Given that NO3RR is inherently sensitive to electrolyte composition, it would be desirable to identify/establish a standard reaction environment for catalyst testing. However, the applications of interest for NO3RR span a wide range of reaction environments, and one catalyst may not represent the best performer in every scenario. We here introduce three model systems for representative catalyst assessment and comparison and note particular complexities inherent to them for further investigation (Table ). Low-level nuclear waste. The highest NO 3 -concentrations are typically attributed to low-level nuclear waste, in many cases exceeding 1 M. These waste streams are also high pH, often exceeding 14. Typical streams also include appreciable amounts of NO 2 -, with potential implications on reaction mechanism and product distribution. Other complexities include the presence of sulfate (SO4 2-) and chloride (Cl -) anions, which may compete for surface sites and reduce NO3RR rates . In addition, streams may include heavy metal ions, such as from Cr and Pb, that could plate out on the electrocatalyst at the reducing potentials of NO3RR.