Materials
All the chemicals were of analytical grade and were used as received without further purification. Deionized water (DIW) was used in all experiments.
Material characterization
Scanning electron microscopy (SEM) images were taken with a Hitachi S − 4800 scanning electron microscope. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM − 200 microscope. The X − ray diffraction (XRD) patterns were recorded with a Bruker D8 Focus Diffraction System by using a Cu Kα radiation source (λ = 0.154178 nm). X − ray photoelectron spectroscopy (XPS) data were collected on a Thermo Fisher Scientific K − Alpha instrument using monochromatic Al Kα (hv = 1486.6 eV) radiation. All the spectra were collected at a vacuum pressure of < 2 × 10− 7 Pa, and the cumulative scanning number of each sample was 20. The sample was prepared and transferred into an Ar environment to avoid oxidation. All binding energies were referenced to the C 1s peak at 284.6 eV.32 An inductively coupled plasma optical emission spectrometer (ICP‒OES) was used with an Agilent 7700x instrument. The ultraviolet‒visible (UV‒Vis) absorbance spectra were measured on a Beijing Purkinje General T6 New Century spectrophotometer. Fourier transform infrared (FTIR) spectroscopy was carried out with a MAGNA1IR 750 (Nicolet Instrument) FTIR spectrometer. Nuclear magnetic resonance (NMR) spectra were recorded on Varian Mercury Plus instruments at 600 MHz (1H NMR). The pH values of the electrolytes were determined using a pH meter (LE438 pH electrode, Mettler Toledo, USA). The Raman spectra were obtained on a Renishaw inVia reflex Raman microscope under excitation with 532 nm laser light with a power of 20 mW.
Preparation of Ru-Co gel. Taking Ru9Co91 as an example, 790 mg of Co(NO3)2·6H2O, 54 mg of RuCl3, 450 mg of glycine and 1.89 g of citric acid were added to 20 mL of H2O under continuous stirring. After 10 min at room temperature, 2 mL of ammonia solution (25 ~ 28 wt%) was added to the above solutions. The mixed solutions were subsequently transferred to a 5×5 cm2 porcelain boat and placed in an 80°C oven. After 12 h, the Ru9Co91 gel was obtained. The methods used for other gels with different Co/Ru ratios were similar to those used for the Ru9Co91 gel. The special mole fractions of Ru and Co in RuxCoy were adjusted by changing the addition amounts of RuCl3 and Co(NO3)2·6H2O, and the total number of moles of Ru and Co was kept unchanged.
Preparation of RuxCoyOz. The RuxCoy gel was put into a muffle furnace and heated at 600°C for 8 hours (heating rate of 10°C per minute), after which the organic molecules in RuxCoyOz were fully removed.
Preparation of RuxCoy catalysts. RuxCoyOz samples were annealed at 600°C for 2 h in 3% H2/Ar (hydrogen content: 3%) to produce RuxCoy, and the heating rate was 2°C min−1.
Plasma catalysis measurements
Plasma-driven nitrogen fixation to produce NOx gas was carried out by means of a custom quartz reactor. A schematic diagram of the device is shown in Fig. 2b. Its length is 24 cm, and its inner and outer diameters are 2.8 and 3 cm, respectively. An arc was generated between the copper mesh wrapped outside the quartz tube and the stainless steel rod penetrating the inside of the quartz tube. Among them, stainless steel (36 cm) served as a high-voltage electrode, and copper mesh (10 cm) served as a high-voltage electrode. The alternative catalyst (50 mg) was filled between the stainless steel and copper mesh inside the quartz tube. To prevent powder leakage, a small amount of quartz wool can be filled as the catalyst horder. Before running this instrument, air gas was introduced at a rate of 20 mL min− 1 for 20 minutes to stabilize the gas flow rate. The working voltage and current are 50 V and 0.5 A, respectively, which can be controlled by a power unit (CTP-2000K). The product NOx can be quantified using online mass spectrometry or gas chromatography.
Electrochemical NOxRR measurements
Electrochemical NOxRR measurements were performed with an Ivium − n−Stat electrochemical workstation (Ivium Technologies B.V.). A typical three − electrode H − cell was used, which included a working electrode, a Ag/AgCl electrode (saturated KCl solution) as the reference electrode, and a carbon rod counter electrode in 0.1 M PBS electrolyte (the pH of the solution was 7 unless otherwise stated); these electrodes were separated into a cathode cell (50 mL) and an anode cell (50 mL) by a membrane. For catalytic potential, we did not use iR correction, except when special instructions were used. For the chronoamperometry test, carbon paper (0.5×0.5 cm2) decorated with 0.15 mg of catalyst was used as the working electrode. In a typical procedure, an evenly distributed catalyst suspension was prepared by ultrasonically mixing 20 mg of catalyst into 8 mL of H2O, 2 mL of isopropyl alcohol, and 50 µL of Nafion. The 75 µL suspension was covered on the carbon paper surface (0.25 cm2). The current density was normalized by the catalyst mass, the geometric area of the electrode, and the electrochemical surface area (ECSA). All the electrochemical data (except for the stability test data) were repeated more than 3 times, and the error bars represent the standard deviation of the data. All potentials were calibrated to the standard hydrogen electrode (SHE) by the following equation:
$${E}_{\text{S}\text{H}\text{E}}={E}_{\text{A}\text{g}/\text{A}\text{g}\text{C}\text{l}}+{\phi }_{\text{r}\text{e}\text{f}\text{e}\text{r}\text{e}\text{n}\text{c}\text{e}}$$
where EAg/AgCl represents the experimental applied potential.
