Self-Growing NiFe-Based Hybrid Nanosheet Arrays on Ni Nanowires

Subscriber access provided by GUILFORD COLLEGE

Article

Self-Growing NiFe-Based Hybrid Nanosheet Arrays on Ni Nanowires for Overall Water Splitting Xue Teng, Lixia Guo, Lvlv Ji, Jianying Wang, Yanli Niu, Zhibiao Hu, and Zuofeng Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00584 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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.

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 8 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

Self-Growing NiFe-Based Hybrid Nanosheet Arrays on Ni Nanowires for Overall Water Splitting Xue Teng,† Lixia Guo,† Lvlv Ji,‡ Jianying Wang,† Yanli Niu,† Zhibiao Hu,§ and Zuofeng Chen*,† †Shanghai

Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China. ‡Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, China. §College of Chemistry and Materials Science, Longyan University, Longyan Fujian, 364012, China. Supporting Information ABSTRACT: Exploring highly efficient and low-cost electrocatalysts is important for boosting the efficiency of water splitting to obtain clean and renewable energy. In this study, the NiFe layered double hydroxide nanosheets (NiFe-LDH NSs) were developed on Ni nanowires (NWs) via the in situ assembly of oriented 2D nanosheets. Because of its unique composition and hierarchical NS@NW structure with the conductive Ni NW core as electron conduit, this material catalyst performs as a superior electrocatalyst toward the oxygen evolution reaction (OER), exhibiting a low onset overpotential of 230 mV in 1 M KOH. Remarkably, NiFe-LDH NSs can be readily converted to Fe-doped Ni2P nanosheets (Fe-Ni2P NSs) or Fe-doped NiSe2 nanosheets (Fe-NiSe2 NSs) upon phosphorization or selenization treatments, which are both excellent electrocatalysts toward the hydrogen evolution reaction (HER) in alkaline solution. Consequently, two-electrode alkaline water electrolyzers have been constructed using NiFe-LDH NSs as anode and Fe-Ni2P NSs or FeNiSe2 NSs as cathode, which can afford a current density of 30 mA cm−2 at cell voltages of 1.69 V and 1.76 V, respectively, with excellent long-term durability. KEYWORDS: water splitting; hierarchical structure; NiFe layered double hydroxides (LDHs); phosphide; diselenide

Introduction The rapid consumption of fossil fuels and serious environmental problems have stimulated research and development of renewable energy systems. Electrochemical water splitting for the generation of hydrogen offers a promise method.1,2 However, water splitting suffers from the large overpotential, which arises mainly from the sluggish oxygen evolution reaction (OER) generating one oxygen molecule via the coupled transfer of four protons and four electrons from two water molecules on the anode and hydrogen evolution reaction (HER) on the cathode.3-5 Currently, the most efficient catalysts to split water are noble metal oxides such as IrO2 or RuO2 for OER and Pt for HER.6,7 However, the scarcity and high cost of such precious metals limit their widespread use on scale-up deployment. A great deal of efforts and progress have thus been made to develop cost-effective and earth-abundant OER and HER catalysts with high activity and stability, especially in alkaline solutions. Several families of earth-abundant materials, including perovskites8,9 and transition metal hydroxides/oxides10-14 for OER and transition metal chalcogenides,15-18 nitrides,19-21 phosphides,22-24 borides,25,26 and heteroatom-doped carbons27,28 have been shown to be effective electrocatalysts for HER. Among various promising alternatives, NiFe oxides/hydroxides,29-32 especially NiFe layered double hydroxides (LDHs) for OER and transition metal phosphides/selenides33,34 for HER have attracted tremendous interest owing to their earth abundance and remarkable electrocatalysis performance. As the catalysis process commonly takes place on the surface of a material catalyst, an inefficient exposure of active sites and poor electron/ion transport ability of powdered samples tend to induce inferior catalysis performance. Therefore, many research efforts have been devoted to design the rational structure to increase the catalytic active sites and accelerate the electron transport. Compared with the conventional one- or two-dimensional

architecture, well-ordered or oriented hybrid structure of nanosheets@nanowires (NSs@NWs) can offer several critical advantages, such as increasing the electrochemically active surface area, promoting electrolyte permeation and facilitating electron transportation. In particular, conductive metal NWs servicing as the catalyst support should greatly enhance the charge mobility to drive proficient catalysis. Inspired by these perspects, we developed here a selfgrowing strategy to fabricate hierarchical-structure NiFeLDH nanosheets (NSs) on Ni nanowires (NWs). Ni NWs were utilized as the Ni source and catalyst skeleton, and after a fast and well-controlled redox reaction with iron(III) salt, the coprecipitation process generates a NiFe-LDH NS shell around the conductive Ni NW core. This unique core-shell structure overcome the problem of low conductivity in the conventional LDH film. Owing to their desirable compositions and unique structural advantages, the asprepared NiFe-LDH NSs manifest enhanced electrocatalytic activity toward alkaline water oxidation, achieving a current density of 10 mA cm−2 at 280 mV. Furthermore, NiFe-LDH NSs are readily converted to Fe-doped Ni2P nanosheets (FeNi2P NSs) or Fe-doped NiSe2 nanosheets (Fe-NiSe2 NSs) upon phosphorization or selenization treatments, both of which exhibit excellent HER performance in alkaline solutions. Consequently, the two-electrode alkaline water electrolyzers, constructed with NiFe-LDH NSs as anode and Fe-Ni2P NSs or Fe-NiSe2 NSs as cathode, can achieve a current density of 30 mA cm–2 at cell voltages of 1.69 V and 1.76 V, respectively. These findings highlight the ability of the Ni-based electrocatalysts for future efficient electrochemical overall water splitting devices. Moreover, our approach provides an operable route to the development of NSs@NWs hybrid structure, which holds promise for a wide range of energy conversion and storage applications.

