Engineering Two-Phase Bifunctional Oxygen Electrocatalysts with Tunable and Synergetic Components for Flexible Zn–Air Batteries

Highlights A novel heterostructured bimetallic Co/CoFe nanomaterial supported on nanoflower-like N-doped graphitic carbon (NC) is prepared through a strategy of coordination construction-cation exchange-pyrolysis. The Co/CoFe@NC exhibits high bifunctional activities with a remarkably small potential gap of 0.70 V between oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which can be used in liquid and flexible quasi-solid-state rechargeable Zn–air batteries. The density functional theory calculations reveal optimized adsorption energies for intermediates of ORR and OER on heterostructured Co/CoFe@NC. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00650-2.

this process, protons produced from the slow hydrolysis of Fe 3+ can induce partial Co 2+ to be dissociated from Co-PPD, which is accompanied with Fe 3+ coordination to the organic ligand as nodes. This cation exchange process is evidenced by supernatant in pink color, caused by Co 2+ released during the reaction. Finally, the resulting product (Co/Fe-PPD) was collected by centrifugation, washed with ethanol for several times and dried at 60 °C overnight. We also prepared a couple of samples by varying Fe 3+ contents introduced into the Co-PPD complex. The products were denoted as Co/FeL-PPD and Co/FeH-PPD, respectively, where subscript L and H represent a low (70 μL) and high (420 μL) amounts of added Fe 3+ . In general, these Fe 3+ -incorporated samples (Co/Fe-PPD, Co/FeL-PPD and Co/FeH-PPD) maintained the nanoflower structure of Co-PPD. However, when the amount of added Fe 3+ was over 700 uL, the nanoflower structure of the resultant sample would be destroyed, tending to form featureless nanospheres.

Synthesis of the Co/CoFex@NC catalyst:
The Co/CoFex@NC catalyst was synthesized by pyrolyzing the Co/Fex-PPD complex in a tube furnace under an Ar atmosphere at 800 °C for 2 h with a ramp rate of 5 °C/min. In the paper, the Co/CoFe@NC signifies a sample with optimized content of Fe, while the Co/CoFeL@NC and Co/CoFeH@NC denote samples with low and high contents of Fe, respectively.
Monometallic Co@NC and Fe@NC catalysts were synthesized by pyrolyzing the Co-PPD and Fe-PPD complex in a tube furnace under an Ar atmosphere at 800 °C for 2 h with a ramp rate of 5°C/min, respectively.

Physical characterization:
The structures and compositions of samples were analyzed by powder X-ray diffraction (XRD) using an X-ray diffractometer (Bruker Foucs D8) with Cu Kα radiation (λ=1.54178 Å, 40 kV and 40 mA). Fourier transform infrared (FTIR) spectra were recorded from KBr disks using a Thermo Scientific Nicolet 6700 spectroscopy instrument with a scan range of 400-4000 cm -1 . The scanning electron microscopy (SEM) images were collected on a Hitachi S-4800 (Hitachi, Japan). The transmission electron microscopy (TEM) images, high resolution transmission electron microscopy (HRTEM) images, the selected-areaelectron-diffraction (SAED) pattern and energy-dispersive X-ray spectroscopy (EDS) elemental mapping images were collected on a microscope (JEM2100F, JEOL, Japan) with an accelerating voltage of 200 kV. The Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model were utilized to calculate the specific surface areas and pore size distributions, respectively. The chemical state and elemental composition of samples were analyzed using X-ray photoelectron spectroscopy (XPS ESCALAB 250) technique, all data were corrected using the C 1s peak at 284.8 eV as an internal standard. Raman spectra were recorded on a confocal microscope laser Raman spectrometer (Rainshaw invia).
Density functional theory (DFT) calculation details: Calculations were carried out with DFT implanted in the Vienna Ab-initio Simulation Package (VASP) to give a better understanding for the superior activity of Co/CoFe@NC. The Perdew-Burke-Ernzerhof (PBE) parametrization of the generalized gradient approximation (GGA) is for the exchange-correlation functional. The energy cut-off (ECUT) of the plane wave basis was set as 500 eV and a gamma centered 7*7*3 Monkhorst-Pack k-point (KPOINTS) mesh was applied for the k-point samples in the Brillouin zone for the surface model geometry optimization. The tolerance of electronic and ionic relaxation is 10 -5 eV and 0.01 eV/A respectively.

