From left, Professor Jin Woo Lee of the Department of Chemical and Biomolecular Engineering at KAIST and PhD candidate Ji-won Kim (first author). Provided by the National Research Foundation of Korea
A new catalyst design principle has been proposed that can increase the efficiency of hydrogen production via water electrolysis without using expensive platinum catalysts currently employed in water electrolysis hydrogen production.
The National Research Foundation of Korea announced on the 13th that a research team led by Professor Jin Woo Lee of the Department of Chemical and Biomolecular Engineering at KAIST, in joint research with a team led by Professor Jung Woo Han of Seoul National University, a team led by Professor Woo-Yeol Kim of the Korea Institute of Energy Technology, and a team led by Principal Researcher Sung-gi Cho of the Hydrogen & Fuel Cell Research Center at the Korea Institute of Science and Technology (KIST), has identified the surface state under operating conditions of ruthenium nanocluster catalysts and the structure of water at the electrode–electrolyte interface. Based on this, they improved the hydrogen production performance of anion exchange membrane water electrolysis. The research results were published online on June 4 in the international journal Energy & Environmental Science.
Anion exchange membrane water electrolysis, which produces hydrogen by electrically decomposing water, operates in an alkaline environment. It can use low-cost cell materials and non-precious metal anode catalysts, making it a key technology for green hydrogen production. However, the hydrogen evolution reaction at the cathode must first split water molecules to form hydrogen intermediates, which makes the reaction slower than under acidic conditions.
The research team uniformly loaded ruthenium, a metal cheaper than platinum, onto a carbon support and controlled the heat-treatment temperature to synthesize a series of catalysts with different sizes, including single atoms, subnanometer clusters of about 1 nm (nanometer, where 1 nm is one-billionth of a meter), and larger nanoclusters. Subnanometer clusters refer to aggregates of metal atoms smaller than 1 nm.
Subsequent X-ray absorption spectroscopy analysis showed that ruthenium nanoclusters of about 1 nm maintained a partially oxidized surface, including ruthenium–oxygen bonds, in a stable manner even under reductive hydrogen evolution conditions. Infrared spectroscopy analysis confirmed that this partially oxidized surface state influences the adsorption characteristics of reaction intermediates and the arrangement of water molecules around the catalyst, thereby promoting the alkaline hydrogen evolution reaction.
In half-cell tests used to evaluate initial activity, the developed catalyst initiated the reaction at a very low voltage (an overpotential of just 20 mV). The mass activity per unit weight of precious metal reached a very high value of 11.05 A (amperes) per milligram of precious metal at 100 mV.
In single-cell tests under conditions similar to commercial environments, the catalyst achieved a high current density of 5.34 A/㎠ and operated stably for more than 400 hours at a current density of 1 A/㎠, which is comparable to practical levels, demonstrating its applicability to real devices.
Professor Lee explained, “This study serves as an important starting point for designing high-performance water electrolysis catalysts, as it clarifies how the size and atomic structure of ruthenium catalysts alter the surface oxidation state and interfacial water structure under actual operating conditions,” adding, “The role of the operating surface state identified in this study will provide a crucial foundation for future design of high-performance alkaline water electrolysis catalysts and the development of green hydrogen production technologies.”
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