Divergent A-site regulation mechanisms in rare-earth and alkaline-earth nickelates for unlocking bifunctional oxygen electrocatalysis

Abstract

The rational design of efficient, cost-effective bifunctional electrocatalysts for the oxygen reduction and evolution reactions (ORR/OER) is crucial for renewable energy technologies. Perovskite nickelates (ANiO₃), with their highly tunable A-site, hold significant promise, yet a comparative understanding of how different A-site cation families—specifically rare-earth (La, Ce, Pr, Nd, Sm, and Eu) versus alkaline-earth (Ca, Sr, and Ba) ions—govern the catalytic mechanisms remains incomplete. Herein, through systematic DFT + U calculations, we decipher the divergent A-site regulation mechanisms in these nickelates, focusing on their (001) surfaces with BO (Ni–O) and AO terminations (exposing A–O bonds). We reveal that the BO termination (exposing Ni–O octahedra) universally enables superior bifunctional activity due to balanced intermediate adsorption energies (−2.5 to −1.8 eV), whereas the AO termination typically leads to adsorption imbalance and inferior performance. Crucially, two distinct regulatory paradigms are uncovered: in alkaline-earth nickelates, a linear “ionic radius → Ni–O bond length → d-band center → *OH adsorption → OER activity” pathway dominates, with BaNiO₃ emerging as the optimal OER catalyst (η_OER = 0.57 V). In contrast, rare-earth nickelates are governed by a volcano-type regulation, where the f-electron count synergistically modulates the d-/p-band centers and cooperates with the e_g orbital occupancy (optimal range: 2.3–2.4) to achieve balanced O/OOH adsorption, rendering EuNiO₃ the top-performing bifunctional catalyst (η_ORR = 0.56 V, η_OER = 0.48 V). Bader charge analysis identifies balanced charge transfer as the electronic origin of peak activity. This work establishes a complete “A-site characteristics → electronic structure → bonding → activity” framework, unlocking design principles for high-performance perovskite oxygen electrocatalysts through element-specific mechanistic insights.