Efficient C–N coupling for urea electrosynthesis on defective Co3O4 with dual-functional sites

Urea electrosynthesis under ambient conditions is emerging as a promising alternative to conventional synthetic protocols. However, the weak binding of reactants/intermediates on the catalyst surface induces multiple competing pathways, hindering efficient urea production. Herein, we report the synthesis of defective Co3O4 catalysts that integrate dual-functional sites for urea production from CO2 and nitrite. Regulating the reactant adsorption capacity on defective Co3O4 catalysts can efficiently control the competing reaction pathways. The urea yield rate of 3361 mg h−1 gcat−1 was achieved with a corresponding faradaic efficiency (FE) of 26.3% and 100% carbon selectivity at a potential of −0.7 V vs. the reversible hydrogen electrode. Both experimental and theoretical investigations reveal that the introduction of oxygen vacancies efficiently triggers the formation of well-matched adsorption/activation sites, optimizing the adsorption of reactants/intermediates while decreasing the C–N coupling reaction energy. This work offers new insights into the development of dual-functional catalysts based on non-noble transition metal oxides with oxygen vacancies, enabling the efficient electrosynthesis of essential C–N fine chemicals.

excitation mode using a Lytle detector.

Preparation of cathode electrode
The catalyst ink was prepared by ultrasonic dispersion of 1 mg of the catalyst powder with 5 μL Nafion solution (5 wt %) in 1 mL acetone for 30 min.Next, the as-prepared ink was drop-coated on a carbon fiber paper (1 x 0.5 cm 2 ) achieving the catalyst loading of 0.5 mg cm -2 .The electrode was then dried in the atmosphere for the subsequent electrochemical testing experiments.

Electrochemical measurements
Electrochemical studies were conducted in an electrochemical H-cell separated by a Nafion 117 membrane (Alfa).The saturated Ag/AgCl electrode and Pt foam were used as the reference and counter electrodes, respectively.The electrolysis was conducted using a CHI 660e electrochemical workstation.The Ar saturated 0.1 M KHCO 3 + 0.02 M KNO 2 , CO 2 saturated 0.1 M KHCO 3 and CO 2 saturated 0.1 M KHCO 3 + 0.02 M KNO 2 were used as the cathode electrolyte for nitrite electroreduction, CO 2 reduction reaction, and urea electrosynthesis, respectively.0.1 M KHCO 3 aqueous solution was used as the anodic electrolyte.Electrochemical CO 2 reduction reaction and urea electrosynthesis were carried out with CO 2 bubbling.All potentials were converted to the reversible hydrogen electrode (RHE) reference scale using the relation E RHE = E Ag/AgCl + 0.197 + 0.059 × pH and compensated with the solution resistance.Controlled potential electrolysis was then performed at each potential ranging from -0.5 to -0.9 V vs. RHE for 20 min.

Gaseous and liquid products analysis
The gaseous product in the electrochemical experiment was collected by using a gas bag and analyzed by gas chromatography (GC, HP 4890D).The NH 3 was quantified using the indophenol blue method using UV-vis spectrophotometry.Briefly, electrolyte (400 μL, pipetted from the cathodic chamber), coloring solution (2 mL) containing salicylic acid (5 wt%) + sodium citrate (5 wt%) + NaOH (0.75 M), oxidizing solution containing NaClO (1 mL, 0.05 M), and sodium nitroferricyanide (III) dihydrate solution (200 uL, 1 wt%) were added in turn and mixed in a sample tube, and the above solution was then diluted to 10 mL with fresh electrolyte.The UV-vis measurements were performed within a range of 550 to 800 nm after the solution was left in the dark at room temperature for 2 h.The maximum UV-vis absorption peak was obtained at 665 nm.The concentration absorbance curve was calibrated using the standard NH 4 Cl solution with different concentrations (0.5, 1.0, 1.5, 2.0, 2.5 μmmol/mL) in 0.1 M KHCO 3 + 0.02 M KNO 2 solution and measuring the absorbance at 665 nm of the samples.A fitting curve (y =0.795x, R 2 = 0.9996) with ideal linear relation was obtained.Urea was decomposed by urease (C urease = 5 mg/mL; V urea /V urease = 10/1) into CO 2 and two ammonia molecules at 37°C for 30 min.After the decomposition, NH 3 concentration of urea electrolyte with urease was detected via above indophenol blue method.At the same time, NH 3 moles (m NH3 ) contained in urea electrolyte without urease was also quantified by same indophenol blue method.The total moles (m urease ) of NH 3 in the electrolyte were measured by the UV-vis spectrophotometry and shown as 2m urea +m NH3 , where 2m urea represents the moles of ammonia coming from the decomposition.Therefore, the moles of urea (m urea ) produced were calculated by (m urease -m NH3 )/2.The Faradaic efficiency (FE) of the product is: Where Q is the charge (C), F is the Faradaic constant (96485 C mol -1 ), N is the number of electrons required to generate the product, and m is the moles of products.For the H 2 , NH 3 , and urea, the N is 2, 6, and 12, respectively.

