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Electronic Supporting Information

In-Situ Formation of Highly Active Ni-Fe based Oxygen-Evolving Electrocatalysts via Simple Reactive Dip-Coating Guofa Donga,b,c,†, Ming Fanga,d,†, Jianshuo Zhangc, Renjie Weia,d, Lei Shua,d, Xiaoguang Lianga,d,

SenPo Yipa,d, Fengyun Wange, Lunhui Guanc, Zijian Zhengb,* and Johnny C. Hoa,d,f,* a Department of Physics and Materials Science, City University of Hong Kong, Kowloon Tong, Kowloon, Hong Kong. b Nanotechnology Center, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Ho, Kowloon, Hong Kong. c Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350108, People's Republic of China. d Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, People's Republic of China. e Cultivation Base for State Key Laboratory, Qingdao University, No. 308 Ningxia Road, Qingdao 266071, People’s Republic of China. f State Key Laboratory of Millimeter Waves, City University of Hong Kong, Kowloon Tong, Kowloon, Hong Kong. * Corresponding authors.

E-mail addresses: johnnyho@cityu.edu.hk, tczzheng@polyu.edu.hk.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2017

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1. Experiment and methods 1.1 The loading of RuO2 on glass carbon electrode (GCE)

Before loading, the GCE surface was first polished for 30 min in turn with the slurry of α-Al2O3 powder with particle size of 500nm, 100nm, and 50 nm, and then rinsed with DI water. Next, the electrode was ultrasonicated with DI water, ethanol, and DI water accordingly for 5 minutes and dried in hot air. A homogeneous ink of RuO2 was prepared as following: 2.0 mg RuO2 nanoparticles was dispersed in 1.0 mL isopropanol with 0.1% v/v Nafion by ultrasonication in a weighing bottle. Then, 20 μL of the ink was drop-casted on a clean GCE surface with a catalyst loading of 0.2 mg cm-2. 1.2 Valuation of Electrochemical Surface Area of Ni foam and NiFeOH@NF

Here a double-layer charging method was used to value Electrochemical Surface Area (ECSA) of the electrodes. The electrochemical capacitance of the electrode surface layer was determined by measuring the non-Faradaic capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammograms (CVs)[1, 2]. The capacitance of the double-layer was calculated by the following equation: jc = vCDL

where v is the scan rate, jc is the current density at 0 V vs. SCE and CDL is electrochemical double-layer capacitance. The scan range was from -0.05 V to + 0.05 V vs. SCE and the scan rates were 0.01, 0.025, 0.05, 0.1, 0.2, 0.4, and 0.6 V/s and All current is assumed to be due to capacitive charging. Before beginning every CV sweep the working electrode was held at each potential vertex for 20 s. The CV plots swept on Ni foam were shown in Figure S8a. A plot of jc as a function of v yields a straight line with a slope equal to CDL (Figure S8b). Here CDL of Ni foam was 0.4505 mF calculated by

CDL (Ni foam) = (0.432-(-0.469))mF/2= 0.4505 mF. Then, ECSA (Ni foam) = CDL/Cs

= 0.4505mF/0.040 mF cm-2 = 11.26 cm2

where Cs is the ideal specific capacitance and here for estimation of the ESCA the Cs value in 1 M NaOH (0.040 mF cm-2) was adopted. Using the same method the ESCA of NiFeOH@NF was estimated to be 15.94 cm2 (Figure S9). 1.3 Survey on the uniformity of the samples

In order to test the uniformity of the samples prepared with this reactive dip-coating method, a piece of Ni foam with size of 30×100 mm was first wash with 1 M HCl, water and ethanol alcohol, accordingly, under the aid of ultrasonication and then dried with pure N2. Next the Ni foam was dipped into 1 mM FeCl3 with the long side for 10 s and annealed on the hot plate at 300 ℃ for 30 min. Then the big piece of coated Ni foam was cooled down to the room temperature and cut into ten pieces with size of 10×30 mm. 7 pieces were randomly selected for the fabrication of NiFeOH@NF electrodes to determine the OER activity. The fabrication procedure was the same to that mentioned previously in this work. Next the OER activity was studied under the same condition and the results were shown in figure S5. It was found that the potential differences under current density of 10 mA cm-2 between these samples were less than 10 mV, and even under the current density of 100 mA cm-2 the potential differences were less than 15 mV.

