ChemicalsCopper chloride, copper nitrate, sodium hydroxide, cyanamide, Iridium (IV) Oxide and hydrazine were purchased from Adamas. Anion-exchange membrane (Fumasep-FAA-3-50) and anion-exchange membrane solution was purchased from Fumatech, German. Proton exchange membrane (N212) and Nafion perfluorinated resin (5 wt%) were purchased from DuPont, USA. All chemicals were used directly from the manufacturer without further purification.Synthesis of Cuδ+NCNThe procedure for the fabrication of Cuδ+NCN was modified based on methodologies delineated in prior literature24. At room temperature, 426.6 mg of copper chloride was dissolved in 45 mL deionized water. Next, 2.5 mL of 3.5 M sodium hydroxide and 3 mL of 2 M cyanamide were added in order. The mixture was stirred for 3 minutes, then 5 mL of hydrazine was quickly poured in. After 2 h of reaction, the mixture was centrifuged, washed with deionized water, centrifuged again, and the final product was obtained by freeze-drying.Synthesis of CuNCNCuNCN was synthesized in a similar manner to Cuδ+NCN, except that hydrazine was not added during the synthesis.Synthesis of CuOThis approach is in accordance with the methodologies delineated in prior studies49. Copper Oxide (CuO) were synthesized utilizing the hydrothermal technique, employing copper (II) nitrate (Cu(NO₃)₂) as the precursor. An aqueous sodium hydroxide solution with a molarity of three moles per liter (3 M, 2 mL) was incrementally introduced into a copper (II) nitrate solution of one mole per liter (1 M, 2 mL). The resultant mixture was subjected to vigorous stirring for a duration of one hour to ensure homogeneity. Subsequently, the mixture was subjected to a thermal treatment at a temperature of 120 °C for 4 h. Upon completion of the heating phase, the system was allowed to equilibrate to room temperature. The final stage encompassed a series of purification steps including centrifugation, meticulous washing, and a drying process.Materials characterizationThe surface textures and elemental distribution of the catalysts were meticulously delineated utilizing a field-emission scanning electron microscope (FE-SEM, Zeiss Gemini 300). The assessment of elemental composition and quantification was conducted through an Energy Dispersive X-ray Spectroscopy (EDS, JEOL-2010) apparatus integrally connected to the FE-SEM. Scanning transmission electron microscopy (STEM) images alongside energy-dispersive X-ray spectroscopy (EDS) mappings were procured using a JEOL ARM300 microscope. This state-of-the-art instrument boasts the capability of capturing ultrahigh-resolution STEM images with an exceptional spatial resolution of 63 picometers. The microscope is outfitted with a dual spherical aberration (CS) corrector, enhancing image clarity and precision. Additionally, it is equipped with an advanced X-ray energy dispersive spectrometer (JED-2300 Series), which incorporates a pair of 158 mm2 solid-state detectors (SSD) for superior spectral sensitivity and precise elemental analysis. To decipher the crystalline architecture of the samples, X-ray diffraction (XRD) profiles were acquired using a Bruker D8-Advance X-ray diffractometer, employing Cu-Kα radiation. The catalyst samples were aerated and methodically surveyed across a range of 5 to 80 degrees at a rate of 5 degrees per minute. The KPFM characterization was carried out with atomic force microscope (nanoIR2-FS). Inductively coupled plasma atomic emission spectroscopy (ICP-OES) was performed on an Agilent 5110 ICP spectrometer. Analysis of the valence states of the elemental constituents was executed via X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). Prior to engaging in curve fitting and background attenuation, a standardization of the XPS spectra was performed referencing the C 1s peak. The Fourier-transform infrared (FTIR) spectra were captured using a Thermo Scientific Nicolet iS20 spectrophotometer; each sample was meticulously prepared by compressing it into a pellet with KBr powder.Electrochemical measurementThe assessment of CO2 reduction reaction (CO2RR) efficacy was meticulously conducted within both a flow cell and a membrane electrode assembly (MEA) electrolytic cell. apparatus.Within the flow cell measurements, an electrolytic solution of 1 M KOH, exhibiting a pH of 13.8, was utilized as both the anolyte and catholyte. The gaseous environments perfusing the cathodic compartment were composed of CO2 and Argon, tailored to the specific exigencies of the reaction conditions. The trifecta of electrodes comprised a gas-diffusion layer measuring 1 cm by 3 cm, a platinum sheet of identical dimensions, and a silver/silver chloride (Ag/AgCl) reference electrode immersed in saturated KCl, each meticulously arranged, with the active operative surface area precisely defined at 1 cm2. The catalyst loading on the cathode is 0.7 mg cm−2. An anion exchange membrane of type FAA-3-50 provided a discrete partition between the cathode and anode chambers. The regulation of gaseous flow was achieved with a mass flowmeter, maintaining a rate of 40 mL min−1, while a peristaltic