Developing highly stable and efficient catalysts for oxygen evolution reaction (OER) is extremely important to sustainable energy conversion and storage, but improved efficiency is largely hindered by sluggish reaction kinetics. Dense and bimetal ruthenates have emerged as one of the promising substitutes to replace single-metal ruthenium or iridium oxides, but the fundamental understanding the role of A-site cations is still blurring. Herein, a family of lanthanides (Ln = all the lanthanides except Pm) are applied to synthesize pyrochlore lanthanide ruthenates (Ln₂Ru₂O₇), and only Ln₂Ru₂O₇ (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) with pure phase can be obtained by the ambient-pressure calcination. Compared with the perovskite ruthenates (SrRuO₃) and rutile RuO₂, the [RuO₆] units in these Ln₂Ru₂O₇ present the largely distorted configurations and different energy level splitting to prevent the excessive Ru oxidation and dissolution, which leads the primary improvement in the electrocatalytic OER performance. In the similar crystalline field split states, the charge transfer between [RuO₆] units and Ln³⁺ cations also affect catalytic activities, even in the Ln₂Ru₂O₇ surface reconstruction during the OER process. Consequently, Tb₂Ru₂O₇ showed the highest OER performance among all the prepared Ln₂Ru₂O₇ with similar morphologies and crystallization. This systematic work gives fundamental cognition to rational design of high-performance OER electrocatalysts in proper water electrolysis technologies.

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.