Influence of Oxygen Impurities on the Electrochromic Response of

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The Influence of Oxygen Impurities on the Electrochromic Response of Viologen Based Plastic Films Amerigo Beneduci, Giuseppe Chidichimo, Bruna Clara De Simone, Daniela Imbardelli, Maurizio De Benedittis, Marianna Barberio, and Loredana Ricciardi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp503740u • Publication Date (Web): 02 Jun 2014 Downloaded from http://pubs.acs.org on June 16, 2014

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

1

The Influence of Oxygen Impurities on the

2

Electrochromic Response of Viologen Based Plastic

3

Films

4

Giuseppe Chidichimo1, Bruna Clara De Simone1, Daniela Imbardelli1, Maurizio De Benedittis1,

5

Marianna Barberio2, Loredana Ricciardi1, Amerigo Beneduci1,*

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1

7 8 9

Dipartimento di Chimica e Tecnologie Chimiche , Università della Calabria, Ponte P. Bucci, 87030 Arcavacata di Rende (CS), Italy

2

Dipartimento di Fisica, Università della Calabria, Ponte P. Bucci, 87030 Arcavacata di Rende (CS), Italy

10 11

ABSTRACT: This paper concerns the coloring kinetics of electrochromic films obtained by

12

dispersing viologen molecules in a thermoplastic polyacrylate matrix. The experimental data

13

show that the oxygen molecules, that are originally dissolved as ubiquitous impurities in film

14

chemical precursors, and are not eliminated during the film preparation, play a relevant role, with

15

respect to the speed of the electrochromic response in the coloring stage. The presence of oxygen

16

can speed up the electrochromic response of the viologen cations or slow it down, depending on

17

the degree of polymerization of the plastic matrix. In stiffer films oxygen accelerates the

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electrochromic response, while the contrary occurs in less stiff films. A theoretical model of the

2

viologen kinetic electrochromic response, has been developed in order to justify the experimental

3

findings. This takes into account the possible electron exchange reactions with the oxygen

4

molecules. A coherent interpretation of the experimental data has been obtained on the basis of

5

this model.

6 7

Keywords: polymer electrochromic films; coloration kinetics; electrochemical processes;

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molecular oxygen impurities; molecular diffusion

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1. INTRODUCTION

2

Electrochromism is the property of materials that exhibit reversible changes in the electron

3

absorption spectrum associated with an oxidation–reduction reaction occurring when an external

4

potential is applied. In the past years, many efforts have been made to develop electrochromic

5

(EC) devices such as EC windows, rearview mirrors for cars and display panels.1-8

6

These devices are generally characterized by the presence of one or more active electrochromic

7

layers sandwiched between inert plastic or glass supports where the contact surfaces are coated

8

by ITO semiconductor layers. Specially interesting, are systems where a single plastic active

9

layer is sandwiched between the conductive external supports,9-11 for their applications in large

10

size, low cost, widespread, electrochromic devices production. In these cases the active films are

11

prepared by inserting into a polymer matrix a proper amount of well known electrochromic

12

organic molecules. Different techniques can be used to obtain these films. One possibility

13

consists in mixing the different components (electrochromic molecules, thermoplastic polymers,

14

plasticizers, etc.) and warming the mixture above the glass transition temperature (Tg) of the

15

polymer. Solid films of the active plastic material, can then be obtained by using appropriate

16

extruders. These films can be then laminated by suitable lamination technologies between plastic

17

or glass conductive supports.

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As an alternative technique, film formation can be induced by in situ polymerization. In this

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case a fluid solution of the electrochromic molecules and other additives, dispersed in a suitable

20

polymerizable monomer, is inserted between conductive supports and afterwards the monomer is

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polymerized thermally or by means of UV light.


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In this paper, it is shown that almost fast switching electrochromic devices can be obtained even

2

in the case where the support polymer becomes quantitatively relevant with respect to the

3

electrochromic active molecules or other additives.

4

One issue that needs to be understood in these systems is the way how electrons interact with the

5

electrochromic molecules during the coloring process if the supporting polymer matrix is

6

intrinsically non electron conductive. Of course, semiconducting polymers could be used to

7

disperse the electrochromic active components, but these molecules generally present high

8

absorption peaks in the visible region, and do not allow to obtain electrochromic films

9

transparent to light in their off state.12 It must be considered that in some cases the films Tg can

10

be very high and, as a consequence, the translational motion of the quite large electrochromic

11

molecules hardly occurs.

12

It is demonstrated in this paper that a relevant role for the electron transmission in the film is

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played by the oxygen molecules. They are always ubiquitously present, as a natural impurity, in

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the components of the film and can also be introduced during the manufacturing process.

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We have investigated electrochromic films prepared by inserting the EC ethyl viologen

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molecule, displaying different colors depending on its oxidation state,13,14 into an acrylate

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thermoplastic polymer obtained by in situ polymerization of the monomer 2-hydroxyethyl

18

acrylate. The components of the films are illustrated in Figure 1.

