Electrochromism Explained

Electrochromism is a phenomenon in which a material displays changes in color or opacity in response to an electrical stimulus.[1] In this way, a smart window made of an electrochromic material can block specific wavelengths of ultraviolet, visible or (near) infrared light. The ability to control the transmittance of near-infrared light can increase the energy efficiency of a building, reducing the amount of energy needed to cool during summer and heat during winter.[2] [3]

As the color change is persistent and energy needs only to be applied to effect a change, electrochromic materials are used to control the amount of light and heat allowed to pass through a surface, most commonly "smart windows". One popular application is in the automobile industry where it is used to automatically tint rear-view mirrors in various lighting conditions.

Principle

The phenomenon of electrochromism occurs in some transition metal oxides which conduct both electrons and ions, such as tungsten trioxide (WO3).[4] These oxides have octahedral structures of oxygen which surround a central metal atom and are joined together at the corners. This arrangement produces a three-dimensional nanoporous structure with "tunnels" between individual octahedral segments. These tunnels allow dissociated ions to pass through the substance when they are motivated by an electric field. Common ions used for this purpose are H+ and Li+.[5] [6]

The electric field is typically induced by two flat, transparent electrodes which sandwich the ion-containing layers. As a voltage is applied across these electrodes, the difference in charge between the two sides causes the ions to penetrate the oxide as the charge-balancing electrons flow between the electrodes. These electrons change the valency of the metal atoms in the oxide, reducing their charge, as in the following example of tungsten trioxide:[7]

+ n(+ e−) →

This is a redox reaction since the electroactive metal accepts electrons from the electrodes, forming a half-cell. Strictly speaking, the electrode as a chemical unit comprises the flat plate as well as the semiconducting substance in contact with it. However, the term "electrode" often refers to only the flat plate(s), more specifically called the electrode "substrate".[8]

Photons that reach the oxide layer can cause an electron to move between two nearby metal ions. The energy provided by the photon causes the movement of an electron which in turn causes optical absorption of the photon.[9] For example, the following process occurs in tungsten oxide for two tungsten ions a and b:[10]

+ + photon → +

Electrochromic materials

Electrochromic materials, also known as chromophores, affect the optical color or opacity of a surface when a voltage is applied. Among the metal oxides, tungsten oxide (WO3) is the most extensively studied and well-known electrochromic material.[11] Others include molybdenum,[12] titanium[13] and niobium oxides,[14] although these are less effective optically.

Viologens are a class of organic materials[15] [16] that are being intensively investigated for electrochromic applications.[17] These 4,4′-bipyridine compounds display reversible color changes between a colorless and a deep-blue color due to redox reactions. Researchers can "tune" them to a deep blue or intense green.

As organic materials, viologens are seen as promising alternatives for electronic applications, compared to metal-based systems, which tend to be expensive, toxic, and a problem to recycle.[15] Possible advantages of viologens include their optical contrast, coloration efficiency, redox stability, ease of design, and potential to scale up for large-area preparation.[17]

Viologens have been used with phenylenediamine by Gentex Corporation, which has commercialized auto-dimming rearview mirrors[17] and smart windows in Boeing 787 aircraft.[18] Viologen has been used in conjunction with titanium dioxide (TiO2, also known as titania) in the creation of small digital displays.[19] [20] A variety of conducting polymers are also of interest for displays, including polypyrrole, PEDOT, and polyaniline.[21]

Synthesis of tungsten oxide

Many methods have been used to synthesize tungsten oxide, including chemical vapor deposition (CVD), sputtering, thermal evaporation, spray pyrolysis (from a vapor or sol-gel), and hydrothermal synthesis (from a liquid).[22] In industry, sputtering is the most common method for the deposition of tungsten oxide. For material synthesis, sol-gel process is widely used due to its advantages of simple process, low cost, and easy control.[23]

Sol-gel process

In the sol-gel process of tungsten trioxide, is dissolved in alcohol and then oxidized by purging into its solution:

+ → +

The formation of is performed by the reaction of alcohol and chlorine that used for the reduction of to obtain a blue solution of :

