Activity and Durability of ALD-ZrO2 Coated IrO2 Film in Sulfuric Acid

Abstract

The technology of power-to-gas (P2G) is drawing attention in and outside Japan for its ability to store renewable energy as hydrogen [1]. For the aspect of P2G, the water electrolyzer plays important role to produce “Green Hydrogen” [2]. On Sep. 26, 2022, Hydrogen energy ministerial meeting was held as online special event. In the open session, the water electrolysis is one of the main topics of the session, and due to the demand for “Green Hydrogen”, installed capacity of water electrolyzer has been required for the huge size of its module [3].In Japan, proton exchange membrane water electrolysis (PEMWE) with small size produced by Kobelco Eco-Solutions Corp have released already installed over 200 units for 25 years. Recently, the PEMWE with MW-class as installed in Yamanashi Hydrogen Company has also applied for the demonstration project [3]. However, the conventional electrocatalysts for PEMWE uses precious metal materials, and especially, the iridium oxide (IrO2) is used as an anode. As the iridium is by-product of platinum, its resources is very poor. Moreover, it is reported that the low loading of IrO₂ easily causes the degradation under dynamic operation condition simulated variable renewable energies (VRE) [4]. From this point of view, the durability of anode against VRE should be required for “Green Hydrogen” production. We focused on ZrO2 which has high stability in acidic solution [5]. Moreover, we found that an oxide-based film which is formed by Atomic Layer Deposition (ALD) showed bi-functional effects on the electrocatalyst for the oxygen reduction reaction; it can reflect the electrochemical properties of the substrate and it can also protect the consumption of the substrate by potential scan. In order to apply for PEMWE, we have investigated the catalytic activity of IrO2 electrodes deposited with ZrO2 using ALD for the oxygen evolution reaction (OER).IrO2 film on Ti rod as substrate was fabricated by thermal decomposition under air atmosphere at 400°C for 10 min. The loading amount of IrO2 was 2.35 mg cm-2 at constant. ZrO2 was deposited on IrO2 film by using ALD at 225°C, the film thickness of ZrO2 was varied in the range of 2 to 10 nm that calculated from the number of ALD cycles. After coating, the heat treatment was performed under air atmosphere at 400oC for 10 min. We used conventional three electrode cell with each sample as working electrode while the reversible hydrogen electrode (RHE) and carbon plate were used as reference and counter electrode to demonstrate the electrochemical measurement in 1 M H2SO4 at 303 K. After the pretreatment, the cyclic voltammetry was demonstrated in the range of 0.05 to 1.2 V. In order to evaluate the OER activity of samples, the slow scan voltammetry (SSV) was performed from 1.2 to 2.0 V vs. RHE.Figure 1 shows the polarization curves of OER on the ALD-ZrO2 of 5 nm coated on IrO2 with and without heat treatment (5nm, 5nm(HT)). The result of IrO2 with and without heat treatment (0nm, 0nm(HT)) are also shown in the Fig. 1. These results are IR-free data. The vertical axis is based on geometric current density (i geo). In the case of IrO2, the i geo of 0nm was larger than 0nm(HT). On the other hand, the i geo of 5nm(HT) was obviously larger than that 5nm in the case of ALD-ZrO2 of 5 nm coated on IrO2, and the i geo of 5nm(HT) at 1.55 V was almost same as that of 0nm(HT). The redox peak of Ir(IV)/Ir(V) or Ir(IV)/Ir(VI) was observed in the cyclic voltammogram (CV) of 5nm(HT) around 1.2 V. This peak also obtained in the CVs of both 0nm and 0nm(HT). However, it is not clearly shown in the CV of 5nm. From the results of SEM-EDX, the coating of ZrO2 of 5nm(HT) was remained after electrochemical measurement while the that of ZrO2 of 5nm was removed after electrochemical measurement. Therefore, the heat treatment after ALD coating was effective for enhancing OER activity.Acknowledgement: This work is partially supported by the Suzuki Foundation and JFE 21st Century Foundation.Reference Fuel Cell RD & D in Japan 2022, Fuel Cell Development Information Center (FCDIC), pp.8. Fuel Cell Development Information Center (FCDIC), Tolyo (2023).K. Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010).https://www.meti.go.jp/english/press/2020/1015_001.html M. Alia, and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010). Figure 1