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2. Figures and Tables

Fig. S1. The SEM images and the photographs of bare Ni foam (a, c) and NiFeO@NF (b, d). The insets in (a, b) are photos of the bare Ni foam and the sample, respectively.

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Fig. S2 The locations of EDS measurement performed on the as-produced NiFeO@NF (a) or the surface layer clusters of NiFeO@NF (b,c). The results of Fe/Ni atomic ratio is shown in table S1.

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Table S1. Fe/Ni atomic ratio determined by EDS As-obtained NiFeO@NF Surface layer clusters of

NiFeO@NF

Spot Atomic ratio (Fe/Ni) Spot Atomic ratio (Fe/Ni)

1 4.03/58.22 1 11.38/14.77

2 3.37/70.68 2 12.05/38.24

3 2.66/60.05 3 10.86/47.60

4 9.14/25.59

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Fig. S3. The cross-sectional elemental mapping of Ni, Fe and O in NiFeO@NF. The scale bar in (b,c,d) is 2 μm.

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Fig. S4. Analysis on the SAED of the surface layer of NiFeOH@NF. The radius (R) of the diffraction rings and the d-space between crystal planes are listed in Table S5.

Table S2. The radius (R) of the of the diffraction rings and the d-space

between crystal plane*.

*note: 1) d is the distance between crystal planes and it equals to 1/R.

2) The hkl values are assigned by pairing with the standard database, and it has been found they match

well with the PDF#29-0713 data corresponding to the structure of Gothite FeOOH crystals.

Rx Length (unit: 1/nm) d (unit: nm) hkl

R0 2.4 0.416 110 R1 4.5 0.220 140 R2 7.0 0.142 112 R3 7.9 0.126 242

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Table S3. Peak attribution of the Ni 2p and Fe 2p spectra of NiFeOH

Ni 2p[3, 4] Fe 2p[5-7] O 1s[6, 8]

BE* (eV) Attribution BE (eV) Attribution BE (eV) Attribution

852.7 Ni 2p3/2, Ni metal 706.5 Fe 2p3/2, Pre-Peak 530.1 O 1s oxides

853.9 Ni 2p3/2, NiO 709.9 Fe 2p3/2, Fe2+ 531.6 O 1s (OH)

855.7 Ni 2p3/2, Ni(OH)2 711.6 Fe 2p3/2, Fe3+

533.2

O 1s

absorbing

H2O 857.4 Ni 2p3/2, NiOOH 715.3 Fe 2p3/2, satellite, Fe2+

861.9 Ni 2p3/2, satellite 719.4 Fe 2p3/2, satellite, Fe3+

724.8 Fe 2p1/2

734.4 Fe 2p1/2, satellite

* BE is the abbreviation of binding energy.

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Table S4. The weight ratio and atomic ratio of Fe/Ni determined from the NiFeO@NF

samples produced with different successive raction times

Catalysts η10 (mV) Tafel slope (mV dec

-1)

Reference

NiFeO@Ni foam 210 31 This work

(NiCo-UMOFNs) 250 42 [9] Commercial RuO2 279 N/A

Ni0.5Co0.5/NC 300 62 [10]

FeSe2 nanoplatelet 330 48.1 [11]

NiFe/NF 215 28 [12]

NiFe-LDH/CNT 247 31 [13]

Fe6Ni10Ox 286 48 [14]

nickel oxysulfide fullerene-like

hollow nanospheres 290 62.38 [15]

