One-step electrosynthesis of NiFe-NF electrodes for highly efficient overall water splitting

Journal Pre-proofs Full Length Article One-step electrosynthesis of NiFe-NF electrodes for highly efficient overall water splitting Li Xu, Liangliang Cao, Wei Xu, Zhihao Pei PII: DOI: Reference:

S0169-4332(19)32938-1 https://doi.org/10.1016/j.apsusc.2019.144122 APSUSC 144122

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

13 June 2019 16 September 2019 19 September 2019

Please cite this article as: L. Xu, L. Cao, W. Xu, Z. Pei, One-step electrosynthesis of NiFe-NF electrodes for highly efficient overall water splitting, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144122

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One-step electrosynthesis of NiFe-NF electrodes for highly efficient overall water splitting Li Xu,⁎ab, Liangliang Caoab, Wei Xuc, Zhihao Peiab ⁎ Corresponding author. E-mail address: [email protected]. a. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China. b. Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin 300072, People’s Republic of China. c. Tianjin Mainland Hydrogen Equipment Co., Ltd. Tianjin 301609, People’s Republic of China. Keywords: water splitting, HER, OER, nickel foam, CV

ABSTRACT: As a promising hydrogen production method, alkaline water electrolysis has the main disadvantage of high energy consumption. Although some electrodes prepared by hydrothermal synthesis have excellent performance, the cumbersome preparation process can not meet the future application. Here, we fabricated the electrode with schistose stacked hemispherical structure by DC electroplating at high current density for a long time, subsequently, the composition of the electrode was changed by continuous redox reaction. The electrode combine the excellent hydrogen and oxygen evolution characteristics of Ni and Fe

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elements, and has a large specific surface area, which means that it can provide more active site in HER and OER, thus, the overpotential can be greatly reduced. The electrode has excellent performance of HER, with overpotential of 41 mV and 132 mV at current density of 10 and 100 mA cm-2, respectively, in 1 M potassium hydroxide solution. In OER, the overpotential is 270 mV at current density of 100 mA cm-2. In overall water splitting test, the electrode only needs 1.54 V to reach 10 mA cm-2. At the same time, the prepared electrodes have good stability, and the cell voltage increases by only 1.4% even at 300 mA cm-2 current density for 40 h.

Introduction Nowadays, the excessive dependence on fossil energy and the resulting environmental degradation make people realize that the development of green and sustainable energy is crucial to the future of mankind. Solar energy and hydrogen energy are considered as the most ideal green and sustainable energy sources.[1] Hydrogen production from alkaline water electrolysis is a potential method, but high energy consumption is its main drawback.[2, 3] Hydrogen production from water electrolysis using "clean" electricity generated by solar or wind energy or excessive electricity generated by power plants is considered as an attractive method to obtain hydrogen energy. Alkaline water electrolysis includes two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).[4, 5] Their overpotential directly affects the rate of water electrolysis reaction. The advantage of bifunctional electrodes is that the factory can use only one production line of electrodes to produce water electrodes, which reduces the production cost. So far, Pt-based[6] and Ru/Ir-based[7] materials have low HER and OER

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overpotential, respectively, but they cannot be used in large-scale industrial water electrolysis because of their limited reserves and high prices on the earth. Transition metal elements (such as nickel, iron and cobalt) have been found to have certain catalytic activity for HER and OER.[3-5] Especially Ni-based materials are considered to be one of the most ideal materials for hydrogen and oxygen production by alkaline water electrolysis because of their excellent corrosion resistance, reasonable price and high water electrolytic catalytic activity.[8-11] Transition metal sulfides, phosphides and nitrides also have high catalytic activity.[12-19] According to convention, the corresponding overpotential of the electrodes at the current density of 10 mA cm-2 (denoted as η10) and 100 mA cm-2 (denoted as η100) was compared. Li et al.[20] prepared an electrode with high catalytic activity for OER, they found that the combination of CoNi and CoFe2O4 could improve the reactivity better. Hydrothermal process is widely used in the preparation of traditional electrodes.[21, 22] The electrodes synthesized by this method have uniform catalytic layer, large surface area and high catalytic activity for HER and OER.[23, 24] However, because of its high synthesis temperature and pressure, it is not conducive to large-scale industrial production. Herein, we report a nickel foam with nickel and iron doped electrode prepared by primary electrodeposition（denoted as NiFe/NF ） , we synthesized a kind of bifunctional electrode consisting of schistose stacked hemispherical structure. The electrode shows superior catalytic activity toward the HER and OER in alkaline media. η 10 = 39 mV，η 100 = 139 mV in HER，and η 100 = 283 mV in OER. Moreover, the electrode requires only 1.54 V to deliver 10 mA cm-2 in overall water splitting. Experimental

