Supporting Information

Trimurti Heterostructured CoO/CoxP Hybrid within 2D Hierarchically Porous Nanosheet

toward Boosted Oxygen Electrocatalysis for Rechargeable Zn-air Batteries

Yue Niu1, Meiling Xiao1, Jianbing Zhu, Taotao Zeng, Jingde Li, Wenyao Zhang, Dong Su, Aiping Yu, Zhongwei Chen*

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2020

Experimental Section

Preparation of the Co-HMT precursor

Typically, 2.07g cobalt nitrate (Co(NO3)2·6H2O) and 1g hexamine (HMT: C6H12N4) with a molar

ratio of 1:1 were separately dissolved in 30ml and 50ml absolute ethanol. Then the

Co(NO3)2·6H2O solution was dropwise added into the HMT under vigorous stirring. The pink

precipitate was immediately generated, which demonstrated the formation of Co-HMT nanosheets.

The mixed solution was further kept stirring for 12 h for full reaction. Finally, the designated Co-

HMT bulk crystal was gained by vacuum filtration and washed by ethanol for several times to

remove unreactive species, and dried at 80℃ overnight in a vacuum oven. We also investigated

the ratios of Co(NO3)2/HMT.

Preparation of the heterostructured CoO/CoxP nanoparticles

Sodium hypophosphite (NaPH2O2) was selected as the P source. The Co-HMT nanosheets and

NaPH2O2 were mixed with a certain ratio (Co/P=1:10), and then directly calcinated at 400 ℃ for

2 h in an Ar atmosphere at a heating rate of 5℃ min-1. The obtained final products were designated

CoO/CoxP. For comparative study, CoP/Co2P was prepared with a similar method unless a higher

temperature of 500 ℃ was applied. Besides, CoO was also synthesized by pyrolyzing Co-HMT

directly.

Materials characterization

The microscopic morphology and structure of the as-prepared catalyst materials were carried out

by transmission electron microscopy (TEM, JEOL 2010F) and scanning electron microscopy

(UltraPlus FESEMs (with EDX/OIM); FEI Quanta Feg 250 ESEM (with EDX)). X-Ray

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Diffraction (A Rigaku MiniFlex 600 X-ray diffractometer with the source of a Cu K irradiation) 𝛼

was applied to study the crystal structures. X-ray photoelectron spectroscopy (XPS) data was

collected by a Thermal Scientific K-Alpha spectrometer to investigate the surface chemical

environment of the samples. A Gaussian-Lorentzian mix is used for analyzing XPS peaks.

“Shirley” type background was selected as the background while analyzing. Meanwhile, the

binding energy scale was calibrated to fix the C sp3 peak at 284.8eV. Nitrogen adsorption-

desorption isothermals were obtained by a Micromeritics ASAP 2020 analyzer to study the pore

structures while the surface area of the samples was evaluated by Brunauer-Emmett-Teller (BET)

analysis.

Electrochemical performance measurement

The electrochemical performance of as-prepared catalysts on ORR and OER was carried out on

an electrochemical workstation (Biologic VSP 300). The oxygen reactions were measured in a

three-electrode glass cell system in 0.1M KOH solution under standard pressure and temperature.

A saturated calomel electrode (SCE) and graphite rod were used as the reference and counter

electrodes, respectively. The working electrode is a glassy carbon rotating disk electrode (RDE)

with an effective surface area of 0.196 cm-2. All of the tested results were finally calibrated to the

reversible hydrogen electrode (RHE) based on the Nernst equation:

ERHE = ESCE + 0.241 + 0.059 × pH

To obtain a homogeneous catalyst ink, 5mg of the as-developed catalyst was dispersed into the

1000µL solution consisting of 960µL ethanol and 40µL Nafion, followed by ultrasonication for

30min. Then, 21µL of the as-prepared ink was pipetted dropwise on the glassy carbon surface to

achieve a loading of 0.53mg cm-2. Commercial precious Pt/C (28wt.% Pt) and Ir/C (20wt.% Ir)

catalysts were used as the benchmark references and prepared according to the same procedure.

