Double-Sided Electrochromic Device Based on Metal–Organic

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 39930-39934

Double-Sided Electrochromic Device Based on Metal−Organic Frameworks Issam Mjejri,†,‡ Cara M. Doherty,§ Marta Rubio-Martinez,§ Glenna L. Drisko,†,‡ and Aline Rougier*,†,‡ †

ICMCB, UPR 9048, CNRS, F-33600 Pessac, France ICMCB, UPR 9048, Université de Bordeaux, F-33600 Pessac, France § CSIRO Manufacturing and Minerals, Research Way, Clayton, Victoria 3168, Australia ‡

S Supporting Information *

ABSTRACT: Devices displaying controllably tunable optical properties through an applied voltage are attractive for smart glass, mirrors, and displays. Electrochromic material development aims to decrease power consumption while increasing the variety of attainable colors, their brilliance, and their longevity. We report the first electrochromic device constructed from metal organic frameworks (MOFs). Two MOF films, HKUST-1 and ZnMOF-74, are assembled so that the oxidation of one corresponds to the reduction of the other, allowing the two sides of the device to simultaneously change color. These MOF films exhibit cycling stability unrivaled by other MOFs and a significant optical contrast in a lithium-based electrolyte. HKUST-1 reversibly changed from bright blue to light blue and ZnMOF-74 from yellow to brown. The electrochromic device associates the two MOF films via a PMMA-lithium based electrolyte membrane. The color-switching of these MOFs does not arise from an organic-linker redox reaction, signaling unexplored possibilities for electrochromic MOF-based materials. KEYWORDS: metal−organic frameworks, thin films, electrochromism, device, HKUST-1, color switching

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MOFs constructed from Cu2+ and Zn2+ and carboxylates (HKUST-1 and ZnMOF-74) that show unexpected redox activity. In the four previous reports of electrochromic MOF materials the organic linker was responsible for the reversible color change.15,17−19 We now show that electrochromic materials can be achieved from commercially available organic linkers and metal salts, which makes these materials suitable for this type of application.20 We then built a device out of these two MOFs, showing a unique double sided color change, opening up a field of applications in displays and infrastructure. Micrometer-thick electrochromic films were prepared from the as-synthesized MOF powders using doctor blading, a simple and cost-effective method. Doctor blading can be performed at ambient temperature and can be applied to a large variety of substrates.3,21 Homogeneous MOF films were prepared by dispersing a small quantity of powder in a few mLs of distilled water. The resulting dispersion was stirred at room temperature until a smooth paste was formed, prior to deposition on an ITO glass substrate. MOF films show an average thickness of 1.5 μm. The electrochromic activity of the two MOF films were recorded in a lithium-based ionic liquid. Figure 1a shows the cyclic voltammograms of the HKUST-1 film deposited onto

mart materials are the salient feature of our modern econnected society. Optical materials are continuously evolving and finding more areas of application, such as the electrochromic windows in the Boeing 787 Dreamliner and Gentex Corporation’s antiglare mirrors.1 Electrochromic smart windows can be used in cars or buildings to adjust brightness or in spacecraft to moderate the intense thermal fluctuations by switching between light/infrared transmission and reflection.2,3 Electrochromic materials and devices change their optical properties in a reversible and persistent way under an applied voltage.4 The most frequently used electrochromic compounds include transition metal oxides,5−10 such as WO3, MoO3, TiO2, IrO2, V2O5, NiO, Prussian blue (iron ferrocyanide) analogues,11 and organic polymers,12 such as polyaniline, polypyrrole, and polythiophen.13 Inorganic electrochromic materials are stable and WO3 is central to most applications;14 however, their range of available colors and brightness are limited. On the contrary, organic polymers show high color efficiency and a huge range of colors but suffer from limited stability, particularly when exposed to the ambient environment. In 2013, Dincǎ et al. first reported using metal organic frameworks (MOFs) as an electrochromic material.15 MOFs offer enormous structural and chemical diversity, being constructed from metal/metal cluster centers and organic linkers in countless combinations. Their use in redox active applications has thus far been fairly limited as the metal centers (e.g., Zn2+) and the organic linkers (e.g., carboxylates) are typically redox-inactive.16 In this contribution, we use two © 2017 American Chemical Society

Received: September 8, 2017 Accepted: October 18, 2017 Published: October 18, 2017 39930