Fe(OH)3:Cu(OH)2 core–shell nanowires

~365 42 [16]

Co3O4CNA 210 61 [17]

N-doped graphene-CoO 340 71 [18]

FeNi-GO LDH 206 39 [19]

NiFe-LDH 300 40 [20]

IrO2 nanoparticles 340 46

NiFe LDH/oGSH ~270 N/A [21]

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1.25 1.30 1.35 1.40 1.45 1.50

0

50

100

150

200j (

mA

cm

-2)

E (V vs. RHE)

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7

(a) with iR compesation

1.2 1.3 1.4 1.5 1.6 1.7-40

-20

0

20

40

60

80

100

120

140(b) without iR compesation

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7

j (m

A c

m-2)

E (V vs. RHE)

Fig. S5. The CV plots of OER at the samples prepared in the same batch for the uniformity survey.

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Fig. S6. The SEM images of NiFeO@NF samples prepared with 1, 2, 4 or 6 dip-coating times (a to d). The white lines were used for EDS determination on the samples.

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Table S5. OER activities of typical electrocatalysts in 1 M KOH

Reaction times

wt% Average Fe/Ni atomic ratio Fe Ni

1

Line 1 2.96 80.57

1/20.27

Line 2 4.14 81.24

Line 3 3.93 78.67

Line 4 4.11 82.60

average 3.79 80.77

2

Line 1 6.77 75.73

1/10.79

Line 2 6.18 75.09

Line 3 7.09 80.46

Line 4 7.07 76.45

average 6.78 76.93

4

Line 1 9.70 65.44

1/7.06

Line 2 12.00 69.17

Line 3 8.73 71.50

Line 4 7.17 72.88

average 9.40 69.75

6

Line 1 15.33 57.86

1/4.66

Line 2 9.94 67.46

Line 3 11.97 63.53

Line 4 14.46 64.45

average 12.93 63.33

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Fig. S7. The OER activity of NiFeO@NF samples prepared with different dip-coating times. The CV curves were determined in 1 M KOH aqueous solution in a scan rate of 5 mV s-1.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

(b)

Slope = -0.469 mFR2 = 0.995

Slope = 0.432 mFR2 = 0.997

Ni foam

j (m

A c

m-2)

Scan Rate (V/s)-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Ni foam(a)

j (m

A c

m-2)

E (V vs. SCE)

10 mV s-1 25 mV s-1 50 mV s-1 100 mV s-1 200 mV s-1 400 mV s-1 600 mV s-1

Fig. S8. Double-layer capacitance measurements for determining the ESCA of pure Ni foam from CV scans performed in 1 M KOH. (a) CV plots were measured in a non-Faradaic region of the voltammogram at the following scan rate: 0.01, 0.025, 0.05, 0.1, 0.2, 0.4, and 0.6 V/s.

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(b) The cathodic (red circle) and anodic (black square) charging currents measured at 0.00 V vs SCE plotted as a function of scan rate.

-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0j (

mA

cm

-2)

E (V vs. SCE)

25mV s-1 50 mV s-1

100 mV s-1 200 mV s-1

400 mV s-1 600 mV s-1

(a) NiFeO@Ni foam

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

NiFeO@Ni foam(b)

Slope = -0.633 mFR2 = 0.984

Slope = 0.642 mFR2 = 0.997

j (m

A c

m-2)

Scan Rate (V/s) Fig. S9. Double-layer capacitance measurements for determining the ECSA of NiFeOH@NF from CV scans performed in 1 M KOH. (a) CV plots were measured in a non-Faradaic region of the voltammogram at the following scan rate: 0.025, 0.05, 0.1, 0.2, 0.4, and 0.6 V/s. (b) The cathodic (red circle) and anodic (black square) charging currents measured at 0.00 V vs SCE plotted as a function of scan rate.

Fig. S10. Typical SEM images of the NiFeO@Ni foam before (a) and after (b) the chronopotentiometry measurement.

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