Regulating Interfacial Microenvironment in Aqueous Electrolyte via a N2 Filtering Membrane for Efficient Electrochemical Ammonia Synthesis

Abstract Electrochemical synthesis of ammonia (NH3) in aqueous electrolyte has long been suffered from poor nitrogen (N2) supply owing to its low solubility and sluggish diffusion kinetics. Therefore, creating a N2 rich microenvironment around catalyst surface may potentially improve the efficiency of nitrogen reduction reaction (NRR). Herein, a delicately designed N2 filtering membrane consisted of polydimethylsiloxane is covered on catalyst surface via superspreading. Because this membrane let the dissolved N2 molecules be accessible to the catalyst but block excess water, the designed N2 rich microenvironment over catalyst leads to an optimized Faradaic efficiency of 39.4% and an NH3 yield rate of 109.2 µg h−1 mg−1, which is superior to those of the most report metal‐based catalysts for electrochemical NRR. This study offers alternative strategy for enhancing NRR performance.

solution was drop-added on a carbon paper with 1 x 1 cm 2 area.Then the PDMS membrane was covered on as-prepared working electrode in a homemade device.Typically, the working electrode was placed on a support in a petri dish, then certain amount of water was added to make the liquid surface 3 mm higher than the working electrode.Various volume of PDMS-hexane precursor was dropped on water surface.After curing for 3 days, water was removed and formed PDMS membrane was covered on the top side of WE.The procedure was repeated to cover the other side of the WE to get the final product (FeSA@NC-Px, where x stands for the volume of PDMS-hexane solution).

Material characterization:
The scanning electron microscopy (SEM) images and transmitting electron microscopy (TEM) images were collected using a field emission scanning electron microscope (Zeiss, Ultra 55, 15 kV) and a transmitting electron microscope (FEI, Tecnai G2 20 TWIN, 200 kV), respectively.An aberration-corrected EM-ARM300F (JEOL, 200 kV) was utilized to obtain the scanning transmission electron microscopy (STEM) images.Powder X-ray diffraction (XRD) was investigated using a D8 Advance diffractometer (Bruker) with Cu Kα radiation λ = 1.5406 A. Fourier Transform Infra-Red (FTIR) data was obtained on Nicolet IS50 spectrometer (Thermofisher Scientific).XPS spectra was collected by a scanning X-ray microprobe (ULVAC-PHI, PHI 5000 C and PHI 5300), using C 1 s (284.6 eV) as a reference.Contact angle experiments were measured by a TBU 95 contact goniometer (Dataphysics, German).Fe contents were measured by an Agilent 5110 inductively coupled plasma-optical emission spectrometer (ICP-OES).Fe K-edge analysis was performed with Si(111) crystal monochromators at the BL11B beamlines at the Shanghai Synchrotron Radiation Facility (SSRF) (Shanghai, China).The XAFS spectra were recorded at room temperature using a 4-channel Silicon Drift Detector (SDD) Bruker 5040.Negligible changes in the line-shape and peak position of both Fe K-edge XANES spectra were observed between two scans taken for a specific sample.The XAFS spectra of these standard samples (Fe foil, FePc, Fe2O3 and Fe3O4) were recorded in transmission mode.The spectra were processed and analyzed by the software codes Athena and Artemis.

Electrochemical measurement:
The electrochemical experiments were carried out in an Hcell using a three electrodes system in 0.1 M Na2SO4 with a WaveDrive2000 electrochemical workstation (Pine, United States).The H-cell was separated with a DuPont 117 proton exchange membrane where working electrode and AgCl reference electrode were placed in cathodic chamber and Pt counter electrode was placed in anodic chamber.
The N2 feed gas was purified with 0.1 M H2SO4 to remove any possible external NH3 followed with 0.1 M KOH to get rid of other N-containing pollutants and acid.At last, the molecular sieves were used to eliminate remaining impurities and H2O.The purified feed gas was purged into the cathodic chamber for 30 min before any electrochemical measurements and kept purging during the test.The linear sweep voltammetry curves were carried out with a scanning rate of 10 mV s -1 .Electrochemical impedance spectroscopy (EIS) was carried out with an AC amplitude of 10 mV and frequency range from 0.1 Hz to 1 MHz.Chronoamperometry plots were measured for 60 min.The following Nernst equation was used to convert the potentials to the reversible hydrogen electrode (RHE): A PHS-3C pH meter (Shanghai Leici Instrument Co., China) was used for the test of pH of the electrolytes.

