Amorphous NiFe Nanotube Arrays Bifunctional Electrocatalysts for

Subscriber access provided by UCL Library Services

Article

Amorphous NiFe Nanotube Arrays Bifunctional Electrocatalysts for Efficient Electrochemical Overall Water Splitting Lu Xu, Fu-Tao Zhang, Jia-Hui Chen, Xian-Zhu Fu, Rong Sun, and Ching-Ping Wong ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00313 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Energy Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Amorphous NiFe Nanotube Arrays Bifunctional Electrocatalysts for Efficient Electrochemical Overall Water Splitting Lu Xu,a,

b

Fu-Tao Zhang,a,

b

Jia-Hui Chen,a Xian-Zhu Fu,*,

a, c

Rong Sun,

*, a

Ching-Ping Wongd, e

a

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences,

Shenzhen 518055, China. b

Nano Science and Technology Institute, University of Science and Technology of

China, Suzhou 215123, China. c

College of Materials Science and Engineering, Shenzhen University, Shenzhen

518055, China. d

Department of Electronics Engineering, The Chinese University of Hong Kong,

Shatin, N.T., Hong Kong, Hong Kong 999077, China. e

School of Materials Science and Engineering, Georgia Institute of Technology,

Atlanta, GA 30332, United States. * Corresponding author: Xian-Zhu Fu, E-mail address: [email protected]; Tel: +86-755-86392151; Fax: +86-755-86392299; Rong Sun, E-mail address: [email protected].

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

It is still a challenge for design and fabrication of cost-effective and efficient bifunctional electrocatalysts for both cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) for overall water splitting. Herein, we design and synthesize amorphous NiFe nanotube arrays on nickel foam (NiFe NTAs-NF) with high electrocatalytic activity and excellent durability for both OER and HER in overall water splitting. The as-synthesized NiFe NTAs-NF only requires relatively low overpotentials of 216 mV for the OER and 181 mV for the HER to reach a current density of 50 mA cm-2 and 10 mA cm-2, respectively. Moreover, when used as bifunctional catalysts for water splitting, the designed electrode only needs a low cell voltage of 1.62 V to obtain 10 mA cm-2 for the overall water splitting, with an extremely excellent durability. The excellent performance of the NiFe NTAs-NF might be attributed to the synergistic effect and amorphous phase of NiFe alloy as well as the well-defined nanotube array architecture with large surface area, abundant active sites, sufficient gas and electrolyte diffusion channels.

KEYWORDS: nanotube arrays, amorphous, oxygen evolution reaction, hydrogen evolution reaction, water splitting

INTRODUCTION The shortage of fossil fuels and environmental crisis have motivated scientists to search for green energy resources.1 Hydrogen (H2) is a clean, efficient, and renewable energy which is an attractive alternative to fossil fuels.2-3 Electrochemical overall water splitting is an effective way to generate pure hydrogen fuels.

4-6

It is driven by two half-cell reactions, including the hydrogen evolution

reaction (HER) and the oxygen evolution reaction (OER). However, in practice, the 2 ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


two half-cell reactions require active and robust electrocatalysts to decrease the unavoidable dynamic overpotentials due to the intrinsically sluggish kinetics.7-8 Currently, the most effective electrocatalysts for HER and OER are Pt-based materials and Ru/Ir-based compounds, respectively.9-11 But their high cost and scarcities greatly limit their large-scale application.12-13 In addition, bifunctional catalysts for overall water splitting are necessary, which are good at HER and OER in the same electrolyte. Because bifunctional catalysts can avoid using different equipment and processes to produce different electrocatalysts, which could decrease the cost.14-16 Hence, it is important to develop inexpensive and earth-abundant effective bifunctional electrocatalysts of overall water splitting for widespread applications.17 To meet these challenges, transition bimetallic materials have attracted increasing interests such as iron, cobalt, and nickel based electrocatalysts for water splitting.18-21 In particular, NiFe-based compound catalysts are highly active for the OER and HER of water electrolysis.15, 19, 22-29 Although there are a lot of work about NiFe based electrocatalyst for water splitting, there is few report about NiFe amorphous alloy nanotube bifunctional electrocatalysts for hydrogen and oxygen evolution reactions. Furthermore, it still remains challenging to develop highly attractive NiFe catalysts with unique architecture through facile and large-scale fabrication methods. It is also very significant to rationally design electrocatalytic electrode architecture for efficient overall water splitting. For example, 3D nanostructured arrays electrodes without the use of binder have attracted great interest due to the following advantages: (1) ordered structures and the large exposed surface area; (2) efficient charge transfer and gas escaping; (3) lower aggregation and collapse; (4) higher electric conductivity of electrodes in comparison to nanoparticles.30 Up to now, there are various nanostructures as building block arrays to fabricate 3D nanostructured arrays electrode, including nanowires,31 nanotubes, nanorods,32-33 and nanoplates.30 Especially, considerable studies have demonstrated that the nanotube arrays can provide a short diffusion path for electrolyte ions, large specific surface 3 ACS Paragon Plus Environment


Page 4 of 24

area and abundant active sites, owing to a higher utilization rate for electrode materials than the other kinds of building blocks.34-38 In addition, one of the most popular substrates to load catalysts is nickel foam because of its multiple levels of porosity, enormous open space, good electrical conductivity, low cost and good structural stability. Moreover, comparing to the traditional crystalline electrocatalyt, amorphous nanomaterial is a strategy to enhance the catalytic activity and durability of electrocatalysts in water splitting. The short-range order of amorphous materials can increase active site density, due to the abundant randomly oriented bonds than crystalline structure.39-45 And structural flexibility of the amorphous material can make it more durable in water splitting process.40,

46

Nevertheless, it remains

challenging to develop highly attractive and bifunctional advanced catalysts by taking advantages of the above considerations. Herein, we fabricate amorphous NiFe nanotube arrays on nickel foam (NiFe NTAs-NF) by electrodeposition method as a high performance non-noble metal bifunctional electrocatalyst for overall alkaline water splitting. Our work has taken advantages of the facile method of NiFe amorphous nanotube arrays electrocatalysts with excellent performance for full water splitting. NiFe NTAs-NF displays excellent water splitting performances in 1.0 m KOH solution with low overpotentials of 216 mV for OER at 50 mA cm-2 and 181 mV for HER at 10 mA cm−2. Thanks to this bifunctional activity, NiFe NTAs-NF also shows efficient overall water splitting performance with 10 mA cm−2 at low potential of only 1.62 V and high durability at high current density of 10 mA cm−2 in the alkaline water electrolysis.

