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Atomic species derived CoOx clusters on nitrogen doped
mesoporous carbon as advanced bifunctional electro-catalyst
for Zn-air battery
1. Experimental Section
1.1 Materials.
Cobalt(II) chloride hexahydrate (CoCl2·6H2O, 99.998% Co metal basis, Alfa Aesar), sodium
borohydride (NaBH4, 99.5%, Sinopharm Chemical Reagent Co. Ltd.), potassium hydroxide (KOH,
85%, Sinopharm Chemical Reagent Co. Ltd.), absolute ethanol (C2H5OH, 99.8%, Aladdin), nitrogen-
doped mesoporous carbon (NMC) powders (8.0 at.% Nitrogen, XFNANO) and Ultrathin carbon film
on holey carbon (400mesh, Cu, Ted Pella Inc.) were used as received without any further purification.
1.2 Preparation of clustered CoOx/NMC sample.
Firstly, 1g NaBH4 powder was directly dissolved in a mixed solvent system (10 ml of ultrapure water
and 30 ml of absolute ethanol) at -40 oC (solution A). Then, 10 ml of solution B (CoCl2 in water, 2 mg
ml-1) was added dropwise into solution A with an injection rate of 50 uL min -1 controlled by a syringe
pump system at room temperature (RT). By mixing with 40 mL of NMC dispersion (1.75 mg ml -1,
Vultrapure water/Vabsolute ethanol=1:3) for another 12 h under stirring at -40 oC, we collected atomically dispersed
CoOOH species on NMC substrates (CoOOH/NMC) sample by rinsing, vacuum filtration and followed
naturally drying at RT. According to previous work, CoOx cluster on NMC substrates (CoOx/NMC) was
transformed from CoOOH/NMC by annealing at 500 oC for 1 h with a heating rate of 1 oC min-1 under
flowing Ar gas. For comparison, we also performed a control experiment by varying the temperature
from -40 oC to RT through the synthesis process.
1.3 Characterizations of as-prepared samples.
Powder XRD patterns were acquired at room temperature using an X-ray diffractometer (D/max
2500V), a typical scan range of a scanning speed of 8 o min-1, an operating voltage of about 40 kV and
corresponding current of 150 mA were employed. Aberration-corrected HAADF-STEM images were
acquired using a JEM-ARM200F transmission electron microscope operated at 200 kV. XPS
measurements were obtained using an X-ray photoelectron spectrometer (Escalab 250Xi) equipped
with an Al Kα radiation source (1487.6 eV) and hemispherical analyzer with pass energy of 30.0 eV
and an energy step size of 0.05 eV. The binding energy of the C 1s peak at 284.8 eV was considered as
an internal reference. Spectral deconvolution was performed by Shirley background subtraction by
using a Voigt function convoluting the Gaussian and Lorentzian functions. Inductively coupled plasma-
mass spectrometry (ICP-MS, ELAN DRC-e) measurements were obtained, confirming the final Co
loading contents on NMC of CoOx/NMC was 11.51%. XAFS measurements at the Co K-edge (7709
eV) in both transmission (for Co foil) and fluorescence (for samples) mode were performed at the
BL14W1 in Shanghai Synchrotron Radiation Facility (SSRF). The electron beam energy was 3.5 GeV
and the stored current was 260 mA (top-up). A 38-pole wiggler with the maximum magnetic field of
1.2 T inserted in the straight section of the storage ring was used. XAFS data were collected using a
fixed-exit double-crystal Si(111) monochromator. A Lytle detector was used to collect the fluorescence
signal, and the energy was calibrated using Co foil. The photon flux at the sample position was
2.1×1012 photons per second. The raw data analysis was performed using IFEFFIT software package
according to the standard data analysis procedures. The spectra were calibrated, averaged, pre-edge
background subtracted, and post-edge normalized using Athena program in IFEFFIT software package.
