DOI: 10.1002/celc.201402265

Polyvinyl Alcohol–Borax Slime as PromisingPolyelectrolyte for High-Performance, Easy-to-MakeElectrochromic DevicesYolanda Alesanco, Jesffls Palenzuela, Ana ViÇuales, Germ�n CabaÇero, Hans J. Grande, andIbon Odriozola*[a]

1. Introduction

Electrochromic devices (ECDs) can change their optical proper-ties reversibly, as a result of the electrochemical oxidation andreduction of a chemical species, known as an electrochromicmaterial. The nature of such electrochromic materials can bequite broad, ranging from inorganic systems to organic mole-cules or conducting polymers.[1–4] The simplest configuration ofan electrochromic device comprises two electrodes sandwich-ing an electrochromic mixture or electrolyte. The electrochro-mic material can be either dispersed in the electrolyte, togeth-er with a suitable redox pair, or deposited over an electrode.

Transparency and high performance are highly sought inelectrochromic devices for their applications in displays, anti-glare rearview mirrors in cars or in smart windows for energy-efficient architecture. Curiously, one of the major problems oftransparency in electrochromic devices arises from the chemi-cal nature of the electrolytes. Thus, the search of high-per-formance electrolytes with high transparency is an active fieldof research.

Traditionally, electrolytes for ECDs are based on organic sol-vents, where all the electroactive components are dissolved.Examples can be found in previously published and patentedwork by our group.[5, 6] Despite their extensive use, liquid elec-trolytes are not exempted from several disadvantages, such assolvent leaking, presence of bubbles or complicated industriali-zation due to solvent-related safety issues. Several efforts to-

wards the development of solid electrolytes have been recent-ly proposed in the literature for batteries, solar cells and elec-trochromic applications.[7, 8] Typically, such solid systems havebeen based on polymers,[9–15] gels[16–18] or composites.[19, 20]

However, in many cases solid electrolytes are not fully trans-parent, and they usually lead to poor performance of electro-chromic devices, due to the poor mobility of the ionic speciesin the solid matrix.

For practical reasons, many of the solid electrolytes are as-sembled in their liquid form and subsequently cured by irre-versible chemical crosslinking.[20] Such irreversible curing cangenerate some non-desirable defects on the device, such asbubbles. An interesting approach to overcome these problemswould be the use of electrolyte mixtures which could be curedby dynamic covalent chemistries.[21, 22] These reversible chemis-tries have recently been demonstrated to be very useful in thefields of dynamic combinatorial chemistry and self-healingpolymers.[23–25]

Polyol–borax slimes, first developed by the Mattel Toy Cor-poration in the 1970s,[26–28] behave as viscoelastic fluids due tothe reversible nature of their borate ester crosslinks. In thepresent work we have developed an electrochromic mixturebased on PVA-borax slime (Figure 1 a,b), with the aim of over-coming the abovementioned problems associated with solidpolyelectrolytes. Due to its non-Newtonian nature, the electro-chromic slime combines the advantages of both liquid andsolid electrolytes, including gels, such as being easy to apply,offering good wettability and excellent contact to the upperand lower electrodes, or optimizing the ionic conductivity andcharge transmission. Polyelectrolyte mixtures made with PVA-borax slime in combination with ethyl viologen and ferrocya-nide/ferricyanide complementary redox species (Figure 1 c)have been evaluated in ECDs. The good results obtained in

A novel slime-type electrochromic system based on polyvinylalcohol (PVA) and borax, in combination with a viologen anda redox pair has been developed. On top of overcoming thelimitations of both liquid electrolytes, such as the risk of leak-age and difficulty of assembly and of solid electrolytes, such aslimited transparency and slow response time, this easy-to-make slime electrochromic system offers an excellent wettabili-ty and transparency and achieves high-performance in terms

of optical contrast (>65 % at 550 nm), switching time (<5 s asestimated for 90 % of the total transmittance change at550 nm) and cyclability (8 000–10 000). For the electrochromicdevices shown here, the CIELAB 1976 color space coordinatesat the “on” state were L* = 7.12, a* = 18.08, and b* =�21.02,corresponding to a purple color. The color efficiencies were75.5 cm2 C�1 and 149.3 cm2 C�1 for coloration and bleachingprocesses respectively.