The measurement was conducted by linear sweep voltammetry (LSV) from -1 to 0 V (versus SCE)

for ORR and from -0.1 to 0.8 V (versus SCE) for OER with the scan rate of 5 mV s-1 and a rotating

speed of 1600 rpm. During the process of testing, O2 or N2 was always purged into 0.1M KOH

solution for 30min before ORR or OER measurements, respectively. Furthermore, a series of LSV

at a scan rate of 5 mV s-1 with the rotating speed ranging from 400 rpm to 1600 rpm were carried

out to study the kinetics of the as-synthesized and commercial catalysts. All the measured

polarization curves were calibrated by IR compensation in the alkaline electrolyte. The electron-

transfer number per molecule is calculated based on the Koutechy-Levich (K-L) equation:

1J

=1JL

+1JK

=1

Bω1/2+

1JK

B = 0.62nFC0(D0)2/3v - 1/6

where: J is the measured current density; JK is the kinetic current density; JL is the diffusion-

limiting current density; B is Levich slope; ω is the rotating speed (rad s-1); n is electron transfer

number; F stands for Faraday constant (96485 C mol-1); C0 (1.2×10-3 mol L-1) is the bulk

concentration of oxygen; D0 (1.9×10-5 cm2 s-1) is the diffusion coefficient of oxygen; v (0.01 cm2

s-1) refers to the kinetic viscosity of the electrolyte. To investigate the electrochemically active

surface area of the catalysts, the double-layer capacitance ( ) was determined by CVs in the 𝐶𝑑𝑙

potential range of -0.26 to -0.14 V (versus SCE) within the non-faradic region and was calculated

via the following equation:

ic = υCdl

( and are stands for the charging current and the scan rate, respectively).𝑖𝑐 𝜐

Assembly of Zn-Air batteries

The as-prepared catalyst materials were directly fabricated into a bifunctional air electrode through

spraying the catalyst ink onto a gas diffusion layer with a catalyst loading of 2mg cm-2. The catalyst

ink was prepared by dispersing 18mg catalysts into solution mixed with 144µL 5% Nafion, 3.24ml

IPA, and 1.08ml DDI. A polished zinc foil was used as an anode. Besides, a stain steel mesh and

copper foil were used as current collectors for the cathode and anode, respectively. The electrolyte

consisting of potassium hydroxide and zinc acetate was regularly purged into the battery.

Commercial Pt/C and Ir/C catalysts were employed as air cathode according to the same method

for comparative study.

Computational Details

The computational simulations were carried out by Vienna ab-initio simulation package (VASP),

which applied projector augmented wave (PAW) pseudo-potentials to reveal the interaction

between nuclei and electrons under the direction of density functional theory (DFT). Within the

generalized gradient approximation (GGA), Perdew-Burke-Ernzerhof (PBE) equation was used to

describe the electronic exchange and correlation effects. The vacuum height is set to 15Å. The

planewave cutoff was set to 400 eV. The k-space was sampled using a 2×2×1 Monkhorst–Pack

grid. Structures are fully relaxed until the forces acting on the atoms are smaller than 0.03 eV/Å.

The Co2P, CoP and CoO nanoparticles were modeled using the Co2P (112), CoP (211) and CoO

(111) surface, respectively. These surfaces were chosen because they are the surfaces observed in

HRTEM analysis. The Co2P (112) and CoP (211) slab was modeled using a 1×1 unit cell with five

atomic layers. The CoO (111) has a 3×3 super cell with six atomic layers. In all the models, the

bottom two atomic layers were fixed and the reminding atoms were set free to relax. The

adsorption energies of surface adsorbates are defined as follows:(Eads)

Eads = EA/slab - EA - Eslab

where represents the total energy of adsorbed system, is the energy of isolated adsorbate EA/slab EA

in gas phase; is the energy of a clean relaxed Co2P (112), CoP (211) and CoO (111) slabs.Eslab

Figure S1. The XRD patterns of Co-HMT MOFs with a series of Co(NO3)2/HMT molar ratios as well as pristine HMT.

Figure S2. SEM images of Co-HMT with molar ratios of (a)0.5, (b)1, (c)2, and (d) the structure after sonication treatment

Figure S3. XRD pattern of Co-HMT phosphorized at 300℃.

Figure S4. XRD patterns of (a) Co2P/CoP, (b) pure CoO.

Figure S5. TEM images of CoO/CoxP.

Figure S6. N2 adsorption-desorption isotherm and pore size distribution of (a) CoO/CoxP, (b) CoP/Co2P, (c) CoO

Figure S7. (a)OER curvers of CoO/CoxP with different Co/HMT ratios of 0.5, 1, and 2; (b)ORR curves of CoO/CoxP with different Co/HMT ratios of 0.5, 1, and 2.

Figure S8. LSV curves of (a)Pt/C, (b) CoP/Co2P, (c) CoO, (d) CoO+CoxP catalyst at different rotating speeds.

Figure S9. Cyclic voltammetry (CV) graph of (a) CoO/CoxP, (b) CoP/Co2P, (c) CoO+CoxP, and (d) CoO at the non-Faradic region at scan rates of 5, 10, 20 and 40 mV s-1. All the CV results are obtained in N2-saturated 0.1 M KOH solution.

