morphology as a high-performance bifunctional electrocatalyst for overall water splitting

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Contents lists available atScienceDirect

Applied Surface Science

journal homepage:www.elsevier.com/locate/apsusc

Full length article

Nickel-iron selenide polyhedral nanocrystal with optimized surface

morphology as a high-performance bifunctional electrocatalyst for overall water splitting

Xianbiao Hu

^a

, Qingwei Zhou

^c

, Pengfei Cheng

^a

, Shaoqiang Su

^a

, Xin Wang

^b

, Xingsen Gao

^a

, Guofu Zhou

^b

, Zhang Zhang

^a,b,⁎

, Junming Liu

^a,c

aInstitute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, PR China

bNational Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, PR China

cLaboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, PR China

A R T I C L E I N F O

Keywords:

Nickel‑iron selenide Bifunctional electrocatalyst Oxygen evolution reaction Hydrogen evolution reaction Overall water splitting

A B S T R A C T

Constructing earth-abundant and eﬃcient electrocatalysts for overall water splitting are still in the research and exploration stage. In this work, the Fe-doped NiSe2polyhedron nanocrystals grown on Ni foam (Ni0.75Fe0.25Se2@ NF) with controllable surface morphology are developed as high-performance bifunctional electrocatalysts. The surfaces of the optimized polyhedron nanocrystals provide high density of catalytic active sites. NiSe₂is ori- ginally a catalytic active substance, and the Fe doping in NiSe2can greatly enhance the conductivity. The optimized Ni0.75Fe0.25Se2@NF (6 h) electrode are performed in an alkaline solution, exhibiting outstanding OER and HER activity with small overpotentials of 210 mV and−117 mV to reach current density of 10 mA cm⁻²and

−10 mA cm⁻², respectively. Remarkably, the Ni0.75Fe0.25Se2@NF (6 h) electrode serve as cathode and anode in water splitting electrolyzer also demonstrates highly eﬃcient overall water electrolysis performance, with a relatively low cell voltage of 1.61 V at 10 mA cm⁻²(1.57 V for RuO₂(+)//Pt/C(−) couple) and excellent long- term stability of 50 h for overall water electrolysis.

1. Introduction

With the aggravation of energy crisis and environmental pollution, ﬁnding new clean energy has become a diﬃcult problem for human society. Electrochemical water splitting, as a simple approach to produce hydrogen and oxygen, has been extensively researched [1–4].

Water splitting, including two half reactions: hydrogen evolution reaction (HER) on the cathode, and oxygen evolution reaction (OER) on the anode, has been known as a promising sustainable technique for energy supply. However, in an electrolytic water splitting cell, the production of H2is severely limited by the OER on the anode, which is a kinetically sluggish four electron transfer reaction [5]. As a con- sequence, highly eﬃcient catalysts for overall water splitting (OWS) are required to overcome the activation energy barriers to improve the energy conversion eﬃciency [6,7]. Currently, noble metal based materials, like Ir/Ru-based oxides for OER and Pt-based materials for HER, are usually acted as the benchmarks due to their outstanding catalytic

performances [8–12]. However, scarcity and exorbitant price of these noble-metal materials impede their widespread commercial applications [13–15]. In addition, producing different single functional catalysts for OER or HER usually requires different processes, which further increase the cost [16,17]. Hence, much effort has been devoted to explore bifunctional electrocatalysts with high activity toward both OER and HER in the same electrolyte to achieve the overall water electrolysis [18].

In recent decades, tremendous works have been reported to explore non-precious OER or HER electrocatalysts with earth-abundance and high activity, such as cobalt oxide (Co3O4), other transition metal hy- droxides/oxides and Ni0.77Fe0.23Se2 for OER [12,19–24], as well as metal compounds MoS2, CoP, WS2, CoSe2for HER [25–28]. Actually, ideal catalysts for both HER and OER must operate in the same pH range so as to work together for water splitting. Nevertheless, it is diﬃcult to pair the two electrode reactions together in an electrolytic water splitting cell for practical applications, because the activity and

https://doi.org/10.1016/j.apsusc.2019.05.220

Received 28 March 2019; Received in revised form 30 April 2019; Accepted 18 May 2019

⁎Corresponding author at: Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, PR China.

