Analysis of Open Circuit Voltage Decay in Titanium Dioxide Electrode
Authors: Nay Wunn Htoo , Sakamoto Yutaro , Hashidaka Isamu , Motoda Shinichi , Morita Motoaki 
Published: December 2020

Titanium Dioxide (TiO2) has been studied for its important photocatalytic semiconductor characteristics. Among many applicable areas of TiO2 photocatalysts, we have been studying to use as an anode electrode of the marine wet solar cells or marine microbial fuel cell. These studies include the analysis of TiO2 electrode prepared by the screen-printing method, and also the dye sensitizing effect on TiO2 electrode. In those studies, it was noticed that there is an open circuit potential that shifted to noble direction after TiO2 electrode was irradiated by the light source. This phenomenon usually occurred after a few minutes the electrode showed its photocatalytic characteristics. It is unclear which caused voltage loss during the measurement. In this study, we have been trying to understand the mechanism affecting the voltage decay of the TiO2 electrode and the factors influencing this.

The TiO2 electrodes were prepared by screen printing method. Type 329J4L stainless steel which exhibits duel phase properties was used as a base substrate for the electrode. Firstly, the surface of the substrate was polished by sandpaper and the substrate was cleaned with acetone using the ultrasonic cleaner for 10 minutes. Then, the substrate was passivated with 10% nitric acid (HNO3) solution at 60 ºC for 30 minutes. After that, the substrate was screen printed with TiO2 paste followed by heat treating for 60 minutes and the first layer of TiO2 was formed. For the second layer forming, the substrate was again screen printed with TiO2 paste and heat treated for 30 minutes. With these processes, double-layered heterojunction TiO2 electrode was prepared. The electrode was then coated with insulating epoxy resins and prepared ready for the electrochemical measurements and analysis.

The electrochemical measurements of TiO2 electrodes were performed in an artificial seawater used as an electrolyte. The irradiation was carried out on the electrodes using a calibrated xenon lamp with 150 W (wavelength ranging from 250 nm to 800 nm). The lamp was set to produce an output light intensity of 10.5 mW/cm2. The saturated calomel electrode (SCE) was used as the reference electrode. During the photo potential measurement, the potentials dropped as the electrodes were exposed to the light irradiation. But the potential values were ennobled after a few minutes the electrode was irradiated. Preliminary examinations showed that there is a relation between the ennobled potential values (open circuit voltage loss when coupled with a counter electrode) and the heat treatment temperatures of the electrodes. For example, the rebound rate of the photo-potentials of the electrode with heat treatment temperature of 150 ºC (second layer) was higher and longer compared to that of the electrode with 550 ºC (second layer). Detailed mechanism was yet under investigation. Those investigation includes the photopotential measurement and the electrochemical impedance spectroscopy (EIS). The surfaces of the electrodes before and after the measurements were compared and checked using scanning electron microscope (SEM). The X-ray powder diffraction (XRD) analysis and differential thermal analysis (DTA) were also included for analysis of the mechanism on the TiO2 electrode. The results of those analyses and the mechanism affecting the open circuit voltage degradation in the TiO2 electrode will be presented to the upcoming ECS conference.