Supporting Information
Improving Electrocatalysts for Oxygen Evolution Using NixFe3-xO4/Ni Hybrid Nanostructures Formed by Solvothermal Synthesis Jinzhen Huang,1 Jiecai Han,1 Ran Wang,1 Yuanyuan Zhang,2 Xianjie Wang,2 Xinghong Zhang,*,1 Zhihua Zhang,3 Yumin Zhang,1 Bo Song*,1,2,4,5 and Song Jin*,6 1
Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin
150001, China 2
Department of Physics, Harbin Institute of Technology, Harbin 150001, China
3
School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028,
China 4
Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology,
Harbin 150001, China; 5
School of Advanced Study, Taizhou University, Taizhou, 317000, China
6
Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue,
Madison, Wisconsin 53706, USA
E-mail:
[email protected],
[email protected],
[email protected]
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Experimental Section. Materials. Sodium borohydride (NaBH4, 99.99%), nickel chloride hexahydrate (NiCl2⋅6H2O, 99.99%), and iron(III) chloride hexahydrate (FeCl3⋅6H2O, 99.99%) were purchased from Alfa-Aesar and used as received without any further purification. Synthesis of NixFe3−−xO4/Ni nanoparticle samples. All the NixFe3−xO4/Ni (y = 0, 0.05, 0.15, 0.30, 0.50, 0.75, and 1) NP samples were synthesized by a two-step procedure, where y is the iron precursor ratio: Fe/(Fe + Ni). The approximate x value corresponding to y in the each nanocomposite is listed in Table S6. First, the intermediates were obtained by the reduction of metal chlorides with NaBH4. The total concentration of both precursors was maintained for 2.5 mM in each case. In a typical run, take y = 0.30 for instance, NiCl2⋅6H2O (1.75 mM) and FeCl3⋅6H2O (0.75 mM) (total concentration: 2.5 mM) were dissolved in ethanol (50 mL) under sonication. Then, aqueous solution (100 mL) containing NaBH4 (7.5 mM) and sodium hydroxide (NaOH, 10 mM) was added into the above mentioned solution under vigorous magnetic stirring. The resulting black precipitate was collected and washed with ethanol for at least three times. In the second step, the obtained intermediates were re-dispersed into ethanol (25 mL) and transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 24 h. The final products were collected by centrifugation, washed with ethanol for three times, and dried overnight at 60 °C. The transformation during the solvothermal treatment from NizB to Ni can be described in equation (1) as follows:1 NizB + 3z C2H5OH → Ni + z (C2H5O)3B + 3z/2 H2
(1)
Structural Characterization. To investigate the morphological features of the samples, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were recorded using a Hitachi SU8020 and JEM-2100F microscope, respectively. The elemental mappings by energy-dispersive X-ray spectroscopy (EDS) were obtained using a JEM-2100F TEM equipped with an Oxford energy-dispersive X-ray analysis system. X-ray diffraction (XRD) measurements were performed using a Rigaku D/max 2500 X-ray diffractometer with Cu Kα radiation at the range of 10 to 90 degree. Raman spectra were collected on a Renishaw inVia confocal Raman microscopy system with a TE air-cooled 576×400 CCD array using 532 nm laser as the excitation source. X-ray photoelectron spectroscopy (XPS) measurements were obtained using an ESCALAB MKII spectrometer with Mg Kα radiation.
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Electrochemical Measurements. All the electrochemical measurements were conducted on a CHI 760E electrochemical workstation (Chenhua, Shanghai). Before tests, the catalyst ink was prepared by dispersing samples (10 mg) into a mixture (1 mL) of ethanol (0.95 mL) and Nafion (0.05 mL, 5 wt.%) followed by sonication for 1 h. The working electrode was prepared by dropcasting the catalyst ink (10 µL) onto a glassy carbon electrode (GCE) with a diameter of 5 mm (area, 0.19625 cm2) and then dried in the air. Thus, the mass loading was calculated to be ~ 0.50 ± 0.05 mg cm−2. During the tests, a conventional three electrodes system using Ag/AgCl and graphitic electrode (Alfa Aesar, 3.05 mm, 99.99%) as reference electron and counter electrode, respectively, was applied. The potentials measured against Ag/AgCl electrode were converted to the potential versus reversible hydrogen electrode (RHE) according to the equation ERHE = EAg/AgCl + 0.059 pH + 0.197. All the tests were performed in 1 M KOH aqueous solution and at a rotating speed of 1,600 rpm. The linear sweep voltammograms for oxygen evolution reaction (OER) and were obtained in the potential ranges of 1.0 – 1.8 V vs. RHE, using a scan rate of 5 mV s−1. The electrochemical impedance spectroscopy (EIS) was measured at 1.50 V vs. RHE over a frequency range from 100 kHz to 0.1 Hz with a 5 mV AC dither. For the double-layer capacitance (Cdl) measurements, cyclic voltammograms (CV) were collected at various scan rates, i.e., 10, 15, 20, 25, and 30 mV s−1 at the potential window of 0.85–0.95 V vs. RHE. For OER stability tests, potentials at 1.5 V vs. RHE and −0.2 vs. RHE were applied and the tests were kept running for 10 h. The linear voltammograms before and after the test were also collected.
