Deep Eutectic Solvents as suitable electrolytes for Electrochromic

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Deep Eutectic Solvents as suitable electrolytes for Electrochromic Devices Hugo Cruz, Noémi Jordão, Patricia Amorim, Madalena Dionísio, and Luis Branco ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03684 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Sustainable Chemistry & Engineering

Deep Eutectic Solvents as suitable electrolytes for Electrochromic Devices Hugo Cruz*, Noémi Jordão, Patrícia Amorim, Madalena Dionísio, Luís C. Branco* E-mail: [email protected]; [email protected]; Fax: +351 212948550; Tel.: +351 212948300. LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829516 Caparica, Portugal. ABSTRACT: The development of chloride free Deep Eutectic Solvents (DES) based on lithium acetate, triflate, bistriflimide and sodium triflate combined with glycerol, ethylene glycol and polyethylene glycol (PEG400) as suitable electrolytes were developed and successfully applied in electrochromic devices (ECD). Reversible ECD incorporating selected DES as electrolyte and bipyridinium as electrochromic probes display a comparable performance than conventional systems, with slower transition times (2.5 times), improving the coloration contrast and efficiency (147.1 cm2.C-1 at 520 nm) and is stable for at least 1250 redox cycles.

KEYWORDS: deep eutectic solvents, electrochromic materials, electrolytes; coloration efficiency; electrochemical redox probes.

INTRODUCTION Recently, Deep Eutectic Solvents (DES) as Room Temperature Ionic Liquid (RTIL) analogues have attracted much attention due to the possibility to use as greener alternative solvents. Abbott and co-workers defined eutectic mixtures as "deep" when a large depression in melting point occurs comparing to the pure components.1 DES can be formed from eutectic mixture between Lewis and Brønsted acids and bases and it can be comprised by a variety of anionic and/or cationic species. In this way, they are prepared by the complexation between hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD).1-3 Generally, DES showed several properties similar to RTIL, such as high solvation ability and wide electrochemical windows. Contrarily to RTIL, DES can be synthesized by simple mixing of the low-cost and environmental friendly starting materials just heat up most of the times and without further purifications.1,2 The main applications of DES include: a) chemical synthesis as alternative reaction media in organic synthesis4 b) electrochemical applications, where it has been used for the determination of its physical-chemical properties,5-6 electroplating and electrodeposition,7-8 or involve environmentally hazardous processes,2,7,8 as well as in electrocatalysis and for supercapacitors and batteries.9-12 Also, it was reported the use of DES for the preparation and stabilization of nanoparticles, metal-organic frameworks, colloidal assemblies, thermochromic composite films, nanostructures metal films, hierarchal porous carbons, bioinspired functional materials,9-11 application for selective extraction processes (e.g. bioactive or biopolymers) as DNA/RNA architectures, DNA concentration and dissolution have been reported13-20 as well as solubility of biomaterials (e.g. chitin or cellulose).1, 21-23 Previously, we reported the preparation and characterization of DES based on choline chloride or lithium chloride with ethylene glycol and glycerol as low-cost, recyclable and green electrolytes for electrochromic devices.24 Nowadays, electrochromic materials for academic and industrial applications have been explored due to their interesting properties. Electrochromic materials exhibit a distinct visible color switch

between transparent (bleached state) and colored state, or between at the least two distinct coloration states.25, 26 Several applications such as electrochromic windows,27 electrochromic displays,28, 29 anti-glare car mirrors,30, 31 eye-glasses,32 solar attenuated windows,33, 34 and flexible electrochromic devices,35 have been reported. The most important classes of electrochromic materials include viologen derivatives, transition metal/lanthanide coordination complexes, metal phthalocyanines, transition metal oxides, metallopolymers, Prussian blue systems, and conducting polymers, among others.36-42 Two-electrode configuration cell for an electrochromic system comprises a conductive surface (electrode) such as ITO (indium tin oxide) or FTO (fluorine-doped tin oxide) coated in a PET or glass surface combined with another conductive surface (electrode) separated by a thickness lower than one millimeter using a double sided adhesive tape. One possible approach corresponds to the possibility of the electrochromic material to be dissolved in the electrolyte and the mixture placed between those two conductive layers. Thus, the choice of a suitable solvent/electrolyte is essential, and it can be considered one of the most important parameters in electrochemical or chemical processes. Recently, greener and safer solvents/electrolytes with peculiar properties (e.g. negligible vapor pressure at room temperature) gained much attention in academia, since it can be an alternative to replace the classical organic solvents, which could present several environmental, health and safety challenges, including safety hazard, waste management as well as human and eco-toxicity issues. Herein, we reported the application of DES based on the suitable combination of several HBD such as ethylene glycol (EG), glycerol (Gly) and PEG 400 and HBA such as lithium triflate (LiOTf), lithium bistriflimide (LiNTf2), lithium acetate (LiAcO) and sodium triflate (NaOTf) as alternative greener electrolyte for electrochemical studies as well as electrochromic devices as indicated in Figure 1.

