âSuperaerophobicâ Nickel Phosphide Nanoarray Catalyst for Efficient

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A “Superaerophobic” Nickel Phosphide Nanoarray Catalyst for Efficient Hydrogen Evolution at Ultra-High Current Densities Xingxing Yu, Zi-You Yu, Xiao-Long Zhang, Ya-Rong Zheng, Yu Duan, Qiang Gao, Rui Wu, Bing Sun, Min-Rui Gao, Guoxiu Wang, and Shu-Hong Yu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02527 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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Journal of the American Chemical Society

A “Superaerophobic” Nickel Phosphide Nanoarray Catalyst for Efficient Hydrogen Evolution at Ultra-High Current Densities Xingxing Yua,b, Zi-You Yub, Xiao-Long Zhangb, Ya-Rong Zhengb, Yu Duanb, Qiang Gaob, Rui Wub, Bing Suna, Min-Rui Gaob,, Guoxiu Wanga, and Shu-Hong Yub, aCenter

for Clean Energy Technology, School of Mathematical and Physical Science, Faculty of Science, University of Technology Sydney, NSW 2007, Australia. bDivision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, CAS Centre for Excellence in Nanoscience, Hefei Science Centre of CAS, University of Science and Technology of China, Hefei 230026, China. KEYWORDS nickel phosphide • “superaerophobic” structure • hydrogen evolution • high current densities • water splitting. Supporting Information Placeholder ABSTRACT: The design of highly efficient non-noble metal electrocatalysts for large-scale hydrogen production remains an ongoing challenge. We report here a Ni2P nanoarray catalyst grown on a commercial Ni foam substrate, which demonstrates an outstanding electrocatalytic activity and stability in basic electrolyte. The high catalytic activity can be attributed to the favorable electron transfer, superior intrinsic activity and the intimate connection between the nanoarrays and their substrate. Moreover, the unique “superaerophobic” surface feature of the Ni2P nanoarrays enables a remarkable capability to withstand internal and external forces and release the in-situ generated H2 bubbles in a timely manner at large current densities (such as > 1000 mA cm-2) where the hydrogen evolution becomes vigorous. Our results highlight that an aerophobic structure is essential to catalyze gas evolution for large-scale practical applications.

INTRODUCTION Electrocatalytic water splitting (2H2O → 2H2 + O2) that produces hydrogen offers a promising route to provide renewable, sustainable and environment-friendly clean energy sources for the replacement of traditional fossil fuels1-5. However, considerable challenges still exist in the quest to boost catalytic hydrogen evolution reaction (HER) to the industrial scale. Currently, the most highly active HER reaction catalysts are scarce and costly noble metal platinum (Pt) or Pt-based materials6-8, thus dramatically hampering practical applications. Great efforts have been devoted to accelerating the advance of low-cost and abundant HER catalysts. Many transition metal-based electrocatalysts such as MoS29, CoSe210, transition metal phosphides11, nitrides12, carbides13, oxides14, or bimetallic materials15 have been reported. Besides, metal-organic frameworks have also demonstrated great potential in this research area16,17. Owing to the low catalytic performance and the poor stability of these promising non-noble electrocatalysts when performing at large current densities, a variety of strategies has been designed to promote the reaction kinetics and improve the catalytic activities, such as improving electron transfer, designing advantageous defects18,19, creating new active sites and tailoring the intrinsic properties20-22. Previous efforts have

