Computational Design of Two-Dimensional Boron-Containing Compounds as Efficient Metal-free Electrocatalysts toward Nitrogen Reduction Independent of Heteroatom Doping.

As metal-free carbon based catalysts, boron (B)-doped carbonaceous materials have proved to exhibit superior catalytic performance toward nitrogen reduction reaction. However, this strategy of heteroatom doping encounters the synthesis challenges of precise control of the doping level and homogeneous distribution of the dopants, and in particular, these materials cannot be utilized in electrochemical N2 reduction because of poor electrical conductivity. Accordingly, via first-principles calculations, we here predicted two stable two-dimensional crystalline compounds: BC6N2 and BC4N, which have small band gaps and uniform distribution of NRR active sp2-B species and holey structures. Between them, the BC6N2 monolayer originally possesses nice NRR activity with limiting potentials of -0.47 V. In the proton-rich acid medium, the electronic properties of these two B-C-N monolayers could be further tailored to exhibit a metallic characteristic by H pre-adsorption. This drastically improves the conductivity and enhances their NRR performances as reflected by the limiting potentials of -0.15, -0.34, and -0.34 V for BC6N2 via enzymatic, distal, and alternating mechanisms, respectively. Besides, NRR on BC4N through enzymatic mechanism proceeds as the limiting potential moderated from -1.20 to -0.90 V. More than that, the competing hydrogen evolution reaction can be effectively suppressed. The current investigation opens an avenue of designing a 2D crystalline phase of MFC catalysts independent of heteroatom doping and gives insightful views of surface functionalization as an impactful strategy to improve the electrocatalytic activity of metal-free catalysts.