Enhanced reversible electrochromism via in situ phase transformation

in tungstate monohydrate

Basila Kattouf, Gitti L. Frey, Arnon Siegmann and Yair Ein-Eli*

Received (in Cambridge, UK) 7th July 2009, Accepted 19th October 2009

First published as an Advance Article on the web 3rd November 2009

DOI: 10.1039/b913461a

This study demonstrates that realizing the correlation between

in situ crystallographic structure modifications of an electro-

chromic material and its functionality leads to improved

performances, which can then contribute to a variety of

energy-efficient applications.

Applying a small bias to electrochromic (EC) materials

induces a change in their appeared color making them amenable

for many applications.1 The operation of a conventional EC

device is based on the reversible electrochemical injection of

both positive ions (H+, Li+, Na+) and electrons into a

multivalent transition metal oxide host lattice.2 While electron

injection into the host matrix is very rapid, cation diffusion is

often kinetically slow, limiting the rate of coloration. Because

the effective cation diffusion coefficient of protons (DH+) is

estimated to be an order of magnitude higher than that

of lithium ions (DLi+),3 EC systems based on aqueous

electrolytes containing free protons (e.g., aqueous H2SO4)

are most appealing.

Among inorganic EC materials, amorphous and crystalline

tungsten trioxide (WO3) have been extensively studied and

integrated as the active component in a variety of EC

applications.4 In addition to WO3, the monohydrated species

WO3�H2O has also shown reversible Li-ion insertion and EC

performance. Moreover, it was recently reported that the

diffusion coefficient of Li-ions into WO3�H2O is 1–2 orders

of magnitude higher than that in WO3.5 The authors

speculated that the water molecules present in the hydrated

species strongly contribute to the ionic conductivity. It was

suggested that, in contrast to the isotropic penetration of Li+

into WO3 grains, Li+ ion propagation into WO3�H2O is

preferentially enhanced along water-decorated plains. The

effective cation diffusion coefficient could further increase by

adding indium tin oxide (ITO) nanoparticles, as recently

demonstrated for a composite WO3�H2O-based electrode in

a non-aqueous electrolyte.6

Here, we show that the capacity value and charge-exchange

quantities of WO3�H2O can be dramatically increased upon

conversion of WO3�H2O to the di-hydrate WO3�2H2O. The

crystallographic structure of WO3�2H2O provides additional

spacious, water-paved pathways for cation insertion/ exclusion.

Accordingly, proton diffusion into and out of the dihydrated

species is facile compared to that in the monohydrate,

and hence charge capacity and charge-exchange quantities

are enhanced upon phase transformation. Finally, it is

demonstrated here that the desired phase transition can be

judiciously induced in situ during EC cycling.

Green/yellow WO3�H2O particles were synthesised according

to the Freedman method,7 while films on glass/indium-tin-oxide

(ITO) substrates were prepared as described in ref. 6. Electro-

chemical characterizations (273A PAR potentiostat) were

performed in a three-electrode glass electrochemical cell using

a Pt foil (counter electrode), a saturated calomel electrode

(SCE, reference electrode) and a solution of H2SO4 (pH 2)

as the electrolyte. X-Ray diffraction (XRD, Philips PW 1820

diffractometer (Cu-Ka), spectrophotometry (reflection mode,

Cary 500 Varian) and high resolution scanning electron

microscopy (HRSEM, LEO982) were employed.

Homogeneous and continuous films of 90 : 10 wt%

WO3�H2O : PVDF were spray-coated onto an ITO/glass

substrate, as shown in the SEM micrograph (Fig. 1a). Higher

magnifications (Fig. 1b) reveal that the film is composed of

plate-like particles, B200 nm in size, in agreement with the

reported layered-like morphology of WO3�H2O.8 A HR-SEM

micrograph of the same sample (Fig. 1c), shows that the

particles are embedded in—and connected by—the PVDF

polymeric matrix (indicated with the white arrow).