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2.1. Preparation NiSO4 · 6H2O, NiCl2 · 6H2O , Fe2(SO4)3, KOH, absolute ethanol were purchased from Aldrich Ltd. (Shanghai, China). Pt/C (20 wt%, Macklin) and IrO2 (Ir ≥ 84.5%, Energy Chemical) and Nafion solution (5 wt%, Aldrich) were also used. All the reagents are analytical grade and used without any further purification. Deionized water were purchased from Tianjin Yongqing Ultrapure Co. Nickel foam (NF) was purchased from Changsha Li Yuan Co. Ltd. The experimental electrodes were prepared by one-step electrodeposition. First of all, Nickel foam and nickel plates (20 mm x 40 mm x 1.5 mm) were soaked in 3 M potassium hydroxide solution for 3 hours and then soaked in 3 M hydrochloric acid for 30 minutes at room temperature. Next, dissolve Fe2(SO4)3 (4 mM), KOH (5 mM), NiSO4·6H2O (0.39 M), NiCl2·6H2O (0.13 M) in deionized water to configure electroplating bath. Finally, the treated NF was used as cathode and the nickel plate as anode are placed in the electroplating bath. The experimental electrode (denoted as NiFe/NF) was prepared by electroplating for 1 hour at the cathode current density of 36 mA cm-2, the temperature of 30 ℃ and the stirring rate of 80 r/min, and then the electrode prepared by electrodeposition was placed in 1 M KOH solution as working electrodes after cleaning for cyclic voltammetrys (CVs) test, the CVs test was carried out at a scan rate of 1 V s-1 with a potential range from 1.0 V to 1.5 V vs RHE for 50 cycles using a standard three-electrode system at an electrochemical workstation. For comparison, dissolve Fe2(SO4)3 (4 mM), KOH (5 mM) and NiSO4·6H2O (0.39 M), NiCl2·6H2O (0.13 M) in deionized water as plating bath, respectively, the electrodes prepared under the same plating conditions and the same CVs experiment as NiFe/NF are named as Fe/NF and Ni/NF. Moreover, the Pt-C/NF and IrO2/NF electrodes were prepared in the following way: 5 mg of Pt/C (or IrO2),

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0.4 mL of deionized water, 0.52 mL ethanol and 80 uL of Nafion solution were ultrasonically dispersed for 0.5 h, Then, let the mixture drip into the NF with a mass loading of ~2 mg cm-2.[2] 2.2. Characterization The crystal structure of the prepared electrodes were determined by a X-ray diffractometer (XRD, Bruker D8-Advance , Cu Kα, λ = 0.154178 nm). The surface morphology and element content were obtained by scanning electron microscopy (SEM, S-4800, Hitachi, Japan) and energy dispersive spectrum (EDS). The corresponding valence states of the elements were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi , Al Kα). The electrochemical performance was tested by VersaSTAT3 electrochemical workstation (USA). Unless otherwise specified, all electrochemical tests were carried out at room temperature (25 ℃ ) in 1 M KOH solution without any iR-compensation. Cyclic voltammetry (CV), Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were measured by a Standard three-electrode system with the electrodes prepared above (cut into 1×1 cm2), carbon rod and Hg/HgO (1 M KOH) electrode as a working electrode, a counter electrode and a reference electrode, respectively. The measured potential (EHg/HgO) was converted to the potential relative to the reversible hydrogen electrode (ERHE): ERHE = EHg/HgO + 0.098 + 0.0591 × pH.[1] LSV tests were measured at a scan rate with 1mV s-1 to obtained the polarization curves. Chronopotentiometry tests were used to study the stability of the NiFe/NF electrodes. The electrochemical surface area (ECSA) was estimated by measuring the doublelayer capacitance (Cdl) of the electrodes (NiFe/NF, Ni/NF and Fe/NF), which can be obtained by CVs at a different scan rate (10 mV s-1, 20 mV s-1, 40 mV s-1, 80 mV s-1, 100 mV s-1, 200 mV s-1, 400 mV s-1). Electrochemical impedance spectroscopy (EIS) tests were performed at -0.1 V vs

