Ultrathin CoNiP@Layered Double Hydroxides CoreâShell

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Ultrathin CoNiP@Layered Double Hydroxides Core-Shell Nanosheets Arrays for Largely Enhanced Overall Water Splitting Lei Zhou, Shan Jiang, Yunke Liu, Mingfei Shao, Min Wei, and Xue Duan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00151 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Energy Materials

Ultrathin

CoNiP@Layered

Double

Hydroxides

Core-Shell Nanosheets Arrays for Largely Enhanced Overall Water Splitting Lei Zhou, Shan Jiang, Yunke Liu, Mingfei Shao*, Min Wei and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China E-mail: [email protected] KEYWORDS: Transition metal phosphides; Layered double hydroxides; Water splitting; Bifunctional electrocatalyst; Synergistic reaction.

ABSTRACT Electrocatalytic water splitting to give a continuous production of oxygen and hydrogen is one of goals of energy research aiming at sustainable energy demands, in which the development of bifunctional electrocatalysts towards both oxygen evo-lution reaction (OER) and hydrogen evolution reaction (HER) with high activity, good durability and low cost remains a challenge. In this work, a bifunctional electrocatalyst based on transition metal phosphides CoNiP@layered double hydrox-ides (LDHs) core-shell nanosheets arrays for both OER and HER has been design and synthesized. The resulting CoNiP@LDHs nanoarrays exhibit excellent

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electrocatalytic activity and a remarkable long-term durability (~6 mV degradation after 500 h OER stability test). Especially, this hierarchical nanostructure needs a cell potential as low as 1.44 V at a current density of 10 mA cm−2, which is the best performance of bifunctional electrocatalysts among transition metal phos-phides to our knowledge.

1. Introduction Electrochemical water splitting is an appealing route to generate hydrogen fuels as renewable and clean energy source to conquer the present energy and environmental problems caused by fossil fuels.

[1-4]

In this area, highly efficient electrocatalysts for both hydrogen and

oxygen evolution reactions (HER and OER) are extremely necessary to lower the energy barrier of overall water splitting. Although noble metal based catalysts have shown excellent electrocatalytic performance in both HER and OER, [5-8] the limited reserves and exorbitant price hinder their commercial applications. Thus, much recent efforts have been devoted to the exploration of electrocatalysts based on earth-abundant element with satisfactory catalytic activity and stability.

[9-11]

However, most HER catalysts exhibit high activities in acidic

environment; while OER catalysts are more active in alkaline electrolyte. This results in a great conundrum when applying these two counterparts in a single electrolyser with operability and cost effectiveness. Therefore, an efficient bifunctional electrocatalyst toward both HER and OER simultaneously are highly required and remains a challenge. To achieve highly-performed overall water-splitting, a coupling of HER and OER active materials in one electrocatalyst with a synergistic performance is important and promising. [12-14] Transition metal phosphides (TMPs) have shown excellent HER catalytic behavior in universal pH media;

[15-17]

unfortunately, their OER performance suffers from a large overpotential and


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sluggish kinetic, which are far from satisfaction in overall water splitting. Moreover, the leaching of P makes TMPs unstable during a long-term employment, especially under alkaline conditions. [18-21]

On the contrary, layered double hydroxides (LDHs) display surprising OER performance in

basic media but offer rather poor HER activity.

[22-24]

This inspires us to explore the idea of

assembling LDHs nanoplatelets onto the surface of TMPs to obtain a new electrocatalyst with a core−shell structure, by which a highly desirable overall water splitting can be achieved based on the synergistic effect of these two components. The resulting hybrid electrocatalyst would possess the following advantages: (i) the superior HER and OER performance of CoNiP and NiFe-LDH would guarantee an overall water splitting reaction; (ii) the core–shell structure enables a strong electronic coupling and enhanced electron transportation at the interface, which would promote the water splitting process. In addition, the anchoring of LDH onto TMPs can prevent electrolyte erosion and reduce the dissolution of phosphor, giving rise to an improved catalytic stability and durability. Herein, we demonstrate the design and fabrication of ultrathin nanoarrays electrode consisting of TMP (CoNiP) core and LDHs shell by direct growth of NiFe-LDH on CoNiP nanosheets arrays (NSAs) via a facile electrosynthesis method, which exhibits largely enhanced performance toward overall water splitting. The as-obtained electrode displays well-defined hierarchical core-shell structure, in which NiFe-LDH nanosheets shell (with thickness of ∼10 nm) are uniformly anchored onto the surface of conductive CoNiP NSAs (with a lateral size of ∼1 µm and thickness of ∼5.5 nm). The resulting CoNiP@NiFe-LDH core−shell nanoarray electrode shows excellent electrocatalytic activity towards both OER and HER under alkaline conditions. Furthermore, a remarkable long-term durability (~6 mV degradation after 500 h stability test) for the relatively sluggish OER process has been demonstrated, in comparison with