pump assiduously controlled the electrolyte flow at a rate of 10 mL min−1. The calibration of potentialities was scrupulously performed in reference to the reversible hydrogen electrode (RHE), utilizing the equation: E (vs. RHE) = E (vs Ag/AgCl) + 0.197 V + (0.0592 × pH). Linear sweep voltammetry (LSV) was executed within the gas diffusion cell at a scanning velocity of 10 mV s−1, traversing a potential range from 0 to −1.7 V versus RHE. The electrochemical active surface area (ECSA) of the catalyst was appraised by gauging electrochemical capacitance over scanning velocities ranging from 20 to 100 mV s−1, with increments of 20 mV s−1, within a non-Faradaic potential window. The electrochemical double-layer capacitance (Cdl) of the specimen was estimated by the differential current (Δj) at the varying scanning rates. All voltages were not subjected to iR compensation.Within the MEA electrolytic cell measurements, using a home-made flow channel plate as a jig, a Nafion N212 membrane as the PEM, and Fumasep-FAA-3-50 as the AEM, an APMA-MEA system was constructed. The anode catalyst employed was iridium dioxide with a loading of 2 mg cm−2, which was applied to the pre-treated PEM through the CCM (Catalyst Coated Membrane) method and thermally pressed before use. The anode gas diffusion layer utilized a platinum-coated titanium mesh. The cathode catalyst was Cuδ+NCN with a loading of 2 mg cm−2, which was spray-coated onto YLS-30T carbon paper using the CCS (Catalyst Coated Substrate) method and was not thermally pressed. The anolyte is deionized water with a flow rate controlled at 30 milliliters per minute, while the cathode gas is humidified with deionized water at 50 °C before entering the cathode chamber, with its flow rate controlled at 30 standard cubic centimeters per minute (sccm). The MEA testing is performed using chronoamperometry. All electrochemical tests were conducted at room temperature.Products analysisThe electrochemical reduction of CO2 was meticulously conducted at ambient temperature, employing a saturated 1 M KOH electrolytic solution across a potential range of −0.8 V to −1.6 V with respect to the reversible hydrogen electrode (RHE). The cathodic electrolysis was methodically sustained for a duration of 20 minutes at each discrete potential setting. Concurrently, oxygen evolution at the anode was expelled along with the electrolyte via the methodical action of a peristaltic pump. The identification and quantification of gaseous byproducts emanating from the cathodic domain of the electrocatalytic CO2 reduction were assiduously monitored through online gas chromatography equipped with both a flame ionization detector (FID) and a thermal conductivity detector (TCD) (Model A91 Plus, Panna Instruments, China), with analyses conducted at five-minute intervals.Throughout the CO2 reduction reaction, the gaseous outputs from both the flow cell and the MEA electrolytic cell were quantitatively ascertained via online chromatographic analysis on a bi-temporal basis of five minutes, utilizing the dual-detection system.The faradaic efficiency (FE) of the gaseous products was calculated using the equation:$${FE}\left(\%\right) =\frac{{Q}_{{products}}}{{Q}_{{total}}}\times 100\%\\ =\frac{v\times x\times N\times F}{j}\times 100\%$$
(1)
where (\(v\)) denotes the volumetric flow of CO2 through the cathodic chamber (volume per second), (\(x\)) represents the product concentration as determined from a 1 ml sample loop calibrated against a standard gas via online GC, (\(N\)) is the number of electrons transferred in the reduction process, (\(F\)) signifies the Faraday constant (96,485 C mol−1), and (\(j\)) is the current density at the given moment.Post-electrolysis, the cathodic liquid was diligently collected and subjected to a 400 MHz nuclear magnetic resonance (NMR) spectrometer for quantitation of the aqueous products. Following a 20-minute CO2RR session at the specified potential, the electrolyte was gathered, and a 500 μL aliquot was mixed with 100 μL of a 10 mM DMSO solution and 100 μL of D2O for diagnostic analysis of the liquid product profile via a 400 MHz 1H-NMR spectrometer. To construct calibration curves, a series of liquid-phase products standards in DMSO and D2O were assayed using NMR. Within the one-dimensional ¹H NMR spectra, the water signal was attentively suppressed, placing the DMSO and liquid-phase products proton resonances, respectively. The liquid-phase products concentration within the electrolyte was deduced from the standard curve.Faradaic efficiency of the liquid-phase products was determined by the equation:$${FE}\left(\%\right) =\frac{{Q}_{{{\rm{liquid}}}-{{\rm{phase}}}\, {\rm {products}}}}{{Q}_{{total}}}\times 100\%\\ =\frac{V \times x\times N\times F}{\int_{0}^{t}\, jdt}\times 100\%$$
(2)
where (\(V\)) is the volume of the cathode electrolyte, (\(x\)) is the concentration of liquid-phase products, (\(N\)) is the number of electrons involved in the reduction process, (\(F\)) is the Faraday constant (96,485 C mol−1), and (\({Q}_{{total}}\)) is calculated by integrating the current over time.