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RHE (HER) and 1.48V vs. RHE （ OER ） with a frequency range from 100 kHz to 0.01 Hz. Overall water splitting tests were performed by a two-electrode system with the distance between cathode and anode of 2 cm. Results and discussion The current density has a great influence on the plating process. The surface morphology of the electrodes prepared at high current density of 36 mA cm-2 is excellent. Chloride ion can not only improve the conductivity of electroplating bath, but also affect the morphology of electroplating bath.[25] Potassium hydroxide is introduced to regulate the PH of electroplating bath, so as to reduce the side reaction of hydrogen evolution in electroplating process. As is illustrated in Scheme. 1, when the nickel is electrodeposited on foam nickel, there will be obvious schistose structure (Fig. S1), which will be stacked together to form a hemispherical

NiFe/NF Codeposition (Ni and Fe(OH)3)

CVs

Nickle foam (NF)

Scheme. 1 Schematic illustration of the synthesis process of the NiFe/NF composite electrodes. shape. when the iron is deposited on the nickel foam alone, spherical particles will be formed and the spherical particles will be stacked together (Fig. S2), when both nickel and iron are deposited

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at the same time, the micro-morphology of the two structures is formed. From the comparison of Fig S3 and Fig S4, we can find that the CVs test did not change the surface morphology of the electrode.

a

c

b

500 nm 500 nm

d 50 μm

e

5 μm

2 μm

O

Fe

Ni

2 μm

500 nm

2 μm

2 μm

Fig. 1 SEM images of (a - d) NiFe/NF.(e) EDS mapping of NiFe/NF.

As is illustrated in Fig. 1b, There is much hemispheric structure on NF substrate after the electrodeposition. From Fig. 1c-d, one can see the stacked and schistose structrue. Fig. 1e is the elemental mapping analysis of the NiFe/NF, It indicates that Fe and Ni elements are evenly distributed. At the same time, it can be seen that nickel exists not only in nickel spheres, but also in iron, the same is true of iron. The distribution of oxygen elements implies the existence of nickel hydroxide and iron hydroxide. Fig. 2a is the XRD pattern of NiFe/NF, because of the loading amount of Fe element and Ni(OH)2 is very low, The XRD peaks just correspond to Ni, but Fig. 2b indicates the existence of Fe element, this is due to the dominance of the following reactions: Ni2+ + 2e- → Ni. In subsequent CVs experiments, the Ni(OH)2/NiOOH redox couple

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a

b

c

d

Fig. 2 (a) XRD pattern and (b) XPS spectra of NiFe/NF.(c) High-resolution Ni 2p spectrum and (d) Fe 2p spectrum. will appear,[26] which makes part of Ni0 in the electrode convert to Ni(OH)2 and NiOOH. Fig. 2c is the high resolution spectrum of Ni 2p, the peaks at 874 eV and 856 eV are corresponding to Ni 2p1/2 and Ni 2p3/2,[19] respectively, that is Ni2+ in the Ni(OH)2. The peaks at 852.4 and 869.4 eV indicates the existence of Ni0. [19] Comparing the Ni2p diagram before and after CVs, we can find that Ni0 in the electrode decreases, and Ni3+ begins to appear at the same time, it means that part of Ni0 is converted to NiOOH or Ni(OH)2. Fig. 2d is the high resolution spectrum of Fe 2p, the peaks at 725 and 712 eV are corresponding to Fe3+.[27] We can see that the content and valence state of Fe element have no obvious change before and after CVs. The introduction of Fe element actually depends on electrophoresis. Fe3+ forms Fe(OH)3 colloid under the promotion of potassium hydroxide, and the Fe(OH)3 colloid adhering to Ni2+ moves to the cathode during

