Ionic additive in an ionogel for a large area long lived high contrast electrochromic device
Graphical abstract
Introduction
Electrochromic devices (ECDs) find diverse architectural and non-architectural applications. Energy efficient glazing, privacy glass, partitions, skylights, etc., fit into the architectural category and the non-architectural ones include fast switching electrochromic anti-glare rearview (ECRA) mirrors, sunglasses, protective eyewear for military, automotive sunroofs and so forth [[1], [2], [3], [4]]. EC automotive mirrors are commercially available and have been strongly successful with the consumers [5]. ECDs can be also used as large area displays, where one such commercial installation is at the Seto Bridge Museum in Japan. In the past, besides Gentex corporation which has multiple patents, other groups have also worked extensively on viologen based electrochromic materials primarily for commercial mirrors [[6], [7], [8], [9]]. Viologens are formed by the di-quaternization of 4,4′-bipyridine to form 1,1′-disubstituted-4,4′-bipyridinium salts [10,11]. But, large area ECRA mirrors with heptyl viologen (HV) as the cathodically coloring solution phase electrochrome are relatively less studied in literature [12,13]. ECDs require electrolyte for their operation, and therefore for developing durable, long lasting ECDs, chemical compatibility between the electrolyte and the electrochromic films without the loss of optical transparency is crucial for display applications. One of the fundamental factors affecting the ECD performance (optical contrast, switching time and cycle life) is the selection of an appropriate electrolyte. Liquid electrolytes exhibit low stability, high flammability, low safety and can leak out as well thus limiting their applicability for long lived ECDs. These concerns can be alleviated by the use of a solid polymeric gel electrolyte in lieu of the liquid electrolyte in ECDs [14,15]. Gel polymeric electrolytes present multiple advantages such as good interfacial contact with the electrodes, improved safety, stability, simple preparation procedure, low production cost and good mechanical properties, thus enabling the development of leak proof ECDs [16]. In this regard, electrolytes typically used for the fabrication of ECDs are ionic liquids (ILs) or inorganic salts dissolved in organic solvents and ionogels [15,17,18]. ILs exhibit good thermal stability, are intrinsically conductive and have been shown to have electrochemical stability windows as wide as 5.0 V in some cases [19]. A polymer gel is defined as an interconnected polymer network formed within a liquid phase. When the polymer network is generated in the presence of IL, the resultant gel has been termed as an ionogel within literature [20]. Ionogels are therefore a new class of hybrid material that combine the physicochemical properties of both the polymer and the physically entrapped IL within the solid matrix [14,20]. The present work details the use of ionogels by synthesizing them using in-situ thermal polymerization techniques for ECDs that can double up as ECRA mirrors as well [21,22]. HV is a type-II class of electrochrome, wherein the process of electrochromism involves the formation of a solid deposit on the surface of an electrode or FTO (SnO2:F) glass upon application of potential from the viologen containing electrolyte [23]. In the viologen molecular solution, the dimerization of viologen radical cations occurs readily in aqueous and concentrated solutions but it is restricted in organic media [24,25]. Dicationic HV2+ based planar molecules, when reduced to radical cations, they re-arrange themselves on the electrode surface to form a stack like assembly, wherein each molecule lies directly above the other. Such a compact structure is resistive to oxidation and within a few cycles, the redox process becomes irreversible and the device acquires a permanently colored state. In the literature, the popular approach attempted to prevent the non-erasure of films of HV+• salt, was to add redox mediators such as hydroquinones, ferrous ions or hexacyanoferrate (II) species [26]. In the present report, we added ferrocene as the redox mediator to the electrolyte solution containing HV salt. The advantage of adding ferrocene as redox mediator in the electrolyte is that the ferricenium ions produced by its oxidation at the blank FTO electrode, chemically oxidizes the viologen dimers and the radical cations.
