Activating Graphyne Nanosheet via sp-Hybridized Boron Modulation for Electrochemical Nitrogen Fixation.

Abstract

Exploring new metal-free catalysts with high activity for nitrogen reduction reaction (NRR) is highly desirable but remains a big challenge. Graphyne (GY) is a typical two-dimensional carbon material with many excellent properties. However, the NRR has rarely been envisaged on a GY-based metal-free catalyst up to now. Density functional theory calculations reveal that although pristine GY is inactive for N2 reduction, boron modulation can endow it with efficient activity toward NRR. Natural bond orbitals analysis, spin/charge density distributions, and free energy change diagrams are performed and discussed. Three boron doping formats including sp2-substituted, sp-substituted, and adsorbed configuration are considered. The obtained data show sp-substitution will induce local moderate spin and charge densities at the boron site on the GY surface, which is convenient for N2 adsorption and activation, and conductive to N-related intermediates formation and transformation. Moreover, the incorporated sp-hybridized boron can provide one empty p orbital and one occupied p orbital around itself, which plays a key role as an electron reservoir to accept electrons from and donate electrons to the adsorbed N-related species, and thus facilitate N2 reduction and ammonia synthesis. Henceforth, it provides more opportunities for preparing GY and other carbon materials as efficient catalysts toward renewable energy conversion and storage.