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The electrochromic coloring kinetics of four polymeric films with different molecular oxygen

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content and stiffness were studied. This last parameter was controlled by varying the mononomer

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to polymer conversion degree of the acrylate component. More precisely, we have investigated

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films having different mechanical complex modulus E* and loss tangent Tan(δ), obtained by

23

exposing the precursor fluid solutions to UV light for 10 minutes and 30 minutes respectively.


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1

The experiments confirmed that the electrochromic kinetics is strongly affected by both oxygen

2

diffusion and sample stiffness, and that these two dependences are strictly connected. The kinetic

3

equation for the viologen reduction in the whole volume of the electrochromic cells has been

4

solved, under reliable assumptions, in order to explain the experimental data.

(a) R N

N R

+e-e-

R N v.+ blue

v++ (not colored)

R N

N R

+e-e-

R

v.+ blue

5

N R

N R v0 (not colored)

(b) O-

HO O

2-hydroxyethylacrylate

6 7

Figure 1. (a) Electrochemical reduction of viologen salts (one of the many canonical forms of

8

the viologen radical cation is given); (b) 2-hydroxyethyl acrylate monomer.

9 10

2. EXPERIMENTAL SECTION

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2.1. Materials. Propylene carbonate (PC), ethyl viologen diperchlorate (EV), ferrocene, and 2-

12

hydroxyethyl acrylate were purchased from Aldrich Chemical. All chemicals were used

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anhydrous.


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2.2. EC devices preparation. A mixture of 69.31 wt % of 2-hydroxyethyl acrylate, 1.98 wt % of

3

ethyl viologen diperchlorate, 0.99 wt % of ferrocene, 26.73 wt % of propylene carbonate and

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0.99 wt% of UV photoinitiator (Irgacure 651 Ciba) was stirred at room temperature. The mixture

5

was then introduced by capillarity into homemade cells, whose thickness was fixed by glass

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spheres spacers of 120 µm. The cell walls had an indium tin oxide (ITO) conductive substrate,

7

which was 120 nm thick; it was sealed by a butyl rubber. The samples so obtained were

8

polymerized by exposure to 10 mW/cm2 UV source (Philips, model HPK 125). Electrochromic

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films with different stiffness have been obtained by varying the UV irradiation time during

10

polymerization: 10 minutes for Low polymerized (LP) films and 30 minutes for High

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polymerized (HP) films. Oxygen depleted electrochromic films were prepared by polymerizing

12

precursor solutions where nitrogen was bubbled for one hour. The content of oxygen in all film

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precursor solutions was measured by means of a Hanna Instruments HI 9828 multi parameters

14

probe. The oxygen concentration resulted to be equal to 7.2x10-4 mmol/L for the deoxygenated

15

solutions, while was equal to 2.46x10-1 mmol/L for the aerated solutions. In the case of the

16

deoxygenated mixtures, the remaining operations required for electrochromic device

17

preparations were performed under nitrogen atmosphere.

18 19

2.3. Determination of the oxygen content in the polymerized EC films. Rose Bengal sodium

20

salt (Sigma-Aldrich, 95%) (RB) was used as luminescent probe to detect the oxygen content. It

21

has been chosen since its emission intensity strongly depends on the concentration of oxygen

22

molecules dissolved in a medium. RB has been dissolved in anhydrous propylene carbonate and

23

added to the electrochromic mixture at a concentration of 10-5 mol/L.


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Steady-state emission spectra were recorded on a HORIBA Jobin-Yvon Fluorolog-3 FL3-211

2

spectrometer equipped with a 450 W xenon arc lamp, double-grating excitation and single-

3

grating emission monochromators (2.1 nm/mm dispersion; 1200 grooves/mm), and a Hamamatsu

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R928 photomultiplier tube. Emission and excitation spectra were corrected for source intensity

5

(lamp and grating) and emission spectral response (detector and grating) by standard correction

6

curves.

7 8

2.4. UV/Visible Spectroscopy. The electro-optical properties of ECD were measured with an

9

Horiba Jobin Yvon VS-140 linear array spectrometer equipped with a white led light source

10

working in the 400-750 nm range. Spectra were collected with an acquisition time of 50 ms at a

11

resolution of 2.3 nm. This very short acquisition time allowed us to collect spectra in real time

12

while the coloration process was in progress during the voltage supply. The light intensity with

13

no sample in place was assumed to be the full-scale intensity. Measurements were performed at

14

25 °C, under the application of pulses of -1.20 V (vs. Fc+/Fc).

15

An AMEL 2049 model potentiostat/galvanostat and an AMEL 568 programmable function

16

generator were used to apply a square potential during electrochromic measurements.