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Hydrogen Bond Aceptor (HBA) Anion

Hydrogen Bond Donor (HBD) Cation

OH

Glycerol (Gly)

F O O F F S N S F F O O F Bis((trifluoromethyl)sulfonyl)amide (NTf2) CH3COO Acetate (AcO)

OH HO

F O F S O F O Trifluoromethanesulfonate (OTf)

Na Sodium

OH HO Ethylene Glycol (EG) HO O H n Poly(Ethylene Glycol) 400 (PEG 400)

Li Lithium

Electrochemical Redox Co

PF6

Cobaltocinium Hexafluorophosphate (Cc) Cl

Cl

N

N

N,N'-dimethyl-[4,4'-bipyridinium] dichloride (MVCl2)

S

Fe Ferrocene (Fc)

O

O

S

S S Tetrathiafulvalene (TTF) I

I

N

N

O

O

N,N'-[di(methoxyethoxy)ethane-[4,4'-bipyridinium] di-iodide

Figure 1. Chemical structures of the Hydrogen Bond Acceptors and Donors selected to prepare Deep Eutectic Solvents (DES); and the tested electrochemical redox probes.

A combination of electrochemical techniques like cyclic voltammetry (CV) and electrolysis at a controlled potential as well as thermal and conductivity studies allowed us a more detailed characterization of the prepared DES. The incorporation of the prepared DES in efficient and reversible electrochromic devices have been performed.

EXPERIMENTAL SECTION Chemicals: All the commercial available reactants were purchased in high purity level without further purification. Anhydrous triflate compounds (LiOTf, NaOTf, KOTf) and lithium bistriflimide (LiNTf2) from Solchemar (99%). Glycerol (Gly) and Polyethylene glycol (PEG400) were purchased from SigmaAldrich (ReagentPlus 99% GC). Ethylene Glycol (EG), Tetrathiafulvalene (TTF, 97%) from Alfa-Aaeser (99%), Ferrocene (Fc) from BDH, Cobaltocinium hexafluorophosphate from Aldrich and N,N’-(methyl)-4,4’-bipyridinium dichloride [(CH3)2bpy]Cl2 from Fluka (98%). 1,1’-[di-(methoxyethoxyethane)]-4,4’-bipyridinium di-iodide [(C5O2)2bpy)]I2 was prepared according to the synthetic route reported in previous work.43 Synthesis of DES: The Eutectic mixture were prepared under vigorous stirring of the both components (HBA and HBD) in the different proportions at a temperature between 60 to 70ºC. After 6 hours of mixing the compound a colorless viscous liquid is obtained, adapted from literature. 1,24 Electrochemical measurements: All the electrochemical studies were performed on an Autolab PGSTAT 12 potentiostat/galvanostat, controlled with GPES software version 4.9 (Eco-Chemie, B.V. Software Utrecht, The Netherlands). 3-electrode configuration Cyclic Voltammetry and Electrochromic Experiments: All the electrochemical Cyclic Voltammetry (CV) and Electrolysis at a controlled potential, using a 3electrode configuration cell (3 mL). A glassy carbon electrode (ET074 eDAQ) was used as working electrode and a Pt wire as counter or auxiliary electrode. All potentials refer to a Ag/AgCl leakless (3.5 M KCl) reference electrode (ET072 eDAQ). Prior to use, the working electrode was polished in aqueous suspensions