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Supplementary Figures and Tables:
Figure S1. SEM images of various reaction intermediates, A) y = 0, B) y = 0.15, and C) y = 1.
Figure S2. SEM images of NixFe3−xO4/Ni nanocomposites, A) y = 0.05, B) y = 0.30, C) y = 0.50, and D) y = 0.75.
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Figure S3. XPS spectra of the reaction intermediate (y = 0), A) Ni 2p, B) B 1s. Signals of XPS survey emerged at around 860 and 190 eV, corresponding to Ni, and B elements, respectively.2 The Ni spectrum (Figure S3A) shows no apparent peaks ascribed to zero valence metal (Ni0) at 852.2 eV, indicating oxidation state of Ni. A weak signal at 192.8 eV in B spectrum is subjected to the B–O bonding in borides or boron oxide (Figure S3B).3
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Figure S4. Elemental mapping of the reaction intermediate made with precursor ratio: A) y = 0 and B) y = 0.15.
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Figure S5. XRD patterns of NixFe3−xO4/Ni (y = 0.05, 0.30, 0.50, and 0.75) samples.
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Figure S6. A) High resolution TEM image, B) SAED pattern of NixFe3−xO4/Ni (y = 0), C) High resolution TEM image and D) SAED pattern of NixFe3−xO4/Ni (y = 1).
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Figure S7. A1) to D1): the low-resolution TEM images of NixFe3−xO4/Ni (y = 0.05, 0.30, 0.50, and 0.75) samples, respectively (insets are the corresponding size distributions histograms). A2) to D2): the high-resolution TEM images of NixFe3−xO4/Ni (y = 0.05, 0.30, 0.50, and 0.75) samples, respectively (insets are the FFT of the corresponding selected area). A3) to D3): the SAED patterns of NixFe3−xO4/Ni (y = 0.05, 0.30, 0.50, and 0.75) samples, respectively.
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Figure S8. Raman spectra of various NixFe3−xO4/Ni (y = 0, 0.05, 0.15, 0.30, 0.50, 0.75, and 1) samples In order to obtain more information about the crystal structures, Raman spectra were collected. A broad peak ranging from ~ 400 to 700 cm−1 and centered at 550 cm−1 is observed for NixFe3−xO4/Ni with y = 0, ascribed to the first longitudinal optical-phonon modes for the vibration of Ni–O bond.4 No distinguished bands ascribed to the Fe–O vibration can be found when the Fe content is less than 0.50, probably due to the overlap between Fe–O and Ni–O vibrations.2 With the increase in the Fe content, typical vibration modes for spinel NiFe2O4 or Fe3O4 are observed. Both NiFe2O4 or Fe3O4 are inverse spinel, which can be described as A(BA)O4 with the composition of A–O tetrahedrons (AO4) and B–O octahedrons (BO6), where A represents Fe and B denotes Fe for Fe3O4 or Ni for NiFe2O4.5 Peaks located at 320 and 490 cm−1 are ascribed to the Eg and F2g vibration modes in AO4 sites. However, the most intense peak located at 690 cm−1 is the A1g vibration mode in BO6 sites.6
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Figure S9. A) Survey XPS spectra of NixFe3−xO4/Ni (y = 0, 0.05, 0.15, 0.30, 0.50, 0.75, and 1) samples. B) the magnified spectra in the regions of B 1s.
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Figure S10. A) Ni 2p, B) Fe 2p, and C) O 1s XPS spectra of NixFe3−xO4/Ni (y = 0.05, 0.30, 0.50, and 0.75) samples.
