The Impact of Mn addition on Ta Oxide-Based Electrocatalysts for Oxygen Evolution Reaction in Acid

Abstract

Hydrogen energy ministerial meeting (H2 EM 2020) was held on Oct. 14, 2020 by the ministry of economy, trade and industry (METI) of Japanese government as online special event with cabinet members and officials from 23 countries, regions, and organizations. In the open session, presenter from industrial company of water electrolysis introduced to improve its technologies for producing hydrogen from renewable energies [1]. In our previous study, the hydrogen producing from renewable energies is called as “Green Hydrogen” [2].Polymer electrolyte water electrolysis (PEWE) 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. Moreover, it is reported that the IrO₂ is afraid to cause the degradation operating by variable renewable energy (VRE) because the catalytic activity of oxygen evolution reaction became lower by potential cycling of VRE simulation [3]. From this point of view, the non-precious metal oxide anode with high durability against VRE should be required for green hydrogen production. We focused on tatanium oxide-based electrocatalyst (TaOx) and have studied its catalytic activity for oxygen evolution reaction (OER) [4]. In this study, we have investigated the catalytic activity of TaOx with and Mn addition for the OER in acidic solution.Ti rod was used as substarate. TaOx was prepared by RF magnetron sputtering method. In the case of fabrication for Mn-TaOx, Ta metal piece for 60% surface area was set on the Mn disc as additional material. Each partial pressure of Ar and O2 gas was adjusted to 0.23 Pa in fabrication of Mn-TaOx. 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 order to evaluate the OER activity of samples, the slow scan voltammetry was performed from 1.2 to 2.5 V vs. RHE in 1 M H2SO4 solution with saturated nitrogen atmosphere at 303 K. We also carried out electrochemical impedance spectroscopy (EIS) to obtain resistance components and semiconducting properties.Figure 1 shows the polarization curve of OER on Ta oxide-based electrocatalysts with Mn addition. The results of Ti oxide-based electrocatalysts with Mo addition (Mo-TiOx) that is our past study [4] is also shown in this figure. The current density was based on geometric surface area. According to figure, the Mn-TaOx has obviously higher OER current than that on Mo-TiOx, and the Tafel slope of OER on Mn-TaOx was 105 mV dec-1 the while the that on the Mo-TiOx was 91 mV dec-1. The peak over 1.5 V was detected from polarization curve of Mn-TaOx in Fig. 1. This peak corresponded to Mn ion, and it suggests that Mn species affected on the OER activity while the amounts of Mn on the sample after electrochemical measurement showed below detection limit from XPS analysis. According to these results, the surface of Mn-TaOx was consisted of Ta oxide mainly with very few amount of Mn after electrochemical measurements. From the results of EIS, resistances of film and charge transfer on Mn-TaOx were lower than that of Mo-TiOx. In addition, the slope of Mott-Schottky plots on the Mn-TaOx was much gentle than that on the Mo-TiOx, and it means that the carrier concentration in Mn-TaOx was higher than that in Mo-TiOx. Therefore, the reason why the Mn-TaOx showed higher OER current than the Mo-TiOx is that both factors of electrical conductivity and catalytic activity were improved.Acknowledgement: This work is partially supported by Toyota Mobility Foundation and ENEOS Tonen General Research / Development Encouragement & Scholarship Foundation.Reference https://www.meti.go.jp/english/press/2020/1015_001.htmlK. Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010).S. M. Alia, and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).K. Matsuzawa, S. Hirayama, K. Sumi, A. Ishihara, Abst. PRiME2020, I01F-2492, Online (2020). Figure 1