Iron-Doped Nickel Phosphide Nanosheet Arrays: An Efficient

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Iron-doped nickel phosphide nanosheet arrays: An efficient bifunctional electrocatalyst for water splitting Pengyan Wang, Zonghua Pu, Yanhui Li, Lin Wu, Zhengkai Tu, Min Jiang, Zongkui Kou, Ibrahim Saana Amiinu, and Shichun Mu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06305 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 18, 2017

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ACS Applied Materials & Interfaces

Iron-Doped Nickel Phosphide Nanosheet Arrays: An Efficient Bifunctional Electrocatalyst for Water Splitting Pengyan Wang,a Zonghua Pu,a Yanhui Li,a Lin Wu,a Zhengkai Tu,a Min Jiang,a Zongkui Kou,a Ibrahim Saana Amiinu,a and Shichun Mua* a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, Wuhan 430070, China *Corresponding author. E-mail: [email protected] ABSTRACT

Exploring efficient and earth-abundant electrocatalysts for water splitting is crucial for various renewable energy technologies. In this work, iron (Fe)-doped nickel phosphide (Ni2P) nanosheet arrays supported on nickel foam (Ni1.85Fe0.15P NSAs/NF) are fabricated through a facile hydrothermal method followed by phosphorization. The electrochemical analysis demonstrates that the Ni1.85Fe0.15P NSAs/NF electrode possesses high electrocatalytic activity for water splitting. In 1.0 M KOH, the Ni1.85Fe0.15P NSAs/NF electrode only needs overpotentials of 106 mV at 10 mA cm-2 and 270 mV at 20 mA cm-2 to drive hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Furthermore, the assembled two-electrode

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(Ni1.85Fe0.15P NSAs/NF||Ni1.85Fe0.15P NSAs/NF) alkaline water electrolyzer can produce a current density of 10 mA cm-2 at 1.61 V. Remarkably, it can maintain stable electrolysis over 20 h. Thus, this work undoubtedly offers a promising electrocatalyst for water splitting.

KEYWORDS: Ni1.85Fe0.15P nanosheet arrays; electrocatalysts; hydrogen evolution reaction; oxygen evolution reaction; water splitting INTRODUCTION Due to the increasingly environmental issue and diminishing fossil fuels, a lot of effort has been put into searching for renewable energy resources. Hydrogen is an ideal candidate due to its outstanding energy storage density and zero carbon emission1-3. Currently, one of effective methods for large-scale hydrogen production is by electrochemical water splitting4, 5. However, the water splitting often requires a large overpotential due to the multiple electrons transfer of oxygen evolution reaction (OER)6. Therefore, to achieve high efficiency, it is necessary to utilize an efficient catalyst to decrease the overpotential7-9. Up to now, the most efficient hydrogen evolution reaction (HER) and OER catalysts are usually Pt and IrO2/RuO2, respectively. However, the practical and large-scale applications of these catalysts are limited by their high costs and poor electrochemical stability10. Therefore, it is an urgent need to develop costeffective and highly active noble metal-free HER and OER catalysts. Nickel (Ni) is a sufficient transition metal element on earth. Ni-based materials, such as Ni-S films11, NiSe2, NiO12, NiB13, Ni2P14 and NiFe(OH)2 15 have been extensively reported as highly efficient HER and (or) OER catalysts. Among them, nickel phosphide (Ni2P) catalyst exhibits unique advantage due to its high theoretical and practical activities14, 16, 21. During the past years, to further enhance the HER and (or) OER activity, researchers have devoted their effort by

