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Electronic supplementary information (ESI)for

Solvent dispersion triggered the formation of NiFe-gel

as an efficient electrocatalyst for enhancing the

oxygen evolution reactionHongkai Wang, Weihuang Zhu, Qi Xue, Changhao Wang*, and Kaiqiang Liu*

aKey Laboratory of Northwest Water Resources, Environment and Ecology, Ministry of Education,

Xi'an University of Architecture and Technology, Xi’an 710055, China.

bKey Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of

Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China

E-mail: changhaowang@snnu.edu.cn; kaiqiangliu@snnu.edu.cn

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

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Experimental Section

Materials

Iron (III) chloride (FeCl3), cobalt (II) chloride (CoCl2) ethanol (≥99.5%),

propylene oxide (≥99%) were purchased from shanghai Macklin Biochemical

Co. Ltd. Nickel (II) chloride hexahydrate (NiCl2·6H2O) (98%) was bought

from Energy Chemical (Shanghai, China), Saturated Calomel Electrode was

purchased from IDA Corporation (Tianjin). All the chemicals are commercially

sourced and used without further purification.

Synthesis of NiFe-gel

The NiFe-gel catalysts were synthesized by solvent dispersion-triggered

gelation reported. Firstly, FeCl3 (0.45 mmol), and NiCl2·6H2O (0.9 mmol) were

dissolved in ethanol (1 mL) in one tube, simultaneously. And another tube is

loaded with deionized water (DI) (0.1 mL) mixed with ethanol (1.0 mL).

Secondly, the two tubes were put into the water bath at a constant temperature

(4 oC) for 1~2 hours. During the stirring, a clear solution was formed rather

than a gel. Thirdly, the precursor solution was mixed with the ethanol-water

mixture and efficiently stirred. Then the mixed solution was transferred into a

vial sealed by the foil paper with some small holes. Finally, the vial was put

into a lager vial filled with propylene oxide (≈2 mL) and the larger via was

sealed. After 12 hours, a brown gel was formed, and the liquid above the gel

was exchanged to acetone in order to absorb organic solvent inside the gel. The

acetone exchanged was preceded efficiently and the gel was overnight freeze-

dried for the following measurements.

Characterization methods

Powder X-ray diffraction (XRD) was performed on Bruker D8 Advance

diffractometer, (Germany) with a scan rate of 10o per minute in the 2θ range

from 5o to 70o. Brunauer-Emmett-Teller (BET) was used for mass-specific

surface areas measurement and the result obtained by analyzing nitrogen

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adsorption and desorption isotherms. The isotherm analysis in the relative

pressure was used to obtain the total surface areas of the catalysts.

Transmission electron microscope (TEM) and Energy Dispersive X-Ray

Spectroscopy (EDX) characterization were conducted with Field Transmittance

Electron Microscope (Tecnai G2 F20, USA). X-ray photoelectron spectra (XPS,

AXIS ULTRA) were used for the material composition investigation of the

samples. Inductive Coupled Plasma Emission Spectrometer (ICP, Bruker M90)

was used to detect the elemental composition of the catalysts. Freeze-drying

procedure was recorded with the model of FD8-6P (GLOD-SIM).

Electrochemical characterization

The electrocatalytic properties of the samples were performed using a three-

electrode system connected to an electrochemical workstation (CHI 660D) at

30±1 oC. A Glassy-Carbon Electrode (GCE) (diameter: 3 mm, area: 0.072 cm2)

was used as a working electrode with a sample modified on it. After polished in

alpha alumina powders mixed in deionized water on a Nylon polishing pad, the

GCE was washed in deionized water and ethanol. A Calomel electrode and

carbon rod Calomel electrode were used as the reference electrode and the

counter electrode, individually.

The working electrode was prepared by the following steps. First, 4 mg of

the sample was dissolved in 1 mL mixture of water and ethanol (4:1, v/v), with

70 μL Nafion (5 wt % in water). Then 5 μL of the above solution was dropped

onto the GCE surface and left to dry in air. Before the measurement, the 1

mol/L KOH solution was bubbled with N2 for 15 min. The scan rate of the CV

measurements remained at 50 mV s-1 and 1 mV s-1 for OER polarization curves.

After 20-cycle CV scanning, the EIS test was performed in 1 mol/L KOH. The

current signals were remained at amplitude of 4 mV, and the frequency scan

was determined from 100 mHz to 100 kHz (All test conditions are guaranteed

to be carried out at temperatures around 25 oC).