+ → +

+ →

nanoparticles can also be obtained by precipitation of ammonium tungstate para pentahydrate,, or nitric acid,, under acidic conditions from aqueous solutions.[24]

Working principle of electrochromic windows

Multiple layers are needed for a functional smart window with electrochromic characteristics.[3] The first and last are transparent glass made of silica, the two electrodes are needed to apply the voltage, which in turn will push (or pull) ions from the ion storage layer, through the electrolyte into the electrochromic material (or vice versa). Applying a high voltage (4 V or more) will push lithium-ions into the electrochromic layer, deactivating the electrochromic material. The window is fully transparent now. By applying a lower voltage (2.5 V for example) the concentration of Li-ions in the electrochromic layer decreases, thus activating (N)IR-active tungsten oxide.[25] [3] This activation causes reflection of infrared light, thus lowering the greenhouse effect, which in turn reduces the amount of energy needed for air conditioning.[26] Depending on the electrochromic material used, different parts of the spectrum can be blocked, this way UV, visible and IR light can be independently reflected at the will of a user.[3]

Applications

Several electrochromic devices have been developed. Electrochromism is commonly used in the production of electrochromic windows or "smart glass",[3] [2] and more recently electrochromic displays on paper substrate as anti-counterfeiting systems integrated into packaging.[27] NiO materials have been widely studied as counter electrodes for complementary electrochromic devices, particularly for smart windows.[28] [29]

ICE 3 high speed trains use electrochromic glass panels between the passenger compartment and the driver's cabin. The standard mode is clear, and can be switched by the driver to frosted.[30] Electrochromic windows are used in the Boeing 787 Dreamliner in the form of a dimmable panel between the exterior window and interior dust cover,[31] allowing crew and passengers to control the transparency of the windows.

See also

Further reading

. Claes-Göran Granqvist . Handbook of Inorganic Electrochromic Materials . 2002 . . 978-0-08-053290-5. 1995.

