Cu3P–Ni2P Hybrid Hexagonal Nanosheet Arrays for Efficient

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Cu3P−Ni2P Hybrid Hexagonal Nanosheet Arrays for Efficient Hydrogen Evolution Reaction in Alkaline Solution Xin Jin,†,‡,∥ Jing Li,‡,∥ Yuting Cui,*,† Xiaoyuan Liu,‡ Xinglai Zhang,‡ Jinlei Yao,§ and Baodan Liu*,‡ †

College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 400047, China Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR) Chinese Academy of Sciences (CAS), No. 72 Wenhua Road, Shenyang 110016, China § Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy Application, School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou 215009, China Downloaded via NOTTINGHAM TRENT UNIV on August 16, 2019 at 04:31:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: The development of efficient and low-cost hydrogen evolution reaction electrocatalysts has been regarded as a promising approach to produce sustainable and clean fuels to solve the energy crisis and environmental problems. Herein, 3D hybrid Cu3P−Ni2P hexagonal nanosheet arrays are successfully prepared on nickel foam (Cu3P−Ni2P/NF). Benefiting from synergistic effects and strong chemical coupling existing at the interface, the Cu3P−Ni2P/NF electrode exhibits a low overpotential of 103 mV at a current density of 10 mA cm−2, which is 47 and 100 mV less than that for Ni2P/NF and Cu3P/NF, respectively. It also shows excellent electrochemical durability for long-term reaction in alkaline medium. The excellent electrocatalytic activity makes the Cu3P−Ni2P/NF as a promising cathode toward efficient hydrogen evolution via electrochemical water splitting.

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INTRODUCTION As clean and sustainable energy, hydrogen has been regarded as a promising alternative to fossil fuels in the hydrogen-economy paradigm.1−4 Its high energy density and zero carbon emission can significantly decrease our dependence on fossil fuels and reduce the greenhouse effect.5 Alkaline electrolyzed water using suitable electrocatalysts provides an effective way for industrial hydrogen production, but the huge energy consumption caused by the large overpotential adds additional cost for practical application.6 Although Pt is the most efficient electrocatalyst for the hydrogen evolution reaction (HER), its high cost and limited storage in earth greatly limit its scalable application in industry.7,8 Therefore, it is highly desired and extremely important to develop efficient, economic, and durable HER catalysts in alkaline electrolytes. Up to now, a variety of non-noble HER catalysts, such as transition-metal nitrides,9,10 sulfides,11−14 carbides,15,16 and especially phosphides17−22 have been developed and widely used as alternatives to Pt-based catalysts. Among them, copper phosphide (Cu3P), as a typical transition-metal phosphide, has aroused ever-growing research interest due to its high conductivity, metalloid characteristic, excellent stability, and significant abundance.23 For example, Tian et al.24 reported the synthesis of self-supported Cu3P nanowire arrays on commercially available porous copper foam which exhibits an overpotential of 143 mV at the current density of 10 mA cm−2 toward HER. Even so, the catalytic performance of copper phosphide is relatively poor compared to those of novel metals toward HER, © XXXX American Chemical Society

especially in an alkaline environment. To overcome this challenge, bimetallic hybrid catalysts have been proposed and extensively studied due to their excellent activity on a range of reactions such as oxygen reduction reaction25 and hydrogen evolution.26 Previous works have confirmed that bimetallic hybrid catalysts, such as Ni−Co−P,27,28 W−Mo−P,29 and Ni− Mo−P,30 can effectively improve the HER performance compared with the single component. This is mainly due to the synergistic effect between different metal elements, which can either change the surface morphology of hybrids to expose more active sites or tune the intrinsic electrical properties of the hybrids. Following this path, we aim to enhance the HER performance of pure Cu3P by introducing proper metal into Cu3P to form a heterostructure, which can improve the intrinsic electrochemical properties of the hybrid catalyst and thus impressively contribute to the high HER activity and excellent stability. In this work, we report the design and integration of 3D Cu3P−Ni2P hexagonal nanosheet arrays on Ni foam (NF) for enhanced HER performance in alkaline medium. Benefiting from the strong chemical coupling and synergistic effects at the heterostructure interface, Cu3P−Ni2P/NF hybrid electrode exhibits superior catalytic HER performance with an overpotential of only 103 mV at a current density of 10 mA cm−2 in 1 M KOH, which is 47 and 100 mV less than that for single Ni2P/ Received: May 28, 2019

A

DOI: 10.1021/acs.inorgchem.9b01567 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. SEM images of (a) hydroxide precursor hydrothermally grown on NF and (b) Cu3P−Ni2P/NF; (c) XRD pattern and (d) EDS spectrum of Cu3P−Ni2P/NF.

