Research ArticleChemistry

Catalyst-free, highly selective synthesis of ammonia from nitrogen and water by a plasma electrolytic system

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Science Advances  11 Jan 2019:
Vol. 5, no. 1, eaat5778
DOI: 10.1126/sciadv.aat5778
  • Fig. 1 Catalyst-free, electrolytic NH

    3 production from N2 and water using a plasma electrolytic system. (A) Schematic of the plasma electrolytic system operated by a dc power supply and galvanostatically controlled using a resistor (R) in series. The direction of electron flow (e) is indicated. (B) Total NH3 produced after 45 min at 6 mA and pH 3.5 for various gas configurations and controls. (C) Potentially important species contained in the plasma, such as vibrationally excited N2 [N2(v)], and in the water, such as solvated electrons [e(aq)], and their involvement in reactions, such as the generation of hydrogen radicals (H·), that lead to NH3 formation. The overall reactions for N2 reduction to NH3 and H2 evolution (under acidic conditions) at the cathode are shown.

  • Fig. 2 NH

    3 yield and efficiency in the plasma electrolytic system. (A) Total NH3 produced and corresponding faradaic efficiency after different processing times at 6 mA and pH 3.5. No NH3 is produced at 0 min based on the untreated electrolyte solution. (B) Total NH3 produced and corresponding faradaic efficiency as a function of current after 45 min at pH 3.5. No NH3 is produced at 0 mA based on the control experiment (see table S1A).

  • Fig. 3 Stability and trapping of NH

    3 in the plasma electrolytic system. (A) Comparison of total NH3 produced in split-compartment versus single-compartment cells at 6 mA and pH 3.5 after different processing times. (B) Total NH3 captured in the main reaction cell and a secondary trap vessel as a function of pH in the main reaction cell. The secondary trap was a strongly acidic H2SO4 bath (pH 2).

  • Fig. 4 Influence of NO

    x on NH3 formation and in situ NOx production in the plasma electrolytic system. (A) Potential scavenging reaction pathways of NO3 (green) and NO2 (purple) before and after solvation of electrons in water. (B) Comparison of total NH3 produced after 45 min at 6 mA and pH 3.5 in the presence of NO3 and NO2 at 10 mM and 1 M concentrations. The total NH3 produced for the same conditions in the absence of any scavenger is included for reference. (C) NOx concentration measured after different processing times at 6 mA and pH 3.5. (D) NOx concentration measured as a function of current after 45 min and pH 3.5.

  • Table 1 Comparison of electrically driven N

    2 reduction to NH3 demonstrations at ambient temperature and pressure.

    ReferenceProduction rate
    (mg/hour)
    Demonstration size
    (geometric area,
    catalyst loading)
    Faradaic
    efficiency
    (%)
    (38)0.0211 cm2, 1 mg/cm28
    (32)0.00476.25 cm2, 3.5 mg/cm22
    (39)0.0000630.25 cm2, 146 μg/cm260
    (40)0.000161 cm2, 0.33 mg/cm24
    This study0.441 mm2, catalyst free100

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/1/eaat5778/DC1

    Equation for faradaic efficiency calculation for NH3 from NH3 measurements

    Equation for instantaneous faradaic efficiency calculation for NH3 from H2 measurements

    Equation for faradaic NH3 concentration

    Equation for energy consumption

    Fig. S1. Representative current waveforms measured in the plasma electrolytic system during NH3 synthesis.

    Fig. S2. RGA measurements of mass/charge ratio (m/z) = 2 and 17, corresponding to H2 and NH3 partial pressure, at 2 mA in a plasma electrolytic reactor by analyzing exhaust gas from cell as a function of time.

    Fig. S3. H2 production in the plasma electrolytic system.

    Fig. S4. Schematic of the hybrid plasma electrolytic system in a split H-cell geometry.

    Fig. S5. Comparison of NOx produced with that predicted from O2 gas evolution in plasma electrolytic system.

    Fig S6. Heating of solution in the plasma electrolytic system.

    Fig. S7. Representative fluorescence assay calibration used to determine NH3 produced.

    Fig. S8. Representative fluorescence assay calibration used to determine NOx produced.

