Oxygen Evolution Reaction on Zirconium Oxide Film in Alkaline Medium

Introduction

     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 [1]. However, in the case of connection of the AWE to electric fluctuant power from renewable energy, the degradation of the electrodes is accelerated [2]. 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 [3].

     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.

Experimental

     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 700^oC 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 30^oC. 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 (E_onset) 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_550^oC had the highest geometric current density (i­_geo) in this study. The ZrO₂ / Ti_APD had smaller current density than the ZrO₂ / Zr_550 and 600^oC.

     Figure 2 shows the Tafel plot of the ZrO₂ / Zr_550^oC and the ZrO₂ / Ti_APD. The Tafel slope of OER on the ZrO₂ / Zr_550^oC 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_550^oC. The i_geo of ZrO₂ / Ti_APD was crossed that of the ZrO₂ / Zr_550^oC 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 E_onset of OER on the ZrO₂ electrodes as a function of the oxidation temperature. The E_onset decreased with the increase of oxidation temperature in the range from 400^oC to 550^oC. Above 550^oC, the E_onset was almost constant. Thus, the active site of OER on the ZrO₂ / Zr would be formed above 550^oC. The E_onset 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 i_geo than the ZrO₂ / Zr electrode in higher i_geo region, the E_onset 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.

Reference

[1].  S. Mitsushima and K. Matsuzawa, J. Hydr. Ener. Sys. Soc. Jpn., 36, 11 (2011) (in Japanese).

[2].  H. Ichikawa, K. Matsuzawa, Y. Kohno, I. Nagashima, Y. Sunada, Y. Nishiki, A. Manabe, and S. Mitsushima, ECS Trans., 58(33), 9 (2014).

[3].  A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).