The Fourier transformation of the k3-weighted EXAFS oscillations, k3·χ(k), from k space to R space was
performed over a range of 2.55-11.76 Å-1 (2.62-12.81 for CoOOH/NMC) to obtain a radial distribution
function. And data fitting was done by Artemis program in IFEFFIT. The position of E0 is identified as
the corresponding energy at 0.5 of normalized absorption spectra. The valence of Co is zero and E0 is
7709eV, the cobalt in CoO is bivalent and E0 is 7717.5eV, the valence of Co in Co3O4 is 2.67 and E0 is
7719.5eV. Then, we perform a linear fitting between valence and E0. At last, the valence of samples are
obtained according to the relationship between E0 and valence. E0 of CoOx is 7718.34eV,and E0 of
CoOOH is 7719.56eV.
1.4 Oxygen electrode electrochemical measurements.
All electro-catalytic tests were conducted in a conventional three-electrode electrochemical system
containing 1 M KOH solution electrolyte at room temperature, using an Autolab PGSTAT-204
potentiostat equipped with the Nova 1.11 software. A rotating-disk glassy-carbon (area 0.196 cm 2)
electrode coated with the catalyst ink served as the working electrode, an Ag/AgCl (3 M KCl, +0.214 V
vs. standard hydrogen electrode) and a graphite rod were used as a reference and a counter electrode,
respectively. All potentials applied herein were calibrated to the RHE using the following equation:
ERHE = EAg/AgCl + 0.214 + 0.059×pH. The working electrode was prepared by the following procedure:
catalysts (5 mg for cobalt-based nonprecious catalyst while 2 mg for Pt/C and Ir/C commercial
catalysts) was dispersed in a mixture of alcohol (250 μL), water (700 μL), and Nafion solution (50 μL,
5%) for 20 min to form homogeneous catalyst inks. Then certain amount of the catalyst ink was
pipetted onto the GC surface by several times, with the loading of nonprecious catalyst, Pt/C (20%) and
Ir/C (20%) were 1.0, 0.2 and 0.4 mg cm-2. For oxygen reduction reaction (ORR), RDE tests were
performed in O2-saturated 1 M KOH solution at 1600 rpm with a sweep rate of 10 mV s -1 at room
temperature, after cyclic voltammetry (CV) activation for 30 cycles with a scan rate of 50 mV s -1 in N2-
saturated electrolyte. The accelerated durability test (ADT) were carried out at the voltage range of
0.57 to 1.07 V (vs. RHE) for 1000 cyclic voltammetry cycles with a scan rate of 100 mV s -1. Nyquist
plots obtained from EIS measurements at 0.85 V (vs. RHE) in O2-saturated electrolyte. For oxygen
evolution reaction (OER), RDE tests were performed in N2-saturated 1 M KOH solution at 1600 rpm
with a sweep rate of 10 mV s-1 at room temperature, after cyclic voltammetry (CV) activation for 30
cycles with a scan rate of 50 mV s-1. The accelerated durability test (ADT) were carried out at the
voltage range of 1.02 to 1.72 V (vs. RHE) for 1000 cyclic voltammetry cycles with a scan rate of 100
mV s-1. Nyquist plots obtained from EIS measurements at 1.55 V (vs. RHE) in N2-saturated electrolyte.
1.5 Zn-air batteries experiments.
The Zn-air battery tests were performed with a homemade cell configuration using a NEWARE CT-
3008 system to carry out the cycling test (1 h for each discharge and charge period), where a mixed
solution of 0.2 M ZnCl2 + 6 M KOH and a fresh polished Zn plate (1 mm thick) were used the
electrolyte and anode, respectively. The air cathode consisted of a hydrophobic carbon paper with a gas
diffusion layer (1.5 cm in diameter) on the air-facing side and a catalyst layer on the water-facing side.
The catalyst layer was made by loading catalyst ink onto the carbon paper by drop-casting with a
loading of 10 mg cm-2 for all catalysts.