17 18

2.5. Cyclic Voltammetry (CV). Cyclic Voltammetry experiments have been performed

19

a) in a propylene carbonate (PC) degassed solution containing ethyl viologen perclorate 10-3 M,

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tetrabutyl ammonium perclorate 5 · 10-2 M, ferrocene (Fc) 10-3 M ; b) in a PC solution ∽ 1mM

21

O2 and tetrabutyl ammonium perclorate 0.1 M electrolyte. The measurements were done at a

22

platinum working electrode in a three-electrode cell, using a platinum auxiliary electrode, an

23

Ag/AgCl reference electrode and a potentiostat configuration from AMEL s.r.l. (Mod. 7050).


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CVs in the solid phase have been performed with a two-ITO electrode cell in which the polymer

2

film has been sandwiched. In this case, the Fc+/Fc redox couple has been used as reference

3

internal standard.

4 5

2.6. Electrochemical Impedance Spectroscopy. Impedance spectra were acquired with an

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Amel 7200 frequency response analyzer connected to an AMEL 7050 potentiostat, in the

7

frequency range 1Hz-30 KHz and using an ac level of 0.05 V. Spectra were acquired at room

8

temperature under the application of a dc voltage bias. Data were modeled with a Randles circuit

9

consisting of a resistance (Rs) that describes ITO contact resistance, followed by a charge

10

transfer resistance (Rct) in parallel with a constant phase element (CPEdl) capacitor that describes

11

the double layer capacitance related to the electrode/electrolyte interface. The low frequency

12

impedance behavior has been modeled by an infinite-length Warburg diffusion process (Wo).

13

Data were fitted by the ZSimpWin 3.50 (Princeton Applied Research, USA) software.

14 15

2.7. Rheological characterization. Electrochromic films, prepared as mentioned above, were

16

cut into strips of 10 mm × 7 mm and left at rest for at least 24 h at room temperature before being

17

submitted to mechanical analyses.

18

These film strips were submitted to normal small amplitude strains in a cyclic manner using a

19

Dynamic Mechanical Analyzer (TTDMA, 125 Triton Technology, UK), equipped with tension

20

clamps. One end of the strand was attached to a superior mobile clamp and the other end was

21

attached to a lower fixed clamp.


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During the tests a periodic sinusoidal strain was applied to the sample and the resulting

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sinusoidal force was measured in terms of amplitude and phase angle as a function of the

3

oscillation angular frequency (frequency sweep tests).

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The resulting dynamic modulus was obtained. This has been divided into two components: the in

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phase storage modulus, E' (elastic response), and the out-of-phase loss modulus E''(viscous

6

response ). The loss tangent, Tan(δ), defined as E’’/E’ and the complex modulus E*, defined as

7

E ' ' 2 + E ' 2 , could be than calculated as a function of the frequency.

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The frequency sweep tests were carried out in the linear viscoelastic region of the material,

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where a linear relationship existed between strain and stress and, thus, the constant strain value

10

used in the measurements was chosen within this region for each sample.

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The linear viscoelastic region was previously determined by performing amplitude sweeps tests

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where the normal storage (E’) and loss (E’’) moduli as well as strain were recorded as a function

13

of stress, at a constant frequency of 1 Hz and a temperature of 25 °C.

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The frequency sweep tests were performed at the same temperature and in the frequency range of

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0.1-10 Hz.

16 17

3. RESULTS AND DISCUSSION

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3.1. Electrochromic response of the films. The rheological properties of the investigated films

19

are summarized in Table 1. From the rheological point of view no difference appeared between

20

the films obtained at the same polymerization time. The rheological properties resulted to be

21

almost invariant with respect to the oscillation frequency, used in the Dynamic Mechanical

22

Analysis, but the values of the mechanical modules resulted to be quite different for the Low

23

polymerized (LP) and High polymerized (HP) films. Looking at the values of E* and Tan(δ)


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(Table 1), it is possible to state that the HP samples are characterized by a more structured

2

polymer network with respect to that of LP samples, as evidenced by the smaller loss tangent and

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higher complex modulus values. The behavior is solid-like for both the films (values of Tan(δ)

4

lower than 1 define a solid-like behavior, while values greater than 1 define a liquid-like

5

behavior), but the predominance of the solid-like behavior for the HP films is increased by a

6

factor two. In other words, a longer polymerization time produces, as expected, an increase of

7

the film stiffness and the structuring degree.

