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 (ANiO 3 ), 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, Eu) versus alkaline-earth (Ca, Sr, 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 BaNiO 3 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 EuNiO 3 the top-performing bifunctional catalyst (η ORR /η OER = 0.56/0.48 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 the design principles for high-performance perovskite oxygen electrocatalysts through element-specific mechanistic insights.