Engineering Ru(IV) charge density in Ru@RuO2 core-shell electrocatalyst via tensile strain for efficient oxygen evolution in acidic media

The design of efficient Ru-based electrocatalysts with high intrinsic activities for acidic water oxidation is highly desirable and challenging for water splitting in proton exchange membrane electrolyzers. Here, for the first time, we engineer the charge density of Ru(IV) by creating tensile strains in the RuO2 shell of Ru@RuO2 core-shell nanoparticles, viz. Ru@RuO2-L. High-resolution spectroscopic characterizations confirm the presence of av. 6% tensile strain in Ru–O bonds, which results in an effective reduction of the Ru(IV) charge density. The resultant RuX+ (4 < X < 5) active sites greatly accelerate the oxygen evolution reaction (OER) in an acidic electrolyte, leading to a remarkably low overpotential of 191 mV at 10 mA cm–2. These values are lower than those for the benchmark RuO2 catalyst and are also among the lowest for efficient Ru-based electrocatalysts reported thus far. The specific activity and mass activity are also greatly enhanced 4.2-fold and 17.7-fold compared to those of RuO2, respectively. The acidic OER activity improvement is ascribed to the lowered adsorption energy of *OOH, owing to the reduced charge density of Ru(IV), and the rapid charge transport owing to the Ru core. Ru@RuO2-L also demonstrates high feasibility as the anode catalyst for the overall water splitting in acidic media.