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zirconium oxides as thin film counter electrodes for all solid state electrochromic transmissive devices
Electrochimica Acta 44 (1999) 3137±3147
Mixed cerium/titanium and cerium/zirconium oxides as thin ®lm counter electrodes for all solid state electrochromic transmissive devices Guglielmo Macrelli, Elisabetta Poli* Isoclima SpA ± R&D Department via A. Volta, 14, 35042 Este (PD), Italy Received 7 September 1998; received in revised form 3 November 1998
Abstract Optically passive thin ®lms of mixed cerium/titanium and cerium/zirconium oxides have been prepared by electron beam reactive evaporation. Dierent oxidation levels have been achieved using dierent oxygen ¯ows in the deposition process. The samples have been optically and electrochemically characterized. Performances are discussed in view of a future utilization in a electrochromic device as thin ®lm counter electrode. Dierent materials have been tested to identify the best solution to be used as an optically passive ion storage layer in an existing electrochromic device in alternative to an optically active V2O5 counter electrode. Process conditions and materials performances are reported, related to each other and discussed. Development studies are still in progress towards the optimization of the device and its future scaling up. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Electrochromism; Counter electrode; Mixed oxides; Thin ®lm; e-beam evaporation
1. Introduction This paper reports preliminary results of a research work performed to develop optically passive mixed oxide counter electrode for electrochromic devices. As previously reported [1] we developed a medium area (300 by 300 mm2) electrochromic device based on an all solid state inorganic thin ®lm multilayer structure. In that solution the counter electrode was a thin ®lm of Vanadium pentoxide deposited by plasma assisted electron beam evaporation. There were at least two main drawbacks to overcome: . The counter electrode was an optically active cathodic material that reduced the optical modulation window,
* Corresponding author.
. The counter electrode was a thin ®lm deposited by plasma assistance to achieve the correct stoichiometry: this complicated greatly the deposition technology. Recently, studies have been reported [2] in which stoichiometrically correct vanadium pentoxide was deposited without plasma assistance, but, even in this case, the optical activity of Vanadium pentoxide is still a problem. These motivations together with the increasingly interest in mixed oxide thin ®lm [3±7] were the background to start this research work. We focused our attention to cerium/titanium and cerium/ zirconium mixed oxides materials. The deposition method we have used is electron beam vacuum evaporation; we ®xed the ratio of the starting oxides and we changed the oxygen partial pressure at two dierent levels.
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 0 3 1 - 6
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G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
Table 1 Process parameters and coatings characteristics Material
WO3
CeO2/TiO2
CeO2/TiO2+O2
CeO2/ZrO2
CeO2/ZrO2+O2
Process parameters Final pressure (mbar) O2 pressure P1 (mbar) Rate (nm/s) Source weigh ratio Source molar ratio
2 10ÿ5 4 10ÿ5 30 ± ±
2 10ÿ5 0 15 50%/50% 31.7%/68.3%
2 10ÿ5 5 10ÿ5 15 50%/50% 31.7%/68.3%
2.5 10ÿ5 0 15 50%/50% 41.7%/58.3%
2.8 10ÿ5 5 10ÿ5 15 50%/50% 41.7%/58.3%
Physical characteristics Thickness (nm) Density (g/cm3)
300 6.0
200 5.7
200 5.7
200 6.4
200 6.4
Compositional and structural studies have not been performed at this preliminary level but, for sure, they will be of paramount importance in any further development. At ®rst we decided to concentrate our attention to determinate electrochemical kinetic and thermodynamic intercalation characteristics of the materials. The electrochemical characterization methods we have used, re¯ect the design concept of our electrochromic system [1]: it is a charge balanced system and the coloring/bleaching potentials are the equilibrium ones resulting from the dierence of the corresponding coulometric titration curves. 2. Thin ®lm preparation The mixed oxide thin ®lms were deposited using a Baltzers BA 510 evaporator. This machine is equipped with a thermal source and a multipocket (4 pocket) electron beam evaporation source. During deposition careful gas inlet control system allows reactive evaporation, the machine is also equipped with a quartz microbalance monitor for thickness and deposition rate control. The quartz monitor feed back the power supply of the electron beam source to achieve a careful PID control at ®xed deposition rate. Indium tin oxide thin ®lm were previously deposited on cleaned optical glass substrates, the coating was about 200 nm thick and had a sheet resistance ranging from 8 to 9 Ohm square. The ITO coated glass substrate were then coated with the following dierent materials: tungsten trioxide, cerium oxide/titanium oxide, cerium oxide/zirconium oxide. Source materials (from Plasmaterials with purity higher than 99.5%) in powder form were mixed in 50% by weight and placed in a molybdenum crucible and than evaporated by electron beam gun. Before deposition a suitable soak heating phase of the source material was performed under a shutter.
