Electron Modulation and Morphology Engineering Jointly Accelerate Oxygen Reaction to Enhance Zn‐Air Battery Performance

Abstract Combining morphological control engineering and diatomic coupling strategies, heteronuclear Fe—Co bimetals are efficiently intercalated into nitrogen‐doped carbon materials with star‐like to simultaneously accelerate oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The half‐wave potential and kinetic current density of the ORR driven by FeCoNC/SL surpass the commercial Pt/C catalyst. The overpotential of OER is as low as 316 mV (η 10), and the mass activity is at least 3.2 and 9.4 times that of mononuclear CoNC/SL and FeNC/SL, respectively. The power density and specific capacity of the Zn‐air battery with FeCoNC/SL as air cathode are as high as 224.8 mW cm−2 and 803 mAh g−1, respectively. Morphologically, FeCoNC/SL endows more reactive sites and accelerates the process of oxygen reaction. Density functional theory reveals the active site of the heteronuclear diatomic, and the formation of FeCoN5C configuration can effectively tune the d‐band center and electronic structure. The redistribution of electrons provides conditions for fast electron exchange, and the change of the center of the d‐band avoids the strong adsorption of intermediate species to simultaneously take into account both ORR and OER and thus achieve high‐performance Zn‐air batteries.

S3 the reaction, the filter cake was collected by filtration, washed alternately with ultrapure water and methanol three times, and then vacuum-dried at 60 °C for 6 h to obtain ZIF-8. Nitrogendoped carbon material (named NC) was obtained by treating ZIF-8 according to the heat treatment procedure for preparing FeCoNC/SL.
The preparation method of FeCoNC/DL is the same as that of FeCoNC/SL except that CTAB was not added.
The method of preparing FeNC/SL and CoNC/SL was the same as the method of preparing FeCoNC/SL, only the following modifications were made: When preparing FeNC/SL, Co(NO 3 ) 2 ⸱6H 2 O was not introduced and the input amount of Fe(NO 3 ) 2 ⸱9H 2 O is 104 mg. Fe(NO 3 ) 2 ⸱9H 2 O was not introduced when preparing CoNC/SL, and the input amount of Co(NO 3 ) 2 ⸱6H 2 O is 80 mg.

Characterization method
The morphology of the material was collected by a field emission scanning electron microscopy (SEM, Zeiss Sigma300), a transmission electron microscopy (TF20), and an aberration corrected scanning transmission electron microscope (AC-STEM, FEI Themis Z) equipped with a spherical aberration corrector and an energy dispersion component. The powder X-ray diffraction signal was collected on an X-ray diffractometer named Rigaku Ultima IV, where the anode target was a Cu target and the collection speed was 10°/min. Xray absorption spectroscopy (XAS) was collected at the Taiwan Synchrotron Radiation Research Center (NSRRC), and the beam was TLS07A1. The XAS raw data was subjected to background subtraction, normalization, k3-weighted Fourier transform and wavelet transform by Athena software. X-ray photoelectron spectroscopy (XPS) signals were collected on an energy spectrometer named Thermo Scientific K-Alpha+, and the binding energy was corrected by standard carbon. The magnetic intensity of the material was collected on a hysteresis loop measuring instrument (LakeShore, 7404). The nitrogen adsorption and desorption experiment was collected on a fully automatic specific surface and porosity S4 analyzer named ASAP2020, where the pre-degassing temperature was 300 °C. The metal loading in the material was determined by inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 7700 s). Among them, 0.1 g of the sample was dispersed into 10 mL of aqua regia, and ultrasonically treated for 10 min to completely dissolve the metals in the material into aqua regia. Then, the residue was removed by filtration and the mixed solution was diluted 100 times for testing ICP-OES. In the results, the loadings of Co in CoNC/SL, Fe in FeNC/SL, Co in FeCoNC/SL and Fe in FeCoNC/SL are 1.01wt%, 0.79wt%, 0.53wt% and 0.41wt%, respectively (Table S5).

Electrochemical experiment
The electrochemical experiment was carried out in a single-chamber electrolytic cell equipped with a rotating disk electrode (RDE), and electrochemical signals were recorded via an electrochemical workstation (Metrohm Autolab). In the three-electrode system, the reference electrode is a Hg/HgO electrode filled with 1 M KOH solution, and the counter electrode is a high-purity graphite rod (purity 99.999wt%). The preparation method of the RDE as the working electrode is as follows. 10 mg of catalyst was fully ultrasonically dispersed in a 1 mL dispersion solution containing 650 μL isopropanol, 300 μL ultrapure water and 50 μL 5% Nafion to form a uniform ink. Then pipette 10 uL ink drop-coated on the 5 mm diameter glassy carbon electrode and air dry. Before starting the electrochemical experiment, first bubbling the electrolyte with O 2 for 1 h to saturate the dissolved oxygen in the electrolyte, and then run a 10-cycle cyclic voltammetry experiment (sweep rate of 100 mV/s) to stabilize the working electrode. The linear sweep voltammetry (LSV) curves of different catalysts were collected at a speed of 1600 rpm. Only when the electron transfer number was studied, the speed of the RDE was controlled to be 400, 625, 900, 1225, 1600 and 2025 rpm, respectively. Note: In consideration of the actual application environment and fair comparison between different reports, the potentials involved in this report are not IR compensated.

S5
The zinc-air battery consists of a positive electrode and a negative electrode. The negative electrode is a high-purity zinc sheet (with a thickness of 0.5 mm), the positive electrode is an air cathode composed of foamed nickel, a catalyst and a water-proof and breathable membrane, and the battery filling liquid is a 6 M KOH solution containing 2 M Zn(CH 3 COO) 2 . The preparation process of the air cathode is as follows: 1) Thoroughly grind and mix 10 mg catalyst, 30 mg carbon black and 20 μL 5% Nafion solution; 2) Place the catalyst in the interlayer between the foamed nickel and the water-proof and breathable membrane (the effective area is about 1 cm 2 ), and then compact by a pair of rollers; 3) Place the foamed nickel side close to the battery filling fluid, and the water-proof and breathable membrane side to contact the air, assemble the air cathode into the Zn-air battery. When testing the specific capacity of zinc-air potential, the discharge current was set at 7.8 mA and the cut-off voltage was 0.1 V. The charge and discharge current density was set 10 mA, the discharge cut-off voltage was set to 0.5 V, and the charge cut-off voltage was set to 2.0 V.

The electron transfer number of ORR is calculated by the Koutecky-Levich equation,
where the Koutecky-Levich equation is as follows: Where j, j k , j L are current density, dynamic current density and limiting current density respectively; w, F, C 0 , D 0 and v are the angular velocity of the working electrode, the Faraday constant (96485 C/mol), the number of electron transfer, the concentration of O 2 in the electrolyte, the diffusion coefficient of O 2 and the dynamic viscosity, respectively.
The electrode potential calibration procedure of the reference electrode was as follows.
The reference electrode is Hg/HgO filled with 1M KOH solution, the working electrode and the counter electrode are Pt discs (diameter 3 mm), and the electrolyte is 1 M KOH solution saturated with H 2 . The collected CV curves were shown in Figure S26, the potential at which S6 hydrogen evolution begins is about 0.96 V, which is 0 V vs RHE when converted to a reversible hydrogen electrode (vs RHE). Therefore, the electrode potential of the reference electrode in this study was 0.134 V.
The acquisition conditions of Nyquist plots were as follows: the electrolyte is a mixed solution containing 0.1 M K 3 Fe(CN) 6    S33 Supporting Information / Table   Table S1