[a] Y. Alesanco, Dr. J. Palenzuela, Dr. A. ViÇuales, G. CabaÇero, Dr. H. J. Grande,Dr. I. OdriozolaMaterials DivisionIK4-CIDETEC Research CenterPaseo Miram�n 196, 20009 Donostia-San Sebasti�n (Spain)E-mail : iodriozola@cidetec.es

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/celc.201402265.

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terms of transmittance, conductivity and cyclability suggestthat the slime material could have a promising potential forthe fabrication of high-performance, easy-to-make inexpensiveECDs.

2. Results and Discussion

2.1. Optimizing Viologen Concentration

Optimization of the final amounts of the electrochromic andredox species is crucial to assure good performance of the re-sulting ECD. For that purpose, different amounts of an aque-ous solution of PVA and ethyl viologen dibromide (to havefinal concentrations from 2.5 mmol L�1 to 25 mmol L�1) weremixed with a fixed concentration of the complementary redoxmixture. The corresponding slimes were then formed with theaddition of the borax solution (Slimes 3 a–h). Such electrochro-mic slimes were then used for the fabrication of devices asshown in Figure 2. The fabrication of the ECDs was very easy, itonly being necessary to spread the slime on the substrates like“jelly-on-toast”, and then covering it with the other electrodesubstrate. Interestingly, if the slime was left to settle on itsown for a few minutes before covering with the second elec-trode, a completely bubble-free device could be obtained.

After studying the variation in transmittance (%T) as a func-tion of wavelength for different applied voltages in the opera-tional range (�2 V to �2.4 V) it was concluded that, in order toobtain the highest optical contrast, the most suitable voltagefor the ON state was �2.3 V. Then, the spectrophotometricmeasurements were performed in transmission mode usinga constant concentration of 0.8 mmol L�1 of ferro/ferricyanide,connecting the devices to a direct current source.

The electrochemical window found for these slime-basedtwo-electrode devices ranged from �2.8 V to + 2.8 V. Beyondthese values, gas generation was observed due to the waterelectrochemical decomposition. Hence, the operational voltageemployed for these systems is within the electrochemicalwindow.

Figure 3 a shows transmittances vs. wavelength plots ob-tained for eight devices with different viologen concentrations.The transparency at the OFF state was similar and equivalentfor all the concentrations, indicating that, for this system, theinitial transmittance is not related to the viologen concentra-tions, at least not in the studied concentration range, but relat-ed to the bare FTO glass electrodes employed for device fabri-cation (Figure S1, Supporting Information). In the ON state, thetransmittance kept on decreasing as the viologen concentra-tion increased, arriving to full absorbance between 500 and600 nm at a concentration of 20 mmol L�1, with an optical con-trast of 68 % at 550 nm. Thus, from the results obtained weconcluded that the most suitable concentration of ethyl violo-gen dibromide to be employed in further experimentationwould be 20 mmol L�1 (Slime 3 g).

2.2. Optimizing Concentration of Redox Species

The following step consisted of optimizing the device in termsof response time, by adjusting the appropriate amount of thecomplementary redox salts. A series of nine ECDs were fabri-

Figure 1. a) Synthesis and chemical structure of the PVA–borax slime;b) photographs showing the physical aspects and texture of the slime;c) ferrocyanide/ferricyanide complementary redox species used for the violo-gen electrochromic reaction and chemical structure of ethyl viologen in itsoxidized state (left) and its reduced state (right).

Figure 2. Assembly of electrochromic device and final coloration after apply-ing a switching voltage. a) FTO glass electrodes and electrochromic slime;b) jelly-on-toast application of electrochromic slime on the electrode;c) sandwiching with the second electrode; d) clipping the electrodes; e) ECDin the bleached state; f) ECD in its colored state, after applying a voltage of�2.3 V.