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of 1.0 and 0.3 µm alumina (Buehler) over 2-7/8” micro-cloth (Buehler) polishing pads, then rinsed with water and methanol or Acetonitrile. This cleaning procedure was applied always before any electrochemical measurement. CV measurements were performed at 20-100 mV.s-1 scan rate and it was used to characterize the electrochemical responses (I-E curves) in the desired electrochemical window. 2-electrode configuration Cyclic Voltammetry and Electrochromic Experiments: In the 2-electrode configuration cell (area of electroactive surface is 2 x 1.5 cm2) the conductive layers were rinsed with ethanol. CV measurements and electrolysis at a controlled potential at applied potential (Eapp) 100 mV higher then peak potential (Ep) predetermined by CV (Eapp=Ep+100 mV) were performed to characterize the electrochromic material. Conductivity measurements: The conductivity of the different DES was performed by dielectric relaxation spectroscopy (DRS). For the DRS measurements, samples were placed between two stainless steel electrodes (10 mm diameter) in a BDS 1200 parallel plate capacitor, using two 50 µm silicon spacers to maintain sample thickness and the sample cell was mounted on a BDS 1100 cryostat. All measurements were carried out using an AlphaN analyser from Novocontrol, covering a frequency range from 10−1 Hz to 1 MHz at 22ºC. Spectroelectrochemical and cycling measurements: Spectroelectrochemistry studies were performed using UV-Vis- NIR spectrophotometer Varian Cary 5000. The potential was controlled with a device potentiostat/galvanostat Model 20 Autolab. The device was placed in the spectrophotometer compartment perpendicularly to the light beam. Cycling stability tests were also performed in the same setup. Water measurements by Karl-Fischer: The water content was measured using an 831 KF Coulometer, 230 Metrohm with generator electrode and without diaphragm. The water content is presented as average values of at least three independent measurements. DSC measurements: DSC analysis is carried out using a TA Instruments Q-series TM Q2000 DSC with a refrigerated cooling system. The sample is continuously purged with 50 ml.min-1 nitrogen. About 2-15 mg of salt was crimped in an aluminum standard sample pan with lid. Glass transition temperature analysis using a heating/cooling rate of 10ºC.min-1 was determined in the 2nd heating after a thermal treatment up to 150ºC. Crystallization (Tc) were determined by DSC analysis using a heating/cooling rate of 10ºC min-1 at 2nd cycle. Viscosity measurements: The viscosity of DES was measured using a Brookfield DV-II+ Pro Viscometer with a cone spindle CPE-40, at 25ºC.

RESULTS AND DISCUSSION Our previous work reported the preparation of five DES as well as its first application as greener electrolytes for electrochromic devices.24 In this work, we develop a series of DES based on lithium and sodium salts for application as efficient electrolytes tested in different electrochemical redox probes (see Figure 1). The preparation and characterization of new DES and their thermal, physical and electrochemical properties are presented and


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ACS Sustainable Chemistry & Engineering extended plateau, a decrease is observed (see ESI).51 This arises from ions accumulation in the sample/electrode interface without discharging, blocking ionic conduction.51 To compare the electrical behavior of the different systems studied, five frequencies were selected (as indicated in Table 2).

discussed. Finally, the stability of electrochromic devices (ECD) using the most promissory DES were evaluated. Thermal analysis: All prepared DES were studied by differential scanning calorimetry (DSC). The glass transition temperature (Tg) as well as melting and crystallization temperatures are summarized in Table 1. Additionally, the water content (determined by Karl Fischer titration) and viscosity are also presented. As expected, the water content contributes to change both thermal (e.g. glass transition due to plasticizing effect) and physical-chemical properties such as viscosity and conductivity. In the case of Gly (see Figure 2), under the experimental conditions tested in this work, neither crystallization nor melting are detected, the only observed thermal event is the glass transition indicated by the discontinuity in the heat flow occurring below 70ºC. This general behavior is followed by the Gly based DES with a shift to higher temperatures according the following order: Tg(LiAcO:3Gly) > Tg(LiOTf:3Gly) > Tg(Gly). This observation can be related to the strength of the hydrogen bonding interactions. In fact, acetate anion can establish stronger hydrogen bonds with the OH groups of Gly as already described for biopolymers such as cellulose derivatives.44, 45 The DES containing EG did not showed any melting and crystallization in the selected experimental conditions contrarily to pure EG; it seems reasonable to assume, at least in the temperature range explored, that the addition of the LiOTf, NaOTf or LiNTf2 salts originates the suppression of both crystallization and melting. This observation can be explained by an anti-freeze behavior. The thermal behavior of the PEG solvent is highly cooling rate dependent. While vitrification process is avoided upon cooling at 10 ºC.min-1, it emerges in the hydrated state when the cooling rate increases to 20ºC.min-1 (close to -70ºC); crystallization and melting are always observed occurring in reasonable agreement with the values reported in the literature.46 Concerning the LiOTf:6PEG, the most striking effect is the depression of the crystallization temperature, whereas the melting temperature undergoes almost no changes (2ºC decrease). The glass transition shifts towards higher temperatures by 5ºC. Viscosity: In general, the viscosity of DES (at 25ºC) is higher than the reported for classical ILs41 and it is strongly influenced by the nature of the solvent and salt, once its determine the nature of intermolecular interactions between the molecules and ions present in solution.47-49 In the case of DES based on EG, it was observed a decrease on the viscosity value with an increase of EG content. Comparing LiOTf:6EG and LiOTf:6PEG, higher viscosity values are observed due to polymeric nature of PEG. Conductivity: Ionic conductivity is another property that is strongly affected by the nature of the components to form eutectic mixture. For all prepared DES, the dependence of ionic conductivity with frequency from 10-1 to 106 Hz was measured at 22ºC. Usually in these disordered systems, the profile of the real component (′ (f)) from the complex conductivity obeys to a universal behavior with a frequency dependent conductivity at high frequencies, exhibiting a plateau at low frequencies; the latter corresponds to the direct current (dc) conductivity (′dc), i.e. translational motion of charge carriers.50 This is not obeyed by the prepared DES, at room temperature. In general, for the systems here investigated, the conductivity response shows a similar frequency dependence being mainly dominated by electrode polarization; therefore, instead of an