Results and discussion

ACS Paragon Plus Environment

1

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

Page 2 of 8

Scheme 1. Schematic illustration of the synthesis of Ni NWs, and NiFe-LDH NSs, Fe-Ni2P NSs and Fe-NiSe2 NSs on Ni NWs.

Preparation and characterization. The fabrication of NiFeLDH NSs for OER and their phosphorization or selenization products for HER is illustrated in Scheme 1 and detailed in the Supporting Information. Briefly, the Ni NWs precursor is first synthesized via a hydrazine hydrate reduction method. The NiFeLDH NSs are then obtained by dispersion of Ni NWs in Fe(NO3)3 solution. In Figure 1A,B, scanning electron microscopy (SEM) images of different magnifications show uniform Ni NWs with a diameter of 300 nm and reveal rough surface over these NWs. In Figure 1C, the high-resolution transmission electron microscopy (HRTEM) image shows clear lattice fringes with an interplanar spacing of 0.203 nm, corresponding to the (111) plane of the metallic Ni. The inset in Figure 1C shows the selected area

Figure 1. SEM images (A, B) and HRTEM image (C) of Ni NWs; SEM images (D, E) and (F) HRTEM image of NiFe-LDH NSs; insets in (C) and (F) show corresponding SAED patterns. (G) EDX elemental mapping images of Ni, Fe, and O in a NiFe-LDH NS.

electron diffraction (SAED) of a Ni NW, confirming its polycrystalline nature. In Figure S1, the energy dispersive X-ray analysis (EDX) reveals that Ni is the major element with a small amount of O on the surface of NWs. The diffraction peaks related to (111), (200) and (220) of metallic Ni are illustrated by the Xray diffraction (XRD) pattern in Figure S2. After the redox reaction with iron(III) salt, the NiFe-LDH NSs grew uniformly on Ni NWs, as shown in Figure 1D. A magnified SEM image in Figure 1E clearly shows that the Ni NW precursor is turned into a hierarchical core-shell structure. This change of substance and structure is presumably attributed to the following reactions35:

2Fe3+ + Ni → 2Fe2+ + Ni2+ (1) Fe3+ + 3H2O → Fe(OH)3 + 3H+ (2) 2Fe(OH)3 + 3Ni2+ → 2Fe3+ +3Ni(OH)2 (3) Fe3+ in the solution can replace Ni2+ in the Ni(OH)2 lattice, forming a stable LDH structure. The excessive cationic charge of Fe3+ is balanced by anion intercalation between the hydroxide layers.36 The obtained NiFe-LDH NSs are further analyzed by HRTEM in Figure 1F, which shows clear lattice fringes with an interplanar spacing of 0.26 nm, corresponding to the (012) plane of NiFe-LDH. The polycrystalline nature of NiFe-LDH NSs is proved by SAED in the inset of Figure 1F. The elemental mapping is performed on a single NiFe-LDH NS to reveal the spatial distribution of different elements in the unique hybrid nanostructure. In Figure 1G, the mapping image exhibits a more dense distribution of Ni element in the core and homogeneous distribution of Fe and O elements throughout the whole NiFe-LDH NS, which is consistent with the hybrid NS@NW structure. In Figure 2A, the XRD pattern of NiFe-LDH NSs manifests that all the diffraction peaks, except for those of metallic Ni inside the core, can be indexed to NiFe-LDH (JCPDS No. 400215), representing a R3m symmetry and hexagonal lattice,37 which is consistent with the lattice fringes shown in Figure 1F. The survey scan the X-ray photoelectron spectroscopy (XPS) is shown in Figure 2B and a strong peak of O 1s is observed at 531 eV in the full spectrum, which is associated with the oxygen of the hydroxyl groups on the surface of hierarchical-structure NiFe-LDH NSs.38 For Ni 2p XPS in Figure 2C, the peaks of Ni 2p3/2 and Ni 2p1/2 are located at 855.4 eV and 873.4 eV along with two satellite peaks, which are indicative of Ni2+ oxidation states in the composites and attributed to the Ni-OH species. For Fe 2p XPS in Figure 2D, the two peaks located at 712.8 and 726.1 eV correspond to Fe 2p3/2 and Fe 2p1/2, implying an Fe3+ oxidation state.39-41 Electrocatalytic OER performance. The OER activity of the NiFe-LDH NSs and pure Ni NWs was examined in 1 M KOH. The as-prepared catalysts were uniformly coated on a glassy carbon electrode with Nafion as binding agent and used as a working electrode. Figure 3A shows the iR-corrected linear sweep voltammetry (LSV) curves at a scan rate of 5 mV s−1. The oxidation peaks between 1.35 - 1.42 V versus reversible hydrogen electrode (RHE) are assigned to the oxidation of Ni2+ to Ni3+. The NiFe-LDH NSs electrode exhibits a low onset overpotential of 230 mV which is far below 300 mV of Ni NWs. The horizontal dotted line in the figure indicates the overpotentials of the two electrodes required to achieve a


