Robust Analysis of 4e− vs 6e− reduction of Nitrogen on metal surfaces and single atom alloys

The electrochemical synthesis of hydrazine is an exciting avenue in the sustainable production of commonly used chemicals. Taking inspiration from the mechanistic selectivity of reactions such as 2e- vs 4e- ORR, we explore how to fine tune catalysts for hydrazine synthesis through the 4e- electrochemical nitrogen reduction reaction (NRR) over the popular 6e- (NRR) used for ammonia synthesis. Optimal 4e- NRR performance requires sufficient activity as well as selectivity over 6e- (NRR), other mechanistic NRR reaction branching points and the hydrogen evolution reaction. In this study, we perform first principles calculations in conjunction with uncertainty quantification on various monometallic and single atom alloy surfaces to study activity and selectivity of 4e- NRR. Through free energy diagrams, estimation of scaling relations and a theoretical activity volcano, we observe that catalysts exhibiting low activity due to weak binding for NH3, favor hydrazine synthesis. We also find that single atom alloys follow the same scaling relations as monometallic surfaces. Through uncertainty quantification, we form distributions of limiting potentials and establish a correlation between the activity of a catalyst with the skewness of its limiting potential distribution. We further quantify first principles calculations uncertainty for branching points within various 4e- NRR branching points. Reaction branching point analysis and the tradeoff between activity and selectivity of the catalysts points to the significant challenges of pushing NRR towards hydrazine synthesis.