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Page 1: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Electronic Supplementary Information (ESI)

Phosphorus and oxygen co-doped composite electrode with

hierarchical electronic and ionic mixed conducting networks for

vanadium redox flow batteries

Wei Ling,‡a Zhian Wang,‡c Qiang Ma,a Qi Deng,a Jian-Feng Tang,a Lei Deng,a Liang-Hong Zhu,d Xiong-Wei Wu*a, Jun-Pei Yue*b and Yu-Guo Guo*b

a.College of Science, Hunan Agricultural University, Changsha, Hunan 410128, China. E-mail:

[email protected] Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy

of Sciences (CAS), Beijing 100190, China. E-mail: [email protected]. School of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China.d.Automotive & Transportation Engineering, Shenzhen Polytechnic, Shenzhen, Guangdong 518055, China;

‡The authors equally contributed to this work.

ExperimentPreparation of the GF-TCNs electrode The GO solution was fabricated for standby application according to Hummers’ method.1 Firstly, 1.2 mL phytic acid solution (70 %) was added to 45 mL GO solution (2 mg mL-1) under intensive mixing, and the GFs (3 cm × 4 cm) were immersed in the aboved solution with 30 min ultrasonic dispersion. Next, the mixed compounds were sealed in an 80 mL Teflon-lined autoclave and underwent a hydrothermal reaction at 180 °C for 12 h. After vacuum freeze drying for 24 h, the resulting products were annealed in a quartz tube furnace under the argon atmosphere at 800 °C for 1 h. Finally, the obtained GFs and black solid were washed three times with deionized water and dried to constant weight at 80 °C for 10 h, and labeled GF-TCNs and TCNs, respectively.

For comparison purposes, the treated GFs named GF-rGO were prepared by an identical process as the GF-TCNs without the addition of phytic acid, and the residual black solid labeled rGO.Structural characterizationThe morphologies and element distribution of the electrode materials were characterized from scanning electron microscope (SEM, JSM-6701F) operated at 10 kV with energy dispersive spectrometer (EDS). The structural properties of the electrode materials were analyzed by X-ray diffraction spectra (XRD, D/max 2500), and Raman spectra (Lab RAM HR Evolution) with a 532 nm laser excitation. The element compositions of the samples were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB250XI) based on a Thermo Scientific ESCALab 250Xi with 200 W Al Kα radiation. The electrolyte wettability of electrodes was tested using 100 μL electrolytes (0.1 mol L-1 (M) VOSO4 in 3 M H2SO4) drop on the surface of electrodes.Electrochemical tests

The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze the electrode reaction process via a three-electrode system2 in the 0.1 M VOSO4 + 3 M H2SO4 electrolyte, and the working electrode area is 0.5 cm× 0.5 cm. The impedance experiments (EIS) are conducted with a frequency range of 0.01-100 kHz at an amplitude of 5 mV under the same test condition as CV, and the data were fitted based on equivalent circuit diagram. The VRFBs assembled pristine GFs and GF-TCNs electrodes (2 × 2 cm2) were performed for the galvanostatic charging and discharging tests with Nafion 115 (DuPont, USA) as the separator, and the electrolyte was 0.75 M VOSO4 + 0.375 M V2(SO4)3 + 3 M H2SO4 (15 mL) (Hunan Yinfeng Co. Ltd.). The voltage windows of the dischargeand charge measurements were set as 1.60–0.95 V, 1.65–0.85 V, 1.65-0.80 V and 1.70-0.80 V at current densities range of 100-150 mA cm-2, 200-225 mA cm-2, 250-300 mA cm-2 and 350 mA cm-2, respectively.

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2019

Page 2: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S1. SEM image and corresponding EDS mappings of the GF-TCNs with the elements distribution of C, O

and P.

Page 3: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S2. (a) XPS survey, (b) High-resolution XPS O 1s spectrum of the GF-rGO.

Page 4: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S3. The electrolyte wettability of GF, GF-rGO and GF-TCNs.

Page 5: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Table S1. The relative element amount of the GF, GF-rGO and GF-TCNs based on XPS.