The mixed oxide coatings were deposited in two dierent conditions: no oxygen gas ¯ow, oxygen gas ¯ow introduced during deposition (reactive evaporation); in this way we had at our disposal 5 dierent types of half cells. The deposition conditions of the dierent coatings are reported in Table 1. Glass substrates were carefully cleaned in an ultrasonic batch, visual inspection at grazing incidence under a bright light was performed for each substrate before deposition. The vacuum evaporation system was pumped down to 1 10ÿ5 mbar, the chamber walls were heated during the pumpdown and than cooled during the deposition. In case of reactive evaporation, oxygen was introduced from an external bottle (oxygen purity 99.999%) and its partial pressure inside the vacuum chamber was regulated and kept constant to 5 10ÿ5 mbar. 3. Experimental methods Physical thickness of the layers were measured using an Alpha Step 200 Stylus pro®lometer, the density of the layers was directly calculated from the measured physical thickness and the weight dierence of a controlled area of the substrate determined by a Sartorious analytical balance with a 0.00001 g sensitivity. Electrochemical experiments were performed using a potentiostat/galvanostat (EG&G Princeton Applied Research model 363) suitably controlled by an interface and data acquisition board (National Instruments Assy 180950±01 Rev. D), connected to a multifunction I/O board (National Instruments AT-MIO-16). The electrochemical tests were driven by a computer program in LabWindows environment. The electrochemical cell was con®gured with three electrodes: a working electrode, a counter electrode and a reference electrode. There were performed two types of electrochemical experiments: galvanostatic intermittent titration (GITT) and voltage step response (VSR). In the ®rst
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
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Fig. 1. GITT results for not oxidized CeTiOy: (a) coulometric titration curve, (b) chemical diusion coecient.
case the working electrode consisted in the deposited thin ®lm of mixed oxides on the ITO coated glass substrate, reference and counter electrode consisted in a Lithium metal wire in contact with an electrolyte solution 1 M LiClO4/propylene carbonate (PC). In the second case (VSR) the counter electrode consisted in
the deposited thin ®lm of mixed oxides on the ITO coated glass substrates and the working electrode was a tungsten trioxide thin ®lm deposited on ITO coated glass substrates. The two electrodes were interfaced by a contact with the same electrolyte solution used in the GITT experiments.
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Fig. 2. GITT results for oxidized CeTiOy: (a) coulometric titration curve, (b) chemical diusion coecient.
Galvanostatic intermittent titration technique (GITT) was introduced in Ref. [8] to measure thermodynamic and kinetic characteristics of mixed ionic/electronic conductors. The main advantages of GITT are: negligible polarization of the electrode and no current transients resulting from charge accumulation at the electrolyte/electrode interface; GITT tests results are
evaluated considering the following assumptions: constant molar volume of the electrode during intercalation/de-intercalation and diusion characteristic length small compared to the characteristic dimension (physical thickness) of the layer. We used GITT to measure the chemical diusion coecient of Lithium ions during intercalation/de-intercalation processes.
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
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Fig. 3. GITT results for oxidized CeZrOy: (a) coulometric titration curve, (b) chemical diusion coecient.
