Atomically Dispersed ZnN4 Sites Anchored on P‐Functionalized Carbon with Hierarchically Ordered Porous Structures for Boosted Electroreduction of CO2

Abstract Tuning the coordination structures of metal sites is intensively studied to improve the performances of single‐atom site catalysts (SASC). However, the pore structure of SASC, which is highly related to the accessibility of active sites, has received little attention. In this work, single‐atom ZnN4 sites embedded in P‐functionalized carbon with hollow‐wall and 3D ordered macroporous structure (denoted as H‐3DOM‐ZnN4/P‐C) are constructed. The creation of hollow walls in ordered macroporous structures can largely increase the external surface area to expose more active sites. The introduction of adjacent P atoms can optimize the electronic structure of ZnN4 sites through long‐rang regulation to enhance the intrinsic activity and selectivity. In the electrochemical CO2 reduction reaction, H‐3DOM‐ZnN4/P‐C exhibits high CO Faradaic efficiency over 90% in a wide potential window (500 mV) and a large turnover frequency up to 7.8 × 104 h−1 at −1.0 V versus reversible hydrogen electrode, much higher than its counterparts without the hierarchically ordered structure or P‐functionalization.

simple calcination of the ZIF-8@PS at 400 °C for 5 h.Synthesis of ZIF-8.Typically, 2.9 g Zn(NO3)2•6H2O was dissolved in 150 mL methanol to prepare solution A; 9.6 g 2-methylimidazole and was dissolved in 150 mL methanol to prepare solution B. Subsequently, solution A was added to solution B, and kept stirring for 12 h at room temperate.After centrifugation, the final product was dried at 60 °C for 5 h.
Preparation of H-3DOM-ZnN4/P-C.100 mg 3DOM-ZIF-8 was added into 50 mL methanol solution containing 100 mg phytic acid solution, and then stirred in an oil bath at 45 °C for 2 h.After filtration, the solid was placed in an oven and dried at 60 °C for 8 h to afford H-3DOM-ZIF-8/Zn-PA.Then, the H-3DOM-ZIF-8/Zn-PA was heated at 950 °C for 2 h in a tubular furnace with a heating rate of 5 °C min −1 under flowing Ar atmosphere.After cooling to room temperature, the final product H-3DOM-ZnN4/P-C was obtained.
Preparation of 3DOM-ZnN4/C.100 mg 3DOM-ZIF-8 was heated at 950 °C for 2 h in a tubular furnace with a heating rate of 5 °C min −1 under flowing Ar atmosphere.After cooling to room temperature, the product 3DOM-ZnN4/C was obtained.
Preparation of 3DOM-ZnN4/P-C. 100 mg 3DOM-ZnN4/C was added into 50 mL methanol solution containing 100 mg phytic acid solution, which was stirred in an oil bath at 45 °C for 2 h.After filtration, the black solid was placed in an oven at 60 °C and dried for 8 h.Then, the black solid was heated at 950 °C for 30 min in a tubular furnace with a heating rate of 5 °C min −1 under flowing Ar atmosphere.After cooling to room temperature, the product 3DOM-ZnN4/P-C was obtained.
Preparation of ZnN4/P-C.The preparation process is the same as that of 3DOM-ZnN4/P-C, except that 3DOM-ZIF-8 was replaced by conventional ZIF-8.

Catalyst characterization
Powder X-ray diffraction (XRD) patterns of the samples were collected with a Rigaku diffractometer (D/MAX/IIIA, 3 kW) employing Cu Kα radiation (40 kV, 30 mA, λ = 0.1543 nm).Raman spectra were recorded on a LabRAM Aramis Raman spectrometer (HORIBA Jobin Yvon).N2 adsorption/desorption isotherms were obtained at 77 K on an ASAP 2460 instrument.The morphology of the materials was investigated by high-resolution scanning electron microscopy (SEM, SU 8220, and 8100 of HITACHI).The structure and element mapping was determined by a high-resolution transmission electron microscope (TEM, JEOL, JEM-2100F) with EDS analysis (Bruker Xflash 5030T) operated at 200 kV.The atomic structure of the catalyst was characterized using a Titan Cubed Themis G2300 (FEI, Netherlands) transmission electron microscope operated equipped with double spherical aberration correctors at 200 kV.X-ray photoelectron spectroscopy (XPS) was performed by using a Thermo Scientific Escalab 250Xi system with a base pressure of 210 −9 Torr.Synchrotron-based X-ray absorption fine structure (XAFS) spectra at the Zn K-edge were collected at BL14W1 station in Shanghai Synchrotron Radiation Facility (SSRF).The metal contents of the samples were determined by atomic absorption spectroscopy (AAS) on a HITACHI Z-2300 instrument.The C, N, and P elemental contents of the samples were measured on a Euro Vector EA3000 instrument and Inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 8300, PE).Static contact angles (CA) were measured on a Dataphysics OCA40 Micro instrument.The Fourier transform infrared spectroscopy (FT-IR) of the samples was measured on a Nicolet IS10.The Zeta potential was determined by Zetasizer ULTRA.

Electrochemical measurements
Electrode preparation.Typically, 1 mg of catalyst was suspended in 250 µL of CH3CH2OH and dispersed uniformly by ultrasonic treatment.Then, the solution was spread onto the carbon cloth (11 cm 2 ) surface by a micropipette.Finally, 10 μL of Nafion D-521 dispersion (0.5 wt%) was pipetted onto the carbon cloth to immobilize the catalyst.

