Fig. 4: Mechanistic analysis of the NO₃RR via a NO₂⁻ intermediate, NO₂RR performance and experimental correlations between lining NO₃RR selectivity and NO₂RR performance.

a–c Isotopic analysis of competing NO₃RR and NO₂RR reactions at −0.4 V. a Schematic for NO₃RR electrolysis in which small amounts of ¹⁵NO₂⁻ are doped in 0.16 M ¹⁴NO₃⁻. b NMR spectra for the electrolysis with 10 ppm of ¹⁵NO₂⁻ doped in 0.16 M ¹⁴NO₃⁻ (10,000 ppm), sampled at 2, 4, and 6 h. c Time course analysis of ¹⁴NH₃ and ¹⁵NH₃ concentration at 2, 4, and 6 h for the NO₃RR with 10 ppm ¹⁵NO₂⁻ (left) and 100 ppm ¹⁵NO₂⁻ (right) in 0.16 M ¹⁴NO₃⁻ (10,000 ppm). d Electrochemical NO₂RR for M-N-C catalysts in 0.05 M PBS + 0.01 M KNO₂ for 0.5 h. Faradaic efficiency for NH₃ as a function of the applied potential between −0.2 V and −0.8 V (top). Bottom section shows the corresponding NH₃ yield rate. The chronoamperometry plots and UV-Vis detection curves for NH₃ are shown in Figs. S50–56. Note, FE’s over 100% arise from electrolyte dilution to bring the concentration of NH₃ into the UV-vis calibrated range. Error bars are determined from three replicate trials. Correlations between the NO₃RR Faradaic efficiency for NH₃ in Fig. 3d, where R is the correlation coefficient (y-axis) and the dividend of the NH₃ yield rate and Faradaic efficiency of the NO₂RR in Fig. 4d at (e) −0.20 V and (f) −0.40 V. The NO₂RR, Yield_NH3 /FE_NH3 is represented as the NO₂RR total current (NO₂RR, j_total). The correlation between the NO₂RR, Yield_NH3 and NO₃RR, FE_NH3 is shown in Fig. S57. Ru and Rh outliers (gray shades) were excluded from the linear fit due to the dominant NO₃RR gaseous products as shown in Fig. S41 as well as Co-N-C. g Summary of the correlation coefficients, R, for varying experimental NO₃RR-NO₂RR correlations. Correlations not shown in Fig. 4e, f, are shown graphically in Figs. S58–60.