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cated with varying concentrations of ferro/ferricyanide potassi-um pair, while keeping the concentration of ethyl viologenconstant at a value of 20 mmol L�1 (Slimes 3 g1–9).

The transmittance at 550 nm was measured for the nine sys-tems and plotted against time (Figure 3 b). The optimumswitching potentials and times for these ECDs were establishedas �2.3 V for the ON state for 30 s and 0 V for the OFF statefor 90 s. In the light of these results, several conclusions couldbe obtained. 1) For a change in the percent transmittance>65 %, a fast system could be obtained even for a relativelyhigh (20 mmol L�1) viologen concentration. 2) Interestingly, andprobably due to the dynamic character of the PVA–boraxslime, the transport of material and thus the speed of switch-ing of these ECDs was considerably higher than when usingstandard solid electrolytes. 3) The response time could beeasily customized, by adjusting the amount of ferro/ferricya-nide potassium salts. This result can be of special interest inthe field of electrochromic windows, as the pane could bekept in the colored state with little or no input of electricpower.

The ECD showing the fastest switching response resulted tobe that made from Slime 3 g9. This system was selected forthe further characterization.

It should be noted that the absorption band of the ferro/fer-ricyanide pair at 420 nm is visible at the bleached state (seethe Supporting Information). However, at the chosen concen-tration of ferro/ferricyanide (6 mmol L�1), its absorption banddoes not affect the color range of the viologen while devicecycling, rendering its contribution to the overall colorationproperties of the device negligible.

2.3. Electrochromic Properties

For Slime 3 g9, the electrochromic performance was character-ized in terms of optical contrast (D %T), switching times for col-oration and bleaching (tc and tb), color coordinates (CIE L*a*b)and coloration efficiency (h). Thus, the optical contrast, calcu-lated as the difference between the transmittance in thebleached state (%Tb) and the colored state (%Tc) was 58 % at550 nm. The switching times were 5 and 4 s for coloration andbleaching, respectively, as estimated for 90 % of the total trans-mittance change at that wavelength. These values are in therange of switching times for ECDs having gel or liquid electro-lytes (1–8 s)[29, 30] and overrunning solid-state organic ECDs(4–60 s).[15, 31–33]

Color efficiency is defined as the change in optical density,DOD = log (Tb/Tc), at a given wavelength, divided by thecharge density consumed during the coloration or bleachingprocesses (Q/A), where A is the device area and Q the injected/ejected charge. For 4 cm2-sized devices based on Slime 3 g9

and Q values corresponding to a time when 90 % of the fulloptical change occurs, an efficiency of h= 75.5 cm2 C�1 was ob-tained at 550 nm during coloration. Higher coloration efficien-cies (h= 149.3 cm2 C�1) were obtained during the bleachingprocess. The difference in the coloration efficiencies could beascribed to the ferricyanide mediator, which causes a veryrapid chemical oxidation of the radical-cation form of the viol-ogen, to reform the dication.[34] Possible entry of oxygen in thedevice could also lead to the same effect.

The CIELAB 1976 color space coordinates when the devicewas in the ON state (�2.3 V) were L* = 7.12, a* = 18.08 andb* =�21.02, corresponding to a purple color (combination ofred and blue according to the a* and b* values, respectively).The perceived purple color of the reduced ethyl viologen is ac-tually the result of color mixing of the blue ethyl viologen radi-cal cation, which is in equilibrium with the red ethyl viologenradical cation dimer in the aqueous electrolyte solution.[3, 35]

This electrochromic Slime 3 g9 was also selected to carry outthe following rheology and conductivity studies.