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electroplating. With the reaction of Ni2+ (Ni2+ + 2e-

→ Ni), the Fe(OH)3 colloid gradually

deposits in the cathode, and some of them reacts with oxygen to convert FeOOH. The electrochemical performance of the electrodes was tested in 1 M potassium hydroxide solution. As shown in Fig. 3(a), NiFe/NF and Ni/NF exhibit excellent HER activity, with overpotential of 41 and 46 mV at current density of 10 mA cm-2, respectively, 132 and 149 mV at current density of 100 mA cm-2, respectively, which are much better than other samples, and also better than those of catalytic electrodes reported recently, such as Ni(OH)2/Ni3S2[28] (52 mV at 10 mA cm-2), MoS2-Ni3S2[12] (98 mV at 10 mA cm-2), CoS2-MoS2[3] (154 mV at 10 mA cm-2). Fig S5 shows that compared with carbon rod as a counter electrode, the HER activity of the prepared electrode with Pt as the counter electrode has not been significantly improved or

a

b

c

d

e

f

Fig. 3 HER performance of the NF, Fe/NF, Ni/NF, NiFe/NF and Pt/C on NF samples measured in 1 M KOH. (a) Polarization curves, and (b) corresponding Tafel plots. (c) Overpotentials obtained from HER polarization curves at the current density of 10 and 100 mA cm-2. (d) Nyquist plots (overpotential = 100 mV) for the samples. (e) Plots of the capacitive currents at 0.1 V vs. RHE as a function of scan rate for the samples. (f) Chronopotentiometry plots of NiFe/NF at the current density of 10 and 100 mA cm-2.

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even changed. Fig. 3(b) is the Tafel plots of the samples. The Tafel slopes of the NF, Fe/NF, Ni/NF, NiFe/NF and Pt/C on NF are 163.8 mV dec-1, 172.0 mV dec-1, 38.9 mV dec-1, 33.2 mV dec-1,79.7 mV dec-1, respectively. In order to further study the kinetics of the electrodes, the sample was tested by EIS, and the results were shown in Fig. 3(d), the charge-transfer resistance (Rct) were determined by fitting, the Rct of the Fe/NF, Ni/NF, NiFe/NF are 11.60 Ω, 0.86 Ω, 1.03 Ω, respectively. It can be seen that the NiFe/NF electrode has a small resistance to charge transfer. The electrochemical surface area (ECSA) was estimated by measuring the double-layer capacitance (Cdl) of the electrodes (NiFe/NF, Ni/NF and Fe/NF), which can be obtained by CVs at a different scan rate (Fig. S6). Cdl is proportional to ESCA,[1] and larger Cdl means more active surface area, so it can provide more active sites. As shown in Fig. 3(e), the function diagram of scan rate and current density Δj ( Δj = (ja - jc)/2,[2] that is the difference of current density at 0.1 V vs. RHE) was drawn, and the Cdl of the sample is determined by calculating its slope. The Cdl of the Fe/NF, Ni/NF, NiFe/NF are 13.9 mF cm-2, 47.1 mF cm-2, 41.0 mF cm-2, respectively. In fact, after multi-loop cyclic voltammetry, not only Ni is transformed into Ni(OH)2 and NiOOH, which makes water molecules more effectively decomposed into Had and OHintermediate,[4] but also the initial α-Ni(OH)2/γ-NiOOH redox couple will be transformed into β-Ni(OH)2/β-NiOOH couple. Because β-Ni(OH)2 and β-NiOOH are more ordered and compact, the catalytic activity of the electrode is enhanced.[26, 29] HER includes Volmer (H2O + e- → Had + OH-), Heyrovsky (H2O + Had + e- → H2+ OH-) and Tafel (Had + Had → H2) reactions, their corresponding Tafel slopes are 120，40，30 mV dec-1, respectively.[20] Volmer reaction is the most important step in HER. Ni2+ in the electrode has an unfilled d-orbital, which is conducive to OH- adsorption. At the same time, NiOOH and Ni0 promote the adsorption of Had.[30] It is the