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tuning the structure and component of Ni2P, such as Ni2P nanoparticle films, NiP2 nanosheet arrays, NiCoP quasi-hollow nanocubes, NiCoP films, and NiCoP nanosheet arrays16-20. However, only NiCoP nanosheet arrays have been used as a bifunctional catalyst20. In addition, previous experimental results have proved that iron (Fe)-doping cannot only increase the surface roughness but expose the active sites of materials22. On the other hand, theoretical calculation has demonstrated that Fe-doping can also adjust the electronic structure of materials23. Therefore, Fe-doped nickel phosphide is expected to exhibit high HER and OER activity. However, as far as we know, rare work has been concerned in this direction. Herein, we report on our recent effort in designing and synthesizing Fe-doped nickel phosphide nanosheet arrays supported on nickel foam (Ni1.85Fe0.15P NSAs/NF) through a facile hydrothermal method followed by phosphorization. When used as a binder-free catalyst, the asprepared Ni1.85Fe0.15P NSAs/NF electrode merely needs overpotentials of 106 mV to drive 10 mA cm-2 for HER and 270 mV to drive 20 mA cm-2 for OER in alkaline solutions. When Ni1.85Fe0.15P NSAs/NF was used as both the cathode and anode of water splitting, a current density of 10 mA cm-2 can be obtained at a cell voltage of 1.61 V, as well as strongly stability. EXPERIMENTAL SECTION Materials Iron nitrate nonahydrate (Fe(NO3)3·9H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O) ， hexamethylene tetramine (HMT), potassium hydroxide (KOH), sodium hypophosphite (NaH2PO2·H2O) and ethanol were purchased from Aladdin Reagent. Nafion (5 wt%), IrO2 and Pt/C (20 wt%) were purchased from Sigma-Aldrich. All the reagents in the experiment were

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analytical grade and used without further treatments. Deionized Mini-Q water was used as solvent. Preparation of Ni2-xFexP NSAs/NF First, a piece of nickel foam (NF) (2×3 cm) was carefully cleaned with acetone, deionized water and absolute ethanol in an ultrasound bath. Then, the weight of pretreated NF was recorded. The Ni2-xFexP NSAs/NF electrode was prepared by the following procedure: 20 mmol hexamethylene tetramine (HMT), iron nitrate nonahydrate (Fe(NO3)3·9H2O) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) with different mole ratios of Fe/Ni (0/1, 1/9, 2/8, 3/7 at a constant metal ion concentration of 10 mmol) were dissolved in 70 ml deionized water to form homogeneous solution. Then, the solution and pretreated NF were transferred into a 100 ml Teflon lined stainless steel autoclave. The autoclave was heated at 100 ℃ for 10 h. After cooling to room temperature, the Ni2-xFex precursor was washed with deionized water and dried in vacuum at 60 ℃ for 8 h. To obtain Ni2-xFexP NSAs/NF, the Ni2-xFex precursor and NaH2PO2·H2O (Metal ion: P=1: 5) were put into a porcelain boat with NaH2PO2·H2O at the upstream side of the furnace. Subsequently, the samples were annealed at 300 ℃ for 2 h under Ar atmosphere. The mass loading of Ni2-xFexP NSAs on NF was about 5.0 mg cm-2. Structural characterizations X-ray power diffraction (XRD) patterns were collected on D8 Advance. The field emission scanning electron microscopy (FE-SEM Zeiss Ultra Plus) and transmission electron microscopy (TEM JEM2100F) were used to characterize morphology and structure. X-ray photoelectron spectroscopy (XPS) was obtained on an ESCALAB 250Xi X-ray photoelectron spectrometer.

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Inductively coupled plasma atom emission spectrometry (ICP-AES) analysis was performed on Optima4300DV. Electrochemical measurements The electrochemical test for HER and OER was performed on a CHI 660E electrochemical workstation in a standard three-electrode system using Ni2-xFexP NSAs/NF as working electrode, a graphite rod as counter electrode and a Ag/AgCl as reference electrode. Pt/C or IrO2 ink was prepared by ultrasonication the mixture of 5 mg Pt/C or IrO2, 1 mL ethanol and 50 uL 5 wt% Nafion. Then, the Pt/C or IrO2 ink was loaded on NF to achieve a mass loading of 5 mg cm-2. Polarization curves were recorded in 1.0 M KOH with a scan rate of 5 mV s-1 at 25 ℃. The measured potentials were converted to RHE using the following equation: E (RHE) = EAg/AgCl + 1.023 V. The obtained HER and OER polarization curves were iR-compensated according to the following equation: Ecorr = Emea – iR (where Ecorr is the iR-compensated potential, Emea is the experimentally measured potential, R (3Ω) is the solution resistance). The long-term stability was tested by a potentiostatic method at fixed potentials. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range from 10-2 Hz to 105 Hz at the applied potential of -1.157 V. The double layer capacitance (Cdl) was measured by cyclic voltammetry curves with scanning rates of 20, 40, 60, 80 and 100 mV s-1. RESULTS AND DISCUSSION Figure 1a shows the X-ray diffraction (XRD) patterns of bare NF and Ni2-xFexP NSAs/NF with a Fe/Ni feeding ratio of 2/8. The diffraction peaks at 40.7°, 44.6°, 47.3°, 54.3°, 55.0°, 66.3°, 72.7°, and 74.9° are indexed to (111), (021), (210), (002), (211), (310), (311) and (212) planes of

