Oxygen Evolution Reaction of Mo Oxide-Based Electocatalysts Doped with Different Metals in Acid Solution
Authors: Matsuzawa Koichi , Hirayama Soma , Sumi Kyogo , Ishihara Akimitsu 
Published: December 2020

At end of February in 2020, Fukushima Hydrogen Energy Research Field (FH2R) has started to operate with a renewable energy-powered 10MW-class hydrogen production unit, which is the largest-class in the world. These kinds of “Green Hydrogen” [1] production technologies will be accelerated not only in Japan but also all over the world because “Global Action Agenda” was released and many presentations were also given by international organizations in Hydrogen energy ministerial meeting (H2 EM 2019) which was held in Tokyo, last year with cabinet members and officials from 35 countries, regions, and organizations.
In FH2R, the newest system of water electrolysis has installed. In Japan, not only alkaline water electrolysis (AWE) but also polymer electrolyte water electrolysis (PEWE) have been developed. Compared with AWE, PEWE is partly commercialized procedure to produce the hydrogen with high purity. However, one of the issue of PEWE is iridium oxide (IrO2) anode that is one of the precious metal oxide with poor resources. In order to utilize the PEWE system applying for green hydrogen production in large scale with low cost, we focused on the molybdenum-oxide based electrocatalyst (MoOx) as alternative anode materials because of its low cost compared with precious metal oxide [2-4]. In this study, we have investigated the catalytic activity of oxygen evolution reaction (OER) on MoOx with and without doping.
Ti rod was used as substarate. Mo oxide-based electrocatalyst was prepared by RF magnetron sputtering method. Mo disc was used as the sputtering target. In the case of fabrication for Zr- and Ta-MoOx, Zr or Ta piece for 10% surface area was set on the Mo disc as doping material. Each partial pressure of Ar and O2 gas was adjusted to 0.23 Pa in fabrication of MoOx, Zr-MoOx and Ta-TiOx. 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 at 303 K. We also carried out electrochemical impedance spectroscopy (EIS) to obtain resistance components and semiconducting properties.
Figure 1 shows the slow scan voltammograms of OER on Zr-MoOx, Ta-MoOx, and MoOx. The Zr-MoOx in this study have larger current densities than that of Ta-MoOx and MoOx. It was found that that Zr doping was effective for enhancing the OER activity of MoOx. According to Mott-Schottky plots, all samples whether doping or non-doping showed n-type semiconductor while flat band potential of Zr-MoOx was smaller than that of Ta-MoOx and MoOx.
Acknowledgement: This work is partially supported by Toyota Mobility Foundation.
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Figure 1