Operando Raman spectroscopyOperando Raman spectroscopy analyses were performed using a Horiba LabRAM HR Evolution system. The experimental arrangement for the electrode mirrored that of the antecedent electrochemical tests, with the modification of the electrolyte to a 0.1 M KHCO3 solution. This modification was intended to mitigate the absorption of CO2 by KOH. Spectral acquisition was performed under 532 nm laser excitation, operated at 10% of the laser potential intensity, and the exposure duration was set to 20 s. Open circuit voltage Raman spectra is the spectra collected by the sample directly immersed in 0.1 M KHCO3. Operando Raman spectra were collected using chronoamperometry at −0.3 – −0.8 V vs. RHE without iR drop compensation.
Operando ATR-SEIRA spectroscopyThe catalytic layer was applied onto a chemically prepared Au film situated atop a Si ATR prism, with subsequent ATR-SEIRAS assessments conducted using a PerkinElmer Spectrum FTIR spectrometer, integrated with a MCT detector. A spectral resolution of 4 cm−1 was selected. Spectral acquisition was conducted within the wavenumber range of 400 to 4000 cm−1, with the number of scans set to four. During the testing process, Au film was utilized as the working electrode onto which the ink was drop-cast and dried prior to testing. A platinum slice served as the counter electrode, and Ag/AgCl electrode was used as the reference in a three-electrode setup. Electrolyte 0.5 M KHCO3 was employed for the electrochemical measurements. Chronoamperometry was the technique used for the electrochemical test, with the test voltage range spanning from −0.1 V to −1.5 V vs. RHE. Spectral data were collected twice after a reaction time of 30 s at each potential. Finally, spectral data were continuously acquired for 12 min at a potential of −1.6 V vs. RHE.
Operando XAFSThe Cu K-edge XAFS spectra were measured at BL17B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF), China. The storage ring of the SSRF were operated at 2.5 GeV with a maximum electron current of 250 mA. Operando XAFS measurements were performed in a homemade cell (Supplementary Fig. 29). Catalyst-loaded carbon paper as working electrode with polyimide film on the back side and then glued to the surface of the operando electrolytic cell, with the catalyst in direct contact with the electrolyte. A 0.5 M KHCO3 solution was used as the electrolyte. All X-ray was monochromatized by a Si (111) double-crystal monochromator with the energy calibrated using Cu foils.XAFS analysis and resultsThe acquired EXAFS data were processed according to the standard procedures using the ATHENA module of Demeter software packages50.The EXAFS spectra were processed by first removing the post-edge background from the total absorption and then normalizing it relative to the edge-jump step. Afterward, the χ(k) data were Fourier transformed into real (R) space using a Hanning window with a width of dk = 1.0 Å−1 to distinguish the EXAFS contributions from various coordination shells. To extract the quantitative structural parameters surrounding the central atoms, least-squares curve fitting was carried out using the ARTEMIS module within the Demeter software suite50.The following EXAFS equation was used51$$\chi \left(k\right)={\sum}_{j}\frac{{N}_{j}{S}_{0}^{2}{F}_{j}(k)}{k{R}_{j}^{2}}\cdot \exp \left[-2{k}^{2}{\sigma }_{j}^{2}\right]\cdot \exp \left[\frac{-2{R}_{j}}{\lambda (k)}\right]\cdot \sin \left[2k{R}_{j}+{\phi }_{j}(k)\right]$$
(3)
the theoretical calculations included scattering amplitudes, phase shifts, and photoelectron mean free paths for all considered paths. The amplitude reduction factor is represented by S02, while Fj(k) denotes the effective curved-wave backscattering amplitude. Nj represents the number of neighboring atoms in the jth atomic shell, and Rj is the distance between the X-ray absorbing central atom and the atoms in the jth atomic shell. The mean free path, denoted as λ, is expressed in Å. The phase shift, ϕj(k), encompasses both the individual shell phase shifts and the total phase shift for the central atom. The Debye-Waller factor, σj, characterizes the variation in distances around the average Rj within the jth shell. The functions Fj(k), λ, and ϕj(k) were computed using the ab initio software FEFF9. Further