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Ni2P (JCPDS no. 65-3544), respectively. In addition, the diffraction peaks at 44.5°, 51.8° and 76.4° arise from NF (JCPDS no. 65-2865)24. Note that no additional diffraction peaks of other Fe phases can be observed, suggesting that Fe is incorporated into the Ni2P lattices. The diffraction peaks appear to slightly shift to larger angles with Fe-doping, also indicating substitutional incorporation of Fe ions into the Ni2P lattices. The inductively coupled plasma atom emission spectrometry (ICP-AES) analysis of Ni2-xFexP NSAs scraped off from the NF with a Fe/Ni feeding ratio of 2/8, exhibits that the atomic ratio of Ni: Fe: P is 1.85:0.15:1 (Table S1), demonstrating the formation of the Ni1.85Fe0.15P NSAs/NF structure. Therefore, the substitution of Fe for Ni does not alter the crystal structure of Ni2P due to the similar atomic sizes. Figure 1b and 1c show the low and high resolution SEM images of Ni1.85Fe0.15 precursor/NF, indicating that nanosheet arrays grow evenly on the NF substrate. After phosphorization, the morphology of the nanosheet arrays remains unaffected and well preserved (Figure 1d and 1e). The highresolution TEM image (Figure 1f) shows well-resolved lattice fringes with an interplanar spacing of 0.189 nm, slightly less than that of Ni2P (0.192 nm) due to the Fe substitution, which can be assigned to the (210) plane of Ni2P. The energy dispersive X-ray (EDX) elemental mapping images (Figure S1) confirm the uniform distribution of Ni, Fe and P elements throughout the Ni1.85Fe0.15P NSAs. All these results demonstrate that Ni1.85Fe0.15 precursor/NF has been successfully converted into Ni1.85Fe0.15P NSAs/NF. In addition to the Ni1.85Fe0.15P NSAs/NF, the crystal structure, morphology and composition of the Ni2-xFexP NSAs/NF catalyst with other Fe/Ni feeding ratios were also investigated, as shown in Figure S2, Figure S3 and Table S1. It is revealed that Ni2-xFexP NSAs/NF catalysts have the similar crystal structure and morphology.

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X-ray photoelectron spectroscopy (XPS) measurements were then carried out to identify the valence states of the Ni1.85Fe0.15P NSAs/NF. The XPS survey spectrum of Ni1.85Fe0.15P NSAs/NF confirms the presence of elemental Ni, Fe, and P (Figure 2a). The signals of C and O can be attributed to surface adsorption or surface oxidation of Ni1.85Fe0.15P NSAs/NF due to exposure to air2, 8. Figure 2b exhibits the high resolution XPS spectrum of Ni 2p. The three main peaks at 853.0, 856.9 and 861.4 eV can be attributed to Ni 2p3/224. The other two main peaks at 870.5 and 874.6 eV correspond to Ni 2p1/225. The additional peak at 880.4 eV can be assigned to the shakeup (marked as “Sat.”)26. In the high-resolution Fe 2p region (Figure 2c), the two main peaks at 707.3 and 713.3eV are characteristic signatures of Fe 2p3/227-29. The high-resolution P 2p region (Figure 2d) exhibits two peaks at 129.3 and 130.0 eV, corresponding to the binding energy of P 2p3/2 and P 2p1/2, along with one broad peak at 134.1 eV. The two peaks at 129.3 and 130.0 eV can be assigned to metal phosphides8, 23, while the peak at 134.1 eV reflects the surface oxidized P species due to the exposure of surface catalyst to air30. We directly used Ni1.85Fe0.15P NSAs/NF as the working electrode to evaluate the electrocatalytic activity toward HER in 1.0 M KOH at a scan rate of 5 mV s-1. For comparison, the catalytic performance of Pt/C (20 wt %), Ni2P NSAs/NF and bare NF were also examined under the same conditions. The mass loading of all catalysts was about 5 mg cm-2. IR compensation was applied to eliminate the effect of ohmic resistance to reflect the intrinsic catalytic activity of catalysts. Figure 3a shows the polarization curves of catalysts. Unquestionably, the commercial Pt/C exhibits the highest HER catalytic activity, while bare NF shows poor HER activity. It is worth noting that by incorporating Fe, the Ni1.85Fe0.15P NSAs/NF electrode exhibits higher HER catalytic activity than that of Ni2P NSAs/NF. For example, the Ni1.85Fe0.15P NSAs/NF electrode merely requires an overpotential of 106 mV at a current density of 10 mA cm-2. This