EXPERIMENTAL SECTION Materials: All chemicals used in this study were of analytical reagent (AR) without any further purification. Deionized (DI) water, ethanol, and 5% HCl solution were used as solvents and for washing. Ni foam (0.5 cm × 2 cm) was pretreated with 5% HCl solution, ethanol and deionized water for 5 min each other prior to using as a 4 ACS Paragon Plus Environment

Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


substrate. Electrodeposited process was performed on a CHI 760E electrochemical workstation in a simple three-electrode configuration with a Pt plate (2 cm × 2 cm) as the counter electrode and Ag/AgCl as the reference electrode. The details of the preparation of NiFeP NTAs-NF Electrocatalysts are described as follows: Synthesis of NiFe NTAs-NF Electrocatalysts: ZnO nanorod arrays (NRAs) were firstly prepared on NF by galvanostatic electrolysis in solution of 0.01 M Zn(NO3)2 and 0.05 M NH4NO3 at 2.0 mA/cm2 for 90 min at 80 °C. The NiFe layer was then coated on the surfaces of ZnO NRAs-NF to form ZnO@NiFe NRAs-NF by galvanostatic electrodeposition in the solution containing 0.0175 M nickel(II) sulfate hexahydrate

(NiSO4·6H2O),

0.00875

M

iron(III)

nitrate

nonahydrate

(Fe(NO3)3·9H2O), 0.0014 M sodium hypophosphite monohydrate (NaH2PO2·H2O), 0.002 M sodium citrate (pH was adjusted to 6.0 by using Na2CO3) at a current density of 0.25 mA/cm2 for 120 min at 50 °C. To remove ZnO NRAs template, the fabricated ZnO@NiFe NRAs-NF was then immersed into 1.0 M NaOH solution for 3 h and then washed with deionized water for three times. Finally, the fabricated NiFe nanotube arrays (NiFe NTAs) was dried at vacuum oven. For comparison of catalytic activities, the Ni NTAs-NF and the Fe NTAs-NF were fabricated with the same method and parameters described above, but without the NiSO4·6H2O and Fe(NO3)3·9H2O, respectively. The reduced NiFe NTAs-NF and oxidized NiFe NTAs-NF: The reduced NiFe NTAs-NF and oxidized NiFe NTAs-NF were prepared by calcination treatment in a tube furnace under H2 and air atmosphere at 350 °C for 3 h, respectively. Characterization: The morphologies of the as-prepared samples were characterized by field emission scanning electron microscopy (FE-SEM, FEI Nova Nano SEM 450). The phase analysis and structure were investigated by X-ray diffraction measurements (XRD, Rigaku D/Max 2500, Japan) with radiation from a Cu target. The surface chemical element composition and chemical states of the elements in the samples were identified by X-ray photoelectron spectroscope (XPS, PHI-1800 XPS system). More detailed information regarding the morphologies, the 5 ACS Paragon Plus Environment


various elements present in the nanotubes, and the crystallinity was obtained using transmission electron microscopy (HTEM) and energy dispersive X-ray spectroscopy (EDS) mapping investigations (JEOL JEM 2100F instrument). Elemental analysis was performed on a Perkin Elmer 7000DV inductively coupled plasma atomic emission spectrometer (ICP-AES). Electrochemical Measurements: Electrochemical tests were conducted with an electrochemical workstation (CHI 760E Chenhua Corp., Shanghai). The various catalysts grown on Ni foam were used as the working electrode; a graphite rod was the counter electrode; and a Ag/AgCl electrode was the reference electrode. All measurements were performed in 1 M KOH aqueous electrolyte at a scan rate of 5 mV s–1 to minimize the capacitive current. All of the potentials in the LSV polarization curves were iR-corrected with respect to the ohmic resistance of the solution, unless specifically indicated. EIS was performed with the working electrode nonbiased or biased at a certain potential while sweeping the frequency from 100 kHz to 10 mHz. The electrochemical surface area for working electrode was measured at various scan rates. The overall water splitting test was performed in a two-electrode system, with two symmetric catalyst electrodes as both the anode and cathode. The durability test was performed using chronopotentiometric measurements. All potentials reported in this work were against the RHE, which was converted from the Ag/AgCl scale using a calibration. The surface roughness and hydrophilicity of the samples were measured in terms of droplet wettability (static contact angle method). All experiments were carried out at room temperature (~25 °C)

RESULTS AND DISCUSSION The electrodeposition fabrication process of NiFe NTAs-NF is shown in Scheme 1, and the details are described in the Experimental Section. Ni foam is selected as a substrate (current collector) because of its excellent electrical conductivity and 3D macroporous networked structures (Figure S1). ZnO nanorod arrays (NRAs) are used as a template fabricated by electrodeposition on the Ni foam, and their SEM images 6 ACS Paragon Plus Environment

Page 6 of 24

Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with different magnifications are shown in Figure 1a, b. The ZnO nanorods are homogeneously coated on the Ni foam surface and separate from each other. As typical FESEM image of hexagonal ZnO nanorods shown in the inset in Figure 1b, the diameters and lengths of ZnO nanorods are about 300 nm and 2 µm, respectively. The NiFe nanotube arrays (NTAs) are obtained by electrodepositing NiFe alloy on the surfaces of ZnO nanorod arrays and then removing ZnO template by chemical dissolution in 1.0 M NaOH. Figure 1c, d display the SEM images of NiFe nanotube arrays with different magnifications. It is clearly observed the nanotube array structures. The nanotube diameters, wall thicknesses, length are ≈ 300 nm, ≈ 25 nm and 2.1 µm, respectively. The loading of NiFe is ≈ 0.62 mg cm-2.

