Computational methodsThe spin-polarized calculations of the reactants, intermediates, products, and reaction process involved in CO2RR on the Ni single-atom catalysts were carried out with the density functional theory (DFT) within the PBE41 exchange-correlation functional with the generalized gradient approximation (GGA)42, as implemented in the VASP43. The ion-electron interaction was described with the projector-augmented plane-wave (PAW) method44. To simulate the Ni single-atom catalysts, a 5 × 5 graphene supercell was chosen and two adjacent C atoms were removed to construct a divacancy to load the single Ni atom, which is coordinated to 2–4 N/C atoms through strong covalent bonds, denoted as Ni-N2C2, Ni-N3C1, and Ni-N4. To avoid the interlayer interaction, the vacuum layer is set to be 15 Å between two graphene layers. To simulate the role of K+ and explicit H2O in the process of CO2RR, one K+ with 7 H2O molecules around was placed nearby the active site of catalysts. For geometry optimization, the cut-off energy was set to be 500 eV and the Brillouin zone was sampled with 5 × 5 × 1 k-points. The systems were relaxed until the energy and force reached the convergence threshold of 10−6 eV and 0.015 eV/Å, respectively. We describe the van der Waals (vdW) interactions by utilizing the DFT-D3 method45. Atomic coordinates of structures can be found in Supplementary Data 1.In the ab initio molecular dynamics (AIMD) simulation model, 68 water molecules, leading to an average density of ∼ 1 g cm–3 were placed to construct the solvent water environment. Under the canonical ensemble (NVT), the AIMD simulations last 10 ps with a timestep of 1 fs, in which the temperature is controlled at 300 K using Nosé–Hoover thermostats46,47.For the elementary reaction barriers, free energy for each point along the NEB48 reaction path was calculated by VASP, thus, the configuration of the transition state could change adiabatically with the applied potential. To obtain the transition states and reaction pathway, the transition state tools for VASP (VTST) package49 with climbing image nudged elastic band (CI-NEB)50 was used.After obtaining the optimized geometry, we performed the single-point jDFTx51 calculations to obtain the combined DFT and solvation free energy. The jDFTx computations were elected due to the accuracy of including constant potentials for electrochemical reaction and in describing continuum solvation. However, because the obtained configurations are similar in both jDFTx and VASP, so a single-point computation is sufficient for the stable species. In addition, to describe the solvation implicitly, the CANDLE solvation model was applied in jDFTx. μe;SHE = 4.66 eV was applied for all structures. With a k-point mesh of 5 × 5 × 1, we applied a plane wave basis set with an energy cutoff of 20 Hartree. The systems were relaxed until the free energy convergence threshold of 10−8 Hartree.ChemicalsAll the chemicals were analytical grade and used without further purification. Analytical grade Sodium chloride (NaCl, 99.5%), Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99.0%), Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, 98.0%), Ferric nitrate hexahydrate (Fe(NO3)3·9H2O, 98.5%), 2-methyl imidazole (98.0%), Potassium hydroxide (KOH, 85.0%) and Potassium bicarbonate (KHCO3, 99.5%) were obtained from Shanghai Chemical Reagents, China. Iridium (III) chloride hydrate (IrCl3·xH2O, 99.9%) was purchased from Alfa Aesar. Nafion solution (5%) was purchased from Aldrich. Deionized (DI) water from a Milli-Q System (Millipore, Billerica, MA, USA) was used in all experiments.Synthesis of ZIF-8Typically, 1.116 g Zn(NO3)2·6H2O was dissolved in 30 ml methanol and 1.232 g 2-methyl imidazole was dissolved in 30 ml methanol, followed by adding Zn(NO3)2 solution into the other one under ultrasound for 100 min at room temperature. Then the ZIF-8 was set aside for growing overnight. The as-obtained precipitates were centrifuged and washed three times with methanol, then dried at 65° C