Electronic properties and oxygen reduction reaction catalytic activity of h-BeN₂ and MgN₂ by first-principles...

Currently, identifying suitable oxygen reduction reaction (ORR) catalysts in novel two-dimensional (2D) materials has attracted more and more research attention. Here, we have studied the catalytic activities of 2D h-BeN₂ and MgN₂ monolayers for ORR by using first-principles calculations. The calculated results reveal that the direct quasiparticle bandgap of BeN₂ monolayer is 3.32 eV, and the indirect bandgap of MgN₂ is 3.42 eV. 2D h-BeN₂ and MgN₂ exhibit high exciton binding energies of 1.07 and 0.83 eV respectively, and their optical properties are determined by bound exciton transitions due to the strong quantum confinement effects. Importantly, h-BeN₂ and MgN₂ monolayers with positive-charged (+1.6 e) metal atom (Be/Mg) on the surface exhibit excellent adsorption ability for O₂ and ORR intermediates, and show better CO tolerance than Pt(111). The calculated free energy plots are always downhill for ORR catalyzed by BeN₂ in both acid and alkaline environments, and by MgN₂ in alkaline environments. The detailed reaction mechanism analyses show that high-efficient four-electron pathway is the optimal pathway for ORR catalyzed by BeN₂ in acid environments. Surprisingly, there is a low overpotential of 0.45 eV for ORR catalyzed by BeN₂ in the acid solution and no overpotential in the alkaline solution. Our studies found for the first time that 2D h-BeN₂ shows huge potential as a non-precious metal ORR catalyst in acid and alkaline environments.