Facet-specific nitrogen vacancy engineering in BaMO2N (M = Ta, Nb) for enhanced electrochemical ammonia production: Insights from first-principles calculations

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Developing efficient electrocatalysts for the electrochemical nitrogen (N 2 ) reduction reaction (eNRR) under ambient conditions is essential for sustainable ammonia (NH 3 ) production. In this study, we have used density functional theory (DFT) calculations to investigate the eNRR performance of two perovskite oxynitrides, BaTaO 2 N and BaNbO 2 N. We have systematically analyzed the reduction pathways and free energy profiles along both distal and alternating pathways on the (0 0 1) and (1 0 0) facets to evaluate the influence of surface orientation on catalytic performance. Our results show that the pristine surfaces exhibit weak N 2 adsorption and require a high Gibbs free energy (ΔG > 1.8 eV) for the initial protonation step, thereby limiting their intrinsic catalytic activity for direct NH 3 formation. We further explore defect engineering via the Mars-van Krevelen (MvK) mechanism, wherein lattice anions (nitrogen and oxygen) participate in vacancy formation and subsequent N 2 activation. On the BaNbO 2 N (0 0 1) surface, lattice nitrogen can be readily protonated and reduced to NH 3 , forming nitrogen vacancies that act as catalytic sites to facilitate N 2 adsorption and activation, thereby restoring the catalytic surface for sustained NH 3 production. Notably, the nitrogen-vacant surface (N v -BaNbO 2 N (0 0 1)) exhibits significantly enhanced N 2 adsorption, with a lower Gibbs free energy (ΔG = 0.18 eV) for the first protonation step, and a thermodynamically favorable NH 3 desorption process. Furthermore, the reduced surface strongly suppresses the competing hydrogen evolution reaction (HER), thereby promoting high selectivity for NH 3 production under ambient conditions. This theoretical study offers valuable insights into the design of perovskite oxynitride-based electrocatalysts, offering a promising strategy for sustainable and economically viable NH 3 synthesis. • Explored the eNRR activity of BaTaO 2 N and BaNbO 2 N perovskite oxynitrides using DFT. • Analyzed N 2 activation pathways on (001) and (100) facets via distal, alternating, and MvK mechanisms. • Pristine surfaces exhibit weak N 2 adsorption and high free energy for initial protonation. • Nitrogen vacancies generated via MvK mechanism on BaNbO 2 N (001) enhance N 2 activation and enable continuous NH 3 production. • N v -BaNbO₂N (001) demonstrates enhanced eNRR selectivity and suppresses competing HER.

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