For the ECSA, we used the double − layer capacitance method in 0.1 M PBS solution in the non − Faradaic potential range with different scan rates ranging from 10 to 100 mV s− 1. The ECSA of the working electrodes was calculated according to the following equation:
$${I}_{\text{c}}=\upsilon {C}_{\text{d}\text{l}}$$
$$ECSA=\frac{{C}_{\text{d}\text{l}}}{{C}_{\text{s}}}$$
where Ic represents the charging current at different scan rates. ν is the scan rate. Cdl is the double − layer capacitance. Cs represents the specific capacitance for a flat metallic surface, which is generally in the range of 20 − 60 µF cm− 2. As shown in the above formula, Cdl and the ECSA have a linear relationship. Therefore, the multiple relationships of the ECSA for different catalysts are consistent with those of Cdl.
The current density conversion formula from mass − normalized to geometric area − normalized is:
$${j}_{\text{g}\text{e}\text{o}.}=\frac{{j}_{\text{m}\text{a}\text{s}\text{s}}\times m}{S}$$
where jgeo. represents the current density normalized by the geometric area, and jmass represents the current density normalized by mass. m is the mass of the supported catalyst (0.15 mg). S is the carbon paper geometric area of the supported catalyst (0.25 cm− 2).
The ammonia Faradaic efficiency was calculated according to the following equation:
$${\text{F}\text{E}}_{\text{N}\text{H}3}=\frac{{Q}_{\text{N}\text{H}4+}}{Q}=\frac{{n}_{\text{N}\text{H}4+}V{c}_{\text{N}\text{H}4+}F}{Q}$$
where Q represents the applied overall coulomb quantity (C). QNH4+ represents the coulomb required to produce ammonia. n is the electron transfer number for 1 mol of ammonia. V is the volume of the catholyte in the cathode chamber, which is 25 mL. CNH4+ is the concentration of ammonia produced. F is the Faraday constant (96,485 C∙mol− 1).
For the single-cell measurements (as shown in Fig. 4a), we used chronopotentiometry (90 mA) to demonstrate the industrial application potential of the proposed device. The concentration of electrolyte in a single chamber cell was 30 mM NH4NO3 solution. The rate of injected water and outflow solution was 9 mL h− 1.
Electrochemical pulse method tests were carried out in a three − electrode H-cell. The step E1 was − 1.3 V vs. SHE, and step E2 was the open-circuit potential with a step-time of 0.1 s. The number of cycles was related to the running time of step-E1. The cumulative time of step-E1 for different controlled experiments was 30 min.
Preparation of PBS solution
A buffer solution with a pH of 7 was obtained by mixing two solutions of 0.1 M K2HPO4 and 0.1 M KH2PO4, and the volume ratio was approximately 60 to 40. A pH meter can be used for monitoring during the mixing process, and the specific volume is subject to the pH meter results. The preparation of PBS at pH 5 or 3 was the same as above, except that 0.1 M KH2PO4 was replaced with 0.1 M H3PO4.
Electrochemical in situ Raman tests
The in situ Raman measurements were carried out by the aforementioned Raman microscope and electrochemical workstation. The cell was made up of Teflon with a quartz window between the sample and the objective. The working electrode was immersed in the electrolyte through the wall of the cell, and the electrode plane was kept perpendicular to the laser. A platinum wire and Ag/AgCl served as the counter and reference electrodes, respectively. LSV curves were obtained from − 0.3 to − 1.5 V vs. SHE with a scan rate of 2 mV s− 1. Electrochemical intermittent in situ Raman measurements were carried out in 0.1 M PBS (pH = 7) solution at − 1.3 V vs. SHE. After the first Raman spectrum was collected, 0.5 mL of 0.067 M KNO3 or 0.033 M KNO2 solution was added to the electrolyte.
