Ag‐Co3O4‐CoOOH‐Nanowires Tandem Catalyst for Efficient Electrocatalytic Conversion of Nitrate to Ammonia at Low Overpotential via Triple Reactions

Abstract The electrocatalytic conversion of nitrate (NO3‾) to NH3 (NO3RR) offers a promising alternative to the Haber–Bosch process. However, the overall kinetic rate of NO3RR is plagued by the complex proton‐assisted multiple‐electron transfer process. Herein, Ag/Co3O4/CoOOH nanowires (i‐Ag/Co3O4 NWs) tandem catalyst is designed to optimize the kinetic rate of intermediate reaction for NO3RR simultaneously. The authors proved that NO3‾ ions are reduced to NO2‾ preferentially on Ag phases and then NO2‾ to NO on Co3O4 phases. The CoOOH phases catalyze NO reduction to NH3 via NH2OH intermediate. This unique catalyst efficiently converts NO3‾ to NH3 through a triple reaction with a high Faradaic efficiency (FE) of 94.3% and a high NH3 yield rate of 253.7 μmol h−1 cm−2 in 1 M KOH and 0.1 M KNO3 solution at ‒0.25 V versus RHE. The kinetic studies demonstrate that converting NH2OH into NH3 is the rate‐determining step (RDS) with an energy barrier of 0.151 eV over i‐Ag/Co3O4 NWs. Further applying i‐Ag/Co3O4 NWs as the cathode material, a novel Zn‐nitrate battery exhibits a power density of 2.56 mW cm−2 and an FE of 91.4% for NH3 production.


Synthesis of i-Ag/Co3O4 nanowires (Ag/Co3O4/CoOOH NWs).
The carbon paper (CP) loaded with Ag/Co3O4 NWs was used as the working electrode in a typical three-electrode system.Ag/AgCl (saturated KCl) and a platinum mesh were employed as the reference and counter electrodes, respectively.The carbon paper loaded with Ag/Co3O4 NWs was polarized by cyclic voltammetry (CV) range from 0.05 to 2.05 V versus RHE in an Ar-saturated 1 M KOH solution for 4 cycles, followed by gentle rinsed with water and acetone and drying in Ar flow.

Synthesis of Co(OH)2 NSs and CoOOH NSs.
The electrochemical deposition was carried out by galvanostatic electrolysis in a twoelectrode cell. [2]A carbon paper (2 cm × 1 cm) and a graphite electrode (1.8 cm 2 , spectral grade) were used as the working and counter electrodes.Co(OH)2 NSs was electrodeposited on carbon paper in 15 mL of 0.02 M Co(NO3)2 + 0.1 M NH4Cl solution at -10 mA cm -2 for 20 min.And CoOOH NSs was fabricated by in situ anodic oxidation of Co(OH)2 NSs in a solution of 0.01 M (NH4)2SO4 at 2 mA cm -2 for 30 min.

Material characterization.
SEM was performed using a Nova Nano SEM 200 (FEI, USA) scanning electron microscope.TEM images were carried out on an FEI Tecnai G2F 20 TEM system using copper grids (JEOL, Japan).Raman spectra were measured from a Renishaw in Via Raman spectrometer (Renishaw, UK).A 50× long-working distance objective (NA, 0.5) was used to focus the laser beam onto the sample and to collect the Raman signals in the backscattering mode.The 785 nm line from an argon ion laser, with a power of 50 mW was used as the excitation source.Fourier-transform infrared (FT-IR) spectra were measured on an IRTracer-100 spectrometer (Shimadzu, Japan).UV-visible absorption spectra were recorded on a UV-1800 UV-visible spectrophotometer (Shimadzu, Japan).XPS was recorded using an ultrahigh-vacuum setup (SES 2002, Gammadata-Scienta) equipped with a monochromatic Al Kα X-ray source (15 kV, 10 mA emission current).
The binding energies were calibrated based on the C 1s feature at 284.8 eV.The nuclear magnetic resonance (NMR) spectroscopy was performed on an AVANCE III AV500 spectrometer.