Scheme 1. Illustration of the fabrication process of NiFe NTAs-NF electrocatalysts.



Page 8 of 24

Figure 1. (a, b) SEM images of ZnO nanorod arrays on NF with different magnification (inset in b: single ZnO nanorod); (c, d) SEM images of NiFe NTAs-NF with different magnification (inset in d: single NiFe nanotube).

The TEM image (Figure 2a) of the material scraped from NiFe NTAs-NF reveals a clear nanotube morphology, which is in good agreement with the SEM images. The high-resolution TEM image shown in Figure 2b exhibits no obvious crystal lattice, indicating the amorphous structure of NiFe NTAs. The corresponding selected-area electron diffraction (SAED) pattern (inset to Figure 2b) shows a broad and diffused halo ring, further confirming that the NiFe NTAs are amorphous. The TEM–energy dispersive X-ray spectroscopy (EDX) elemental mapping results (Figure 2c) indicate the homogeneous distribution of Ni, Fe elements over the examined detection range at the surface of the NiFe NTAs. XRD pattern of NiFe NTAs-NF shown in Figure S2 (Supporting Information) exhibits only three diffraction peaks of the Ni foam at 44.5°, 51.8° and 76.4°, respectively47, without the observation of any new diffraction peaks, further indicating the amorphous structure of NiFe NTAs, which coincides with the results of the HRTEM and SAED study of NiFe NTAs. 8 ACS Paragon Plus Environment

37, 48-49

The inductively

Page 9 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


coupled plasma atomic emission spectroscopy (ICP-AES) shows that the atomic ratio of Ni to Fe is ≈0.57: 0.43 in amorphous NiFe NTAs scratched off NF (Table S1, Supporting Information).

Figure 2. (a) TEM image of NiFe NTAs-NF; (b) HRTEM image of the wall of NiFe NTAs-NF (inset in b: SAED pattern); (c) EDS mappings of NiFe NTAs-NF.

Figure 3a presents the entire XPS spectrum of NiFe NTAs-NF, indicating the presence of the elements Ni and Fe. The high-resolution Fe 2p spectrum (Figure 3b) exhibits two peaks at 711.9 eV and 725.3 eV, corresponding to Fe 2p3/2 and 2p1/2, respectively. As shown in high-resolution Ni 2p spectrum (Figure 3c), the peaks at 855.7 eV and 873.3 eV can be attributed to Ni 2p3/2 Ni 2p1/2, respectively.47 Compared with those of Ni NTAs-NF, the Ni 2p1/2 of NiFe NTAs-NF shifts to higher binding energies of ≈0.2 eV (Figure 3d). Therefore, compared with those of NiFe NTAs-NF and Ni NTAs-NF, the peak shifts of Ni 2p1/2 confirm the strong electron interactions 9 ACS Paragon Plus Environment


between Ni and Fe in the NiFe NTAs-NF, which might affect the catalytic activity.

Figure 3. (a) The entire XPS spectrum of NiFe NTAs-NF; XPS spectra of (b) Fe 2p, (c) Ni 2p of NiFe NTAs-NF; (d) The magnified XPS spectra of Ni 2p between 868 and 877 eV for the NiFe NTAs-NF and Ni NTAs-NF.

The OER performances of NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF are first evaluated in a conventional three-electrode electrochemical cell in the alkaline aqueous solution (1.0 M KOH, pH = 14 ) at room temperature. Figure 4a and 4c show the linear sweep voltammetry (LSV) curves with iR correction and overpotentials at current density of 50 mA cm−2 of NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF ( polarization curve without iR correction was shown in Supporting Information, Figure S3a). At the current density of 50 mA cm−2, NiFe NTAs-NF exhibits excellent activity and only demands overpotential of 216 mV, which is much lower than that of Ni NTAs-NF (277 mV), Fe NTAs-NF (290 mV) and NF (495 mV). The NiFe 10 ACS Paragon Plus Environment

Page 10 of 24

Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NTAs-NF only demands overpotential 238 mV to reach 100 mA cm–2, and this overpotential are much lower than those for Ni NTAs-NF (η100 mA cm–2 = 345 mV) and Fe NTAs-NF (η100 mA cm–2 = 357 mV) (Figure 4d). NiFe NTAs-NF’s high catalytic ability at large current densities makes it a promising material to serve as nonprecious oxygen evolution electrocatalysts for viable industrial alkaline water splitting. NiFe NTAs-NF also demonstrates comparable or even better OER activity compared to many previously reported catalysts (Table S2, Supporting Information). Figure 4b shows the Tafel slopes of the four samples calculated from Figure 4a. The Tafel slopes of the NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF samples are 64.5 mV dec-1, 182.8 mV dec-1, 131.8 mV dec-1 and 178.6 mV dec-1, respectively. Resulting from the strong synergistic effect of two active metals – Ni and Fe, which might enhance intrinsic activities of active sites, NiFe NTAs-NF shows the smallest Tafel slope. That indicated the fast OER kinetics and inherent excellent OER activity of NiFe NTAs-NF in alkaline media.50 A multi-step chronopotentiometric curve for NiFe NTAs-NF with current density being started from 10 to 40 mA cm-2 (ca. 3.75 mA cm-2 per 500 s) is shown in Figure 4e. The response potentials remained constant at each step, implying the high stability of the NiFe NTAs-NF within a wide range of current densities. Moreover, the response of potentials to current ramps are immediate and the slightly decrease in the step height with the increasing of current density indicated efficient conductivity and mass transportation of this electrode. The electrochemical long-term stability is another critical criterion to evaluate catalyst performance. The stability of the electrodes for OER is evaluated by chronopotentiometric measurement at current densities of 100 mA cm−2 for 20 h under the alkaline conditions as shown in Figure 4f. The observation of almost unchangeable maintaining of overpotential during 20 h indicates the excellent stability of NiFe NTAs-NF. Figure S4a (Supporting Information) shows that there is no distinct morphological change of NiFe NTAs-NF after the stability test, and the nanotube arrays morphology is well maintained, demenstating its good stability. The XRD pattern and XPS spectrum after durability test are shown in Figure S5 and Figure S6 11 ACS Paragon Plus Environment


