Ruthenium might sound like a material from science fiction, but it’s found in a surprising range of everyday items, from electronic chips to fountain pen nibs. In its oxidised form, ruthenium’s unique properties could also make it a potent electrocatalyst in the oxygen evolution reaction (OER), a key step in the water splitting (electrolysis) process for making green fuels and chemicals.
However, various studies have shown that ruthenium oxide (RuO2) catalysts don’t always perform consistently. When tasked with boosting the OER in the acidic conditions typical of many industrial water electrolysers, some RuO2 samples stay stable throughout repeated use, while others rapidly dissolve.
“Although many research papers refer to 'RuO2 catalysts,' these materials can look very different at the atomic level,” said Jiajian Gao, a Senior Scientist at the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE²). “Some are amorphous, meaning their atoms are arranged loosely like a pile of stones; others are crystalline, meaning their atoms are packed neatly like bricks in a wall.”
To figure out how these structural differences affect RuO2’s catalytic performance, Gao and A*STAR ISCE² colleagues worked with researchers from National Yang Ming Chiao Tung University, Taiwan, in a systematic side-by-side comparison of several commercial RuO2 catalysts in acidic OER electrolysis.
The team used cyclic voltammetry, hydrogen peroxide (H₂O₂) probes, in situ spectroscopy and other techniques to examine the catalysts’ structures, catalytic performance activity and stability. They also observed minute changes to the catalysts as they operated, such as the way amorphous RuO2 dissolved almost instantly under voltage.
“Using an ultra-sensitive microbalance, we confirmed that amorphous RuO2’s inherent structural instability, not electrochemical degradation, was causing it to disappear before producing any meaningful oxygen,” said Gao.
The researchers also tested catalysts created by heating (calcining) RuO2 at different temperatures, revealing that samples calcined at 300–400 °C hit a performance ‘sweet spot’ of high activity and reasonable stability.
“We used H₂O₂ as a test molecule that mimics a key step in the OER; if a catalyst can handle H₂O₂ well, it often means it has the right surface chemistry to promote the OER efficiently," said Gao.
“When we tested our catalyst series with H₂O₂, we found that the 300–400 °C samples reacted at lower voltages and with faster kinetics.”
Mass-tracking tools and in situ spectroscopy measurements revealed why this was the case: moderately calcined samples had just enough crystallinity to remain stable, but were still ‘open’ enough at their surface to interact effectively with reaction intermediates. In contrast, highly crystalline catalysts calcined at 700 °C showed excellent stability but weaker intermediate adsorption, limiting their activity.
Gao noted that future strategies for fine-tuning RuO2 could include doping the material with other elements or designing nanostructures that use each ruthenium atom more efficiently. "Overall, we now have a reliable benchmark that helps judge whether new ruthenium-based catalysts are true improvements in both activity and durability," said Gao.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE²).