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overpotential is reduced about 85 mV for Ni2P NSAs/NF. The lower overpotential for Ni1.85Fe0.15P NSAs/NF implies superior HER activity. This overpotential is lower than that of some other Ni-based HER catalysts in alkaline solution, including NiS/NF31, Ni3S2/NF32, Ni2P33 and Ni5P4 films34 (Table S2). The Tafel plots in Figure 3b were applied to study the HER mechanism of Ni1.85Fe0.15P NSAs/NF. Pt/C on NF shows a Tafel slope of 47.8 mV dec-1. Ni1.85Fe0.15P NSAs/NF and Ni2P NSAs/NF exhibit Tafel slopes of 89.7 and 105.1 mV dec-1, respectively. The Tafel slopes suggest that the Volmer reaction is the rate determining step and the HER occurs on Ni1.85Fe0.15P NSAs/NF via the Volmer-Heyrovsky mechanism1, 35. Note that the Tafel slope of Ni1.85Fe0.15P NSAs/NF is less than that of Ni2P NSAs /NF, indicating higher inherent electrocatalytic activity for Ni1.85Fe0.15P NSAs/NF. The stability is a very important parameter for catalyst application. Therefore, we tested the accelerated stability of the Ni1.85Fe0.15P NSAs/NF catalyst by continuous cycle voltammetry (CV) at the scan rate of 100 mV s-1 in 1.0 M KOH. Only a negligible degradation can be observed after 1000 CV cycles (Figure 3c), indicating the good stability of this catalyst in alkaline solutions. In addition, the electrolysis at a fixed overpotential of 134 mV was used to evaluate the long-term durability of Ni1.85Fe0.15P NSAs/NF. It is determined that Ni1.85Fe0.15P NSAs/NF possesses good catalytic stability over 10h (Figure 3d). After durability tests, the XRD analysis (Figure S4a) shows no obvious change in the pattern. In addition, SEM images (Figure S4b) indicate that the sample almost maintains the pristine morphology of nanosheet arrays. Furthermore, XPS spectra (Figure S5) are very similar to those before the stability test. These results further demonstrate that Ni1.85Fe0.15P NSAs/NF has a good HER stability.

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Notably, the catalytic activity for HER can be adjusted by changing the Fe/Ni ratio. The HER activities of Ni1.92Fe0.08P NSAs/NF and Ni1.79Fe0.21P NSAs/NF were also investigated as shown in Figure S6. Ni1.85Fe0.15P NSAs/NF electrode exhibits superior HER performance than Ni1.92Fe0.08P NSAs/NF and Ni1.79Fe0.21P NSAs/NF. Moreover, this electrode has a smaller Tafel slope (89.7 mV dec-1) than Ni1.92Fe0.08P NSAs/NF (95.1 mV dec-1) and Ni1.79Fe0.21P NSAs/NF (91.1 mV dec-1). These results demonstrate that proper Fe doping can provide the highest HER activity. We further assessed the OER catalytic activity of Ni1.85Fe0.15P NSAs/NF in 1.0 M KOH by applying iR compensation at a scan rate of 5 mV s-1. For comparison, the OER catalytic activity of IrO2, Ni2P NSAs/NF and bare NF were also investigated under the same conditions. As observed in Figure 4a, the Ni1.85Fe0.15P NSAs/NF electrode exhibits excellent OER catalytic performance. Note that this performance is better than that of bare NF and Ni2P NSAs/NF, even commercial IrO2. To obtain a current density of 20 mA cm-2, the Ni1.85Fe0.15P NSAs/NF electrode needs an overpotential of 270 mV. This overpotential is lower than that for Ni2P NSAs/NF (323 mV), IrO2 (308 mV) and some other Ni-based OER catalysts in alkaline solutions, such as Ni(OH)2/NF36, NiO37 and NiCo LDH38 (Table S3). The excellent OER activity of Ni1.85Fe0.15P NSAs/NF can be attributed to: (i) The presence of OER intermediates such as Ni(OH)2 · 0.75 H2O and NiOOH in the XRD pattern displayed in Figure S7a. These intermediates are critical during the oxygen evolution process and originate from the oxidation of NiII to NiIII, which can be proved by the oxidation peaks at 1.38 V vs RHE in Figure 4a and the disappearance of phosphide peaks in the P 2p region and two characteristic Ni 2p peaks at 853.0 and 870.5 eV after OER tests (Figure S8 and S9)2, 6, 39. (ii) As displayed in Figure 4c, the Tafel slope of Ni1.85Fe0.15P NSAs/NF (96 mV dec-1) is lower than that of IrO2 (103 mV dec-1), Ni2P