Table 1. Physical-Chemical properties of prepared DES as electrolytes. DSChydrated [a] DES (HBA: HBD)

DSCafter 150ºC [b] H2O

Visco sity (Pa.s)[d]

EW

Tg-mid (oC)

Tc/Tm (oC)

Tg-mid (oC)

Tc /Tm o ( C)

Gly

-79.6

- /-

-76.8

- /-

0.5

0.725

n.a.

LiOTf: 3Gly

-70.7

- /-

-62.8

- /-

3.5

0.683

1.8 / -2.0

LiAcO: 3Gly

-67.1

-/-

-51.3

- /-

5.1

1.713

1.6 / -2.5

EG

-

-53.3 / -15.3

-

-42.1 / -12.3

0.2

0.016

n.a.

LiNTf2: 4EG

-

-/-

-

- /-

1.0

0.055

2.5 / -2.3

LiOTf: 4EG

-

-/-

-

-/ -

5.0

0.060

1.9 / -2.0

LiOTf: 6EG

-

-/-

-

- /-

3.7

0.036

1.9 / -2.1

LiOTf: 10EG

-

-/-

-

- /-

7.2

0.025

1.9 / -2.1

PEG400

-

-12.6/ 4.0

-

-16.8 /5.7

0.5

0.077

n.a.

PEG400

-70.9

-49.1[f]/ -7.1

-

-20.1 /5.3

-66.2

31.3; -48.1 [f] / 3.2

1.4

0.163

2.2 / -2.5

-

- /-

3.1

0.025

(20ºC/min)

LiOTf: 6PEG

-65.4

NaOTf: 6EG

-

-28.5; -41.5[f] / -1.13

- /-

/wt% [c]

(V) [e]

1.9 / -2.3

[a]

Glass transition (Tg-mid), melting and crystallization temperatures were determined in the 1st cycle scanned at 10ºC. min-1 (hydrated). [b] Glass transition (Tgnd cycle mid), melting and crystallization temperatures were determined in the 2 scanned at 10oC. min-1 (after thermal treatment up to 150oC). [c] Measurement by Karl Fischer titration at 25oC. [d] Measurement by microviscometer at 25oC. [e] Electrochemical window (EW) is defined by the anodic or cathodic limited (onset of DES oxidation or reduction processes). The EW can be calculated by the difference between reduction potential and the oxidation potential of the DES. [f] Cold crystallization was detected upon the heating run.

It was previously observed a correlation between the emerging of the plateau region and the surpassing of the glass transition,52, 53 being interpreted as a conductivity mechanism enabled by structural motion, i.e. a dynamic glass transition assisted hopping mechanism of charge transport.52-54 Therefore, it is important to note that for all the DES studied, conductivity measurements were carried out above Tg, which for



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all the DES occurs below -50ºC (in some cases, possibly below 90ºC). The experiments were performed at least 70ºC above Tg, in a temperature region where electrode polarization strongly displays highly masking the dc plateau. It should be mentioned that stainless steel electrodes were tested (even though originating a higher electrode polarization), since redox reaction was observed in gold plated electrodes.55 Table 2. Real part of complex conductivity at plateau region of the prepared DES. σ' (mS/cm) at 22ºC DES

LiAcO:3Gly

102 Hz

103 Hz

104 Hz

105Hz

9.08x105Hz

0.034

0.081

0.157[a]

0.175[a]

0.175[a]

[a]

[a]

[a]

LiOTf:3Gly

0.018

0.104

0.248

LiNTf2:4EG

0.015

0.069

0.394

2.702[a]

2.614[a]

LiOTf:4EG

0.018

0.077

0.403

2.540[a]

2.600[a]

LiOTf:6EG

0.016

0.071

0.354

1.920[a]

2.297[a]

LiOTf:10EG

0.015

0.053

0.126[a]

0.169[a]

0.165[a]

LiOTf:6PEG

0.029

0.086

0.199[a]

0.228[a]

0.226[a]

NaOTf:6EG

0.025

0.096

0.632

3.417[a]

2.430[a]

0.291

0.294

Figure 3. (Left) General scheme of sample preparation for conductivity measurements; (Right) Real part of complex conductivity in function of the frequency for LiAcO:3Gly and LiOTf:3Gly.