Electrochemical measurements:
The electrocatalytic activities of ORR and OER were evaluated by employing a CHI 760E electrochemical workstation (Shanghai Chenhua instrument, China) coupled with a Pine rotator in a standard three-electrode system. For a working electrode, 5 mg of catalyst was ultrasonically dispersed in a mixed solution containing 0.49 mL of isopropanol and 10 μL of 5 wt% Nafion solution to form a homogeneous suspension. The 10 μL of the as-prepared catalyst ink was evenly dropped on a clean rotating disk working electrode (RDE) or rotating ring-disk electrode (RRDE) with a catalyst loading of 0.25 mg cm -2 . The Hg/HgO (1 M KOH) and carbon rod electrodes were used as reference and counter electrodes, respectively and all measured potentials (vs. Hg/HgO) in this work were converted to a RHE scale via calibrated equation = Hg/HgO + 0.059 + 0.098.
ORR measurements: All the measurements were carried out in O2-saturated 0.1 M KOH solution. Linear sweep voltammetry (LSV) tests were conducted at a scan rate of 5 mV s -1 with different rotation speeds (400, 625, 900, 1225, 1600 and 2025 rpm) from 0.2 to 1.05 V versus RHE. Koutecky-Levich plots (J -1 vs. ω -1/2 ) were fitted into the linear curves, where the slopes and intercepts can be used to calculate the electron transfer number (n) and kinetic current density (Jk) number according to the Koutecky-Levich equation: Where J is the measured current densities (mA cm -2 ), JK is the kinetics current density (mA cm -2 ) and JL is the diffusion-limiting current densities (mA cm -2 ), ω is the rotating speed (rpm), F is the Faraday constant (96485 C mol -1 ), C0 is the bulk concentration of O2 (1.2 × 10 -6 mol cm -3 ), ν is the kinematic viscosity of the electrolyte (0.01 cm 2 s -1 ), D0 is the O2 diffusion coefficient (1.9 × 10 -5 cm 2 s -1 ), and k is the electron-transfer rate constant.
The electron transfer number (n) and the yield of hydrogen peroxide (H2O2%) can also be calculated based on the rotating ring-disk electrode (RRDE) measurements using the following equation: Where Id represents the disk current and Ir represents the ring current. N is the current collection efficiency of the Pt ring which was determined to be 0.37. The ring potential is fixed at 1.5 V vs. RHE.
OER measurements: Prior to measurements, cyclic voltammetry (CV) scanning was conducted at a scan rate of 50 mV s -1 for 100 cycles to activate the catalyst in N2saturated 1 M KOH electrolyte. After that, the electrochemically active surface areas (ECSA) of the samples were roughly evaluated based on the electrochemical doublelayer capacitance (Cdl), which was determined from CV curves recorded at different scan rates from 40 to 160 mV s -1 in the non-Faradaic potential region (1.0-1.1 V). The value of Cdl was calculated according to the following equation: Where is the difference between anodic and cathodic current densities recorded at the middle of selected potential range, and is the scan rate.
The OER polarization curves were measured at a potential window of 0 -1 V (vs. RHE) with a sweep rate of 2 mV s -1 and all the LSV curves were presented without any iR correction. Tafel slopes were calculated from the LSV curves by plotting overpotential (η) against log (j). The electrochemical impedance spectroscopy (EIS) measurements were carried out at various overpotentials from 10 kHz to 0.1 Hz with an amplitude of 5 mV. The stability of optimized catalyst was characterized by chronopotentiometric test with a constant current at 10 mA cm -2 for 12 h.

Assembly and test of aqueous Zn-air batteries:
A home-made aqueous Zn-air battery was assembled with the optimized catalyst loaded carbon paper with gas diffusion layer as the air cathode (mass loading: 1 mg cm -2 ), a polished Zn foil (thickness: 0.3 mm) as the anode, and 6 M KOH + 0.2 M Zn (CH3COO)2 as the electrolyte. For comparison, commercial Pt/C and RuO2 mixed catalyst (mass ratio of 1:1) was also prepared by the same procedure to fabricate a liquid primary Zn-air battery. All tests were operated under an ambient atmosphere (oxygen from air). The polarization curves were recorded using a CHI 760E electrochemical working station with the open circuit potential as the initial potential at a scan rate of 2 mV s -1 and both the current density and power density were normalized to the geometric surface area of the cathode. The galvanostatic discharge-charge curves were collected on a LAND testing station at a current density of 20 mA cm -2 with 40 min per cycle (20 min for charging and 20 min for discharging).

Assembly and test of flexible quasi-solid-state Zn-air batteries:
The home-made flexible quasi-solid-state Zn-air battery was fabricated by using zinc-deposited carbon cloth as the anode, and the as-prepared catalyst loaded on a hydrophilic carbon cloth (0.5 × 2.0 cm -2 , catalyst loading: 1 mg cm -2 ) as the air cathode. The hydrogel polymer electrolyte was prepared as follows:[1] A concentrated sodium hydroxide solution (5 mL, 20 M) was slowly dropped into an aqueous solution of acrylic acid monomer (7.2 mL, 47 wt%) under vigorous stirring. Thereafter, 110 mg of ammonium persulfate and 4 mg of N, N′-methylenebis-acrylamide were added to the above mixture solution followed by stirring for 30 min at room temperature. Before polymerization, the above mixture solution was degassed with N2. Subsequently, the free-radical polymerization was initiated to proceed at 45 °C for 30 h. Finally, the as-prepared polymer was peeled off and fully dried in an 80 °C oven and then soaked in a mixed solution containing 6 M potassium hydroxide. The flexible quasi-solid-state Zn-air battery was assembled by attaching as-prepared air cathode and Zn anode on the two sides of gel electrolyte. The cycling test of Zn-air battery was conducted using a recurrent galvanostatic pulse method for 5 min of discharge followed by 5 min of charge at a current density of 5 mA cm -2 . The power density of Zn-air battery was calculated by P = IV, where I is the discharge current density and V is the corresponding voltage. The specific capacity was calculated according the following equation: Specific capacity = current * service hours weight of consumed zinc The following equation is used to calculate hydrogel electrolyte ionic conductivities (σ):

= /
Where d represents the thickness of electrolyte, S is the area of the electrolyte, and R is the ohmic resistance obtained from the electrochemical impedance spectrum. Figures  Fig. S1 a FTIR spectra of p-phenylenediamine and Co-PPD complex. b XRD pattern, c-e SEM images and f-h corresponding EDS elemental mapping images of C, Co, and N elements of Co-PPD complex