External links

Notes and References

  1. Book: Chua . Ming Hui . Tang . Tao . Ong . Kok Haw . Neo . Wei Teng . Xu . Jian Wei . Electrochromic Smart Materials: Fabrication and Applications. Chapter 1 Introduction to Electrochromism . Smart Materials Series . 2019 . Royal Society of Chemistry . 1–21 . 10.1039/9781788016667-00001 . 978-1-78801-143-3 . 139718051 . 29 July 2022 . en.
  2. Encyclopedia: Mortimer . R.J. . Electrochromic Materials . Annu. Rev. Mater. Res. . 2011 . 41 . Pt 3 . 241–268 . 10.1146/annurev-matsci-062910-100344 . 12449538 . 2011AnRMS..41..241M.
  3. Miller . Brittney J. . How smart windows save energy . Knowable Magazine . 8 June 2022 . 10.1146/knowable-060822-3 . free . 15 July 2022.
  4. Electrochromic materials and devices: present and future . Somani . Prakash R. . Radhakrishnan . S. . Materials Chemistry and Physics . Elsevier . 26 September 2001 . 22 August 2019 . 77 . 117–133. 10.1016/S0254-0584(01)00575-2 . 36550231 .
  5. Book: Granqvist, C.G. . Fenestration for reducing building cooling needs . Eco-efficient Materials for Mitigating Building Cooling Needs . 2015 . Elsevier . 460–464. 978-1-78242-380-5.
  6. Brus . Jiri . Czernek . Jiri . Urbanova . Martina . Rohlíček . Jan . Plecháček . Tomáš . Transferring Lithium Ions in the Nanochannels of Flexible Metal–Organic Frameworks Featuring Superchaotropic Metallacarborane Guests: Mechanism of Ionic Conductivity at Atomic Resolution . ACS Appl. Mater. Interfaces . 2020 . 12 . 42 . 47447–47456 . 10.1021/acsami.0c12293 . 32975402 . 221918602 . free .
  7. Book: Electrochromism and Electrochromic Devices . Monk . P.M.S. . Mortimer . R.J. . Rosseinsky . D.R. . 2007 . 978-0-521-82269-5 . Cambridge University Press.
  8. Book: NASA Reference Publication . 1979 . National Aeronautics and Space Administration, Scientific and Technical Information Branch . x . 29 July 2022 . en.
  9. Web site: Colours of Transition Metal Ions in Aqueous Solution . Compound Interest . 29 July 2022 . 5 March 2014.
  10. Deepa . M. . Joshi . A. G. . Srivastava . A. K. . Shivaprasad . S. M. . Agnihotry . S. A. . Electrochromic Nanostructured Tungsten Oxide Films by Sol-gel: Structure and Intercalation Properties . Journal of the Electrochemical Society . 2006 . 153 . 5 . C365 . 10.1149/1.2184072 . 2006JElS..153C.365D . 10.1.1.949.7131 .
  11. Can . Fabien . Courtois . Xavier . Duprez . Daniel . Tungsten-Based Catalysts for Environmental Applications . Catalysts . 2 June 2021 . 11 . 6 . 703 . 10.3390/catal11060703 . 29 July 2022. free .
  12. Jinmin . Wang . Lijun . H. O. U. . Dongyun . M. A. . Molybdenum Oxide Electrochromic Materials and Devices . Journal of Inorganic Materials . 20 May 2021 . 36 . 5 . 461 . 10.15541/jim20200416 . 229203829 . en . 1000-324X. free .
  13. Eyovge . Cavit . Deenen . Cristian S. . Ruiz-Zepeda . Francisco . Bartling . Stephan . Smirnov . Yury . Morales-Masis . Monica. Mónica Morales Masis . Susarrey-Arce . Arturo . Gardeniers . Han . Color Tuning of Electrochromic TiO 2 Nanofibrous Layers Loaded with Metal and Metal Oxide Nanoparticles for Smart Colored Windows . ACS Applied Nano Materials . 27 August 2021 . 4 . 8 . 8600–8610 . 10.1021/acsanm.1c02231 . 34485847 . 8406417 . en . 2574-0970.
  14. Ong . Gary K. . Saez Cabezas . Camila A. . Dominguez . Manuel N. . Skjærvø . Susanne Linn . Heo . Sungyeon . Milliron . Delia J. . Electrochromic Niobium Oxide Nanorods . Chemistry of Materials . 14 January 2020 . 32 . 1 . 468–475 . 10.1021/acs.chemmater.9b04061 . 213992620 . en . 0897-4756.
  15. Striepe . Laura . Baumgartner . Thomas . Viologens and Their Application as Functional Materials . Chemistry - A European Journal . 1 December 2017 . 23 . 67 . 16924–16940 . 10.1002/chem.201703348 . 28815887 . 29 July 2022 . en.
  16. Kathiresan . Murugavel . Ambrose . Bebin . Angulakshmi . Natarajan . Mathew . Deepa Elizabeth . Sujatha . Dhavamani . Stephan . Arul Manuel . Viologens: a versatile organic molecule for energy storage applications . Journal of Materials Chemistry A . 2021 . 9 . 48 . 27215–27233 . 10.1039/D1TA07201C . 244388428 . 29 July 2022 . en . 2050-7488.