Figure S10. TEM images of CoO/CoxP after long-term oxygen reaction test.

Figure S11. EDX spectrum of the catalyst (a) before, and (b) after long-term oxygen reaction test.

Figure S12. Optimized configurations and adsorption energy of ORR intermediates (a) O2, (b) OOH, and (c) OH adsorbed on Co2P (112) surface.

Figure S13. Optimized configurations and adsorption energy of ORR intermediates (a) O2, (b) OOH, and (c) OH adsorbed on CoP (211) surface.

Figure S14. Optimized configurations and adsorption energy of ORR intermediates (a) O2, (b) OOH, and (c) OH adsorbed on CoO (111) surface.

Figure S15. Cyclability of Zn-air batteries between this work and other recently reported results.

Table S1. Summary of the bifunctional activities of as-prepared catalysts for ORR and OER

ORR OER Bifunctionality

CatalystsOnset

potential(V)

Half-wave potential(E1/2, V)

Diffusion-limited current

density(mA cm-2)

Potential at 10 mA cm-2

(Ej=10, V)∆𝐸= 𝐸𝑗= 10 ‒ 𝐸1/2

Pt/C+Ir/C 0.98 0.88 5.7 1.64 0.76CoO/CoxP 0.93 0.86 5.6 1.60 0.74CoO+CoxP 0.88 0.82 4.8 1.61 0.79

CoO 0.87 0.81 5.1 1.65 0.84Co2P/CoP 0.86 0.82 3.6 1.66 0.84

Note: All potentials presented in this table are demonstrated versus reversible hydrogen electrode (RHE) and obtained in 0.1 M KOH solution using the glassy carbon working electrode.

Table S2. Summary of the ORR and OER activities of recently reported bifunctional electrocatalysts

Catalyst

ORR: half-wave potential(E1/2, V)

OER: potential at10 mA cm-2

(Ej=10, V)

∆𝐸= 𝐸𝑗= 10 ‒ 𝐸1/2 Reference

CoO/CoxP 0.86 1.60 0.74 This workCoS2/SKJ 0.84 1.58 0.74 [S1]

CoO0.87S0.13/GN 0.83 1.59 0.76 [S2]3DOM-

Co@TiOxNy0.84 1.62 0.78 [S3]

Co/Co3O4@PGS 0.89 1.58 0.69 [S4]CoFe/N-GCT 0.79 1.67 0.88 [S5]

Co@Co3O4/NC 0.80 1.65 0.85 [S6]NC-Co SA 0.87 1.59 0.72 [S7]CoP@CC 0.67 1.68 1.01 [S8]

CoP@mNSP-C 0.90 1.64 0.74 [S9]CoxOy/NC 0.80 1.66 0.86 [S10]

Note: All potentials presented in this table are demonstrated versus reversible hydrogen electrode (RHE) and obtained in 0.1 M KOH solution using the glassy carbon working electrode.

Table S3. Comparision of the weight percent and atomic percent of the electrocatalyst before and after long-term oxygen reaction test.

Element Before AfterWeight% Atomic% Weight% Atomic%

C k 42.30 56.76 28.71 47.12N k 14.14 16.27 2.84 3.99O k 18.77 18.91 28.27 34.84P k 5.18 2.70 2.01 1.28

Co k 19.61 5.36 38.18 12.77Totals 100.00 100.00

Table S4. Summary of the cyclability of rechargeable Zn-air batteries assembled with state-of-the-art bifunctional oxygen electrocatalysts.

Catalyst Cyclability Reference

CoO/CoxPBattery cyclability of over

200 h at 5 mA cm-2 This work

c-CoxMn3-xO4Battery cyclability of over

17 h at 10 mA cm-2 [S11]

Co-N-CNTs Battery cyclability of over 15 h at 2 mA cm-2 [S12]

Co3O4/NPGCBattery cyclability of over

80 h at 5 mA cm-2 [S13]

Co@Co3O4Battery cyclability of over

40 h at 20 mA cm-2 [S14]

NCNT-CoO-NiO-NiCo Battery cyclability of 17 h at 20 mA cm-2 [S15]

CoPS Battery cyclability of over 100 h at 10 mA cm-2 [S16]

A-Co@CMK-3-D Battery cyclability of over 45 h at 5 mA cm-2 [S17]

NiCoOS Battery cyclability of over 170 h at 5 mA cm-2 [S18]

1 nm-CoOx/N-RGOBattery cyclability of 10 h

at 6 mA cm-2 [S19]

CoP-PBSCF Battery cyclability of over 32 h at 10 mA cm-2 [S20]

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