Chemical diusion coecient, D , is de®ned as the proportionality factor between ions ¯ux Ji and ion concentration gradient (Hci) according to:
ion ¯ux to the total electrochemical potential gradient HZi):
Ji ÿD rci
Ji ÿ
1
and it is related to the ion diusivity D, introduced in the most general Nerst±Planck equation (connecting
ci D rZ kT i
by the Wagner factor W according to [8,9]:
2
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Fig. 4. Voltage step response results for not oxidized CeTiOx: (a) current density vs. time, (b) optical transmittance response. Bleaching voltage 1.7 V, coloring voltage ÿ1.7 V.
D D W:
3
We used GITT also to determinate the coulometric titration curve which is the equilibrium open circuit voltage OCV of the working electrode (towards the Li/ Li+ reference) against the concentration of the interca-
lated ions. Details of the test method can be found in the literature [8±10]. In the VSR test a voltage step between two potential values was ®xed between the WE and CE for a certain ®xed duration and the current ¯owing in the cell was recorded against time. Potentials were chosen accord-
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
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Fig. 5. Voltage step response results for oxidized CeTiOx: (a) current density vs. time, (b) optical transmittance response. Bleaching voltage 1.7 V, coloring voltage ÿ1.7 V.
ing to the coulometric titration curves of the electrodes to have lithium ions insertion in the WE and than in the CE. With this technique the current density (electric current divided by the surface of the electrode/electrolyte interface) can be determined as a time function and it can be integrated to get the surface density of charge Q (mC/cm2).
Before the VSR tests Lithium ions were previously electrochemically intercalated in the working electrode (WE±WO3) up to a surface density of charge of 12 mC/cm2; VSR tests were then performed with a ®rst bleaching phase (Li+ ions moving from WE to CE) and a second coloring phase (Li+ ions moving from CE to WE).
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Fig. 6. Voltage step response results for not oxidized CeZrOx: (a) current density vs. time, (b) optical transmittance response. Bleaching voltage 1.7 V, coloring voltage ÿ1.7 V.
In both the electrochemical test con®gurations integrated light transmittance through the cell was measured and recorded using an optical bench consisting of a collimated light source (illuminant C) and a silicon photodetector with a ®lter for photopic vision.
4. Experimental results The as deposited thin ®lm were submitted to GITT. In these tests all the potentials were measured against Li/Li+ in the range between 1.0 and 4.0 V and the
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
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Fig. 7. Voltage step response results for oxidized CeZrOx: (a) current density vs. time, (b) optical transmittance response. Bleaching voltage 1.7 V, coloring voltage ÿ1.7 V.
measurements were carried out at a current density of 0.1 mA/cm2. The intercalation properties of cerium/zirconium mixed oxide thin ®lms were dierent in respect to the de-intercalation properties: it was possible to introduce lithium ions but their extraction was very incomplete (de-intercalation potentials exceeded the 4 V upper
limit), in other words the cerium/zirconium ®lms deposited in reactive way (with O2 ¯ow) and the ones deposited without oxygen ¯ow were both not fully reversible. The cerium/titanium mixed oxide thin ®lms exhibited dierent behavior according to the deposition conditions. The ®lms obtained in reactive way were com-
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G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
