Tuning surface curvature in B and N co-doped CNT-derived Fe, Ru and Ir catalysts for electrochemical hydrogenation of N2 to NH3†

Abstract

Single-atom catalysts (SACs) have tremendous applications in enhancing the catalytic performance in the electrocatalytic nitrogen reduction reaction (NRR). Carbon-based substrates have superior properties that improve the catalytic performance either by forming defects or by doping heteroatoms, such as B,N-doped graphene, S-doped graphene, and defective carbon nanotubes. However, the carbon nanotube (CNT)-based electrocatalysts for NRR study are currently less explored. Here, we use the FeB₂N₂-(n,0) CNTs (n = 3–8) as representative electrocatalysts to study the different CNT curvatures and reveal their effects on the NN triple bond activation and adsorption free energy (ΔG) of the *N₂ molecule, with changes in the potential-determining step in NRR. Zigzag B₂N₂-(6,0) CNTs were selected as the efficient substrate, with three transition metal atoms (TM = Fe, Ru and Ir) anchored on the B₂N₂-(6,0) CNT to construct the NRR catalysts. Using first-principles calculation and the computational hydrogen electrode (CHE) model, we investigated their electrocatalytic performance in NRR. FeB₂N₂-(6,0) CNT is the most efficient catalyst and has a low limiting potential (U_L) of −0.551 V for NRR. Further, the projected partial density of states and projected crystal orbital Hamilton population analyses illustrate that the N₂ activation is due to strong π*-backbonding, which leads to effective charge transfer between the active site (metal d-orbital) and N₂ molecule (p-orbital). The FeB₂N₂-(6,0) CNT also showed high NRR selectivity, inhibiting the competitive hydrogen evolution reaction. Our study provides a detailed mechanism of catalysis by the carbon-based, high-efficiency electrocatalyst for NRR and opens up the possibility for experimentalists to further explore the carbon-based one-dimensional electrocatalyst for NRR.