    Table S1A. Summary of NH3 produced by plasma electrolytic synthesis for the following configurations: N2 both flowing through the cathode tube where the plasma is normally generated and bubbled through the solution to purge, but no electrical power applied, Ar as both the supply gas in the plasma and purge gas, Ar as the supply gas in the plasma and N2 as the purge gas, and N2 as both the supply gas in the plasma and purge gas.

    Table S1B. Summary of one-sample and two-sample t tests carried out on datasets in table S1A.

    Table S2A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis after different processing times.

    Table S2B. Summary of one-sample and two-sample t tests carried out on datasets in table S2A.

    Table S3A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis at different currents.

    Table S3B. Summary of one-sample and two-sample t tests carried out on datasets in table S3A.

    Table S3C. Summary of H2 produced in plasma electrolytic reactor by analyzing exhaust gas using GC.

    Table S4A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis in a split cell.

    Table S4B. Summary of two-sample t tests carried out on datasets in table S4A (and tables S2A and S3A).

    Table S5A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis at different pH values.

    Table S5B. Summary of one-sample and two-sample t tests carried out on datasets in table S5A.

    Table S6A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis in the presence of NO3 and NO2 scavengers.

    Table S6B. Summary of one-sample and two-sample t tests carried out on datasets in table S6A.

    Table S7A. Summary of NOx produced by plasma electrolytic synthesis after different processing times and at different currents.

    Table S7B. Summary of one-sample and two-sample t tests carried out on datasets in table S7A.

    Table S8. Summary of voltages measured between the plasma cathode and the Pt anode as a function of the plasma current in the plasma electrolytic system.

  • Supplementary Materials

    This PDF file includes:

    • Equation for faradaic efficiency calculation for NH3 from NH3 measurements
    • Equation for instantaneous faradaic efficiency calculation for NH3 from H2 measurements
    • Equation for faradaic NH3 concentration
    • Equation for energy consumption
    • Fig. S1. Representative current waveforms measured in the plasma electrolytic system during NH3 synthesis.
    • Fig. S2. RGA measurements of mass/charge ratio (m/z) = 2 and 17, corresponding to H2 and NH3 partial pressure, at 2 mA in a plasma electrolytic reactor by analyzing exhaust gas from cell as a function of time.
    • Fig. S3. H2 production in the plasma electrolytic system.
    • Fig. S4. Schematic of the hybrid plasma electrolytic system in a split H-cell geometry.
    • Fig. S5. Comparison of NOx produced with that predicted from O2 gas evolution in plasma electrolytic system.
    • Fig S6. Heating of solution in the plasma electrolytic system.
    • Fig. S7. Representative fluorescence assay calibration used to determine NH3 produced.
    • Fig. S8. Representative fluorescence assay calibration used to determine NOx produced.
    • Table S1A. Summary of NH3 produced by plasma electrolytic synthesis for the following configurations: N2 both flowing through the cathode tube where the plasma is normally generated and bubbled through the solution to purge, but no electrical power applied, Ar as both the supply gas in the plasma and purge gas, Ar as the supply gas in the plasma and N2 as the purge gas, and N2 as both the supply gas in the plasma and purge gas.
    • Table S1B. Summary of one-sample and two-sample t tests carried out on datasets in table S1A.
    • Table S2A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis after different processing times.
    • Table S2B. Summary of one-sample and two-sample t tests carried out on datasets in table S2A.
    • Table S3A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis at different currents.
    • Table S3B. Summary of one-sample and two-sample t tests carried out on datasets in table S3A.
    • Table S3C. Summary of H2 produced in plasma electrolytic reactor by analyzing exhaust gas using GC.
    • Table S4A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis in a split cell.
    • Table S4B. Summary of two-sample t tests carried out on datasets in table S4A (and tables S2A and S3A).
    • Table S5A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis at different pH values.
    • Table S5B. Summary of one-sample and two-sample t tests carried out on datasets in table S5A.
    • Table S6A. Summary of NH3 produced and faradaic efficiencies by plasma electrolytic synthesis in the presence of NO3 and NO2 scavengers.
    • Table S6B. Summary of one-sample and two-sample t tests carried out on datasets in table S6A.
    • Table S7A. Summary of NOx produced by plasma electrolytic synthesis after different processing times and at different currents.
    • Table S7B. Summary of one-sample and two-sample t tests carried out on datasets in table S7A.
    • Table S8. Summary of voltages measured between the plasma cathode and the Pt anode as a function of the plasma current in the plasma electrolytic system.

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