demonstrated several catalyst systems for high-current operations. These include the hybrid bimetallic phosphides (FeP/Ni2P) for oxygen evolution reaction (OER) and HER in basic solutions23, hexagonal α-MoB2 for HER in acidic solutions24, Fe(PO3)2 formed on Ni2P/Ni foam and Ni-FeOH@Ni3S2/NF for OER in basic solutions25,26. Although impressive progress, there is no report on studies of solidliquid interfaces and gas-liquid-solid interfaces that permit the generation and release of large amount of H2 bubbles from the electrode. During large-scale catalytic hydrogen evolution, tremendous quantities of H2 bubbles will be rapidly formed at high current densities. The aggregation of gas bubbles on the contact surface between catalysts and electrolyte severely hinders liquid mass transport, slowing the electron transfer and decreasing the number of exposed active sites, which leads to inferior electrocatalytic activity and stability27. The challenging dilemma in advancing industrial hydrogen production is to detach the formed H2 bubbles to retain the catalytic ability of the electrode. This expectation has been described by the pioneering works from the Jin’s28 and Sun’s29 groups. It was observed that “superaerophobic” surface structures could be assembled via the formation of an array architecture30, which is indispensable for large-current HER

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due to their superior ability to release the formed gas bubbles28,29. Herein, we report the preparation of a Ni2P nanoarray catalyst on a Ni foam substrate (denoted as Ni2P/NF), where Ni2P nanosheets are evenly distributed on Ni2P nanowires arrays, forming a nanoarray structure. The Ni2P/NF catalyst performs highly active and stable for HER, even exceeding commercial noble metal Pt/C at large current densities (> 170 mA cm-2). The unique assembly of the Ni2P/NF provides a preferred “superaerophobic” structure to release the in-situ generated gas bubbles. Using Ni2P||Ni-Fe LDH (LDH = layered double hydroxide) as catalyst electrodes, our Labmade overall water-splitting cell outperforms the Pt/C||Ir/C

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cell at high current densities (> 600 mA cm-2). These findings demonstrate a promising route that meets the requirements of scalable H2 production at large current densities by using the “superaerophobic” material architecture. RESULTS AND DISCUSSION Material Synthesis and Characterization. The Ni2P/NF catalyst was synthesized via annealing NaH2PO2·H2O with Ni(OH)2 nanoarrays that were grown on Ni foam (denoted as Ni(OH)2/NF) under a flowing argon atmosphere. The Ni(OH)2/NF precursor was obtained via a hydrothermal method as described in the experimental section. Figure 1a

Figure 1. (a) Schematic illustration of the synthetic process for Ni2P nanoarrays. (b, c) SEM, (d) TEM, (e) HRTEM, (g) STEM-EDX mapping images of the obtained Ni2P/NF. (f) XRD patterns of Ni2P/NF and blank NF. The inset in (d) is the corresponding SAED pattern.

illustrates the whole synthesis process. In the first step, the free-standing precursor was prepared through a direct thermal reaction between (NH4)2HPO4 and Ni foam. Scanning electron microscopy (SEM) images show that smooth nanowire arrays are evenly-distributed on the Ni foam (Figure S1 in the Supporting Information). Transmission electron microscopy (TEM) images confirm the formation of nanowires with a diameter of ~250 nm. Elemental mapping and energy dispersive X-ray (EDX) spectra evidence the existence of Ni, O and P elements (Figure S2). In the second step, the precursors were hydrothermally treated with KOH aqueous solution. The morphology transforms from smooth nanowires

to a hybrid three-dimensional (3D) structure consisting of nanowires and nanosheets, where the nanosheets are uniformly embedded in nanowires (Figure S3). X-ray powder diffraction (XRD) patterns and Raman spectra reveal that the precursor has been gradually changed into Ni(OH)2 during the hydrothermal process. At a higher temperature, the precursors were completely transformed into Ni(OH)2 with a hexagonal phase (JCPDS 14-0117) (Figure S4)31,32. The transformation process was speculated as follows. First, (NH4)2HPO4 directly reacted with nickel foam, which provides sites for oriented growth of the nanowires during the reaction process, forming the nanowire array precursor. Then, the obtained precursors