E-mail address:[email protected](Z. Zhang).

Available online 20 May 2019

T

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Herein, via a two-step method, the Fe-doped NiSe2polyhedron nanocrystals grown on 3D NF (Ni0.75Fe0.25Se2@NF) with controllable surface morphologies were constructed as high-performance bifunctional electrocatalysts. The Ni0.75Fe0.25Se2@NF electrode exhibits both excellent OER and HER activity in an alkaline solution. The constructed bifunctional electrocatalyst has low overpotentials of 210 and

−117 mV at a current density of 10 and−10 mA cm⁻², respectively, as well as a stability of 30 h at ± 100 mA cm⁻²for both OER and HER. In addition, compared with a RuO2(+)//Pt/C(−) couple, the Ni0.75Fe0.25Se2@NF (6 h) couple demonstrates a promising performance toward OWS, being with a relatively low potential of 1.61 V at 10 mA cm⁻²(1.57 V for RuO2(+)//Pt/C(−) couple) and a long-term stability of 50 h for OWS.

2. Experimental section 2.1. Chemicals

Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, > 98%), iron nitrate nonahydrate (Fe(NO3)3·9H2O, > 98.5%), sodium hydroxide (NaOH, analytical reagent), potassium hydroxide (KOH, analytical reagent), hydrazine (N2H4·H2O,≧50%), dimethylformamide (DMF, 99.5%), se- lenium (99.999%) were purchased from Aladdin reagent (Shanghai) Co. Ltd., 5 wt% Naﬁon solution was purchased from Sigma Aldrich, commercial RuO2(99.9%Ru) and Pt/C (Pt 20%) were purchased from Macklin Reagent.

2.2. Synthesis of Ni3Fe(OH)9on Ni foam

Prior to the synthesis, the NF was sonicated in 3 M HCl solution for 15 min to remove NiO layer on the surface. Then, the NF was rinsed with deionized water and absolute ethanol for 5 min each to ensure the clean surface. The cleaned NF (1 cm × 4 cm × 0.5 mm) was dried under high pressure nitrogen gasﬂow. And then, we carried out the electrodeposition process by a CHI660E electrochemical workstation at room temperature, using a three-electrode system, with the cleaned NF as working electrode, a Pt plate as counter electrode, and Ag/AgCl (3 M KCl) as reference electrode. The electrolyte bath contains 3 mM Ni (NO3)2·6H2O and 3 mM Fe(NO3)3·9H2O. The constant potential electrodeposition was carried out at−1 V (versus Ag/AgCl). The deposition time has been determined to be 300 s. After deposition, the Ni foam was carefully withdrawn from the electrolyte, rinsed with water and ethanol, then sonicated gently in ethanol and left dry in air. Mass loading of Ni3Fe(OH)9precursor is ~0.9 mg cm⁻²(Speciﬁc data details see Table S1).

2.3. Ni0.75Fe0.25Se2@NF (3 h,6 h,12 h) prepared through a solvothermal selenylation process

A piece of as-prepared NF coated with Ni3Fe(OH)9was submerged into a 50 mL Teﬂon lined stainless steel autoclave containing Se (0.5 g), NaOH (0.3 g), hydrazine (0.3 mL) and dimethylformamide (DMF 30 mL). After keeping at 180 °C for diﬀerent time (3 h, 6 h and 12 h),

(SEM, ZEISS Ultra 55). The SEM images were obtained using a sec- ondary electron detector, and an accelerating voltage of 5 kV.

Transmission electron microscopy (TEM) images were obtained by a JEOL JEM-2100HR at an acceleration voltage of 200 kV. Energy dis- persion spectrum (EDS) elemental mapping was recorded by an Oxford instrument IET250 spectrometer. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Fischer ESCALAB 250Xi spectro- photometer.