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other electrocatalytic materials reported to date. A symmetric two-electrode cell assembled by CoNiP@NiFe-LDH exhibits excellent performance that only requires a cell potential of 1.44 V to drive a current density of 10 mA cm−2; this is the lowest potential of TMPs-based bifunctional catalysts for water splitting to our knowledge. This work provides a facile approach for the design and preparation of highly-efficient bifunctional electrocatalysts, which can serve as a promising candidate in electrochemical energy storage and conversion. 2. Results and Discussion 2.1. Structural and Morphological Characterization of CoNiP@LDH array The synthesis process for the CoNiP@NiFe-LDH core–shell nanoarrays involves a two-step deposition procedure, as shown in Figure 1. Firstly, well-defined CoNi-LDH NSAs on a Ni foam substrate were prepared via a facile hydrothermal method, followed by a phosphidation process to produce CoNiP NSAs. Subsequently, a uniform layer of NiFe-LDH nanosheets was directly grown onto the surface of as-prepared CoNiP NSAs by a fast eletrosynthesis process. Four samples of CoNiP@NiFe-LDH NSAs with various shell thickness and morphology were prepared with a fine control over the eletrosynthesis time (t = 20, 50, 100, and 200 s, denoted as CoNiP@LDH-20, CoNiP@LDH-50, CoNiP@LDH-100 and CoNiP@LDH-200, respectively).

Figure 1. Schematic illustration for the synthesis of CoNiP@NiFe-LDH hierarchical arrays.


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The X-ray diffraction (XRD) patterns of CoNi-LDH, CoNiP and CoNiP@NiFe-LDH samples are illustrated in Figure S1. The XRD pattern of CoNi-LDH can be indexed to a rhombohedral LDH phase with an interlayer distance (d003) of 0.78 nm, in accordance with the CO32−-LDH phase

[25]

. After phosphidation, the (111), (210), and (300) reflection of a typical

CoNiP phase are clearly observed (JCPDS No. 71-2336), with the absence of original CoNiLDH reflections, indicating a successful structural transformation to well-crystallized CoNiP material.

[26]

After the following electrosynthesis of NiFe-LDH onto the surface of CoNiP

nanosheets, an LDHs phase is observed with increasingly enhanced intensity along with the synthesis time. The superimposition of XRD reflections for both LDH and CoNiP confirms the co-existence of these two components. The scanning electron microscopy (SEM) image of CoNiLDH arrays precursor displays an interconnected porous structure in which the LDH nanosheets (with a lateral size of ~1 µm) stand vertically on the substrate (Figure 2a). A further phosphidation treatment results in the well-organized CoNiP NSAs with a similar morphology to CoNi-LDH NSAs precursor (Figure 2b and Figure S2). The final core−shell CoNiP@LDH NSAs display intercrossing NiFe-LDH nanoflakes shell perpendicularly grafting onto the CoNiP core (Figure 2c−f). It is found that the electrosynthesis duration show a significant influence on the LDHs shell thickness and morphology: a prolonged time leads to a thicker and denser LDH shell (Figure 2c−f). Transmission electron microscopic (TEM) image of CoNiP@LDH reveals that the electrodeposited NiFe-LDH nanosheets are vertically aligned on the plane of CoNiP nanosheets; the core–shell nanostructure is further confirmed by the side-view of TEM image (Figure 2g), from which the thickness of CoNiP core (~5.5 nm) and LDH shell (~10 nm) are identified obviously. In addition, the d-spacing from high resolution TEM (HR-TEM) is 0.22 nm in core and 0.15 nm in shell, corresponding to the CoNiP (111) and LDH (110) plane,


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respectively. Energy dispersive X-ray spectroscopy (EDS) and TEM mapping of CoNiP@LDH NSAs reveals a uniform dispersion of O, P, Fe, Co and Ni (Figure 2h and Figure S3), indicating a homogeneous distribution of NiFe-LDH shell on the CoNiP NSAs.