pletely reversible in intercalation and de-intercalation, reversibility was not so good for the ®lms deposited without oxygen. During the galvanostatic tests, the optical response of all the materials was completely passive, changes in the optical transmittance were well below the sensitivity of the measuring system (0.5%). Coulometric titration curves and chemical diusion coecients are reported in Figs. 1±3 for the as deposited thin ®lms, results are not reported for the cerium/ zirconium thin ®lm deposited without oxygen ¯ow because of its strong irreversible behavior. Reversibility properties of the oxidized and not oxidized cerium/zirconium and of the cerium/titanium non oxidized thin ®lm were improved after a certain number of charging and discharging cycles. Voltage step response results (current density vs. time and normalized visible transmittance vs. time) are reported in Figs. 4±7 for all the materials under examination after at least 10 charge/discharge cycles. Tests performed with the oxidized cerium/titanium and cerium/zirconium exhibited good charging/discharging and optical balancing properties in the coloring and bleaching phases; on the contrary for not oxidized counter electrodes charge extraction and coloration was not balanced to charge injection and bleaching levels. Balanced conditions for charge and coloration levels can be achieved for the not oxidized cerium/titanium counter electrode enlarging the coloring phase (Li+ ions extracted from CE and intercalated in WO3) from the original 200 s to 500 s. 5. Discussion Deposition conditions (oxygen partial pressure) strongly in¯uence the intercalation properties of the electrode mixed oxide materials considered here. Reversibility properties in intercalation and de-intercalation processes in galvanostatic and potentiostatic conditions are mainly in¯uenced by the oxidation level of the thin ®lms during deposition. The most probable causes that can lead to reversibility problems can be addressed to charge trapping eects and dierent kinetic behavior of the materials to charge intercalation/ de-intercalation. Charge trapping eects are not new for electrochromic materials [11,12] and are mainly related to material non-stoichiometry [11]. Improvements in reversibility as material is cycled is, generally, a good indication of charge trapping eects. This is true, in our case, for both the not oxidized materials and for the oxidized cerium/zirconium thin ®lm. Charge trapping can be generated by non stoichiometry through grain boundary activity [11] or can be simply a structural thin ®lm problem. Anyway, cycling recovered reversibility was complete, in our testing conditions, only for the oxidized
cerium/zirconium coating. The not oxidized cerium/ titanium thin ®lm could be reversible modifying its discharging time in respect to the charging time, this indicates that even if the trapping eect can be compensated, kinetic intercalation/de-intercalation problems are still present in the not oxidized Ce/Ti electrode. Further structural and compositional studies should clarify more in depth the non reversibility mechanisms. Chemical diusion coecient values are in good agreement with the relevant literature data considering that dierent measuring methods have been used and materials were prepared by electron beam gun evaporation instead of sol±gel or DC magnetron sputtering techniques. All the ®lms prepared exhibited no optical absorption in the visible range, this is consistent with the model of inserted electrons accommodation in 4f states of Ce [6,7].
6. Conclusion Thin ®lm mixed oxides (Ce/Ti and Ce/Zr) materials have been prepared with dierent levels of oxidation by electron beam gun reactive and not reactive evaporation. Optical and electrochemical measurements indicate that oxidation levels and oxides combinations have an in¯uence in the reversibility behavior of the thin ®lm materials when used as counter electrode in electrochromic devices. This can be addressed to charge trapping eects and to dierent intercalation/ de-intercalation kinetic properties. Future research work based also on structural and compositional studies are needed to correlate electrochemical properties to the deposition conditions and to verify the nonreversibility mechanism. Thermodynamic and kinetic electrochemical characteristics have been measured according to the design scheme based on exact, fully reversible charge balance between the active layers and coloring/bleaching potentials resulting from equilibrium OCV vs. concentration curves [1]. Among the four materials studied, the most suitable to be used as optically passive counter electrode in the electrochromic system previously developed [1] is the oxidized Ce/Ti mixed oxide.
References [1] G. Macrelli, E. Poli, in: K.C. Ho, C. Greenberg, D.M. MacArthur (Eds.), Electrochromic Materials III, Proceedings Volume 96±24, The Electrochemical Society Inc, Pennington, NJ, 1997, p. 338.