8 9 10

Table 1. Mechanical properties and electrochromic kinetics of the different investigated

11

films as a function of UV curing time and oxygen content EC Filma

Curing time

Film stiffness

Kinetics of coloration

(min)

Tan (δ)

E*(×10-5) (Pa)

C

γ (×103) (sec-1 )

LP-ox

10

0.150 (0.005)

5.24 (0.12)

122.95 (0.06)

65.93 (0.05)

LP-deox

10

0.147 (0.004)

5.31 (0.10)

153.40 (0.07)

77.57 (0.09)

HP-ox

30

0.080 (0.009)

7.61 (0.09)

118.90 (0.13)

27.40 (0.096)

HP-deox

30

0.083 (0.010)

7.49 (0.08)

97.90 (0.18)

22.31 (0.099)

a

LP = Low polymerized; HP = High polymerized; ox = polymerization under aerated conditions; deox =

polymerization under oxygen-depleted conditions

12


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1

Besides the role played by the film stiffness, here we study the effect of molecular oxygen

2

impurities on the electrochromic kinetics. However, oxygen can readily reacts with the carbon

3

centered radicals during free radical polymerization, acting as scavenger of both initiating and

4

propagating species.15 Therefore, though polymerization can proceed in the presence of oxygen,

5

the free oxygen molecules initially present in the EC solution, could be incorporated either in

6

monomeric hydroperoxide acids or in the polymer structure as polymeric peroxides, affecting the

7

polymer properties.15 However, the data reported in Table 1 clearly show that oxygen has not

8

significant effect on the final rheological properties of the polymer films. In order to show that

9

the films obtained under air-equilibrated and degassed conditions have different free oxygen

10

content, we incorporated into the EC mixture the luminescent probe Rose Bengal (RB) whose

11

emission intensity is highly sensitive to the amount of oxygen in the medium, being enhanced in

12

absence of dioxygen.16 The excitation and emission spectra of the dye-incorporating EC

13

polymeric matrix are displayed in Figure 2.

14 15 16 17 18 19 20 21 22

Figure 2. Excitation (black curve) and emission spectra of RB in deoxygenated (HP-deox; red

23

line) and air-equilibrated film (HP-ox; blue line).

24


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The emission spectra in presence or absence of oxygen show a maximum at 569 nm and a low-

2

intensity band at about 687 nm, while total luminescence is enhanced when oxygen is removed.

3

By dissolving RB in air-equilibrated electrochromic solution (Supporting Information, Figure

4

S1), a similar behavior is observed on a comparable spectral range slightly blue shifted in

5

polymer matrix respect to the solution, due to the more rigid environment experienced by the

6

chromophore, confirming that RB has been preserved after the polymerization process. By

7

analogy with the band attribution of solution spectrum,16 transition at 569 nm is due to a

8

fluorescence deactivation, while the band around 700 nm is identified with the triplet state

9

phosphorescence of the dye. Degassed sample shows an increased fluorescence intensity respect

10

to the air-equilibrated one, because O2, being a paramagnetic species, promotes the singlet-triplet

11

intersystem crossing, and its absence favors fluorescence deactivation path.17 Moreover, at room

12

temperature the limited mobility of the photosensitizing agent in rigid matrix respect to solution,

13

allowed the detection of a more intense phosphorescence18,19 with respect to that in solution

14

(Supporting Information, Figure S1); phosphorescence is in turn slightly enhanced in degassed

15

sample respect to air-equilibrated one (Figure. 2), due to the sensitation quenching effect exerted

16

by O2 on the RB triplet state.16 These evidences can be rightly explained by the different free

17

oxygen content in the two films, that has a clear quenching effect. Therefore, even thought we

18

cannot provide quantitative data on the concentration of free oxygen in the EC films, we are

19

however sure that a certain amount of O2 molecules is present either in aerated or in

20

deoxygenated samples, but a significant concentration difference exists between them.

21

Figure 3a shows the optical absorption spectra of the four different EC films after a -1.20 V (vs.

22

Fc+/Fc) square DC pulse with a step duration of 60 seconds. In the same spectral region, the off

23

state spectra is completely flat, as already observed previously.11 Electrochromic switching at


12

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1

this potential involves only the first reduction of the viologen, as can be seen in the cyclic

2

voltammograms (CV) of Figure 3b acquired on the HP-ox EC film sandwiched between two ITO

3

electrodes. In such a case, the ITO plate, where electrons are injected, represents the working

4

electrode while the other one the counter electrode.20 As pseudo-reference electrode, the Fc+/Fc

5

couple can be suitably used. The two-electrode cell CV shows indeed, the reversible process

6

associated to the Fc+/Fc couple at a half-wave potential of +0.032 V and two strongly reversible

7

redox processes attributed to the viologen couples V++/V•+ and V•+/V0 at half-wave potentials of -

8

0.74 V vs. Fc+/Fc and -1.26 V vs. Fc+/Fc. Therefore, the spectral bands shown in Figure 3a have

9

been assigned to the optical transitions of the V•+ viologen species, so they can be related only to

10

the kinetic evolution of this species during the electrochromic process. According to our previous

11

work11 where a detailed analysis of the absorption has been carried out, we have found that each

12

of these bands results from the overlapping of four different transitions. In Figure 3c, an

13

experimental spectrum, obtained from the LP-deox sample, is reported together with the fitting

14

by Gaussian deconvolution. For the other three samples, an analogous behavior has been

15

detected and a best fit band has been obtained after deconvolution.