Prussian blue (PB) films are widely used as anodic electrochromes as they undergo reversible change from Prussian white (transparent) to Prussian blue upon oxidation and also offer a charge balancing feature, the latter being beneficial for long-term cycling stability [27]. A gel polymeric electrolyte or an ionogel is used for fabricating an electrochromic device (ECD). The gel is based on an ionic liquid namely, 1-butyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide and the electrochromic viologen salt: HV(BF4)2 was dissolved therein. An in-situ thermal polymerization technique is employed for solidifying the liquid electrolyte, with MMA as the monomer, as we found in a previous study that this method leads to devices with good interfacial contacts, which effectively translates into fast switching [21]. High ionic conductivity, good thermal stability and a wide electrochemical potential stability window for the semi-solid gel electrolyte, characteristics which are quintessential for any practical application, were ensured by the use of the imide based IL. Furthermore, another issue with HV2+ based ECDs face, is the uncontrolled formation of HV0 species during reduction, which is difficult to oxidize. To prevent the formation of this reduced product, an ionic additive, ethylenediamine tetra-acetic acid, EDTA was added to the electrolyte, and role of EDTA in improving all ECD performance parameters of transmission modulation, coloration efficiency, switching kinetics, chromaticity coordinates and cycle life is unambiguously demonstrated by comparing ECDs without and with EDTA and referred to as HV/gel/PB and HV/EDTA in gel/PB ECDs. Large area ECDs of ~8 cm × 6 cm dimensions have been fabricated, and in particular, the ability of the HV/EDTA in gel/PB ECD to color and bleach uniformly and reversibly, and its’ application as a ECRA mirror comparable or even better than commercial mirrors are demonstrated.
Section snippets
Materials
4,4′-Bipyridyl, 1-bromoheptane, methylmethacrylate (MMA) were procured from Sigma Aldrich. Potassium ferricyanide (K3[Fe(CN)6]), iron (III) chloride (FeCl3), sodium tetrafluoroborate, ferrocene, acetonitrile, N,N-dimethyl formamide, 1-butyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BuMePy][TFSI]), disodium salt of ethylenediamine tetra-acetic acid (Na2EDTA), activated neutral alumina, benzoyl peroxide (25% H2O), hydrochloric acid (35% HCl), ethyl acetate, acetone and methanol
Redox reactions in ECDs
The CV plots for the HV/gel/PB and HV/ETDA in gel/PB ECDs, were recorded over a potential range of −1.8 to +1.5 V by taking the bare FTO as the working electrode, at a slow scan rate of 2 mV s−1 (Fig. 1a). The CV profiles obtained are characteristic of the redox reactions experienced by HV2+ species and ferrocene. The reversible reaction corresponding to reduction and oxidation of ferrocene is given by the following equation, with Eox/Ered for this redox couple at −0.15 V/−0.82 V and at
Conclusions
The quest for high performance viologen based ECDs originates from their huge potential in the commercial sector. While heptyl viologen is easy to synthesize and ECD fabrication based on the same is also facile, but the radical cations: HV+•, that are formed during reduction are susceptible to the following unwanted reactions: they have a strong propensity to self-assemble into molecular stacks and they form dimers or undergo deep reduction to yield the pale colored neutral viologen (HV0). The
CRediT authorship contribution statement
Rambabu Sydam: Investigation, Resources, Visualization, Writing - original draft. Manoranjan Ojha: Investigation, Visualization. Melepurath Deepa: Conceptualization, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
M.O. is thankful to the University Grants Commission (UGC) for the grant of senior research fellowship. Reflection data was obtained on an instrument procured through a Department of Technology (DST), India, sponsored project: DST/TSG/PT/2007/69.
References (44)
- et al.
Highly stable viologens-based electrochromic devices with low operational voltages utilizing polymeric ionic liquids
Chem. Phys. Lett.
(2020) - et al.
An electrochromic device based on all-in-one polymer gel through in-situ thermal polymerization
Sol. Energy Mater. Sol. Cells
(2016) - et al.
Dimer formation of viologen derivatives and their electrochromic properties
Dyes Pigments
(1997) The effect of ferrocyanide on the performance of heptyl viologen-based electrochromic display devices
J. Electroanal. Chem.
(1997)- et al.
Electrodeposited prussian blue films: annealing effect
Electrochim. Acta
(2006) - et al.
Quantification of colour stimuli through the calculation of CIE chromaticity coordinates and luminance data for application to in situ colorimetry studies of electrochromic materials
Displays
(2011) - et al.
In situ spectroelectrochemistry and colour measurement of a complementary electrochromic device based on surface-confined Prussian blue and aqueous solution-phase methyl viologen
Sol. Energy Mater. Sol. Cells
(2012) - et al.