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NSAs/NF (110 mV dec-1) and bare NF (131 mV dec-1), indicating that Ni1.85Fe0.15P NSAs/NF has more excellent inherent electrocatalytic activity. In addition, we evaluated the long-term durability of Ni1.85Fe0.15P NSAs/NF catalyst by the electrolysis at a fixed overpotential of 338 mV. As shown in Figure 4e, this electrode can maintain its catalytic activity for at least 10 h without significant changes. Furthermore, SEM images displayed in Figure S7b indicate that Ni1.85Fe0.15P NSAs/NF electrode preserves the initial morphology after the stability test, which also demonstrates the good stability of Ni1.85Fe0.15P NSAs/NF. Figure S10a shows the polarization curves for Ni1.92Fe0.08P NSAs/NF and Ni1.79Fe0.21P NSAs/NF. Ni1.85Fe0.15P NSAs/NF exhibits the lowest overpotential at current density of 20 mA cm -2 and 100 mA cm

-2

(Figure S10b), indicating that Ni1.85Fe0.15P NSAs/NF owns the best OER activity among the three electrodes. Due to high electrocatalytic performance for Ni1.85Fe0.15P NSAs/NF toward HER and OER in alkaline solutions, Ni1.85Fe0.15P NSAs/NF was used as both anode and cathode for water splitting in 1.0 M KOH. Figure S11 shows the photo of the two-electrode system. For comparison, Ni2P NSAs/NF||Ni2P NSAs/NF (Ni2P NSAs/NF as both anode and cathode), and Pt/C||IrO2 (Pt/C as the cathode and IrO2 as the anode) were also examined. As shown in Figure 4b, the Pt/C||IrO2 system needs 1.55 V to obtain 10 mA cm-2. In contrast, Ni1.85Fe0.15P NSAs/NF||Ni1.85Fe0.15P NSAs/NF offers a current density of 10 mA cm-2 at 1.61 V, which is smaller than the voltage required for Ni2P NSAs/NF||Ni2P NSAs/NF (1.68 V) and other Ni-based materials, including Ni2.3%-CoS2||Ni2.3%-CoS240, NiCo2S4||NiCo2S441 and Ni3S2||Ni3S232 (Table S4). Movie S1 illustrates the gas evolution on both electrodes at 1.70 V. The Tafel slope for Ni1.85Fe0.15P NSAs/NF||Ni1.85Fe0.15P NSAs/NF (299 mV dec-1) is lower than that for Ni2P NSAs/NF||Ni2P NSAs/NF (320 mV dec-1) and higher than that for Pt/C||IrO2 (219 mV dec-1), as shown in Figure

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4d. Moreover, we utilized chronoamperometric to check the stability of Ni1.85Fe0.15P NSAs/NF for alkaline water electrolysis. Remarkably, this Ni1.85Fe0.15P NSAs/NF||Ni1.85Fe0.15P NSAs/NF system can maintain stable electrolysis over 20 h (Figure 4f). Figure S10c shows the polarization curves for Ni1.92Fe0.08P NSAs/NF||Ni1.92Fe0.08P NSAs/NF and Ni1.79Fe0.21P NSAs/NF||Ni1.79Fe0.21P NSAs/NF. Ni1.85Fe0.15P NSAs/NF||Ni1.85Fe0.15P NSAs/NF exhibits the lowest voltage at current density of 10 mA cm