2.5. Electrochemical measurements

All electrochemical measurements were conducted on a CHI 660E electrochemical workstation (Chenhua Corp., China), using a standard three-electrode system. All potentials measured were calibrated to re- versible hydrogen electrode (RHE) using the Nernst equation:

E(RHE)= E(Ag/AgCl)+ 0.059 × pH + 0.197. As-prepared Ni3Fe(OH)9@ NF and Ni0.75Fe0.25Se2@NF samples were used as working electrodes without further treatments. A graphite rod and Ag/AgCl (3 M KCl) electrode was used as counter electrode and reference electrode, respectively. The electrolyte was 100 mL of 1.0 M KOH, which was prepared using deionized water (18 MΩcm). Before recording, the potentials of Ni3Fe(OH)9@NF and Ni0.75Fe0.25Se2@NF samples were scanned for 20 cycles in KOH solution until stable cyclic voltammograms were recorded. Linear sweep voltammetry (LSV) polarization curves were conducted from 0.8 to 0 V and−1.45 to−0.9 V with a scan rate of 2 mV s⁻¹. IR drop was compensated at 90% for OER and HER. Tafel slopes were derived from OER and HER polarization curves obtained at 2 mV s⁻¹. Stability tests were conducted from−0.5 to 0 V vs RHE for 1000 cycles using a CV technique with a scan rate of 100 mV s⁻¹. Chronopotentiometric measurements were obtained at a constant current density without iR-compensation. Electric impedance spectroscopy (EIS) measurements were carried out at open-circuit potential from 10⁵ to 0.01 Hz with an AC potential amplitude of 5 mV. The electrochemical active surface areas (ECSAs) were determined from the capacitance measurements with various scan rates (20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 mV s⁻¹) and collected in the−0.05 to 0.05 V vs SCE region. OWS tests were performed in a two-electrode conﬁguration with Ni0.75Fe0.25Se2@NF (6 h) as both anode and cathode by LSV polarization curve with 90% iR-compensation and chronoamperometric curve at a constant current density without iR-compensation.

3. Results and discussions

As illustrated inScheme 1, NieFe selenide supported on Ni foam (Ni0.75Fe0.25Se2@NF) was created by a two-step method of electrodeposition and post solvothermal selenylation. The electrodeposition process leads to a brown composite of amorphous NieFe hydroxide (Ni3Fe(OH)9) deposited on the Ni foam substrate, and selenylation process turns it into black (see Fig. S1).

The morphologies of as-prepared products before and after selenylation were characterized by SEM. We have provided low magniﬁcation SEM images of pure nickel foam and Ni0.75Fe0.25Se2attached to the nickel foam substrate (see Fig. S2). It can be seen from the full view

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image that the Ni0.75Fe0.25Se2is uniformly attached to the surface of nickel foam substrate.Fig. 1a demonstrates the surface morphologies of NieFe hydroxide before selenylation. Obviously, high-density nanospheres have covered homogenously on the surface of NF with a uni- form size around 100 nm in diameter. Fig. 1(b–d) correspond to the SEM images of NieFe selenides with diﬀerent selenization time (3 h, 6 h, 12 h), respectively. As shown inFig. 1b, we conﬁrm that the ori- ginal nanospherical NieFe hydroxide precursor transforms into the octahedral NieFe selenide (3 h).Fig. 1c is an overview SEM image of NieFe selenide nanocrystals formed on NF after a 6 h selenylation.

Obviously, most of the nanocrystals have polyhedral nanostructures with the side lengths ranging from several tens to several hundreds nanometers. With the selenization time up to 12 h, as shown inFig. 1d, most NieFe selenide nanocrystals grow larger, and change back to the octahedron morphology. Moreover, we found obvious irregular defects on the surfaces. The inset illustration is a schematic diagram of the morphologies corresponding to the SEM observations. Compared with

the spherical and octahedral nanostructures, polyhedrons with a 6 h selenylation may provide more edge-related active sites for catalysis [41].