2. Supplementary Figures
Figure S1. Selective STEM images of CoOx/NMC at different magnifications
Figure S2. (a) HAADF-STEM images image of CoOx/NMC and (b) the
corresponding size distribution of CoOx clusters.
Figure S3. (a) HAADF-STEM and the corresponding EDS elemental mapping images
of CoOx/NMC, (b) Co, (c) O e and (d) Co and O overlay.
Figure S4. (a) SEAD pattern and (b) EDS spectrum of CoOx/NMC.
Figure S5. HAADF-STEM images at different magnifications of (a, b) CoOOH/NMC-RT
and (c, d) CoOx/NMC-RT.
Figure S6. HAADF-STEM images of (a-b) CoOx/NMC-750 and (c-d) CoOx/NMC-
900.
Figure S7. XRD patterns of (a) CoOOH/NMC, CoOOH/NMC-RT and pure NMC
substrate and (b) CoOx/NMC, CoOx/NMC-RT with respect to pure NMC substrate.
Figure S8. High-resolution B 1s XPS spectra of CoOx/NMC and CoOOH/NMC.
Figure S9. High-resolution C 1s XPS spectra of CoOx/NMC and CoOOH/NMC.
Figure S10. High-resolution N 1s XPS spectra of CoOx/NMC and CoOOH/NMC.
Figure S11. Normalized X-ray absorption pre-edge structure spectra at Co K-edge for
different samples.
Figure S12. Fourier transformed k3 weight EXAFS oscillations measured at Co K-edge and fitted by different model. (a)Co K edge of CoOx/NMC fitted by structure of CoO. (b) Co K edge of CoOOH/NMC fitted by structure of Co and CoOOH. (c) Co K edge of CoOOH/NMC fitted by structure of CoOOH.
Figure S13. The fitting line of the valence of different samples.
Figure S14. Oxygen electrode catalytic performance of different catalysts in 1 M
KOH. (a) Bifunctional ORR polarization curves, (b) Tafel Plots of ORR, (c)
Bifunctional OER polarization curves and (d) Tafel Plots of OER for CoOx/NMC,
CoOOH/NMC, CoOx/NMC-RT and CoOOH/NMC-RT.
Figure S15. Oxygen electrode catalytic performance of various catalysts in 1 M
KOH. (a) ORR polarization curves, (b) OER polarization curves.
Figure S16. LSV curves of (a) CoOOH/NMC and (b) Pt/C before and after 10,00 potential cycles under ORR conditions. LSV curves of (a) CoOOH/NMC and (b) Pt/C before and after 10,00 potential cycles under OER conditions.
Figure S17. Discharge demonstration to power the LED using two primary Zn-air
batteries in series using CoOx/NMC as the cathode catalyst.
Figure S18. The galvanostatic discharge curves of different Zn-air batteries at a
current density of 10 mA cm−2.
Table S1. EXAFS structural fitting parameters for CoOx/NMC. CN, coordination
number; R, distance between absorber and backscatter atoms; σ2, the Debye-Waller
factor value; The Fourier transformation of the k3-weighted EXAFS oscillations, k3·
χ(k), from k space to R space was performed over a range from 2.55 to 11.76 Å -1 to
obtain a radial distribution function.
Path CN R (Å) σ2
1 Co-O 3.99 2.08 0.01
2 Co-Co 5.63 3.03 0.01
3 Co-O 3.25 3.59 0.01
4 Co-Co 7.34 4.39 0.009
5 Co-O 10.46 4.58 0.01
Table S2. EXAFS structural fitting parameters for CoOOH/NMC. CN, coordination
number; R, distance between absorber and backscatter atoms; σ2, the Debye-Waller
factor value; The Fourier transformation of the k3-weighted EXAFS oscillations, k3·
χ(k), from k space to R space was performed over a range from 2.62 to 12.81 Å-1 to
obtain a radial distribution function (red part belong to Co).