-2

and 100 mA cm

-2

(Figure S10d), indicating

that Ni1.85Fe0.15P NSAs/NF||Ni1.85Fe0.15P NSAs/NF possesses the best water splitting performance among three systems. The double layer capacitance (Cdl) was also measured to estimate the electrochemical surface area42. As shown in

Figure

S12, Cdl for Ni1.85Fe0.15P NSAs/NF is 8.48 mF cm-2, which is larger

than that of 4.01 mF cm-2 for Ni2P NSAs/NF. Therefore, the electrochemical surface area (ECSA) of Ni1.85Fe0.15P NSAs/NF is higher than Ni2P NSAs/NF, which resulting in more catalytic active sites for Ni1.85Fe0.15P NSAs/NF. This indicates that Fe-doped is beneficial to promotion of electrocatalytic activity23. In addition, it can be seen from Figure S13 that the Cdl of Ni1.85Fe0.15P NSAs/NF is larger than that of Ni1.79Fe0.21P NSAs/NF (6.04 mF cm-2) and Ni1.92Fe0.08P NSAs/NF (5.77 mF cm-2), revealing larger ECSA and more catalytic active sites for Ni1.85Fe0.15P NSAs/NF. Furthermore, the charge transfer resistance (Rct) was investigated by an electrochemical impedance spectrum (EIS). As displayed in Figure S14, Ni1.85Fe0.15P NSAs/NF exhibits the smallest Rct compared to other two catalysts, indicating that Ni1.85Fe0.15P NSAs/NF has faster charge transfer43. Thus, the higher ECSA, the more catalytic active sites and the faster charge transfer of Ni1.85Fe0.15P NSAs/NF contribute to its superior electrocatalytic activity.

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CONCLUSION In summary, we report a two-step synthesis of Ni1.85Fe0.15P nanosheet arrays supported on nickel foam (Ni1.85Fe0.15P NSAs/NF). The obtained Ni1.85Fe0.15P NSAs/NF can act as a bifunctional catalyst for both HER and OER in alkaline solutions. The resulting Ni1.85Fe0.15P NSAs/NF electrode only requires overpotentials of 106 mV for achieving 10 mA cm-2 toward HER and 270 mV for achieving 20 mA cm-2 toward OER. The alkaline water electrolyzer based on this bifunctional electrode attains a current density of 10 mA cm-2 at 1.61V with good stability. It is no doubt that the facile and low-cost preparation, along with the high HER and OER catalytic activity and long-term stability make the iron-doped nickel phosphide nanosheet arrays applicable in water splitting and other electrochemical devices. ASSOCIATED CONTENT Supporting Information. SEM images; EDX element mapping images; XRD patterns; XPS and EIS spectra; Polarization curves; Tafel plots; CVs; Photo and movie of the evolution of H2 and O2 on Ni1.85Fe0.15P NSAs/NF electrodes in a two-electrode system driven by a DC power supply with a cell voltage of 1.70 V in 1.0 M KOH; Tables. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51372186, 51672204), and the Fundamental Research Funds for the Central Universities (No.2016-YB001). The authors express thanks to Xiaoqing Liu and Tingting Luo for HR-TEM measurement support, in the Materials Analysis Center of Wuhan University of Technology.

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2016, 8, 8500-8504. (9) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (10) Pu, Z. H.; Wang, M.; Kou, Z. K.; Amiinu, I. S.; Mu, S. C. Mo2C Quantum Dot Embedded Chistom-Derived Nitrogen-Doped Carbon for Efficient Hydrogen Evolution in a Broad PH Range. Chem. Commun. 2016, 52, 12753-12756. (11) Jiang, N.; Bogoev, L.; Popova, M.; Gul, S.; Yano, J.; Sun, Y. J. Electrodeposited NickelSulfide Films as Competent Hydrogen Evolution Catalysts in Neutral Water. J. Mater. Chem. A 2014, 2, 19407-19414. (12) Gong, M.; Zhou, W.; Tsai, M. C.; Zhou, J. G.; Guan, M. Y.; Lin, M. C.; Zhang, B.; Hu, Y. F.; Wang, D. Y.; Yang, J.; Pennycook, S. J.; Hwang, B. J.; Dai, H. J. Nanoscale Nickel Oxide/Nickel Heterostructures for Active Hydrogen Evolution Electrocatalysis. Nat. Commun.2014, 5, 4695. (13) Los, P.; Lasia, A. Electrocatalytic Properties of Amorphous Nickel Boride Electrodes for Hydrogen Evolution Reaction in Alkaline Solution. J. Electroanal. Chem. 1992, 333, 115125. (14) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak. R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267-9270. (15) Li, X. H.; Walsh, F. C.; Pletcher, D. Nickel Based Electrocatalysts for Oxygen Evolution in High Current Density, Alkaline Water Electrolysers. Phys. Chem. Chem. Phys. 2011, 13, 1162-1167. (16) Pu, Z. H.; Liu, Q.; Tang, C.; Asiri, A. M.; Sun, X. P. Ni2P Nanoparticles Films Supported

