Interfacial engineering of Cu-Fe 2 O 3 nanotube arrays with built-in electric-field and oxygen vacancies effects for boosting electrocatalytic reduction of nitrate

and the mixed solution was placed in the dark without disturbance for 24 h, finally the liquid supernatant was transferred into a Teflon bottle refrigerated for use. For colorimetric assay, a certain amount of electrolyte was taken out from electrolytic cell and diluted to 5 mL to detection range. Next, 0.1 mL potassium sodium tartrate solution (ρ = 500 g L -1 ) was added and mixed thoroughly, then 0.1 mL Nessler’s reagent was put into the solution. The absorption intensity at wavelength of 420 nm was recorded after sitting for 20 min. The concentration-absorbance curve was made using a series of standard ammonium chloride solutions from 0 to 2.00 ppm and the ammonium chloride crystal was dried at 105 °C for 2 h in advance. the electrolyte, and a the electrolytic cell the target reactant. All the electrochemical measurements performed using CHI 660E electrochemical workstation The potential is recorded under a

electrolytic cell and diluted to 5 mL to detection range. Next, 0.1 mL color reagent was added into the aforementioned 5 mL solution and mixed uniformity, and the absorption intensity at a wavelength of 540 nm was recorded after sitting for 20 min. The concentration-absorbance curve was calibrated using a series of standard sodium nitrite solutions.

Detection of ammonium-N:
The Nessler's reagent was prepared by dissolving 0.35 g KI, 0.5 g HgI 2 in 5 mL 4.0 M NaOH solution successively and then the mixed solution was placed in the dark without disturbance for 24 h, finally the liquid supernatant was transferred into a Teflon bottle refrigerated for use. For colorimetric assay, a certain amount of electrolyte was taken out from electrolytic cell and diluted to 5 mL to detection range. Next, 0.1 mL potassium sodium tartrate solution (ρ = 500 g L -1 ) was added and mixed thoroughly, then 0.1 mL Nessler's reagent was put into the solution. The absorption intensity at wavelength of 420 nm was recorded after sitting for 20 min. The concentration-absorbance curve was made using a series of standard ammonium chloride solutions from 0 to 2.00 ppm and the ammonium chloride crystal was dried at 105 °C for 2 h in advance.

Text S2. Materials characterization
Morphology of the catalysts were characterized using scanning electron microscopy (SEM, Zeiss Supra 40) and transmission electron microscopy (TEM, JEOL JEM-2010) with an energy-dispersive spectroscopy (EDX) for elemental mapping. X-ray Diffraction (XRD, MAP18AHF) was performed to determine the crystal structure of the samples. The chemistry composition was characterized by X-ray photoelectron spectroscopy (XPS, ESCA Lab MKII). The surface potential images were measured using Kelvin probe force microscopy techniques (AFM5500M, HITACHI) under an ambient atmosphere. By EMX nano electron paramagnetic resonance (EPR) spectrometer, the concentration of OVs were detected. The ultraviolet-visible (UV-Vis) absorbance spectra were measured on Shimadzu UV-3900 spectrophotometer. The isotope labeling experiments were measured by 1 H NMR measurement (JNM-ECZ600R). The reaction intermediate information was studies by In-situ Raman spectroscopy (in Via-Reflex).
The electrochemical nitrate reduction reaction experiments were carried out using a standard three-electrode system in a single-chamber electrolytic cell. The catalyst loaded on CF, saturated calomel electrode (SCE), and platinum foil were used as the working electrode, reference electrode, and counter electrode, respectively. 0.5 M Na 2 SO 4 solution was used as the electrolyte, and a certain concentration of NaNO 3 was added to the electrolytic cell as the target reactant. All the electrochemical measurements were performed using CHI 660E electrochemical workstation (CHI 660E, Chenhua, Shanghai). The potential is recorded under a standard hydrogen electrode, and the conversion formula is E(RHE)=E(SCE)+0.0591pH+0.2438. Before conducting the nitrate electroreduction test, the linear sweep voltammetry was performed to make the polarization curve reach a steady state. A constant potential test was carried out at different potentials for 2 hours.
Text S4. Pilot-Scale Tests of Electrochemical Nitrate Reduction.
The industrial electrocatalytic nitrate reduction experiment was carried out using a two-electrode system in a pilot-scale reactor. The catalyst loaded on CF and titanium plate were used as the working electrode and counter electrode, respectively. 0.5 M Na 2 SO 4 solution was used as the electrolyte, and a certain concentration of NaNO 3 was added to the electrolytic cell as the target reactant. The size of the reactor is 77×59×40cm, and it can process 180L of wastewater. In addition, in order to enhance the catalytic efficiency, the catalyst is connected in series into an electrode group. The size of all electrodes is maintained at 20 × 30 cm。A constant potential test was carried out at different potentials for 4 hours.

