Isolated Boron Sites for Electroreduction of Dinitrogen to Ammonia

Exploring electrocatalysts with high activity is essential for the production of ammonia via an electrochemical routine. By employing density functional theory calculations, we investigated the electrochemical nitrogen reduction reaction (eNRR) activity on binary metal borides, a model system of metal borides. To elaborate the mechanisms, molybdenum borides (Mo2B, alpha-MoB, and MoB2) were first modeled; the results indicate that the crystal structures greatly impact the N-2 adsorption and therefore the electrocatalytic activity. Our electronic structure investigation suggests that boron p-orbital hybrids with dinitrogen pi*-orbital, and the population on p-pi*-orbital determine the N-2 adsorption strength. Therefore, the isolated boron site of Mo2B with less filled p(z)-orbital benefits the activation of N-2 and weaken the triple bond of dinitrogen. This isolated boron sites concept was successfully extended to other metal borides in the form of M2B (M stands for Ti, Cr, Mn, Fe, Co, Ni, Ta, W). Mo2B, Fe2B, and Co2B were discovered as the most promising candidates with low limiting potentials due to appropriate adsorption strength of reaction intermediates led by moderate p(z), filling. This work provides insights for designing metal borides as promising eNRR electrocatalysts.