NF and Cu3P/NF, respectively. Importantly, the hybrid catalyst shows excellent long-term electrochemical durability with negligible catalytic activity degradation for at least 48 h.

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RESULTS AND DISCUSSION To synthesize the Ni2P−Cu3P hybrid catalyst in situ grown on NF, the simple hydrothermal reaction and subsequent lowtemperature phosphidation method are proposed. Figure S1 describes each key step involved in the growth of the Cu3P− Ni2P/NF electrode. The scanning electron microscopy (SEM) image of the hydroxide precursor (Figure 1a) reveals that the bare NF is completely covered with the hexagonal nanosheet arrays after hydrothermal reaction, and the original sheet-like morphology can be well maintained after the phosphidation process (Figure 1b). The X-ray diffraction (XRD) analysis (Figure 1c) demonstrates that the as-synthesized phosphides are mixed phases of Cu3P and Ni2P. The diffraction peaks located at 28.31°, 35.92°, 38.82°, 41.37°, 44.94°, 46.03°, and 78.14° can be well indexed to the (111), (112), (202), (211), (300), (113), and (314) planes of the hexagonal Cu3P phase (JCPDS no. 712261), respectively, and these diffraction peaks centered at 40.57°, 47.21°, 54.13°, 54.82°, 66.34°, 72.57°, and 74.58° can be assigned to the (111), (210), (300), (211), (310), (311), and (400) planes of hexagonal Ni2P phase (JCPDS no. 03-0953), respectively. The other additional peaks correspond to metallic nickel substrate. The composition analysis of Cu3P−Ni2P/NF using energy dispersed X-ray spectroscopy (EDS) (Figure 1d) and elemental mapping images (Figure S2) confirm the existence of Cu, Ni, and P elements and their uniform distribution throughout the whole NF. Detailed transmission electron microscopy (TEM) analysis (Figure 2) further reveals that the single Cu3P−Ni2P hexagonal nanosheet is actually composed of numerous tiny nanosheets, as shown in Figure 2a and 2b. The high-resolution TEM (HRTEM) (Figure 2c and 2e) image indicates that the tiny nanosheet has two different

Figure 2. (a, b) Typical TEM images of the Cu3P−Ni2P nanosheet; HRTEM image and FFT of the (c, d) Ni2P catalyst and (e, f) Cu3P catalyst; (g) STEM image and (h−j) the corresponding elemental mappings of the Cu3P−Ni2P nanosheet.

lattice fringes with typical interplane spacing of 0.22 and 0.23 nm, corresponding to the (111) plane of Ni2P and (022) plane of Cu3P, respectively, in good agreement with the XRD result. Additionally, the corresponding fast Fourier transform (FFT) pattern (Figure 2d and 2f) of the HRTEM image also matches well with the electron diffraction patterns of a standard hexagonal Ni2P and Cu3P taken along the 132 and 133 zone axis, respectively. The scanning transmission electron microscopy (STEM) image (Figure 2g) and corresponding EDS B

DOI: 10.1021/acs.inorgchem.9b01567 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) XPS survey spectrum of Cu3P−Ni2P/NF and high-resolution XPS spectra of (b) Ni 2p, (c) Cu 2p, and (d) P 2p.

Figure 4. (a) IR-corrected LSV polarization curves for HER in 1.0 M KOH and (b) their corresponding Tafel plots. (c) Polarization curves of Cu3P− Ni2P/NF before and after 2000 CV cycles. (d) Time-dependent current density of Cu3P−Ni2P/NF for the HER with an overpotential of 153 mV.

