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

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH) is a promising alternative route for an NH 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 (N), and for future targeted improvement of NRR catalysts, it is of utmost importance to get detailed insights into structure-performance relationships and mechanisms of N activation in such structures. Here, we report density functional theory studies on the NRR catalyzed by single Au and Fe atoms supported in graphitic CN materials. Our results show that the metal atoms present in the structure of CN are the reactive sites, which catalyze the aforesaid reaction by strong adsorption and activation of N. We further demonstrate that a lower onset electrode potential is required for Fe-CN than for Au-CN. Thus, Fe-CN is theoretically predicted to be a potentially better NRR catalyst at ambient conditions than Au-CN owing to the larger adsorption energy of N 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.