A job for vacancies

Electrocatalysts based on ruthenium are very active for water oxidation (the reaction that occurs at the anode of an electrolyser) but typically suffer from poor stability. In the harsh acidic and oxidative conditions found at the anode of a proton exchange membrane electrolyser, metal ions can leach out of the catalyst and structural collapse may occur, leading to reduced performance. Now, Zhanwu Lei, Shuhong Jiao, Jing-Li Luo, Ruiguo Cao and colleagues across China and the Republic of Korea report a Ru_0.5Mn_0.5O₂ water oxidation catalyst, which — through a self-limiting surface leaching mechanism — achieves long-term durable operation.

The researchers find that during an initial period of water oxidation, although Ru ions leach out of the catalyst to an extent, Mn ions leach out much more so. This leads to reconstruction of the catalyst, with a surface layer forming that is rich in Mn vacancies. The results suggest that the presence of these vacancies suppresses further leaching of Mn ions and stabilizes the Ru catalytically active sites in the reconstructed layer. The team report that the activity of the catalyst (normalized to the amount of Ru) is 525 A g_Ru⁻¹ at 1.45 V (versus the reversible hydrogen electrode) and that it maintains 95% of its initial activity after 2,500 h at 10 mA cm^–2 in a chronopotentiometric measurement in 0.5 M H₂SO₄. A comparable catalyst without Mn had approximately 11 times lower mass-normalized activity and failed after 180 h. In a full proton exchange membrane electrolyser, the catalyst performed stably for 330 h at 80 °C and 200 mA cm^–2, also demonstrating promise under more realistic conditions.