(Supporting Information), respectively. There was no obvious change in crystal phase and chemical composition of the NiFe NTAs-NF. In addition, Figure S7 shows the linear sweep voltammetry (LSV) curves after 20 h chronopotentiometric measurements is consistent with that formerly, further evidencing the excellent durability.

Figure 4. (a) The OER linear sweep voltammetry (LSV) curves of NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF with iR correction; (b) Tafel plots of different electrocatalysts; The required overpotentials to achieve current densities of (c) 50 mA cm−2 and (d) 100 mA cm−2 for different electrocatalysts; (e) Multi-current process of NiFe NTAs-NF. The current density started at 10 mA cm-2 and ended at 40 mA cm-2, 12 ACS Paragon Plus Environment

Page 12 of 24

Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with an increment of about 3.75 mA cm-2 per 500 s without iR correction; (f) Chronopotentiometric measurement of long-term stability of NiFe NTAs-NF at the current density of 100 mA cm−2 for 20 h.

The HER catalytic activities of the electrocatalysts are next assessed in the same alkaline electrolyte employing a conventional three-electrode configuration. Figure 5a and 5c show the linear sweep voltammetry (LSV) curves and overpotentials at current density of 10 mA cm−2 of NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF (polarization curve without iR correction was shown in Supporting Information, Figure S3b). The NiFe NTAs-NF only requires overpotential of 181 mV to achieve current density of 10 mA cm−2, and Tafel slope is 147.0 mV dec-1. However, the Ni NTAs-NF, Fe NTAs-NF and NF afford a current density of 10 mA cm−2 at overpotentials of 222 mV, 223 mV, 236 mV, and the corresponding Tafel slopes are 175.5 mV dec-1, 200.3 mV dec-1 and 217.3 mV dec-1, respectively (Figure 5b). The lower overpotential and lower Tafel slope demonstrate that the NiFe NTAs-NF has better HER catalytic activity than monometallic catalysts. The long-term durability of NiFe NTAs-NF is also tested by the chronopotentiometric measurement for 20 h in 1.0 M KOH (pH = 14) at current densities of 10 mA cm−2. The overpotential almost remains stable for 20 h (Figure 5d). In addition, the XRD pattern and XPS spectra after HER stability test were similar to that characterized initially (Figures S5 and S8, Supporting Information). The LSV curves of NiFe NTAs-NF exhibit no obvious change after chronopotentiometric measurement for 20 h (Figure S9, Supporting Information), and the nanotube arrays morphology is well maintained after testing (Figure S4b, Supporting Information). The above results demonstrate that NiFe NTAs-NF has good stability in a long-term HER process.



Figure 5. (a) The HER LSV curves of NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF with iR correction; (b) Tafel plots of different electrocatalysts; (c) The required overpotentials to achieve a current density of 10 mA cm−2 for different electrocatalysts; (d) Chronopotentiometric measurement of long-term stability of NiFe NTAs-NF at the current density of 10 mA cm−2 for 20 h.

To get a deeper understanding of such efficient OER and HER performances of NiFe NTAs-NF, we measured the capacitances of the double layer at the solid–liquid interface for the samples to estimate the electrochemical surface areas (ECSAs) (Figure S10 a-c, Supporting Information). The double layer capacitance (Cdl) of the NiFe NTAs-NF, Ni NTAs-NF, and Fe NTAs-NF samples are 2.18 mF cm-2, 1.46 mF cm-2, and 1.35 mF cm-2, respectively (Figure S10d, Supporting Information). The observation of the largest ECSAs of NiFe NTAs-NF as compared with other samples indicate more active sites and more efficient mass and charge transport capability of NiFe NTAs-NF, which coincides with the lower overpotential, the lowest Tafel slope and the smallest charge-transfer resistance. Electrochemical impedance spectroscopy 14 ACS Paragon Plus Environment

