Mitigation of RuO6 octahedron distortion by enhanced A-site electronegativity in pyrochlore for acidic water oxidation

Abstract

Ruthenium-based pyrochlore oxides (A₂Ru₂O₇) have emerged recently as state-of-the-art catalysts for acidic water oxidation; however, their stability still needs to be further improved. Exploring the relationship between the A-site cation and the structure of the active site (Ru) is highly desirable for designing efficient electrocatalysts. Herein, we rationally manipulate the substitution of the A-site atoms in Y₂Ru₂O₇ (YRO) by Ho³⁺, which has an identical ionic radius to Y³⁺ but higher electronegativity due to the 4f electron effect. It was demonstrated that the higher electronegativity could enlarge the Ru–O–Ru bond angle and reduce the Ru–O bond length, mitigating the RuO₆ octahedral distortion in Ho₂Ru₂O₇ (HRO) for enhancing the intrinsic OER activity. Compared with other pyrochlore oxides, HRO displayed an ultralow overpotential of 215 mV @ 10 mA cm⁻² with lower Ru content and higher mass activities, showing long-term (>60 h) stability in acid media. Density functional theory (DFT) calculations revealed that the higher electronegativity of Ho could strengthen the Ru–O covalency, thereby optimizing the free energy of oxygen species (ΔG_OOH^* − ΔG_O2) for better catalytic activity. In addition, the higher electronegativity could reduce the oxygen vacancies and improve the formation energy of oxygen vacancies for better resistance to Ru dissolution. This work reveals the inherent relationship of the A-site atom electronegativity, the lattice structure of the active site, and the activity–stability of the catalysts.