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147 [2] M. Green, K. Pita, J. Appl. Phys. 81 (1997) 3592. [3] D. Keomany, Doctorate thesis LIES, Grenoble, 1993. [4] D. Keomany, C. Ponsignon, D. Deroo, Sol. Energy Mater. Sol. Cells 33 (1994) 429. [5] D. Camino, D. Deroo, J. Salardenne, N. Treull, Sol. Energy Mater. Sol. Cells 39 (1995) 350. [6] M. Veszelei, L. Kullman, M. Stromme Mattson, A. Azens, C.G. Granqvist, Zr±Ce Oxides as candidates for optically passive counter electrodes, paper presented at the IME-2 Conference, San Diego, CA (September 29, 1996). [7] A. Azens, L. Kullman, D.D. Ragan, M. Stromme Mattson, C.G. Granqvist, in: K.C. Ho, C. Greenberg, D.M. MacArthur (Eds.), Electrochromic Materials III,
[8] [9] [10]
[11] [12]
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Proceedings Volume 96±24, The Electrochemical Society Inc, Pennington, NJ, 1997, p. 218. W. Weppner, R.A. Huggins, J. Electrochem. Soc. 124 (1977) 1569. W. Weppner, in: P.G. Bruce (Ed.), Solid State Electrochemistry, Cambridge University Press, Cambridge, 1995, p. 199. W. Weppner, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), The CRC Handbook of Solid State Electrochemistry, CRC Press Inc, Boca Raton, FL, 1997, p. 295. M. Green, K. Pita, Sol. Energy Mater. Sol. Cells 43 (1996) 393. M.S. Burdis, J.R. Siddle, Thin Solid Film 237 (1994) 320.
Mixed cerium/titanium and cerium/zirconium oxides as thin ®lm counter electrodes for all solid state electrochromic transmissive devices Guglielmo Macrelli, Elisabetta Poli* Isoclima SpA ± R&D Department via A. Volta, 14, 35042 Este (PD), Italy Received 7 September 1998; received in revised form 3 November 1998
Abstract Optically passive thin ®lms of mixed cerium/titanium and cerium/zirconium oxides have been prepared by electron beam reactive evaporation. Dierent oxidation levels have been achieved using dierent oxygen ¯ows in the deposition process. The samples have been optically and electrochemically characterized. Performances are discussed in view of a future utilization in a electrochromic device as thin ®lm counter electrode. Dierent materials have been tested to identify the best solution to be used as an optically passive ion storage layer in an existing electrochromic device in alternative to an optically active V2O5 counter electrode. Process conditions and materials performances are reported, related to each other and discussed. Development studies are still in progress towards the optimization of the device and its future scaling up. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Electrochromism; Counter electrode; Mixed oxides; Thin ®lm; e-beam evaporation
1. Introduction This paper reports preliminary results of a research work performed to develop optically passive mixed oxide counter electrode for electrochromic devices. As previously reported [1] we developed a medium area (300 by 300 mm2) electrochromic device based on an all solid state inorganic thin ®lm multilayer structure. In that solution the counter electrode was a thin ®lm of Vanadium pentoxide deposited by plasma assisted electron beam evaporation. There were at least two main drawbacks to overcome: . The counter electrode was an optically active cathodic material that reduced the optical modulation window,
* Corresponding author.
. The counter electrode was a thin ®lm deposited by plasma assistance to achieve the correct stoichiometry: this complicated greatly the deposition technology. Recently, studies have been reported [2] in which stoichiometrically correct vanadium pentoxide was deposited without plasma assistance, but, even in this case, the optical activity of Vanadium pentoxide is still a problem. These motivations together with the increasingly interest in mixed oxide thin ®lm [3±7] were the background to start this research work. We focused our attention to cerium/titanium and cerium/ zirconium mixed oxides materials. The deposition method we have used is electron beam vacuum evaporation; we ®xed the ratio of the starting oxides and we changed the oxygen partial pressure at two dierent levels.