DOI: 10.1021/acsami.7b13647 ACS Appl. Mater. Interfaces 2017, 9, 39930−39934

Letter

ACS Applied Materials & Interfaces

conductive substrate, which is probably due to film thickness. Variation in current during Li ion insertion and deinsertion with respect to the applied potential is measured and significant changes were observed in the electrochromic properties of the films. As shown in the inset of Figure 1a, the HKUST-1 films switch reversibly between a reduced light blue state (−0.4 V) and an oxidized-bright blue state (0.8 V). This change in color is most likely due to a variation in the oxidation state of the copper metal from Cu(II) to Cu(I) after lithium insertion into the structure. The diffuse reflectance spectra of HKUST-1 films corresponding to the oxidized and reduced states are illustrated in Figure 1b. The color-switching from the reduced to oxidized state corresponds to reflectance values of about 27% and 91% at 460 nm, respectively, leading to an optical reflectance modulation, ΔR, of about 64%. Figure 1c shows the Coulombic efficiency vs the number of cycles for the HKUST-1 film. The electrochemical reversibility defined as the ratio of the capacity in oxidation (Qox) to the capacity of reduction (Qred), is close to 99.5%, after 100 cycles, confirming excellent reversibility and stability of the HKUST-1 film. This good electrochemical performance is ascribed to the highly porous hierarchical platelike architecture that facilitates electrolyte penetration and Li+ diffusion. The porous-framework of HKUST-1 contains large cavities having windows of ∼6 Å in diameter, which is considerably larger than the radius of a lithium ion (0.74 Å). Uninhibited lithium diffusion facilitates a fast electrochemical reaction. The switching kinetics of the HKUST-1 film was investigated by chronoamperometric (CA) measurements. Figure 1d shows current transient (j−t) for the HKUST-1 film for voltage steps of (−0.4 V/60 s) and (0.8 V/60 s). Both response times are on the order of a few seconds; 5 s for the oxidation and 6 s for the reduction. In CIE colorimetric space, the color is represented by three parameters, the luminance axis (L*) and two hue axes (a*) and (b*), which can be used to define and compare quantitatively

Figure 1. (a) Cyclic voltammograms of the HKUST-1 film cycled 100 times in 0.3 M LiTFSI in EMITFSI with a 20 mV s−1 scan rate and a photograph of the HKUST-1 film in the reduced and oxidized states. (b) Diffuse reflectance spectra for the HKUST-1 films cycled ex situ in 0.3 M LiTFSI in EMITFSI, initial state (As-dep.), reduced, and oxidized state. (c) Charge capacity evolution with the number of cycles and (d) chronoamperogramms of ITO/HKUST-1/0.3 M LiTFSI in EMITFSI cycled at −0.4 V for 60 s and 0.8 V for 60 s.

ITO and cycled, at a scan rate of 20 mV s−1 between the potential range of −0.4 to 0.8 V, in a three electrodes cell using ionic liquid based (0.3 M) LiTFSI in EMITFSI as a supporting electrolyte and Satured Calomel Electrode as reference electrode (SCE). The cycling of the HKUST-1 film shows good reversibility, and stability during the 100 cycles tested. Previous MOF-based electrochromic materials could not be cycled more than 6 to 25 times due to delamination of the MOF film.15,17−19 Thus, the MOFs we prepared using doctor blading have better longevity than those directly grown on a

Figure 2. (a) Cyclic voltammograms for the ZnMOF-74 film cycled in 0.3 M LiTFSI in EMITFSI with a 20 mV −1 scan rate and a photo of the ZnMOF-74 film in the reduced and oxidized state. (b) diffuse reflectance spectra for the HKUST-1 films cycled ex situ in 0.3 M LiTFSI in EMITFSI, initial state (as-dep.), reduced and oxidized state. (c) charge capacity evolution with the number of cycles and (d) chronoamperogramms of ITO/ ZnMOF-74/0.3 M LiTFSI in EMITFSI cycled at −1 V for 60 s and 2 V for 60 s. 39931

