Oxygen Evolution Reaction on Mo Oxide-Based Binary Electrocatalysts in Acidic Solution

Matsuzawa, Koichi

Hydrogen energy ministerial meeting (H₂ EM 2021) was held on Oct. 4, 2021 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 industrial session, 4 sessions were held, and the water electrolysis was one of the themes of industrial session. Panelists from several countries introduced state-of-art technologies of water electrolyzer [1]. In our previous study, the hydrogen producing from renewable energies is called as “Green Hydrogen” [2].

Proton exchange membrane water electrolysis (PEMWE) has partly commercialized and recently applied its system for the Power-to-Gas (PtG). However, conventional anode is precious metal oxide such as IrO₂ and the cost of raw material is expensive. From this point of view, the non-precious metal oxide anode should be required for green hydrogen production. We focused on tantalum and molybdenum oxide-based electrocatalyst (TaOx and MoOx) and have studied its catalytic activity for oxygen evolution reaction (OER) [3]. Moreover, TaOx with Mn addition showed much higher the OER activity than TaOx [4]. In this study, we have applied for MoOx with Mn addition for investigating the catalytic activity for the OER in acidic solution.

Ti rod was used as substarate. MoOx was prepared by RF magnetron sputtering method. In the case of fabrication for Mn-MoOx, Mo metal piece for 40% surface area was set on the Mn disc as additional material. Each partial pressure of Ar and O₂ gas was adjusted to 0.15 Pa in fabrication of Mn-MoOx. The substrate heating temperature was constant at 673 K during sputtering. 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 solution with saturated nitrogen atmosphere at 303 K. Several samples were demonstrated pretreatment from 0.05 to 1.2 V vs. RHE before the measurement of OER. The slow scan voltammetry was performed from 1.2 to 2.0 V to evaluate the OER activity. We also carried out electrochemical impedance spectroscopy (EIS) to obtain resistance components and semiconducting properties. In addition, samples before and after electrochemical measurement were analyzed by SEM-EDX and XPS.

Figure 1 shows the Tafel plots of OER on Mo oxide-based electrocatalysts with and without Mn addition. The current density was based on geometric surface area. In the case of sample performing pretreatment, the Mn-MoOx with pretreatment (Mn-MoOx(Pre)) has obviously higher OER current than MoOx with pretreatment (MoOx(Pre)). Moreover, Mn-MoOx has higher OER current than Mn-MoOx(Pre), and the Tafel slope of OER on Mn-TaOx was 79 mV dec^-1 the while the that on the Mn-MoOx(Pre) was 147 mV dec^-1. From the results of EIS, the charge transfer resistance of Mn-MoOx was much smaller than that of Mn-MoOx(Pre) while the film resistance of was similar to that of Mn-MoOx(Pre). The results from SEM-EDX revealed that Mn amount in Mn-MoOx after the electrochemical measurement was constant compared to that before electrochemical measurement while Mn amount in Mn-MoOx(Pre) after electrochemical measurement was decreased compared to that before electrochemical measurement. From results from XPS analysis, Mn ratio in the Mn-MoOx after electrochemical measurement was constant compared to that before electrochemical measurement while Mn amount in Mn-MoOx(Pre) after electrochemical measurement was decreased compared to that before electrochemical measurement. Therefore, Mn species still remained on both surface and bulk in the case of Mn-MoOx while it was decreased in the case of Mn-MoOx(Pre). In Fig. 1, The peak over 1.5 V was detected from Tafel plots of both Mn-MoOx and Mn-MoOx(Pre). This peak corresponded to redox peak of Mn(III)/Mn(IV) [5], and it implied that Mn species affected on the OER activity. However, the electric charge of Mn(III)/Mn(IV) of Mn-MoOx was smaller than that of Mn-MnOx(Pre), and it is suggested that not only Mn oxide but also other species such as Mn-Mo compound oxide reacts as an active site of OER on Mn-MoOx.

Acknowledgement: This work is partially supported by the Suzuki Foundation.

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