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 0 3 1 - 6
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G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
Table 1 Process parameters and coatings characteristics Material
WO3
CeO2/TiO2
CeO2/TiO2+O2
CeO2/ZrO2
CeO2/ZrO2+O2
Process parameters Final pressure (mbar) O2 pressure P1 (mbar) Rate (nm/s) Source weigh ratio Source molar ratio
2 10ÿ5 4 10ÿ5 30 ± ±
2 10ÿ5 0 15 50%/50% 31.7%/68.3%
2 10ÿ5 5 10ÿ5 15 50%/50% 31.7%/68.3%
2.5 10ÿ5 0 15 50%/50% 41.7%/58.3%
2.8 10ÿ5 5 10ÿ5 15 50%/50% 41.7%/58.3%
Physical characteristics Thickness (nm) Density (g/cm3)
300 6.0
200 5.7
200 5.7
200 6.4
200 6.4
Compositional and structural studies have not been performed at this preliminary level but, for sure, they will be of paramount importance in any further development. At ®rst we decided to concentrate our attention to determinate electrochemical kinetic and thermodynamic intercalation characteristics of the materials. The electrochemical characterization methods we have used, re¯ect the design concept of our electrochromic system [1]: it is a charge balanced system and the coloring/bleaching potentials are the equilibrium ones resulting from the dierence of the corresponding coulometric titration curves. 2. Thin ®lm preparation The mixed oxide thin ®lms were deposited using a Baltzers BA 510 evaporator. This machine is equipped with a thermal source and a multipocket (4 pocket) electron beam evaporation source. During deposition careful gas inlet control system allows reactive evaporation, the machine is also equipped with a quartz microbalance monitor for thickness and deposition rate control. The quartz monitor feed back the power supply of the electron beam source to achieve a careful PID control at ®xed deposition rate. Indium tin oxide thin ®lm were previously deposited on cleaned optical glass substrates, the coating was about 200 nm thick and had a sheet resistance ranging from 8 to 9 Ohm square. The ITO coated glass substrate were then coated with the following dierent materials: tungsten trioxide, cerium oxide/titanium oxide, cerium oxide/zirconium oxide. Source materials (from Plasmaterials with purity higher than 99.5%) in powder form were mixed in 50% by weight and placed in a molybdenum crucible and than evaporated by electron beam gun. Before deposition a suitable soak heating phase of the source material was performed under a shutter.
The mixed oxide coatings were deposited in two dierent conditions: no oxygen gas ¯ow, oxygen gas ¯ow introduced during deposition (reactive evaporation); in this way we had at our disposal 5 dierent types of half cells. The deposition conditions of the dierent coatings are reported in Table 1. Glass substrates were carefully cleaned in an ultrasonic batch, visual inspection at grazing incidence under a bright light was performed for each substrate before deposition. The vacuum evaporation system was pumped down to 1 10ÿ5 mbar, the chamber walls were heated during the pumpdown and than cooled during the deposition. In case of reactive evaporation, oxygen was introduced from an external bottle (oxygen purity 99.999%) and its partial pressure inside the vacuum chamber was regulated and kept constant to 5 10ÿ5 mbar. 3. Experimental methods Physical thickness of the layers were measured using an Alpha Step 200 Stylus pro®lometer, the density of the layers was directly calculated from the measured physical thickness and the weight dierence of a controlled area of the substrate determined by a Sartorious analytical balance with a 0.00001 g sensitivity. Electrochemical experiments were performed using a potentiostat/galvanostat (EG&G Princeton Applied Research model 363) suitably controlled by an interface and data acquisition board (National Instruments Assy 180950±01 Rev. D), connected to a multifunction I/O board (National Instruments AT-MIO-16). The electrochemical tests were driven by a computer program in LabWindows environment. The electrochemical cell was con®gured with three electrodes: a working electrode, a counter electrode and a reference electrode. There were performed two types of electrochemical experiments: galvanostatic intermittent titration (GITT) and voltage step response (VSR). In the ®rst
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
3139
Fig. 1. GITT results for not oxidized CeTiOy: (a) coulometric titration curve, (b) chemical diusion coecient.
case the working electrode consisted in the deposited thin ®lm of mixed oxides on the ITO coated glass substrate, reference and counter electrode consisted in a Lithium metal wire in contact with an electrolyte solution 1 M LiClO4/propylene carbonate (PC). In the second case (VSR) the counter electrode consisted in
the deposited thin ®lm of mixed oxides on the ITO coated glass substrates and the working electrode was a tungsten trioxide thin ®lm deposited on ITO coated glass substrates. The two electrodes were interfaced by a contact with the same electrolyte solution used in the GITT experiments.
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Fig. 2. GITT results for oxidized CeTiOy: (a) coulometric titration curve, (b) chemical diusion coecient.
