Role of A-sites in pyrochlore lanthanide ruthenate for electrocatalysis of oxygen evolution reaction

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. [Display omitted] •Possibility of the lanthanides acted as A sites in pyrochlore ruthenates is verified by experimental and theoretical studies.•Ln2 (from Sm to Lu) Ru2O7 are prepared with the similar morphologies and crystallization for catalyzing OER.•Different configurations and energy levels of [RuO6] dominate the electrocatalytic performance.•Compared with other pyrochlore lanthanide ruthenates, Tb2Ru2O7 shows the best electrochemical performance for OER.•The secondary performance difference in Ln2Ru2O7 is caused by the charge transfer between A-site ions and [RuO6] units.