Strategies to activate inert nitrogen molecules for efficient ammonia electrosynthesis: current status, challenges, and perspectives

The electrocatalytic N-2 reduction reaction (NRR) offers an alternative to the traditional Haber-Bosch (H-B) process for the synthesis of ammonia (NH3) and has received a surge of interest recently. However, as the prerequisite step for an efficient NRR, the N-2 activation process over an electrocatalyst is rather difficult to be realized under mild conditions due to the thermodynamic stability and chemical inertness of the N-2 molecule, which greatly limits the selectivity and activity of the NRR process, as well as the development of this renewable synthesis route for NH3. To date, a variety of electrocatalysts have been developed for effective N-2 activation in the past five years, although the presented activation abilities for N-2 molecules remain unsatisfactory even on the laboratory scale, and the corresponding design concepts/principles are still at the trial-and-error stage. Instead of focusing on labeling and classifying NRR electrocatalysts that have been extensively reviewed elsewhere, we herein present a timely and comprehensive review of emerging strategies to activate the inert N-2 molecule for NH3 electrosynthesis at the microscopic and macroscopic level on the basis of an in-depth understanding of the physicochemical properties and microelectronic structure of the N-2 molecule. We initially analyze the physicochemical properties and the microelectronic structure of the N-2 molecule from the perspective of molecular orbital theory. On the basis of this, we then emphasize the microscopic electronic effects of electrocatalysts for enhancing N-2 activation at length, typically covering sigma-donation, pi-backdonation, and sigma-donation/pi-backdonation effects, along with the design concepts/principles of electrocatalysts. Subsequently, the driving forces of macroscopic external fields (such as light, plasma, etc.) and the local microenvironment regulation-induced built-in electrostatic fields for assisting N-2 activation are introduced. In addition, the methodologies for studying the N-2 activation process over electrocatalyst surfaces are also presented from theoretical and experimental perspectives. Finally, we look at the future research directions and opportunities for improving N-2 activation and stimulating the practical application of NRR technology, covering electrocatalyst engineering, process intensification, and device architecture.