode is Li in anhydrous (2.5 M) LiClOa-dioxolane electrolyte. The schematic diagram-of the cell as well as the optical and electrical measurement techniques used were previously described (1,3).
3. RESULTS AND DISCUSSION 3.1. Cyclic voltammetric study Typical voltammograms at indium-tin oxide (ITO)-WOR electrode in 2.5 M LiClO.-dioxolanewater erectrolytes ar~ shown in ~ig.l. The scan rate is 50 mV.s -I. C u r v e Q i s relative to an anhydrous electrolyte while c u r v e s O ~ a n d caracterize hydrated solutions. As previously mentioned (i) we shall assume that coloration of the WO 3 thin films is due to a simultaneous+ double +injection +of ~lectrons and + cations M (e.g. H , Li , Na , K ...) into WO 3 to form a blue bronze according to the follo ~ wing reversible reaction : xM + + xe-+ WO 3 transparent
~---- MxWO 3 blue
268
O. Bohnke et al. / Electrochromism in WO 3 thin films
curves of Fig. 2 are directly obtained from computations presented in a previous paper (3). It can be seen that the higher the potential, the faster the electrochromic reactions.
J mA/¢m I
t !
.
0••
CO~,OaATION V i I . I WLI I U 0
-U -I -I
141
Illl
Fig.l Cyclic voltammetric curves of ITO-WO_ o d (4000 A) electrode in 2.5M LiClO4-dioxolane electrol~tes containing 9ifferent amounts of water. Scan rate 50 mV.s- .
In Fig. 1 the cathodic current is associated with the WO 3 coloration and therefore with an insertion process into WO 3 while anodic current is associated with the WO_~ bleaching and with a deinsertion process from ~03. As observed in PC-water mixtures, no chan ge in the shape of the cathodic current curve is observed as water content increases in the solution. On the other hand, the form of the anodic current curve changes considerably and a large peak appears as water content increases. Moreover, a shouldering near + 2 V versus Li is observed on the anodic curves. As an interpretation of such a phenomenon we could suggest a double ionic injection of Li + and H + ions into WO^ during coloration. The fact that no inflexion ~ is observed on the cathodic side could be attributed to the high concentration of Li + ions in the electrolyte and/ or to the relatively high surface area of the electrode (~icmL). Experiments with less concentrated solutions are being carried out.
3.2. time
Variation
of
the optical
density versus
The variations of the optical density of WOq as a function of time is represented in Fi~. 2 durin E both the coloration and the bleachin E processes. The parameter used in this experiment is the potential applied to the electrode according to t h e voltammograms of FIE.1. Coloration is performed with applied potential varying from + 1.5 V to - 4 V versus Li reference electrode, and up to an optical density of WO 3 film of 0.46. Bleaching is performed with a potential varying from + 3.5 V to + 5.5. V versus Li. The bleached samples were previously colored up to an optical density of 0.30. The experimental
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Fig.2 Variation of o the optical density of W O _ thin films (4000 A) as a function of time during coloration and bleaching processes. V is the potential applied to the electrochromic electrode.
3.3. Coloration characteristics 3.3.1. Influence of the thickness, of the water content in the electrolyte and of the applied potential to ~he electrode on the optical density of the th~n film. Fi E . 3 shows the variation of the optical density versus the injected charge per square centimeter durin E coloration of WO_. The thickness of the film is the parameter. As observed in PC-water solvent, there is a saturation phenomenqn with a 2400 thick film. However for 5000 A and 6600 ~ thick films the variation is linear. The above results have been obtained with 0.5 % water in the electrolyte. The same behavior is observed with 1 % , 5 % and lO % water contents. The charge needed to2have an Optical density of 0.30 is 6-8 mC/cm depending on the film thickness. The optical colgration efficiency may be evaluated to 45 cm~/C. Fi E . 4 represents the variation of the optical density versus time. The overpotential applied to the electrochromic electrode as well as the water content of the electrolyte are used as the parameters. The overpotential n is defi ned as the difference between the applied po-
O. Bohnke et al. / Electrochromism in WO3 thin films
269
the water content, the faster the coloration kinetics. The variations of (O.D) are linear w i t h time. For exam21e, a variation of (O.D) of 0.30 for a d000 A thick film is obtained in 260 ms in 5 % water-dioxolane solvent instead of 640 ms in 0.5 % water-dioxolane solvent. I-
|
3.3.2. Optimization electrochromic cell.
<]
/
THICKN.: A ~
A
A ~ A
(} iN , . C / a . ' Fig.3 Variation of the optical density of WO 3 electrode as a function of injected charge during coloration. Influence of the thickness.
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~) .2~'/!