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Journal of the American Chemical Society were transformed into the Ni(OH)2 in a KOH aqueous solution via another hydrothermal treatment. This process causes the morphology change from nanowire array of the precursor to the unique Ni(OH)2 nanoarray. The optimal Ni(OH)2 nanoarrays obtained at 120 ºC were chosen to be further investigated (Figure S5). TEM image also confirms that the external ultra-thin nanosheets are arranged regularly on the nanowires (Figure S6a). Corresponding selected area electron diffraction (SAED) patterns reveal single-crystalline-feature of the as-prepared Ni(OH)2. High-resolution TEM (HRTEM) images demonstrate the high crystallinity of Ni(OH)2 with the resolved lattice fringes of (110), (002) and (101) planes. Scanning TEM (STEM) and elemental mapping display the uniform distribution of Ni and O (Figure S6b-e). The analysis of Ni 2p and O 1s spectra elucidated that the main peaks at 855.6 eV and 873.4 eV could be assigned to Ni 2p3/2 and Ni 2p1/2. While, the metal-O peak is at 530.7 eV (Figure S7) 33.

Figure 2. (a) HER polarization curves for different catalysts at high current density (left) and low current density (right). Scan rate: 10 mV s-1. (b)Tafel plots for different catalysts derived from Figure 2a (right). (c) Multi-chronoamperometric responses curve from 240 mV to 650 mV overpotentials for the Ni2P/NF catalyst.

The

superaerophobic

Ni2P/NF

nanoarrays

were

obtained via a phosphorization process by annealing the prepared Ni(OH)2/NF with NaH2PO2·H2O at 300 ºC for 0.5 h under a Ar flow at a heating rate of 5 ºC/min (Figure S8). NaH2PO2.H2O was placed at the upper location, which can be decomposed to PH3 species in situ during the annealing process, enabling the formation

XRD patterns (Figure 1e, 1f, S10 and S11). EDX and XPS survey spectra verify the existence of Ni and P, which are uniformly distributed, as shown in our STEM elemental mapping (Figure S12, S13a and 1g). Ni 2p spectra analysis shows that the two obvious peaks at 857.2 eV and 875.1 eV can be attributed to Ni 2p3/2 and Ni 2p1/2. The peak at 134.3 eV originates from P 2p. These results verify the generation of Ni2P (Figure S13b-c)34-36. The loading of Ni2P on NF support was determined to be ~3.83 mg cm-2 (Table S1). We note that this phosphorization approach can be extended to produce other transition metal phosphides, such as Ni2P nanosheets, FeP and Cu3P (Figure S14 and S15). The unique structural features and solid adhesive connection with 3D Ni foam will synergistically function together to provide excellent electrocatalytic properties. Electrocatalytic HER Activities. Electrocatalytic HER activities of the as-prepared catalysts were evaluated via a three-electrode system in 1 M KOH electrolyte (pH 13.95, Figure S16). As shown in Figure 2a (left), Ni2P/NF catalyst exhibits an excellent electrocatalytic activity, ever showing better performance than the commercial Pt/C (20 wt%) catalyst at high current densities. For example, the Ni2P/NF can sustain current densities of 1000 mA cm-2 and 1500 mA cm-2 at overpotentials of 306 mV and 368 mV, respectively, However, much higher overpotentials of 575 mV and 758 mV are required for the commercial Pt/C catalyst. The electrocatalytic activity of Ni2P/NF always overwhelmingly surpassed that of Ni(OH)2/NF and blank NF. Tafel analysis was performed from the small polarization area shown in Figure 2a (right). The corresponding Tafel plots give slopes of 76, 44, 121 and 204 mV dec-1 for Ni2P/NF, Pt/C, Ni(OH)2/NF and blank NF, respectively (Figure 2b). The HER activities for the catalysts at different constant overpotentials were also recorded. The Ni2P/NF catalyst demonstrates a superior electrochemical performance with a current density > 1800 mA cm-2 at an overpotential of 0.4 V (Figure S17). EIS Nyquist plots reveal that the charge transfer resistance (Rct) of Ni2P/NF is 3.5 Ω, which is much lower than that of Ni(OH)2/NF (25.2 Ω) and blank NF (69.5 Ω) at an overpotential of 100 mV, indicating accelerated charge transfer kinetics for Ni2P/NF catalyst (Figure S18a). To estimate the effective electrochemically active surface area of the catalysts, we calculated double-layer capacitances (Cdl). The Cdl of Ni2P/NF (55.2 mF cm-2) is larger than Ni(OH)2/NF (3.1 mF cm-2) and blank NF (1.0 mF cm-2) (Figure S18b and S19), suggesting more exposed active sites on Ni2P/NF catalyst.