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Figure S11. Cyclic voltammetry curves at the scan rate of 10(black curve), 15 (red curve), 20(red curve), 25(magenta curve) and 30(green curve) mV s−1 for NixFe3−xO4/Ni samples, A) y = 0, B) y = 0.05, C) y = 0.15, D) y = 0.30, E) y = 0.50, F) y = 0.75, and G) y = 1). The voltage window is 0.85–0.95 V vs. RHE. S-13
Figure S12. ECSA-normalized LSV curves for NixFe3-xO4@Ni nanocomposites with y = 0.50, 0.75 and 1 samples. The samples are composed of both metallic Ni and spinel oxide NixFe3−xO4 nanoparticles. The double-layer capacitance (Cdl) behaviors of this two types of materials are different, since spinel oxide is less conductive at lower potential due to its semiconductor character. To compare the electrochemical active surface area normalized activity of the as-prepared samples, we divided all the samples into two groups: for the first group, metallic Ni is the main component (y = 0, 0.05, 0.15 and 0.30); for the second group, the spinel oxides are the main component (y = 0.50, 0.75 and 1), respectively. The LSV curves were normalized by the ECSA according to previously reported work.7 It has been pointed out that Cdl is only a reliable indicator of ECSA when the catalyst has excellent electronic conductivity.8 So ECSA-normalized current density for y = 0, 0.05, 0.15 and 0.30 might be more reliable. For the second group samples (y = 0.50, 0.75 and 1), it can be observed distinctly that the intrinsic activity all decrease as the increase of y value.
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Figure S13. Equivalent circuit model derived from the Nyquist plots. A) Randles circuit for NixFe3−xO4/Ni (y = 0, 0.05, 0.15, 0.75, and 1) samples, and B) Voigt circuit for NixFe3−xO4/Ni (y = 0.30 and 0.50) samples. CPE1 and CPE2 are the two constant phase elements, while Rs, Rf, and Rct are electrolyte resistance, electrode porosity (or interfacial resistance), and charge transfer resistance, respectively. Notably, all catalysts have a similar morphology with little difference in mean particle size. Thus, the emergence of Rf in NixFe3−xO4/Ni (y = 0.30 and 0.50) samples is not ascribed to mass transport resistance due to electrode porosity but the ohmic drop caused by interfacial resistance between metal and semiconductor.9
Figure S14. OER performance of the NixFe3−xO4/Ni (y = 0.15) samples synthesized at different temperatures (iR-corrected).
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Figure S15. TEM images of NixFe3-xO4/Ni with y = 0.15 after OER test, which show that metallic Ni and NiFe2O4 nanoparticles still existed after the OER test. A) Low magnification TEM image, B) and C) high magnification TEM image (inset in B is the SAED pattern).
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Figure S16. The comparison of XPS spectra of NixFe3−xO4/Ni (y = 0.15) before and after OER stability test. A) Ni 2p, B) Fe 2p, and C) O 1s XPS spectra.
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Table S1. Detail information about the three crystal structures taken from their PDF cards.
Ni (PDF#04-0850) NiFe2O4 (PDF#45-0964) Fe O (PDF#45-0629) 3
4
Crystal
Space group
a (Å)
c (Å)
cubic cubic cubic
Fm-3m(225) Fd-3m(227) Fd-3m(227)
3.5238 8.336 8.396
-
Table S2. Element compositions of NixFe3−xO4/Ni (y = 0, 0.05, 0.15, 0.30, 0.50, 0.75, and 1) samples estimated from XPS survey. NixFe3−−xO4/Ni
O (at.%)