Figure 2. SEM images of (a) CoNi-LDH, (b) CoNiP, (c) CoNiP@LDH-20, (d) CoNiP@LDH50, (e) CoNiP@LDH-100 and (f) CoNiP@LDH-200 NSAs; (g) TEM image and (h) EDS mapping of CoNiP@LDH-100 NSAs. To better understand the interaction between NiFe-LDH shell and CoNiP core, a fine-scan X-ray photoelectron spectroscopy (XPS) was conducted for the NiFe-LDH, CoNiP and CoNiP@LDH NSAs. The high-resolution XPS spectra of Ni 2p3/2 for CoNiP NSA can be deconvolved into three peaks (Figure S4a). The peaks at 853.1 and 855.9 eV can be assigned to


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the metallic Ni and Ni–P, respectively; while the peak at 862.3 eV is related to the satellite peak.[35] As for the XPS spectra of P 2p (Figure S4b), the two peaks located at 129.0 and 130.0 eV are well matched to the P3−, which is a typical situation for transition metal phosphides.[17] The binding energies of CoNiP at 781.9 and 797.9 eV correspond to Co 2p3/2 and Co 2p1/2, respectively (Figure 3a).

[27]

After coating with LDH shell, these values shift negatively by 0.36

and 0.35 eV for Co 2p3/2 and Co 2p1/2, respectively (Figure 3a). Some reports confirmed that TMPs share a similar catalytic mechanism to the metal complex catalysts and hydrogenases, in which metal centers usually serve as active sites for HER.[28] In this work, the Co sites with increased electron density are beneficial to the adsorption of H atoms, thus facilitating the first step of HER process. The binding energies of Fe 2p3/2 and Fe 2p1/2 in CoNiP@LDH NSAs are located at 712.2 and 725.8 eV, with a positive shift by 0.46 eV compared with those of pristine NiFe-LDH NSAs (Figure 3b). However, after the electrosynthesis of LDH shell, both the Ni 2p1/2 and Ni 2p3/2 peaks have no obvious change (873.6 and 855.8 eV, respectively) compared with those of pristine NiFe-LDH NSAs (Figure S5).[29] It has been reported that the high valence state of Fe in NiFe-LDH leads to enhanced OER performance.[30] In this case, the positively shifted binding energies of Fe 2p suggest an increased Fe valence state in CoNiP@NiFe-LDH NSAs, which would be beneficial to OER performance.


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Figure 3. The XPS spectra of (a) Co 2p orbital for CoNiP NSAs and CoNiP@LDH-100 NSAs, and (b) Fe 2p orbital for NiFe-LDH nanoflakes and CoNiP@LDH-100 NSAs. 2.2. Oxygen Evolution Reaction (OER) The OER activities of the CoNiP@LDH core–shell NSAs were first evaluated in 1 M KOH using a standard three-electrode system. As demonstrated in Figure 4a, the hierarchical CoNiP@LDH NSAs electrode shows largely enhanced current density and significantly reduced overpotential (η) compared with that of pristine CoNiP NSAs. Typically, with the increase of electrosynthesis time of LDH from 20 s to 200 s, the onset potential of CoNiP@LDH firstly decreases from 1.51 to 1.41 V and then slightly enhances to 1.43 V vs. RHE. Remarkably, CoNiP@LDH-100 NSAs presents the smallest overpotential (216 mV) to reach a current density of 10 mA cm−2 among these core–shell electrocatalysts (Figure 4b). Moreover, CoNiP@LDH100 NSAs afford a high current density of 209 mA cm−2 at η = 300 mV, much superior to that of other samples (Figure 4c). The slight decrease in activity of CoNiP@NiFe-200 relative to CoNiP@NiFe-100 NSAs can be ascribed to the blocked mass transfer path caused by the overloaded LDH nanoflakes in CoNiP@NiFe-200. In addition, the cyclic voltammogram (CV)


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results exhibit similar conclusion according to Figure S6. The Tafel slope of the CoNiP@LDH100 NSAs is 45 mV dec−1 (Figure 4d), much smaller than that of other CoNiP@LDH NSAs, further indicating its enhanced OER process. The stability test of the CoNiP@LDH-100 NSAs was carried out by means of a chronopotentiometry measurement at 50 mA cm−2 (Figure 4e), which showed a surprisingly stable OER behavior with a nearly unchanged overpotential after a long time test of 500 h. In addition, the CoNiP@LDH-100 NSAs electrode after 500 h measurement maintains its initial hierarchical morphology without visible shedding and pulverization, demonstrating a remarkable structural robustness (Figure 4e, insert). It is reported that the TMPs materials usually suffer from easy dissolution of P in alkaline electrolyte,

[18-21]

which destroys its surface phosphor-based species (efficient to mediate water oxidation), leading to a decreased activity during long-term application. Fortunately, CoNiP@LDH NSAs display marginal leaching of P species due to the strong core−shell interaction by LDH coating, as proved by the EDS analysis (Figure S7 and Table S1). These results indicate that CoNiP@LDH NSAs possess an excellent durability in alkaline solution toward OER process, which accounts for the largely enhanced activity and durability.


