Oxygen Evolution Reaction on Zirconium Oxide Film in Alkaline Medium
Alkaline water electrolysis (AWE) is one of the hydrogen production methods without any carbon dioxide emission with renewable electric power supply. The AWE has several advantages compared with polymer electrolyte water electrolysis such as utilize of inexpensive materials . However, in the case of connection of the AWE to electric fluctuant power from renewable energy, the degradation of the electrodes is accelerated . For the practical use for AWE with renewable energies, a durable alternative material should be required. We focused on zirconium compounds as the alternative material because of their high activity for oxygen reduction reaction and high chemical stability .
In this study, the relationship between the catalytic activity for oxygen evolution reaction (OER) and preparing method of the zirconia film has been investigated as a fundamental study of the OER activity in alkaline solution.
A zirconium plate (Zr purity: 99.2%) was used as a starting material, and oxidized for 10 minutes in the range of temperature from 400 to 700oC in air to prepare ZrO2 / Zr electrode (ZrO2 / Zr_Temp. of oxidation). To prepare ZrO2 / Ti electrode, a Zr target was used in deposition method of an arc plasma gun (APD). ZrO2 was deposited on Ti rods for 6000 shots applied 100 V with a 360 μF capacitor at room temperature (ZrO2/ Ti_APD). After evacuation of the chamber, oxygen was introduced with 1 mL / min during deposition. Oxygen partial pressure in the chamber was 0.083 Pa. The frequency of the pulse was 3 Hz.
The ZrO2 electrode, a reversible hydrogen electrode (RHE), and a glassy carbon plate were used as a working, reference, and counter electrodes. All measurements were performed under N2 atmosphere in 0.1 M KOH at 30oC. Cyclic voltammetry (CV) was conducted for 100 cycles between 0 and 1.0 V vs. RHE with the scan rate of 100 mV s-1 as a pretreatment. The OER activity of the ZrO2 electrodes was evaluated by slow scan voltammetry (SSV) between 1.0 and 2.0 V vs. RHE with 5 mV s-1 of the scan rate. The onset potential (Eonset) defined by the potential at (geometric current density) in Tafel region was used as the index for OER activity.
Results and discussion
Figure 1 shows the polarization curves of the ZrO2 / Zr and ZrO2 / Ti electrodes. The ZrO2 / Zr_550oC had the highest geometric current density (igeo) in this study. The ZrO2 / Ti_APD had smaller current density than the ZrO2 / Zr_550 and 600oC.
Figure 2 shows the Tafel plot of the ZrO2 / Zr_550oC and the ZrO2 / Ti_APD. The Tafel slope of OER on the ZrO2 / Zr_550oC was 70 mV dec-1, and it was smaller than that of the ZrO2 / Ti_APD of 108 mV dec-1. However, Tafel region of the ZrO2 / Ti_APD was at almost same potential as that of the ZrO2 / Zr_550oC. The igeo of ZrO2 / Ti_APD was crossed that of the ZrO2 / Zr_550oC at about 1.7 V. This might be affected by contact resistance between ZrO2films and Ti substrates or oxide film resistance.
Figure 3 shows the Eonset of OER on the ZrO2 electrodes as a function of the oxidation temperature. The Eonset decreased with the increase of oxidation temperature in the range from 400oC to 550oC. Above 550oC, the Eonset was almost constant. Thus, the active site of OER on the ZrO2 / Zr would be formed above 550oC. The Eonset of OER on the ZrO2 / Ti_APD was the smallest potential of 1.59 V vs. RHE in the ZrO2electrodes in this study.
Though the ZrO2 / Ti had the lower igeo than the ZrO2 / Zr electrode in higher igeo region, the Eonset of OER on the ZrO2 / Ti_APD was the lowest in this study. Therefore, the ZrO2/ Ti_APD has potential for the high OER catalytic activity in alkaline solution and would have higher current density by improvement conductivity.
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