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.