Nano Focus: Electrolyte-free electrochromic device fabricated using graphene quantum dot-viologen nanocomposites
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Nano Focus Electrolyte-free electrochromic device fabricated using graphene quantum dot-viologen nanocomposites
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he uniqueness of electrochromic materials lies in their ability to undergo a reversible change in optical properties with applied voltage. These electro-optical properties can be used to fabricate novel, technologically advanced electrochromic devices (ECDs) ranging from e-paper to smart windows to display panels. Conventional ECDs require the use of an electrolyte to support electrochromic reactions. Now E. Hwang, H. (Hanleem) Lee, and their colleagues from Sungkyunkwan University, South Korea, have introduced an electrolyte-free ECD that functions using graphene quantum
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ion-selective membranes; this functions like a diode for ionic currents. The group then puts this material through the rigors of a typical four-diode full-wave rectifier and found that it performed well and was able to maintain an overall rectifying ionic current efficiency of 86%, with higher efficiencies of 95% during steady-state operation. To demonstrate its suitability as a drug delivery system, the researchers constructed a cation-selective channel inside the ionic four-diode bridge. They then used this channel to deliver a common neurotransmitter called acetylcholine (ACh) from a source to a target
electrolyte. They find that this system permits a nearly undisturbed delivery of ACh over an extended period of time without the production of adverse side reactions. While their device suffers from some voltage and frequency limitations, Berggren and colleagues have demonstrated that this type of four-diode bridge could be used to improve select types of electrokinetic devices. Particularly when compared to similar systems that utilize moving parts for ion transport, this simple approach paves the way for smaller ionic circuits with no moving parts, perfect for implantable devices. Ian J. McDonald
dot-viologen nanocomposites. They published their research in the August 13 issue of Advanced Materials (DOI: 10.1002/adma.201401201; p. 5129). According to the researchers, the use of electrolytes in an ECD system could lead to the unwanted decomposition of metal-ion containing electrochromes at high voltages. In order to combat the negative effect of electrolytes on device stability and performance, the researchers developed a flexible ECD where the electrochrome, methyl-viologen (MV2+) is combined with electrostatically strong, conductive graphene quantum dots (GQDs). There is strong adherence between the MV2+ (cation) and GQDs (anion) as a result of strong electrostatic and π–π interactions. The resultant ECDs demonstrate stable electrochromic performance without the use of an electrolyte.
Panel (a) of the figure shows an illustration of an electrolyte-free flexible electrochromic device with MV2+-GQDs. The researchers used a three-electrode electrochemical cell to demonstrate the electrochromic behavior of MV2+ in a GQD solution using cyclic voltammetry, as depicted in the inset in panel (b) of the figure. The color change of th
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