To further confirm nanostructure and crystallinity, TEM and corresponding selected area electron diffraction (SAED) have been performed.Fig. 2a displays a TEM image of agglomerate NieFe hydroxide, which are comprised of nanospheres with diameters around 100 nm, being in accordance with the SEM image ofFig. 1a. The corresponding high-resolution (HR) TEM image (Fig. 2b) of the thinner edge displays no visible lattice fringes. And the inset is the corresponding SAED pattern, which displays broad and diffused halo rings, further sup- porting that the amorphous nature of as-prepared NieFe hydroxide nanospheres. As shown inFig. 2c, TEM image of several randomly or- iented NieFe selenide (6 h) polyhedral nanocrystals displays the quadrangle projections. Moreover, the thinner edge of one sample was examined by HRTEM. InFig. 2d, a large amount of lattice fringes were observed with different orientations. The lattice space of one set of Scheme 1.Schematic illustration of the two-step method for Ni_0.75Fe_0.25Se₂@NF.

Fig. 1.The SEM images of a) NieFe hydroxide precursor, b) NieFe selenide (3 h), c) NieFe selenide (6 h), d) NieFe selenide (12 h).

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fringes in pole is measured to be 0.26 nm, as illustrated in the selected square region, which should correspond to the (210) plane. The inset is the corresponding Fast-Fourier transform (FFT) image from the selected square region, showing a [111] viewing direction. Elemental mapping images (Fig. 2e) reveal the homogeneous distributions of Ni, Fe and Se in one single nanocrystal, and further conﬁrm the elemental composi- tions of the nanocrystals.

InFig. 3a, XRD patterns were obtained from Ni0.75Fe0.25Se2@NF (6 h) sample. Peaks at 44.5, 51.8 and 76.4° are detected, which can be ascribed to Ni foam (JCPDS No.04-0850). The amorphous nature of as- prepared Ni3Fe(OH)9@NF (Fig. S3(a)) has been conﬁrmed without any new feature peak observed except for those from the metallic NF substrate. The XRD characterization is consistent with the TEM result. After selenylation, additional peaks appeared in Ni0.75Fe0.25Se2@NF (6 h) indicate the formation of a metal selenide, being similar to NiSe2

(JCPDS No.88-1711). The diﬀraction peak at 33.8°of Ni0.75Fe0.25Se2

was shift to a higher angle compared with the counter- part corresponding to (210) in NiSe2(Fig. S2(a)).And the other peaks at 30.1, 37.0, 50.9, 55.6, 57.9,62.5 and 72.7° can be indexed to (200), (211), (311), (023), (321), (400) and (421) of NiSe2, respectively [30]. In

addition, the XRD results of Ni0.75Fe0.25Se2 samples with diﬀerent heating time (3 h,6 h,12 h) show that the rest peaks belonging to Ni0.75Fe0.25Se2 remain unchanged except the characteristic peaks of nickel substrate (Fig. S3(b)). The composition of Ni3Fe(OH)9can be determined by XPS (see Fig. S4(a)), which conﬁrms the existence of Ni and Fe elements. Furthermore, inFig. 3b, the high resolution Ni 2p spectrum shows two peaks at 874 and 856 eV, corresponding to Ni 2p1/

2and Ni 2p3/2, respectively, and with two shakeup satellites, demon- strating that Ni is in the Ni²⁺oxidation state [29].Fig. 3c exhibits the high resolution Fe 2p spectrum. The binding energies at 725 and 712 eV correspond to Fe 2p1/2and Fe 2p3/2, respectively, and with the presence of a shakeup satellite at 720 eV indicates that Fe is mostly in the Fe³⁺

oxidation state [41]. The Ni: Fe stoichiometric ratio can be obtained from the ICP-AES test (Table S2), that is 2.87:1. Based on this, the electrodeposited NieFe hydroxide could be speciﬁed as Ni3Fe(OH)9

[29,41].

The surface element chemical states of the bimetallic NieFe selenide are determined by XPS analysis. The XPS survey demonstrates that Ni, Fe as well as Se elements are existed after solvothermal selenylation (see theFig. 4(b)). The high-resolution XPS spectrum of Ni 2p (Fig. 3d) Fig. 2.a) TEM and b) HRTEM image of Ni3Fe(OH)9nanospheres, the inset is the corresponding SAED pattern of b). c) TEM and d) HRTEM image of Ni0.75Fe0.25Se2

(6 h) nanocrystals, the inset is the corresponding FFT image. e) The corresponding elemental mapping of Ni_0.75Fe_0.25Se₂(6 h).