2

B

-2 Ni 2p1/2

O 1s

Fe 2p1/2

-1

152 mV dec -1

68 mV dec

0.3

-2

1.3

0

1.7

0.8

-2

Fe 2p1/2

Fe 2p3/2

1.5

Figure 2. (A) XRD pattern and (B) survey XPS, (C) Ni 2p XPS, and (D) Fe 2p XPS of NiFe-LDH NSs.

current density of 10 mA cm−2. In Figure 3B, the NiFe-LDH NS electrode exhibits a smaller Tafel slope of 68 mV dec−1 than 152 mV dec−1 of Ni NWs, indicating that the NiFe-LDH NSs proceed a faster OER kinetics than Ni NWs. Table S1 shows comparison of OER performance of NiFe-LDH NSs and various NiFe LDH catalysts reported in other literature. To assess the stability of the NiFe-LDH NS electrode for the OER, a multistep chronopotentiometric curve was recorded under the alkaline condition. The applied current increases from 10 to 90 mA cm‒2 with an increment of 10 mA cm‒2 per 10 min. As shown in Figure 3C, the response potentials hold steady at each step, indicating the high stability of the electrode within a wide range of current densities. The robustness of the material catalyst is further proved by long-term electrolysis measurement at a fixed potential of 1.53 V for 12 h and by the LSV curves of NiFe-LDH NSs before and after OER test, as shown in Figure 3D. Moreover, the existence of NiFe-LDH NSs after electrolysis is proved by SEM image in Figure S3 and XRD pattern in Figure S4. To rationalize the high performance of the NiFe-LDH NSs, we calculate their double-layer capacitances (Cdl) to estimate the electrochemically active surface area (ECSA) of the catalysts. In Figure S5, the capacitative charging currents of NiFe-LDH NSs and Ni NWs were recorded in the non-Faradaic potential region between 0.9 - 1.2 V at scan rates from 10 to 50 mV s–1. As shown in Figure 3E, the linear slope of NiFe-LDH NSs is twice of Ni NWs, which benefits from their unique hybrid structure. In addition to the large surface area, it contributes to diffusing water molecules, ensuring intimate contact between the catalyst and electrolyte. Moreover, the interlayer spacing is propitious to release gas.39 In Figure 3F, the electrochemical impedance spectroscopies (EIS, presented by Nyquist plots) exhibit two semi-circles in the high and low-frequency regions, which correspond to the electrolyte diffusion and charge transfer resistance (Rct) within the electrodes, respectively. It reveals that NiFe-LDH NSs have a smaller Rct (100 Ω) than Ni NWs (145 Ω), indicating a faster charge transfer and favorable reaction kinetics on the hybrid electrode.39,42 In Figure S6, the charge transfer resistance decreases as the overpotential increases, conforming to the faster OER kinetics at larger overpotentials. Besides the above advantages of the hybrid structure, the synergy between Fe and Ni of the NiFe-LDH materials has been intensively investigated recently. Under positive potential, Fe (IV) with a d4 configuration and partially unoccupied d orbitals is formed, which is stabilized by  bonding to a terminal oxo-ligand.

10 mA cm

0

20

E 0.50

40 60 t (min)

0.25

0.00

60

-2

2.35 mF cm

10

20 30 40 -1 Scan rate (mV s )

50

10

0

0

2

1.3

16 12 8 4 0

120

-2

5.54 mF cm

15

5

F 160

Ni NWs NiFe-LDH NSs

20

-2

40

0 1.2

80

1.8

25

20

-2

1.2 1.4 1.6 -2 log|j (mA cm )|

Initial After electrolysis

80

-2

1.6

-2

720 710 Binding energy (eV)

90 mA cm

1.7

j (mA cm )

730

1.0

D 100

1.8

j (mA cm )

C

1.4 1.5 1.6 E (V vs. RHE)

4

6 t (h)

8

10

12

1.4 1.5 1.6 E (V vs. RHE)

1.7

- Z" / (ohm)

1000 800 600 400 200 Binding energy (eV)

1.4

880 870 860 Binding energy (eV)

0.4

10 mA cm

0 1.2

Counts

Sat.

50 25

Ni 2p3/2 Ni 2p1/2

Ni NWs NiFe-LDH NSs

C 1s

D Sat.

75

j (mA cm )

C

Fe 2p3/2

0.5

B

Ni NWs NiFe-LDH NSs

- Z" / (ohm)

10 20 30 40 50 60 70 80 2 (degree)

j (mA cm )

 Ni NWs

(012) (200) (101)  (110) (006)  (220)  (003)  (113)   (015) 

A 100

Ni 2p3/2

  NiFe-LDH NSs

E (V vs. RHE)

(111)

Counts

Intensity (a.u.)

A

Counts

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


 (V)

Page 3 of 8

80

0

4

8

12

Z' / (ohm)

16

40 0

Ni NWs NiFe-LDH NSs

0

40

80 120 Z' / (ohm)

160

Figure 3. (A) Polarization curves and (B) Tafel plots for OER on Ni NWs and NiFe-LDH NSs. (C) Multicurrent process obtained with NiFe-LDH NSs in 1 M KOH. (D) OER polarization curves for NiFe-LDH NSs before and after electrolysis under an overpotential of 300 mV for 12 h; inset shows the time-dependent current density curve. (E) Capacitive currents at 1.05 V as a function of scan rate and (F) Nyquist plots for different electrodes in 1 M KOH; inset shows the magnified view of Nyquist plots in the high frequency domain.