Galvanostatic intermittent titration technique (GITT) was introduced in Ref. [8] to measure thermodynamic and kinetic characteristics of mixed ionic/electronic conductors. The main advantages of GITT are: negligible polarization of the electrode and no current transients resulting from charge accumulation at the electrolyte/electrode interface; GITT tests results are
evaluated considering the following assumptions: constant molar volume of the electrode during intercalation/de-intercalation and diusion characteristic length small compared to the characteristic dimension (physical thickness) of the layer. We used GITT to measure the chemical diusion coecient of Lithium ions during intercalation/de-intercalation processes.
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
3141
Fig. 3. GITT results for oxidized CeZrOy: (a) coulometric titration curve, (b) chemical diusion coecient.
Chemical diusion coecient, D , is de®ned as the proportionality factor between ions ¯ux Ji and ion concentration gradient (Hci) according to:
ion ¯ux to the total electrochemical potential gradient HZi):
Ji ÿD rci
Ji ÿ
1
and it is related to the ion diusivity D, introduced in the most general Nerst±Planck equation (connecting
ci D rZ kT i
by the Wagner factor W according to [8,9]:
2
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Fig. 4. Voltage step response results for not oxidized CeTiOx: (a) current density vs. time, (b) optical transmittance response. Bleaching voltage 1.7 V, coloring voltage ÿ1.7 V.
D D W:
3
We used GITT also to determinate the coulometric titration curve which is the equilibrium open circuit voltage OCV of the working electrode (towards the Li/ Li+ reference) against the concentration of the interca-
lated ions. Details of the test method can be found in the literature [8±10]. In the VSR test a voltage step between two potential values was ®xed between the WE and CE for a certain ®xed duration and the current ¯owing in the cell was recorded against time. Potentials were chosen accord-
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
3143
Fig. 5. Voltage step response results for oxidized CeTiOx: (a) current density vs. time, (b) optical transmittance response. Bleaching voltage 1.7 V, coloring voltage ÿ1.7 V.
ing to the coulometric titration curves of the electrodes to have lithium ions insertion in the WE and than in the CE. With this technique the current density (electric current divided by the surface of the electrode/electrolyte interface) can be determined as a time function and it can be integrated to get the surface density of charge Q (mC/cm2).
Before the VSR tests Lithium ions were previously electrochemically intercalated in the working electrode (WE±WO3) up to a surface density of charge of 12 mC/cm2; VSR tests were then performed with a ®rst bleaching phase (Li+ ions moving from WE to CE) and a second coloring phase (Li+ ions moving from CE to WE).
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Fig. 6. Voltage step response results for not oxidized CeZrOx: (a) current density vs. time, (b) optical transmittance response. Bleaching voltage 1.7 V, coloring voltage ÿ1.7 V.
In both the electrochemical test con®gurations integrated light transmittance through the cell was measured and recorded using an optical bench consisting of a collimated light source (illuminant C) and a silicon photodetector with a ®lter for photopic vision.
4. Experimental results The as deposited thin ®lm were submitted to GITT. In these tests all the potentials were measured against Li/Li+ in the range between 1.0 and 4.0 V and the
G. Macrelli, E. Poli / Electrochimica Acta 44 (1999) 3137±3147
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Fig. 7. Voltage step response results for oxidized CeZrOx: (a) current density vs. time, (b) optical transmittance response. Bleaching voltage 1.7 V, coloring voltage ÿ1.7 V.