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Fig. 3.a) XRD pattern of the Ni0.75Fe0.25Se2@NF (6 h) sample. b) High-resolution Ni 2p and c) Fe 2p XPS spectra for Ni3Fe(OH)9@NF. d) High-resolution Ni 2p, e) Fe 2p and f) Se 3d XPS spectra for Ni0.75Fe0.25Se2@NF (6 h).

Fig. 4.a) The polarization curves for the Ni0.75Fe0.25Se2@NF (3 h, 6 h, 12 h), Ni3Fe(OH)9@NF, commercial RuO2and bare NF for OER. Histograms of b) overpotentials required for J = 10 mA cm⁻², and c) current densities atη= 270 mV of Ni_0.75Fe_0.25Se₂@NF (3 h, 6 h, 12 h), Ni₃Fe(OH)₉@NF, RuO₂and NF in 1.0 M KOH with a scan rate of 2 mV s⁻¹. d) The corresponding Tafel plots. e) The multi-step chronopotentiometric curve of Ni_0.75Fe_0.25Se₂@NF (6 h) without iR-compensation. f) The chronopotentiometric measurement of Ni0.75Fe0.25Se2@NF (6 h) at 10, 50, 100 mA cm⁻²without iR-compensation.

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NieFe selenide could be speciﬁed as Ni0.75Fe0.25Se2[30,33].

Electrocatalytic activities of the Ni0.75Fe0.25Se2@NF for OER were evaluated in a three-electrode electrochemical cell at room temperature. To minimize the interference of diffusion limitation and capacitive current, all samples were subjected to the LSV measurement for OER evaluation at a slow scan rate of 2 mV s⁻¹. Due to the effect of ohmic resistance, the measured reaction currents cannot directly reflect the intrinsic behavior of catalysts. IR-correction was applied to initial data for further analysis. Fig. 4a shows the LSV curves on the RHE scale.

Typically, Ni foam is also an active OER catalyst, and an overpotential of 435 mV was applied to achieve 10 mA cm⁻². The polarization curves demonstrate that all the three Ni0.75Fe0.25Se2@NF samples possess the higher OER activities than NF and Ni3Fe(OH)9@NF. And the Ni0.75Fe0.25Se2@NF (6 h) exhibits the highest OER activity, being with the lowest overpotential of 210 mV at 10 mA cm⁻². For the commercial RuO2 catalyst, the overpotential required to reach 10 mA cm⁻² was measured to be 321 mV. Remarkably, as compared in a histogram of Fig. 4b, Ni0.75Fe0.25Se2@NF (6 h) has a much improved OER catalytic activity than RuO2. Meanwhile, their current densities at the same overpotential η= 270 mV are also compared in Fig. 4c, where the maximum current is 204 mA cm⁻²for Ni0.75Fe0.25Se2@NF (6 h). The superior OER activity of Ni0.75Fe0.25Se2@NF (6 h) can be supported by a small Tafel slope of 39.4 mV dec⁻¹(Fig. 4d), which is also as the in- dicator of high performance electrocatalyst. The low overpotential and small Tafel slope of Ni0.75Fe0.25Se2@NF (6 h) all demonstrate its outstanding catalytic activity for OER, which are comparable to or better than those of other transition metal-based OER catalysts (see Table S3) [4–12,14,16–18,24,37]. Fig. 4e demonstrates a multi-step chronopotentiometric curve for Ni0.75Fe0.25Se2@NF (6 h) in 1.0 M KOH, with increased current from 50 to 500 mA cm⁻²(50 mA cm⁻²per 600 s).

The potential was instantaneously leveled oﬀat 1.535 V with the start current of 50 mA cm⁻², and remained constant for the rest 600 s. The following steps all show a similar behavior up to 500 mAcm⁻², indicating the excellent mass transport property, conductivity and me- chanical robustness of the Ni0.75Fe0.25Se2@NF (6 h) electrode. Con- sidering the stability is also critical to high-performance electrocatalyst, the long-term stability of Ni0.75Fe0.25Se2@NF (6 h) for OER was as- sessed by continuous chronopotentiometric measurement at 10, 50, 100 mA cm⁻², respectively.Fig. 4f illustrates the three corresponding chronopotentiometric curves, all suggesting a stable catalytic activity during 30 h electrolysis.