Text S5. N isotope labeling experiments
The N isotopic labeling experiments were carried out using the aforementioned electrochemical nitrate reduction methods in the electrolyte (50 ppm NO 3 --N) with Na 15 NO 3 and Na 14 NO 3 as N source, respectively. The amount of produced 15 NH 4 + and 14 NH 4 + was quantified by the 1 H-Nuclear Magnetic Resonance (NMR) spectroscopy. For quantitative, we prepared a series of standard solutions and plotted the standard curve. First, a series of 15 NH 4 + solutions with known concentration were prepared in 0.5 M Na 2 SO 4 as standards; Second, 50 mL of the 15 NH 4 + and standard solution with different concentration was mixed with 50 ppm maleic acid; Third, 50 μL deuterium oxide (D 2 O) was added in 0.5 mL above mixed solution for the NMR detection; Fourth, the calibration was achieved using the peak area ratio between 15 NH 4 + and maleic acid because the 15 NH 4 + concentration and the area ratio were positively correlated. Similarly, the amount of 14 NH 4 + was quantified by this method when Na 14 NO 3 was used as the feeding N-source.

Text S6. In situ Raman characterization
To detect the intermediates of nitrate reduction reaction, a Raman electrochemical cell with Pt wire and an Ag/AgCl electrode were used as the counter and the reference electrodes, respectively, for in situ Raman measurements. Raman spectroscopy was performed on a Laser Micro-Raman spectrometer at room temperature with an Ar + laser of 514.5 nm excitation. A proper electrochemical cell was selected to fit the Raman spectrometer to perform the in-situ Raman test. Laser beams focus on the sample through a hole in the middle of the cell to collect Raman information. To study the intermediates on the surface of Cu-Fe 2 O 3 nanotubes in the nitrate reduction process, the amperometry i-t curve (i-t) test method was employed to apply different voltages to the electrode. In addition, the spectra were obtained by applying single potential steps of 0.1 V from 0 to -1.0 V vs. RHE.
Text S7. Calculation of the conversion, yield, selectivity, and Faradaic efficiency.
The NO 3conversion rate was calculated as follows: The selectivity of the product can be calculated by: The yield of NH 4 + (aq) was calculated using equation: The Faradaic efficiency was calculated as follows: where C NH4+ is the concentration of NH 4 + (aq) , C NO2-is the concentration of NO 2 -(aq) , ∆C NO3-is the concentration difference of NO 3before and after electrolysis, C 0 is the initial concentration of NO 3 -, V is the electrolyte volume, t is the electrolysis time, m is the mass of catalyst, F is the Faradaic constant (96485 C mol -1 ), Q is the total charge passing the electrode.

Text S8. Theoretical Simulation
Density functional theory (DFT) calculations were performed as implemented in the plane wave set Vienna ab initio Simulation Package (VASP) code 7 . Generalized gradient approximation (GGA) with the exchange-correlation functional in the Perdew-Burke-Ernzerhof (PBE) form was adopted 8 . DFT + U method was used to better describe the on-site coulomb (U) correlation of the localized 3d electrons for transition metal Cu with U -J = 3.42 eV 9 . Spin polarization was considered for all calculations. The kinetic-energy cut off was set as 500 eV. The convergence threshold of 10 -4 eV was set for self-consistent field (SCF) iteration between two electronic steps. Conjugate gradient method was adopted for geometry optimization with forces on each atom less where * represents the active site. Then, the reaction free energy change can be obtained with the equation below: where E DFT   Scalability. The size of the materials can be flexibly controlled, which can not only meet the experimental level, but also be appropriately enlarged to meet the industrial level.