2.4. Rheology

Rheological behavior of the bare PVA-borax slime, (Slime 1)was as expected for a viscoelastic fluid, as shown in Figure 4.At low frequencies, Slime 1 presented a liquid-like behaviorwith G’<G“. Then, a liquid-to-gel transition (G’= G’’) was ob-served at a frequency of 2 Hz. At higher frequencies, Slime 1exhibited solid-like behavior, with G’>G”. A similar responsewas observed in a fully formulated Slime 3 g9 (20 mmol L�1 vi-ologen and 6 mmol L�1 ferro/ferricyanide potassium salts). In

Figure 3. a) UV/Vis transmittance responses of different ECDs made fromSlimes 3 a-h at �2.3 V and a constant ferro/ferricyanide concentration of0.8 mmol L�1. b) Transmittance changes of ECDs made from Slimes 3 g1�9

subjected to potential steps from �2.3 V (30 s) and 0 V (90 s) measured at550 nm.

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this case, the presence of viologen and redox pair salts shiftedthe liquid-to-gel transition to lower frequencies (0.4 Hz), indica-tive of a more solid-like character. Such particular rheologicalbehavior permits the facile manipulation of this electrolytemixtures during the device assembling process (SupportingMovie SM1).

2.5. Conductivity

It would be expected that the labile bonding between the PVAhydroxyl groups and the negatively charged central boronatoms could offer some ionic conductivity, also permitting theflow of the rest of the electroactive materials, thus enhancingthe necessary transport of material through the polymer net-work. The variation of the ionic conductivity of different slimesis presented as a function of 1000 T�1 in Figure 5. As expected,

the conductivity of the electrolyte slimes was highly depen-dent on the content of ionic species and also on the tempera-ture. In order to evaluate the contribution of the different ionicspecies present in the final ECDs, the conductivities of neatPVA–borax slime (Slime 1), the slime containing just the redox

pair (Slime 2) and the fully formulated slime (Slime 3 g9) werecompared. As expected, the conductivity increased with thepresence of redox pair and viologen, in the following order:Slime 1<Slime 2<Slime 3 g9. Again as expected, the ionicconductivity also increased with the temperature.

2.6. Cyclability

Cyclability of ECDs is a very important issue as far as their in-dustrial applicability is concerned. In order to analyze the cy-clability of our devices, three identical ECDs were fabricatedfrom Slime 3 g9. These ECDs were hermetically sealed usinga UV-curable adhesive. The selected switching potentials andtimes for these cyclability tests were established as �2.3 V forthe ON state, for 10 s and 0 V for the OFF state, for 180 s. Thethree tested ECDs could be cycled up to 8 000–10 000 times.Above these cycles, the bleached state started showinga slightly pink coloration, leading to a decrease in the opticalcontrast. Thus, the optical contrast was 58 % for up to10 000 cycles, being diminished to 50 % after 15 000 cycles andto 33 % after 25 000 cycles (Figure 6).

3. Conclusions

We have demonstrated that high-performance electrochromicdevices can easily be fabricated using a new concept of PVAslime as electrolyte. The response of such ECDs could bemodulated according to the users’ needs by selecting the ap-propriate concentrations of viologen and ferro/ferricyanide po-tassium salts. The viscoelastic fluid character of the electrolytemixture was very convenient to avoid the formation of bub-bles during the assembly. Finally, the obtained ECDs showedfast response and good cyclability, making them potential can-didates for industrial applications, such as smart windows forbuildings and vehicles, displays, and so forth.

Figure 5. Variation in the conductivity of Slime 1 (bare slime), Slime 2(6 mmol L�1 of 1:1 mixture of ferro/ferricyanide potassium salts) andSlime 3 g9 (6 mmol L�1 of 1:1 mixture of ferro/ferricyanide potassium saltsand 20 mmol L�1 of ethyl viologen) as a function of temperature.

Figure 6. Evolution of the cycling performance after 10 000, 15 000 and25 000 cycles.

Figure 4. Dynamic rheological behaviors of Slime 1 and Slime 3 g9. Filledand empty symbols correspond to storage (G’) and loss (G’’) modulus,respectively.