Electrocatalytic performance for HER was evaluated by LSV in a standard three-electrode system. For comparison, electrocatalytic performances of the commercial Pt/C and the bare NF were demonstrated under the same conditions.Fig. 5a illustrates the polarization curves of diﬀerent electrodes for HER, at a scan rate of 2 mV s⁻¹. Obviously, the commercial Pt/C catalyst possesses the best HER performance, whereas the bare NF demonstrates a low catalytic activity toward HER. Com- pared with the overpotentials in a histogram of Fig. 5b, Ni0.75Fe0.25Se2@NF electrodes exhibit excellent catalytic performance for HER. It is evident that the Ni0.75Fe0.25Se2@NF (6 h) electrode achieves a current density of −10 mA cm⁻² at the overpotential of

−117 mV, which was 22, 28, 78 and 163 mV less than that of

HER. In addition, the stability is conﬁrmed by the continuous CV measurements in the range of−0.5 to 0 V vs RHE at 100 mV s⁻¹for 1000 cycles. Fig. 5e demonstrates the polarization curve after the 1000 cycles, which is still very close to the ﬁrst cycle. The chronopotentiometric measurement was performed at −10, −50, and

−100 mA cm⁻² for 30 h without iR-compensation, respectively. As plotted inFig. 5f, the potential curves are with negligible increase- ments, suggesting that the Ni0.75Fe0.25Se2@NF (6 h) electrode has excellent stability in an alkaline solution. As shown in Fig. S5, the bifunctional catalytic performance of Ni0.75Fe0.25Se2@NF (6 h) electrode for hydrogen and oxygen production was studied by LSV curve.

EIS measurements were performed to further interpret the electrochemical performance of the Ni0.75Fe0.25Se2@NF (6 h), which could give insight into the internal resistance of an electrode material and the resistance between electrode and electrolyte. Fig. 6a illustrates the Nyquist plots of Ni0.75Fe0.25Se2@NF (6 h), Ni3Fe(OH)9@NF and NF, which are all composed of a depressed semicircle in the high-frequency region, that can be assigned to the charge transfer resistance (Rct) from the electrode/electrolyte interface. Moreover, the semicircle of the Ni0.75Fe0.25Se2@NF (6 h) is much smaller than those of Ni3Fe(OH)9@ NF and NF, indicating that the Ni0.75Fe0.25Se2@NF (6 h) electrode has the lowest Rct. The ECSAs of the Ni0.75Fe0.25Se2@NF (6 h) electrode was estimated by measuring double layer capacitance (Cdl) with CV at different scan rates (Fig. S6). As illustrated inFig. 6b, the Cdlvalue increases from 5.58 mF cm⁻²for pure NF to 6.85 mF cm⁻²for Ni3Fe (OH)9@NF, and to its maximum of 16.52 mF cm⁻²for Ni0.75Fe0.25Se2@ NF (6 h). Obviously, the ECSAs of the three samples follow the trend Ni0.75Fe0.25Se2@NF (6 h) > Ni3Fe(OH)9@NF > NF, which confirms that the introduction of Se can effectively modulate the ECSAs of Ni0.75Fe0.25Se2@NF (6 h). Since Ni0.75Fe0.25Se2@NF (6 h) can perform as a high-performance electrocatalyst for both OER and HER in 1.0 M KOH solution, a two-electrode electrolyzer was assembled with Ni0.75Fe0.25Se2@NF (6 h) as both anode and cathode. Additionally, even though noble metal Pt/C and RuO2are identified as the high-efficient electrocatalysts to produce H2and O2, respectively, their bifunctional properties have hardly been reported. For comparison, RuO2(+)//Pt/

C(−) couple was also investigated. LSV curves of the overall water electrolysis on Ni0.75Fe0.25Se2@NF (6 h) couple, Ni3Fe(OH)9@NF couple, NF couple and RuO2(+)//Pt/C(−) were presented inFig. 6c.

