Robust Analysis of 4e- Versus 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 the 2e(-) versus 4e(-) oxygen reduction reaction (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 branching points, and the hydrogen evolution reaction (HER). In this study, we perform first-principles calculations in conjunction with uncertainty quantification on various monometallic and single-atom alloy surfaces to study the 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 the uncertainty of first-principles calculations for branching points within 4e(-) NRR. Reaction branching point analysis reveals the difficulty of overcoming the HER for weakly binding catalysts and the affinity toward 6e(-) NRR for stronger binding catalysts. This underlines the significant challenges of pushing NRR toward hydrazine synthesis.

Automated summary

The electrochemical synthesis of hydrazine is an exciting avenue in sustainable production. This study explores the optimization of catalysts for hydrazine synthesis through the 4e(-) electrochemical nitrogen reduction reaction (NRR). The research uses first-principles calculations and uncertainty quantification to analyze various catalyst surfaces, finding that catalysts with weak binding for NH3 are more favorable for hydrazine synthesis. Additionally, single-atom alloys exhibit the same scaling relations as monometallic surfaces. The study also quantifies the uncertainty of branching points within 4e(-) NRR and highlights the challenge of overcoming the hydrogen evolution reaction (HER) for weakly binding catalysts and the dominance of 6e(-) NRR for strongly binding catalysts.