16

The real-time integrated absorbance obtained from the four samples at different times has been

17

used to study the kinetics of the EC coloring process (Figure 3d). An interesting feature of the

18

electrochromic kinetics is that, even though a large number of factors would have been expected

19

to influence the integral absorption spectra, the kinetic data (Figure 3c) are very well interpolated

20

(all the errors do not exceed 0.44%) by a single exponential function (eq. 1):

21

(

A(t) = C 1− e−γt

)

(1)


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1

where, γ is the rate of the coloring process and C is the integrated absorbance at infinite time.

2

The value of the fit parameters are reported in Table 1.

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Figure 3. a) Visible absorption spectra of the analyzed EC films recorded after 60 s of a square

14

pulse application of -1.20 V (vs. Fc+/Fc). b) CV of the HP-ox film acquired in a two-ITO

15

electrode cell (electrode area 1.5 cm2; cell gap 120 µm; scan at 50 mV/s). c) Spectral

16

deconvolution (green) of a LP-deox absorption spectrum (black) acquired in the same conditions

17

as in a) and best spectral fit (red). d) Real time integrated absorbance of the four investigated

18

samples, calculated every 50 ms during 120 s application of a dc voltage of -1.20 V (vs. Fc+/Fc).

19

The straight lines are the exponential functions (eq. 1) that best interpolate the data (Table 1).

20


14

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1 2 3

These data show that the kinetics of the electrochromic response is primarily influenced by the

4

stiffness of the film (LP vs. HP). In general, it can be seen that either γ or the integrated

5

asymptotic absorption C are larger in the films with lower stiffness (LP) than in those with

6

higher stiffness (HP) (Table 1). This was of course expected. Indeed, the reduction process

7

which leads to the formation of the V•+colored species, occurs at the cathode. Therefore, the

8

gradual increase of light absorption involves the diffusion of the V++ to this electrode and the

9

diffusion of the V•+from the cathode to the bulk of the film. The greater the diffusion coefficient

10

of the viologen species throughout the film, the faster is the coloration process. On the other

11

hand, the diffusion coefficient of the molecules is expected to increase with decreasing film

12

stiffness. This explains, on a qualitative basis, the faster electrochromic kinetics of the LP films.

13

It is more complex, however, to explain the role played by oxygen on the electrochromic kinetics

14

and the dependence of this role on the sample stiffness. The data reported in Figure 3 and in

15

Table 1 clearly show that deoxygenation speeds up the coloration process in the LP films while

16

the reverse effect occurs in the case of the stiffer films. Therefore, depending on the stiffness of

17

the polymer matrix, oxygen can positively or negatively influence the kinetics of the

18

electrochromic processes.

19

In order to gain insight into this issue, we studied the reduction reaction of O2 in propylene

20

carbonate (PC) by cyclic voltammetry (CV) (Figure 4a). In agreement with other literature

21

reports,21-23 here we find that the O2/O2•- process occurs at the cathode at a reduction potential of

22

-0.85 V (vs. AgCl/Ag) (Figure 4a). The absence of the anodic wave evidences the irreversible

23

character of this reaction and the relative stability of the reduced oxygen species, O2•-. Inspection


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Page 16 of 31

1

of Figure 4a and b reveals also that the oxygen molecule can compete with the V++species in

2

cathodic reduction since its reduction potentials is very close to that of the V++/V•+ couple.

3

Therefore, the heterogeneous reduction of oxygen must be considered among the reactions

4

occurring at the cathode electrode during the electrochromic response achieved in the conditions

5

reported in Figure 3a.

6

Furthermore, the possibility of electron transfer (ET) from the HOMO energy level of the O2•-

7

superoxide anion to the LUMO level of V++, should be considered. This can occur only if the

8

energies of these orbitals are quite similar, or if the energy of the HOMO of the superoxide ion is

9

higher than that of the LUMO of the V++.

10 11 12 13 14 15

Figure 4. CVs of a) ~ 1 mM O2 and b) ethyl viologen perchlorate (4×10-3 M) and ferrocene

16

(4×10-3 M), in propylene carbonate (PC) solutions. CVs were acquired with an Ag/AgCl

17

reference electrode at scan rate of 50 mV/s. Tetrabutyl ammonium perclorate 0.1 M was used as

18

electrolyte.

19 20

This last has been determined by the CV data of Figure 4b and with calibration referred to the

21

standard (Fc) redox system. The first reduction potential of ethyl viologen dication in PC


16

Page 17 of 31

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1

solution is about -0.9 V vs. Fc+/Fc (Figure 4b). Assuming a value of -4.8 eV for the HOMO

2

energy level of ferrocene with respect to the zero vacuum level,24 a LUMO value of -3.9 eV for

3

V++ can be derived. This is in agreement with literature data for similar azo-compounds.25,26

4

On the other hand, a HOMO energy value of -3.5 eV with respect to vacuum, has been calculated

5

for

6

double excitations (QCISD).27

7

These data support the idea that the superoxide anion formed in the cathodic reduction can easily

8

transfer the acquired electron to the viologen dication. In other words, oxygen impurities can be

9

efficiently reduced at the cathode, at the same voltages inducing the viologen dication reduction.