As expected, RuO2(+)//Pt/C(−) couple is indeed the best performed catalytic system at low current density, which only required 1.57 V to reach 10 mA cm⁻². And the Ni0.75Fe0.25Se2@NF (6 h)//

Ni0.75Fe0.25Se2@NF (6 h) couple required 1.61 V to reach 10 mA cm⁻². However, such a potential of 1.61 V is comparable to or lower than the values from recently reported state-of-the-art bifunctional catalysts for OWS (see Table S5) [4,5,12,17,36,48–56]. By contrast, the Ni3Fe (OH)9@NF//Ni3Fe(OH)9@NF and NF//NF couples demonstrate a cell voltage of water splitting of 1.72 V and 1.89 V to reach 10 mA cm⁻², respectively, being much inferior to the Ni0.75Fe0.25Se2@NF (6 h)//

Ni0.75Fe0.25Se2@NF (6 h) couple. A chronopotentiometric test at 10 mA cm⁻²for 50 h displays a slight overpotential loss of about 10 mV (Fig. 6d), suggesting its good stability in the long-term water electrolysis. As shown in the SEM image of Ni0.75Fe0.25Se2@NF (6 h) after 50 h

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Fig. 5.a) The polarization curves for the Ni_0.75Fe_0.25Se₂@NF (3 h, 6 h, 12 h), Ni₃Fe(OH)₉@NF, commercial Pt/C on Ni foam and bare Ni foam for HER. b) overpotentials required at J = -10 mA cm⁻²and c) Tafel plots for Ni0.75Fe0.25Se2@NF (3 h, 6 h, 12 h), Ni3Fe(OH)9@NF, NF and Pt/C in 1.0 M KOH at the scan rate of 2 mV s⁻¹. d) The multi-step chronopotentiometric curve of Ni0.75Fe0.25Se2@NF (6 h) without iR-compensation. e) LSV curves for Ni0.75Fe0.25Se2@NF (6 h) before and after 1000 CV cycles. f) chronopotentiometric measurement of HER at−10,−50,−100 mA cm⁻²using Ni_0.75Fe_0.25Se₂@NF (6 h) as catalyst.

Fig. 6.a) Nyquist plots at an open-circuit potential measured from EIS in the frequency range from 10⁵Hz to 0.01 Hz. The solid lines are thefits to the data using the simplified Randles circuit. b) Capacitive J versus scan rate for Ni_0.75Fe_0.25Se₂@NF (6 h), Ni₃Fe(OH)₉@NF and NF. c) Polarization curve of Ni_0.75Fe_0.25Se₂@NF(6 h) couple, Ni₃Fe(OH)₉@NF couple, NF//NF and RuO₂(+)//Pt/C (−) in 1.0 M KOH for overall water splitting. d) Chronopotentiometric curve of water electrolysis for Ni0.75Fe0.25Se2@NF (6 h)//Ni0.75Fe0.25Se2@NF (6 h) in a two-electrode configuration at the constant current density of 10 mA cm⁻².

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catalyst, the electrode only requires−117 mV to reach−10 mA cm⁻². Electrochemical measurements indicate that the introduction of Se into the NieFe hydroxide could eﬃciently lower the kinetic energy barrier of Volmer step and eventually enhance the OER and HER activities. In addition, compared with a high-performance RuO2(+)//Pt/C(−) couple, the Ni0.75Fe0.25Se2@NF (6 h)//Ni0.75Fe0.25Se2@NF (6 h) couple demonstrates promising activities toward OWS. Therefore, such a bimetallic NieFe selenide supported on nickel foam not only provides a promising alternative to noble catalysts for OER and HER, but also indicates a new direction for exploring low-cost commercial catalysts for OWS.

Author contributions

The manuscript was written through contributions of all authors.

All authors have given approval to theﬁnal version of the manuscript.

Funding sources

This work was supported by the National Key R&D Program of China (2016YFB0401501), National Natural Science Foundation of China (51561135014), The Guangdong Natural Science Foundation (2018A030313377), Cultivation project of National Engineering Technology Center (2017B090903008), Xijiang R&D Team (X.W.), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology Grant (2017B030301007), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (IRT_17R40), and MOE International Laboratory for Optical Information Technologies and the 111 Project.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://

doi.org/10.1016/j.apsusc.2019.05.220.

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