Path CN R (Å) σ2
1 Co-O 3.94 1.90 0.006
1 Co-Co 0.79 2.56 0.0053
2 Co-H 10.17 2.58 0.01
3 Co-Co 3.19 2.81 0.008
4 Co-O 5.43 3.43 0.005
2 Co-Co 1.81 3.68 0.007
5 Co-O 6.40 3.79 0.009
6 Co-H 18.36 4.094 0.008
Table S3. Structural parameters for CoO standard sample. CN, coordination number;
R, distance between absorber and backscatter atoms.
Path CN R (Å)
1 Co-O 6 2.13
2 Co-Co 12 3.01
3 Co-O 8 3.69
4 Co-Co 6 4.26
5 Co-O 24 4.77
6 Co-Co 24 5.22
Table S4. Structural parameters for Co3O4 standard sample. CN, coordination
number; R, distance between absorber and backscatter atoms.
Path CN R (Å)
1 Co-O 12 1.93
2 Co-O 12 2.73
3 Co-O 12 3.04
4 Co-Co 12 3.38
5 Co-Co 4 3.53
6 Co-O 24 4.51
7 Co-O 24 4.71
8 Co-O 24 4.9
9 Co-O 24 5.09
Table S5. Structural parameters for CoOOH standard sample. CN, coordination
number; R, distance between absorber and backscatter atoms.
Path CN R (Å)
1 Co-O 6 1.90
2 Co-H 6 2.75
3 Co-Co 6 2.86
4 Co-O 6 3.43
5 Co-O 6 3.83
6 Co-H 6 3.94
7 Co-Co 2 4.40
8 Co-O 12 4.46
9 Co-O 6 4.77
10 Co-H 12 4.89
Table S6. Structural parameters for Co foil. CN, coordination number; R, distance
between absorber and backscatter atoms.
Path CN R (Å)
1 Co-Co 12 2.49
2 Co-Co 6 3.52
3 Co-Co 24 4.31
4 Co-Co 12 4.98
Table S7. Comparison between CoOx/NMC and other recently reported bifunctional
cobalt oxides nanocatalysts for oxygen-electrode activity.
Catalyst Electrolyte
Ej10
(V vs.
RHE)
OER Tafel
Slope
(mV dec-1)
E1/2
(V vs.
RHE)
ORR Tafel
Slope
(mV dec-1)
Ref
CoOx/NMC 1M KOH 1.499 59.8 0.907 71.5This
work
1nm CoOx/N-
RGO0.1M KOH 1.50 76 0.896 NA 1
CoOx/NC 0.1M KOH 1.578 NA 0.80 NA 2
Co@CoOx/
NCNT0.1 M KOH NA NA 0.80 80 3
Co-CoO/N-
rGO 0.1M KOH 1.62 68 0.780 NA 4
single-crystal
CoO
nanorods
1M KOH 1.56 NA 0.85 NA 5
CoO/N-
graphene1M KOH 1.57 71 0.81 NA 6
Ni-doped
CoO NSs1M KOH NA NA 0.825 121 7
Co/
[email protected] KOH 1.58 76.1 0.89 52.6 8
Co3O4/MnO2- 0.1M KOH 1.62 NA ~0.86 NA 9
CNTs -350
Co3O4-doped
Co/CoFe-9000.1M KOH ~1.57 72.8 0.79 NA 10
Co@C
o3O4@NC-
900
1M KOH
OER
0.1M KOH
ORR
1.60 94 0.80 NA 11
CeO2/
Co3O4@NC-
600
0.1M KOH 1.504 58.3 0.86 65.3 12
N-
Co3O4@NC1M KOH 1.56 NA 0.77 NA 13
Table S8. The fitted data of Nyquist plots. Corresponding equivalent circuits
[Rs(Rct1Q1)(R2Q2)]. Rs is the solution resistance, the high-frequency (Rct1, Q1) element
is associated with the charge-transfer process through electrode-electrolyte interface,
and the low frequency (R2, Q2) element can be attributed to the O2 mass-transport.