DOI: 10.1021/acsami.7b13647 ACS Appl. Mater. Interfaces 2017, 9, 39930−39934

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ACS Applied Materials & Interfaces the colors. The relative luminance (L*), the hue (a*) and (b*) values of HKUST-1 films in different states were measured. Indeed, for the light blue-reduced state (at −0.4 V), the L*a*b* parameters are 75, −8, and 5, respectively, whereas for the bright blue-oxidized state (at 0.8 V), the L*a*b* parameters are 87, −7, and 1, respectively. The contrast ΔE* = [(L*2 − L*1)2 + (a*2 − a*1)2 + (b*2 − b*1)2]1/2 is 13. Figure 2a shows the cyclic voltammogramms (CVs) of ZnMOF-74/0.3 M LiTFSI in EMITFSI/Pt vs SCE taken between −1 V and 2 V, at a cycling rate of 20 mV s−1. The shape of the CVs remains practically unchanged over 100 cycles. The repeated cycles are well superposed, which indicates that the MOF are electroactive and stable under these conditions. The insertion/deinsertion of Li+ in the MOF films is accompanied by a color change. The ZnMOF-74 films switch reversibly between a reduced brown state (−1 V) and an oxidized-yellow state (2 V) (Figure 2a). As Zn and the 2,5dihydroxyterephthalic acid linker should both be electrochemically inactive, the mechanism of the brown⇄yellow electrochromism is likely due to a charge transfer between the ligand and the Zn center or to an interaction with the electrolyte. Figure 2b exhibits the ex situ evolution of the reflectance spectra for ZnMOF-74 films cycled in LiTFSI in EMITFSI and progressively colored at various potentials. The reduced-brown state (−1 V) and the oxidized-yellow state (2 V) are associated with reflectance values of about Rox ≈ 56% and Rred ≈ 25% at 600 nm, respectively. At 20 mV s−1, the electrochemical capacity of the reduction process is equal to the one measured under oxidation (Qox/Qred = 99%) (shown in Figure 2c). This indicates that all inserted Li+ within the MOF film during the reduction process are reversibly and totally deinserted upon oxidation. The switching times deduced from chronoamperometry performed by applying either −1 or 2 V for 60 s are of about 10 and 7 s, respectively (Figure 2d). For the brown, reduced state (at −1 V), the L*a*b* parameters are 35, − 1, and 12, whereas for the yellow, oxidized state (at 2 V) the L*a*b* parameters are 76, 2, and 34. The contrast ΔE* is 47. The phase stability of the MOF frameworks during electrochemical cycling and film preparation was confirmed using X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The XRD pattern of the HKUST-1 powder indicates that the d-spacing values of all diffraction peaks belong to the MOF, thus no crystalline impurities are detected (Figure 3a). The quality of the baseline indicates that no amorphous impurities were present. High purity HKUST-1 is easily synthesized via flow chemistry22 using mild reaction conditions and commonly available laboratory equipment. The XRD pattern of the HKUST-1 powder (Figure 3a) exhibits the same peaks before and after suspension in water and after film deposition (Figure 3b). This confirms the crystalline stability of HKUST-1 during aqueous suspension and doctor blading. Figure 3c shows the XRD of the HKUST-1 film after 50 cycles, ending with oxidation. All peaks are indexed to the crystal phase of HKUST-1 showing a remarkable stability upon repeated lithium insertion and deinsertion events. The crystallite size, calculated using the Scherrer formula, for the HKUST-1 powder, film as deposited and oxidized HKUST-1 film after 50 cycles is about 156, 140, and 106 nm, respectively. SEM images of HKUST-1 powders show a homogeneous phase with uniform polyhedron (Figure 3d). The morphologies of the HKUST-1 film and the as-made powder differ due to aging in

Figure 3. XRD patterns of (a) the HKUST-1 powder, (b) the HKUST-1 film as-deposited on a ITO/glass substrate, (c) ex situ XRD patterns of the oxidized HKUST-1 film after the 50th cycle, (d) SEM of the HKUST-1 powders, (e) the HKUST-1 film as-deposited, (f) oxidized HKUST-1 film after the 50th cycle, (g) XRD patterns of the ZnMOF-74 powders, (h) ZnMOF-74 film as-deposited on ITO/glass substrate (i), ex situ XRD patterns of the oxidized ZnMOF-74 film after the 50th cycle, (j) SEM of the ZnMOF-74 powders, (k) the ZnMOF74 film as-deposited, and (l) the oxidized ZnMOF-74 film after the 50th cycle.

water. The SEM image of the HKUST-1 film before cycling (Figure 3e) shows that the film consists of needles and platelets arranged randomly, with the surface being relatively densely covered. It seems that elongated needles and platelets form via the coalescence of adjacent nanospheres with similar crystallographic orientation during aging. SEM imaging after 50 cycles (Figure 3f) shows that the morphology changes significantly. Interestingly, the morphological changes of the crystals have no impact on the electrochromic performance. For the yellow ZnMOF-74 powder, the XRD pattern reveals good crystallinity where the peaks are indexed to those of the crystalline phase of ZnMOF-74 with two major peaks typically indexed as (110) and (300)23 (Figure 3g). No peaks of any other phase or impurity are detected in the XRD pattern before and after cycling (Figure 3h, i). Thus, it seems that ZnMOF-74 39932