O2• − with ab initio calculations based on quadratic configuration interaction with single and

10

The superoxide anion generated in this way can then assist the electrochromic process by

11

transferring the electron to the viologen dication. It must be considered that superoxide anion,

12

being very small with respect to the solvated viologen molecules (nearly two orders of magnitude

13

smaller), is able to diffuse very efficiently also through very stiff polymer media and can act as

14

an efficient charge carrier into the polymer matrix. This could explain why in stiffer systems the

15

electrochromic processes become faster when the oxygen content has been increased. We have to

16

face now the following question: how can the opposite role played by oxygen in films having a

17

different degree of polymerization be explained? To find an explanation we have developed the

18

theoretical model reported below.

19 20

3.2. Reaction Kinetics Model. It has been shown here that in the electrochromic films the

21

second reduction process illustrated in Figure 1 does not occur when using driving voltages

22

below -1.3 V vs. Fc+/Fc (Figure 3b). In this case, the amount of V0 is negligible with respect to


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

1

that of the V•+species, even after long driving pulses. Moreover, taking into account the possible

2

role played by the oxygen molecules, we could consider all the kinetic reactions involved in the

3

formation and the consumption of the radical cation:

4

a)

++

V

+ e →V • + −

5

b)

k1c

−

k2c

O2 + e → O2• − +

−

k3c

An + e → An

6

c)

7

at the cathode electrochemical double layer;

8 k1b

9

V ++ + O2• − →V • + + O2 d) V

•+

k r 1b

+ O2 →V ++ + O2• −

10

e)

11

f) V • + + An + → V + + + An

k2b

•− 2

+

k3b

+ An → O2 + An

12

g) O

13

in the bulk of the electrochromic film;

14

15 16

h) i)

V O

•+

•− 2

k r1

→V + + + e− kr 2

→ O2 + e −


18

Page 19 of 31

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k3a

1

j)

An → An + + e −

2

at the anodic double layer.

3

Here the anodic substance (ferrocene in this paper) has been generically indicated as An. In order

4

to simplify the kinetic equations it is possible to make some further reasonable assumptions

5

based on the diffusion of viologen, ferrocene, oxygen molecules across the film.

6

To the best of our knowledge, no diffusion coefficient data of viologens or ferrocene molecules

7

embedded in soft plastic of the type used here, are available in literature. However, we can

8

roughly estimate it by considering that the diffusion coefficient of pyrene molecule in

9

metacrylate polymers is of the order of 10-12 cm2sec-1, at temperatures above the glass transition

10

temperature (Tg).28 Here we determined (by DSC) a Tg for the HP films of -74.2° C, while our

11

experimental temperature was 25°C, that is to say 99.2 degrees above Tg. Taking into account

12

that viologen and ferrocene are smaller than pyrene and that the solvent propylene carbonate is

13

contained in the electrochromic films, we can assume that the diffusion of these molecules in the

14

EC films is faster than the pyrene's diffusion in metacrylate and it is possible to hypothesize a

15

value of 10-10-10-11 cm2sec-1 for the diffusion coefficient, in agreement with data reported for

16

viologen and its derivatives in polyethylene glycol.29 Even faster diffusion coefficients (i.e. 10-8 -

17

10-9 cm2sec-1) are expected in the LP films for which the mechanical consistency was of gel type,

18

in agreement with data concerning viologen diffusion in nafion films (D=0.8x10-9 cm2sec-1).30

19

The electrochromic films here investigated have thicknesses of about 120 µm. This means that,

20

even after a long driving pulse (let us say 100 seconds), the V•+and the An+ species are not able to

21

diffuse more than ~2 µm regarding the longer time cured film, and more than ~20 µm in the

22

short time cured one. Conditions are very different when taking into consideration the diffusion


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

1

of the oxygen molecule in the film. For this small molecules, it is expected to be fast in both

2

types of films with a weakly dependence on the structuring degree of the polymer. In glassy

3

polyacrilate films the oxygen diffusion coefficient is of the order of 10-8 cm2sec-1.31 Therefore,

4

we can hypothesize that it could be in the range 10-5-10-6 cm2sec-1 for the HP and LP films, in

5

agreement to literature data concerning diffusion coefficient measurements in various

6

solutions.32-35 Therefore, oxygen molecules are able to travel across the greater part of the film

7

thickness, in a few seconds. This is not the case of viologen and ferrocene molecules, that can

8

travel across only a small part of the film and do not have the possibility to transfer electrons

9

directly to each other according to eq. f).

10

On the basis of the above considerations, we can neglect reactions c), f) and h) listed above, at

11

first approximation. Another argument strengthening our hypothesis is that equation f), if

12

efficiently occurring, would partially confer to the kinetic evolution of the V•+ species a second

13

order character, that is to say a partially hyperbolic trend. This comes from the fact that V•+ and

14

An+ are produced without any charge accumulation in the film (at least for the greater part of the

15

time of the electrochromic evolution of the film) and therefore their concentrations should be

16

proportional. Let us treat now the coloring process, disregarding eqs. c), f), h).