At slightly higher potentials, oxidation of Fe(IV)=O to Fe(V) or Fe(VI) occurs, and maintenance of the positive potential leads to form cis-dioxo-Fe(VI) species and then produce an Fe(IV)peroxide under internal redox rearrangement to release oxygen. The nickel-hydroxide lattice stably and tightly binds the catalytic Fe centers, and Ni(III) centers serve as conduits that deliver electrons to the electrode during the process. It’s noteworthy that the reactive cis-dioxo-Fe(VI) fragment is only produced by the structural rearrangements in the corner sites of the LDH lattice.43 Further experimental and computational investigations are necessary to elucidate the catalysis mechanism of NiFe-LDH.44 Electrocatalytic HER performance. With the NiFe-LDH NSs material available, it was subject to phosphorization and selenization treatments and the resultant materials were utilized for HER. As seen from SEM images in Figure 4A,B, the NiFe-LDH NSs are converted into Fe-Ni2P NSs with wellretained structure after phosphorization. The HRTEM image of a single Fe-Ni2P NS in Figure 4C shows a well-resolved lattice fringe spacing of 0.224 nm for the (111) plane, which is slightly larger than that of pure Ni2P (0.221 nm) probably owing to the substitution of Fe for Ni.45,46 The SAED pattern in the inset of Figure 4C exhibits that the Fe-Ni2P NSs are of polycrystalline structure. Another HER electrocatalyst, Fe-NiSe2 NSs, was synthesized by a similar procedure. As shown by SEM images in Figure 4D,E, upon selenization Fe-NiSe2 NSs keep the integrated nanosheet structure inherited from the NiFe-LDH NSs precursor. In Figure 4F, the HRTEM image of a single FeNiSe2 NS reveals well-resolved lattice fringe with interplanar distance of 0.246 nm, slightly larger than the (211) plane of pure NiSe2 (0.243 nm) due to the substitution of Fe for Ni.45,46 The polycrystalline structure of the material is also proved by the SAED in the inset of Figure 4F. In Figure 4G,H, the elemental


3


mapping images are consistent with the formation of hierarchical-structure Fe-Ni2P NSs and Fe-NiSe2 NSs on Ni NWs.

Figure 4. SEM images (A, B) and HRTEM image (C) of Fe-Ni2P NSs; SEM images (D, E) and (F) HRTEM image of Fe-NiSe2 NSs; insets in (C) and (F) show corresponding SAED patterns. (G) EDX elemental mapping images of Ni, Fe, and P in Fe-Ni2P NSs; (H) EDX elemental mapping images of Ni, Fe, and Se in Fe-NiSe2 NSs.

These HER electrocatalysts were further characterized by XRD and XPS. As shown in Figure S7, the XRD pattern of Fe-Ni2P NSs is consistent with a Ni2P standard pattern (JCPDS No. 89-2742). The Ni2P phase, including (111), (201), (210), (300), (211) and (400), and the metallic Ni phase, such as (111), (200) and (220) are clearly observed, indicating the existence of Fe-Ni2P NSs@Ni NWs in the products. In Figure S8A, the survey XPS spectrum further corroborates the presence of Ni, Fe and P elements. For the high-resolution Ni 2p3/2 XPS in Figure S8B, there are two main peaks at binding energies of 853.0 and 856.2 eV with a shake-up satellite peak 864.5 eV, which are assigned to Ni-P and Ni-O of the surface oxidation product, respectively.46 The Fe 2p spectrum in Figure S8C is fitted with two peaks at 711 and 724.5 eV assigned to Fe2+ and a peak at 715.2 eV assigned to Fe3+, implying the coexistence of Fe2+ and Fe3+. There is no peak characteristic of FeP, which indicates the formation of a ternary Fe-Ni2P compound rather than a mixture of two solid phases.46,47 The binding energies of P 2p spectrum in Figure S8D at 129.8 and 133.6 eV are consistent with a metal phosphide and a metal phosphate on the surface, the latter of which is presumably caused by surface oxidation in air.47,48 The XRD pattern of Fe-NiSe2 NSs in Figure S9 demonstrates that the product is consistent with cubic pyritephase NiSe2 (JCPDS No. 88-1711). The formation of FeNiSe2 NSs is further supported by XPS. In Figure S10A, the survey spectrum reveals the presence of Ni, Fe and Se elements. In Figure S10B, the high-resolution Ni 2p3/2 XPS of Fe-NiSe2 NSs shows Ni-Se related peak at 852.7 eV and Ni-O related peak at 855.3 eV for the surface oxidation product with a shake-up satellite peak at 860.3 eV. In Figure S10C, the two fitting peaks at 711.5 and 725.3 eV and the peak at 715.7 eV are assigned to Fe2+ and Fe3+, respectively. In Figure S10D, the binding energies of 54.2 and 55.7 eV are characteristic of the Se 3d5/2 and Se 3d3/2, respectively, and the