Page 14 of 24

Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(EIS) analyses of NiFe NTAs-NF are also performed. Nyquist plots of NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF are presented in (Figure S11, Supporting Information). An equivalent circuit is shown in the inset of Figure S11a (Supporting Information). Rs represents the resistance of electrolyte, Rct is the charge transfer resistance, and Cdl is the double-layer capacitance. The intercept of X axis for NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF are almost equivalent, confirming that the four kinds of electrode have the almost equal mass-transfer resistance. However, the radius of the semicircle of NiFe NTAs-NF is smaller than that of Ni NTAs-NF, Fe NTAs-NF and NF, indicating its smaller charge transfer resistance (Rct). Rct is usually used to describe the rate of the redox reactions, which react at the electrode and electrolyte interfaces51. Therefore, the synergistic effect between Ni and Fe of NiFe NTAs-NF could lead to the decrease of Rct and the acceleration of the charge transport. The contact angles of water with bare Ni foam and NiFe NTAs-NF are measured to study the wettability characteristics of catalysts. The contact angle of the bare Ni foam is about 110.4°due to its hydrophobic nature (Figure S12a, Supporting Information). However, a water droplet is hard to take shape on its surface and infiltrated into NiFe NTAs-NF quickly (Figure S12b, Supporting Information). Any specific contact angle of NiFe NTAs-NF is not obtained. The results indicate that the surface of NiFe NTAs-NF is rougher and more hydrophilic than the bare Ni foam, a favorable structure for greater access of the electrolyte into the catalyst.52 3D nanoarray electrode and rough electrode surface could benefit the electrolyte diffusion and gas evolution reaction. Therefore, the OER and HER performance are enhanced.32 Based on the above studies, it is reasonable to suggest that the NiFe NTAs-NF would serve as an OER and HER bifunctional electrode for water splitting in strong alkaline electrolytes. Hence, we constructed a two-electrode configuration using the NiFe NTAs-NF as both the anode and cathode (NiFe NTAs-NF// NiFe NTAs-NF) for overall water splitting in 1 M KOH (Figure 6a). For comparison, we also prepared a two-electrode cell having the structure of bare Ni foam//bare Ni foam. The liner 15 ACS Paragon Plus Environment


sweep voltammetry of

the NiFe NTAs-NF//NiFe

Page 16 of 24

NTAs-NF two-electrode

configuration shows that the water electrolyzer requires only a cell potential of 1.62 V to afford a current density of 10 mA cm-2 (Figure 6b). The voltage difference (∆V) between HER and OER is close to the voltage for overall water splitting at the same current density of 10 mA cm-2 (Figure 6c)53. The ∆V is slightly smaller than 1.62 mV, due to the existence of internal resistance which is measured about 5.8 ohm (Figure S13, Supporting Information). In contrast, the bare Ni foam//bare Ni foam alkaline water electrolyzer required higher cell voltages of 1.80 V to reach current densities of 10 mA cm–2. The performance at the current density of 10 mA cm-2 is also comparable or even better than most previously reported non-noble metal bifunctional catalysts for overall alkaline water splitting (Table S3, Supporting Information). Furthermore, we also investigate the long-term durability of the NiFe NTAs-NF//NiFe NTAs-NF electrolyzer at current densities of 10 mA cm–2 for 20 h in 1 M KOH electrolyte. As Figure 6d shown, the electrolysis potential remains stable during the 20 h, only with slight change (25 mV). Furthermore, to determine the Faradic efficiency, the gas generated from the overall water splitting was collected by a water drainage method at a current density of 50 mA cm-2. The ratio of H2 and O2 was 2 : 1, and the amounts of H2 and O2 matched well with the theoretically calculated value (Figure S14, Supporting Information). The Faradaic efficiency is determined to be about 98%. The results indicate that NiFe NTAs-NF is an efficient bifunctional electrocatalysts for water splitting, due to their high electrocatalytic activity and excellent long-term stability.


Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Overall splitting performance. (a) Illustration of the two-electrode water electrolysis device (b) LSV curves of the NiFe NTAs-NF//NiFe NTAs-NF and bare Ni foam//bare Ni foam two-electrode configuration system in 1 M KOH without iR correction; (c) Steady-state polarization curves of NiFe NTAs-NF, Ni NTAs-NF, Fe NTAs-NF and NF. (d) Chronopotentiometric curve of water electrolysis for the NiFe NTAs-NF electrode serving as both cathode and anode under a constant current density of 10 mA cm−2 for 20 h.

The high catalytic activity and durability of amorphous NiFe NTAs-NF might be attributed to following intrinsic merits: (1) The synergistic effect between Ni and Fe with strong electron interactions, which could lead to more active sites and more efficient mass and charge transport capability. (2) The amorphous phase could increase active site density for the short-range order39,

41-45

, thus would improve

charge-transfer rate by providing easy pathways for charges54, and make it more durable to the structural tensions occurring water splitting due to the larger structural flexibility than crystalline structure40, 46. As shown in Figure S15 and S16 (Supporting 17 ACS Paragon Plus Environment


Information), the performance of NiFe NTAs-NF with calcination treatment is slightly lower than the original amorphous NiFe NTAs-NF. The crystallinity of amorphous NiFe would be improved after treatment at high temperature55, which would result in worse electrocatalytic performance. (3) The well-defined nanotube array architecture grows uniformly on the Ni foam with good separation, which provides a short diffusion path for electrolyte ions and gas, large specific surface area and abundant exposed active sites. In addition, the nanotubes have a higher utilization rate for electrode materials than the other kinds of building blocks. (4) Ni foam is used as a substrate of electrocatalysts, its advantages of enormous open space, good electrical conductivity, low cost and good mechanical stability could be beneficial for enhancing the OER and HER performance. (5) The surface roughness of amorphous NiFe NTAs-NF is very high, allowing it to act as a highly hydrophilic catalyst in the electrolyte solution (Figure S12, Supporting Information).

CONCLUSIONS Amorphous NiFe nanotube arrays are directly grown on Ni foam via a facile electrodeposition method. The NiFe nanotube arrays exhibit excellent bifunctional OER/HER electrocatalytic activities and stabilities in alkaline solutions, which only require an overpotential of 216 mV for OER at 50 mA cm-2 and 181 mV for HER at 10 mA cm-2. The excellent electrocatalytic performance might be attributed to synergistic effect of two active metals – Ni and Fe, well-defined nanotube array architecture, and amorphous phase. Moreover, a two-electrode alkaline water electrolyzer for overall water splitting needs only a cell voltage of 1.62 V to achieve the current density of 10 mA cm-2. Meanwhile, the water electrolyzer exhibits good electrochemical stability for at least 20 h. Our work provides a strategy for design and easy fabrication of highly efficient and durable bifunctional catalysts for alkaline full water splitting.