Electrochemical in situ ATR − FTIR tests
Electrochemical in situ ATR − FTIR measurements were performed on a Linglu Instruments ECIR − II cell mounted on a Pike Veemax III ATR with a single bounce silicon crystal covered with an Au membrane in internal reflection mode. Spectra were recorded on a Thermo Nicolet Nexus 670 spectrometer. The electrolyte was degassed by bubbling N2 for 30 min before the measurement. A single − bounce silicon crystal covered with a Au membrane was prepared through the following procedure. (1) NaOH (0.12 g), NaAuCl4·2H2O (0.23 g), NH4Cl (0.13 g), Na2SO3 (0.95 g), and Na2S2O3·5H2O (0.62 g) were dissolved in H2O (100 mL) (denoted Solution A). (2) Monocrystalline silicon was immersed in aqua regia (Vconcentrated HCl:VHNO3 = 1:1) for 20 min and then polished using Al powder for 10 min. After washing three times with water and acetone, clean monocrystal silicon was obtained. (3) The above monocrystalline silicon was immersed in a mixture of H2SO4 and H2O2 (Vconcentrated H2SO4:VH2O2 = 1:1) for 20 min. (4) After washing three times with water, the above monocrystalline silicon was immersed in 40% NH4F aqueous solution and washed three times with water. (5) Monocrystalline silicon was immersed in a mixture of 15 mL of solution A and 3.4 mL of 2% NH4F aqueous solution. (6) After 5 min, Au − coated monocrystal silicon was obtained.
Electrochemical online DEMS test
First, 1.2% NOx was injected into the electrolyte (0.1 M PBS + 0.03 M KNO2 + 0.06 M KNO3) for 20 minutes. Then, this electrolyte flowed into a custom-made electrochemical cell through a peristaltic pump. Glassy carbon electrodes coated with the Ru9Co91 catalyst, Pt wire, and Ag/AgCl electrodes were used as the working electrode, counter electrode, and reference electrode, respectively. Then, an applied voltage (− 1.3 V vs. SHE) was applied alternately. After the electrochemical test was complete and the mass signal returned to baseline, the next cycle started using the same conditions to avoid accidental error. After six cycles, the experiment ended.
Determination and quantitation of ammonia using UV − vis spectroscopy
We used the Nessler’s reagent method to determine the concentration of ammonia after chronoamperometry measurements at different potentials. A certain amount of electrolyte was removed from the electrolytic cell and diluted to 5 mL to reach the detection range. Then, 0.1 mL of potassium sodium tartrate solution (500 g/L potassium sodium tartrate) and 0.1 mL of Nessler’s reagent solution (including 160 g/L NaOH, 70 g/L KI and 100 g/L HgI2) were added to the aforementioned solution. After 20 min, the absorption spectrum was tested using an ultraviolet − visible spectrophotometer, and the absorption intensities at a wavelength of 420 nm were recorded. The concentration − absorbance curve was calibrated using a series of standard ammonium chloride solutions (0, 0.02, 0.04, 0.06, 0.08, 0.1 mM), and the ammonium chloride crystals were dried at 105 ~ 110°C for 2 h in advance.
Determination and quantitation of ammonia using 1H NMR
After the electrochemical chronoamperometry measurements, we first adjusted the pH of the electrolyte to neutral (pH = 7) and then added maleic acid (for a final mixed solution concentration of 0.4 mg/mL). Subsequently, 0.5 mL of mixed solution, 50 µL of H2SO4 (4 M), and 50 µL of DMSO − d6 were transferred to an NMR tube. The concentration − peak area curve from the NMR spectrum was calibrated using a series of standard ammonium chloride solutions (5, 10, 15, 20, 25 mM), and the ammonium chloride crystals were dried at 105 ~ 110°C for 2 h in advance.
Determination and quantitation of nitrite using UV‒vis spectroscopy
A certain amount of electrolyte was removed from the electrolytic cell and diluted to 5 mL, which was within the detection range. Then, 0.1 mL of nitrite color reagent was added to the aforementioned solution. After 20 min, the absorption spectrum was tested using an ultraviolet‒visible spectrophotometer, and the absorption intensities at a wavelength of 540 nm were recorded. The nitrite color reagent consisted of 40 g L−1 p − aminobenzenesulfonamide, 100 mL/L phosphoric acid, and 2 g L−1 N−(1 − naphthyl) − ethylenediamine dihydrochloride. The concentration − absorbance curve was calibrated using a series of standard potassium nitrite solutions (0, 0.005, 0.01, 0.015, 0.02, 0.03 mM), and the potassium nitrite crystal was dried at 105 ~ 110°C for 2 h in advance.
Determination and quantitation of nitrate using UV‒vis spectroscopy
A certain amount of electrolyte was removed from the electrolytic cell and diluted to 5 mL, which was within the detection range. Then, 0.1 mL of 1 M HCl and 0.01 mL of 0.8 wt% sulfamic acid solution were added to the aforementioned solution. The absorption spectrum was tested using an ultraviolet‒visible spectrophotometer, and the absorption intensities at wavelengths of 220 and 275 nm were recorded. The final absorbance value was calculated by the following equation:
A = A220nm – 2A275nm
The concentration‒absorbance curve was calibrated using a series of standard potassium nitrate solutions, and the potassium nitrate crystals were dried at 105–110°C for 2 h in advance.