in a vacuum for overnight.Synthesis of Ni-N4-HMIn a normal procedure, 200 mg Nickel (II) nitrate hexahydrate was dissolved in 250 mL ethanol, and the powder of ZIF-8 (100 mg) and powder of NaCl (3 g) were mixed by grinding in mortar, then 1.5 ml Nickel (II) nitrate hexahydrate solution was added to be adsorbed into the ZIF-8. Next, the dried powder was transferred into a tube furnace and heated to 950 °C (heating rate 10 °C/min) for 2 h in a stream of Ar (20 mL/min). The annealed powder was washed with a mixture of ethanol and water three times and then dried in a vacuum to obtain the Ni-N4-HM. The Ni content was measured to be 0.52 wt% based on ICP-AES analysis.Synthesis of Ni-CN-ZIFNi-CN-ZIF was synthesized by using an ionic exchange method15. The ZIF-8 powder was uniformly dispersed in 10 ml of n-hexane by ultrasonic treatment for 5 min at room temperature. Once a homogeneous solution was obtained, 50 µL of Ni(NO3)2 aqueous solution(100 mg/mL) was slowly added into the homogeneous solution, followed by ultrasonication for 2 min at room temperature. To make the Ni species adsorbed, the obtained solution was stirred for 3 h. The mixture was then centrifuged, and the powder was dried in a vacuum. Next, the dried sample was transferred to a tube furnace and heated at 1000° C (with a heating rate of 10° C/min) for 2 h at an Ar atmosphere (20 mL/min) to yield Ni-CN-ZIF. The Ni content was measured to be 1.13 wt% based on ICP-AES analysis.Synthesis of Ni-CN-BLEThe powder of ZIF-8 (100 mg) was mixed with NaCl (3 g) by grinding, followed by annealing at 950° C for 2 h in tube finance, the product was then washed with the mixture of ethanol and water and dried in vacuum to obtain the CN-substrate. Next, the powder of CN-substrate (100 mg) was dispersed in 50 mL Ni(NO3)2 aqueous solution (800 mg/L) and stirred for 3 h. Then the substrate was centrifuged and dried. Finally, the sample was transferred into a tube finance and annealed at 950° C (heating rate 10° C/min) for 2 h in a stream of Ar (20 mL/min). The cooled powder was Ni-CN-BLE. The Ni content was measured to be 0.67 wt% based on ICP-AES analysis.Synthesis of NiPc-MDEIn a normal procedure14,52, the Carbon nanotubes (CNTs) were cleaned by calcination at 500° C in air for 2 h followed by washing with a 5 wt% HCl aqueous solution. The purified CNTs were filtered and washed with DI water. Then, 30 mg of the purified CNTs was dispersed into 30 ml of DMF under sonication. The DMF solution with NiPc dissolved was subsequently added to the CNT suspension in sonication for 30 min to make sure it was a well-dispersed solution which was further stirred at room temperature overnight. Subsequently, the product was centrifuged, and washed with DMF and ethanol, followed by lyophilization. The Ni content was measured to be 0.23 wt% based on ICP-AES analysis.Material characterizationsPowder X-ray diffraction patterns of samples were collected using a Rigaku Miniflex-600, operating at 40 kV voltage and 15 mA current with CuKα radiation (λ = 0.15406 nm). Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping of samples were provided by a JEOL JEM-2010 LaB6 high-resolution transmission electron microscope operated at 200 kV. Elemental content analysis of Ni of all samples was identified by inductively coupled plasma atomic emission spectrometry (ICP-AES) on an Optima 7300 DV. Micromeritics Tristar II3020 M was used to determine the Brunauer-Emmett-Teller (BET) specific surface area, samples were degassed at 300° C for 3 h to prepare for adsorption-desorption isotherm measurements. X-ray photoelectron spectroscopy (XPS) spectra were obtained using a PHI 5000 Versa microprobe with Al Kα radiation, referencing the C1s peak at 284.8 eV. During chronoamperometry, effluent gas from the cell was analyzed using a PANNA A91 gas chromatograph. The gas chromatograph, equipped with molecular sieve 5 A, Porapak Q 80/100 mesh, SE-30, and HP-Al2O3/S capillary columns, used ultra-high purity helium as the carrier gas to determine gas product