Usually, for electrolytes a correlation between conductivity and viscosity is reported 56. This is investigated through a Walden’s plot, where the molar or molal conductivity () and fluidity (the inverse of viscosity) are plotted on log−log scales. This plot for the prepared DES is presented in figure 4, in which the conductivity was normalized by molality (ratio between the number of moles of solute and 1000 g of solvent); the ideal conductivity behavior of a non-interacting ionic material is given as a dashed line with slope equal to 1 obtained for high diluted (0.01 M) of KCl aqueous solution.1,2,56

[a]

′ values taken from the plateau (see ESI)

For all investigated systems, the plateau always falls in the high frequency range (see sigma at ~105 – 106 Hz in Table 2 and Figure 3).

Figure 4. Walden plot for prepared DES. dashed line corresponds to ideal behavior using a diluted (0.01 M) of KCl aqueous solution.

Figure 2. (Left) General schematic of the DSC. (Right) Heat flow thermogram for Gly (full line), LiOTf:3Gly (dot line) and LiAcO:3Gly (dash dot line) obtained at 10 oC.min-1 (heating/cooling rate) after a thermal treatment up to 150ºC.

It is possible to conclude that the EG based DES exhibit higher dc values, by a factor of approximately 20. DES based LiOTf:nEG (n=4, 6, 10) allowed to infer about the influence of salt concentration and it seems that an increase of LiOTf salt originates an increase dc.

Indeed, the expected correlation is obeyed in general for the studied DES, with LiOTf:6PEG, LiOTf:3Gly, LiAcO:3Gly falling in the ideal line. The other DES, fall close but under the ideal line which can be rationalized as result of ion-ion interaction. The applicability of Walden’s rule to DES it is validated by Abbott and co-workers using the hole theory, which assumes that the predicted relationship occurs due to an almost infinite dilution of suitable sized empty spaces/voids in a material, determining ion mobility. 2 Electrochemical Measurements: The solubility of electrochemical compound in the presence of specific electrolyte is a key parameter. In our case, the prepared DES were firstly tested in the presence of four different electrochemical redox probes: N,N’-dimethyl-4,4’-bipyridinium di-chloride ([(CH3)2bpy]Cl2) or MVCl2), ferrocene (Fc), tetrathiafulvalene (TTF) and cobaltocinium hexafluorophosphate (Cc). Cyclic voltammetry and electrolysis at a controlled potential were used to study the electrochemical and electrochromic behavior of the probes. Those probes presented a well-known electrochemical behavior in the reduction (MVCl2 and Cc) or oxidation (Fc and TTF) processes.57, 58 The


