ULSAN — Korean researchers have developed a water electrolysis catalyst that combines cheap iron with a trace amount of ruthenium and outperforms costly platinum catalysts. By splitting the tasks of water dissociation and hydrogen generation between the two metals, the catalyst sharply cuts power consumption, and its simple manufacturing process is expected to help lower the cost of producing green hydrogen.
A team led by professors Jong-Beom Baek and Young-Kook Kwon of the Department of Energy and Chemical Engineering at Ulsan National Institute of Science and Technology (UNIST), together with professor Sang-Kyu Kwak of Korea University, said on the 21st that it had developed a division-of-labor electrolysis catalyst, dubbed RuNC@Fe1NC, in which single iron atoms and ruthenium nanoclusters each handle a different reaction.
Water electrolysis produces hydrogen by splitting water with electricity. Using renewable power yields green hydrogen with no carbon emissions, but heavy electricity consumption has been a hurdle to commercialization. Anion exchange membrane water electrolysis (AEMWE), seen as a next-generation technology, has faced a particular problem: in an alkaline environment, the first step of stripping hydrogen from a water molecule — the Volmer step — proceeds slowly, dragging down overall process efficiency.
The team applied a spatial division-of-labor design that assigns roles according to each metal's intrinsic properties. Single iron atoms, which have a strong affinity for water, break the bonds of water molecules and detach hydrogen atoms, and the adjacent ruthenium nanoclusters then combine those hydrogen atoms and release them as hydrogen gas. Computational chemistry analysis showed the activation energy for water dissociation at the iron active site was 0.08 electron volts, far lower than ruthenium's 0.22 eV.
The catalyst showed higher activity than commercial platinum catalysts even though ruthenium, a precious metal, accounted for only about 2% of its total weight. In a 1.0-molar potassium hydroxide (KOH) solution, the overpotential required to pass a current of 10 milliamperes per square centimeter of electrode area was 13.8 millivolts, less than half the 29.4 mV needed by a commercial platinum catalyst. Activity per unit mass of precious metal was 6.5 times that of the platinum catalyst.
The catalyst also proved its performance and durability in an actual device. Applied to an anion exchange membrane electrolysis cell, it reached an industrial current density of 1.0 ampere per square centimeter at 1.66 volts, the lowest level among previously reported systems. A commercial platinum catalyst required 1.93 V under the same conditions. The catalyst also retained its initial performance without structural deformation after running continuously for more than 1,100 hours at 80 degrees Celsius.
The team also secured mass production capability. Without any solvent or additional precursor, it used a mechanochemical ball-milling process in which graphite powder, nitrogen gas and steel balls are placed in a sealed container and rotated. Drawing on the principle that iron atoms detached through friction with the steel balls become anchored in defect sites in the carbon lattice, the team synthesized more than 10 grams of the catalyst uniformly in a single run.
"By designing common iron atoms and a trace amount of ruthenium to work together, we overcame the chronic reaction-rate limits of alkaline water electrolysis," Baek said. "Since it is easy to mass-produce and has also demonstrated long-term operational stability in an actual device, it will make a substantive contribution to lowering the cost of commercializing green hydrogen."
UNIST researchers Baek Jae-hoon, Kwon Sung-hyun and Cha Soon-kwan participated as co-first authors. The study was supported by the National Research Foundation of Korea under the Ministry of Science and ICT and by the national hydrogen core research laboratory for alkaline water electrolysis. The findings were published online in Advanced Materials, an international materials science journal.