The EC properties of the spray-coated composite films were

studied by performing cyclic voltammetry measurements

(CVs). The yellow-green films turn dark blue as the potential

is reduced to �0.6 V. Subsequently switching the potential to

1.1 V induces a color change back to the pristine color,

indicating that the EC process in these electrodes is reversible.

EC durability, related to the cycling stability/degradation, is

examined by continuously CV-cycling the electrode. The first

150 continuous CV cycles are presented in Fig. 2a. The shape

of all CV curves are similar to those previously reported for

WO3�H2O.9 However, it can clearly be seen that the overall

area of the CV curves increases with cycling, indicating a

capacity growth. Furthermore, cycling induces a shift in the

anodic peak toward higher potentials, accompanied by an

increase in the measured currents. The peak-shift and increase

in currents is listed in the inset table of Fig. 2a. Galvanostatic

measurements (Fig. 2b) performed at a relatively high current

density (1 mA cm�2) subsequent to 1, 50, 100 and 150 cycles

show a dramatic increase of B50%, in the overall charge

during the first 150 cycles.

The origin of the observed increase in charge capacity

during cycling was investigated by analyzing the chemical

composition of the electrode before and after cycling.

Fig. 3a shows the XRD patterns of the as-prepared electrode

(black line), after 3 h immersion in the solution without

the application of any bias or current (green line) and

after 150 continuous cycles (red line). The pattern of the

Department of Materials Engineering, Technion–Israel Institute ofTechnology, Haifa 32000, Israel. E-mail: eineli@tx.technion.ac.il.;Fax: +972-4829-5677

7396 | Chem. Commun., 2009, 7396–7398 This journal is �c The Royal Society of Chemistry 2009

COMMUNICATION www.rsc.org/chemcomm | ChemComm

Publ

ishe

d on

03

Nov

embe

r 20

09. D

ownl

oade

d by

Uni

vers

ity o

f N

orth

Car

olin

a at

Gre

ensb

oro

on 0

2/10

/201

3 22

:37:

09.

View Article Online / Journal Homepage / Table of Contents for this issue

as-prepared film reflects a WO3�H2O film with peaks at

2y B 16.81, 261 and 30.51. However, immersion in the acidic

solution generates additional peaks centered at 2y B 131, 241

and 27.51 (green line), which dramatically increase upon

continuous EC cycling (red line). The new peaks are in

agreement with the presence of tungstate dihydrate WO3�2H2O,

indicating a phase transition from the monohydrated phase to

the dihydrated phase. Indeed, previous studies have reported

on such phase transformation upon immersion in acidic

solutions.10 Notably, the remarkable increase in the intensity

of the WO3�2H2O pattern upon cycling indicates that

continuously cycling of the film dramatically accelerates phase

transformation.

Theoretically, the phase transformation is not expected to

induce an increase of the charge obtained from the EC process.

However, the electrochemical studies (Fig. 2) have shown a

dramatic increase in the overall charge during EC electrode’s

cycling. Since proton insertion reactions are controlled by the

diffusion of charged species into the guest lattice, it is suggested

that the proton diffusion coefficient value in WO3�2H2O is

higher than that in WO3�H2O, providing an increase in charge

intercalation upon phase transformation. Thus, under the

conditions of high currents, protons are capable of in-depth

intercalating into the interstitial lattice sites, reaching higher

capacities for WO3�2H2O compared to WO3�H2O. In-depth

intercalation could also be the reason for the anodic potential

shift associated with the cycling (Table inset, Fig. 2a). The

higher proton diffusion coefficient value could be realized from

the atomic structure of the lattices, as shown in Fig. 3b. The

structure of the dihydrated phase (right pane in Fig. 3b), is

more receptive to protons intercalation than the mono-

hydrated phase (left pane in Fig. 3b). This is due to the

increase of the distance by the incorporation of the extra

H2O molecules in the lattice. Thus, the larger spacing

facilitates diffusion/migration of protons into and out of the

lattice during coloration and bleaching processes, respectively.

