Salt Effect Engineering Single Fe‐N2P2‐Cl Sites on Interlinked Porous Carbon Nanosheets for Superior Oxygen Reduction Reaction and Zn‐Air Batteries

Abstract Developing efficient metal‐nitrogen‐carbon (M‐N‐C) single‐atom catalysts for oxygen reduction reaction (ORR) is significant for the widespread implementation of Zn‐air batteries, while the synergic design of the matrix microstructure and coordination environment of metal centers remains challenges. Herein, a novel salt effect‐induced strategy is proposed to engineer N and P coordinated atomically dispersed Fe atoms with extra‐axial Cl on interlinked porous carbon nanosheets, achieving a superior single‐atom Fe catalyst (denoted as Fe‐NP‐Cl‐C) for ORR and Zn‐air batteries. The hierarchical porous nanosheet architecture can provide rapid mass/electron transfer channels and facilitate the exposure of active sites. Experiments and density functional theory (DFT) calculations reveal the distinctive Fe‐N2P2‐Cl active sites afford significantly reduced energy barriers and promoted reaction kinetics for ORR. Consequently, the Fe‐NP‐Cl‐C catalyst exhibits distinguished ORR performance with a half‐wave potential (E1/2) of 0.92 V and excellent stability. Remarkably, the assembled Zn‐air battery based on Fe‐NP‐Cl‐C delivers an extremely high peak power density of 260 mW cm−2 and a large specific capacity of 812 mA h g−1, outperforming the commercial Pt/C and most reported congeneric catalysts. This study offers a new perspective on structural optimization and coordination engineering of single‐atom catalysts for efficient oxygen electrocatalysis and energy conversion devices.


Synthesis of the Electrocatalysts
943 mg PA was dispersed in 16 mL water, and 4 g NaCl was added into the solution to promote the ionization of PA. 2.475 g Fe(NO)3•9H2O was added with stirring for 3 h to chelate with PA to obtain SE-PA-Fe.Then 325 mg OPD and 5mL H2O was added into the solution and assembled with SE-PA-Fe to achieve SE-PA-Fe-OPD intermediate.The resulted suspension was filtered and washed deionized water 3 times to remove the NaCl salt completely.The mixture was dried at 60 °C and subsequently carbonized at 900 °C for 2 h (with a heating rate of 5 °C min -1 ) under N2 atmosphere.The obtained sample was treated with 1 M HCl at 90 °C for 24 h, and the precipitate was rinsed with deionized water

Material Characterization
Scanning electron microscopy (SEM) images (Hitachi S-4800) and transmission electron microscopy (TEM) images (JEM-2100F) were captured to observe the microstructures and morphologies of samples.Energy dispersive X-ray (EDX) mapping were recorded on the JEM-2100 microscope operating at 200 kV.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were carried out on a spherical aberration corrected Titan 80-300 operated at 300 kV.Raman analysis was conducted on a Jobin-Yvon Labram-010 Raman spectrometer with a wavelength of 532 nm.X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra equipment (Chestnut Ridge) with Mg Kα radiation (1486.6 eV).The specific surface area and pore structure were measured by the Brunauer-Emmett-Teller (BET) method on a sorptometer (Micromeritics, ASAP 2020).
The metal loading was measured on by an inductively coupled plasma-atomic emission spectroscopy (ICP-AES, AGILENT730ES).X-ray absorption near edge structure (XANES) and extend X-ray absorption fine structure (EXAFS) were performed at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF).

Electrochemical Measurements
Electrochemical measurements were performed on an electrochemical workstation (CHI 760E) with a three-electrode cell system, using an Ag/AgCl (saturated KCl) electrode as reference electrode and carbon rod as the counter electrode.All the potentials were referred to a reversible hydrogen electrode (RHE): ERHE= EAg/AgCl + 0.197 V + 0.0591pH Equation S1 A rotating ring-disk electrode (RRDE) with a glassy carbon disk (GCE, 4 mm in diameter) and a Pt ring was loaded with the as-prepared catalysts, serving as the working electrode.2 mg of samples were dispersed in 0.8 mL ethanol and 5 μL of 5 wt.%Nafion solution under sonication for 30 min.15 μL of the suspension was pipetted onto the GCE and dried in air at a catalyst loading of 0.30 mg cm -2 .O2 was saturated in the electrolyte before the tests.Cyclic voltammetry (CV) tests were conducted in an O2 saturated 0.1 M KOH electrolyte at a scan rate of 0.1 V s −1 .Linear-sweep voltammetry (LSV) measurements were performed at a scan rate of 5 mV s −1 under an electrode rotation rate of 1600 rpm.The applied potential at the ring electrode is 0.5 V.
The peroxide percentage (H2O2%) and the electron transfer number (n) are determined by the following equations: where ID is the disk current, IR is the ring current, and N is current collection efficiency of the Pt ring.N is determined to be 0.40.
The ORR tests were performed in an O2 saturated 0.1 M KOH aqueous solution (70 mL).As to the methanol tolerance test, 47.5 μL methanol dissolved in 1 mL of 0.1 M KOH was added to the O2saturated 0.1 M KOH aqueous solution at 300 s.
A primary Zn-air battery was assembled to verify the catalysts for application in energy conversion and storage devices. 2 mg catalyst was dispersed in 0.8 mL ethanol and 5 μL of 5 wt.%Nafion solution under sonication for 30 min to form a homogeneous catalyst ink.410 μL of the suspension was loaded on the carbon cloth (1*1 cm -2 ) and dried in air at a catalyst loading of 1 mg cm -2 .Polished zinc plate (with a thickness of 0.5 mm), 6 M KOH (containing 0.2 M Zn(OAc)2) solution and carbon cloth loaded with 1 mg cm -2 catalyst were used as the negative electrode, electrolyte and positive electrode, respectively.Charge-discharge cycle experiments were carried out at 10 mA cm -2 for 30 minutes every cycle (15 mins charge, 15 mins discharge).