The TOF (Turnover frequency) values of the catalysts for OER were

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calculated by the following equation.

n4FJATOF

J means current density at an overpotential of 0.3 V in A (0.072 cm-2), A is

the geometric area of GCE (0.072 cm2), F is the Faraday constant; n is the mole

number of cobalt and iron atoms on the electrode.

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Table S1. Performance comparison of different methods for NiFe catalyst.

Reference MaterialsOverpotential at 10 mA cm-2

(mV)BET (m2 g-1)

Onset potential(V)

Tafel (mV dec −1)η= b log (j / j0)

This work NiFe-gel 245 216.9 1.41 50

1 NiFe-LDH 350 60 1.58 47

2 Fe-Ni(OH)2/NF 267 - - 51.5

3NiFe–NiCoO2

polyhedron286 - - 49.3

4 NiFe2O4 381 6.60 1.48 46.5

5 NiFeOx 350 - - -

6 Ni5Fe1/RGO 245 80.04 1.46 -

7 Fe0.5Ni0.5Ox 584 38.4 1.63 72

8 Ni2FeCo-LDH 420 85.41 1.53 78.7

9 NiFe-LDH 210 - - 40

10Ni0.75Fe0.25O

OH200 - - -

11 NiFe-NM@G 208 - - -

12Ni2Fe1

nanometer pearl necklaces

240 - - -

13 NiFe−VM−O 371 - 1.53 28

14 Fe2Ni

MOF/NF222 - - 42.4

15 FeNiP-NP 180 7.53 1.35 76

16Ni0.75Fe0.25Se2

@NF210 - - 39.4

Note: Symbol - refers to no measurements in the corresponding references.

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Table S2. ICP measurements for various NixFe gels.

Atomic ratio Ni0.7Fe-gel Ni1.0Fe-gelNi1.5Fe -

gelNi2.0Fe-gel Ni2.5Fe-gel

Fe (%) 52 44 36 31 27

Ni (%) 38 42 52 60 63

Experimental

ratio of Ni/Fe0.73:1 0.95:1 1.44:1 1.94:1 2.33:1

Theoretical ratio

of Ni:Fe0.70:1 1.00:1 1.50:1 2.00:1 2.50:1

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Table S3. pH change with time upon addition of propylene oxide

Solvent pH change with time upon addition of propylene oxide0 min 30 min 60 min 90 min 120 min

H2O + Ethanol pH = 7.66 pH = 7.89 pH = 8.11 pH = 8.25 pH = 8.44

Notes: The ethanol-water mixture is pure solvent without dissolving two metal precursors.

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Fig. S1. a) N2 adsorption-desorption isotherm curve of the Ni2.0Fe-gel, b) Pore size distribution of

the Ni2.0Fe-gel.

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Fig. S2. XRD and EDX patterns of the Ni2.0Fe-gel.

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Fig. S3. a) EDX and b) XRD measurements for various NixFe gels.

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Fig. S4. OER polarization curves of Ni2.0Fe-gel, Ni- and Fe-gels.

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Fig. S5. XPS characterizations of Ni2.0Fe-gel, Ni-gel and Fe-gel.

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Fig. S6. The theoretical oxygen production and oxygen content collected of Ni2.0Fe-gel in 1M

KOH at 0.95 V for 60 min.

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Fig. S7. a) LSV polarization curves and b) Stability measurementat of Ni2.0Fe-gel and

commercial RuO2.

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Fig S8. Stability test for Ni2.0Fe-gel in 0.1M KOH and 1M KOH electrolyte.

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Fig. S9. The TEM image of Ni2.0Fe-gels (a) before 8 hours of stability test, (b) after 8 hours of

stability test

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Fig. S10. Tafel curves of Ni2.0Fe-gel and RuO2.

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Fig. S11. Gel-method and solution-method with or without addition of propylene oxide during the

synthesis of the catalysts.

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Fig. S12. TEM images of the materials from a) solution method, b) dropping propylene oxide into

the solution, c) infiltrating propylene oxide into the solution.

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Fig. S13. XRD of the resulting materials from a) the gel-method, b) the solution-method

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Fig. S14. Effect of propylene oxide on the performance of the gel catalyst. Gel method (1): infiltrating propylene oxide into the solution; Gel method (2): dropping propylene oxide into the solution; solution method: without addition of propylene oxide.

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Fig. S15. Possible gelation mechanism triggered by the dispersion of propylene oxide into the

aqueous solution of two metal precursors.

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