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Experimental Section

Materials

Polyvinyl alcohol (PVA, Mw 61 000), sodium tetraborate (borax,99.5 %), 1,1’-diethyl-4,4’-bipyridinium dibromide (ethyl viologen di-bromide, 99 %), potassium ferrocyanide (98.5 %) and potassium fer-ricyanide (99 %), were purchased from Sigma–Aldrich and used asreceived. Fluorine-doped tin oxide (FTO) coated glass substrates(TEC7, Rs 6–8 W sq�1) were supplied by Solems and UV-curing ad-hesive NOA-65 was provided by Norland Products.

Methods

UV/Vis spectra were obtained in transmission mode on a JascoV-570 spectrophotometer. The spectra were registered using a filmholder accessory for solid samples, while the devices were connect-ed to Biologic MPG potentiostat–galvanostat as a direct currentsource. Before recording the transmittance (%T) of ECDs as a func-tion of the wavelength at a suitable voltage, the devices were ex-posed to the same voltage during 40 seconds with the purpose ofreaching the maximum colored state before starting themeasurement.

Cyclability assays were carried out with the sealed devices in a Bio-logic MPG potentiostat–galvanostat. Measurements were made forthree identical devices. The devices were sealed using a colorlessUV-curing adhesive. The curing process was carried out in a TECNI-GRAF AKTIPRINT T/A 40-2 UV-curing tunnel.

Color coordinates of the devices were determined by performingcolorimetry measurements using a spectroradiometer Konika Min-olta CS-1000 A, with a D65 illumination source. CIE 1976 L*a*bcolor space was used as the quantitative scale to define the color.The three coordinates of CIELAB represent the lightness of thecolor (L* = 0 yields black and L* = 100 indicates diffuse white; spec-ular white may be higher), its position between red/magenta andgreen (a*, negative values indicate green while positive values in-dicate magenta) and its position between yellow and blue/cyan(b*, negative values indicate blue and positive values indicateyellow).

Rheological testing was carried out in a TA instruments AR2000exrheometer using a 20 mm plate-plate geometry. A 5 % strain stepwas applied and storage and loss modulus (G’ and G’’, Pa) weremonitored at room temperature.

Conductivity values were monitored in a Crison CM35 + con-ductimeter over a temperature ramp from 20 to 90 8C.

Preparation of Slime 1

A 4 % solution of PVA was prepared by placing 4 g of PVA and96 mL of distilled water in an appropriate flask equipped with me-chanical stirrer. A silicon oil bath was employed to heat the dissolu-tion to the desired temperature, lower than 100 8C to avoid theboiling of the solution, until the complete dissolution of the poly-mer was achieved. Separately, 4 % solution of borax was similarlyprepared. The slime was obtained by mixing the two solutions ina 4:1 (PVA:borax) volumetric ratio followed by vigorous stirringwith a spatula until the gel was obtained.

Preparation of Slime 2

The slime electrolyte (Slime 2), containing just the redox species,was prepared as follows: to a 4 % solution of PVA, an aqueous6 mmol L�1 solution of a 1:1 mixture of potassium ferrocyanide and

ferricyanide salts was added. The resulting mixture was stirred untila homogeneous solution was obtained. Then, this solution wasmixed with a 4 % borax aqueous solution in a 4:1 volumetric ratioby vigorous stirring with a spatula, until a gel was obtained.

Preparation of Slimes 3 a–h

The electrochromic Slimes 3 a-h, containing both electrochromicand redox species, were prepared as follows: to a 4 % solution ofPVA, aqueous solutions containing appropriate amounts of ethylviologen dibromide and a 1:1 mixture of potassium ferrocyanideand ferricyanide were added. The resulting mixtures were stirreduntil a homogeneous solution was obtained. Then, each of thesesolutions were mixed with a 4 % borax aqueous solution in a 4:1volumetric ratio by vigorous stirring with a spatula, until the forma-tion of a gel. The final concentrations of viologen for Slime 3 a,Slime 3 b, Slime 3 c, Slime 3 d, Slime 3 e, Slime 3 f, Slime 3 g andSlime 3 h were 2.5, 5.0, 7.5, 10.0, 15.0, 17.5, 20.0 and 25.0 mmol L�1,

respectively. The final concentration of redox salts was0.8 mmol L�1 in all cases (see Table 1).

