Effect of Specific Surface Area of Counter Electrode in Hybrid Capacitor Type Electrochromic Device

Abstract

We have already reported a novel hybrid capacitor multicolor electrochromism (EC) in a single device by introducing a porous capacitive counter electrode, ITO particle-modified electrode.1 Due to the formation of the electrical double layer on the counter electrode successfully compensated the charge consumption of EC reaction enables certain colors to be represented selectively both anodic and cathodic EC molecules by just using two electrodes and sandwiched electrolyte. In our another paper, we also found that the capacitance has a greater impact on the appearance of multiple colors.2 Beyond those, in practical EC applications, the performance of coloration, response time also play very important roles. In this regard, the effect of the increase of specific surface area of counter electrode on the EC properties of the hybrid capacitor ECD has not been studied in detail. In this paper, we fabricated several ECDs having hybrid capacitor structure with flat ITO electrode as the working side and ITO particle-modified electrodes which having different thickness (aim to increase the specific surface area) as the counter side. As the EC molecule, we employed the molecule of 10-methylphenothiazine (MPT). We discussed the effect of increased specific surface area of the counter electrode on the EC properties in the hybrid capacitor ECD. We measured the capacitance of the ITO particle-modified electrodes having different thickness. Fig. 1 shows the relationship between electrostatic capacitance and thickness of ITO particle-modified electrodes. From the result, we can clearly know that with the increase of thickness, the capacitance showed an increasing trend. Fig. 2 shows the transmission spectra of the ECDs under application of positive voltage of 1.2 V for 10 s. The transmittance of ECDs decreased strongly due to oxidized MPT molecule appeared at around 515 nm. As the result, color of ECDs changed from transparent state to pink color. At the same driving condition, with the increase of thickness, the amount of charge consumed by EC reaction increased, leading to more obvious ΔT. This result can be explained that with the larger capacitance due to increased specific surface area leading to larger accumulation of electrical charge on the counter electrode, oxidative reaction of the MPT was promoted on the working electrode, toward increased density of pink color.