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Figure 4. (a) OER polarization curves in 1.0 M KOH at a scan rate of 2 mV s−1 (b) overpotential at current density of 10 mA cm−2, (c) current densities at η = 300 mV, (d) Tafel plots of CoNiP NSAs, CoNiP@LDH-20, -50, -100 and -200 NSAs, (e) chronopotentiometric measurements at 50 mA cm−2 of CoNiP@LDH-100 NSAs for 500 h (inset: SEM images after durability test.). 2.3. Hydrogen Evolution Reaction (HER) Recently, TMPs have been intensively studied as efficient HER electrocatalysts due to their suitable hydrogen adsorption ability and good electrical conductivity. Some strategies have been adopted to further improve their HER behavior, such as doping secondary metal atoms or coupling with carbon materials.[31-33] In this work, a facile decoration by LDH coating onto the CoNiP nanosheets can efficiently enhance its HER performances. As shown in Figure 5a, CoNiP@LDH NSAs demonstrate a dramatically higher electrocatalytic activity toward HER compared with CoNiP NSAs. Typically, the CoNiP@LDH-100 NSAs only require 83 mV to


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produce a HER current density of 10 mA cm−2, significantly lower than that of CoNiP (111 mV), CoNiP@LDH-20 (98 mV), CoNiP@LDH-50 (89 mV), and CoNiP@LDH-200 NSAs (148 mV) (Figure 5b). The current density of CoNiP@LDH-100 NSAs at η= 300 mV is 230 mA cm−2, which is much larger than the other hierarchical electrodes (Figure 5c). Moreover, the CoNiP@LDH-100 NSAs also demonstrate the fastest HER kinetics with a Tafel plot value of 80 mV dec−1 (Figure 5d), much smaller than that of CoNiP NSAs (97 mV dec−1), CoNiP@LDH-20 (85 mV dec−1), CoNiP@LDH-50 (82 mV dec−1), and CoNiP@LDH-200 NSAs (84 mV dec−1). To further evaluate the electrocatalytic activity and durability of CoNiP@LDH-100 NSAs under continuous operating conditions, chronopotentiometric tests were carried out at current density of 10 mA cm−2. The overpotential for CoNiP@LDH-100 NSAs only deteriorate about 7 mV, much less than that for pristine CoNiP (~41 mV) (Figure 5e). The SEM image for the CoNiP@LDH after HER durability test (20 h) indicates a well retained core-shell architecture with LDH nanosheets perpendicularly grown on the CoNiP nanoarrays (Figure S8), which is consistent with the morphology of just prepared CoNiP@LDH NSAs. Moreover, the CoNiP@LDH-20, -50 and -200 NSAs deteriorate about 21, 10 and 5 mV after 20 h durability test (Figure S9), respectively. According to these results, the erosion of the CoNiP NSAs can be prevented along with the prolonged electro-synthesis time of LDHs, which demonstrates that the LDHs layer can markedly enhance HER durability of the electrodes. In addition, an obvious exfoliation of active materials on the surface of electrode is found in pristine CoNiP NSAs during the long term HER test, which is absent in CoNiP@LDH core−shell NSAs (Figure S10). This further demonstrates the excellent stability of the CoNiP@LDH core−shell NSAs during the HER process.


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Figure 5. (a) HER polarization curves, (b) overpotential at current density of 10 mA cm−2, (c) current densities at η = 300 mV, and (d) Tafel plots of CoNiP NSAs, CoNiP@LDH-20, -50, -100 and -200 NSAs, (e) chronopotentiometric measurements at 10 mA cm−2 of CoNiP and CoNiP@LDH-100 NSAs for 20 h. Electrochemical impedance spectroscopy (EIS) and electrochemical surface areas (ECSA) of these electrocatalysts were measured to gain insight into their electron transportation ability and effective reactive surfaces. The EIS measurements at 1.05 V vs. RHE are conducted and the corresponding Nyquist plots of the EIS spectra are shown in Figure 6a. All of the curves for CoNiP@LDH NSAs catalysts consist of a semicircle in high frequency region and a straight line in low frequency region. The diameter of semicircle and the slope of straight line in the Nyquist plot represent the charge–transfer resistance (Rct) and ion diffusion resistance of the electrode, respectively. The CoNiP@LDH-100 NSAs show a smaller Rct and a larger slope, indicative of its


