Controlling of coordination state of Ru_xN_y clusters for efficient oxygen reduction electrocatalysis

Ruthenium (Ru) is an attractive potential alternative to platinum as an electrocatalyst for the oxygen reduction reaction (ORR), in virtue of its high catalytic selectivity and relatively low price. In this work, a series of well-dispersed nitrogen-coordinated Ru-clusters on carbon black (Ru_xN_y/C) were prepared by pyrolyzing different Ru-containing sandwich compounds as the Ru sources. The higher thermal stability of these complexed sandwich precursors (bis(1,2,3,4,5-pentamethylcyclopentadienyl) Ru(II) monomer, dichloro(p-cymene) Ru(II) dimer, and chloro(1,2,3,4,5-pentamethylcyclopentadienyl) Ru(II) tetramer) affords the control of coordinated state for the resulting Ru-clusters, in comparison of that derived from ruthenium chlorides. After the pyrolysis treatment, the Ru coordinated state in Ru_xN_y/C, with the Ru–N and Ru–Ru bonds, still showed the structural inheritance from the Ru(II) monomer, dimer, and tetramer, but using ruthenium chlorides as the Ru source resulted in the nanoscale Ru agglomerations. The ORR testing exhibited that the Ru_xN_y/C sample derived from the Ru(II) tetramer (Ru_xN_y/C-T) presents the higher catalytic activity than the other obtained samples in either alkaline or acidic electrolytes. Even in the acidic electrolyte, Ru_xN_y/C-T shows the comparable ORR activity to that of Pt/C catalysts, and it shows the superior tolerance against methanol and CO. The X-ray absorption spectroscopy and density functional theory calculations demonstrate that these tetra-nuclear Ru-clusters could be the most active site due to their broadened d-orbital bands and lower energy d-band center than those of other subnano species and nanocrystals, and their favorable Yeager-type adsorption of O₂-molecules is also contributed to promoting O–O bond cleavage and accelerating the ORR process.