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the
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To optimize the coloration time of an electrochromic cell, we studied the influence of different parameters on the coloration in order to obtain an optical density of 0.30. Such a value Elves a sufficient contrast for a visualisation application point of view. Fig. 5,6 and 7 show the variation of coloration time for A(O.D)=0.30 versus the overpotential. Water content in the electrolyte is used as a p~rameter. These figu[es are rela tire to 2400 A, 4000 ~ and 5000 A thick films respectively. First, we can see that the higher the overpotential, the faster the response time. Second, for each set of curves and then for each film thickness there is an optimum in the water content in the electrolyte. For 2400 ~ thick film, the coloration time decreases from 0.5 % to 5 ~ ; however, for i0 % water content, the time increases dramatically. Moreover, for 0.5 % water it seems that the film saturation has been reached. For a 4000 ~ thick film, the same phenomenon is observed but the increase of the coloration time with i0 % water ~s much less important. Finally, for a 5000 A thick film, the coloration time decreases from 0.5 % to i0 % water con tent. This result is visualized in Fi E . 8 where coloration time is recorded as a function of % water content in the electrolyte at a gi-
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FIE.4 Variation of the optical density of WO 3 electrode (4000 ~) as a function of time. Influence of the overpotential and •L i C l O . 4- d i o xolane electrolyte hydration (solld shapes : 0,5 % H20 ; hollow shapes : 5 % H20)
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tential to the elm~trode (V) and the equili• l=u . brlum potential (E ) of the elgc~rochromlc electrode in the electrolyte. E x=u is commonly equal to + 3.5 V versus Li in the above considered electrolyte. As with PC-water solvents, the hiEher the overpotential and the higher
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Fig.5 Coloration time for A(O.D)=0.30 as a function of the overpotential applied to the WO_ electrode (2400 ~). Influence of
Li~lO4-dioxol~e electrolyte hydration.
O, Bohnke et al. / Electrochromism in WO3 thin films
270
stoichiometry of the film which may be different from a thickness to the other or to the water content in the film which may influence the electrochromic coloration time. The influence of the thickness parameter is shown in Fig. 9. In 5 % water, for example, the shortest coloration time is obtained with a 4000 thick film. This behavior has also been obser-
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Fig.6 Coloration time for A(O.D)=0.30 as a function of the over~otential applied to the WOR electrode (4000 A). Influence of LiCIO.diSxolane electrolyte hydration.
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Fig.8 Coloration time for A(O.D)=0.30 a s a function of water content in LiClO_-dioxolane electrolyte. Influence of WO 3 4 thickness (n=-3V)
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Fig.7 Coloration time for A(O.D)=0.30 as a function of the over~otential applied to the WO. electrode (5000 A). Influence of LiCIO.dloxolane electrolyte hydratlon. . J
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yen overpotential (n =-3V) for different film thicknesses. The maximum of each curve is function of both the water content and the thickness of the film. The thicker the film, the higher the % water content to reach the maximum and the shorter the coloration time. Such a behavior may be attributed either to the
' -i ' -i ' -i OVERPOTENTIAL IN VOLTS
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Fig.9 Coloration time for A(O.D)=0.30 as a function of the overpotential a p p l i e d to the WO_ electrode. Influence of WO_ thickness. (hydration=5 % water in LiClO4-dioxolane electrolyte)
O. Bohnke et aL / Electrochromism in WO3 thin jWms
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271
water and dioxolane are 78.5 and 6.9 respectively at 25°C. So the water-dioxolane mixtures will have strongly different dissociation properties from those of the pure Solvents. On the other hand, the dielectric constants of water and PC (¢_^=64.4 at 25°C) are very high • . F~ and very slmllar. I n that later case, the water-PC mixtures will have dissociation properties very similar to those of the pure solvents. More over, dioxolane is a stronger solvating solvent than PC. Finally it has also a strong basic character and then water is more acid in this solvent than in PC. Since H + ions might play an important role in the electrochromic process through a double ionic insertion phenomenon into WO_, this property may be of considerable imporfance on the rate of coloration. 3.4. Bleaching characteristics 3.4.1. Optimization electrochromic cell.
Fig. lO Coloration time for a function of water content lene carbonate electrolyte. overpotential applied to WO 3
W
A(O.D)=0.30 as in LiClO4-propyInfluence of the (4000 ~).
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bleaching
of
an
To optimize the bleaching rate, we studied the decoloration of a film colored up to an optical density of 0.30. The reversibility of the coloration process is observed only when water is present in the solvent as in the case of PC (1). As for coloration, a lot of parameters influence the bleaching kinetics. The higher the applied potential, the thinner the film and the higher the water content, the shorter the bleaching time. Fig. 12, 13 and 14 show the variation of the bleaching time as a function of the applied potential to the electrode with % water content as a parameter for X, and thick film .respectively. We observe the same behavior as for the coloration process in regard to thickness and water content parameters. For each thick-
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Fig.ll Coloration time for A(O.D)=0.30 as a function of water content in LiClO4-dioxolane electrolyte. Influence of the overpotential applied to WO 3 (4000 ~).
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i ved in PC solvent (i). However, the influence of water seems to be more important in PC than in dioxolane as shown in Fig. i0 and Fig. ii, for comparison. This might be due to the different solvent'properties of dioxolane and PC and therefore of dioxolane-watsr and PC-water mixtures. Indeed, the dielectric constants of
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APPEItO POTI[NTIAL VS. ti (V) Fig.12 Erasing time for A(O.D)=0.30 as a function of t h e potential applied to W O ~ e l e c trode (2400 ~). Influence of the "electrolyte hydration.
