Oxygen Evolution Reaction of Ti and Mo Oxide-Based Electocatalysts in Acid Solution
Authors: Matsuzawa Koichi , Sumi Kyogo , Hirayama Soma , Kuroda Yoshiyuki , Mitsushima Shigenori , Ota Ken-ichiro , Ishihara Akimitsu 
Published: May 2020

On Oct. 25, 2019, Hydrogen energy ministerial meeting (H2 EM 2019) was held in Tokyo, Japan with cabinet members and officials from 35 countries, regions, and organizations. In the meeting, “Global Action Agenda” was released as the chairman’s summary, and many presentations were also given by international organizations. In the sector-integration workshop in this meeting, several presenters from industrial company of water electrolysis expected to improve its technologies for producing hydrogen from renewable energies. In our research center, the hydrogen producing from renewable energies is called as “Green Hydrogen” [1].
Polymer electrolyte water electrolysis (PEWE) is partly commercialization procedure to produce the hydrogen with high purity. Recently, it has been developed to be large scale for applying the Power-to-Gas (PtG) in Europe. However, the conventional anode uses an iridium oxide that is one of the precious metal oxide with poor resources. In order to utilize the PEWE system applying for PtG and "Hreen hydrogen" production in large scale with low cost, we focused on the titanium- and molybdenum-oxide based electrocatalyst (TiOx and MoOx) as alternative anode materials because of its low cost with abundant resources compared with precious metal oxide. In addition, it is reported that the overpotential of oxygen evolution reaction (OER) on molybdenum (Mo) doped TiOx (Mo-TiOx) was predicted to be similar to that of IrO2 from the density functional theory calculation [2-3]. In this study, we have investigated the catalytic activity of TiOx and MoOx with and without doping for the OER.
Ti rod was used as substarate. Ti and Mo oxide-based electrocatalyst was prepared by RF magnetron sputtering method. Ti or Mo disc was used as the sputtering target. In the case of fabrication for Mo-TiOx, a piece of Mo metal as 10% surface area for Ti metal was set on the Ti disc as doping material. Each partial pressure of Ar and O2 gas was adjusted to 0.23 Pa in fabrication of TiOx and Mo-TiOx. In the case of fabrication MoOx, partial pressure of O2 was fix at 0.10 Pa. The substrate heating temperature was constant at 373 K during sputtering. The output was set on 150 W, and sputtering time was constant for 20 minutes. 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 2.0 V vs. RHE in 1 M H2SO4 solution at 303 K.
Figure 1 shows the slow scan voltammograms of OER on Ti and Mo oxide-based electrocatalysts. The result of ZrO2 on conductive oxide (ZrO2/ConOx) [4] is also shown in this figure. The Mo-TiOx/Ti and MoOx/Ti in this study have obviously larger current densities than that of ZrO2/ConOx and TiOx/Ti. The Tafel slope of OER on Mo-TiOx/Ti was 91 mV dec-1 the while the that on the ZrO2/ConOx, MoOx/Ti and TiOx/Ti were larger than 180 mV dec-1. Because of the smallest Tafel slope and the largest current density on Mo-TiOx/Ti in this study, the Mo doping is effective for enhancing OER activities. According to Mott-Schottky plots, the Mo-TiOx/Ti seemed to have higher carrier concentration than that of TiOx/Ti while both materials showed as n-type semiconductors. Above all, the Mo-TiOx/Ti has higher activity for the OER than other materials in this study.
Acknowledgement: This work is partially supported by Toyota Mobility Foundation.
Reference
1. K. Ota, A. Ishihara, K. Matsuzawa, and S. Mitsushima, Electrochemistry, 78, 970 (2010).
2. M. Garca-Mota, A. Vojvodic, H. Metiu, I. C. Man, H-Y. Su, J. Rossmeisl, and J. K. Norskov, ChemCatChem, 3, 1607 (2011).
3. X. Huang, J. Wang, H. B. Tao, H. Tiana and H. Xu, Chem. Sci., 10, 3340 (2019).
4. K. Matsuzawa, K. Sumi, Y. Kuroda, S. Mitsushima, and A. Ishihara, Abst. 16th Annual Meeting of Soc. Nano Sci. Tech., #O-33, Tokyo, Jpn (2018) (in Japanese).

Figure 1