(Digital Presentation) Mo Added Zr Oxide-Based Thin Film for Oxygen Evolution Catalyst in Alkaline Solution
Authors: Matsuzawa Koichi , Nozaka Atsushi , Ishihara Akimitsu 
Published: July 2022

On Oct. 4, 2021, Hydrogen energy ministerial meeting (H2 EM 2021) was held by the ministry of economy, trade and industry (METI) of Japanese government as online special event with cabinet members and officials from 29 countries, regions, and organizations. In the meeting, “Global Hydrogen Review 2021” and was released to propose the policy toward hydrogen-based society. In the open session, the water electrolysis was one of the main session, and participants shared their commitment to improve the performance of water electrolysis units and optimize the relevant systems with a view toward reducing the costs of “Green Hydrogen”, and exchanged views on topics including the future prospects for the “Green Hydrogen” market [1]. Here, “Green Hydrogen” is the hydrogen which is produced by the electricity from renewable energies introducing in our previous study [2].

Alkaline water electrolysis (AWE) has already commercialized all over the world and recently applied its system for the Power-to-Gas (PtG) in Europe and Japan such as Fukushima hydrogen energy research field (FH2R). In the FH2R, one of the largest module of AWE has operated from 2020. However, we found that the degradation of Ni anode occurs operating by variable renewable energy (VNE) from our previous report [3]. From this point of view, the alternative anode with high durability against VNE should be required for green hydrogen production. We focused on zirconium oxide-based electrocatalyst (ZrOx) and have studied its catalytic activity for oxygen evolution reaction (OER) [4]. In this study, we have investigated the activity and durability of ZrOx with and without Mo addition for the OER in alkaline solution.

Zr compound films were formed on Ti rods as a base material, using the R.F. magnetron sputtering. Gas atmosphere in the chamber was a mixture of Ar and O2 gas. After polishing and washing, the Ti rods were heated at 300 oC for Zr oxide-based film without any addition (ZrOx), and 350 oC for Zr oxide-based film with Mn addition (Mn-ZrOx). Sputtering power and sputtering time were fixed at 150 W and 20 min, respectively. All electrochemical measurements were carried out using a three-electrode cell at 30 oC in 7 M KOH. A reversible hydrogen electrode (RHE) and a carbon plate were used as a reference and a counter electrode, respectively. ZrOx or Mo-ZrOx was used as working electrode. In order to evaluate the catalytic activity for the OER, slow scan voltammetry (SSV) was performed under N2 atmosphere. Current density (i
geo) was based on the geometric surface area of the working electrode. The resistance of the film (R
film) and charge transfer (R
ct) was evaluated by AC impedance spectroscopic measurements in the frequency range from 105 to 10−1 Hz.

Figure 1 shows polarization curves of ZrOx and Mo-ZrOx. The i
geo for the OER on Mo-ZrOx has obviously larger than that of ZrOx. According to the cyclic voltammograms (CV) of both sample, the electric double layer (C
dl) of Mo-ZrOx from CV was larger than that of ZrOx. This trend was similar to the result reported in previous study [5], and it is suggested that the C
dl becomes larger by the effect of Mo addition. From the results of electrochemical impedance spectroscopy (EIS), both ZrOx and Mo-ZrOx are n-type semiconductor. Moreover, the slope of Mott-Schottky plot on Mo-ZrOx was more gentle than that on ZrOx and it is suggested that the electric conductivity of Mo-ZrOx was larger than that of ZrOx. Thus, the reason why the i
geo for the OER on Mo-ZrOx has larger than that of ZrOx is that both improvement of the increase of C
dl and the electric conductivity of sample.

Acknowledgement: This work is partially supported by Iketani Science and Technology Foundation.

Reference

[1] https://www.meti.go.jp/english/press/2021/1008_001.html

[2] K. Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010).

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

[4] K. Matsuzawa, A. Ishihara, A. Ohshi, S. Mitsushima, and K. Ota, Mater. Sci. Eng. B, 267, 115112 (2020).

[5] Y. Takasu, T. Ohnuma, W. Sugimoto, and Y. Murakami, Electrochemistry, 67, 1187 (1999).

Figure 1