Particle size-control enables extraordinary activity of ruthenium nanoparticles/multiwalled carbon nanotube catalysts towards the oxygen reduction reaction

Catalysts with high efficiency for the oxygen reduction reaction (ORR) play a vital important role in fuel cells and metal-air batteries. Herein, Ru nanoparticles are highly dispersed on functional multi-walled carbon nanotubes (MWCNTs) by a facile impregnation-reduction method. The particle sizes of Ru nanoparticles are simply and effectively adjusted by the concentration of the Ru precursor. Benefiting from the optimal Ru particle size (2.1 nm), a large electrochemically active surface area and fast electron transport, the Ru/MWCNT catalyst shows outstanding ORR activity and durability via a four-electron pathway, producing a diffusion-limited current density of 4.7 mA cm^-2 with a half-wave potential of 0.72 V (vs. RHE). Such performance is better than that of a commercial 10 wt% Pt/C catalyst. Density functional theory calculation results reveal that the O_ads adsorption on the surface of Ru increases gradually with the addition of the RuO_x layer. The Ru/MWCNT catalyst with a particle size of 2.1 nm features appropriate O_ads adsorption energy due to the formation of an optimal RuO_x/Ru interface for the facilitation of the ORR.