Optimizing the NRR activity of single and double boron atom catalysts using a suitable support: a first principles investigation.

Designing cost effective transition-metal free electrocatalysts for nitrogen fixation under ambient conditions is highly appealing from an industrial point of view. Using density functional theory calculations in combination with the computational hydrogen electrode model, we investigate the N activation and reduction activity of ten different model catalysts obtained by supporting single and double boron atoms on five different substrates ( GaN, graphene, graphyne, MoS and g-CN). Our results demonstrate that the single/double boron atom catalysts bind favourably on these substrates, leading to a considerable change in the electronic structure of these materials. The N binding and activation results reveal that the substrate plays an important role by promoting the charge transfer from the single/double boron atom catalysts to the antibonding orbitals of *N to form strong B-N bonds and subsequently activate the inert NN bond. Double boron atom catalysts supported on graphene, MoS and g-CN reveal very high binding energies of -2.38, -2.11 and -1.71 eV respectively, whereas single boron atom catalysts supported on graphene and g-CN monolayers bind N with very high binding energies of -1.45 and -2.38 eV, respectively. The N binding on these catalysts is further explained by means of orbital projected density of states plots which reflect greater overlap between the N and B states for the catalysts, which bind N strongly. The simulated reaction pathways reveal that the single and double boron atom catalysts supported on g-CN exhibit excellent catalytic activity with very low limiting potentials of -0.67 and -0.36 V, respectively, while simultaneously suppressing the HER. Thus, the current work provides important insights to advance the design of transition-metal free catalysts for electrochemical nitrogen fixation from an electronic structure point of view.