% C O N PGF 85.1 12.12 2.78

GF-rGO 84.62 12.26 3.13GF-TCNs 58.71 32.46 2.76 6.06

Page 6: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S4. (a) The positive and (b) negative CV curves of the GF-rGO at a scan rate of 10 mV s-1 in the 0.1 M

VOSO4 + 3 M H2SO4 electrolyte

Page 7: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Table S2. The CV and EIS data of the GF, GF-rGO and GF-TCNs

Positive half-cell Negative half-cell

mA cm-2 V mV mA cm-2 V mV

electrode Ipa Ipc Vpa Vpc

Ipc/

Ipa

ΔE Ipa Ipc Vpa Vpc

Ipc/

Ipa

ΔE

GF 123 88.6 1.21 0.644 0.720 566 18.3 -0.355

GF-rGO 152 128 1.17 0.675 0.842 495 40.6 247 -0.376 -0.743 6.08 367

GF-TCNs 160 140 1.09 0.706 0.875 384 99.4 173 -0.356 -0.667 1.74 311

Page 8: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S5 Multi-sweep tests of (a) GF, (b) GF-rGO, (c) GF-TCNs at various rate of 5,8,10,12 and 15 mV/s, (d)

the relationship between the peak current ratio and scan rate of the GF, GF-rGO and GF-TCNs

Page 9: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S6. The Nyquist plots of the GF-rGO in the 0.1 M VOSO4 + 3 M H2SO4 electrolyte with the fitting equivalent

circuit based on the Nyquist curve

Page 10: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S7. (a) Charge-discharge curve, (b) discharge specific capacity, (c) voltage efficiencye and (d) energy

efficiency of the VRFBs assembled GF-rGO electrodes

Page 11: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S8. The discharge/charge curves of (a) GF, (b) GF-rGO and (c) GF-TCNs under various rates

Page 12: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Figure S9. The cycling performances of the GF-rGO electrode at 200 mA cm−2

Page 13: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Table S3. The cycle efficiency data of GF, GF-rGO and GF-TCNs

Current density (mA cm-2)Sample

Efficiency

(%)100 150 200 225 250 300 350

EE 81.7 73.5 66.6 63.0 58.3

GF

VE 83.6 75.1 67.9 64.2 59.1

EE 81.9 74.7 67.8 64.6 60.6

GF-rGO

VE 83.4 76.1 69.2 65.7 61.5

EE 83.0 76.6 70.6 67.6 64.6 58.1 54.0

GF-TCNs

VE 85.5 78.9 72.7 69.6 66.0 59.1 55.1

Page 14: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

Table S4. Comparison of the rate capability and lifespan of the GF-TCNs electrode with previous work on

electrodes materials for VRFBs

Reference Electrode Cycle

number

Current density

(mA cm-2)

Discharge capacity(Ah L-1)

Energy efficiency

Maximum Current density

(mA cm-2)

This work GF-TCNs 1000100

200

30.0

25.1

83.0

71.0350

3Adv. Funct.

Mater., 2019,

1903192

Exfoilated

-GF100

100

200

21.0

10.0

86.4

60.0200

4J. Mater. Chem. A,

2019, 7, 5589-5600

NiCoO2/G

F50

100

150Not Given

73.7

72.5150

5Nano Energy,

2018, 43, 55-62PGF 500

150

200

28.0

(24.0)

73.0

(68.0)300

6Energy Storage

Mater., 2018, 13,

66-71.

GO-

rGO/GF50

50

100Not Given

87.0

65.0100

7J. Mater. Chem. A,

2018, 6, 6625-

6632.

TiC-GF 6580

10026

67.0

62.0100

8J. Mater. Chem. A,

2018, 6, 41-44.PF-GF 1000

120

200Not Given

79.2

65.0250

9Adv. Energy

Mater., 2017, 7,

1700461

rGO-GF 500150

200

23.0

19.0

75.0

71.0300

10Nano Energy,

2016, 28, 19-28.NCS/GF 350

150

200

27.0

23.0

71.0

67.0300

11ACS Appl. Mater.

Interfaces, 2016, 8,

15369-15378.

ZrO2-GF 200100

200Not Given

78.0

66.0250

12Adv. Sci., 2016, 3,

1500276CF-G-1 100

25

125

20.5 87.0

72.0125

13J. Mater. Chem.

A, 2015, 3, 12276-

12283.

NGF-Co 50100

150

20

(17.5)

80.0

75.0150

14Energy Environ.

Sci., 2014, 7, 3727-

3735.

N-CB-GF 8050

150

19.2

10.1

85.2

70.0150

15Nano Lett., 2014,

14, 158-165Nb-GF 50

50

150

21.5

14.4

75.0

88.0150

Page 15: vanadium redox flow batteries hierarchical electronic and ... · The cyclic voltammetry (CV) measurements performed on the electrochemical workstation (CHI760D) were applied to analyze

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