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faster electron transport kinetics and ion diffusion rate (Figure 6a). The results are in high agreement with their HER and OER behavior (Figure 4a and 5a). Furthermore, the intrinsic active site exposure of the core-shell nanosheet arrays can be demonstrated by the electrochemical specific area (ECSA), according to their double-layer capacitance (Cdl, Figure S11 and S12). The CoNiP@LDH-100 NSAs possess a largest ECSA, which gives 3 times higher Cdl than that of the pristine CoNiP NSAs (Figure 6b). This result indicates that more active sites can be utilized for water splitting reactions due to the in-situ grown of NiFe-LDH nanosheets. The increase in ECSA from CoNiP@LDH-20 to CoNiP@LDH-100 can be attributed to the enhanced loading of NiFe-LDH on the surface of CoNiP NSAs. From CoNiP@LDH-100 to CoNiP@LDH-200, an increased resistance and a sharply decreased ECSA are observed in Figure 6a and 6b. Therefore, a suitable coating of LDH flakes onto CoNiP NSAs can facilitate a fast electron transfer and efficient reactant diffusion.

Figure 6. (a) EIS results (Insert: corresponding enlarged image of high frequency area and corresponding equivalent circuit) and (b) double-layer capacitance of CoNiP NSAs, CoNiP@LDH-20, -50, -100 and -200 NSAs.


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2.4. Overall water splitting As demonstrated above, the well-uniformed CoNiP@LDH core-shell NSAs material shows both significantly improved HER and OER activities. This can be ascribed to the synergistic effect between CoNiP core and NiFe-LDH shell: CoNiP serves as a highly-efficient HER catalyst; while NiFe-LDH provides abundant active sites for OER process. Moreover, the core– shell nanostructure produces an interface with strong electronic coupling (as demonstrated by XPS results), which reduces the energy barriers (with obviously decreased overpotential for both OER and HER) and promotes electron transfer during water splitting (Figure 6a). In addition, the uniform LDHs coating prevents phosphor dissolution of CoNiP by the strong core-shell interaction, giving rise to largely improved catalytic stability and durability (Figure 4e and 5e). By merits of the promising overall water splitting performances and in order to demonstrate their practical applications of CoNiP@LDH NSAs, a two-electrode water splitting cell was fabricated utilizing CoNiP@LDH-100 NSAs electrode as both anode and cathode. As displayed in Figure 7a, the electrolyzer requires only a potential of 1.44 V to achieve a current density of 10 mA cm−2, much smaller than that of the cells assembled by CoNiP||NiFe-LDH (1.55 V), CoNiP||CoNiP (1.61 V) and NiFe-LDH|| NiFe-LDH (1.72 V) (Figure 7b). This is, to the best of our knowledge, the lowest potential needed among TMPs-based bifunctional electrocatalysts for overall water splitting (Figure 7c and Table S2).[34-54] Furthermore, the electrolysis potential remains unchanged for an overall water splitting test as long as 20 h, demonstrating its remarkable long-term stability. As observed in Figure 7a, plenty of bubbles at the surface of both anode and cathode are released at a cell potential of 1.50 V. We collected the gases generated by the electrolysis cell and calculated the faradaic efficiency. It is found that the CoNiP@LDH gives an efficiency of 99 %, indicating a high level of electron utilization (Figure S13).


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Figure 7. (a) photographs of water-splitting system, (b) LSV results of two-electrode cell assembled by various materials, (c) potentials comparison at 10 mA cm−2 for this work and the reported TMPs-based bifunctional electrocatalysts in two-electrode water splitting systems, (d) the durability test of electrolyzer constructed by CoNiP@LDH-100 NSAs at 10 mA cm−2 for 20 h. 3. Conclusion In summary, a hierarchical, well-organized nanostructure with NiFe-LDH shell grown on the surface of the CoNiP NSAs was synthesized successfully. The resulting CoNiP@LDH coreshell NSAs electrode displays overwhelming electrocatalytic activity and robust durability toward overall water splitting. A potential of 1.44 V is needed to drive a current density of 10 mA cm−2 in a two-electrode cell, much superior to NiFe-LDH, CoNiP and the TMPs-based