concentrations.In situ XANES measurementsThe Ni K-edge (8333 eV) XANES spectra were measured at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF). The storage ring of SSRF was operated at 3.5 GeV with a maximum electron current of 250 mA. The hard X-ray beam was monochromized using a Si(111) double-crystal monochromator and further suppressed for harmonics by 30% detuning. In situ, XANES measurements were performed in the fluorescence mode using a custom-designed reaction cell. The Ni K-edge (E0) position was calibrated using a Ni foil reference, and all XANES data were collected within a single beam time to ensure consistency. The collected XANES spectra were processed using the ATHENA module of the IFEFFIT software package53, which includes background subtraction, normalization, and energy calibration. Further analysis, including linear combination fitting and principal component analysis, was used to elucidate the evolution of Ni species during the reaction process.XANES simulationsReasonable parameters are selected for the convolution of the calculated spectra employing an energy-dependent arctangent shape of the Lorentzian profile (details can be found in the manual for the FDMNES program54). The calculated convolution spectrum after the common shift is compared with the p-state projection of the Ni absorber (pDOS). The maximum white line peak of the XANES spectrum is aligned with the maximum p-state projection of the Ni absorber to evaluate the rationality of parameters.In situ Raman measurementsThe Raman spectroscopy measurements were performed on a Horiba LabRAM HR Evolution Raman microscope with a 785 laser. We measure in static mode with an Olympus LUMFLN60XW water-dipping objective, 2.25 mW laser power, and an acquisition time of 10 s, we measured two different Raman windows separately (1200–1450 cm–1, 1450–1800 cm–1). All measurements were carried out in the solution of 1 M KCl with CO2 gas purged continuously.Preparation of the cathode electrodesFor the H-type cell, the area of the working electrode was fixed at 1 cm−2. The catalyst ink consisted of 5 mg catalyst, 40 \(\mu L\) Nafion solution, and 1 mL 1:1 (vol/vol) ethanol/water mixed solvent was sonicated for 60 min at room temperature to form the homogeneous ink. The well-dispersed ink was dip-coated onto the electrode to reach the loading of 0.4 mg/mL.For the Flow cell, the carbon paper (Sigracet 35 BC) with a microporous layer (MPL) was purchased from the Fuel Cell Store. The well-dispersed ink was airbrushed onto the gas diffusion layer using an Anest Iwata RG-3L airbrush (Japan) pumped by an air compressor at a certain pressure of 60 p.s.i to reach a loading of 0.4 mg/cm2 for the Flow cell test. The gas diffusion electrodes (GDEs) were dried in a vacuum chamber before use.For the rotating disk electrode, the well-dispersed ink was dipped onto the glassy carbon RDE to obtain a loading of 0.1 mg/cm2Preparation of the anode IrO2-Ti meshThe IrO2-Ti mesh was prepared by a modified thermal decomposition and dip coating method. Briefly, acetone and de-ionized water were used to degrease the titanium mesh, which was then etched in a 6 M HCl solution for 20 min before coating. The dip coating solution consisted of 30 mg of IrCl3·xH2O, and 1 ml concentrated HCl was dissolved in 9 ml isopropanol. Then the fresh titanium mesh was dipped into the IrCl3 solution followed by calcination in a furnace at 500 °C for 10 min. The process of coating was repeated until a certain loading was achieved (2 mg/cm2).Preparation of FeNi-LDHs/Ni meshThe Ni mesh washed with 5 M HCl aqueous solution was used to electrodeposit. The FeNi-LDHs was electrodeposited on the as-prepared Ni mesh in a 25 mL electrolyte bath containing Ni(NO3)2·6H2O (3 mM) and Fe(NO3)3·9H2O (3 mM) at − 1.00 V (vs. Ag/AgCl) at 25 °C for 120 s. After the reaction, the mesh was washed thoroughly with DI water. The FeNi-LDHs/Ni mesh anode was used in the Flow cell system.Electrochemical measurementsThe electrochemical measurements were conducted with a PGSTAT302N workstation with a 10 A Booster M204 and CHI760E. All the electrolytes are stored in a PFA bottle. For H-type cell tests, the two gas-tight cells were separated by Nafion212 membrane, a Pt foil was used as a counter electrode to generate OER, an Ag/AgCl electrode was used as a reference electrode, which is calibrated in a 0.1 M Na2SO4 Solution. 