Ni (at.%)
Fe (at.%)
Fe/(Ni + Fe)
y=0 y = 0.05 y = 0.15 y = 0.30 y = 0.50 y = 0.75 y=1
33.61 35.88 36.41 32.00 39.88 31.06 40.41
18.32 26.02 22.76 10.64 13.86 5.78 0
0 6.55 8.18 5.84 11.79 7.81 32.59
0 0.20 0.26 0.35 0.46 0.57 1
Table S3. Ratios of different Ni species (metallic Ni and Ni2+) derived from the deconvolution of Ni 2p3/2 XPS spectra. NixFe3−−xO4/Ni
Ni0 2p3/2
Ni2+ 2p3/2
Satellite
y=0 y = 0.05 y = 0.15 y = 0.30 y = 0.50 y = 0.75
0.20 0.18 0.17 0.13 0.10 0.04
0.35 0.44 0.53 0.39 0.49 0.48
0.45 0.38 0.29 0.48 0.41 0.48
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Table S4. Ratios of different Fe species (Fe2+ and Fe3+) derived from the deconvolution of Fe 2p3/2 XPS spectra. NixFe3−−xO4/Ni
Fe2+ 2p3/2
Fe3+ 2p3/2
Satellite
Fe2+/ Fe3+
y = 0.05 y = 0.15 y = 0.30 y = 0.50 y = 0.75 y=1
0.19 0.21 0.22 0.22 0.23 0.21
0.68 0.66 0.62 0.52 0.51 0.44
0.13 0.13 0.16 0.26 0.26 0.35
0.28 0.32 0.35 0.42 0.45 0.48
Table S5. Ratios of different O species derived from the deconvolution of O 1s XPS spectra. NixFe3−−xO4/Ni
Lattice O
Defect sites with low coordination (O1)
–OH (O2)
Adsorbed H2O (O3)
y=0 y = 0.05 y = 0.15 y = 0.30 y = 0.50 y = 0.75 y=1
0.16 0.21 0.22 0.25 0.31 0.35 0.67
0.24 0.37 0.33 0.24 0.12 0.19 0.16
0.45 0.35 0.34 0.39 0.33 0.26 0.08
0.15 0.07 0.11 0.12 0.24 0.20 0.07
Table S6. The value of x in different NixFe3−xO4/Ni nanocomposites deduced from the XPS results. NixFe3−−xO4/Ni
x
y = 0.05 y = 0.15 y = 0.30 y = 0.50 y = 0.75
0.44 0.36 0.30 0.16 0.10
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Table S7. Comparison of OER performance of NixFe3−xO4/Ni (y = 0.15) sample with other OER catalysts in 1 M KOH aqueous solution. Catalyst
Substrate
η10 (mV)
Tafel slope (mV dec−1)
Reference
NixCo3−xO4 CoFe2O4/PANI-MWCNTs Ni@NC NiFe2O4 nanorods Mn3O4@MnxCo3−xO4 Co(OH)F Co/CoP Nanoparticles FeaCo1−aOx/N-rGO hybrid CoSe2 microspheres VOOH hollow nanospheres CoV2O6−V2O5/NRGO-1 Fe3O4 @Co9S8/rGO Fe3O4/Co3S4 nanosheets CoOx-ZIF Fe3N/Fe4N Fe-doped NiOx nanotubes NiCo-UMOFNs NixFe1−xS2 microflower NixFe1−xSe2 Ni–Mo/Cu nanowire G-FeCoW NixFe3−xO4 /Ni (y = 0.15)
GCE GCE GCE GCE GCE GCE GCE GCE GCE GCE GCE GCE GCE GCE NF GCE GCE NF NF CF GCE GCE
330 314 280 342 246 313 340 257 330 270 239 340 270 318 238 310 250 198 195 280 223 225
48 30.69 45 44 46 52.8 79.5 30.1 79 68 49.7 82.8 56 70.3 44.5 49 42 64 28 66 37 44
J. Mater. Chem. A, 2017, 5, 7173 J. Mater. Chem. A, 2016, 4, 4472 Adv. Mater. 2017, 29, 1605957 Electrochimica Acta, 211 (2016), 871 Adv. Mater. 2017, 1701820 Adv. Mater. 2017, 1700286 Adv. Energy Mater. 2017, 1602355 Adv. Mater. 2017, 1701410 J. Mater. Chem. A, 2017, 5, 15310 Angew. Chem. 2017, 129, 588 –592 ACS Energy Lett. 2017, 2, 1327−1333 Adv. Funct. Mater. 2016, 26, 4712 J. Mater. Chem. A, 2017,5, 9210-9216 Adv. Funct. Mater. 2017, 1702546 ACS Catal. 2017, 7, 2052−2057 Nano Energy, 38 (2017), 167–174 Nature Energ, 1(2016), 16184 J. Mater. Chem. A, 2017, 5, 15838 Nat. Commun.7(2016), 12324 J. Mater. Chem. A, 2017, 5, 4207 Science, 2016, 352, 333-337 This study
GCE: glassy carbon electrode, NF: nickel foam, CF: copper foam. Table S8. The OER performance of NixFe3−xO4/Ni (y = 0, 0.15 and 1) samples in comparison with the intermediates before the solvothermal treatment. y=0
y = 0.15
y=1
η10 for intermediate (mV)
313
273
315
η10 for NixFe3−−xO4/Ni (mV)
300
225
315
Reduction in η10 (mV)
13
48
0
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