Ammonia quantification:
The produced NH3 was determined with Indophenol blue method and 1 H NMR method.The indophenol blue method was as follow: 2 mL NH4Cl standard solution was mixed with 2 mL NaOH solution (1 M) containing salicylic acid (5 wt%) and sodium citrate (5 wt%).Next, 1 mL NaClO solution (0.05 M) and 0.2 mL sodium nitroferricyanide solution (1 wt%) were mixed with the above solution and aged for 1 h at room temperature.Then the adsorption of the solutions at 655 nm wavelength was measured with an UV-vis spectrophotometer (HATACHI, U4100).The obtained data was used to plot the calibration curve of absorbance-concentration.The produced NH3 in each NRR experiment was measured by the same procedure.For NMR method, the standard solution was prepared by mixing 0.5 mL NH4Cl as reference sample, 0.1 mL dimethyl sulfoxide-d6 (DMSO-d6) as solvent, 0.1 mL HCl (0.1 M) and certain amount of maleic acid as internal standard.As-prepared standards were measured by a Avance NEO 600 NMR spectrometer (Bruker).The concentration of NH4 + and peak ratio between NH4 + and maleic acid was plotted as calibration curve.The electrolytes after electrolysis were used to replace the standards with the same method to quantitatively determine NH3 produced.
The Faradaic efficiency was calculated using the following equation: where F is the Faraday constant (96485 C mol −1 ), CNH3 is the NH3 concentration (g L −1 ), V is the volume of electrolyte (L), and Q is the total charge (C).
The NH3 yield was obtained as follows: where t is the reduction time (h) and mcat. is the mass of catalyst loaded on working electrode (mg).
Isotope labelling experiment: 15 N2 was utilized to accurately track the N element during the electrochemical process.First, the 15 N2 gas was pre-treated with 1 M H2SO4 and 1 M KOH to remove any impurities or exterior NH3 pollutant.Then it was purged into the reaction chamber before electrolysis for 20 min and kept purging during test.The 1 H NMR of the electrolyte after reaction was carried out with the same procedure described in previous section. 15N2 was also replaced with 14 N2 for comparison.
NOx detection: NOx contaminates were identified via spectrophotometric method and Mass spectrometer (MS).For spectrophotometric method, 0.5 g sulfanilic acid and 5 mL acetic acid were dissolved in 90 mL DI water followed with the addition of 5 mg N-(-1-naphthyl)ethylenediamine dihydrochloride. [2]The color reagent was then prepared by mixing above mentioned solution with water by a volume ration of 1:4.The tested gas was first fed into an oxidation tube than passed through the color reagent for a period of time.The calibration curve was plotted using potassium nitrite as standard.For MS characterization, 50 L feeding gas was transferred using a syringe and analyzed via a GCMS-QP2020 NX (SHIMADZU).

N2H4 quantification:
The N2H4 was determined by The Watt and Chrisp method.Typically, 5.99 g of p-dimethylaminobenzaldehyde was dissolved in HCl (concentrated, 30 mL) and ethanol (300 mL) to get color reagent.Then 5 mL of test solution were mixed with the above solution.A UV-Vis spectrophotometer was used to measure the absorbance at 455 nm of the mixed solution after aging for 20 min at 25 o C. Certain amount of N2H4 was mixed with 0.1 M Na2SO4 as standards, then the absorbance of the standard solutions was measured and used for plotting the concentration-absorbance calibration curve.
NO2 -quantification: First, 0.2 g of N-(1-naphthyl) ethylenediamine dihydrochloride, 4 g of p-aminobenzenesulfonamide, and 10 mL of phosphoric acid (ρ = 1.685 g mL −1 ) was mixed in 50 mL deionized water and stored at 4 o C for further use.Then 5 mL of sample solution was combined with 0.1 mL of abovementioned solution and aged for 20 min at room temperature.The absorbance at 540 nm was spectrophotometrically measured and used for calibration curve.The electrolytes were further tested using the same procedure to quantitatively determine the NO2 -content.
In situ characterization: The in situ Raman characterization was carried out using a tailormade cell on HORIBA, XploRa spectrometer.Ag/AgCl reference electrode and Pt counter electrode were placed in anodic chamber which is separated to cathodic chamber containing working electrode by a Nafion 117 membrane.The electrochemical cell was filled with N2 saturated electrolyte before test.The spectrum was taken under -0.6 V vs RHE at different time.The in situ FTIR measurements were conducted using Attenuated Total Reflectance (ATR) model on Thermo Scientific, Nicolet iS50 spectrometer.The Si ATR crystal is placed on the side of a customized spectro-electrochemical cell.Asprepared catalyst was deposited on a polycrystalline Au film as working electrode, Ag/AgCl and Pt were used as reference electrode and counter electrode, respectively.All spectrums were obtained with 8 cm -1 resolution for 64 scans using the FTIR spectrometer.
Computational details: All spin-polarized density functional theories (DFT) were performed by using the Vienna Ab initio Simulation Package (VASP). [3]The Perdew-Burke-Ernzerhof generalized-gradient approximation functional was used to describe the interaction between electrons. [4]The energy cutoff was set to 400 eV.The calculation for pyridinic and pyrrolic structures were carried out using the 3x3x1 and 4x4x1 Gamma-Centered k-points grids.The vacuum region was set to be 15 Å in z direction to prevent the interaction between two adjacent surfaces.The energy criterion was set to 10 -5 eV.
The adsorption energies (ΔG) were obtained by Δ = Δ  + Δ − ΔS where ΔEDFT is the reaction energy calculated from DFT; ΔZPE is the zero-point energy; ΔS is the change in entropy. [5]e finite element simulation (FES) was carried out using COMSOL Multiphysics software coupled with the transport of the diluted species module.A cubic box with a diameter of 0.1 cm was constructed and was divided into two parts.In the initial FES model, the upper section with 0.08 cm length in the y direction is filled with H2O and N2 molecules based on the actual N2 solubility, and the lower section with 0.02 cm length is filled with pure H2O.Then, the simulations for PDMS and PDMS membrane are performed by using PDMS to replace pure water in the lower part of the box and using 100 nm PDMS membrane to separate the upper and lower part, respectively.FES runs of 300 s were conducted for the models, respectively.
Molecular dynamics (MD) simulations are performed with Gromacs version 2019.6. [6] the system with PDMS, two PDMS films are separated by 2 nm, and each film consists of 100 PDMS chains with a thickness of 1 nm.The outer region of the PDMS films is solvated by a liquid phase containing 10787 water molecules and 250 N2 molecules.In the control system, PDMS films are eliminated for comparison.Above model construction procedures are completed with PACKMOL software. [7]TIP3P model is adopted for water. [8]MS chain is modeled by the general amber force field. [9]N2 is represented by the TraPPE model. [10]Coulombic interactions are computed with the particle-mesh Ewald method.Van         Step meter data of (a) FeSA@NC-P20, (b) FeSA@NC-P30, (c) FeSA@NC-P40, (d) FeSA@NC-P50 and (e) FeSA@NC-P60.