0.5 M KHCO3 aqueous solutions pre-saturated with CO2 were used as electrolytes for each chamber. For the electrolyte with a pH value of 4 (4.0 ± 0.1), 2 M KCl and 0.00005 M H2SO4 dissolved in the DI water with CO2 gas saturated to make the electrolyte. The area of the working electrode was fixed as 1 \(\times\) 1 cm2, and CO2 gas was purged in during the reaction. All potentials were measured against an Ag/AgCl reference electrode (saturated KCl, obtained from Tjaida and stored in a saturated KCl solution before use) and converted to the reversible hydrogen electrode (RHE) scale by$$E\,\left({vs}. \, {RHE}\right)=E\,\left({vs}. \, {Ag}/{AgCl}\right)+0.22\,V+0.0592\times {pH}$$For the gas-tight Flow cell system, a CO2 gas compartment and two liquid compartments with channels of dimensions 2 cm × 0.5 cm × 0.3 cm were separated by carbon paper and an anion-exchange membrane respectively. The gaseous CO2 could pass behind the GDL into the cathode electrolyte and then go out with gas product. The catholyte and anolyte were separated by an anion exchange membrane (Sustainion 37–50, Dioxide Materials). The FeNi-LDHs/Ni mesh was used as an anode, and an Ag/AgCl electrode was used as a reference electrode. The area of the working electrode was fixed as 2 \(\times\) 0.5 cm2, 1 M KOH aqueous solution was used as an electrolyte to be circulated, and the gas flow rate increased along with the current density increase. All potentials were measured as mentioned above. Resistance in the flow cell is 1.48 Ω for Ni-N4-HM, 1.53 Ω for Ni-CN-BLE, and 2.16 Ω for Ni-CN-ZIF. An IR correction was compensated by:$$E\,\left({vs}. \, {Ag}/{AgCl}\right)=E-0.8\,{I}_{{total}}R$$A factor of 0.8 is applied for the ohmic potential drop.The RDE experiment was done using a glassy carbon RDE with a diameter of 5 mm. The electrode was polished by alumina polishing powder (0.05 μm) before use. A Pt foil and an Ag/AgCl electrode were used as counter electrode and reference electrode respectively. 1 M KHCO3 pre-saturated with CO2 gas was used as an electrolyte. The linear sweeping voltammetry curves were collected at a scan rate of 5 mV/S with different rotating speed (400 to 2500 rpm). The electron transfer number (n) and kinetic current density (Jk) can be obtained according to the K-L equation:$$\frac{1}{J}=\frac{1}{{J}_{K}}+\frac{1}{{J}_{L}}=\frac{1}{{J}_{K}}+\frac{1}{B{\omega }^{\frac{1}{2}}}$$$$B=0.62{nFC}{D}^{\frac{2}{3}}{V}^{-\frac{1}{6}}$$Where \(J\) is the total current density from tests, \({J}_{K}\) is the kinetic current density, \({J}_{L}\) is the limiting diffusion current density, \(\omega\) is the angular velocity of the disk, \(n\) is the number of electrons, \(F\) is the Faraday constant, \(C\) is the bulk concentration of CO2, \(D\) is the diffusion coefficient of CO2 in 1 M KHCO3, and \(V\) is the kinematic viscosity of the electrolyte.CO2RR product analysisThe Faradaic efficiency (FE) of gas products was calculated as$${{FE}}_{{gas}}={x}_{i}\times v\times \frac{{P}_{0}}{{RT}}\times \frac{{z}_{i}F}{{I}_{{total}}}\times 100\%$$where \({x}_{i}\) is the volume fraction of gas product \(i,v\) the gas flow rate at the cathode outlet measured by a soap film flow meter. \({z}_{i}\) the number of electrons involved in the reaction to produce one molecule of product \(i\), \(F\) the Faraday constant, \({P}_{0}\) the absolute pressure (101.325 kPa), \(R\) the gas constant, \(T\) the temperature, \({I}_{{total}}\) the total current.The Tafel slope was calculated based on the Tafel equation (ƞ = blog(jCO/jo)), where ƞ is the overpotential, b is the Tafel slope, jCO is the current density for CO formation, and jo is the exchange current density.
