The 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 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.

(a) Illustration of an electrolyte-free flexible electrochromic device of MV²⁺ graphene quantum dots (GQDs). (b) Cyclic voltammogram of 5 mM MV²⁺ at an indium-tin-oxide electrode in an aqueous solution containing 8 mg mL^–1 GQD (blue line) and 0.1 M KCl (red line) at a scan rate of 100 mVs^–1. Inset: A depiction of a three-electrode cell composed of a working electrode (W), a counter electrode (C), and a reference electrode (R). Reproduced with permission.

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 the electrochrome from colorless (MV²⁺) to purple (MV⁺) is represented as two redox peaks in the cyclic voltammogram trace of MV²⁺-GQD (blue line), where the voltage is swept between –1.8 V and 0 V. The researchers also compared the electrochromic behavior of MV²⁺ in GQDs to MV²⁺ in a KCl electrolyte [red plot in the figure (b)]; a comparison of the corresponding cyclic voltammogram traces showed an exact match in the peaks for MV²⁺ in GQDs and the peaks for MV²⁺ in the KCl electrolyte.

The research team concluded that GQDs are stable enough to perform electrolyte-like charge transfer in solution and that they act as an electron transfer medium to facilitate oxidation or reduction of organic species. The researchers also extended their experiments to demonstrate the thermal and mechanical stability of GQDs. The results provide useful guidelines for the fabrication of stable, durable, and flexible electrolyte-free ECDs in the future.