der
Waals interactions are computed by the Lennard Jones (LJ) potential which is truncated at 1.2 nm, and LJ parameters between different atoms are determined by Lorentz-Berthelot mixing rules.Energy-minimization is conducted for initial configurations.Subsequently, 50-ns production runs are conducted at 298.15 K and 1 bar.The motion equations are integrated by leapfrog algorithm.A time step of 1 fs is used.Three-dimensional periodic boundary conditions are applied during all simulations.

Figure
Figure S4.(a) N 1s and (b) C 1s XPS spectrum of FeSA@NC and NC.

Figure S6 .
Figure S6.Calibration curve for determination of Fe content using ICP.

Figure S7 .
Figure S7.Experimental and FT-EXAFS fitting curves of FeSA@NC at Fe K-edge

Figure
Figure S12.(a) Faradaic efficiency and (b) corresponding NH3 yield rate of FeSA@NC in

Figure
Figure S13.LSV curves of FeSA@NC with various thickness of PDMS membrane in N2

Figure
Figure S15.(a) Fe K-edge XANES and (b) FT EXAFS of as-prepared catalyst after stability

Figure S16 .
Figure S16.XRD of as-prepared catalyst before and after stability test.

Figure
Figure S17.(a) Fe 2p, (b) N 1s and (c) C 1s XPS spectrum of as-prepared catalyst after

Figure
Figure S18.FTIR of as-prepared catalyst before and after long tern electrolysis.

Figure S22 .
Figure S22.Detection of (a) NOx and (b) NH3 contaminates in N2 feeding gas and

Figure
Figure S25.NO2 -detection before and after electrolysis.

Figure
Figure S30.(a) Calibration curve of H2 detection via GC.(b) Total Faradaic efficiency of

Figure S31 .
Figure S31.Optical images of contact angle of water over (a) glass, (b) glass with PDMS,

Figure S32 .
Figure S32.Nyquist plots of as-prepared samples and equivalent circuit.Rs and Rct

Figure
Figure S33.(a) Snapshots of N2 diffusion behavior in a box with pure water at the bottom.

Figure S34 .
Figure S34.N2 concentration along with y-direction of FES of (a) pure water at bottom,

Figure S35 .
Figure S35.The van der Waals' force among N2, H2O and PDMS during MD simulation

Figure S36 .
Figure S36.H2O distribution of MD simulation (a) with and (b) without PDMS membrane.

Table S1 .
Determination of Fe content in FeSA@NC.

Table S2 .
EXAFS fitting parameters at the Fe K-edge for various samples.

Table S6 .
Gibbs free energy (eV) for DFT calculations of NRR on FeN4 in FeSA@NC.