Supporting Figures and Tables
to neutral and dried to obtain Fe-NP-Cl-C.As a control, Fe-NP-C and Fe-N-Cl-C was prepared with the same method except for the addition of NaCl salt and PA, respectively.Whereas Fe-N-C was fabricated without the inclusion of NaCl and PA.To investigate the influence of NaCl dosage, Fe-NP-Cl-C-3 and Fe-NP-Cl-C-5 were synthesized by changing to 3 g NaCl and 5 g NaCl, respectively.To distinguish the effect of salt, various Fe-NP-Cl-C-LiCl, Fe-NP-F-C-NaF, Fe-NP-C-NaNO3, Fe-NP-Br-C-NaBr, Fe-NP-Cl-C-KCl and Fe-NP-Cl-C-MgCl2 samples were prepared by changing NaCl into equimolar LiCl, NaF, NaNO3, NaBr, KCl and MgCl2, respectively.
Scheme S1.Illustration for the formation mechanism of SE-PA-Fe and SE-PA-Fe-OPD induced by salt effect.

Figure S7 .
Figure S7.(a) TEM image of Fe-NP-C and (b) corresponding EDX elemental mapping of C, N, P and Fe.

Figure S8 .
Figure S8.TEM image of Fe-NP-Cl-C and corresponding EDX elemental mapping of C, N, P, Fe and Cl.

Figure S15. k 3
Figure S15.k 3 -weighted FT spectra in R space for (a) Fe foil and (b) FeTPPCl and Fe-NP-C.

Figure S16 .
Figure S16.EXAFS fitting curves of (a, b) Fe-NP-C and (c, d) Fe-NP-Cl-C in the R space and k space.Wavelet transform plots of (e) Fe foil and (f) Fe2O3.

Figure S23 .
Figure S23.Electron transfer number and H2O2 yield at various potentials of Fe-NP-C and Fe-NP-Cl-C.

Figure
Figure S24.i-t curves for Fe-NP-Cl-C and Pt/C in O2-saturated 0.1 M KOH solution.

Figure S27 .
Figure S27.Photograph of electric fan powered by three Fe-NP-Cl-C-based Zn-air batteries in series.

Figure S29 .
Figure S29.Long-term cycling performance of the Zn-air batteries based on Fe-NP-C, Fe-NP-Cl-C and Pt/C at the current density of 10 mA cm -2 .

Figure S30 .
Figure S30.DFT calculation analyzes the charge density difference of Fe-N4 between dopants and graphitic carbon layer.Electron accumulation is in blue and depletion in yellow.

Figure S31 .
Figure S31.DFT calculation analyzes the charge density difference of P-Fe-N5 between dopants and graphitic carbon layer.Electron accumulation is in blue and depletion in yellow.

Figure S32 .
Figure S32.DFT calculation analyzes the charge density difference of Fe-N2P2 between dopants and graphitic carbon layer.Electron accumulation is in blue and depletion in yellow.

Figure S35 .
Figure S35.Illustration of the ORR reaction mechanism over Fe-N4.

Figure S37 .
Figure S37.Illustration of the ORR reaction mechanism over P-Fe-N5.

Figure S38 .
Figure S38.Illustration of the ORR reaction mechanism over Fe-N2P2.

Table S1 .
The pH variation of solutions.

Table S2 .
The content of Na, Fe and P in PA-Fe, SE-PA-Fe, PA-Fe-OPD and NaCl-PA-Fe-OPD determined by ICP-AES.

Table S3 .
The content of C, N, O, P, Fe and Cl loading in Fe-NP-C and Fe-NP-Cl-C tested by XPS.

Table S4 .
The content of Fe and P in Fe-NP-C and Fe-NP-Cl-C determined by ICP-AES.

Table S6 .
Structural parameters of samples obtained by fitting the EXAFS data.There are the average coordination number (N), path distance (R), Debye-Waller factor (σ 2 ), threshold energy correction (∆E), and the R-Factor of the fitting.

Table S8 .
Comparison of the Fe-NP-Cl-C-based Zn-air battery performance with recent reported Fe/Cobased catalysts.