ments were carried out to further analyze the surface elemental type and valence state. The survey spectrum shows that Ni, Cu, P, and O signals can be detected from Cu3P−Ni2P/NF samples (Figure 3a), in which the O element mainly comes from the surface oxidation of Cu3P−Ni2P in air and residual lattice oxygen.31 As for the high-resolution XPS spectrum of Ni 2p (Figure 3b), the peak centered at 853.2 eV with a peak satellite corresponds to Ni 2p3/2 of Ni2P, and the peak at 871.1 eV with a satellite belongs to Ni 2p1/2 of Ni2P.32 For the typical highresolution XPS spectrum of Cu 2p (Figure 3c), it is clearly

elemental mapping images (Figure 2h−j) also illustrate the homogeneous distribution of Ni, Cu, and P elements throughout the single nanosheet. Therefore, it can be concluded that our designed nanosheet arrays are Cu3P−Ni2P hybrid catalysts with uniform mixing at the nanoscale, which will facilitate the coupling and synergistic effects for superior HER activity. The surface composition of the Cu3P−Ni2P/NF electrocatalyst is extremely important to study the HER and related processes. X-ray photoelectron spectroscopy (XPS) measureC

DOI: 10.1021/acs.inorgchem.9b01567 Inorg. Chem. XXXX, XXX, XXX−XXX

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the initial data (Figure 4c). To further evaluate the stability of the catalyst, we then operated the HER by chronopotentiometric measurement at a constant current density of 50 mA cm−2 with an overpotential of 153 mV, and Cu3P−Ni2P/NF shows superior long-term electrochemical durability with its catalytic activity stably maintaining for at least 48 h (Figure 4d). To investigate the electrode kinetics at the electrode electrolyte interface during the HER process, electrochemical impedance spectroscopy (EIS) tests were conducted at a constant overpotential of 150 mV in 1.0 M KOH solution. The semicircle of the Nyquist plot is attributed to the charge transfer resistance Rct that is closely related to electrocatalytic kinetics, and we can use the radius of the semicircle to evaluate the reaction rates.35,36 As shown in Figure S4, the Cu3P−Ni2P/NF hybrid electrode has a much lower impedance, indicating the enhanced electron transfer and fast catalytic kinetics compared with Ni2P/NF and Cu3P/NF, which is in good agreement with its best intrinsic catalytic activity toward HER. To further explore the influence of the Ni2P−Cu3P heterostructure on catalytic performance, the electrochemical surface areas (ECSA) of the hybrid electrodes were measured on the basis of electrochemical double-layer capacitances (Cdl) values. Figure S5a display the cyclic voltammetry (CV) curves at the scan rates of 20, 40, 60, 80, 100, and 120 mV s−1 from 0.250 to 0.350 V (vs RHE). It can be seen (Figure S5b) that the capacitance (11 mF cm−2) of Cu3P−Ni2P/NF is much higher than those of Ni2P/ NF (5 mF cm−2) and Cu3P/NF (2.5 mF cm−2), implying more exposed active sites and a higher surface area for Cu3P−Ni2P/ NF, which promoted electrocatalytic performance toward HER. At last, the morphology of Cu3P−Ni2P/NF catalyst was examined after the electrochemical performance and stability test. It is found that the electrode still maintains hexagonal nanosheet arrays after long-term reaction (Figure S6). The EDS analysis (Figure S7) reveals the existence of Ni, Cu, P, O, Al, and K signals, where the K peak originates from residual KOH, the Al peak is from the sample stage, and the change in elemental content is not very large. The molar ratio of Ni, Cu, and P is 10:2:4, which is close to the value of 10:2:5 before the long-time cycling test. Therefore, the designed 3D Cu3P−Ni2P/NF hybrid catalysts are highly efficient and extremely stable toward enhanced HER due to the aforementioned excellent features.24,37 The synthetic strategy to construct hybrid heterostructures using proper metal phosphides also provides some guidance for the design and integration of a variety of metal oxides and sulfides for clean and sustainable hydrogen production on the basis of electrocatalytic water splitting.