Electrochemical tests.
The electrocatalytic tests were performed using a typical three-electrode system connected to the CHI 660E electrochemical workstation (CHI Instrument, China) in a typical H-type cell.The catalysts supported by carbon paper, Ag/AgCl (saturated KCl), and platinum mesh were used as the working electrode, reference, and counter electrodes, respectively.The electrolytes were Ar-saturated 1 M KOH containing different NO3 -or NO2 -concentrations.The LSV curves were collected at a scan rate of 10 mV•s -1 .Tafel slopes were extracted from near static LSV.All potentials were calibrated to the RHE reference scale using ERHE = EAg/AgCl + 0.204 V + 0.0591 × pH.
The current density was normalized to the geometric electrode area (~1 cm 2 ).
Potentiostatic measurements were performed for 1 h in 30 mL cathode electrolyte, and then the electrolyte was stored at 4 °C (no more than 2 days) before analysis.To evaluate the long-term stability of i-Ag/Co3O4 NWs for NO3RR, the electrolyte solution was collected and analyzed for NH3 production after every hour of electrolysis.To maintain consistency, a fresh electrolyte solution was used for each cycle of one-hour electrolysis.The Cdl was determined by CV scanning in a non-faradaic potential window at different scan rates (10-70 mV•s -1 ).The plot of the capacitive anode and cathode current differences [(ja -jc)/2] at a set potential against the CV scan rates shows a linear relationship, and the slope is Cdl.Electrochemical impedance spectroscopy (EIS) tests were performed using an Autolab potentiostat (Metrohm, Switzerland).EIS was performed at different applied potentials versus RHE in the 10 -2 -10 5 Hz frequency range.

Assembly of the zinc-nitrate battery and electrochemical test.
The CP-supported i-Ag/Co3O4 NWs (1×1 cm 2 ) were employed as the cathode and the Zn plate (1.5×2 cm 2 ) was used as the anode for the zinc-nitrate battery.A typical H-type cell that contains 25-mL cathode electrolyte (1 M KOH + 0.1 M KNO3) and 25-mL anode electrolyte (1 M KOH) separated by a bipolar membrane.The discharging polarization curves with a scan rate of 10 mV•s -1 and galvanostatic tests were conducted using CHI 660E workstation and Neware test system at room temperature, respectively.
After the electrochemical test, the electrolyte was diluted for subsequent detection.
The power density (P) of zinc-nitrate battery was determined by P = I ×V, where I and V are the discharge current density and voltage, respectively.

Determination of ion concentrations. NH4 + quantification.
The produced NH3 was quantitatively determined by the indophenol blue method. [3,4]pically, a certain amount of electrolyte was removed from the reaction cell and diluted to 2 mL.Then, 2 mL of 1 M NaOH solution containing citrate dihydrate (5 wt%) and salicylic acid (5 wt%) (stored at 4 °C) and 1 mL of freshly prepared 0.05 M NaClO were added.The resulting mixture was then briefly shaken to ensure proper mixing of the components.Finally, 0.2 mL of 1 wt% sodium nitroferricyanide solution (stored at 4 °C) was added for the color reaction.Following a 2-hour incubation period at room temperature, the resulting solution was measured using an ultraviolet-visible (UV-Vis) spectrophotometer.The absorbance at 655 nm was used to determine the concentration of NH3.To quantify the amount of NH3, a calibration curve was built using a standard NH4Cl solution in 1 M KOH.

NO2 − quantification.
A specific colour reagent for NO2 − quantification was prepared by mixing 0.08 g of N- (1-naphthyl) ethylenediamine dihydrochloride, 1.6 g of sulfonamide and 4 mL of phosphoric acid (85 wt%, ρ =1.7 g/mL) with 20 mL of deionized water. [5,6]In a typical colorimetric test, 1 mL HCl (1 M) was firstly added into the 5 mL of diluted postelectrolysis electrolytes, and then 0.1 mL of colour reagent was added and shaken to obtain a uniform solution.The UV-Vis absorbance at 540 nm was recorded after 30 min at room temperature.The amount of NO2 − was determined using a calibration curve of NaNO2 solutions.

Calculation of the NH3 yield rate and Faradaic efficiency.
The FE was defined as the charge consumed for forming a specific product (e.g.NH3) divided by the total charge passing through the electrodes (Q) during electrolysis.Given that eight electrons are consumed to produce one NH3 molecule, FENH3 and NH3 yield rate (YNH3) was calculated according to the following equation: where F is the Faraday constant (96485 C mol -1 ), Q is the total charge passing the electrode, cNH3 is the molar concentration of detected NH3, VNH3 is the volume of the electrolytes (30 mL), A is the electrode geometric area (1 cm 2 ), and t is the reaction time.
Given that two electrons are consumed to produce one NO2 − molecule, the FE of NO2 − can be calculated as follows: where CNO2− is the molar concentration of detected NO2 − .
The reaction apparent activation energy tests. [7] extract the apparent activation energy (Ea) for the NO3RR, the electrochemical measurements of the catalysts were conducted in 1 M KOH solution containing 0.1 M KNO3 at different temperatures.For heterogeneous electrocatalytic reactions, the current density can be expressed from Ea according to the following Arrhenius equations. [8]= A a exp(- where Aa is the apparent pre-exponential factor, R is the ideal gas constant (8.314J•K - 1 •mol -1 ), T is the temperature in Kelvin (K).Therefore, Ea can be further calculated by fitting the slope of the Arrhenius plot the following equations. [9]∂(log 10 j) while the intercept of log10 j vs. 1/T plot is the logarithm of Aa.