measurements were carried out at a current density of 0.1 mA/cm2. The intercalation properties of cerium/zirconium mixed oxide thin ®lms were dierent in respect to the de-intercalation properties: it was possible to introduce lithium ions but their extraction was very incomplete (de-intercalation potentials exceeded the 4 V upper
limit), in other words the cerium/zirconium ®lms deposited in reactive way (with O2 ¯ow) and the ones deposited without oxygen ¯ow were both not fully reversible. The cerium/titanium mixed oxide thin ®lms exhibited dierent behavior according to the deposition conditions. The ®lms obtained in reactive way were com-
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pletely reversible in intercalation and de-intercalation, reversibility was not so good for the ®lms deposited without oxygen. During the galvanostatic tests, the optical response of all the materials was completely passive, changes in the optical transmittance were well below the sensitivity of the measuring system (0.5%). Coulometric titration curves and chemical diusion coecients are reported in Figs. 1±3 for the as deposited thin ®lms, results are not reported for the cerium/ zirconium thin ®lm deposited without oxygen ¯ow because of its strong irreversible behavior. Reversibility properties of the oxidized and not oxidized cerium/zirconium and of the cerium/titanium non oxidized thin ®lm were improved after a certain number of charging and discharging cycles. Voltage step response results (current density vs. time and normalized visible transmittance vs. time) are reported in Figs. 4±7 for all the materials under examination after at least 10 charge/discharge cycles. Tests performed with the oxidized cerium/titanium and cerium/zirconium exhibited good charging/discharging and optical balancing properties in the coloring and bleaching phases; on the contrary for not oxidized counter electrodes charge extraction and coloration was not balanced to charge injection and bleaching levels. Balanced conditions for charge and coloration levels can be achieved for the not oxidized cerium/titanium counter electrode enlarging the coloring phase (Li+ ions extracted from CE and intercalated in WO3) from the original 200 s to 500 s. 5. Discussion Deposition conditions (oxygen partial pressure) strongly in¯uence the intercalation properties of the electrode mixed oxide materials considered here. Reversibility properties in intercalation and de-intercalation processes in galvanostatic and potentiostatic conditions are mainly in¯uenced by the oxidation level of the thin ®lms during deposition. The most probable causes that can lead to reversibility problems can be addressed to charge trapping eects and dierent kinetic behavior of the materials to charge intercalation/ de-intercalation. Charge trapping eects are not new for electrochromic materials [11,12] and are mainly related to material non-stoichiometry [11]. Improvements in reversibility as material is cycled is, generally, a good indication of charge trapping eects. This is true, in our case, for both the not oxidized materials and for the oxidized cerium/zirconium thin ®lm. Charge trapping can be generated by non stoichiometry through grain boundary activity [11] or can be simply a structural thin ®lm problem. Anyway, cycling recovered reversibility was complete, in our testing conditions, only for the oxidized
cerium/zirconium coating. The not oxidized cerium/ titanium thin ®lm could be reversible modifying its discharging time in respect to the charging time, this indicates that even if the trapping eect can be compensated, kinetic intercalation/de-intercalation problems are still present in the not oxidized Ce/Ti electrode. Further structural and compositional studies should clarify more in depth the non reversibility mechanisms. Chemical diusion coecient values are in good agreement with the relevant literature data considering that dierent measuring methods have been used and materials were prepared by electron beam gun evaporation instead of sol±gel or DC magnetron sputtering techniques. All the ®lms prepared exhibited no optical absorption in the visible range, this is consistent with the model of inserted electrons accommodation in 4f states of Ce [6,7].
6. Conclusion Thin ®lm mixed oxides (Ce/Ti and Ce/Zr) materials have been prepared with dierent levels of oxidation by electron beam gun reactive and not reactive evaporation. Optical and electrochemical measurements indicate that oxidation levels and oxides combinations have an in¯uence in the reversibility behavior of the thin ®lm materials when used as counter electrode in electrochromic devices. This can be addressed to charge trapping eects and to dierent intercalation/ de-intercalation kinetic properties. Future research work based also on structural and compositional studies are needed to correlate electrochemical properties to the deposition conditions and to verify the nonreversibility mechanism. Thermodynamic and kinetic electrochemical characteristics have been measured according to the design scheme based on exact, fully reversible charge balance between the active layers and coloring/bleaching potentials resulting from equilibrium OCV vs. concentration curves [1]. Among the four materials studied, the most suitable to be used as optically passive counter electrode in the electrochromic system previously developed [1] is the oxidized Ce/Ti mixed oxide.
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