ASSOCIATED CONTENT 18 ACS Paragon Plus Environment

Page 18 of 24

Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information Available: SEM images of Ni foam; XRD patterns for the samples; SEM images, LSV curves, XPS spectrum and XRD patterns of NiFe NTAs-NF after OER and HER stability test; comparison of LSV polarization curves with and without iR correction for OER and HER; detail for estimation of electrochemically active surface area; Nyquist plots of samples; detail for contact angle measurement and Faradic efficiency; table about ICP-AES data of sample; table comparing OER and bifunctional water splitting activities of the NiFe NTAs-NF with recently documented materials

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel: +86-755-86392151. Fax: +86-755-86392299. *E-mail: [email protected]. ORCID Xian-Zhu Fu: 0000-0003-1843-8927 Rong Sun: 0000-0001-9719-3563

ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (No.21203236), Guangdong Department of Science and Technology (2017A050501052), Guangdong Provincial Key Laboratory (2014B030301014), and Shenzhen research plan (JCYJ20160229195455154).

REFERENCES (1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T. F. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355. (2) Zhou, H.; Yu, F.; Huang, Y.; Sun, J.; Zhu, Z.; Nielsen, R. J.; He, R.; Bao, J.; Goddard Iii, W. A.; Chen, S.; Ren, Z. Efficient hydrogen evolution by ternary molybdenum sulfoselenide particles on self-standing porous nickel diselenide foam. Nat. Commun. 2016, 7, 12765. (3) Yu, L.; Zhou, H.; Sun, J.; Qin, F.; Yu, F.; Bao, J.; Yu, Y.; Chen, S.; Ren, Z. Cu nanowires shelled with NiFe layered double hydroxide nanosheets as bifunctional electrocatalysts for overall water



splitting. Energy Environ. Sci. 2017, 10, 1820-1827. (4) Chen, P.; Zhou, T.; Zhang, M.; Tong, Y.; Zhong, C.; Zhang, N.; Zhang, L.; Wu, C.; Xie, Y. 3D Nitrogen-Anion-Decorated Nickel Sulfides for Highly Efficient Overall Water Splitting. Adv. Mater. 2017, 29. (5) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. In situ cobalt-cobalt oxide/N-doped carbon hybrids as superior bifunctional electrocatalysts for hydrogen and oxygen evolution. J. Am. Chem. Soc. 2015, 137, 2688-2694. (6) Xiao, Z.; Wang, Y.; Huang, Y.-C.; Wei, Z.; Dong, C.-L.; Ma, J.; Shen, S.; Li, Y.; Wang, S. Filling the oxygen vacancies in Co3O4 with phosphorus: an ultra-efficient electrocatalyst for overall water splitting. Energy Environ. Sci. 2017, 10, 2563-2569. (7) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (8) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K. C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces. Science 2011, 334. (9) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399-404. (10) Kwon, T.; Hwang, H.; Sa, Y. J.; Park, J.; Baik, H.; Joo, S. H.; Lee, K. Cobalt Assisted Synthesis of IrCu Hollow Octahedral Nanocages as Highly Active Electrocatalysts toward Oxygen Evolution Reaction. Adv. Funct. Mater. 2017, 27, 1604688. (11) Yin, H.; Zhao, S.; Zhao, K.; Muqsit, A.; Tang, H.; Chang, L.; Zhao, H.; Gao, Y.; Tang, Z. Ultrathin platinum nanowires grown on single-layered nickel hydroxide with high hydrogen evolution activity. Nat. Commun. 2015, 6, 6430. (12) Fan, K.; Ji, Y.; Zou, H.; Zhang, J.; Zhu, B.; Chen, H.; Daniel, Q.; Luo, Y.; Yu, J.; Sun, L. Hollow Iron-Vanadium Composite Spheres: A Highly Efficient Iron-Based Water Oxidation Electrocatalyst without the Need for Nickel or Cobalt. Angew. Chem., Int. Ed. 2017, 56, 3289-3293. (13) Pei, Z.; Li, H.; Huang, Y.; Xue, Q.; Huang, Y.; Zhu, M.; Wang, Z.; Zhi, C. Texturing in situ: N,S-enriched hierarchically porous carbon as a highly active reversible oxygen electrocatalyst. Energy Environ. Sci. 2017, 10, 742-749. (14) Jin, Y.; Wang, H.; Li, J.; Yue, X.; Han, Y.; Shen, P. K.; Cui, Y. Porous MoO2 Nanosheets as Non-noble Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Mater. 2016, 28, 3785-3790. (15) Xiao, C.; Li, Y.; Lu, X.; Zhao, C. Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting. Adv. Funct. Mater. 2016, 26, 3515-3523. (16) Menezes, P. W.; Indra, A.; Das, C.; Walter, C.; Göbel, C.; Gutkin, V.; Schmeiβer, D.; Driess, M. Uncovering the Nature of Active Species of Nickel Phosphide Catalysts in High-Performance Electrochemical Overall Water Splitting. ACS Catal. 2016, 7, 103-109. (17) Guo, Y.; Yao, Z.; Shang, C.; Wang, E. Amorphous Co2B Grown on CoSe2 Nanosheets as a Hybrid Catalyst for Efficient Overall Water Splitting in Alkaline Medium. ACS Appl. Mater. Interfaces 2017, 9, 39312-39317. (18) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016, 6, 8069-8097.


