A Universal Electrochemical Synthetic Strategy for the Direct Assembly of Single‐Atom Catalysts

Abstract Single‐atom catalysts (SACs) have been one of the frontiers in the field of catalysis in recent years owing to their high atomic utilization and unique electronic structure. To facilitate the practical application of single‐atom, it is vital to develop a sustainable, facile single‐atom preparation method with mass production potential. Herein, a universal one‐step electrochemical synthesis strategy is proposed, and various metal–organic framework‐supported SACs (including Pt, Au, Ir, Pd, Ru, Mo, Rh, and W) are straightforwardly obtained by simply replacing the guest metal precursors. As a proof‐of‐concept, the electrosynthetic Pt‐based catalysts exhibit outstanding activity and stability in the electrocatalytic hydrogen evolution reaction (HER). This study not only enriches the single‐atom synthesis methodology, but also extends the scenario of electrochemical synthesis, opening up new avenues for the design of advanced electro‐synthesized catalysts.

compositions and morphologies of the catalysts were observed using scanning electron microscopy (SEM, Zeiss Sigma 500) and transmission electron microscopy (TEM, FEI TalosF200S).The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and Energy-dispersive X-ray Spectra (EDS) element mapping were acquired using an aberration-corrected JEOLJEMARM300F TEM/STEM and transmission electron microscopy (TEM, FEI TalosF200S), respectively.The metal contents were determined by SHIMADZU ICPE-9820 inductively coupled plasma optical emission spectrometry (ICP-OES).X-ray photoelectron spectroscopy (XPS) was measured on a VG Scientific ESCALAB 250 photoelectron spectrometer equipped with a 300 W aluminum target radiation source (Al Kα).The Raman spectra were tested using a LabRam HR confocal Raman spectrometer with a laser excitation wavelength of 532 nm.Dynamic light scattering (DLS) measurements were performed on a Horiba SZ-100 Nanoparticle Size Analyzer.The nitrogen sorption isotherms were measured using the Belsorp Max automatic volumetric adsorption system.The specific surface areas and pore size distribution were calculated by using the Brunauer-Emmett-Teller (BET) equation and the non-linear density functional theory (NL-DFT) model.Fourier transform infrared (FT-IR) spectra were performed on a Bruker ALPHAǁFT-IR spectrometer.
XAS measurements.Pt L3-edge analysis was performed with Si(311) crystal monochromators at the BL14W1 beamlines at the Shanghai Synchrotron Radiation Facility (SSRF) (Shanghai, China).Before the analysis at the beamline, samples were pressed into thin sheets with 1 cm in diameter and sealed using Kapton tape film.The XAFS spectra were recorded at room temperature using a 4-channel Silicon Drift Detector (SDD) Bruker 5040.Pt L3-edge extended X-ray absorption fine structure (EXAFS) spectra were recorded in fluorescence mode.
Negligible changes in the line-shape and peak position of Pt L3-edge XANES spectra were observed between two scans taken for a specific sample.The XAFS spectra of these standard samples (Pt foil and PtO2) were recorded in transmission mode.
Data reduction, data analysis, and EXAFS fitting were applied through Athena and Artemis software. [1]The energy calibration of the sample was conducted through a standard Pt foil, which as a reference was simultaneously measured.For EXAFS modeling, the global amplitude EXAFS (CN, R, σ 2 and ΔE0) were obtained by nonlinear fitting, with least-squares refinement, of the EXAFS equation to the Fourier-transformed data in R-space, using Artemis software, EXAFS of the Pt foil was fitted and the obtained amplitude reduction factor S0 2 value was set in the EXAFS analysis to determine the coordination numbers (CNs) in the scattering path in samples.The Debye-Waller factors and delta Rs were obtained based on the guessing parameters and constrained for Pt-N.Wavelet transformation (WT) was also employed using the software package developed by Funke and Chukalina using Morlet wavelet with κ = 10, σ = 1. [2, 3]  Electrochemical measurements.The electrochemical tests in this paper are all completed using Shanghai Chenhua CHI 660E electrochemical workstation and standard three-electrode system.The Ag/AgCl electrode (built-in saturated potassium chloride electrolyte) was used as the reference electrode, and the graphite rod was used as the counter electrode.All measured electrode potentials were obtained by the formula E (RHE) = E (Ag/AgCl) + 0.199 + 0.059pH conversion.The working electrode adopts a glassy carbon electrode (GCE) whose surface was modified by catalysts, and its preparation method was as follows: the well-dispersed catalyst ink was prepared by ultrasonication with 5.0 mg catalyst, 490 μL C2H5OH and 10 μL Nafion solution (5 wt%) for 30 min.Then, 5 μL of the catalyst ink was pipetted onto the GCE surface (3 mm in diameter, S = 0.0706 cm 2 ).The loading amount of catalysts was 0.71 mg cm −2 on the GCE.A certain amount of prepared catalyst slurry was applied to the surface of the GCE and the catalyst-modified working electrode was obtained after drying at room temperature.The long-term stability of 1.83-ePt@NC and 0.79-ePt@NC catalysts was tested using the chronoamperometric method.In this procedure, catalysts (0.71 mg cm -2 ) were pipetted onto the GCE (L-shaped, S = 0.0706 cm 2 ) surfaces and the applied potential was 0.3 V (vs.RHE).
Computational Details.The calculations were carried out using density functional theory with the PBEform of generalized gradient approximation functional (GGA). [4]The Vienna ab-initio simulation package (VASP) [5-8] was employed.The plane wave energy cutoff was set as 400 eV.The Fermi scheme was employed for electron occupancy with an energy smearing of 0.1 eV.The first Brillouin zone was sampled in the Monkhorst−Pack grid. [9]The 3×3×1 k-point mesh for the surface calculation.The energy (converged to 1.0 ×10 -6 eV/atom) and force (converged to 0.01eV/Å) were set as the convergence criterion for geometry optimization.The spin polarization was considered in all calculations.
Model.The graphene (001) surface was employed as the support for the PtN3 and Pt clusters.
For Pt clusters, the Pt13 will be used, while the defect site near the PtN3 will be selected as the adsorption site for Pt13.In structural optimization calculations, all atoms were allowed to relax.
A vacuum layer as large as 15 Å was used along the c direction normal to the surface to avoid periodic interactions. [10, 11]  For HER, The Gibbs free-energy change (∆Gads) of H on the catalyst is defined as follows: is the adsorption energy of the atomic H on the catalyst, and is the difference in zero-point energy between the adsorbed hydrogen and hydrogen in the gas phase.∆S is the entropy change of one H atom from the absorbed state to the gas phase.Since the H atom is binding on the surface, the entropy of the adsorbed hydrogen can be negligible.Therefore, the S  can be estimated by -1/2×S0, in which S0 is the standard entropy of H2 with gas phase at a pressure of 1 bar and pH = 0 at 300 K Calculation of turnover frequency (TOF) and mass activity.The TOF value was calculated according to the previous report, and the detail was described below: [12]   TOF = #  ℎ     # The number of total hydrogen turnovers was calculated from the current density extracted from the LSV polarization curve according to the following equation: The number of active sites in x-ePt@NC was calculated from the mass loading on the GCE, assuming each Pt center accounts for one active site: Active sites = ( catalyst loading per geometric area (x g cm 2 × Pt wt%) Pt M w ( g mol ) ) ( 6.022 × 10 23 Pt atoms 1 mol Pt ) For commercial Pt/C, the active sites were calculated according to the following equation:

Active sites = 𝑄 𝑠 /𝐴 𝑑𝑖𝑠𝑘 F
where Adisk was the area of GCE, F was the Faraday constant, and Qs was the amount of transferred charge calculated from the integral area of the CO striping region of catalysts in 0.5 M H2SO4.Before the test, the electrolyte was purged by high-purity N2 gas for 20 min.
Subsequently, high-purity CO was bubbled into the electrolyte for 20 min to achieve maximum coverage of CO on the metal surface.Finally, the dissolved CO in the electrolyte was purged out by bubbling high-purity N2 gas for 15 min.
The mass activity is derived from the current density that was normalized by the mass loading, which was calculated by the below equation: [12]   Mass activity =  ×   The mass of Pt on the electrode                                       Table S4.Comparison of HER activity for 1.83-ePt@NC in acidic solution with previously reported catalysts.

Figure S4 .
Figure S4.TEM images of different magnifications of (a-c) eZIF-Pt and (d) HAADF STEM and corresponding EDS mapping of low magnification.

Figure S5 .
Figure S5.(a) First derivatives of Pt L-edge XANES regions of Pt foil, PtO2, and eZIF-Pt.(b) The fitted average oxidation states of Pt from XANES spectra.

Figure S7 .
Figure S7.Experimental and fitting EXAFS curves for eZIF-Pt in R space.

Figure S8 .
Figure S8.EXAFS fitting curves of eZIF-Pt at the K space.

Figure S9 .
Figure S9.The wavelet transform of the eZIF-Pt, Pt foil and PtO2.

Figure S23 .
Figure S23.(a) First derivatives of Pt K-edge XANES regions of Pt foil, PtO2, and 0.79-ePt@NC.(b) The fitted average oxidation states of Pt from XANES spectra.

Figure S25 .
Figure S25.Experimental and fitting EXAFS curves of 0.79-ePt@NC in R space.

Figure S32 .
Figure S32.(a) TEM and (b, c) HAADF-STEM images of 1.83-Pt@NC after stability testing.(d) Size distribution of Pt clusters in 1.83-ePt@NC after stability testing.

Figure S33 .
Figure S33.Comparison of high-resolution Pt 4f XPS spectra of 1.83-ePt@NC before and after electrocatalytic testing in 0.5 M H2SO4.

Figure S35 .
Figure S35.The top-view and side-view of the H-adsorption configurations of (a, b) Pt cluster/PtN3 and (c) PtN3.

Figure S36 .
Figure S36.The PDOS plots of Pt cluster and PtN3.
S0 2 was fixed to 0.8229, according to the experimental EXAFS fit of the sample foil by fixing CN as the known crystallographic value.This value was fixed during EXAFS fitting, based on the known structure of Pt foil.Data ranges 3.0 ≤ k ≤ 11.0 Å -1 , 1.0 ≤ R ≤ 2.0 Å.The Debye-Waller factors and ΔRs are based on the guessing parameters and constrained for Pt-N.

Table S1 .
Fitting parameters for Pt L-edge EXAFS for the eZIF-Pt.Waller factors are obtained based on the guessing parameters and constrained as 0.004 for Pt-N.The delta R is based on guessing parameters and fixed as -0.13 for Pt-N.

Table S2 .
The amount of guest metal in ZIF-8.

Table S3 .
Fitting parameters for Pt L-edge EXAFS for the 0.79-ePt@NC.Pt foil and fixed as 0.8229.a CN, coordination number; b R, the distance to the neighboring atom; c σ 2 , the Mean Square Relative Displacement (MSRD); d ΔE0, inner potential correction; R factor indicates the goodness of the fit.