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Environmental and energy considerations demand that the Haber-Bosch process for reducing N2 to NH3 be replaced with electrochemical ammonia synthesis where the H atoms come from water instead of from H2. But a practical realization of electrochemical N2 reduction reaction (NRR) requires the development of new generation electrocatalysts with low overpotential and high Faraday efficiency (FE). A major problem here is that the hydrogen evolution reaction (HER) competes with NRR. Herein, we consider new generation dual-site catalysts involving two different metals incorporated into a novel two-dimensional C3N–C2N heterostructure that provides a high concentration of well-defined but isolated active sites that bind two distinct metal atoms in a framework that facilitates electron transfer. We report here the mechanism and predicted kinetics as a function of applied potential for both NRR and HER for the (Fe–Ru)/C3N–C2N dual atom catalyst. These calculations employ the grand canonical potential kinetics (GCP-K) methodology to predict reaction free energies and reaction barriers as a function of applied potential. The rates are then used in a microkinetic model to predict the turn-over-frequencies (TOF) as a function of applied potential. At U = 0 V, the FE for NRR is 93%, but the current is only 2.0 mA cm−2. The onset potential (at 10 mA cm−2) for ammonia on Fe–Ru/C3N–C2N is −0.22 VRHE. This leads to a calculated TOF of 434 h−1 per Fe–Ru site. We expect that the mechanisms for NRR and HER developed here will help lead to new generations of NRR with high TOF and FE. [ABSTRACT FROM AUTHOR]

Single-atom catalysts (SACs), which are low-cost, contain earth-abundant metals, and feature two-dimensional material supports, have shown great potential for us in a wide range of electrochemical reactions, including the CO2 reduction reaction (CO2RR) to convert CO2 to valuable chemicals and fuels. In recent years, substantial advances have been achieved in the preparation methodologies, with improved catalytic performances, but the underlying structure–activity relationship from a general perspective remains elusive. In particular, it is urgent to summarize the progress made on SACs with diatom metal centers toward efficient CO2RR. Based on the recent progress in this area, this review synopsizes the fundamental understandings of non-noble metal-based SACs for CO2RR using selected examples. We also highlight the representative atomic structures of active sites from the latest progress, including M–N–C, heteroatom co-doping, vacancy/edge defects and bimetallic SACs, with the aim of elucidating the nature of active sites on various 2D substrates. Moreover, we summarize the spectroscopic and computational studies to verify the atomic-level regulation of the geometric and electronic properties of SACs. We anticipate that this review will deepen the mechanistic understanding of the structure–performance relationship and inspire future studies on SACs for CO2RR. [ABSTRACT FROM AUTHOR]