17 18

3.2 Kinetics of the coloring process. Taking into account the arguments exposed above, the

19

kinetic equation for the V•+ species can be written:

20

D •+ ∂ • + ∂ •+ ∂2 V = qV • + V V E + DV • + V • + + k 1c f V + + + ∂t RT ∂ x ∂x 2 + k1b V + + O 2• − − k r 1b [O 2 ] V • +

[ ]

[ ]

[ ][ ]

[ ]

[ ]

[ ]

2)


20

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DV • + ∂ • + V E is the contribution from migration due to the movement of charged RT ∂x

[ ]

1

where, qV • +

2

viologen radical-cations under the influence of the electric field E , D V

3

contribution to the time variation of the V•+ due to molecular diffusion; [V

4

[O2 ] represent the molar concentrations of the chemical species; while k1c, k1b, kr1b are the kinetic

5

constants of the reactions listed above. Equation 2 holds under the approximation of

6

electroneutrality condition in the bulk of the electrochromic film where the perturbation in the

7

concentration due to the electric field is negligible. Actually, the electric field effects are

8

expected to be relevant only next to the electrode, in the so called electric double layer, where

9

the electric potential drops very rapidly at increasing distances from the electrode. This distance

•+

∂2 V ∂x 2

[ ] is the

•+

•+

] , [V

++

•−

] , [O2 ] ,

10

is the so called Debye length.36 Here, it has been estimated to be of the order of a few nm (∼ 6

11

nm), by electrochemical impedance spectroscopy of the electrochromic device (Figure S2 and

12

Table S1). Therefore, despite the low mobility and concentration of charge carriers in the film

13

(unsupported situation), bulk electroneutrality approximation is justified by the fast reaction

14

kinetics at the electrodes and the large electrode separation (120 µm).37

15

In order to describe the competition occurring at the cathode between the viologen dications and

16

the oxygen molecules for the electron uptake, it has been necessary to define f as the fraction of

17

the cathode surface interacting on average with the V++ species.

18

Equation 2 could not be solved without taking into account a similar equation for the time

19

variation of [O2• − ] . A simplifying hypothesis for the solution of eq. 2) can be introduced

20

considering that the kinetic trend of the experimental data concerning the electrochromic

21

processes (Figure 3c) is of first order. To take into account this experimental evidence we have

22

supposed that the oxygen molecule can diffuse through the film faster than the viologen, so the


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

O2• − species rapidly reach a stationary state characterized by a constant value of their

1

O2

2

concentrations in the full film volume. It is then possible to express these stationary

3

concentration values in the form:

and

4

[O2 ] = α[O2]0

5

[O ]= (1−α ) [O ] = β [O ] •− 2

2 0

3)

2 0

[O2 ]0 the concentration of oxygen in the not operated film and α a constant to be

6

being

7

determined. Likewise, also the electron density at the cathode, e-, is assumed to reach a

8

stationary state.

9

The hypothesis of stationary state conditions assumed for the concentration of oxygen species,

10

allows us to consider (1-f ), the cathode surface fraction interacting on average with the oxygen

11

molecules, as proportional to the analytic oxygen concentration, through a proportionality

12

constant S0. The surface fraction f can then be expressed as:

13

f = (1− S0[O2 ]0 )

4)

14

Taking into consideration that [V++] = [V++]0 − [ V • + ], where [V++]0 is the equilibrium viologen

15

concentration in the not operated film, equation 2 can be rewritten:

16

∂ •+ V ( x, t ) = ∂t D •+ ∂ •+ ∂2 = qV • + V V E + D 2 V • + (x, t ) + k1c f V + + 0 − V • + (x, t ) + RT ∂x ∂x + k1b β [O2 ]0 V + + 0 − V • + ( x,t ) − k r1bα [O2 ]0 V • + (x,t )

17

where it has been highlighted that the variables depend both on time and on the spatial

18

coordinate along the film thickness (x).

[

]

[ ] {[ ] [

[ ]}

]

{[ ] [ [ ]

]}

5)


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Equation 5) can be integrated over the film thickness (L). Reminding that the third term on the

2

right side represents the viologen radical cation V • + production at the cathode, we can restrict the

3

integration of this term only in the region of the electric double layer next to the cathode. Let us

4

define this length as ∆‹‹L and assume that

5

viologen radical cation can be expressed as:

[V

•+

L

(x,t)] ≈[V•+(0,t)]. The optical absorbance for the

[

]

A+ (t ) = ε ∫ V • + ( x, t ) dx

6

6)

0

7

On the other hand the fitting of our experimental data (Figure 3, Table 1) suggests an exponential

8

behavior for

A+ (t ) : A+ (t ) = C(1− e−γt )

9

7)

10

We integrate equation 5) to get the time-dependent evolution of the absorbance.

