Electrochromic device response controlled by an in situ polymerized ionic liquid based gel electrolyte

Polymer electrolytes were synthesized by two different approaches and applied to electrochromic devices based on electrodeposited tungsten oxide (WO3) or poly(3,4-ethylenedioxythiophene) (PEDOT) films as the cathode, and a Prussian blue (PB) film as the anode. The first method involved the entrapping of an ionic liquid in a polymer host (poly(methylmethacrylate) or PMMA) and the second approach relied on the in situ thermal polymerization of methylmethacrylate (MMA) in the hydrophobic ionic liquid, yielding a solidified transparent gel. The effect of in situ solid polymer electrolyte formation on device performance characteristics was realized in terms of a larger coloration efficiency of 119 cm(2) C-1 (lambda = 550 nm) achieved for the WO3-PB (MMA) device, as compared to a value of 54 cm(2) C-1 obtained for the WO3-PB (PMMA) device. Similar enhancements in electrochromic coloring efficiency, reflectance contrast, and faster switching kinetics were obtained for the PEDOT-PB (MMA) device. The strategy of introducing an electrolyte to the electrochromic device in a liquid state and then subjecting the same to gradual polymerization allows greater accessibility of the electrolyte ions to the active sites on the electrochromic electrodes and superior interfacial contact. As a consequence, larger optical contrast and faster kinetics are achieved in the MMA based devices. While PEDOT films were amorphous, PB films were semi-crystalline but only in the case of WO3; the hexagonal structure of WO3, equipped with three/four/six-coordinated voids was found to affect bleaching kinetics favorably. The performance of PMMA based electrolyte is limited by high resistance at the electrode-electrolyte interface, and a smaller number of ions available for oxidation and reduction. Large area (similar to 10 cm x 4 cm) devices were also fabricated using this simple wet chemistry method and their ability to color uniformly without any pinholes was demonstrated

Ionic additive in an ionogel for a large area long lived high contrast electrochromic device

Heptyl viologen (HV) based electrochromic devices (ECD) suffer from poor write-erase efficiencies and become permanently colored after a few redox switches, thus limiting their use as electrochromic windows or mirrors. This formidable issue is addressed by incorporating an ionic additive: a disodium salt of ethylenediamine tetra-acetic acid (EDTA), in the ionogel electrolyte. EDTA, via electrostatic interactions, binds to the HV+• radical cation and prevents its' further reduction to the undesirable pale colored neutral HV0, by wedging in-between the moieties and inhibiting their undesirable stacking. A large area HV/EDTA in gel/Prussian blue (PB) ECDs of ~8 cm × 6 cm dimensions outperforms the HV/gel/PB ECD which is evidenced in the enhanced transmission modulation (ΔTmax: 73.1% at 606 nm), coloration efficiency (η: 346.2 cm2 C−1) and superior chromaticity coordinates (colored: L*,a,b: 40, 10, −65) compared to lower magnitudes of 69.2%, 270.3 cm2 C−1 and (L*,a,b: 50.3, 8, −48.5) respectively. When evaluated as a mirror, HV/EDTA in gel/PB ECD shows a reflectance modulation of ~65% which does not alter significantly, even at −10 °C or at +60 °C, ratifying its thermal robustness. The HV/EDTA in gel/PB ECD shows color and bleach times of 16 and 35 s, retains ~93% of its’ initial contrast after 104 cycles and after aging for 2 years, continues to deliver a transmission modulation of 68%. This study successfully demonstrates a cost effective, easily implementable, scaled-up HV/EDTA in gel/PB based long-lived durable ECD for energy efficient smart window and rearview mirror applications

A novel 1,1 '-bis[4-(5,6-dimethyl-1H-benzimidazole-1-yl)butyl]-4,4 '-bipyridinium dibromide (viologen) for a high contrast electrochromic device

