Electrochemical N₂ Reduction to Ammonia Using Single Au/Fe Atoms Supported on Nitrogen-Doped Porous Carbon

Abstract

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH₃) is a promising alternative route for an NH₃ synthesis at ambient conditions to the conventional high temperature and pressure Haber-Bosch process without the need for hydrogen gas. Single metal ions or atoms are attractive candidates for the catalytic activation of non-reactive nitrogen (N₂), and for future targeted improvement of NRR catalysts, it is of utmost importance to get detailed insights into structure-performance relationships and mechanisms of N₂ activation in such structures. Here, we report density functional theory studies on the NRR catalyzed by single Au and Fe atoms supported in graphitic C₂N materials. Our results show that the metal atoms present in the structure of C₂N are the reactive sites, which catalyze the aforesaid reaction by strong adsorption and activation of N₂. We further demonstrate that a lower onset electrode potential is required for Fe-C₂N than for Au-C₂N. Thus, Fe-C₂N is theoretically predicted to be a potentially better NRR catalyst at ambient conditions than Au-C₂N owing to the larger adsorption energy of N₂ molecules. Furthermore, we have experimentally shown that single sites of Au and Fe supported on nitrogen-doped porous carbon are indeed active NRR catalysts. However, in contrast to our theoretical results, the Au-based catalyst performed slightly better with a Faradaic efficiency (FE) of 10.1% than the Fe-based catalyst with an FE of 8.4% at -0.2 V vs. RHE. The DFT calculations suggest that this difference is due to the competitive hydrogen evolution reaction and higher desorption energy of ammonia.