shown that the strong characteristic peaks of Cu 2p appear at 932.4 eV (2p3/2) and 952.3 eV (2p1/2), which are close to the binding energy of Cu in Cu3P.33 The other peaks with binding energies at 934.7 and 954.9 eV can be attributed to the Cu2+ arising from the superficial oxidation of Cu3P−Ni2P as a result of direct exposure to air. The high-resolution P 2p spectrum (Figure 3d) shows two peaks at 128.8 and 133.4 eV. The binding energy of 128.8 eV can be assigned to phosphides, and the other peak at 133.4 eV can be attributed to PO43− or P2O5 formed on the surface of the Cu3P−Ni2P nanosheet.34 The higher intensity of the P−O peak may be due to the residual lattice oxygen and the formation of P2O5 on the surface. In fact, the EDS results in Figure 1d also confirm the existence of a strong oxygen signal, and the molar ratio of Ni, Cu, P, and O is close to 10:2:5:5. Moreover, Ni 2p peaks at 870.9 and 853.2 eV of Cu3P−Ni2P show a positive shift of 0.8 and 0.5 eV compared with pure Ni2P; Cu 2p3/2 at a binding energy of 932.4 eV is negatively shifted from that of metallic Cu (932.6 eV), while P 2p at a binding energy of 128.8 eV has a lower binding energy than elemental P (130.0 eV). It can be deduced that there exists strong electron interactions and synergistic effects between Cu3P and Ni2P, which is beneficial for the improvement of the HER catalytic performance of hybrid Cu3P−Ni2P/NF. In order to investigate the HER performance of the Cu3P− Ni2P/NF (loading: ∼2.5 mg cm−2) electrode, an electrochemical test was performed using a typical three-electrode system in an Ar-saturated 1.0 M KOH aqueous solution. Bare NF, Ni2P/NF, Cu3P/NF, and Pt/C@NF were also tested under the same conditions for comparison, and iR-correction from the solution resistance was applied to all initial data. As shown in Figure 4a, the linear sweep voltammetry (LSV) curves indicate that the commercial Pt/C catalyst shows a much superior HER activity and demands only an overpotential of 40 mV to achieve a current density of 10 mA cm−2, while the bare NF electrode shows very poor catalytic activity for HER. Ni2P/NF and Cu3P/ NF are efficient for the HER with overpotential of 150 and 203 mV for 10 mA cm−2, respectively. On the contrary, the Cu3P− Ni2P/NF electrode displays an obviously enhanced HER catalytic activity and demands only an overpotential as low as 103 mV to drive the same current density of 10 mA cm−2. This overpotential is only 63 mV higher than that of the commercial Pt/C catalyst but is 47 and 100 mV lower than those of Ni2P/NF and Cu3P/NF, respectively. It is also lower than those of most reported Ni or Cu phosphide catalysts in alkaline solution (Table S1). The alkaline HER process of hybrid Cu3P−Ni2P nanosheet is shown in Figure S3. Such excellent performance of Cu3P−Ni2P/NF hybrid electrode is mainly due to sufficient active sites provided by the sheet-like morphology with huge surface areas and strong chemical coupling of the two metal phosphides. To accurately assess the HER kinetics, Tafel plots are plotted based on the LSV curves (Figure 4b). In this study, Cu3P−Ni2P/NF yields a Tafel slope of 80 mV dec−1 in 1 M KOH, which is lower than for Cu3P (117 mV dec−1) and Ni2P (93 mV dec−1) but is slightly larger than for Pt/C@NF (35 mV dec−1). Tafel slopes of the Cu3P−Ni2P/NF electrode suggest a Volmer−Heyrovsky mechanism (H2O + e− = Hads + OH− and H2O + e− + Hads = H2 + OH−). Except for the lower overpotential to drive the HER, the catalyst durability in alkaline solution is also a vital issue that should be carefully considered in practical applications. Therefore, we performed continuous cyclic voltammetry scanning with a scan rate of 50 mV s−1 in 1 M KOH. After 2000 cycles, the polarization curve of Cu3P−Ni2P/ NF showed negligible performance degradation compared with

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CONCLUSIONS In summary, 3D hybrid hexagonal Cu3P−Ni2P nanosheet arrays have been successfully prepared on NF through a hydrothermal reaction and phosphidation process. The heterostructure of Cu3P−Ni2P/NF to form hybrid catalyst significantly increases the active sites and fast facile transport of electrons throughout the entire electrode, exhibiting outstanding alkaline HER activity with an overpotential as low as 103 mV for a current density of 10 mA cm−2 in 1.0 M KOH due to the synergistic effect. Moreover, the synthetic approach to construct phosphide heterostructures leads to the drastic enhancement of overpotential and electrochemical durability for long-term reaction, showing a promising application potential for clean and sustainable hydrogen production. D

DOI: 10.1021/acs.inorgchem.9b01567 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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EXPERIMENTAL SECTION