A suggested mechanism for the electrochemically-induced

phase transformation involves initially the incorporation of

hydronium cations (H3O+). Protons are extracted from the

lattice upon bias reversing, leaving water molecules trapped in

the lattice, inducing the phase transformation. These trapped

water molecules pave the way for the next wave of intercalated

hydronium cations into the matrix during subsequent

coloration cycles. Consequently, upon further cycling a phase

transformation occurs, allowing proton intercalation into

deeper sites in the phase-transformed lattice, hence increasing

charge capacity.

The improved EC performance of the phase-transformed

electrode is evident from the reflectance spectra measured

before and after the phase transformation. Fig. 4 shows the

normalized reflectance obtained for an EC film in its blue state

(E= �0.6 V) (blue line), and an EC film after 1 and 150 cycles

Fig. 1 SEM micrographs of a 90 : 10 wt% WO3�H2O : PVDF film

spray-coated onto an ITO/glass substrate. The white arrow in Fig. 1c

points to the PVDF matrix.

Fig. 2 Electrochemical behavior of WO3�H2O EC electrodes

subsequent to a single cycle, 50, 100 and 150 cycles; (a) CVs in

H2SO4 solution (pH 2) at a scan rate of 50 mV s�1. Inset table

summarizes the anodic positive potential shift (Vap) and the anodic

current increase (Iap) during cycling; (b) Charge profiles obtained by

applying a constant current of 1 mA cm�2. The inset table shows the

dramatic capacity increase upon cycling.

This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 7396–7398 | 7397

Publ

ishe

d on

03

Nov

embe

r 20

09. D

ownl

oade

d by

Uni

vers

ity o

f N

orth

Car

olin

a at

Gre

ensb

oro

on 0

2/10

/201

3 22

:37:

09.

View Article Online

at a potential of 0.3 V (black and red lines, respectively). A

potential value of 0.3 V was selected because the difference

between the currents obtained at this potential in the 1st and

150th cycles was the highest (Fig. 2). The reflectance spectrum

of the film in its blue state shows the expected peak in the visible

spectrum. The reflectance spectrum of the film which was cycled

once to�0.6 V (blue) and back to 0.3 V shows no reflectance in

the blue region of the visible spectrum (black line in Fig. 4), and

is in the pale yellow form, (figure inset, black frame). In

contrast, the film that was cycled 150 times and then held at a

potential of 0.3 V is not entirely back to the pale-yellow color,

but is rather, in an intermediate state, between yellow and blue.

This is evident from its reflectance spectrum, which shows non-

negligible intensity in the blue region of the visible spectrum

(red line in Fig. 4) and the film photograph (Fig. 4 inset, red

frame). This noticeable difference in the EC performance and

optical properties of the films cycled once and 150 times is

attributed to the mono- to dihydrated tungstate phase trans-

formation, occurring during cycling. Therefore, continuous

cycling of the as-prepared electrode actually conditions the

electrode for improved EC performance through an in situ

phase transformation to the dihydrated phase. This results in

a more spacious crystallographic structure facilitating proton

intercalation/deintercalation from the matrix.

In summary, the observed improved EC film’s performance

is assigned to an in situ mono- to dihydrated tungstate phase

transformation, introducing water-paved crystallographic

spacious planes, providing facile cation diffusion pathways

into and out of the matrix.

Notes and references

1 (a) M. Green and K. Pita, Sol. Energy Mater. Sol. Cells, 1996, 43,393–411; (b) C. M. Lampert, Sol. Energy Mater. Sol. Cells, 2003,76, 489–499; (c) S.-H. Lee, M. C. Hyeonsik, L. Ping, S. Dave,C. E. Tracy, M. Angelo, J. R. Pitts and K. D. Satyen, J. Appl.Phys., 2000, 88, 3076–3078.

2 (a) C. Bechinger, S. Ferrere, A. Zaban, J. Sprague and B. A. Gregg,Nature, 1996, 383, 608–610; (b) C. G. Granqvist, Handbook ofInorganic Electrochromic Materials, Elsevier, New York, 1995;(c) M. Gratzel, Nature, 2001, 409, 575–576; (d) S.-H. Lee,M. C. Hyeonsik, Z. Ji-Guang, M. Angelo, K. B. David andK. D. Satyen, Appl. Phys. Lett., 1999, 74, 242–244;(e) M. Wagemaker, A. P. M. Kentgens and F. M. Mulder, Nature,2002, 418, 397–399.