Sample OER ORR
Rs (Ω) Rct1 (Ω) R2 (Ω) Rs (Ω) Rct1 (Ω) R2 (Ω)
CoOx/NMC 2.12 1.38 19.90 9.02 37.70 78.60
CoOOH/NMC 2.18 1.56 21.10 10.10 47.00 98.60
Pt/C (ORR)
Ir/C (OER)2.16 2.16 25.30 9.30 80.50 151.50
Table S9. Comparison between CoOx/NMC and other previously reported
bifunctional oxygen catalysts for cathode of Zn-Air Battery.
Catalyst ΔE (V)
Open circuit
potential
(V)
Maximum power density
(mW cm-2)
High energy density
(mW h g-1)
1st Over
potenial (V)
Over potenial
after cycling (V)
Ref
CoOx/NMC-500 0.592 1.476 195.3 849.60.74 @
10mA cm-2
1.03 @ 10mA cm-2
(400h)
This work
Pt/C 0.693 1.410 152.9 806.80.69 @
10mA cm-2
1.12 @ 10mA cm-2
(400h)
This work
Co9S8@MoS2 NA 1.384 NA NA0.8 @ 10mA
cm-2NA 14
Co2P@CNF ~0.89 1.393 121 NA0.81 @
10mA cm-2
1.1 @ 10mA cm-2 (150th)
15
Co-N,B-CSs 0.83 1.430 100.4 NA~1.15 @ 5mA cm-2
1.35 @ 10mA cm-2
(128th)16
P,S-CNS 0.69 1.510 198 8450.8 @ 25mA
cm-2NA 17
NiO/CoN PINWs 0.85 1.460 79.6 9450.84 @
50mA cm-2NA 18
NiFe-LDH/Co,N-CNF
0.752 NA NA NA1 @ 25mA
cm-2NA 19
CoSx@PCN/rGO 0.79 1.380 NA NA1.5 @ 50mA
cm-2NA 20
Co/Co3O4@PGS 0.69 1.450 118.27 NA0.91 @
10mA cm-2
0.92 @ 10mA cm-2
(370h)8
NOGB-800 0.78 NA 111.9 NA0.72 @
10mA cm-2NA 21
CuS/NiS2 INs 0.79 1.440 172.4 NA 0.57 @ NA 22
25mA cm-2
CoZn-NC-700 0.78 1.420 152 NA0.73 @
10mA cm-2
1.1 @ 5mA cm-2 (200th)
23
CuCo2O4/N-CNTs NA 1.360 83.83 653.90.36 @
20mA cm-2NA 24
Mo–N/C @MoS2 0.81 1.46 196.4 846.070.75 @ 5mA
cm-2
1.02 @ 5mA cm-2 (12h)
25
Co–Nx/C NRA 0.65 1.42 193.2 NA0.68 @
50mA cm-2
0.91 @ 50 mA cm-2
(80h)26
S-GNS/ NiCo2S4 0.69 ~1.35 216.3 NA0.8 @ 10mA
cm-2NA 27
Ni0.5Fe0.5@N-GR 0.61 1.482 85 NA0.8 @ 10mA
cm-2NA 28
P-CoSe2/N-C FAs 0.59 1.3 NA NA~0.8 @ 1mA
cm-2
~1.25 @ 1mA cm-2
(20h)29
NCNFs-1000 1.02 1.48 185 8380.73 @
10mA cm-2
0.86 @ 10mA cm-2
(80h)30
Co3FeS1.5(OH)6 0.867 NA NA NA0.86 @
20mA cm-2NA 31
IrMn/Fe3Mo3C 0.63 NA NA NA0.75 @
20mA cm-2
0.86 @ 20mA cm-2
(200h)32
Fe0.5Co0.5Ox 0.74 1.43 86 9040.79 @