Electrocatalytic CO2 reduction.
All the electrochemical experiments were conducted on the electrochemical workstation (CHI 760E).All potentials cited in this work were referenced to the RHE, unless stated otherwise.The reference potentials were converted to RHE using the formulas E(vs.RHE) = E(vs.Ag/AgCl) + 0.197 V + 0.059 × pH.The electrolysis experiments were conducted at 25 °C in an H-type cell with a working cathode, a counter anode (platinum gauze), and a reference electrode (Ag/AgCl with saturated KCl).In the experiment, Nafion-117 membrane was used as the proton exchange membrane that separated the cathode and anode compartments.KHCO3 aqueous solution (0.1 M) was utilized as the electrolyte.In each experiment, 35 mL of the electrolyte was used.Before starting the electrolysis experiment, the catholyte was bubbled with CO2 for 30 min to ensure the solution is saturated.
Product analysis.The gaseous product of electrochemical experiments was analyzed by gas chromatography (GC, HP 4890D), which was equipped with FID and TCD detectors using helium as the internal standard.The liquid product was analyzed by 1  Electrocatalytic hydrazine hydrate oxidation.The test device was consistent with CO2 reduction, except that the electrolyte is replaced with 1 M KOH.Unless otherwise specified, the concentration of hydrazine hydrate is 100 mM.The Energy saving efficiency (ŋ) is calculated from the perspective of the energy of the full cell.The ŋ value is calculated from the equation:

Computational method
The free energies of CO2 reduction states were carried out by the Vienna Ab-initio Simulation Package (VASP) 1,2 , taking advantage of the density functional theory (DFT) with the Projected Augmented Wave (PAW) method 3 .The revised Perdew-Burke-Ernzerh of (RPBE) functional was used to describe the exchange and correlation effects [4][5][6] .For all the geometry optimizations, the cut-off energy was set to 450 eV.The Monkhorst-Pack grids 7 were set to be 3×3×1 and 5×5×1 for adsorption calculations and density of states (DOS) calculations, respectively.A 20 Å vacuum thickness was added in the z-direction of the simulation box, preventing interactions between the adjacent slabs.
In aqueous conditions, the reduction of CO2 to produce CO could occur in the following where ∆Eads is the electronic adsorption energy, ∆EZPE is the zero-point energy difference between adsorbed and gaseous species, T∆Sads is the corresponding entropy difference between these two states, and ∫CPdT is the enthalpy correction.The electronic binding energy is referenced as graphene for each C atom, ½ H2 for each H atom, and (H2O-H2) for each O atom, plus the energy of the clean slab.The corrections of zero-point energy, entropy, and enthalpy of adsorbed and gaseous species can be found in the supporting information."/" indicates that the information provided in the text is insufficient to obtain the value.

Supporting Figures and Tables
H NMR (Bruker Avance III 400 HD spectrometer) in deuterium oxide.The turnover frequency (TOF) value of CO2RR was calculated according to the equation: TOF = jCO / (n × N × F) jCO represents the current density of the CO (mA cm −2 ).n is electron transfer number, which is 2 for CO.F is the Faraday constant (96485 C mol −l ), and N (mol) is the number of active sites involved in the CO2RR.N can be calculated by the equation: N = M/65.4× (ECSA / BET) 65.4 represents the relative atomic mass of Zn.M is the mass of Zn in the catalyst on the electrode surface: M = mcat × w mcat is mass of the catalyst on the electrode (mg), w (%) is the content of single-atom metal in the catalyst.ECSA can be calculated by the equation: ECSA = (Cdl / Cs) × Ageo Cdl is the double-layer capacitance value.Cs is the specific capacitance (typically 0.04 mF cm −2 ) and Ageo is the geometric surface area of the electrode (1 cm 2 ) three elementary steps: CO2 + (H + + e -) + * → *COOH *COOH + (H + + e -) → *CO + H2O *CO → CO + * where * denotes the active sites on the catalyst surface.Based on the above mechanism, the free energy of two intermediate states, *COOH and *CO, are important to identify a given material's activity in catalyzing CO2 reduction.The computational hydrogen electrode (CHE) model 8 proposed by Norskov et al. was used to calculate the free energies of CO2 reduction intermediates, based on which the free energy of an adsorbed species is defined as

Figure S1 .
Figure S1.SEM image of PS template.

Figure S11 .
Figure S11.EXAFS fitting analysis of Zn foil in R (a) and k (b) space.

Figure S12 .
Figure S12.EXAFS fitting analysis of ZnO in R (a) and k (b) space.

Figure S13 .
Figure S13.EXAFS fitting analysis of ZnPc in R (a) and k (b) space.

Figure S19. 1 H
Figure S19.1 H NMR spectra of the electrolyte after electrolysis at −0.6 V vs. RHE.

Figure S25 .
Figure S25.(a) LSV curves and (b) FECO at −0.6 V (vs.RHE) of different catalysts obtained with different amounts of phytic acid.

Figure S27 .
Figure S27.Local structures of ZnN4/P graphene with phosphorous atom in various possible locations evaluated by DFT calculation.

Figure S28 .
Figure S28.Model diagram of catalyst binding with intermediate states in CO2RR process.

Figure S29 .
Figure S29.Model diagram of catalyst binding with intermediate states in HER process.

Figure S30 .
Figure S30.CV comparison before and after long-term electrolysis.

Table S1 .
The pH value of methanol and PA-methanol solution.

Table S2 .
The Zn, N, and P loadings of different catalysts measured by AAS and ICP-OES.

Table S4 .
Comparison of CO2RR performances of H-3DOM-ZnN4/P-C with other reported single-atom catalysts.

Table S5 .
The Cdl value of different catalysts obtained by EIS test.

Table S6 .
The Zn content of catalysts obtained at different calcination temperatures.

Table S7 .
The Bader charge and oxidation state of Zn in different unites.

Table S8 .
Comparison of HzOR performances of H-3DOM-ZnN4/P-C with other reported transition metal-based catalysts.