Page 4 of 8

broad peak near 58.9 eV implies surface oxidation of the Se species.49,50 Table S2 summarizes the atomic ratios of Ni and Fe elements in Fe-Ni2P NSs and Fe-NiSe2 NSs measured by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). For comparison, the Ni2P NWs and NiSe2 NWs were also prepared according to the procedures described in the Supporting Information. The nanowire-structure-remained SEM images and other characterization data such as XRD and EDX are illustrated in Figures S11-S16. These hierarchical-structure electrocatalysts were evaluated in 1 M KOH. Figure 5A shows the polarization curves of 20 wt% Pt/C, Ni NWs, Ni2P NWs and Fe-Ni2P NSs. As can be seen, Fe-Ni2P NSs require an operating potential of −0.116 V to deliver a cathodic current density of 10 mA cm−2, which is much lower than those of Ni2P NWs (−0.166 V) and Ni NWs (−0.25 V). In Figure 5B, Fe-Ni2P NSs exhibit a Tafel slope value of 68 mV dec−1, which is also smaller than those of Ni2P NWs (83 mV dec−1) and Ni NWs (106 mV dec−1), implying the favorable HER kinetics. As shown in Table S3, this potential value also largely outperforms other metal phosphide materials loaded on planar substrates. Similarly, the catalysis performance of Fe-NiSe2 NSs is also evaluated in an alkaline solution. As shown in Figure 5C, it requires an overpotential of approximately 147 mV to reach a current density of 10 mA cm−2, which is lower than 188 mV for the contrast samples NiSe2 NWs and 250 mV for Ni NWs. In addition, Figure 5D shows that the Tafel slope of Fe-NiSe2 NSs (74 mV dec−1) is smaller than those of NiSe2 NWs (90 mV dec−1) and Ni NWs (106 mV dec−1). Table S4 shows that the catalysis performance of Fe-NiSe2 NSs is among the best of NiSe2-based materials reported in literature. As mentioned above, the charge-transfer resistance Rct is known to relate to the electrocatalytic kinetics and its lower value reflects a faster reaction rate, which can be obtained from the diameter of semicircle in the EIS Nyquist plot in the low frequency region. In Figure S17A, the charge transfer resistance of Fe-Ni2P NSs (65 Ω) at ‒0.19 V is lower than those of Ni2P NWs (75 Ω) and Ni NWs (90 Ω). In addition, Figure S17B shows that the charge transfer resistance of Fe-Ni2P NSs decreases with the overpotential increasing, conforming to the faster HER kinetics at larger overpotentials. Similar results are obtained with Fe-NiSe2 NSs as shown in Figure S18. Stability is another vital parameter for the evaluation of electrocatalysts. Remarkably, as shown in Figure 5E,F, both Fe-Ni2P NSs and Fe-NiSe2 NSs are stable over 12 h during long-term electrolysis measurements. The SEM images and XRD patterns of Fe-Ni2P NSs and Fe-NiSe2 NSs after HER electrolysis are provided in Figures S19-S22, which confirm further the high catalytic durability toward the HER. Overall Water Splitting. The overall water splitting performance of the alkaline electrolyzer was evaluated in a twoelectrode configuration using NiFe-LDH NSs as the anode and FeNi2P NSs or Fe-NiSe2 NSs as the cathode, respectively. In Figure 6A, the NiFe-LDH NSs||Fe-Ni2P NSs electrolyzer requires a cell voltage of 1.69 V to reach a current density of 30 mA cm−2, which is in contrast to 1.78 V for NiFe-LDH NSs||Ni2P NWs and 1.91 V for Ni NWs||Ni NWs. Similarly, the polarization curves in Figure 6B indicate that the NiFe-LDH NSs||Fe-NiSe2 NSs electrolyzer exhibits superior overall water splitting activity with a current density of 30 mA cm−2 achieved with a cell voltage of 1.76 V, which is better than NiFe-LDH NSs||NiSe2 NWs electrolyzer requiring 1.81 V. Specifically, the cell voltages of the aforementioned couples at 30 mA cm−2 agree well with the sum of the potentials of HER and OER measured in the three-electrode


4

Page 5 of 8

system as shown in Figure 6C. A detailed comparison of various electrocatalysts couples reported in literature is listed in Table S5.

A

B

-2

j (mA cm )

0

C

20 wt% Pt/C Ni NWs Ni2P NWs

 (V)

-50

20 wt% Pt/C Ni NWs Ni2P NWs

-1

83 mV dec

Fe-Ni2P NSs -1

106 mV dec

-1

68 mV dec

0.1

Fe-Ni2P NSs

-0.3 -0.2 -0.1 E (V vs. RHE)

-1

28 mV dec

-0.5

0.0

D

0

-50

 (V)

-2

j (mA cm )

0.3

0.2

-100 -0.4

20% wt Pt/C Ni NWs NiSe2 NWs

0.0

0.5 1.0 1.5 -2 log|j (mA cm )|

-100 -0.4

-0.3

-0.2

-0.1

Fe-NiSe2 NSs

0.2

-1

90 mV dec

-1

106 mV dec

-1

74 mV dec

0.1 -1

0.0

0.0

0.4 0.8 1.2 1.6 -2 log|j (mA cm )|

-15

-100

-0.2

0

2

4

6 t (h)

8

-0.1 E (V vs. RHE)

10

-2

-10 -15 -20

12

-100 -0.3

0.0

Experimental section and additional information as noted in the text.

