Enhancing Electrochemical Seawater Oxidation with Noble Metal-Decorated Co3O4 Nanocubes
Authors: Park Joon Yong , Park Gisang , Kim MinJeong , Nam Ki Min 
Published: December 2024

The pressing global challenge of overwhelming reliance on nonrenewable fossil fuels demands innovative solutions to decarbonize energy and industrial parts. Green hydrogen is produced through water electrolysis using renewable energy sources such as solar and wind and increasingly recognized as a promising alternative. This method leads to significant technological advancements and cost reductions in renewable electricity.

Given that seawater accounts for 96.5% of Earth’s total water reserves, it presents a nearly inexhaustible resource for hydrogen production. However, the seawater electrolysis suffers from some challenges due to the complexity of seawater including various impurities that lead to corrosive and toxic byproducts. The experimental focus of seawater electrolysis encompasses the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. Additionally, the anodic reaction is accompanied by competing with the chlorine evolution reaction (CER). The CER involving a two-electron reaction demonstrated faster kinetics than the OER (four-electron reaction) along with a decrease in the potential thermodynamic difference in acidic and neutral conditions. Though some research focuses on enhancing OER selectivity over CER, certain industries may prefer chlorine (Cl2) generation for wastewater treatment purposes.

Noble metals for seawater electrolysis have been used with observed stability throughout overall pH values. However, they are a scarce element and quite expensive, so that lead to problems for industrial scale. Therefore, many transition metal-based catalysts have been introduced to reduce contents of noble metals for a long maintenance. Among these, cobalt is cost-effectiveness and showed better synergistic behavior on coupling with precious metals. In this regard, we synthesized spinel type of cobalt oxide, and then performed a simple deposition method with Ir and/or Pd precursor solution.

The Co3O4 nanocubes (denoted as Co3O4NCs) was synthesized by hydrothermal reaction and they exhibited the size 145 ± 30 nm. The photodeposition of noble metals was subsequently conducted for enhancing catalytic activity. This approach aimed to reduce the usage of noble metals and significantly boost the electrochemical catalytic activity for both OER and the chloride oxidation reaction (COR). The 2wt% Ir decorated catalysts showed a considerable improvement in overall catalytic activity, while the catalysts with 6wt% Pd excelled the COR activity under simulated seawater conditions. Especially, metal palladium has been known as an excellent selectivity and activity for chlorine species, but it is too reactive to be a stable. However, the combined deposition of Pd onto Co3O4NCs@IrO2 surface not only demonstrated a synergistic effect, significantly outperforming other metal electrodes under 0.5 M NaCl conditions (FEHOCl ~100%), but also contributed to a notable improvement in catalyst stability at 40 mA/cm2 over 8 hours.

This synergistic interaction between Ir and Pd on the Co3O4NCs surface, and the resultant enhanced stability, indicate the significance of our findings. Furthermore, the effective employment of noble metals would benefit from a design of catalysts based on precious metals. Our research paves the way for advancing hydrogen production technologies and propelling the hydrogen economy forward, illustrating the viability of seawater.