Chemical Reactants. Cu(NO3)2·5H2O (AR), Ni(NO3)2·6H2O (AR), CO(NH2)2 (AR), NH4F (AR), NaH2PO2 (AR), KOH (AR), Nafion (5 wt %), and Ni foam (NF) were purchased from Macklin (Shenyang, China). The Pt/C catalyst (20 wt % Pt on a Vulcan XC72R) was purchased from Johnson Matthey Corporation. The deionized (DI) water was purified by a Millipore system (18.2 MΩ· cm). All chemicals were used as received without further purification. Preparation of Cu3P/NF, Ni2P/NF, and Cu3P−Ni2P/NF Electrodes. NF (1 cm × 2 cm) was first washed with 3 M HCl, acetone, DI water, and ethanol by ultrasonication for about 30 min. Cu(NO3)2· 5H2O (2 mmol), Ni(NO3)2·6H2O (2 mmol), CO(NH2)2 (10 mmol), and NH4F (5 mmol) were then dissolved in 40 mL of DI water in a beaker with continuous stirring for 20 min. The prepared aqueous solution and NF were transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 120 °C for 6 h in a drying oven for hydrothermal reaction. To obtain Cu3P−Ni2P/NF, NaH2PO2 and hydrothermal precursor were placed into two alumina crucibles and heated at 300 °C for 120 min in Ar gas protection. Ni2P/NF was prepared by dissolving Ni(NO3)2·6H2O (2 mmol), urea (10 mmol), and NH4F (5 mmol) in 40 mL of DI water; the other steps were the same as above. Cu3P/NF was prepared by dissolving Cu(NO3)2·5H2O (2 mmol), urea (10 mmol), and NH4F (5 mmol) in 40 mL of DI water; the other steps were the same as above. Morphology, Structure, and Composition Characterizations. A field-emission scanning electron microscopy (SEM, Hitachi, SU-70) instrument equipped with an Oxford Max energy dispersed X-ray spectrometer (EDS) system and a 200 kV transmission electron microscopy (TEM, FEI, TecnaiG2, F20) instrument were used to characterize the morphology, composition, and microstructure of asprepared samples. X-ray diffraction (XRD, Rigaku D/max 2400) with Cu Kα radiation as the X-ray source (λ = 0.154056 nm) was used to analyze the phase and crystallinity of the catalysts. The elemental type and valence state of the catalysts were analyzed using X-ray photoelectron spectroscopy (XPS, Thermal VG/ESCALAB250). Electrochemical Measurements. Electrochemical measurements were performed with an electrochemical analyzer (Autolab 302N) to evaluate the HER performance of Cu3P−Ni2P/NF. The threeelectrode system consisting of a graphite rod as the counter electrode, an Hg/HgO reference electrode as the reference electrode, and the Cu3P−Ni2P/NF as the working electrode was used for the electrochemical measurement. All electrochemical experiments were carried out in 1.0 M KOH electrolyte at room temperature (25 °C). Polarization curves were obtained using linear sweep voltammetry (LSV) with a scan rate of 5 mV s−1. The long-term durability was studied by chronopotentiometric measurement at a constant current density. Cyclic voltammetry (CV) curves were obtained at different scanning rates to evaluate the double-layer capacitance (Cdl) values. Electrochemical impedance spectroscopy (EIS) measurements were performed with a frequency range from 100 kHz to 0.1 Hz in a potentiostatic mode. All measured potentials were converted to the reversible hydrogen electrode (RHE) scale using Nernst equation: E (RHE) = E (Hg/HgO) + 0.098 V + 0.059 × pH. For comparison, a Pt/ C@NF electrode was prepared by dispersing the mixture of 5 mg of Pt/ C and 20 μL of 5 wt % Nafion solution in 600 μL of water/ethanol solvent and then depositing the mixture on NF with a loading of 2.5 mg cm−2.

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and SEM images of the Cu3P−Ni2P/NF electrode after an HER stability test (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (B.L.) *E-mail: [email protected]. (Y.C.) ORCID

Baodan Liu: 0000-0001-8141-8940 Author Contributions ∥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (51702326, 51872296) and the Program for Leading Talents in Science and Technology Innovation of Chongqing City (cstc2014kjcxljrc0023).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01567. Synthetic route of Cu3P−Ni2P/NF; SEM image and EDS elemental mapping images of P, Cu, and Ni in Cu3P− Ni2P/NF; proposed reaction process of Cu3P−Ni2P for the HER in alkaline media; Nyquist plots of Ni foam, Ni2P/NF, Cu3P/NF, and Cu3P−Ni2P/NF electrodes; E

DOI: 10.1021/acs.inorgchem.9b01567 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01567 Inorg. Chem. XXXX, XXX, XXX−XXX