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on a Ti Plate as an Efficient Hydrogen Evolution Reaction. Nanoscale 2014, 6, 1103111034. (17) Jiang, P.; Liu, Q.; Sun, X. P. NiP2 Nanosheet Arrays Supported on Carbon Cloth: An Efficient 3D Hydrogen Evolution Cathode in both Acidic and Alkaline Solutions. Nanoscale 2014, 6, 13440-13445. (18) Feng, Y.; Yu, X. Y.; Paik, U. Nickel Cobalt Phosphides Quasi-Hollow Nanocubes as an Efficient Electrocatalyst for Hydrogen Evolution in Alkaline Solution. Chem. Commun. 2016, 52, 1633-1636. (19) Yu, J.; Li, Q. Q.; Li, Y.; Xu, C. Y.; Zhen, L.; Dravid, V. P.; Wu, J. S. Ternary Metal Phosphide with Triple-Layered Structure as a Low-Cost and Efficient Electrocatalyst for Bifunctional Water Splitting. Adv. Funct. Mater. 2016, 26, 7644-7651. (20) 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. (21) Liu, P.; Rodriguez, J. A. Catalysts for Hydrogen Evolution from the [NiFe] Hydrogenase to the Ni2P (001) Surface: The Importance of Ensemble Effect. J. Am. Chem. Soc. 2005, 127, 14871-14878. (22) Yang, N,; Tang, C.; Wang, K. Y.; Du, G.; Asiri, A. M.; Sun, X. P. Iron-Doped Nickel Disulfide Nanoarray: A Highly Efficient and Stable Electrocatalyst for Water Splitting. Nano Res. 2016, 9, 3346-3354. (23) Tang, C.; Gan, L. F.; Zhang, R.; Lu, W. B.; Jiang, X. E.; Asiri, A. M.; Sun, X. P.; Wang, J.; Chen, L. Ternary FexCo1-xP Nanowire Array as a Robust Hydrogen Evolution Reaction ELectrocatalyst with Pt-like Activity: Experimental and Theoretical Insight. Nano Lett.

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2016, 16, 6617-6621. (24) Tang, C.; Pu, Z. H.; Liu, Q.; Asiri, A. M., Sun, X. P.; Luo, Y. L.; He.Y. Q. In Situ Growth of NiSe Nanowire Film on Nickel Foam as an Electrode for High-Performance Supercapacitors. ChemElectroChem 2015, 2, 1903-1907. (25) Zhang, B.; Xiao, C. H.; Xie, S. M.; Liang, J.; Chen, X.; Tang, Y. H. Iron-Nickel Nitride Nanostructures in Situ Grown on Surface-Redox-Etching Nickel Foam: Efficient and Ultrasustainable Electrocatalysts for Overall Water Splitting. Chem. Mater. 2016, 28, 69346941. (26) Wang, Z.; Cao, X. Q.; Liu, D. N; Hao, S.; Du, G.; Asiri, A. M.; Sun, X. P. Ternary NiCoP Nanosheet Array on Ti mesh: A High-Performance Electrochemical Sensor for Glucose Detection. Chem. Commun.2016, 52, 14438-14441. (27) Liu, Q.; Pu, Z. H.; Asiri, A. M.; Sun, X. P. Nitrogen-Doped Carbon Nanotube Supported Iron Phosphide Nanocomposites for Highly Active Electrocatalysis of the Hydrogen Evolution Reaction. Electrochim. Acta 2014, 149, 324- 329. (28) Fominykh, K.; Chernev, P.; Zaharizva, I.; Sicklinger, J.; Stefanic, G.; Doblinger, M.; Muller, A.; Pokharel, A.; Bocklein, S.; Scheu, C.; Bein, T.; Rohlfing, D. F. Iron-Doped Nickel Oxide Nanocrystals as Highly Efficient Electrocatalysts for Alkaline Water Splitting. ACS Nano 2015, 9, 5180-5188. (29) Zhang, Z.; Lu, B. P.; Hao, J. H.; Yang, W. S.; Tang, J. L. FeP Nanoparticles Grown on Grapheme Sheets as Highly Active Non-Precious-Metal Electrocatalysts for Hydrogen Evolution Reaction. Chem. Commun. 2014, 50, 11554-11557. (30) Bai, J.; Li, X.; Wang, A. J.; Prins, R.; Wang, Y. Hydrodesulfurization of Dibenzothiophene and its Hydrogenated Intermediates over Bulk MoP. J. Catal. 2012, 287, 161-169.