A new electrochromic viologen, 1,1'-bis-[4-(5,6-dimethyl-1H-benzimidazole-1-yl)-butyl]-4,4'-bipyridinium dibromide (IBV) was synthesized by di-quaternization of 4,4'-bipyridyl using 1-(4-bromobutyl)-5,6-dimethyl-1H-benzimidazole. X-ray photoelectron spectroscopy confirmed the formation of the IBV (viologen) salt as distinct signals due to quaternary nitrogen and neutral nitrogen, and ionic-bonded bromide were identified. An electrochromic device encompassing a dicyanamide ionic liquid based gel polymeric electrolyte with high ionic conductivity, a thermal decomposition temperature above 200 degrees C, and a stable voltage window of similar to 4 V with the IBV viologen dissolved therein, was constructed. IBV is a cathodically coloring organic electrochrome and the device underwent reversible transitions between transparent and deep blue hues; the color change was accompanied by an excellent optical contrast (30.5% at 605 nm), a remarkably high coloration efficiency of 725 cm(2) C-1 at 605 nm and switching times of 2-3 s. Electrochemical impedance spectroscopy revealed an unusually low charge transfer resistance at the IBV salt/gel interface, which promotes charge propagation and is responsible for the intense coloration of the reduced radical cation state. The device was subjected to repetitive switching between the colored and bleached states and was found to incur almost no loss in redox activity, up to 1000 cycles, thus ratifying its suitability for electrochromic window/display applications

A WO3-poly(butyl viologen) layer-by-layer film/ruthenium purple film based electrochromic device switching by 1 volt application

A layer-by-layer (LbL) assembly of poly(butyl viologen) (PBV) and poly(peroxotungstate) was used for preparing a cathodically coloring dual electrochrome WO3-PBV film. A new formulation containing ferrocenecarboxaldehyde and potassium hexacyanoruthenate(II) was employed for the first time and anodically coloring ruthenium purple (RP) thin films were electrodeposited. X-ray photoelectron spectroscopy and Fourier transform infrared analyses confirmed the RP film structure to be an inorganic coordination polymer: K-x center dot Fe-x(III)[Ru-II(CN)(6)](y), wherein Fe3+ and Ru2+ color centers are flanked by CN- ligands. Nanoscale electrical conductivities determined by conducting atomic force microscopy were deduced to be 1.03 and 147.4 mS cm(-1), for the RP and WO3-PBV LbL films respectively, which confirmed their ability to function as mixed conductors, attributes pertinent during optical switching. Reversible bias induced transition between purple and colorless hues was attained in the RP film and between deep blue and colorless states was achieved for the WO3-PBV LbL film with coloration efficiencies of 188 and 380 cm(2) C-1 respectively. A complementarily coloring electrochromic device of WO3-PBV/RP was fabricated with an environmentally benign, inexpensive aqueous electrolyte, which switched between blue, purple, and colorless states. The device exhibited an electrochromic efficiency of 476 cm(2) C-1 at 580 nm, an outstanding transmission modulation of 49%, and short switching times (similar to 1 s); all under remarkably low operating voltages of +/- 1V. The WO3-PBV/RP device can deliver high optical contrast under low operating bias, and is highly scalable and durable, which demonstrates it to be ideal for practical optical switching devices like fast changing displays or smart windows

A WO3-poly(butyl viologen) layer-by-layer film/ruthenium purple film based electrochromic device switching by 1 volt application

A layer-by-layer (LbL) assembly of poly(butyl viologen) (PBV) and poly(peroxotungstate) was used for preparing a cathodically coloring dual electrochrome WO3-PBV film. A new formulation containing ferrocenecarboxaldehyde and potassium hexacyanoruthenate(II) was employed for the first time and anodically coloring ruthenium purple (RP) thin films were electrodeposited. X-ray photoelectron spectroscopy and Fourier transform infrared analyses confirmed the RP film structure to be an inorganic coordination polymer: Kx'Fex III[RuII(CN)6]y, wherein Fe3+ and Ru2+ color centers are flanked by CN- ligands. Nanoscale electrical conductivities determined by conducting atomic force microscopy were deduced to be 1.03 and 147.4 mS cm-1, for the RP and WO3-PBV LbL films respectively, which confirmed their ability to function as mixed conductors, attributes pertinent during optical switching. Reversible bias induced transition between purple and colorless hues was attained in the RP film and between deep blue and colorless states was achieved for the WO3-PBV LbL film with coloration efficiencies of 188 and 380 cm2 C-1 respectively. A complementarily coloring electrochromic device of WO3-PBV/RP was fabricated with an environmentally benign, inexpensive aqueous electrolyte, which switched between blue, purple, and colorless states. The device exhibited an electrochromic efficiency of 476 cm2 C-1 at 580 nm, an outstanding transmission modulation of 49%, and short switching times (~1 s); all under remarkably low operating voltages of ±1 V. The WO3-PBV/RP device can deliver high optical contrast under low operating bias, and is highly scalable and durable, which demonstrates it to be ideal for practical optical switching devices like fast changing displays or smart windows