Unravelling the electrochemical mechanisms for nitrogen fixation on single transition metal atoms embedded in defective graphitic carbon nitrideElectronic supplementary information (ESI) available: Full computational details, total energy and lattice parameter, density of states (DOS), thermochemical analysis, Gibbs free energies, computation of activation barriers in electrochemical reactions, and optimized structures. See DOI: 10.1039/c8ta06497k

Electrochemical reduction of nitrogen (N 2 ), in which the conversion of N 2 to ammonia (NH 3 ) takes place under mild conditions, is of timely significance for paving a way toward technological applications in agriculture and the chemical industry. In this work, various single transition metal atoms anchored on graphitic carbon nitride (g-C 3 N 4 ) with nitrogen vacancies (TM@NVs-g-C 3 N 4 ), acting as electrocatalysts for N 2 reduction, were systematically investigated by means of density functional theory (DFT) calculations. Most of the isolated metal atoms (Ti, V, Co, Ni, Zr, Mo, Ru and Pt) can be fixed by the nitrogen vacancies stably after performing the molecular dynamics simulation. For hexagonal close-packed and body centered cubic metal atoms, their N 2 chemisorption activity decreases as the coordination number of the single atom rises. Nevertheless, the anchored cubic close-packed metal atom does not serve as a good site for N 2 adsorption and activation even with a low-coordination number. Among all studied TM single atoms, the single Ti atom is found to be the most promising catalyst for its excellent N 2 reduction performance with a potential-limiting step of 0.51 eV and a rate-determining barrier of 0.57 eV. Atomic level insights are provided to elucidate the electrochemical mechanisms for N 2 reduction. The coordination number of the active center is accountable for the robust N 2 reduction activity with high stability. Overall, this work exemplifies the in-depth investigations of different single TM atoms, including the coordination number and binding mode, which are essential to lay the groundwork for the advancement of single atom catalysis toward practical implementation. Single transition metal atoms supported by defective g-C 3 N 4 are examined by DFT for electrochemical N 2 fixation. The single Ti atom is the most promising candidate for its high activity and stability owing to the coordination number of the active center.