Insights into Electrochemical Nitrate Reduction with Cu Single Crystals in Alkaline Electrolytes

The electrochemical nitrate reduction reaction (NitRR) is an alternative to wastewater treatment, allowing for the conversion of NO₃^- and NO₂^- into NH₃ which can be used to supplement NH₃ formation from the Haber-Bosch process. Cu is a promising material for NitRR catalysis, reaching high rates and Faradaic efficiencies for NH₃. While Cu performs well, there are fundamental gaps in our understanding of which properties are responsible for Cu’s activity. The activity of Cu single crystals has been thoroughly studied in acidic environments, but few studies have investigated the NitRR on Cu single crystals in alkaline electrolytes. To bridge this gap, we have utilized Cu single crystals to determine the most active low-index facet for the reduction of NO₃^- to NO₂^- and the subsequent reduction of NO₂^- to NH₃. To gain more insight into the structure-function relationship, we further investigated the kinetics and thermodynamics to determine which reaction steps are rate-limiting for each surface. This study has provided facet-specific mechanistic insight into the NitRR. These results will help advise toward optimal nanoparticle shape of Cu-based catalysts for the NitRR as well as providing a fundamental understanding of which catalyst attributes we should consider for further catalyst refinement.