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standard (Eo) and peak (Ep) potentials are summarized in Table 3. The shapes of the cyclic voltammogram of the electrogenerated species in aprotic solvents containing bulky organic countercations and in RTILs57 are comparable to the DES. Figure 5 illustrates the respective response I-E curves at a 3-electrode configuration cell using MVCl2 and Fc as probes. The electrochemical behavior of MVCl2 is a well-defined two successive mono-electronic electron-transfer (ET) with a standard potential of -0.651 and -1.085 V vs Ag/AgCl leak less obtained in aqueous media (in a phosphate buffer solution).58 In our case, the electrogenerated species (radical-cation and neutral species) were stable during the time of the experiment and our focus is centered in the first mono-electronic ET due to color switch process (higher electrochromic behavior; see Figure 5 and ESI Figure S1). In the case of the DES using Gly (a viscous liquid) as HBD and lithium salt, such as LiCl, LiAcO and/or LiOTf as HBA, a cathodic peak potential for the first mono-electronic ET decreases in the sequence: -0.60 V vs. Ag/AgCl leak less (Cl), -0.62 V vs. Ag/AgCl leak less ([AcO]) and -0.67 V vs. Ag/AgCl leak less ([OTf]). This observation can be explained by the size of each anion as well as their stronger hydrogen bond acceptor. For the case of EG as HBD combined with LiCl, LiOTf and/or LiNTf2 as HBA, a similar electrochemical behavior, i.e. two successive monoelectronic ET are observed. The study of the first electron transfer presents a standard potential of -0.53 V vs. Ag/AgCl leak less (Cl), -0.59 V vs. Ag/AgCl leak less ([OTf]) and -0.65 V vs. Ag/AgCl leak less ([NTf2]) respectively, see ESI Figure S2. The anion effect also affected the standard potentials in the following order: E0 (Cl) > E0 ([OTf]) > E0 ([NTf2]). In the case of the DES composed by LiOTf:nEG (n= 4, 6 and 10), a dilution effect leads to a decrease in the viscosity and conductivity profile, is affecting the electron transfer process, which is displaced for higher potentials E0(LiOTf:4EG) < E0(LiOTf:6EG) < E0(LiOTf:10EG) as can be seen in Table 1, Figure 5 left upper and ESI Figure S2. In the case of the DES LiOTf:6PEG [E0 (LiOTf:6PEG) = - 0.47 V vs. Ag/AgCl leak less] vs. LiOTf:6EG [E0(LiOTf:6EG) = - 0.57 V vs. Ag/AgCl leak less], the shape of the I-E curve for the twosuccessive ET are similar. However, the polymeric nature of PEG leads to an increase of the standard potential to higher values and the reduction of MVCl2 became easier (100 mV) see Table 1 and ESI Figure S2. Finally, ET became easier around 10 mV in the case of sodium based DES [LiOTf:6EG vs NaOTf:6EG], and about 20 mV for the first cathodic peak as can be seen in Table 1and ESI Figure S2. For the case of Cc as another reduction probe, the DES LiOTf:6PEG was selected because it is one that presents a lower standard potential, see Table 1 and ESI Figure S3. A similar behavior was reported in the literature for other ILs and polar aprotic solvents (E0 in DPA and ILs and comparing to DES; E0= 0.91 V and Ep= -0.96 V vs. Ag/AgCl leak less).18 In the case of oxidation probes TTF, LiOTf:6PEG is suitable to solubilize the TTF and it is the one that presents two successive mono-electronic ET, see Table 1 and ESI Figure S4. Other examples of electrochemical solvents and ILs reported a similar behavior: E0 = 0.316 V (Ep1 = 0.358 V) and E0 = 0.602 V (Ep2= 0.568 V) in the presence of DMF + 0.1M [TBA][BF4]; E0 = 0.175 V (Ep1 = 0.203 V) and E0 = 0.495 V (Ep2 = 0.529 V) in [BMIM][BF4] and E0 = 0.184 V (Ep1 = 0.129 V) and E0 = 0.572 V (Ep2 = 0.649 V) in [BMIM][PF6] (all the potentials are presented V vs. Ag/AgCl leak less). 56 In the selected DES, TTF presents E0 = 0.36 V (Ep1 = 0.43) and E0 = 0.62 V (Ep2 = 0.70 V).

Table 3. Standard (Eº) and peak (Ep) potentials of selected redox electrochemical probes dissolved in different Deep Eutectic Solvents in a 3electrode cell. Compounds

MVCl2

Fc

TTF

DES HBA:HBD

Ep (VvsAg/AgCl)

E0 a (VvsAg/AgCl)

∆Eb (mV)

ChCl:2EG

-0.53

-0.50

70

ChCl:2Gly

-0.56

-0.53

70

LiCl:3EG

-0.56

-0.53

69

LiCl:3Gly

-0.60

-0.57

70

LiAcO:3Gly

-0.62

-0.54

153

LiOTf:3Gly

-0.67

-0.62

82

LiNTf2:4EG

-0.70

-0.65

109

LiOTf:4EG

-0.62

-0.59

72

LiOTf:6EG

-0.61

-0.57

85

LiOTf:10EG

-0.58

-0.55

71

LiOTf:6PEG

-0.55

-0.47

160

NaOTf:6EG

-0.59

-0.56

59

LiOTf:3Gly

0.27

0.22

96

LiNTf2:4EG

0.23

0.18

106 144

LiOTf:6PEG

0.49

0.42

NaOTf:6EG

0.53

0.49

81

LiOTf:6PEG

0.43

0.36

144

Cc LiOTf:6PEG -0.96 -0.91 110 Standard potential, calculated as E0 = (Epc +Epa)/2 measured vs. Ag/AgCl leak less; b∆E = (|Epc| – |Epa|) in mV, with Epa (anodic peak potential) and Epc (cathodic peak potential). a