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(39) Stern, L. A.; Feng, L. G.; Song, F.; Hu, X. L. Ni2P as a Janus Catalyst for Water Splitting: The Oxygen Evolution Activity of Ni2P Nanoparticles. Energy Environ. Sci. 2015, 8, 23472351. (40) Fang, W. Z.; Liu, D. N.; Lu, Q.; Sun, X. P.; Asiri, A. M. Nickel Promoted Cobalt Disulfide Nanowire Array Supported on Carbon Cloth: An Efficient and Stable Bifunctional Electrocatalyst for Full Water Splitting. Electrochem. Commun. 2016, 63, 60-64. (41) Liu, D. N.; Lu, Q.; Luo, Y. L.; Sun, X. P.; Asiri, A. M. NiCo2S4 Nanowires Array as an Efficient Binfunctional Electrocatalyst for Full Water Splitting with Superior Activity. Nanoscale 2015, 7, 15122-15126. (42) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L. S.; S. Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274-10277. (43) Vrubel, H.; Moehl, T.; Gratzel, M.; Hu, X. L. Revealing and Accelerating Slow Electron Transport in Amorphous Molybdenum Sulphide Particles for Hydrogen Evolution Reaction. Chem. Commun. 2013, 49, 8985-8987.

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Figure captions Figure 1. (a) XRD patterns for bare NF and Ni1.85Fe0.15P NSAs/NF. (b) Low- and (c) highresolution SEM images of Ni1.85Fe0.15 precursor on NF. (d) Low- and (e) HR-SEM images of Ni1.85Fe0.15P NSAs/NF. (f) HR-TEM image of Ni1.85Fe0.15P NSAs/NF. Figure 2. (a) XPS survey spectrum for Ni1.85Fe0.15P NSAs/NF. XPS spectra in the Ni 2p (b), Fe 2p (c) and P 2p (d) regions for Ni1.85Fe0.15P NSAs/NF. Figure 3. Polarization curves (a) and corresponding Tafel curves (b) of Ni2P NSAs/NF, Ni1.85Fe0.15P NSAs/NF，Pt/C on NF and bare NF with a scan rate of 5 mV s-1. (c) Polarization curves recorded for Ni1.85Fe0.15P NSAs/NF before and after 1000 CV cycles. (d) Time-dependent current density curve of Ni1.85Fe0.15P NSAs/NF at a static overpotential of 134 mV. The electrolyte is 1.0 M KOH. Figure 4. Polarization curves (a) and corresponding Tafel curves (c) of Ni2P NSAs/NF, Ni1.85Fe0.15P NSAs/NF，IrO2 on NF and bare NF in 1.0 M KOH with a scan rate of 5 mV s-1. (e) Time-dependent current density curve of Ni1.85Fe0.15P NSAs/NF at a static overpotential of 338 mV in 1.0 M KOH. Polarization curves (b) and corresponding Tafel curves (d) of an alkaline electrolyzer using Ni1.85Fe0.15P NSAs/NF||Ni1.85Fe0.15P NSAs/NF, Ni2P NSAs/NF||Ni2P NSAs/NF and Pt/C||IrO2 as two electrodes in 1.0 M KOH with a scan rate of 5 mV s-1. (f) Chronoamperometric electrolysis in 1.0 M KOH at a constant voltage of 1.776 V for Ni1.85Fe0.15P NSAs/NF||Ni1.85Fe0.15P NSAs/NF.

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Graphical abstract (TOC)

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Iron-Doped Nickel Phosphide Nanosheet Arrays: An Efficient

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