Oxygen Evolution Activity on Mo Added Zr Oxide Catalyst in Alkaline Solution
Authors: Matsuzawa Koichi , Nozaka Atsushi , Hirose Kazuya , Ishihara Akimitsu 
Published: September 2023

On Sep. 26, 2022, Hydrogen energy ministerial meeting (H2 EM 2022) was held by the ministry of economy, trade and industry (METI) of Japanese government as online special event with cabinet members and officials from 30 countries, regions, and organizations. In the chair’s summary of meeting, an additional goal also proposed the amount of renewable and low-carbon hydrogen to be produced by 2030 of at least 90Mt H2. In the open session, the water electrolysis was one of the main topics of the session, and due to the demand for “Green Hydrogen” as already described in chairman’s summary, installed capacity of water electrolyzer has been required for the huge size of its module [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 and recently applied its system for the Power-to-Gas (PtG) all over the world. In the Fukushima hydrogen energy research field (FH2R) in Japan, one of the largest module of AWE named “Aqualyzer” has operated from 2020. However, previous literature reported that the degradation of Ni anode occurs operating by variable renewable energy (VNE) [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 the catalytic activity of its thin film fabricated by sputtering procedure for oxygen evolution reaction (OER) [4-5]. In this study, we have investigated the OER activity of Mo added ZrO2 (Mo-ZrO2) fabricated by atomic layer deposition (ALD) and arc plasma deposition (APD) in alkaline solution.
Mo-ZrOx was formed on Ti rods as a base material, using by ALD and APD. In the step of ALD, Tetrakis(dimethylamino)zirconium (TDMAZr) and water were used as precursor and oxidant of ZrO2, and the substrate was heated at 225 oC. The thickness of ZrO2 film was 10 nm for preparation. Then, the MoOx was fabricated by APD using Mo metal as target material at 0.74 Pa of partial pressure of oxygen and the substrate was heated at 300oC. At last, the annealing was performed under Ar atmosphere at 300 oC to produce the Mo-ZrO2 catalyst. In the case of inserting interlayer, TaOx was formed by the R.F. magnetron sputtering before ALD step. 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. Mo-ZrO2 catalyst on Ti rod was used as working electrode. In order to evaluate the catalytic activity for the OER, slow scan voltammetry (SSV) was performed under N2 atmosphere. The 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 Tafel plots of OER on Mo-ZrO2 catalysts with and without interlayer of TaOx (Mo-ZrO2/TaOx or Mo-ZrO2). Compared with previous result, Tafel plot of OER on Mo-ZrOx fabricated by sputtering (Mo-ZrOx_SP) was also showed in Fig. 1. The i
geo at 1.8 V vs RHE on the Mo-ZrO2/TaOx was larger than that on the Mo-ZrO2, and was also similar to that on the Mo-ZrOx_SP. The Tafel slope of Mo-ZrO2/TaOx was 95 mV dec-1, and it was the smallest compared to other catalysts such as Mo-ZrO2 and Mo-ZrOx_SP. From the results of electrochemical impedance spectroscopy (EIS), the R
film and R
ct of Mo-ZrO2 was obviously larger than that of Mo-ZrO2/TaOx and Mo-ZrOx_SP. According to XPS result, ZrO2 around surface of both Mo-ZrO2/TaOx and Mo-ZrO2 were still remained after electrochemical measurement. Above all, the insert of interlayer of TaOx between Mo-ZrO2 and Ti rod (Mo-ZrO2/TaOx) was effective for activity and stability for OER, and it also has similar OER activity to Mo-ZrOx_SP.
Acknowledgement: This work is partially supported by Hitachi Metals-Materials Science Foundation, Yashima Environment Technology Foundation, and Amano Institute of Technology.
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Figure 1