Development of mesoporous tungsten oxide WO 3 thin films for electrochromic applications

Four steps 1) Preparation of the solution

([W12O42H2]10-/EtOH/H2O ) + (Brij-56/EtOH/H2O)

2) Dip-coating

•Controlled RH% •Controlled deposition speed

3) Stabilization at 170

4) Calcination at high temperatures

•Elimination of the surfactant •Preservation of mesoporosity •Solidification of the material

Observation: The presence of surfactant delays crystallization

T

(

oC

)

The electrochemical behaviour of WO3 films depend on the surface chemistry of the material. Mesoporous films possess an open structure with multiple sites for Li+ intercalation/deintercalation improving the electrochemical properties of the film. This is evident by the enhanced diffusion coefficient, the decrease of irreversible traps in the first cycles and the faster deintercalation process. Moreover due to the existence of multiple sites in mesoporous film, current’s decline with time follows a multistep progression during deintercalation with fast and slow regions.

. log (t) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 log( I) -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 mesoporous dense slope -1/2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 mesoporous dense slope -3/4 log( I) log (t)

Dense film: Low reversibility in the first cycle. Slower kinetics in regard to mesoporous

film

Mesoporous film: Improved reversibility in the first cycle. Fast deintercalation.

During intercalation both films decline in a non-linear way.

Mesoporous film possesses fast (B, D) and slow (A, C) regions of current’s decline due to the presence of multiple Li+ sites

Dense film is composed by a compact and smooth surface

Mesoporous film consists of regular pores with diameters less than 4nm

Dense film: Irreversible traps cause substantial modifications in the first few cycles.

At low voltage scan rates, traps are designated by a small cathodic peak.

Mesoporous film: Gradual degradation with cycling. Multiple anodic peaks during

cyclic voltammetry due to multiple sites for lithium intercalation.

Enhanced diffusion coefficient implying that mesopores facilitate deinsertion of lithium cations.

Development of mesoporous tungsten oxide WO

₃

thin films

for electrochromic applications

Group of Research in Energy and Environment from Materials (GreenMat), University of Liege B6C, allee de la Chimie 3, 4000 Liege, Belgium

Electrochromism is the ability of some materials to change their optical properties in a reversible and persistent way under the action of a voltage pulse. This phenomenon finds applications in the area of smart windows and subsequently it contributes to building energy savings. The reaction that takes place between the colored (right part) and bleached state (left part) is :

WO

+ xLi

+ xe

Li

WO

In this work, we prepared amorphous thin films of WO3 via the sol-gel technique and studied their electrochemical behaviour in the presence and absence of mesopores, induced by

the micelles of a non-ionic surfactant. The existence of mesopores in a film results in the increase of the framework’s surface area and consequently it reduces the diffusion length of cations. These advantages could potentially lead to electrochromic materials with better reversibility, coloration efficiency, charge capacity and switching kinetics.

WHAT IS AN ELECTROCHROMIC FILM?

EXPERIMENTAL PROCEDURE CYCLIC VOLTAMMOGRAMS 350o_C,2h STRUCTURAL CHARACTERIZATION dense film 300o_C,1h mesoporous film 350o_C,2h Ewe/Vvs. Ag/AgCl/KCl (3.5M) -1.0 -0.5 0.0 0.5 1.0 J ( m A. cm -2) -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 dense film -1.0 -0.5 0.0 0.5 1.0 1.5 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 Ewe/Vvs. Ag/AgCl/KCl (3.5M) J ( m A. cm -2) mesoporous film -1.0 -0.5 0.0 0.5 1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 20 mV.s-1, 20th cycle 50 mV.s-1, 1st cycle Ewe/Vvs. Ag/AgCl/KCl (3.5M) J ( m A. cm -2) mesoporous film -1.0 -0.5 0.0 0.5 1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 3 mV.sec-1 5 mV.sec-1 7 mV.sec-1 10 mV.sec-1 15 mV.sec-1 Ewe/Vvs. Ag/AgCl/KCl (3.5M) J ( m A. cm -2) dense film -1.0 -0.5 0.0 0.5 1.0 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 3 mV.s-1 7 mV.s-1 Ewe/Vvs. Ag/AgCl/KCl (3.5M) J ( m A. cm -2) mesoporous film Sample Diffusion coefficient (cm2.s-1) 10th _{cycle 20}th _cycle Dense film 2.15x10-10 _{2.29 x10}-10 Mesoporous film 7.31x10-10 7.30 x10-10 KINETIC MEASUREMENTS Sample

Time for 90% of total charge capacity

(10th_/20th _cycle) Reversibility (%)

Coloration time (s) Bleaching time (s) 1st _{cycle 20}th _cycle

Dense film 24/24 18/18 54% 99%

Porous film 21/21 9/10 73% 96%

Intercalation Deintercalation

This work was financially supported by A.S Onassis Public Benefit Foundation (2011-2012) and the Service des bourses d’excellence ‘’Wallonie-Bruxelles International (2012-2013)

ACKNOWLEDGEMENTS CONCLUSIONS

[1] http://ec.europa.eu/clima/policies/package/index_en.htm [2] Solid State Ionics 53-56 (1992), 479-489

[3] Solar Energy Materials & Solar Cells 83 (2004), 115-124 [4] http://sageglass.com/portfolio/ REFERENCES A B C D