Design of Catalysts for Water Splitting and N2/CO2 Reduction

Abstract

Transforming the abundant atmospheric nitrogen (N2) into ammonia (NH3), a widely used industrial feedstock, through a green and sustainable process has gained an increasing interest for both commercial and environmental needs. The current industrial route for the production of NH3 is based on the Haber-Bosch (H-B) process, which is conducted under high temperature (~500℃) and pressure (~200 atm) conditions and consumes about 2% of the world’s energy per year. Thus, it is of paramount significance to search for energy-efficient and environmentally friendly techniques to replace the H-B process. The electrochemical nitrogen reduction reaction (eNRR), which takes place at mild conditions and can be driven by renewable energies such as solar and wind, constitutes a desirable alternative. However, the development of low-cost and high-efficient electrocatalysts for eNRR has remained elusive up to now. In this work, we conduct a comprehensive study to unravel the reaction mechanisms of N2 fixation on SAC-Nb2CN2 and molybdenum nitride by using density-functional-theory (DFT) calculations. For SAC-Nb2CN2, we screen the transition metals (TM), including 26 elements, supported on two-dimensional (2D) Nb2CN2 (TM-Nb2CN2) for their applications into electrochemical reduction of N2 (NRR) based on first-principles calculations. We show that most SACs can bind with Nb2CN2 strongly through a TM-N3 configuration. We find that Mn-Nb2CN2 is a promising candidate for the N2 reduction reaction (NRR), with a low overpotential of 0.51 V through the distal mechanism. Importantly, TM-Nb2CN2 presents high selectivity to NRR by blocking the hydrogen adsorption and preventing the hydrogen evolution reaction (HER). For molybdenum nitride, the activity and selectivity of eNRR on pristine (001) and (110) Mo5N6 surfaces as well as few specific numbers of heteroatom anchored N-terminated surfaces are all evaluated and compared. We find that the Mo and N atoms on the pristine Mo5N6 surface are both active for eNRR, while follow different pathways in mechanism. Moreover, the eNRR catalytic performance of Mo5N6 could be further boost by specific metal atoms anchoring, such as single atom, metal dimer and heterodiatom pair. Finally, a full map of eNRR mechanism on pristine and metal atom-decorated Mo5N6 surfaces are illustrated. Our work not only provides a fundamental understanding of eNRR mechanism on TMNs based materials, but also offers powerful strategies towards the rational design of efficient NRR electrocatalysts.