For Fc, the [Ep] and [E0] are well-known for several electrochemical solvents and electrolytes, including ILs and polar aprotic solvents.57 In the selected DES, a well-defined one monoelectronic ET is observed, the electrogenerated radical cation is formed than LiOTf:Gly and LiOTf:PEG. It is also observed that the LiNTf2:4EG easily form the electrogenerated radical contrarily to NaOTf:6EG see Table 1 and Figure 5 left down and ESI Figure S5. To evaluate the potential application of DES as electrolyte for ECDs (two-electrode system), an additional test was performed as control, using the same conditions of the redox probes, i.e. CV in an electrochemical window of 0/-3/3/0 in a scan rate of 20 mV.s-1 is performed and ET processes are detected in those conditions (see Figure 6). In general, for two-electrode system containing MVCl2 dissolved in DES, a characteristic electrochemical response in the cathodic direct scan (0/-3/3/0) is observed. However, no ET is detected for the reduction of the probe in this electrochemical window. It is required the application of higher potentials, almost -3 V to obtain the corresponding blue coloration, i.e. radical cation formation (see Figures S7-S10 from ESI). In the cases of LiAcO:3Gly, LiOTf:6PEG and NaOTf:6EG, a well-defined ET and blue coloration characteristic for the radical cation are detected at lower peak potentials [Ep (LiAcO:3Gly) = 2.57 V, Ep (LiOTf:6PEG) = -2.87 V and Ep (NaOTf:6EG) = -2.52 V] in the cathodic reverse scan (see Figures S11-S13 from ESI). Following the cyclic voltammetry at the higher potential (+3 V), the MVCl2 increases the blue coloration. The same DES presented a well-defined ET with Ep (LiAcO:3Gly) = 2.46 V, Ep (LiOTf:6PEG) = 2.85 V and Ep (NaOTf:6EG) = 2.42 V also fol-



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lowed by an increase in the blue coloration intensity at lower potentials but don´t return the initial coloration.

Figure 7. RGB coordinates for the ECD containing MVCl2 as redox probe.

Figure 5. (Right) Schematic 3-electrode configuration cell for the study of the DES as electrolyte at a scan rate 50 mV/s, [(CH3)2bpy]Cl2, Fc, Cc and TTF in concentration of 15 mM and (Left) the respective response I-E curves at a 3-electrode configuration cell for the [(CH3)2bpy]Cl2 and Fc in the same conditions as before. Legend: A-DES and Electrochromic; BRef. Electrode (Ag/AgCl leak less); C- Counter Electrode CE (Pt wire); D-Inert gas flow (N2); E- Working Electrodes WE (Glassy Carbon, CV & PET-ITO electrolysis).

Figure 7 illustrates the coloration gradient for the ECD of the MVCl2 between pale brown to five different pallet colors: pale blue (LiOTf:6PEG); blue green (LiOTf:10EG); blue (NaOTf:6EG); dark blue (LiAcO:3Gly); violet (LiCl:3Gly). It is possible to conclude that in the case of DES containing EG, the electrogenerated radical cation becomes blue and in the case of the glycerol becomes more violet.24 neneElectrode surface

To the Surface add spacer

Add drop DESs and electrochromic mixture

Close the cell with another electrode

Figure 6. (Top) General schematic for construction of the 2-electrode cell and (Bottom) Overlapping CV in a 2-electrode configuration cell, electrochromic device, using as electrolyte the DES LiOTf:10EG between 0/3/3/0 at a scan rate of 20 mV/s, green line and using as probe the MVCl2 in 15 mM, blue line 1st cycle and red line 2nd cycle.

To obtain a stable and darker coloration, it is required -2.5 V of potential and a transition time between minutes to hours for each ECD [NaOTf:6EG, LiOTf:10EG, LiNTf2:4EG and LiOTf:6PEG]. For the LiOTf:EG, ECDs gained color slower depending on the amount of HBD used, the higher amount of EG allowed a significant reduction of viscosity and perhaps also increasing the diffusion coefficient of the species. No color change was observed in the case of cobaltocinium as an electrochromic cathodic probe using these experimental conditions. The study of ferrocene as electrochromic anodic probe was tested in five DES, the most promising ones, LiOTf:6PEG, LiNTf2:4EG, NaOTf:6EG, and LiOTf:3Gly. For the case of ECD using LiOTf:6PEG, the color change at 1.8V for a gray green coloration and after four days in open circuit the ECD was permanently gray. LiNTf2:4EG at 2V for 1h, the ECD showed a light blue greenish coloration that increases to a blue gray coloration and returns to gray after one day in open circuit. NaOTf:6EG at -2V showed a light green brown coloration that increases at -3V to dark brownish coloration and after five days in open circuit the coloration remains brown. The ECD using LiOTf:3Gly at 2V for 30 to 60 minutes changed between gray to a blue greenish gray coloration. TTF as an electrochromic probe was tested in the ECD using LiOTf:6PEG at 2V for 1h, it changed between pale brown to a brownish green coloration and after 24h at open circuit almost recover its original color. For all the DES successfully used as electrolyte for the viologen derivatives a well-defined electron transfer at approximately 0V is observed, in both cathodic and anodic back scans, this could be related to an oxidation process that don’t lead to a color change or an adsorption process at the electrode surface, it’s not clear at this time in this study.59,63 Preliminary studies for the determination of stabilities of the ECD were performed using three of the most promising DES as electrolytes (LiOTf:10EG, LiOTf:6PEG and NaOTf:6EG) and N,N’-[di(methoxyethoxyethane)]-4,4’-bipyridinium di-iodide [(C5O2)2bpy]I2 as an alternative electrochromic probe. In the case of the NaOTf:6EG applying a potential of -/+2V for 600 s ECD gained a purple coloration in the first seconds and it required a longer time than MVCl2 to return to original coloration (see ESI Figure S18 -S19). For the case of LiOTf:6PEG, the transition times are shorter to obtain a green-gray coloration (15 minutes) and to recover its original coloration (7 minutes). It is particularly interest the case of LiOTf:10EG as electrolyte in the presence of [(C5O2)2bpy]I2 as probe, only 60 seconds is required to gain purple coloration and to recover its original coloration longer time is necessary (at least 200s).