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synergistic effect between Ni0, Ni(OH)2 and NiOOH that makes the electrode have excellent HER activity. The Tafel slope is only 33.2 mV dec-1. In order to study the long-term stability of hydrogen evolution of NiFe/NF electrodes, Chronopotentiometry tests were carried out at cathode current densities of 10 mA cm-2 and 100 mA cm-2, respectively. As shown in Fig. 3(f), after 30 h of hydrogen evolution at 10 mA cm-2 current density, the overpotential even decreased by 13.5% compared with the initial stage. This may be due to the long-term immersion of electrodes in alkaline solution, which resulted in the transformation of part of Ni into Ni(OH)2, thus enhancing its activity of hydrogen evolution. Hydrogen evolution continued at the current density of 100 mA cm-2 for about 30 h, and the overpotential increased by 4.7%, indicating that the activity decreased due to the shedding of the electrode after hydrogen evolution continued for 60h. The OER performance test of the electrodes is similar to that of HER, which is carried out in 1 M potassium hydroxide solution. As shown in Fig. 4(a), NiFe/NF exhibits excellent oxygen evolution activity, the overpotential of NF, Fe/NF, Ni/NF, NiFe/NF and IrO2/NF is 322 mV, 270 mV, 280 mV, 220 mV, 316 mV at current density of 15 mA cm-2, respectively. Meanwhile, at 100 mA cm-2 current density, the overpotential of NiFe/NF electrode is only 270 mV, which is better than some oxygen evolution electrodes reported previously, such as Ni@Ni(Fe)OOH[19] (310 mV), MoS2/NiS[18] (450 mV). Fig S7 shows that compared with carbon rod as a counter electrode, the OER activity of the prepared electrode with Pt as the counter electrode has not been significantly improved or even changed. Fig. 4(b) is the corresponding Tafel plots of the samples. The Tafel slope of NiFe/NF is 51.2 mV dec-1, slightly higher than that of Fe/NF at 51.5 mV dec-1, while the Tafel slope of Ni/NF and IrO2/NF are 74.1 mV dec-1 and 79.4 mV dec-1, respectively. It can be concluded that the introduction of Fe element greatly enhances its oxygen

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evolution activity, which provides a guarantee for it to be an efficient bifunctional electrocatalyst. The Nyquist plots of the samples are shown in Fig. 4(d), It can be seen that the Rct of NiFe/NF is the lowest, only 1.25 Ω, which is lower than that of Fe/NF (4.09 Ω) and Ni/NF (7.97 Ω), it means that the NiFe/NF electrode has a small resistance to charge transfer. There are many intermediates in the OER process. M is the catalyst, OER can be described as the following process:[31] M + OH- → MOH + eMOH + OH-→ MO + H2O+ eMO + OH- → MOOH + eMOOH + OH- → M + O2 + H2O + e-

a

d

b

c

e

Fig. 4 OER performance of the NF, Fe/NF, Ni/NF, NiFe/NF and IrO2/NF samples measured in 1 M KOH. (a) Polarization curves, and (b) corresponding Tafel plots. (c) Overpotentials obtained from OER polarization curves at the current density of 15 and 100 mA cm-2. (d) Nyquist plots (overpotential = 250 mV) for the samples. (e) Chronopotentiometry plots of NiFe/NF at the current density of 10 and 100 mA cm-2.

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The existence of unfilled d-orbitals in NiOOH and FeOOH is conducive to the adsorption of OH- and thus improves the OER activity of the electrode.[30-32] Chronopotentiometry tests were carried out at adode current densities of 10 mA cm-2 and 100 mA cm-2, As shown in Fig. 4(e), the overpotential increased by 4.9% after 35 h compared with the initial stage, Oxygen evolution continued at the current density of 100 mA cm-2 for about 35h, and the overpotential increased by 1.9%.