10mA cm-2
0.89 @ 10mA cm-2
(60th)33
NGM-Co 0.95 1.44 152 NA1.12 @ 5mA
cm-2
1.15 @ 5mA/cm2
(12h)34
NiS2/CoS2-O NWs
0.765 1.49 NA NA0.95 @ 3mA
cm-2
1.3 @ 3mA cm-2 (25h)
35
Ni6/7Fe1/7S2 NA NA NA NA 0.55 @ ~0.85 @ 36
15mA cm-25mA cm-2
(160th)
1nm CoOx/N-RGO
0.74 1.39 NA NA0.57 @ 6mA
cm-2NA 1
Fe-N4 SAs/NPC 0.775 NA 232 NA1.45 @
50mA cm-2NA 37
Fe3Pt/Ni3FeN 0.665 NA NA NA0.72 @
50mA cm-2
0.97 @ 50mA cm-2
38
meso/micro-FeCo-Nx-CN-30
0.78 ~1.4 150 NA~0.81 @
10mA cm-2
0.8 @ 10mA cm-2 (40h)
39
CoNi/BCF 0.8 1.44 155.1 853.1~0.85 @
10mA cm-2
~0.95 @ 10mA cm-2
(30h)40
Co3O4/Ni foam NA NA 35.7 NA~1.15 @
10mA cm-2
~1.3 @ 10mA cm-2
(20h)41
La2O3/Co3O4/MnO2-CNTs
NA 1.5 295 970~0.7 @
10mA cm-2
~0.8 @ 10mA cm-2
(89h)42
NiCo-air 0.93 1.38 102.08 NA0.98 @
10mA cm-2
1.12 @ 10mA cm-2
(29h)43
U-Fe7C3@NC NA 1.486 105.3 710.3 NA NA 44
Ni1Co3/CN-3 NA 1.16 38.5 NA NA NA 45
Co3O4/MnO2-CNTs -350
0.83 1.47 340 775 NA NA 9
Fe@C-NG/ NCNTs
0.84 1.37 101.2 764.50.89 @
10mA cm-2
1.02 @ 10mA cm-2
(99h)46
Fe/N/C NA NA 250 NA0.66 @ 5mA
cm-2
~1 @ 5mA cm-2
(40h)47
Co@Co3O4@NC-900
0.85 1.06 64 NA~0.7 @
10mA cm-2
<1 @ 10mA cm-2 (98th)
11
Co3O4-doped Co/CoFe-900
0.78 1.43 97 819~0.7 @ 5mA
cm-2
~1.25 @ 10mA cm-2
(65h)10
CeO2/Co3O4@NC-600
0.64 1.41 116.8 805~0.75 @ 5mA cm-2
~1.2 @ 10mA cm-2
(350th)12
CoN4/NG 0.74 1.51 115 6710.84 @
10mA cm-2
0.84 @ 10mA cm-2
(100h)48
(Zn,Co)/NSC NA 1.5 150 NA NA NA 49
Ordered Pd3Pb/C NA NA NA 7100.72 @
10mA cm-2
0.86 @ 10mA cm-2
(135th)50
NPMC 1.02 1.48 55 8351.26 @ 5mA
cm-2NA 51
N-Fe-HPCs NA 1.48 540 >800 NA NA 52
CoNCF-1000-80 0.84 1.44 170 7970.75 @
10mA cm-2
0.83 @ 10mA cm-2
(166h)53
CF-K-A NA 1.4 61.5 NA NA NA 54
Fe-NSDC 0.8 1.53 225.1 NA0.45 @ 4mA
cm-2
0.7 @ 4mA cm-2 (50h)
55
FeNx/C-70020 1.1 1.6 36 NA1.3 @ 5mA
cm-2
1 @ 5mA cm-2 (84h)
56
N, P/CoS2 @TiO2
NP0.78 1.31 NA NA
0.8 @ 10mA cm-2
0.85 @ 10mA cm-2
(133h)57
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