0

-50

0

2

4

6 t (h)

8

10

12

-0.2 -0.1 E (V vs. RHE)

0.0

Figure 5. (A) HER polarization curves and (B) Tafel plots for 20 wt% Pt/C, Ni NWs, Ni2P NSs and Fe-Ni2P NSs; (C) HER polarization curves and (D) Tafel plots for 20 wt% Pt/C, Ni NWs, NiSe2 NSs and Fe-NiSe2 NSs; LSV curves for (E) Fe-Ni2P NSs and (F) Fe-NiSe2 NSs before and after electrolysis under an overpotential of 140 mV and 170 mV for 12 h, respectively; insets show the corresponding time-dependent current density curves.

During electrolysis, the stable i-t curves of NiFe-LDH NSs||FeNi2P NSs and NiFe-LDH NSs||Fe-NiSe2 NSs (Figure 6D), as well as the nearly overlapped polarization curves before and after the durability test (Figure S23) demonstrate a high stability for overall water splitting in an alkaline solution.

B 100 j (mA cm )

Ni NWs||Ni NWs NiFe LDH NSs||Ni2P@Ni NW

80 60 40 20

1.4 1.6 1.8 Cell voltage (V)

40

D Cell Voltage

1.4 1.6 1.8 Cell voltage (V)

2.0

NiFe-LDH NSs||Fe-Ni2P NSs

20

-2

2.0

j (mA cm )

EHER+EOER

1.5

i e Ni||N ||NiS 2 iSe 2 LDH ||Ni 2P iP ||Fe-N LDH NiFe LDH ||Fe-N 2 NiFe LDH NiFe NiFe

0 20 0

Experimental details, XRD patterns, SEM images, XPS spectra, Nyquist plots, EDX elemental analysis, iR-corrected polarization curves, and performance comparisons of various catalysts (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Zuofeng Chen: 0000-0002-2376-2150

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573160, 21872105), the Fundamental Research Funds for the Central Universities, and Science & Technology Commission of Shanghai Municipality (14DZ2261100).

REFERENCES

0 1.2

2.0

2.5

1.0

NiFe LDH NSs||Fe-NiSe2 NSs

60

20

0 1.2

C

Ni NWs||Ni NWs NiFe LDH NSs||NiSe2 NWs

80

-2

NiFe LDH NSs||Fe-Ni2P NSs

-2

j (mA cm )

A 100

ASSOCIATED CONTENT Supporting Information

-5

j (mA cm )

-10

2.0


-2

-2

j (mA cm )

-50

0

j (mA cm )

-5

-2

j (mA cm )

F


0

facilitates the electron transfer through the inner metallic Ni NWs, overcoming the problem of low conductivity of catalyst layers. Compared with pure Ni NWs, the hierarchically structured NiFeLDH NSs and Fe-Ni2P NSs or Fe-NiSe2 NSs are demonstrated as more efficient electrocatalysts, which require overpotentials of 280 mV for OER and 116 or 147 mV for HER, respectively, to afford a current density of 10 mA cm–2 in an alkaline solution. Furthermore, the constructed NiFe-LDH NSs||Fe-Ni2P NSs (or Fe-NiSe2 NSs) electrolyzers exhibit admirable performance toward overall water splitting, which could deliver the current density of 30 mA cm−2 at a cell voltage of 1.69 (or 1.76 V). The discovery of this self-growing hybrid electrocatalysts highlights a unique avenue to tune the material composition structure, which would hold the promise for real-world water splitting electrolyzers.

28 mV dec

0.0

E (V vs. RHE)

E

2.0

20 wt% Pt/C Ni NWs NiSe2 NWs

0.3

Fe-NiSe2 NSs

Voltage (V)

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


NiFe LDH NSs||Fe-NiSe2 NSs

0

2

4

6 t (h)

8

10

12

Figure 6. (A, B) LSV curves of different electrolyzers. (C) Performance comparison of EHER+EOER and cell voltages for different electrolyzers. (D) Controlled potential electrolysis for the labelled electrolyzers.

Conclusions In summary, Ni NWs are synthesized and used as catalyst skeleton and Ni source to fabricate NiFe-LDH NSs@Ni NWs for OER. Upon phosphorization or selenization treatment, this material can be readily converted to Fe-Ni2P NSs@Ni NWs or FeNiSe2 NSs@Ni NWs for HER. The unique hybrid core-shell structure can provide large surface areas and contribute to rapid gas release and intimate electrolyte permeation. Meanwhile, it

(1) Xu, S.; Chen, Y.; Li, Y.; Lu, A.; Yao, Y.; Dai, J.; Wang, Y.; Liu, B.; Lacey, S. D.; Pastel, G. R.; Kuang, Y.; Danner, V. A.; Jiang, F.; Fu, K. K.; Hu, L. Universal, In Situ Transformation of Bulky Compounds into Nanoscale Catalysts by HighTemperature Pulse. Nano Lett. 2017, 17, 5817-5822. (2) Ji, L.; Lv, C.; Chen, Z.; Huang, Z.; Zhang, C. Nickel-Based (Photo)Electrocatalysts for Hydrogen Production. Adv. Mater. 2018, 30, 1705653. (3) Huynh, M.; Bediako, D. K.; Nocera, D. G. A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid. J. Am. Chem. Soc. 2014, 136, 6002-6010. (4) Peng, X.; Wang, L.; Hu, L.; Li, Y.; Gao, B.; Song, H.; Huang, C.; Zhang, X.; Fu, J.; Huo, K.; Chu, P. K. In Situ Segregation of Cobalt Nanoparticles on VN Nanosheets via Nitriding of Co2V2O7 Nanosheets as Efficient Oxygen Evolution Reaction Electrocatalysts. Nano Energy 2017, 34, 1-7. (5) Liang, Y.; Li, Y.; Wang, H.; Dai, H. Strongly Coupled Inorganic/Nanocarbon Hybrid Materials for Advanced Electrocatalysis. J. Am. Chem. Soc. 2013, 135, 2013-2036. (6) 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, 12561260.


