Combined Effects of Concentration, pH, and Polycrystalline Copper Surface on Electrocatalytic Nitrate-to-Ammonia Activity and Selectivity

Datasets for figures included in the results and discussion section in our recently accepted publication at ACS Catalysis: 10.1021/acscatal.2c05136

Figure 3: Cyclic voltammograms for 0.1 M, 0.5 M and 1 M NaNO3 for (a) pH 8, (b) pH 10, and (c) pH 14 using a Cu disk electrode (Figure 1) with a scan rate of 20 mV s-1 and stir rate of 900 rpm. Average current density for the last and steady cycle is shown as a bold line with the shaded regions representing the standard deviation over at least 3 trials computed at every potential. Arrows indicate the direction of the sweep. Data in Figure 3(a) and 3(b) for pH 8 and pH 10 is a selected subset of more experimental trials. Figure S2 shows the same results with potential versus the normal hydrogen electrode (V vs NHE) to illustrate pH-dependent shift in the range of the working electrode potentials. Blanks (predicted H2 evolution) have not been subtracted from the CV data for NaNO3.

Figure 4: Cyclic voltammograms for pH and 1 M NaNO3 and 1 M NaNO2 concentrations for a Cu disk electrode obtained with a scan rate of 20 mV s-1 and a stir rate of 900 rpm at: (a) pH 8, (b) pH 10, and (c) pH 14. The blanks associated with each trial condition are shown as dotted lines and the onset potential of the blank, i.e., hydrogen evolution reaction, is indicated by the vertical dashed line. Average current density for the last and steady cycle is shown as a bold line, with the shaded regions representing the standard deviation over all trials calculated at each potential. Blanks (predicted H2 evolution) have not been subtracted from the CV data for NaNO3 and NaNO2.

Figure 5: Cyclic voltammograms for 0.5 M NaNO3 for a Cu disk electrode with a scan rate of 20 mV s-1 and a stir rate of 900 rpm at (a) pH 8, (b) pH 10 and (c) pH 14. For (a) and (c), two different clusters of measured datasets are observed and average current density values over multiple trials (at least 2) are shown in bold and the shaded regions represent the standard deviation. For (b), data from all trials are shown. Blanks (predicted H2 evolution) have not been subtracted from the shown data.

Figure 6: Chronoamperometry studies for pH 8, 10, and 14 for NaNO3 concentrations of (a) 0.1 M and (b) 1 M. Average current density for all trials is shown in the bold line, with the shaded regions representing the standard deviation of 3 experimental trials. Experiments were completed with planar Cu electrodes with geometric areas in Table S3, where the working electrode was subject to the voltages in Table 1, and with a stir rate of 900 rpm. Blanks without NaNO3 have been subtracted from currents obtained with NaNO3 electrolytes at all concentrations and pH to discount H2 evolution currents at cathodic potentials.

Figure 7: (a) Average and standard deviations in Faradaic efficiency to NH3 (green) and NO2- (purple) at pH 8, 10, and 14 for 0.1 M and 1 M NaNO3. (b) Average and standard deviations in measured NO3- consumption (blue) and estimated NO3- consumption (teal); (c) trial-to-trial breakdown of Faradaic efficiency to NO2-, NH3, predicted H2, and unaccounted products. All data for this figure are from experiments completed with planar Cu electrodes with geometric areas in Table S1, where the working electrode was subject to the voltages in Table 1, and with a stir rate of 900 rpm. Average values in 7(a) and 7(b) are obtained by averaging over all trials in Figure 6, and error bars account for both random (trial-to-trial) and systematic errors (Table 2); 7(c) does not show uncertainties in the Faradaic efficiency to NO2- and NH3. Instead, the red box indicates the portion of the unaccounted products that lies outside of the systematic error coming from the NH3 and NO2- measurements.

Figure 8: Energy intensity for ammonia recovery,  (MJ kgN-1), and rate of ammonia production,  (gN m-2 day-1), as a function of pH 8, 10, and 14 for 0.1 M and 1 M NaNO3. Energy intensity values estimated for the standard biological nitrification-denitrification approach (green star), and for the Sharon-Anammox process (yellow star) are included for comparison. All data for this figure are from experiments completed with planar Cu electrodes (Figure 1). Average values for energy intensity and rate of ammonia production are obtained by averaging over all trials in Figure 6, and error bars account for both random (trial-to-trial) and systematic errors (Table 2).

Figure 9: Correlation, using trial-by-trial experimental data, between the average of initial and final values of the charge-transfer resistance, , and the charge passed for NO3- reduction to NO2- and NH3, , for 0.1 M and 1 M NaNO3 at pH 8, 10, and 14. Best-fit charge-transfer resistance values are obtained from electrochemical impedance spectroscopy data measured with planar Cu electrodes at a potential of -0.1 V. Lower  leads to higher charge passed to NO3- reduction products, as shown by the fit line. Systematic errors in the determination of  (Table 2) are not explicitly shown in this dataset.

Figure 10: Correlations using trial-by-trial experimental data between the net charge passed and the average of the initial and final effective double-layer capacitance, . (a, b) show the total charge, , and (c, d) only consider NO2- and NH3, . The  is split by concentration, with (a, c) showing 0.1 M and (b,d) showing 1 M NaNO3 for pH 8, 10, and 14. Trendlines in the data (dotted and dashed) are included at every pH. Best-fit double-layer capacitance values are calculated from electrochemical impedance spectroscopy data obtained with planar Cu electrodes at a potential of -0.1 V. The error bars for  include the contributions from systematic error (Table 2).