High Performance Bifunctional Catalysts Using RuCoOx Composite Oxide System for Alkaline Water Electrolysis
Authors: Zhang Zihao , Thakur Neha , Kumar Mukesh , Watanabe Toshiki , Yamamoto Kentaro , Matsunaga Toshiyuki , Kuroda Yoshiyuki , Mitsushima Shigenori , Uchimoto Yoshiharu 
Published: December 2024

In recent years, the importance of sustainable energy has rapidly escalated due to increasingly severe environmental concerns and the exhaustion of fossil fuels. Among these, renewable energy sources such as hydrogen, solar, and wind energy have garnered growing attention and research interest as crucial pathways to achieving zero carbon emissions. Hydrogen, being the most plentiful element in the universe, boasts rich resources and widespread sources. Hydrogen gas is also one of the highest energy density fuels and, upon combustion, produces only water, making it clean and non-polluting1. Among the methods for hydrogen production, water electrolysis stands out as the most efficient and cleanest approach, representing a promising direction for breakthroughs in energy technology. In the water electrolysis reaction, oxygen evolution reaction (OER) occurs at the anode while hydrogen evolution reaction (HER) takes place at the cathode. However, due to their involvement in multi-electron transfer processes, both reactions exhibit sluggish kinetics, requiring substantial overpotentials to drive them forward. This necessity has prompted the development of efficient electrocatalysts to expedite the reactions. Catalysts based on Ru and Ir are considered benchmark catalysts for water electrolysis due to their excellent OER activity and long-term stability2. Among them, Ru-based catalysts offer significant advantages in resource abundance and cost, with catalytic activity higher than Ir-based catalysts. Nonetheless, the poor stability and corrosion resistance hinder the development of this type of material.
Recent studies have indicated that introducing transition metal elements into conventional Ru-based oxide catalysts can alter the catalyst's electronic structure and bonding situations to some extent, potentially enhancing the catalytic activity and long-term stability of Ru-based water electrolysis catalysts3. Furthermore, Ru-based catalysts can exhibit excellent HER catalytic performance through appropriate modifications, making them potential bifunctional water electrolysis catalysts. In this study, we synthesized a series of RuCoOx catalysts using a simple glucose-blowing method at different temperatures and heating times (heating for 5 hours at 350 °C, heating for 5 hours at 500 °C, heating for 7 hours at 350 °C, and heating for 7 hours at 500 °C), and focused on investigating the catalytic performance of these catalysts for OER and HER (Figure 1.) in an alkaline environment (1.0 mol·L-1 KOH solution). The X-ray diffraction (XRD) patterns indicate the coexistence of RuO2 and Co3O4 in the catalyst samples describing the successful synthesis (Figure 2.). Linear sweep voltammetry (LSV) results demonstrated the relationship between the catalytic performance of the RuCoOx series catalysts and the heating temperature and duration during synthesis. The electrocatalytic performance of the RuCoOx series catalysts generally exhibited a negative correlation with the calcination time. Among them, the sample heated at 500 °C for 5 hours achieved overpotentials of 268 mV for OER and 92 mV for HER at a current density of 10 mA·cm-2. Additionally, we analyzed the changes in the electronic structure of the catalysts during the water electrolysis reaction using in situ or operando X-ray absorption spectroscopy (XAS) to track active structures for understanding catalytic mechanisms.
Figure 1.: (a) XRD patterns of RuCoOx composite under different conditions (b) LSV curves of OER for RuCoOx; (c) LSV curves of HER for RuCoOx.

Acknowledgment

This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References:

Ibrahim, A. Canan. Review and evaluation of hydrogen production methods for better sustainability. Int. J. Hydrogen Energy, 2015, 40.34: 11094-11111.

Kelsey A., et al. Orientation-dependent oxygen evolution on RuO2 without lattice exchange. ACS Energy Lett., 2017, 2.4: 876-881.

Deqiang, et al. High-density accessible RuOCo moieties with accelerated formation of activated oxygen species for freshwater and seawater electrolysis. Appl. Catal. B: Environ., 2024, 343: 123578.

Figure 1