O. Bohnke et al.
272
Li¢lO,
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Electrochromism i . WO3 thin films
N,O 3.4.2. Comparative study of the bleaching rate in PC, dioxolane, and H2SO 4 solutions
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Fig. 16 shows the result of a comparative study. The recorded bleaching times are relative to a colored film of 4000 ~ thickness and previously colored up to an optical density of
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Fig.13 Erasing time for A(O.D)=0.30 as a function of the ° potential applied to W O s electrode (4000 A). Influence of the elec%rolyte hydration.
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WATER CONTENT
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• II% Fig.15 Erasing time for A(O.D)=0.30 as a function of water content in LiClO.-dioxolane 4 electrolyte. Influence of WO~ thickness. (V is the potential applied to ~he electrochromic electrode during erasing).
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+.. +4 +5 +1 APPLIED POTENTIALVS. Li (VJ Fig.14 Erasing time for A(O.D)=0.30 as a function of t h e p o t e n t i a l applied to WO n electrode (5000 ~). Influence of the electrolyte hydration.
ness, a maximum is observed in the decolo ration curves, as shown in Fig. 15. Decoloration is then performed at a potential of +5.5V versus Li which is the maximum potential to avoid any secondary oxidation reaction. As for coloration, the fastest bleaching time is obtained with 5 % water content in the electrolyte for both 2400 ~ and 4009 ~ thick films and with i0 % water f ~ a 5000 A thick film.
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WATER CONTENT Fig.16 Erasing tion of water lene carbonate electrolytes. also mentioned
time for A(O.D)=0.30 as a funccontent in (0) IM LiClO4-propyand (@) 2.5M LiClO.-dfoxolane Erasing time in H2~O 4 (N) is (---).
O. Bohnke et al. /Eiectrochromism in WO3 thin films
0.46. The applied voltage for bleaching is identical in the three experiments. We can observe that the times are shorter in dioxolane than in PC and even shorter than in sulfuric acid. The double ionic injection of Li + and H + ions into WO_ seems to enhance the kinetics of bleaching of~the thin film. First, the Li + ion injection from an anhydrous electrolyte (either dioxolane with 34 ppm of water or PC with 50 ppm of water) is irreversible. Second the H + ion injection from H SO4 electrolyte • 2 yields a poorer reversibllity o f the process than in a mixture of solvents. 3.5. Performances of an electrochromic cell in (2.5. M) LiCIO 4 dioxolane-water electrolytes The best experimental results obtained are the following : for a 4000 A thick film, with 5 % water content in the electrolyt e , coloration time is performed in 260 ms for an optical density variation of 0.30. Th~ corresponding injected charge is 7.5 mC/cm-. Bleachin~ time is performed in 500 ms. For a 5000 A thick film, in lO % water content, coloration time of 240 ms is obtained, t~e corresponding injected charge is 6.5 mC/cm~ and the blea ching time is 450 ms. These results are obtained by stopping the electrical pulse as soon as an optical density of 0.30 is reached. However, these coloration times can be decreased since we observe that coloration continues a short time after the removal of the electrical pulse. Therefore with a 4000 ~ thick film and with the same experimental conditions as mentioned above, we can achieve an optical density of 0.31 in 490 ms with a 200 ms pulse. T~e correspondin~ injected charge is 6.5 mC/cm-. For a 5000 A thick film in i0 % water, we can reach an optical density of 0.34 in 270 ms with a 200 ms pulse. The injected charge is then 6 mC/cm 2. This observation can lead either to decrease the consumption of energy or to achieve a better contrast.
4. CONCLUSION The experimental results have shown that in dioxolane solvent, the water content in the electrolyte is of considerable importance for both coloration and bleaching kinetics. In an electrolyte made of (2.5M) LiCIO. and 34 ppm of water i n dloxolane, coloration of WO_ thin 3 film is irreversible as previously observed in propylene carbonate solvent. As soon as water is added to the solvent, the process of coloration becomes reversible and the higher the water content, the faster the response time of the electrochromic cell. If for coloration, the same time is obtained in dioxolane as in propylene carbonate (240 ms), decoloration is faster in dioxolane than in PC (450 ms instead of 750 ms). This longer time i n P C is not caused by the Pt counter electrode since the use of a carbon counter electrode gives the same result.
273
5. ACKNOWLEDGMENTS Thanks are due to "SAFT-LECLANCHE" for providing the anhydrous LiClO.-dioxolane electrolyte. We are grateful to MiChel GUIGNARD, Laboratoire d'Optique BesanQon for technical assistance.
6. REFERENCES (i) O. Bohnke, C. Bohnke, B. Carquille, G. Robert (and references herein) (to be published). Solid State Ionics (2) L.P. Klemann, G.H. Newman, J. Electrochem Soc. 128 1 (1981) 13-18. (3) M. Guignard, B. Carquille, C. Bohnke, O. Bohnke (to be published). Displays Technology and Applications.