Research ArticleChemistry

Favoring the unfavored: Selective electrochemical nitrogen fixation using a reticular chemistry approach

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Science Advances  09 Mar 2018:
Vol. 4, no. 3, eaar3208
DOI: 10.1126/sciadv.aar3208
  • Fig. 1 Characterization of as-synthesized ZIF thin film fabricated over Ag nanocube array on Au working electrode (Ag-Au@ZIF).

    (A) Schematic depicting the importance of Ag-Au@ZIF as water repellent and nitrogen (N2) molecular concentrator for subsequent application in electrochemical NRR into ammonia (inset). HCl-treated Ag nanocube is denoted as AgNC. (B) Cross-sectional scanning electron microscopy (SEM) image of Ag-Au@ZIF using an 18-nm Au film as proxy. (C) Substrate XRD diffraction pattern of HCl-treated AgNC, neat ZIF thin film, and as-synthesized Ag-Au@ZIF (bottom to top). a.u., arbitrary units.

  • Fig. 2 Electrochemical reduction of N2 gas using the Ag-Au@ZIF electrode.

    (A) Schematic illustrating the experimental setup of liquid electrolyte-based electrochemical reduction of N2 gas to ammonia (inset). The electrolyte solution is prepared in dry tetrahydrofuran (THF). Various platforms prepared on Au electrodes function as the working electrode (WE). Pt wire and Ag/AgCl (1 M KCl) are used as counter (CE) and reference electrodes (RE), respectively. Cyclic voltammetry analysis using (B) Ag-Au@ZIF electrodes and (C) Ag-Au electrodes, both under argon (Ar) or N2 gas bubbling. (i) Scheme depicting the respective working electrodes. (ii) Cyclic voltammograms recorded from respective working electrodes under Ar or N2 gas bubbling. Directions of forward and backward scans are denoted by the double and single arrows, respectively. All gases are bubbled at 3 sccm using a mass flow controller.

  • Fig. 3 Evaluation of electrochemical performance and selectivity toward NRR using Ag-Au@ZIF electrodes.

    (A) Experimental scheme for the electrochemical reduction of N2 and subsequent quantification of the ammonia generated. Ammonia detection is based on the indophenol blue method. (B) Absorption spectra of indophenol blue formed from the ammonia generated using the Ag-Au@ZIF electrode under N2 bubbling. (C) Effective rate of ammonia formation and Faradic efficiency for various electrode platforms. (D) Comparison of NRR selectivity in the presence and absence of ZIF encapsulation. (E) Overall process mechanism for the selective and improved electrochemical reduction of N2 gas to form ammonia achieved simply by encapsulation with ZIF. ZIF layer functions both as a water-resistant coating and gas concentrator. N2 is electrochemically reduced to generate ammonia (left). Water molecules are deterred from accessing the electroactive surface by a hydrophobic ZIF coating, hence impeding the competing HER (right).

  • Fig. 4 Enhancing and sustaining electrochemical N2 reduction performance using the Ag-Au@ZIF electrode.

    (A) Schematic depicting the continuous accumulation of N2 molecules near the electrocatalyst to boost the performance of NRR. (B) Cyclic voltammograms depicting electrochemical responses indexed to NRR using Ag-Au@ZIF for 45 cycles. The red arrow denotes the systematic evolution of the electrochemical responses. (C) Peak potentials and (D) peak heights derived from the cyclic voltammograms in (B). (E) Magnified cyclic voltammograms near the HER region recorded from Ag-Au@ZIF electrodes for 45 cycles. A control voltammogram recorded from the Ag-Au electrode in the absence of ZIF encapsulation is included to indicate the potential region for HER. All experiments are performed under constant N2 gas bubbling.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaar3208/DC1

    fig. S1. Characterization of as-synthesized and HCl-treated Ag nanocubes.

    fig. S2. Characterization of the Ag-Au@ZIF platform.

    fig. S3. Qualitative comparisons of SERS spectra obtained under various experimental conditions.

    fig. S4. Electrochemical investigations of various electrodes under respective conditions.

    fig. S5. Calibration of the indophenol blue method for subsequent NH3/NH4+ quantification.

    fig. S6. Evaluating the performance of the Ag-Au@ZIF ensemble over control platforms.

    fig. S7. Selectivity of various electrode platforms toward NRR.

    fig. S8. Water-repelling properties of Ag-Au@ZIF and cyclic voltammetry analysis on Au@ZIF.

    fig. S9. Enhancing and sustaining electrochemical N2 reduction performance using the Ag-Au@ZIF electrode.

    fig. S10. Time-dependent electrochemical evaluation of the Ag-Au electrode.

    References (3337)

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Characterization of as-synthesized and HCl-treated Ag nanocubes.
    • fig. S2. Characterization of the Ag-Au@ZIF platform.
    • fig. S3. Qualitative comparisons of SERS spectra obtained under various experimental conditions.
    • fig. S4. Electrochemical investigations of various electrodes under respective conditions.
    • fig. S5. Calibration of the indophenol blue method for subsequent NH3/NH4+ quantification.
    • fig. S6. Evaluating the performance of the Ag-Au@ZIF ensemble over control platforms.
    • fig. S7. Selectivity of various electrode platforms toward NRR.
    • fig. S8. Water-repelling properties of Ag-Au@ZIF and cyclic voltammetry analysis on Au@ZIF.
    • fig. S9. Enhancing and sustaining electrochemical N2 reduction performance using the Ag-Au@ZIF electrode.
    • fig. S10. Time-dependent electrochemical evaluation of the Ag-Au electrode.
    • References (33–37)

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