DOI: 10.1021/acsami.7b13647 ACS Appl. Mater. Interfaces 2017, 9, 39930−39934

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ACS Applied Materials & Interfaces

of ion insertion and deinsertion. Although the MOF crystals changed in morphology, the X-ray diffractograms and the electrochromic properties indicated a robust conservation of the framework. In this study, the electrochromic properties of the metal organic frameworks HKUST-1 and ZnMOF-74 have been discussed. The electrochromic properties of HKUST-1 and ZnMOF-74 films deposited by doctor blading from a water dispersion show good stability and nice reversibility. HKUST-1 reversibly changes from light blue to bright blue, corresponding to an optical contrast of 13. ZnMOF-74 changes from brown to yellow with an optical contrast of 47. Finally, the first MOFbased electrochromic device was built from HKUST-1/ electrolyte/LixZnMOF-74. This double-sided electrochromic device exhibited nice optical contrast and fast color switching. The diversity of MOFs arises from the multitude of combinations of metal ions or clusters and organic linkers. The number of possible MOFs is limited only by human imagination. The first electrochromic MOFs used colorswitching organic-linkers, which certainly limits the range of colors and efficiencies obtainable, as well as their commercial prospects. We proved that any MOF that is (1) easy to synthesize using commercially available materials and (2) colored, should be investigated for electrochromism. We could not have predicted the observed electrochromic activity of the MOFs presented here. MOFs may employ the metal center, the ligand, the electrolyte, or all three of these in the color switch mechanism, hence opening up a vast area of research. Particularly, we are studying the generalizability and the redox mechanisms at work.

is very stable under the cycling conditions. The SEM image of the ZnMOF-74 powders (Figure 3j) shows that the MOF consists of a homogeneous phase of uniform nanorods with smooth surfaces. The morphology of ZnMOF-74 film before cycling (Figure 3k) appears as an assortment of nano and micron sized rods, sometimes arranged in bundles. SEM imaging after 50 cycles (Figure 3l) shows that there is a significant change in the morphology, but once again this has no bearing on the electrochemical performance of this MOF. We then constructed the first two-faced MOF-based electrochromic device by coupling the electrochromic properties of the two MOF films, HKUST-1 and ZnMOF-74. The films were individually deposited on an ITO substrate in 0.3 M LiTFSI-EMITFSI electrolyte. The two films were associated using a hydrophobic electrolyte membrane based of 0.3 M LiTFSI-EMITFSI with PMMA as an ion conductor. Prior to device assembly, the ZnMOF-74 thin film was cycled in the 0.3 M LiTFSI-EMITFSI electrolyte for three cycles and assembled in the brown reduced state to the precycled light blue HKUST1 (Figure 4). The HKUST-1 and ZnMOF-74 simultaneously changed colors upon an applied voltage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13647. Experimental details (PDF)

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Figure 4. (a) Schematic of MOF/electrolyte/MOF electrochromic device combining HKUST-1 and LixZnMOF-74 films on an ITO substrate separated by an electrolyte membrane consisting of 0.3 M LiTFSI−EMITFSI in 40% PMMA. (b) Cyclic voltammograms of the HKUST-1/electrolyte/LixZnMOF-74 device with a 20 mV/s scan rate for 100 cycles. (c) Chronoamperogramms of HKUST-1/electrolyte/ LixZnMOF-74 device at −1.3 V for 60 s and 1.3 V for 60 s.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aline Rougier: 0000-0002-1340-734X Author Contributions

The manuscript was written through contributions of all authors.

The electrochemical stability of the HKUST-1/LiTFSIEMITFSI in PMMA/LixZnMOF-74 proved to be excellent through 100 repeated cycles of oxidation and reduction (Figure 4b). The similarity of the CVs recorded for the first cycle and the 100th cycle at 20 mV/s confirms that the device is highly stable. The CV curves illustrate reversible behavior with good cyclability associated with nice color switches which differ on the two faces, depending on whether the HKUST-1 or the ZnMOF-74 film is visible. Under an alternating potential between −1.3 V/60 s and +1.3 V/60 s (Figure 4c), the colorswitching time was about 8 s upon reduction and 9 s upon oxidation. The optical contrasts of the two faces were evaluated using the CIE L*a*b* color system. For both sides HKUST-1, from light blue to bright blue, and ZnMOF-74, from brown to yellow, ΔE* are of the order of 10 and 13, respectively. MOFs are notorious for their fragility. Here we showed excellent preservation of the MOF framework upon 100 cycles

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.L.D. was supported by the LabEx AMADEus (ANR-10LABX-42) in the framework of IdEx Bordeaux (ANR-10-IDEX03-02), i.e., the Investissements d’Avenir program of the French government managed by the Agence Nationale de la Recherche. Most SEM images were acquired by David Montero in the Institut des Matériaux de Paris Centre. C.M.D. is supported by the Australian Research Council (DE140101359), a CSIRO Julius Career Award and a Veski Inspiring Women Fellowship.

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