High contrast all-solid-state electrochromic device with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), heptyl viologen, and succinonitrile
Sol. Energy Mater. Sol. Cells
(2012) - et al.
Multi-color electrochromic devices based on phenyl and heptyl viologens immobilized with UV-cured polymer electrolyte
Sol. Energy Mater. Sol. Cells
(2018) - et al.
Highly stable ion gel-based electrochromic devices: effects of molecular structure and concentration of electrochromic chromophores
Org. Electron.
(2018)
Multifunctional hydrogel enables extremely simplified electrochromic devices for smart windows and ionic writing boards
Mater. Horiz.
Next-generation multifunctional electrochromic devices
Acc. Chem. Res.
Out of a niche
Nat. Mater.
Transparent wood smart windows: polymer electrochromic devices based on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) electrodes
ChemSusChem
Vehicular Interior Electrochromic Rearview Mirror Assembly
Single-compartment, Self-Erasing, Solution-phase Electrochromic Devices, Solutions for Use Therein, and Uses Thereof
Variable Reflectance Motor Vehicle Mirror
Thermally cured dual functional viologen-based all-in-one electrochromic devices with panchromatic modulation
ACS Appl. Mater. Interfaces
The viologens
Electrochromism and Electrochromic Devices
Novel color-reinforcing electrochromic device based on surface-confined ruthenium purple and solution-phase methyl viologen
Chem. Mater.
Achieving low-energy driven viologens-based electrochromic devices utilizing polymeric ionic liquids
ACS Appl. Mater. Interfaces
Voltage-tunable multicolor, sub-1.5 v, flexible electrochromic devices based on ion gels
ACS Appl. Mater. Interfaces
Cited by (9)
Well-matched materials: Color palettes-like multicolor electrochromic displays
2024, Applied Materials TodayFluorinated benzyl viologens for enhanced electrochromism and remarkable stability in electrochromic devices: An in-situ mass exchange probing through EQCM
2023, Solar Energy Materials and Solar CellsAll-in-One plasticized Ionogel-based stretchable electrochromic devices
2023, Chemical Engineering JournalStable viologen-based electrochromic devices: Control of Coulombic interaction using multi-functional polymeric ionic liquid membranes
2023, Solar Energy Materials and Solar CellsCitation Excerpt :In accordance with the above-mentioned literatures, several key indexes for high performance ECDs, including high ΔT (>70%), fast response time (<5 s), and good long-term stability (over 20k cycles), are hard to get simultaneously by using the viologen-based ECDs. Among these three types of electrolytes listed in Table 1, most of them successfully suppressed the aggregation of alkyl-viologens, including ethyl (blue ECD), heptyl (blue ECD), and nonyl (blue ECD), and provided promising ECD performance as well as long-term stability [11,12,17,18,23]. However, very few electrolytes could prevent the aggregation of aryl-viologens, such as functionalized-phenyl (green ECD), due to the strong intermolecular π-π stacking of phenyl rings [13].
Unveiling the diffusion-controlled operation mechanism of all-in-one type electrochromic supercapacitors: Overcoming slow dynamic response with ternary gel electrolytes
2021, Energy Storage MaterialsCitation Excerpt :However, this issue was addressed using TGE-45. Fig. 6e shows the excellent EC performance of the TGE-45-based ECS compared to previously reported single-layer all-in-one EC systems [26,27,39–44], indicating the effectiveness of binary solvent engineering to achieve high-performance gel electrolytes and electrochemical devices. Long-term stability during cyclic coloration/bleaching transitions was also tested (Fig. 6f), for which the durations for coloration and bleaching were fixed at 5 s (+0.9 V) and 17 s (0 V, short-circuit condition), respectively.
Electrically actuated visible and near-infrared regulating switchable smart window for energy positive building: A review
2021, Journal of Cleaner ProductionCitation Excerpt :A Polymer-based EC window showed 50,000 times switchability (Xu et al., 2004) and a viologen-based EC window offered 2 years of durability (Kubo et al., 2003). Recently heptyl viologen EC showed 68% transmission modulation after 2 years of aging (Sydam et al., 2021). Large EC devices can be made using an array of small EC units to overcome non-uniformity and slow switching behaviors (Lee and DiBartolomeo, 2002).