Preparation of Slimes 3 g1–9

To a 4 % solution of PVA, aqueous solutions containing appropriateamounts of ethyl viologen dibromide and a 1:1 mixture of potassi-um ferrocyanide and ferricyanide were added. The resulting mix-tures were stirred until a homogeneous solution was obtained.Then, each of these solutions was mixed with a 4 % borax aqueoussolution in a 4:1 volumetric ratio by vigorous stirring with a spatula,until the formation of a gel. The final concentrations of redox saltsfor Slime 3 g1, Slime 3 g2, Slime 3 g3, Slime 3 g4, Slime 3 g5,

Table 1. Concentration [mmol L�1] of ethyl viologen dibromide and ferro-cyanide/ferricyanide redox salts (FeII/FeIII) in Slimes 3 a-h.

Slime [Ethyl viologen dibromide][mmol L�1]

[FeII/FeIII][mmol L�1]

Slime 3 a 2.5 0.8Slime 3 b 5.0 0.8Slime 3 c 7.5 0.8Slime 3 d 10.0 0.8Slime 3 e 15.0 0.8Slime 3 f 17.5 0.8Slime 3 g 20.0 0.8Slime 3 h 25.0 0.8

Table 2. Concentration [mmol L�1] of ethyl viologen dibromide and ferro-cyanide/ferricyanide redox salts (FeII/FeIII) in Slimes 3 g1�9.

Slime [Ethyl viologen dibromide][mmol L�1]

[FeII/FeIII][mmol L�1]

Slime 3 g1 20.0 0.4Slime 3 g2 20.0 0.8Slime 3 g3 20.0 1.2Slime 3 g4 20.0 1.6Slime 3 g5 20.0 2.0Slime 3 g6 20.0 3.0Slime 3 g7 20.0 4.0Slime 3 g8 20.0 5.0Slime 3 g9 20.0 6.0

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Slime 3 g6, Slime 3 g7, Slime 3 g8 and Slime 3 g9 were 0.4, 0.8, 1.2,1.6, 2.0, 3.0, 4.0, 5.0 and 6.0 mmol L�1, respectively. The final con-centration of viologen was 20 mmol L�1 in all cases (see Table 2).

In the case of Slime 1, in order to maintain the final concentrationin all samples, the same amount of distilled water used in theSlime 2 and Slimes 3 to dissolve the viologen and the comple-mentary redox species, was added.

Fabrication of Electrochromic Devices

Once the electrochromic slimes were formed, they were used forthe fabrication of devices having an approximate active area of4 cm2. To do so, the electrochromic mixtures were just sandwichedbetween two transparent FTO-coated glasses. A 200 mm adhesivetape was used as spacer between the glass substrates.

Acknowledgements

This work was financially supported by the Basque Governmentunder the ETORTEK 2013 program contract no. IE13-380(ACTIMAT).

Keywords: borax · electrochromic devices · polyelectrolytes ·polyvinyl alcohols · viologen

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Received: August 2, 2014

Published online on && &&, 2014

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CHEMELECTROCHEMARTICLES www.chemelectrochem.org

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Y. Alesanco, J. Palenzuela, A. ViÇuales,G. CabaÇero, H. J. Grande, I. Odriozola*

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Polyvinyl Alcohol–Borax Slime asPromising Polyelectrolyte for High-Performance, Easy-to-MakeElectrochromic Devices

Masters of the universe slime: The useof a slime for the preparation of electro-chromic devices presents various advan-tages, compared to the classic systems

based on liquid or solid polyelectro-lytes: facile jelly-on-toast preparation,high optical performance and goodcyclability.

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