K 15 NO3 isotope labeling experiments.
The isotope labeling experiment was carried out in 1 M KOH solution containing 0.1 M K 15 NO3 (98% 15 N atom) by chronoamperometry measurements for 1 h at -0.25 V (vs.RHE).Briefly, the pH of the processed electrolyte was adjusted to 3 with a 4 M H2SO4 solution. [10]Then, 400 μL of electrolyte and 150 μL of deuterium oxide (D2O) were added into the NMR tube, and 15 NH4 + in the electrolyte was detected using 1 H S7 NMR(500 MHz). [11] situ FTIR spectroscopy.
FTIR measurements were performed with an IRTracer-100 spectrometer.The electrochemical cell was assembled on top of a CaF2 prism, and the electrode was situated against this prism to form a thin layer.The measurements were performed under external reflection (Figure S18).The electrochemical cell was assembled by a three-electrode configuration with a counter electrode of Pt wire, a reference electrode of Ag/AgCl (saturated KCl), and a working electrode prepared by dropping 20 μL ink of electrocatalyst on an Au sheet.Electrolyte was Ar-saturated 1 M KOH with 0.1M NO3 -.FTIR spectra were obtained from an average of 512 scans with a resolution of 8 cm -1 at the selected potentials.The potentiostatic model is adopted, and the potentials are scanned from 0.35 V to -0.45 V (vs.RHE) compared to the reference potential (0.45 V vs. RHE).The spectra were reported as -lg(R/R0), where R is the reflectance at a set potential, and R0 is the reflectance at the reference potential.Thereby the ratio gives positive bands for species formation at the sample potential, and negative bands correspond to the loss of species at the sample potential.

In situ Raman spectroscopy.
Raman spectroscopy was performed with a Renishaw in Via Raman spectrometer (Renishaw, UK) equipped with a 785 nm laser as the excitation source, a 50× objective, a monochromator (1200 grooves/mm grating).The in situ Raman spectra were collected under controlled potentials.The electrolytic cell was homemade by Teflon with a piece of round quartz glass as a cover to protect the objective (Figure S23).The Au electrode modified with the catalyst was used as the working electrode, Pt wire and Ag/AgCl electrode as the counter and reference electrodes, respectively.The surface of the working electrode was positioned 100-200 μm from the glass window.Each spectrum is an average of three continuously acquired spectra with a collection time of 50 s each.

Figure S1 .
Figure S1.(a) SEM images of Ag NWs.(b) Statistic length distribution of Ag NWs

Figure S2 .
Figure S2.XRD patterns of the as-synthesized catalysts.

Figure S6 . 1 S11Figure S7 .
Figure S6.Tafel slopes of the catalysts for HER in 1 M KOH.The LSV curves were

Figure S9 .
Figure S9.NH3 synthesis performance of Ag NWs at a series of potentials.(a)

Figure S10 .
Figure S10.NH3 synthesis performance of Ag/Co3O4 NWs at a series of potentials.

Figure S11 .
Figure S11.NH3 synthesis performance of i-Ag/Co3O4 NWs at a series of potentials.

Figure 12 .
Figure 12.Comparison of NH3 Faradaic efficiency and NH3 yield rate of i-Ag/Co3O4

Figure S13 .
Figure S13.Comparison of the NH3 yield rate and FE on the i-Ag/Co3O4 NWs catalysts

Figure S14 .Figure S15 .
Figure S14.(a) The cycling tests of the i-Ag/Co3O4 NWs for the reduction tests at −0.25

Figure S18 .Figure S19 .
Figure S18.Schematic diagram of the in situ FTIR spectra device.

Figure S20 .
Figure S20.The logarithm of the catalytic current density plotted against 1000 times

Figure S21 .
Figure S21.(a) Electrochemical in situ FTIR spectra of CoOOH NSs at different

Figure S22 .
Figure S22.The equivalent circuit for modeling the measured electrochemical response.

Figure S23 .
Figure S23.Schematic diagram of the homemade in situ Raman device.

Figure S24 .
Figure S24.(a) In situ Raman spectra of Ag NWs at different applied potentials in

Figure S25 .
Figure S25.P roposed mechanism for the NO hydrogenation step catalyzed by

Table S1 .
Comparison of NO3RR activity for i-Ag/Co3O4 NWs with other reported electrocatalysts.