Potential-dependent insights into the origin of high ammonia yield rate on copper surface via nitrate reduction: A computational and experimental study

Abstract

Focusing on revealing the origin of high ammonia yield rate on Cu via nitrate reduction (NO3RR), we herein applied constant potential method via grand-canonical density functional theory (GC-DFT) with implicit continuum solvation model to predict the reaction energetics of NO3RR on pure copper surface in alkaline media. The potential-dependent mechanism on the most prevailing Cu (111) and the minor (100) and (110) facets were established, in consideration of NO2−, NO, NH3, NH2OH, N2, and N2O as the main products. The computational results show that the major Cu (111) is the ideal surface to produce ammonia with the highest onset potential at 0.06 V (until −0.37 V) and the highest optimal potential at −0.31 V for ammonia production without kinetic obstacles in activation energies at critical steps. For other minor facets, the secondary Cu (100) shows activity to ammonia from −0.03 to −0.54 V with the ideal potential at −0.50 V, which requires larger overpotential to overcome kinetic activation energy barriers. The least Cu (110) possesses the longest potential range for ammonia yield from −0.27 to −1.12 V due to the higher adsorption coverage of nitrate, but also with higher tendency to generate di-nitrogen species. Experimental evaluations on commercial Cu/C electrocatalyst validated the accuracy of our proposed mechanism. The most influential (111) surface with highest percentage in electrocatalyst determined the trend of ammonia production. In specific, the onset potential of ammonia production at 0.1 V and emergence of yield rate peak at −0.3 V in experiments precisely located in the predicted potentials on Cu (111). Four critical factors for the high ammonia yield and selectivity on Cu surface via NO3RR are summarized, including high NO3RR activity towards ammonia on the dominant Cu (111) facet, more possibilities to produce ammonia along different pathways on each facet, excellent ability for HER inhibition and suitable surface size to suppress di-nitrogen species formation at high nitrate coverage. Overall, our work provides comprehensive potential-dependent insights into the reaction details of NO3RR to ammonia, which can serve as references for the future development of NO3RR electrocatalysts, achieving higher activity and selectivity by maximizing these characteristics of copper-based materials.