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ECD Stability Studies for ECD stability studies, the [(C5O2)2bpy]I2 as alternative electrochromic redox probe dissolved in LiOTf:10EG as electrolyte was selected as the model system. The electrochromic parameters such as chromatic contrast (∆T, %), transition times (t90 or t70) and coloration efficiency (CE) of this ECD was evaluated. Chromatic contrast (∆T, %) increases until 96.2%. The transition times, t90 or t70 (to reach 90% or 70% of its maximum colour (or bleach state)) increases using the selected DES as electrolyte comparing to previous work using bipyridinium based ILs in a gel electrolyte (t90 9.7s to 21.4s and t70 4.9s to 12.4s).64 The coloration efficiency (CE that corresponds to the amount of colour reached (or bleached) by charge consumed per cm2 of the ECD at a specific wavelength) is higher (147.1 cm2.C-1 at 520 nm). Figure 8 illustrates a preliminary cycling stability study of the ECD using [(C5O2)2bpy]I2 with LiOTf:10EG as electrolyte. The following experimental conditions: coloration time (15 s) and/or bleaching time (90 s) by applying +/-2V and 0Vof potential, respectively were used.

These novel class of DES can open outstanding alternatives to conventional electrolytes for application in potential industrial device applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Cyclic voltammetry in 3- and 2- electrode configuration, RGB coordinates, DSC/DRS measurements (file type, i.e., PDF)

ACKNOWLEDGMENT This work was performed under the project “SunStorage - Harvesting and storage of solar energy”, with reference POCI-010145-FEDER- 016387, funded by European Regional Development Fund (ERDF), through COMPETE 2020 - Operational Programme for Competitiveness and Internationalisation (OPCI), and by national funds (PTDC/QEQ-QFI/1971/2014), through FCT - Fundação para a Ciência e a Tecnologia. H. Cruz and L. C. Branco thanks to financial support of FCT/MCTES (Post-Doc grant SFRH/BPD/102705/2014 and IF/0041/2013/CP1161/CT00). The NMR spectrometers are part of The National NMR Facility, supported by FCT/MCTES (RECI/BBB-BQB/0230/2012).

REFERENCES

Figure 8. Switching cycles of [(C5O2)2bpy]I2 ECDs fabricated with LiOTf:10EG as electrolyte. Cycles 1–14 and 1236–1250 are illustrated for ECD switched at 15s and 90s cycles between -2 and 0 V respectively at 520 nm.

CONCLUSIONS and PERSPECTIVES Different DES based on lithium and sodium salts with glycerol, ethylene glycol and polyethylene glycol as sustainable and suitable electrolytes were developed and successfully applied in electrochromic devices (ECD). Some of the prepared DES could completely solubilize and stabilize different electrochemical probes requiring a lower voltage to enhance the electrochemical and electrochromic processes at the same electrochemical window. The most promising DES are NaOTf:6EG and LiOTf:10EG requiring a lower potential to enhance the blue coloration for the MVCl2 as electrochemical redox probe as well as LiOTf:6PEG which presented the lower standard and peak potentials for the first electron transfer in the case of MVCl2 and Fc probes. Comparing to previous work using conventional gel electrolytes, LiOTf:10EG as DES showed a significant increase in the chromatic contrast as well as coloration efficiency. However, longer transition times are required to achieve a complete colored and bleached states. The reduction of viscosity of the DES seems to improve the original transition times.

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For table of contents use only

Deep Eutectic Solvents (DES) as suitable electrolytes were developed and successfully applied as reversible electrochromic devices (ECD) incorporating bipyridinium derivatives.

0V/-2V


Deep Eutectic Solvents as suitable electrolytes for Electrochromic

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