5


(7) Reier, T.; Oezaslan, M.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal. 2012, 2, 1765-1772. (8) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383-1385. (9) Vojvodic, A.; Norskov, J. K. Chemistry. Optimizing Perovskites for the Water-Splitting Reaction. Science 2011, 334, 1355-1356. (10) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M (Ni,Co,Fe,Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550-557. (11) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt-Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: the Role of Structure And Composition on Activity, Stability, And Mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648. (12) Wang, L.; Lin, C.; Huang, D.; Zhang, F.; Wang, M.; Jin, J. A Comparative Study of Composition and Morphology Effect of Ni(x)Co(1-x)(OH)2 on Oxygen Evolution/Reduction Reaction. ACS Appl. Mater. Interfaces 2014, 6, 10172-10180. (13) Qi, H.; Wolfe, J.; Fichou, D.; Chen, Z. Cu2O Photocathode for Low Bias Photoelectrochemical Water Splitting Enabled by NiFe-Layered Double Hydroxide Co-Catalyst. Scientific Reports 2016, 6, 30882. (14) Wang, J.; Ji, L.; Chen, Z. In Situ Rapid Formation of a Nickel–Iron-Based Electrocatalyst for Water Oxidation. ACS Catal. 2016, 6, 6987-6992. (15) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100-102. (16) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis. Nat. Mater. 2012, 11, 963-969. (17) Teng, X.; Wang, J.; Ji, L.; Lv, Y.; Chen, Z. Ni Nanotube Array-Based Electrodes by Electrochemical Alloying and DeAlloying for Efficient Water Splitting. Nanoscale 2018, 10, 92769285. (18) Tang, S.; Wang, X.; Zhang, Y.; Courte, M.; Fan, H. J.; Fichou, D. Combining Co3S4 and Ni:Co3S4 nanowires as efficient catalysts for overall water splitting: an experimental and theoretical study. Nanoscale 2019, 11, 2202-2210. (19) Cao, B.; Veith, G. M.; Neuefeind, J. C.; Adzic, R. R.; Khalifah, P. G. Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-Noble Metal Electrocatalysts for the hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 19186-19192. (20) Xie, J.; Li, S.; Zhang, X.; Zhang, J.; Wang, R.; Zhang, H.; Pan, B.; Xie, Y. Atomically-Thin Molybdenum Nitride Nanosheets with Exposed Active Surface Sites for Efficient Hydrogen Evolution. Chem. Sci. 2014, 5, 4615-4620. (21) Shi, J.; Pu, Z.; Liu, Q.; Asiri, A. M.; Hu, J.; Sun, X. Tungsten Nitride Nanorods Array Grown on Carbon Cloth as an Efficient Hydrogen Evolution Cathode at all pH Values. Electrochim. Acta 2015, 154, 345-351. (22) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0-14. J. Am. Chem. Soc. 2014, 136, 7587-7590. (23) Teng, X.; Wang, J. Y.; Ji, L. L.; Tang, W. Q.; Chen, Z. F. Hierarchically Structured Ni Nanotube Array-Based Integrated Electrodes for Water Splitting. ACS Sustainable. Chem. Eng. 2018, 6, 2069-2077. (24) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Carbon Nanotubes Decorated with Cop Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution. Angew. Chem., Int. Ed. 2014, 53, 6710-6714.

