Experimental

3.2.1. Synthesis of Fe@C2N

The powder FeCl3 of 1.17 g was added in 35 mL NMP with an ice bath. Hexaketocyclohexane octahydrate of 1.13 g was added in the solution, and then Hexaaminobenzene trihydrochloride of 1.0 g was added in nitrogen (N2) condition. The flask was maintained to ambient temperature for 120 minutes.

The ice bath was taken the place of an oil bath and the flask was maintained to 175 °C with reflux during 8 hours. After finishing the refluxing, the solution was turned down the heat to 80 °C and sodium borohydride (NaBH4) powder was slowly added. The solution was maintained at reflux for 180 minutes.

The solution was turned down the heat to room temperature and then distilled water was slowly added.

PTFE (0.5 μm) membrane was used to collect the precipitated solid product by suction filtration. The black powder was extracted followed Soxhlet with distilled water (3 days) and methanol (3 days). After that, it was rapidly cooled down below -120 oC using freeze-dried method. After drying, the powder was annealed at 600 oC for 120 minutes in argon (Ar) condition. The final product was washed several times with 4 M HCl and distilled water.

3.2.2. Synthesis of NP Co3O4/Fe@C2N

Fe@C2N was dispersed in the anhydrous ethanol (EtOH) with the concentration of the Fe@C2N in EtOH solution was ~0.3 mg/mL. For the first step synthesis, 2.4 ml of 0.125 M Co(Ac)2 aqueous solution was mixed with 48 ml of Fe@C2N EtOH solution, and then 1.2 ml distilled water was added at ambient temperature. Then, the product was maintained at 80 oC with stirring for 10 hours. After finishing the dispersion, the solution (~40 ml) was moved to the autoclave for the hydrothermal process at 150 oC for 180 minutes. After filtration, the product was obtained and then washed several times with distilled water and ethanol, respectively. The resulting NP Co3O4/ Fe@C2N catalyst was ~20 mg after filtration. To prepare 0.2 M, 0.15 M, and 0.1 M Co3O4/ Fe@C2N, 2.4 ml of 0.2 M, 0.15 M, and 0.1 M Co(Ac)2 solution was added to 48 ml of Fe@C2N EtOH solution, respectively. The following steps were the same as above. To prepare NP Co3O4/C2N catalyst, 2.4 ml of 0.125 M Co(Ac)2 aqueous solution was added to 48 ml of C2N EtOH suspension. The following steps were the same as above.

3.2.3. Structural characterization and electrochemical tests

The micromorphology was investigated using scanning electron microscopy (Nova FE-SEM) instrument. The transmission electron microscopy was conducted using a JEOL 2100F with a probe- side spherical aberration (Cs) corrector at an accelerating voltage of 200 kV. HAADF-STEM images and EELS spectra were taken using a FEI Titan3 G2 60-300 microscope equipped with a double-sided Cs corrector operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was conducted on an X-ray Photoelectron Spectrometer Thermo Fisher K-alpha (UK). The crystal structure of NP Co3O4/Fe@C2N and Fe@C2N was investigated by using X-ray powder diffraction (XRD) (Rigagu- diffractometer, Cu Ka radiation) and a scan rate is 0.03 ^o s^-1 for measurements. In a three-electrode system, graphite rod was used to serve as the counter-electrode and the reference electrode was a commercial Ag/AgCl in saturated KCl. The working electrode was the glassy carbon (GC) electrode which was coated by each catalyst. The 10 mg catalyst was firstly added in mixture solution with EtOH/isopropyl alcohol (IPA) with a 1:1 ratio and the 54 L Nafion solution (5 wt. %, Sigma-Aldrich) was added. The addition of the Nafion could lead as a well-dispersed catalyst solution and act binder between the catalyst and GC electrode. In the case of composite Co3O4+Fe@C2N catalyst, 5 mg commercial Co3O4(<50nm particle size, Sigma Aldrich, Lot# MKBR1914V) and 5 mg Fe@C2N were physically mixed. A 5 L catalyst solution was loaded on the GC disk electrode with controlled 0.125 cm². The working electrode was dried at ambient temperature for all the electrochemical measurements.

In RRDE measurements, the number of transferred electrons (n) and hydroperoxide yields (%) were investigated as follows equations

n = 4

% HO2-= 200

where Id means the value of disk current, Ir means the value of the ring current and N means the efficiency of the current collection on the Pt ring in the RRDE. Abbreviation of N was measured to be 0.41 by the reduction of K3Fe[CN]6. All the electrochemical test was conducted using a potentiostat (Biologic VMP3).

3.2.4. Zinc (Zn)-air cell assembly and measurements

A Zn-air cell is composed of Zinc metal / aqueous electrolyte / cathode. 0.2 M zinc acetate in 6 M aq.

KOH solution was used as the aqueous electrolyte between the anode and cathode, and carbon paper was served as a current collector. The zinc acetate material was used for the rechargeable one Zn metal with 0.25 mm thickness (Alfa Aesar) was selected as the anode and gas diffusion layer (Toray TGP-H- 090) was used as the air electrode. Zinc plate and air electrode were separated by a glass fiber separator (EL-CELL, 18 mm x 1mm). The catalyst (10 mg) was dispersed in a 0.95 mL IPA/EtOH mixed solution (1/1) with Nafion (5 wt.%) stock solution (54 uL) for the catalyst ink. The active area was controlled to be 16 mm in diameter and the catalyst solution ink was loaded on the carbon paper. The catalyst loading density was 1 mg cm-2 and binder content in the cathode was 20 wt. %. In the case of Pt/C+IrO2 sample, 5 mg commercial Pt/C (20 wt.%) and 5 mg IrO2 were physically mixed. Battery test was conducted using a meshed CR2032 coin cell (CR2032-CASE-304-MESH, MTI Co.). Full-cell measurements were investigated at room temperature using a Biologic VMP3.

3.2.5. Hybrid lithium (Li)-air cell assembly and measurements

A Li metal (0.2 mm thickness) was used from Honjo metal, cut for use the anode electrode of 1cm. An organic electrolyte was selected as 1 M LiPF6 in tetraethylene glycol dimethyl ether (TEGDME), and an aqueous electrolyte was selected as 1 M LiNO3 + 0.5 M LiOH solution. The solid electrolyte (Li1+x+yTi2-xAlxP3-ySiyO12, 0.15 mm thickness, OHARA Inc., Japan) between anode and cathode was applied as solid lithium super ionic conductor (LISICON). The cathode was controlled by drop- coating each catalyst inks with Nafion binder onto the carbon paper (Toray TGP-H-090). 1 mg cm-2 catalyst was loaded and Nafion binder content in the cathode was 20 wt. %. Full-cell measurements were investigated on a computer controlled potentiostat (Biologic VMP3).