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The overall water splitting performance of NiFe/NF electrode was tested in 1M potassium

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hydroxide solution, the same electrode material was used for cathode and anode. Fig. 5(a) shows

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the polarization curve. It can be seen that the NiFe/NF electrode has the highest catalytic activity for overall water splitting. The current density of 10 mA cm-2 can be achieved by only applying 1.54 V voltage, which is better than previous reports, such as NiFeLDH@Ni[33] (1.57V), CoMnCH[34] (1.68V), Ni2Fe1-xO[35] (1.64 V). Meanwhile, The required voltage of NiFe/NF electrode with 100% IR-compensation is only 1.52V and 1.69V at 10 mA cm-2 and 100 mA cm-2 current densities. The stability of the electrode was first measured in 1 M potassium hydroxide solution at current densities of 10 mA cm-2 and 100 mA cm-2, as shown in Fig. 5(c), the applied voltage only increased by 1.5% after 35 h compared with the initial stage, the test continued at the current density of 100 mA cm-2 for about 35h, and the applied voltage only increased by 0.5%.

a

b

c

d

Fig. 5 (a) Polarization curves for overall water splitting with the NiFe/NF electrodes as both the anode and cathode. (b) Overpotentials obtained from overall water splitting polarization curves at the current density of 10 and 100 mA cm-2. (c) Chronopotentiometry plots of NiFe/NF at the current density of 10 and 100 mA cm-2 in a two-electrode configuration. (d) Chronopotentiometry plots of NiFe/NF at the current density of 300 mA cm-2 in 30 wt% potassium hydroxide solution.

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The SEM and EDS images of the electrode after continuous electrolysis of water for about 70 hours are shown in Fig S8 and S9-11, respectively. From the comparison between Fig 1 and Fig S8, it can be seen that after 70 hours of overall water splitting test, the special structure of the electrode coating still exists without obvious drop. However, compared with the HER of the cathode, the anode coating becomes thinner after OER, which may be related to the strong oxidation of the anode. Comparing Fig S9, S10 and S11, we can find that after a long time of electrolytic water test, the content of nickel element in cathode increases, which may be due to the reduction reaction of nickel in the anode after oxidation and peeling off, and the decrease of iron content in the anode may be due to the peeling off of the coating. What’s more, after a long period of testing, the element content of the electrode has not been significantly reduced, which proves that the electrode has excellent stability. In order to further study the electrolytic stability of NiFe/NF electrodes, The test was carried out under simulated industrial electrolysis conditions (current density of 300 mA cm-2 and 30 wt% potassium hydroxide solution). The experimental device is shown in Fig. S12 (ESI). In order to avoid the influence of external environment, the glass bottle containing electrolyte is refluxed with condensation tube and sealed with glue in a constant temperature water bath pot (25 ℃), in addition, water level calibration line is set to replenish water in time. As shown in Fig. 5(d), due to the high current density, the phenomenon of bubble release from the electrodes was intensified, which made the data fluctuate to a certain extent. 。After 40 hours, the cell voltage increased by 1.4% compared with the initial stable stage (at about 3 hours). It shows that the NiFe/NF electrode has reliable stability and has a promising application prospect.

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Conclusions By electroplating Ni and Fe on the nickel foam substrate at a high current density, then Ni(OH)2 and NiOOH was generated on the surface of the electrode by multi-loop cyclic voltammetry. The preparation process is much simpler than the traditional hydrothermal synthesis method, and the electrode has excellent hydrogen evolution and excellent oxygen evolution performance. In HER, η10 = 41 mV, η100 = 132 mV. In OER, η100 = 270 mV. When the electrode is used for overall water splitting, it only needs 1.54 V voltage to reach 10 mA cm-2 current density without any IR-compensation. Moreover, the stability of the electrode is excellent, it can remain stable for a long time under the high current density of 300mA cm-2. Our research provides a new way for the future application of alkaline water electrolysis to produce hydrogen. Appendix A. Supplementary data

Notes There are no conflicts to declare. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grand No. 21276177). References [1] Z. Pei, L. Xu, W. Xu, Hierarchical honeycomb-like Co 3 O 4 pores coating on CoMoO 4 nanosheets as bifunctional efficient electrocatalysts for overall water splitting, Applied Surface Science 433 (2018) 256-263.

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