Page 20 of 24

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Candelaria, S. L.; Bedford, N. M.; Woehl, T. J.; Rentz, N. S.; Showalter, A. R.; Pylypenko, S.; Bunker, B. A.; Lee, S.; Reinhart, B.; Ren, Y.; Ertem, S. P.; Coughlin, E. B.; Sather, N. A.; Horan, J. L.; Herring, A. M.; Greenlee, L. F. Multi-Component Fe–Ni Hydroxide Nanocatalyst for Oxygen Evolution and Methanol Oxidation Reactions under Alkaline Conditions. ACS Catal. 2016, 7, 365-379. (20) Huang, T.; Chen, Y.; Lee, J. M. A Microribbon Hybrid Structure of CoOx-MoC Encapsulated in N-Doped Carbon Nanowire Derived from MOF as Efficient Oxygen Evolution Electrocatalysts. Small 2017, 1702753. (21) Tang, Y. J.; Liu, C. H.; Huang, W.; Wang, X. L.; Dong, L. Z.; Li, S. L.; Lan, Y. Q. Bimetallic Carbides-Based Nanocomposite as Superior Electrocatalyst for Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2017, 9, 16977-16985. (22) Dong, B.; Zhao, X.; Han, G.-Q.; Li, X.; Shang, X.; Liu, Y.-R.; Hu, W.-H.; Chai, Y.-M.; Zhao, H.; Liu, C.-G. Two-step synthesis of binary Ni–Fe sulfides supported on nickel foam as highly efficient electrocatalysts for the oxygen evolution reaction. J. Mater. Chem. A 2016, 4, 13499-13508. (23) Zhang, B.; Lui, Y. H.; Zhou, L.; Tang, X.; Hu, S. An alkaline electro-activated Fe–Ni phosphide nanoparticle-stack array for high-performance oxygen evolution under alkaline and neutral conditions. J. Mater. Chem. A 2017, 5, 13329-13335. (24) Zou, X.; Liu, Y.; Li, G. D.; Wu, Y.; Liu, D. P.; Li, W.; Li, H. W.; Wang, D.; Zhang, Y.; Zou, X. Ultrafast Formation of Amorphous Bimetallic Hydroxide Films on 3D Conductive Sulfide Nanoarrays for Large-Current-Density Oxygen Evolution Electrocatalysis. Adv. Mater. 2017, 29, 1700404. (25) Wang, J.; Ji, L.; Zuo, S.; Chen, Z. Hierarchically Structured 3D Integrated Electrodes by Galvanic Replacement Reaction for Highly Efficient Water Splitting. Adv. Energy Mater. 2017, 7, 1700107. (26) Gong, M.; Dai, H. A mini review of NiFe-based materials as highly active oxygen evolution reaction electrocatalysts. Nano Research 2014, 8, 23-39. (27) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation. Journal of the American Chemical Society J Am Chem Soc 2014, 136, 6744-6753. (28) Liu, J.; Zhu, D.; Ling, T.; Vasileff, A.; Qiao, S.-Z. S-NiFe 2 O 4 ultra-small nanoparticle built nanosheets for efficient water splitting in alkaline and neutral pH. Nano Energy 2017, 40, 264-273. (29) Han, N.; Zhao, F.; Li, Y. Ultrathin nickel–iron layered double hydroxide nanosheets intercalated with molybdate anions for electrocatalytic water oxidation. J. Mater. Chem. A 2015, 3, 16348-16353. (30) Niu, S.; Jiang, W. J.; Tang, T.; Zhang, Y.; Li, J. H.; Hu, J. S. Facile and Scalable Synthesis of Robust Ni(OH)2 Nanoplate Arrays on NiAl Foil as Hierarchical Active Scaffold for Highly Efficient Overall Water Splitting. Adv. Sci. 2017, 4, 1700084. (31) Wang, Z.; Zeng, S.; Liu, W.; Wang, X.; Li, Q.; Zhao, Z.; Geng, F. Coupling Molecularly Ultrathin Sheets of NiFe-Layered Double Hydroxide on NiCo2O4 Nanowire Arrays for Highly Efficient Overall Water-Splitting Activity. ACS Appl. Mater. Interfaces 2017, 9, 1488-1495. (32) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661-4672. (33) Xie, L.; Zhang, R.; Cui, L.; Liu, D.; Hao, S.; Ma, Y.; Du, G.; Asiri, A. M.; Sun, X. High-Performance Electrolytic Oxygen Evolution in Neutral Media Catalyzed by a Cobalt Phosphate Nanoarray. Angew. Chem., Int. Ed. 2017, 56, 1064-1068. (34) Feng, J. X.; Ye, S. H.; Xu, H.; Tong, Y. X.; Li, G. R. Design and Synthesis of FeOOH/CeO2 Heterolayered Nanotube Electrocatalysts for the Oxygen Evolution Reaction. Adv. Mater. 2016, 28,



Page 22 of 24

4698-4703. (35) Feng, J. X.; Xu, H.; Dong, Y. T.; Ye, S. H.; Tong, Y. X.; Li, G. R. FeOOH/Co/FeOOH Hybrid Nanotube Arrays as High-Performance Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2016, 55, 3694-3698. (36) Feng, J. X.; Xu, H.; Dong, Y. T.; Lu, X. F.; Tong, Y. X.; Li, G. R. Efficient Hydrogen Evolution Electrocatalysis Using Cobalt Nanotubes Decorated with Titanium Dioxide Nanodots. Angew. Chem., Int. Ed. 2017, 56, 2960-2964. (37) Wang, A. L.; Xu, H.; Feng, J. X.; Ding, L. X.; Tong, Y. X.; Li, G. R. Design of Pd/PANI/Pd sandwich-structured nanotube array catalysts with special shape effects and synergistic effects for ethanol electrooxidation. J. Am. Chem. Soc. 2013, 135, 10703-10709. (38) Zhu, Y. P.; Ma, T. Y.; Jaroniec, M.; Qiao, S. Z. Self-Templating Synthesis of Hollow Co3 O4 Microtube Arrays for Highly Efficient Water Electrolysis. Angew. Chem., Int. Ed. 2017, 56, 1324-1328. (39) Li, H.; Chen, S.; Jia, X.; Xu, B.; Lin, H.; Yang, H.; Song, L.; Wang, X. Amorphous nickel-cobalt complexes hybridized with 1T-phase