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bifunctional electrocatalysts for overall water splitting reported to our knowledge. The remarkable activity of the CoNiP@NiFe-LDH can be attributed to the synergetic effects of the CoNiP core and the NiFe-LDH shell, which simultaneously promotes the HER and OER processes. It is expected that the approach can be extended to the fabrication of other ultrafine, highly-active bifunctional catalysts with remarkable electrochemical performances. 4. Experimental Section 4.1. Synthesis of CoNiP@LDH array Preparation of CoNi-LDH arrays: CoNi-LDH arrays on Ni foam were synthesized via a hydrothermal process. In a typical procedure, Co(NO3)2·6H2O (2 mmol), Ni(NO3)3·6H2O (2 mmol) and hexamethylenetetramine (8 mmol) were dissolved and stirred thoroughly to form a clear solution (80 ml). Typically, a piece of Ni foam (3 cm × 5 cm × 1 mm) was pretreated with absolute ethanol, 2.0 M HCl solution and deionized water, respectively (each for 15 min), to ensure a clean surface. The Ni foam was then immersed in the aqueous solution in a 100 mL Teflon-lined stainless-steel autoclave, heating at 100 °C for 10 h, followed by cooling to room temperature naturally. A green thin film on the Ni foam was formed, which was rinsed with distilled water and ethanol with the assistance of ultrasonication, and dried at 60 °C for 6 h. Preparation of CoNiP nanosheet arrays (NSAs): The CoNiP NSAs were prepared by an in situ phosphidation process of the CoNi-LDH NSAs precursor. In a typical procedure, NaH2PO2 was adopt as phosphorus precursor, mixed with CoNi-LDH UNSAs, and followed by an annealing in a N2 stream at 300 °C for 1 h, with an initial heating rate of 5 °C min−1. The phosphidation process results in the chemical transformation from LDHs to transition metal phosphides. The resulting product was slowly cooled to room temperature in a N2 stream.


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Preparation of CoNiP@NiFe-LDH hierarchical NSAs: The CoNiP@NiFe-LDH NSAs were synthesized via an electrosynthesis method. In a typical procedure, the as-prepared CoNiP NSAs were used as the working electrode in a three-electrode electrochemical cell, by using platinum wire as the counter electrode and Ag/AgCl as the reference electrode. The electrolyte for the electrosynthesis of NiFe-LDH UNSAs was obtained by dissolving Ni(NO3)2·6H2O (0.15 M) and FeSO4·7H2O (0.15 M) in 50 mL of distilled water. The potentiostatic deposition was carried out at a potential of −1.0 V vs. Ag/AgCl for 20, 50, 100, 200 s, respectively. The resulting CoNiP@NiFe-LDH NSAs on Ni foam were withdrawn and rinsed with distilled water. 4.2. Characterization X-ray diffraction patterns of samples were collected on a Shimadzu XRD-6000 diffractometer using a Cu Kα source, with a scan step of 10 ° min−1 and a scan range between 3° and 70°. The morphology of the nanosheet was investigated using a scanning electron microscope (SEM; Zeiss SUPRA 55) with an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) images were recorded with Philips Tecnai 20 and JEOL JEM-2010 high-resolution transmission electron microscopes. The accelerating voltage was 200 kV in each case. Highresolution TEM (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images were collected on an FEI Tecnai G2 F20 S-Twin working at 200 kV. X-ray photoelectron spectra (XPS) were performed on a Thermo VG ESCALAB 250 X-ray photoelectron spectrometer at a pressure of ∼2×10–9 Pa using Al Kα X-rays as the excitation source. 4.3. Electrochemical measurements Electrodes were tested on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., China) in a three-electrode electrochemical cell using a 1 M KOH aqueous solution as electrolyte at room temperature. A 1 cm × 1 cm area of CoNiP or CoNiP@NiFe-LDH


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NSAs prepared with various time on Ni foam was used directly as working electrode. A Pt wire and Ag/AgCl electrode were used as the counter and reference electrode, respectively. The distance between the working electrode and the counter electrode was 2 cm. For the OER and HER, cyclic voltammetry and linear sweep voltammetry at a scan rate of 10 mV s−1 were conducted in 1 M KOH solution. AC impedance measurements were carried out in the same configuration at −1.0 V vs. RHE from 10−1 to 106 Hz with an AC voltage of 5 mV. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Natural Science Foundation of China (NSFC), the 973 Program (Grant No. 2014CB932102), the Fundamental Research Funds for the Central Universities (buctrc201506; PYCC1704) and the Higher Education and High-quality and Worldclass Universities (PY201610).


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Founda-tion of China (NSFC), the 973 Program (Grant No. 2014CB932102), the Fundamental Research Funds for the Central Universities (buctrc201506; PYCC1704) and the Higher Education and High-quality and Worldclass Universities (PY201610).

