Fabrication and Characterization of Electrochromic Devices Utilizing CVD Graphene
Authors: Fukuzawa Miki , Watanabe Takeshi , Kuromatsu Sho , Koh Shinji 
Published: December 2024

Electrochromism is a function whereby the optical properties of materials change reversibly and persistently through electrochemical redox reactions driven by small electrical energy. Electrochromic devices are used in a wide range of applications including smart windows for buildings and aircraft, displays, electronic paper, and so on. The advantages of electrochromic devices include the ability to be manufactured at low cost and compatibility with non-planar and flexible surfaces. Electrochromic devices typically have a multilayer structure comprising transparent conductive electrodes, electrochromic film, ion-conducting layer, and ion-storage film. Since electrochromic devices in their uncolored state need to be transparent, the fabrication of flexible electrochromic devices requires flexible transparent conductive films as well as flexible and transparent ion-conducting layers.

In this study, we aim to fabricate flexible film-type electrochromic devices using graphene films as transparent conductive electrodes. Fig. 1 illustrates the schematic images of the fabricated electrochromic devices. The electrochromic devices consist of two graphene films grown using chemical vapor deposition. Prussian blue (PB), the electrochromic material, and silver nanoparticles, which act as the ion-storage material, were electrodeposited onto each graphene film, respectively. A PVDF (polyvinylidene fluoride) porous membrane, soaked in a KCl solution as an electrolyte, was sandwiched between the two graphene electrodes to serve as a separator and ion-conducting layer. The PVDF membrane exhibits a white color due to its porous structure. Therefore, by adjusting the refractive index of the KCl solution to match that of PVDF, a transparent and flexible ion-conducting layer was formed.

The graphene films were grown on copper foils by the CVD method and were then transferred onto a PET substrate using thermal press bonding with PMMA (polymethyl methacrylate) as the adhesive layer. Subsequently, copper was etched using an ammonium persulfate aqueous solution. A gold contact electrode was formed on the edge of the graphene surface by vacuum evaporation. PB was electrodeposited on one of the graphene electrode surfaces by applying a constant current of −20 μA/cm2 for 5 min in an electrolyte solution containing K3[Fe(CN)6] (5 mM), FeCl3 (5 mM), KCl (0.1 M), and HCl (0.1 M). Silver was electrodeposited on other graphene electrode surfaces by applying a constant current of −100 μA/cm2 for 5 min in an electrolyte solution containing NaNO3 (0.1 M) and AgNO3 (1 mM). The mixture of water (nd=1.333) and glycerol (nd=1.473) solution containing 0.1 M KCl was utilized to make PVDF films transparent by matching the refractive index of the electrolyte solution with that of the PVDF. By preparing the electrolyte with a water-to-glycerol ratio of 4:6, resulting in a refractive index of 1.42 for PVDF, the transmittance of PVDF immersed in this electrolyte reached almost 100%. Using this electrolyte, the EC characteristics of the film-type EC device were evaluated by Cyclic voltammetry (CV). CV curves obtained using PB/graphene as a working electrode vs Ag/graphene as a counter electrode are presented in Fig. 2, showing oxidation peaks at 0.5 V and reduction peaks at −0.2 V. It was confirmed that Prussian blue undergoes decolorization during reduction and coloration during oxidation, enabling reversible color changes between blue and transparent. Therefore, this demonstrates that the transparent and flexible PVDF layer functions as the ion-conductive layer even in the presence of 60% glycerol in the electrolyte and that the conductivity of graphene films is maintained even with a stacking structure, indicating their potential as flexible light-dimming devices.

These results reveal that it was possible to fabricate flexible electrochromic devices using graphene. We believe that this work will promote further efforts to develop new multifunctional electrochromic devices that can be attached to existing glass materials such as window glass and eyeglasses to add a dimming function afterward.

Figure 1