11

By using the following definitions:

k3 = ε(k1c f∆ + k1bβ[O2 ]0 L) ; γ

12

=

[ ]

k3 + + V C

8)

0

13

and passing through the equations developed in the Supporting Information; Appendix,

14

we obtain for the decay constant

γ =

15

16

γ the following expression: ε

[V ] {k C ++

0

1c

∆ + (β k 1b L − k1c S 0 ∆ )[O 2 ]0 }

9)

that can be rewritten in terms of the diffusion coefficients DO•− and DV++ of the c and V++ species 2

17 18

respectively:38 γ =

ε C

[V ] {a D ++

0

V ++

(

)

+ bD O • − − cDV + + [O 2 ]0 2

}

(10)


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

1

The factors a, b, and c are defined as:

2

a = e−E1C RT / l; b = 4πrNALβe−E1b RT; c = S 0 a

3

where, l is the thickness of the layer in which the concentration drops from the bulk value V++ to

4

its value near the cathode surface VS ; r is the distance at which the

5

react. NA is the Avogadro number. E1c and E1b are the activation energies of the reactions a) and

6

d), respectively (section 3.1).

7

We want here to underline that equation 10 yields a very simple explanation of the opposite

8

influence of the oxygen content as a function of the sample stiffness. The contribution to γ from

9

oxygen can be positive or negative depending on the sign of the term bDO •− − cDV + + . A

++

O2•− and V++ species can

(

2

)

10

qualitative explanation of the observed dependence of the electrochromic kinetics with respect to

11

the oxygen concentration is achieved by assuming that this term changes from positive to

12

negative when going from films with an enhanced solid-like behavior to less structured films.

13

This would occur if the diffusion coefficient of the viologen decreases much more than that of

14

the oxygen anion, when the stiffness of the film increases.

15 16

4. CONCLUSIONS

17

This paper reports that the presence of oxygen strongly affects the electrochromic kinetics in

18

plastic electrochromic films containing viologen active species. The operational times of the

19

devices depend on the oxygen content in a complex way. In films with higher stiffness the

20

increase of the oxygen concentration reduces the coloring times, while the effect reverses in less

21

structured films. This behavior has been clarified on the basis of a kinetic model which takes into

22

account the competition of V++ and O2 species in the uptake of electrons at the cathode.


24

Page 25 of 31

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1

Assuming that the bulk and electrodes electron exchange reactions are controlled by the

2

diffusion of the reacting species, it was possible to explain the experimental results, at least at a

3

qualitative level, on the basis of a different influence played by the film stiffness on the diffusion

4

coefficients of oxygen and viologen species. All the results can be indeed justified if the

5

diffusion of the viologen grows, in percentage, much more than the diffusion of oxygen, when

6

the stiffness lowers.

7

The approach presented in this paper is susceptible of further inquire improvements, in order to

8

investigate the role played by several chemical and physical factors on the electrochromic

9

response of plastic self supported electrochromic films. Work is in progress in this direction.

10 11

ASSOCIATED CONTENT

12

Supporting Information

13

Figure S1 showing the emission spectra of the electrochromic solution acquired under air-

14

equilibrated and oxygen-depleted conditions. Figure S2 showing the impedance spectra (Nyquist

15

and Bode plots) of the electrochromic device and the equivalent circuit model used for fitting the

16

impedance data. Table S1 reporting the fitting results. Appendix describing all the physical-

17

mathematical steps needed to solve the set of differential equations relative to the reaction

18

kinetics model. This material is available free of charge via the Internet at http://pubs.acs.org.

19

AUTHOR INFORMATION

20

Corresponding Author

21

Amerigo Beneduci, Dept. of Chemistry and Chemical Technologies, University of Calabria. Via

22

P. Bucci, Cubo 17/D, Arcavacata di Rende (CS), Italy. E-mail: [email protected]


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Author Contributions

2

GC and BCD developed the reaction kinetics model. BCD performed optical spectra study. DI

3

performed electrochemical study, HOMO LUMO calculations and contributed to the

4

development of the kinetic model. AB prepared samples, performed luminescence and

5

electrochemical studies in the polymer films and contributed to the development of the kinetic

6

model. LR performed luminescence study. MD prepared samples. The manuscript was written

7

through contributions of all authors. All authors discussed the data and have given approval to

8

the final version of the manuscript.

9 10

Notes The authors declare no competing financial interest.

11

Acknowledgments

Page 26 of 31

12

MIUR is acknowledged for financial support (Grant PRIN 2008 N. 2008 ALLB79). The

13

authors are also grateful to Dr. Lucia Seta for rheological measurements and to Prof. Emilia

14

Sicilia for helpful discussion about the quantum mechanical description of O2/O2•- reduction.

15 16 17 18

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Influence of Oxygen Impurities on the Electrochromic Response of

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