REFERENCES 1. 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, 6430–6435. 2. McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347–4357. 3. Gao, X. H.; Zhang, H. X.; Li, Q. G.; Yu, X. G.; Hong, Z. L.; Zhang, X. W.; Liang, C. D.; Lin, Z. Hierarchical NiCo2O4 Hollow Microcuboids as Bifunctional Electrocatalysts for Overall Water-Splitting. Angew. Chem. Int. Ed. 2016, 55, 6290–6294. 4. Yang, Y.; Fei, H. L.; Ruan, G. D.; Tour, J. M. Porous Cobalt-Based Thin Film as A Bifunctional Catalyst for Hydrogen Generation and Oxygen Generation. Adv. Mater. 2015, 27, 3175–3180.


19


Page 20 of 35

5. Binninger, T.; Mohamed, R.; Patru, A.; Waltar, K.; Gericke, E.; Tuaev, X.; Fabbri, E.; Levecque, P.; Hoell, A.; Schmidt, T. J. Stabilization of Pt Nanoparticles Due to Electrochemical Transistor Switching of Oxide Support Conductivity. Chem. Mater. 2017, 29, 2831–2843. 6. Morales, O. D.; Raaijman, S.; Kortlever, R.; Kooyman, P. J.; Wezendonk, T.; Gascon, J.; Fu, W. T.; Koper, M. T. M. Iridium-Based Double Perovskites for Efficient Water Oxidation in Acid Media. Nat. Comm. 2016, 7, 12363. 7. He, D. S.; He, D. P.; Wang, J.; Lin, Y.; Yin, P. Q.; Hong, X.; Wu, Y.; Li, Y. D. Ultrathin Icosahedral Pt-Enriched Nanocage with Excellent Oxygen Reduction Reaction Activity. J. Am. Chem. Soc. 2016, 138, 1494–1497. 8. Chen, G. Y.; Kuttiyiel, K. A.; Su, D.; Li, M.; Wang, C. H.; Buceta, D.; Du, C. Y.; Gao, Y. Z.; Yin, G. P.; Sasaki, K.; Vukmirovic, M. B.; Adzic, R. R. Oxygen Reduction Kinetics on Pt Monolayer Shell Highly Affected by the Structure of Bimetallic AuNi Cores. Chem. Mater. 2016, 28, 5274–5281. 9. Liu, Y. P.; Li, Q. J.; Si, R.; Li, G. D.; Li, W.; Liu, D. P.; Wang, D. J.; Sun, L.; Zhang, Y.; Zou, X. X. Coupling Sub-Nanometric Copper Clusters with Quasi-Amorphous Cobalt Sulfide Yields Efficient and Robust Electrocatalysts for Water Splitting Reaction. Adv. Mater. 2017, 29, 1606200. 10. Zhou, W. J.; Xiong, T. L.; Shi, C. H.; Zhou, J.; Zhou, K.; Zhu, N. W.; Li, L. G.; Tang, Z. H.; Chen, S. W. Bioreduction of Precious Metals by Microorganism: Efficient Gold@ N-Doped Carbon Electrocatalysts for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2016, 55, 8416–8420.


20

Page 21 of 35 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


11. Zou, X.; Liu, Y.; Li, G. D.; Wu, Y. Y.; Liu, D. P.; Li, W.; Li, H. W.; Wang, D. J.; Zhang, Y.; Zou, X. 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. 12. Zhu, H.; Zhang, J. F.; Yanzhang, R. P.; Du, M. L.; Wang, Q. F.; Gao, G. H.; Wu, J. D.; Wu, G. M.; Zhang, M.; Liu, B. When Cubic Cobalt Sulfide Meets Layered Molybdenum Disulfide: A Core–Shell System toward Synergetic Electrocatalytic Water Splitting. Adv. Mater. 2015, 27, 4752–4759. 13. Li, Y. J.; Zhang, H. C.; Jiang, M.; Kuang, Y.; Sun, X. M.; Duan, X. Ternary NiCoP Nanosheet Arrays: An Excellent Bifunctional Catalyst for Alkaline Overall Water Splitting. Nano Res. 2016, 9, 2251–2259. 14. Huang, H. W.; Yu, C.; Zhao, C. T.; Han, X. T.; Yang, J.; Liu, Z. B.; Li, S. F.; Zhang, M. D.; Qiu, J. S. Iron-Tuned Super Nickel Phosphide Microstructures with High Activity for Electrochemical Overall Water Splitting. Nano Energy 2017, 34, 472–480. 15. Cai, P. W.; Huang, J. H.; Chen, J. X.; Wen, Z. H. Oxygen-containing Amorphous Cobalt Sulfide Porous Nanocubes as High-Activity Electrocatalysts for the Oxygen Evolution Reaction in an Alkaline/Neutral Medium. Angew. Chem. Int. Ed. 2017, 56, 4858–4861. 16. Callejas, J. F.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction. Chem. Mater. 2016, 28, 6017–6044.


