Superior Oxygen Electrocatalysis on RuSex Nanoparticles for Rechargeable Air Cathodes

A one‐step, facile supercritical‐ethanol‐fluid synthesis of Se‐modified Ru nanoparticles nucleated on carbon defects is reported, and it is demonstrated that these nanoparticles provide, with >70% efficiency at 1 A g −1 , a highly active and reversible oxygen‐reduction/oxygen‐evolution reaction on an air cathode in a nonaqueous electrolyte. The Se modification not only prevents Ru oxidation during charge/discharge cycling, but also improves the catalytic activity by promoting Li 2 O 2 versus Li 2 O deposited on the Ru particles during discharge. A computational calculation with density functional theory supports the role of a larger electron transfer to the oxygen of Li 2 O 2 adsorbed on a surface layer of RuSe 2− δ than on a surface layer of RuO 2 , thereby shifting the more stable adsorbent from Li 2 O to Li 2 O 2 .