Page 6 of 8

(25) Masa, J.; Weide, P.; Peeters, D.; Sinev, I.; Xia, W.; Sun, Z.; Somsen, C.; Muhler, M.; Schuhmann, W. Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution. Adv. Energy Mater. 2016, 6, 1502313. (26) Xu, N.; Cao, G. X.; Chen, Z. J.; Kang, Q.; Dai, H. B.; Wang, P. Cobalt Nickel Boride as an Active Electrocatalyst for Water Splitting. J. Mater. Chem. A 2017, 5, 12379-12384. (27) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmeková, E.; Asefa, T. Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at all pH Values. Angew. Chem. 2014, 126, 4461-4465. (28) Qu, K.; Zheng, Y.; Jiao, Y.; Zhang, X.; Dai, S.; Qiao, S.-Z. Polydopamine-Inspired, Dual Heteroatom-Doped Carbon Nanotubes for Highly Efficient Overall Water Splitting. Adv. Energy Mater. 2017, 7, 1602068. (29) Song, F.; Hu, X. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, 4477. (30) 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. J. Am. Chem. Soc. 2014, 136, 6744-6753. (31) Zhou, Y.; Wang, Z.; Pan, Z.; Liu, L.; Xi, J.; Luo, X.; Shen, Y. Exceptional Performance of Hierarchical Ni-Fe (hydr)oxide@NiCu Electrocatalysts for Water Splitting. Adv. Mater. 2019, 31, 1806769. (32) 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. (33) Yu, F.; Zhou, H.; Huang, Y.; Sun, J.; Qin, F.; Bao, J.; Goddard, W. A., 3rd; Chen, S.; Ren, Z. High-Performance Bifunctional Porous Non-Noble Metal Phosphide Catalyst for Overall Water Splitting. Nat. Commun. 2018, 9, 2551. (34) Liu, T.; Asiri, A. M.; Sun, X. Electrodeposited Co-Doped NiSe2 Nanoparticles Film: a Good Electrocatalyst for Efficient Water Splitting. Nanoscale 2016, 8, 3911-3915. (35) Zhao, Z.; Wu, H.; He, H.; Xu, X.; Jin, Y. Self-Standing NonNoble Metal (Ni–Fe) Oxide Nanotube Array Anode Catalysts with Synergistic Reactivity for High-Performance Water Oxidation. J. Mater. Chem. A 2015, 3, 7179-7186. (36) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452-8455. (37) Tang, C.; Wang, H. S.; Wang, H. F.; Zhang, Q.; Tian, G. L.; Nie, J. Q.; Wei, F. Spatially Confined Hybridization Of Nanometer-Sized Nife Hydroxides Into Nitrogen-Doped Graphene Frameworks Leading to Superior Oxygen Evolution Reactivity. Adv. Mater. 2015, 27, 4516-4522. (38) Wang, Y.; Qiao, M.; Li, Y.; Wang, S. Tuning Surface Electronic Configuration of NiFe LDHs Nanosheets by Introducing Cation Vacancies (Fe or Ni) as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction. Small 2018, 14, 1800136. (39) 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, 18201827. (40) Zhang, C.; Shao, M.; Zhou, L.; Li, Z.; Xiao, K.; Wei, M. Hierarchical NiFe Layered Double Hydroxide Hollow Microspheres with Highly-Efficient Behavior toward Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 3369733703. (41) Ni, Y.; Yao, L.; Wang, Y.; Liu, B.; Cao, M.; Hu, C. Construction of Hierarchically Porous Graphitized CarbonSupported NiFe Layered Double Hydroxides with a Core-Shell Structure as an Enhanced Electrocatalyst for the Oxygen Evolution Reaction. Nanoscale 2017, 9, 11596-11604. (42) Zhang, H.; Li, X.; Hähnel, A.; Naumann, V.; Lin, C.; Azimi, S.; Schweizer, S. L.; Maijenburg, A. W.; Wehrspohn, R. B.


6

Page 7 of 8 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 Bifunctional Heterostructure Assembly of NiFe LDH Nanosheets on NiCoP Nanowires for Highly Efficient and Stable Overall Water Splitting. Adv. Funct. Mater. 2018, 28, 1706847. (43) Hunter, B. M.; Thompson, N. B.; Müller, A. M.; Rossman, G. R.; Hill, M. G.; Winkler, J. R.; Gray, H. B. Trapping an Iron(VI) Water-Splitting Intermediate in Nonaqueous Media. Joule 2018, 2, 747-763. (44) Dau, H. Water Splitting: Unexpected Fe(VI) Trapped by Manipulation of Reaction Kinetics. Chem 2018, 4, 668-670. (45) Tang, C.; Gan, L.; Zhang, R.; Lu, W.; Jiang, X.; Asiri, A. M.; Sun, X.; Wang, J.; Chen, L. Ternary FexCo1-xP Nanowire Array as a Robust Hydrogen Evolution Reaction Electrocatalyst with Ptlike Activity: Experimental and Theoretical Insight. Nano Lett. 2016, 16, 6617-6621. (46) Yu, J.; Cheng, G.; Luo, W. Hierarchical NiFeP Microflowers Directly Grown on Ni Foam for Efficient Electrocatalytic Oxygen Evolution. J. Mater. Chem. A 2017, 5, 11229-11235. (47) Zhang, B.; Lui, Y. H.; Gaur, A. P. S.; Chen, B.; Tang, X.; Qi, Z.; Hu, S. Hierarchical FeNiP@Ultrathin Carbon Nanoflakes as

Alkaline Oxygen Evolution and Acidic Hydrogen Evolution Catalyst for Efficient Water Electrolysis and Organic Decomposition. ACS Appl. Mater. Interfaces 2018, 10, 8739-8748. (48) Tang, W.; Wang, J.; Guo, L.; Teng, X.; Meyer, T. J.; Chen, Z. Heterostructured Arrays of NixP/S/Se Nanosheets on CoxP/S/Se Nanowires for Efficient Hydrogen Evolution. ACS Appl. Mater. Interfaces 2017, 9, 41347-41353. (49) Zhou, H.; Wang, Y.; He, R.; Yu, F.; Sun, J.; Wang, F.; Lan, Y.; Ren, Z.; Chen, S. One-Step Synthesis of Self-Supported Porous NiSe2/Ni Hybrid Foam: An Efficient 3D Electrode for Hydrogen Evolution Reaction. Nano Energy 2016, 20, 29-36. (50) Sun, Y.; Xu, K.; Wei, Z.; Li, H.; Zhang, T.; Li, X.; Cai, W.; Ma, J.; Fan, H. J.; Li, Y. Strong Electronic Interaction in DualCation-Incorporated NiSe2 Nanosheets with Lattice Distortion for Highly Efficient Overall Water Splitting. Adv. Mater. 2018, 30, 1802121.


7


Page 8 of 8

Table of Contents Entry


8

Self-Growing NiFe-Based Hybrid Nanosheet Arrays on Ni Nanowires

Recommend Documents