3 (a) P. Baudry, M. A. Aegerter, D. Deroo and B. Valla,J. Electrochem. Soc., 1991, 138, 460–465; (b) C. Bohnke andO. Bohnke, Solid State Ionics, 1990, 39, 195–204; (c) O. Bohnke,M. Rezrazi, B. Vuillemin, C. Bohnke, P. A. Gillet andC. Rousselot, Sol. Energy Mater. Sol. Cells, 1992, 25, 361–374;(d) B. W. Faughnan, S. C. Richard and A. L. Murray, Appl. Phys.Lett., 1975, 27, 275–277; (e) S. J. Golden and B. C. H. Steele, SolidState Ionics, 1988, 28–30, 1733–1737; (f) M. L. Hitchman, ThinSolid Films, 1979, 61, 341–348.

4 (a) C. G. Granqvist, Sol. Energy Mater. Sol. Cells, 2000, 60,201–262; (b) A. Lusis, J. Kleperis and E. Pentjuss, J. Solid StateElectrochem., 2003, 7, 106–112; (c) E. Ozkan Zayim, P. Liu,S.-H. Lee, C. E. Tracy, J. A. Turner, J. R. Pitts and S. K. Deb,Solid State Ionics, 2003, 165, 65–72; (d) S. Badilescu andP. V. Ashrit, Solid State Ionics, 2003, 158, 187–197.

5 P. Judeinstein and J. Livage, J. Chim. Phys. Phys. Chim. Biol.,1993, 90, 1137–1147.

6 O. Lavi, G. L. Frey, A. Siegmann and Y. Ein-Eli, Electrochem.Commun., 2008, 10, 1210–1213.

7 M. L. Freedman, J. Am. Chem. Soc., 1959, 81, 3834–3839.8 A. Bessiere, L. Beluze, M. Morcrette, V. Lucas, B. Viana andJ. C. Badot, Chem. Mater., 2003, 15, 2577–2583.

9 C. Marcel and J. M. Tarascon, Solid State Ionics, 2001, 143,89–101.

10 (a) C. Balazsi and J. Pfeifer, Solid State Ionics, 1999, 124, 73–81;(b) Y. G. Choi, G. Sakai, K. Shimanoe, N. Miura andN. Yamazoe, Sens. Actuators, B, 2002, 87, 63–72; (c) J. Livageand G. Guzman, Solid State Ionics, 1996, 84, 205–211.

Fig. 3 (a) XRD patterns of an as-prepared WO3�H2O EC electrode

before (black line), after 3 h immersion in a sulfuric acid solution

(pH 2) with no application of bias or current (green line) and after

150 continuous cycles (red line); (b) Crystallographic structures of

WO3�H2O (orthorhombic, a = 0.5238 nm, b = 1.070 nm, c =

0.5120 nm;10b left pane), and WO3�2H2O (monoclinic, a = 0.75 nm,

b = 0.693 nm, c = 0.37 nm, b = 90.51;10b right pane).

Fig. 4 Reflectance spectra of a blue colored film at �0.6 V vs. SCE

(blue line, blue frame photograph), and a fresh sample swept from

open circuit potential (0.3 V) to �0.6 V, then back to 0.3 V (black line,

black frame photograph), and a film continuously cycled for

150 cycles, then back to 0.3 V (red line, red frame photograph).

7398 | Chem. Commun., 2009, 7396–7398 This journal is �c The Royal Society of Chemistry 2009

Publ

ishe

d on

03

Nov

embe

r 20

09. D

ownl

oade

d by

Uni

vers

ity o

f N

orth

Car

olin

a at

Gre

ensb

oro

on 0

2/10

/201

3 22

:37:

09.

View Article Online