21


Page 22 of 35

17. Zhang, R.; Wang, X. X.; Yu, S. J.; Wen, T.; Zhu, X. W.; Yang, F. X.; Sun, X. N.; Wang, X. K.; Hu, W. P. Ternary NiCo2Px Nanowires as pH-Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction. Adv. Mater. 2017, 29, 1605502. 18. Tian, J. Q.; Liu, Q.; Asiri, A. M.; Sun, X. P. 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. 19. Li, D.; Baydoun, H.; Kulikowski, B.; Brock, S. L. Boosting the Catalytic Performance of Iron Phosphide Nanorods for The Oxygen Evolution Reaction by Incorporation of Manganese. Chem. Mater. 2017, 29, 3048–3054. 20. Li, D.; Baydoun, H.; Verani, C. N.; Brock, S. L. Efficient Water Oxidation Using CoMnP Nanoparticles. J. Am. Chem. Soc. 2016, 138, 4006–4009. 21. Chang, J. F.; Liang, L.; Li, C. Y.; Wang, M. L.; Ge, J. J.; Liu, C. P.; Xing, W. Ultrathin Cobalt Phosphide Nanosheets as Efficient Bifunctional Catalysts for A Water Electrolysis Cell and The Origin for Cell Performance Degradation. Green Chem. 2016, 18, 2287–2295. 22. Zhang, C.; Shao, M. F.; Zhou, L.; Li, Z. H.; Xiao, K. M.; Wei, M. Hierarchical NiFe Layered Double Hydroxide Hollow Microspheres with Highly-Efficient Behavior toward Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 33697–33703. 23. Liang, H. F.; Li, L. S.; Meng, F.; Dang, L. N.; Zhuo, J. Q.; Forticaux, A.; Wang, Z. C.; Jin, S. Porous Two-Dimensional Nanosheets Converted from Layered Double Hydroxides and Their Applications in Electrocatalytic Water Splitting. Chem. Mater. 2015, 27, 5702–5711.


22

Page 23 of 35 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


24. Zhang, C.; Zhao, J. W.; Zhou, L.; Li, Z. H.; Shao, M. F.; Wei, M. Layer-by-Layer Assembly of Exfoliated Layered Double Hydroxide Nanosheets for Enhanced Electrochemical Oxidation of Water. J. Mater. Chem. A 2016, 4, 11516–11523. 25. Zhou, L.; Shao, M. F.; Zhang, C.; Zhao, J. W.; He, S.; Rao, D. M.; Wei, M.; Evans, D. G.; Duan, X. Hierarchical CoNi-Sulfide Nanosheet Arrays Derived from Layered Double Hydroxides toward Efficient Hydrazine Electrooxidation. Adv. Mater. 2017, 29, 1604080. 26. Wu, C.; Yang, Y. J.; Dong, D.; Zhang, Y. H.; Li, J. H. In Situ Coupling of CoP Polyhedrons and Carbon Nanotubes as Highly Efficient Hydrogen Evolution Reaction Electrocatalyst. Small 2017, 29, 1602873. 27. He, P. L.; Yu, X. Y.; Lou, X. W. Carbon-Incorporated Nickel-Cobalt Mixed Metal Phosphide Nanoboxes with Enhanced Electrocatalytic Activity for Oxygen Evolution. Angew. Chem. Int. Ed. 2017, 56, 3897–3900. 28. Xing, Z. C.; Liu, Q.; Asiri, A. M.; Sun, X. P. Closely Interconnected Network of Molybdenum Phosphide Nanoparticles: A Highly Efficient Electrocatalyst for Generating Hydrogen from Water. Adv. Mater. 2014, 26, 5702–5705. 29. Jia, Y.; Zhang, L. Z.; Gao, G. P.; Chen, H.; Wang, B.; Zhou, J. Z.; Soo, M. T.; Hong, M.; Yan, X. C.; Qian, G. R.; Zou, J.; Du, A. J.; Yao, X. D. A Heterostructure Coupling of Exfoliated Ni–Fe Hydroxide Nanosheet and Defective Graphene as A Bifunctional Electrocatalyst for Overall Water Splitting. Adv. Mater. 2017, 29, 1700017.


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30. Ahn, H. S.; Bard, A. J. Surface Interrogation Scanning Electrochemical Microscopy of Ni1– xFexOOH (0

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