  17. Shah . Kwok Wei . Wang . Su-Xi . Soo . Debbie Xiang Yun . Xu . Jianwei . Viologen-Based Electrochromic Materials: From Small Molecules, Polymers and Composites to Their Applications . Polymers . 8 November 2019 . 11 . 11 . 1839 . 10.3390/polym11111839 . 31717323 . 6918392 . free .
  18. Wang . Yang . Runnerstrom . Evan L. . Milliron . Delia J. . Switchable Materials for Smart Windows . Annual Review of Chemical and Biomolecular Engineering . 7 June 2016 . 7 . 1 . 283–304 . 10.1146/annurev-chembioeng-080615-034647 . 27023660 . 29 July 2022 . en . 1947-5438. free .
  19. De Matteis . Valeria . Cannavale . Alessandro . Ayr . Ubaldo . Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods . Applied Sciences . 13 December 2020 . 10 . 24 . 8896 . 10.3390/app10248896 . 29 July 2022. free .
  20. Zheng . Yanxing . Wang . Jiwei . Tang . Xinqiao . Zhang . Lei . Meng . Fanbao . Liquid-crystalline behavior and ferroelectric property of viologen-based ionic liquid crystals . Journal of Molecular Liquids . 1 March 2020 . 301 . 112369 . 10.1016/j.molliq.2019.112369 . 212774258 . en . 0167-7322.
  21. Ouyang . Jianyong . Application of intrinsically conducting polymers in flexible electronics . SmartMat . September 2021 . 2 . 3 . 263–285 . 10.1002/smm2.1059 . 238642974 . en . 2688-819X. free .
  22. Zheng. Haidong. Ou. Jian Zhen. Michael Strano. Strano. Michael S.. Kaner. Richard B.. Mitchell. Arnan. Kalantar-zadeh. Kourosh. 2011-05-24. Nanostructured Tungsten Oxide – Properties, Synthesis, and Applications. Advanced Functional Materials. 21. 12. 2175–2196. 10.1002/adfm.201002477. 138637143 . 1616-301X. free.
  23. Lai. Wei Hao. Su. Yen Hsun. Teoh. Lay Gaik. Tsai. Yuan Tsung. Hon. Min Hsiung. 2007. Synthesis of Tungsten Oxide Particles by Chemical Deposition Method. Materials Transactions. 48. 6. 1575–1577. 10.2320/matertrans.mep2007057. 1345-9678. free.
  24. Supothina. Sitthisuntorn. Seeharaj. Panpailin. Yoriya. Sorachon. Sriyudthsak. Mana. August 2007. Synthesis of tungsten oxide nanoparticles by acid precipitation method. Ceramics International. 33. 6. 931–936. 10.1016/j.ceramint.2006.02.007. 0272-8842.
  25. News: Woodford . Chris. Chris Woodford (author) . How do electrochromic (smart glass) windows work? . 29 July 2022 . Explain that Stuff . April 23, 2021.
  26. News: Smart windows that protect against solar radiation can help reduce greenhouse gases . 29 July 2022 . Tech Xplore . September 1, 2021 . en.
  27. Glogic . Edis . Futsch . Romain . Thenot . Victor . Iglesias . Antoine . Joyard-Pitiot . Blandine . Depres . Gael . Rougier . Aline . Aline Rougier. Sonnemann . Guido . Development of Eco-Efficient Smart Electronics for Anticounterfeiting and Shock Detection Based on Printable Inks . ACS Sustainable Chemistry & Engineering . 6 September 2021 . 9 . 35 . 11691–11704 . 10.1021/acssuschemeng.1c02348 . 238705710 . en . 2168-0485.
  28. Islam . Shakirul M. . Hernandez . Tyler S. . Michael D. McGehee . McGehee . Michael D. . Barile . Christopher J. . Hybrid dynamic windows using reversible metal electrodeposition and ion insertion . Nature Energy . March 2019 . 4 . 3 . 223–229 . 10.1038/s41560-019-0332-3 . 2019NatEn...4..223I . 1688410 . 186288929 .
  29. Chen . Po-Wen . Chang . Chen-Te . Ko . Tien-Fu . Hsu . Sheng-Chuan . Li . Ke-Ding . Wu . Jin-Yu . Fast response of complementary electrochromic device based on WO3/NiO electrodes . Scientific Reports . 21 May 2020 . 10 . 1 . 8430 . 10.1038/s41598-020-65191-x . 32439890 . 7242463 . 2020NatSR..10.8430C . 29 July 2022 . en . 2045-2322.
  30. Web site: Introducing German Railways InterCity Express trains . The Man in Seat 61 . 29 July 2022.
  31. News: PPG will make electronically dimmed cabin windows for Boeing 787 . 16 December 2005 . Military+Aerospace Electronics . Huntsville, Alabama .

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