details on the EXAFS simulations are provided below.All fits were performed in the R space with k-weight of 3 while phase correction was also applied in the first coordination shell to make R value close to the physical interatomic distance between the absorber and shell scatterer. The coordination numbers of model samples were fixed as the nominal values. The obtained S02 was fixed in the subsequent fitting. While the internal atomic distances R, Debye-Waller factor σ2, and the edge-energy shift Δ were allowed to run freely. The detailed analysis results are illustrated in Supplementary Tables 2, 3.For Wavelet Transform analysis, the χ(k) exported from Athena was imported into the Hama Fortran code. The parameters were listed as follow: R range, 1–4 Å, k range, 0–15 Å−1 for samples; k weight, 3; and Morlet function with κ = 10, σ = 1 was used as the mother wavelet to provide the overall distribution.Density functional theory computationDensity functional theory (DFT) investigations were carried out using the Vienna ab initio simulation package (VASP). The interaction between ions and electrons under the frozen-core approximation was described employing the projector augmented wave (PAW) method52. Kohn-Sham valence states were expanded in a plane-wave basis set with a cut-off energy of 450 eV. Spin-polarized calculations utilized the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approximation (GGA)53,54. The Brillouin zone integration employed a Monkhorst-Pack mesh of 3 × 3 × 1. To separate periodic images, a 15 Å vacuum was added. The atomic structures were relaxed until the forces were less than 0.03 eV Å−1. For the implicit solution model, VASPsol was implemented to balance the net electronic charges introduced by the constant-potential method55. The relative permittivity was set to 78.4, and a linearized Poisson–Boltzmann model with a Debye length of 3.0 Å was employed to mimic the compensating charge. In addition, we obtained the charge values using the Bader charge analysis method.Constant-potential method for obtaining the potential-dependent grand canonical energiesIn the constant-potential calculations, the structures and work functions of the involved reaction intermediates were fully optimized to account for the effect of an applied potential. The optimization method we utilized is developed by Duan et al.56. The work functions of the reaction intermediates are related to the applied potential by referencing them to ΦSHE = − 4.6 eV, which is the work function of the standard hydrogen electrode (SHE).Free energy calculation method under constant potentialsIn this work, the grand free energy changes (ΔG) of the key CO2RR steps under a constant potential (U) were evaluated by Eq. (4):$$\Delta G=\Delta E(U)+\Delta {ZPE}+\Delta {G}_{U}^{{PCET}}+\Delta {G}_{{pH}}+\Delta {\int }_{0}^{T}{C}_{P}{dT}-T\Delta S$$
(4)
where ΔZPE is the zero-point energy change, ΔGUPCET is the free energy contribution of proton-coupled electron transfer (PCET) at electrode potential U. ΔGpH = 2.303 × kBT × pH (or 0.06 × pH) eV. The entropy change is denoted as ΔS, while Cp signifies the constant-pressure heat capacity. The entropy and the integration term are obtained through the vibrational energy calculations of the CO2RR intermediates.In the above equation, E(U) is defined as a grand canonical energy of the system:$$E(U)={E}_{DFT}+\varDelta {n}^{CPS}\cdot \left(U-{V}_{sol}+\frac{{\varPhi }_{SHE}}{e}\right)$$
(5)
where EDFT is the energy calculated from DFT, ΔnCPS is the number of electrons added or removed from the system, which is determined by the constant-potential method. ΦSHE is the work function of the standard hydrogen electrode, SHE (−4.6 eV), and Vsol is the potential deep in the solution.Formation energy of a Cu vacancyThe surface Cu0 and bulk phase Cu1+ vacancy formation energies are defined as:$${E}_{{vf}}={{{\rm{E}}}}_{{vac}}+{E}_{{Cu}}-{E}_{{tot}}$$
(6)
where Evac is total energy of the structure with a Cu vacancy, ECu is the energy of a single Cu atom, Etot is the total energy of the pristine structure without any defects. In this work the energy of single Cu atom refers to an isolated Cu atom in vacuum.
Stabilized Cu0 -Cu1+ dual sites in a cyanamide framework for selective CO2 electroreduction to ethylene
