Influence of a Tiny Quantity of Ir in Ruir Binary Oxide for Oxygen Evolution Reaction Using the Model Electrode Method
Authors: Negishi Mayu , Banno Akichika , Maruyama Takahiro , Saida Takahiro 
Published: December 2024

Polymer electrolyte membrane water electrolysis (PEMWE) is one method for mass hydrogen production. However, the oxygen evolution reaction (OER) of the PEMWE anode has a high overpotential, which is the main factor in increasing electricity requirements. Therefore, it is desired to decrease the overpotential of OER.

RuO2 is known as the most active OER catalyst, but its stability in the high-potential regions is poor. For this reason, IrO2, with relatively high stability, is generally used as the anode catalyst despite its OER activity being lower than that of RuO2. The most significant problem with the IrO2 catalyst is its cost, which is attributed to the rarest element. There are many reports about the Ir-Ru binary oxide catalyst proposed to improve stability in RuO2 and increase OER activity in IrO2. Their Ir/Ru ratio is generally about one, and the Ir content should be reduced as much as possible to suppress the cost.

Recently, it was reported that the combination of only 6% of Ir with a Ru catalyst demonstrated high OER activity and stability [1]. The other research groups haven’t followed the low rate of Ir in the Ir-Ru binary catalyst, although it is very advantageous in terms of commercialization. Therefore, it is hard to judge the true effect of a tiny quantity of Ir to OER activity and stability in Ru-based catalysts.

In the practical catalyst, many factors, such as the diffusivity of the O2 bubble, proton and electron conductivities, and so on, influence the OER activity. Herein, we employed the model electrode method to investigate the essential effect of a small number of Ir for the OER activity of the Ru-based catalyst. The model electrode doesn’t have to consider proton and electron conductivity and O2 bubble diffusivity. Therefore, it can focus on the difference in electron structure, OER activity, and stability in the model electrode with several Ir content rates.

In this study, the additive effect of a tiny quantity of Ir on the Ru-based model electrodes on OER activity and stability was investigated.

The model electrode was composed of a glassy carbon substrate, the Ru nanosheet stacking layer of about 1 – 3 nm, and a few nanometer-sized Ir nanoparticles at the top layer. The amount of the Ir on a model electrode was controlled by a number of pulses in the arc plasma deposition method. The electrochemical measurements were conducted by a three-electrode cell, which is composed of a working electrode, a carbon counter electrode, a reversible hydrogen electrode, and 0.1 M HClO4 (room temp.). All of the model electrodes were subjected to electrooxidation by potential sweeping before their OER activity and stability were evaluated. Furthermore, before and after the electrochemical measurements, the electron structure of the model electrodes was confirmed by XPS analysis.

All model electrodes showed redox peaks at around 0.6 V and 1.0 V, which are attributed to RuO2 and IrO2, respectively. Adding a tiny quantity of Ir seemed to improve the OER activity. In contrast, the stability of RuO2 depended on the amount of Ir. Surprisingly, the dissolution rate of Ru increased with the increase of Ir contents in the model electrode. These results suggested the possibility that a few amounts of Ir contents in RuO2 could improve the OER activity but have a negative effect on its stability.

[1] D. Wu, et al.Nature Communication,12, (2021) 1145-9.