Double layer capacitance (C dl ) measurement
The cyclic voltammetry measurement was conducted using a H-cell, and the other conditions were the same as that of the CO 2 reduction.Cyclic voltammogram measurements of the catalysts were conducted from -0.34 to -0.44 V versus Ag/AgCl with various scan rates to obtain the double layer capacitance (C dl ) of different catalysts.
The C dl was estimated by plotting the Δj (j a -j c ) at -0.39 V versus Ag/AgCl against the scan rates, in which j a and j c are the anodic and cathodic current densities, respectively.The linear slope was equivalent to twice C dl .

N isotope labeling experiment
The 15 N isotopic labeling experiments were conducted using 0.2 M KHCO 3 + 0.02 M K 15 NO 2 (99 atom %) electrolytes with CO 2 as feeding gas.After the potentiostatic electrolysis at -0.7 V (vs.RHE) for 4 h, the electrolyte was concentrated at 60 °C.The 1 H NMR spectra were measured on a Bruker Avance III 400 HD spectrometer.The DMSOd 6 was used as the internal standard.

In situ Raman measurements
In situ Raman measurements were carried out using a Horiba LabRAM HR Evolution Raman microscope in a modified H-cell, which was produced by GaossUnion (Tianjin) Photoelectric Technology Company.The carbon paper loaded with catalyst was used as working electrode, a saturated Ag/AgCl electrode and Pt wire were used as reference electrode and counter electrode, respectively.CO 2 saturated 0.1 M KHCO 3 + 0.02 M KNO 2 aqueous solution and 0.1 M KHCO 3 aqueous solution were used as electrolyte and circulated through the cathodic chamber and anodic chamber, respectively, by peristaltic pumps at a rate of 5 mL min -1 .A 785 nm excitation laser was used and signals were recorded using a 20 s integration and by averaging two scans.The signals were recorded at different applied potentials, and a 10 min electrolysis was conducted to gain the steady state before the collection of Raman spectra.

In situ ATR-FTIRS measurements
A Nicolet 6700 FT-IR equipped with a mercury cadmium telluride detector cooled with liquid nitrogen was employed in the in-situ electrochemical study.The measurement was conducted in a modified electrochemical cell, the catalyst was dropped on the silicon ATR crystal deposited with Au film, which was used as working electrode.The Pt wire and saturated Ag/AgCl electrode were used as counter electrode and reference electrode, respectively.CO 2 saturated 0.1 M KHCO 3 + 0.02 M KNO 2 aqueous solution and 0.1 M KHCO 3 aqueous solution were used as electrolyte and circulated through the cathodic chamber and anodic chamber, respectively, by peristaltic pumps at a rate of 5 mL min -1 .
The signals were recorded at different applied potentials, and a 10 min electrolysis was conducted to gain the steady state before the collection of IR spectra.

Theoretical calculations
All the calculations were performed in the framework of the density functional theory with the projector augmented plane-wave (PAW) method using the Vienna ab initio simulation package (VASP). 1 The generalized gradient approximation proposed by Perdew, Burke, and Ernzerh was selected for the exchange-correlation potential. 2The Grimme's DFT-D3 correction method 3 was included to describe the weak dispersion interactions during surface adsorption.The cut-off energy for the plane wave was set to 500 eV.The energy criterion was set to 10 -5 eV in the iterative solution of the Kohn-Sham equation.A vacuum layer of 15 Å was added perpendicular to the sheet to avoid artificial interaction between periodic images.A 2×2×1 Monkhorst-Pack k-point sampling was set for all models.U (Co 3d ) value of 3.5 eV was applied to the Co 3d state.All the structures were relaxed until the residual forces on the atoms had declined to less than 0.02 eV Å -1 .The free energy change (ΔG) of each elementary reaction can be computed by the following equation: where ΔE, ∆ZPE, T, and ∆S are the reaction energy difference, zero-point energy change, temperature, and entropy change, respectively.

Figure S6 .
Figure S6.The ratio of Co 2+ /Co 3+ vs. the ratio of O 2 /O 1 in the as-prepared catalysts based on XPS semiquantitative analysis.

Figure S10 .
Figure S10.(a) UV-vis absorption spectra of urea solutions with different concentrations after decomposition by urease.(b) The calibration curve used for quantifying urea.

Figure S18 .
Figure S18.(a) 1 H NMR spectra of standard urea solution with various concentrations of 1.0-3.0mg mL -1 .(b) The calibration curve used for quantifying urea.

Figure S20 .
Figure S20.The FEs of urea over Co 3 O 4 -1.0 electrocatalyst under different potentials detected by the UV-vis method and 1 H NMR method.

Figure S25 .
Figure S25.Schematic structures of (a, b) Co 3 O 4 and (c, d) Co 3 O 4 -V o with top and side view.

Table S1
Comparison of FE, yield rate of urea and current density over CO 2 with different N source over Co 3 O 4 -1.0 with some state-of-the-art catalysts in electrochemical urea synthesis.