molybdenum disulfide

via

hydrazine-induced phase

transformation for water splitting. Nat. Commun. 2017, 8, 15377. (40) Gao, Y. Q.; Liu, X. Y.; Yang, G. W. Amorphous mixed-metal hydroxide nanostructures for advanced water oxidation catalysts. Nanoscale 2016, 8, 5015-5023. (41) Indra, A.; Menezes, P. W.; Sahraie, N. R.; Bergmann, A.; Das, C.; Tallarida, M.; Schmeisser, D.; Strasser, P.; Driess, M. Unification of catalytic water oxidation and oxygen reduction reactions: amorphous beat crystalline cobalt iron oxides. J. Am. Chem. Soc. 2014, 136, 17530-17536. (42) Kornienko, N.; Resasco, J.; Becknell, N.; Jiang, C. M.; Liu, Y. S.; Nie, K.; Sun, X.; Guo, J.; Leone, S. R.; Yang, P. Operando spectroscopic analysis of an amorphous cobalt sulfide hydrogen evolution electrocatalyst. J. Am. Chem. Soc. 2015, 137, 7448-7455. (43) Liu, W.; Liu, H.; Dang, L.; Zhang, H.; Wu, X.; Yang, B.; Li, Z.; Zhang, X.; Lei, L.; Jin, S. Amorphous Cobalt-Iron Hydroxide Nanosheet Electrocatalyst for Efficient Electrochemical and Photo-Electrochemical Oxygen Evolution. Adv. Funct. Mater. 2017, 27, 1603904. (44) Yang, L.; Guo, Z.; Huang, J.; Xi, Y.; Gao, R.; Su, G.; Wang, W.; Cao, L.; Dong, B. Vertical Growth of 2D Amorphous FePO4 Nanosheet on Ni Foam: Outer and Inner Structural Design for Superior Water Splitting. Adv. Mater. 2017, 29, 1704574. (45) Lee, S. C.; Benck, J. D.; Tsai, C.; Park, J.; Koh, A. L.; Abild-Pedersen, F.; Jaramillo, T. F.; Sinclair, R. Chemical and Phase Evolution of Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production. ACS Nano 2016, 10, 624-632. (46) Chen, D.; Dong, C. L.; Zou, Y.; Su, D.; Huang, Y. C.; Tao, L.; Dou, S.; Shen, S.; Wang, S. In situ evolution of highly dispersed amorphous CoOx clusters for oxygen evolution reaction. Nanoscale 2017, 9, 11969-11975. (47) Lu, X.; Zhao, C. Electrodeposition of hierarchically structured three-dimensional nickel-iron electrodes for efficient oxygen evolution at high current densities. Nat. Commun. 2015, 6, 6616. (48) Wang, S.; Huang, Z.; Li, R.; Zheng, X.; Lu, F.; He, T. Template-assisted synthesis of NiP@CoAl-LDH nanotube arrays with superior electrochemical performance for supercapacitors. Electrochim. Acta 2016, 204, 160-168. (49) Wang, A. L.; He, X. J.; Lu, X. F.; Xu, H.; Tong, Y. X.; Li, G. R. Palladium-cobalt nanotube arrays supported on carbon fiber cloth as high-performance flexible electrocatalysts for ethanol oxidation. Angew. Chem., Int. Ed. 2015, 54, 3669-3673. (50) Zhang, B.; Lui, Y. H.; Ni, H.; Hu, S. Bimetallic (Fe x Ni 1−x ) 2 P nanoarrays as exceptionally


Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


efficient electrocatalysts for oxygen evolution in alkaline and neutral media. Nano Energy 2017, 38, 553-560. (51) Xiao, Y.; Zhang, P.; Zhang, X.; Dai, X.; Ma, Y.; Wang, Y.; Jiang, Y.; Liu, M.; Wang, Y. Bimetallic thin film NiCo–NiCoO2@NC as a superior bifunctional electrocatalyst for overall water splitting in alkaline media. J. Mater. Chem. A 2017, 5, 15901-15912. (52) Deng, J.; Li, H.; Wang, S.; Ding, D.; Chen, M.; Liu, C.; Tian, Z.; Novoselov, K. S.; Ma, C.; Deng, D.; Bao, X. Multiscale structural and electronic control of molybdenum disulfide foam for highly efficient hydrogen production. Nat. Commun. 2017, 8, 14430. (53) Zhang, Y.; Shao, Q.; Pi, Y.; Guo, J.; Huang, X. A Cost-Efficient Bifunctional Ultrathin Nanosheets Array for Electrochemical Overall Water Splitting. Small 2017, 13. (54) Li, H.; Gao, Y.; Wang, C.; Yang, G. A Simple Electrochemical Route to Access Amorphous Mixed-Metal Hydroxides for Supercapacitor Electrode Materials. Adv. Energy Mater. 2015, 5, 1401767. (55) Wang, T.; Wang, C.; Jin, Y.; Sviripa, A.; Liang, J.; Han, J.; Huang, Y.; Li, Q.; Wu, G. Amorphous Co–Fe–P nanospheres for efficient water oxidation. J. Mater. Chem. A 2017, 5, 25378-25384.



Table of Contents

Amorphous NiFe nanotube arrays are fabricated on nickel foam as high performance non-noble metal bifunctional electrocatalysts for overall alkaline water splitting.


Page 24 of 24

Amorphous NiFe Nanotube Arrays Bifunctional Electrocatalysts for

Recommend Documents