of Ni2P. SEM and TEM images of the obtained Ni2P show a uniform nanoarray morphology, inherited from the original Ni(OH)2 precursor (Figure 1b-d and S9). The corresponding SAED pattern reveals a polycrystalline feature, which can be easily distinguished from the single-crystal-like pattern of the Ni(OH)2 precursor (Inset in Figure 1d). HRTEM images demonstrate distinct lattice fringes with a spacing of 0.13 nm that could be attributed to the (311) plane of hexagonal Ni2P (JCPDS 030953), which is consistent with the results from the



Figure 3. Digital images of hydrogen bubbles on (a) blank NF and (b) Ni2P/NF. (c) Schematic illustration of the adhesion behavior for a H2 bubble on blank NF (left) and Ni2P/NF nanoarrays (right). The contact area is an “isolated zone” due to the bubble effect.

Besides electrocatalytic activities, stability is also a critical parameter for realizing large-scale hydrogen production. We evaluated the HER stability of Ni2P/NF catalyst via multiple testing methods. Multi-step chronoamperometric curves were measured under a wide potential range (from 240 to 650 mV) and multi-step chronopotentiometry curves were recorded across a broad current density (from 100 to 4000 mA cm-2). The results showed that the electrocatalytic activities remain stable at each step during the test, confirming the excellent

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stability of Ni2P/NF catalyst (Figure 2c and S20). Furthermore, the long-term chronoamperometry (j–t) test shows that a high current density of 2500 mA cm-2 can be maintained without decrease after 10 h continuous electrolysis (Figure S21). Additionally, the accelerated cyclic polarization curves (0 to 0.37 V) for Ni2P/NF demonstrate no apparent activity decay even after 5,000 cycles. It should be noted that no visible shifts were observed in polarization curves or EIS Nyquist plots after a long-term chronopotentiometry test (at 500 mA cm-2), clearly indicating the good stability of Ni2P/NF catalyst under high-current electrolysis (Figure S22 and S23). The postmortem SEM and XRD characterizations reveal no apparent changes of the morphology and crystal structure after a long-term high-current electrolysis (Figure S24). The above kinetic and energetic merits demonstrate that the Ni2P/NF is a superior catalyst among the previously reported HER catalysts under basic conditions (Figure S25 and Table S2)23,36-44. Structure mechanisms. The feature of solid-liquid contact interface between catalysts and electrolyte plays an important role in HER catalysis. However, little attention has been paid to this aspect until now. The H2 bubbles generated, especially at high current densities, tend to pin or aggregate on the surface of the catalyst, resulting in decreased solid-liquid contact area and limited electron transfer. Therefore, a unique “superaerophobic” structure may be efficient to solve this challenging dilemma. Figure 3a shows that a H2 bubble is readily pinned on the surface of the bare NF, whereas the “superaerophobic” Ni2P/NF exhibits a tremendous potential to release the in-situ generated H2 bubbles and prevent bubble retention. The contact angle tests showed that H2 bubbles detached with a superfast speed off the surface (Figure 3b). To further analyze the “superaerophobic” property of Ni2P/NF catalyst, a heavier gas bubble (an air bubble) was studied under the same basic conditions, showing that air bubble can


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Figure 4. (a) Polarization curves of the catalysts at a scan rate of 10 mV s-1. Inset shows EIS Nyquist plots of the catalysts at a voltage of 1.7 V. (b) The comparison of overall electrocatalytic water splitting performance of the Ni2P||Ni-Fe LDH catalyst with recently reported highly active catalysts. (c) Chronoamperometric curves at a voltage of 2.1 V. (d and e) H2 amount for Ni2P||Ni-Fe LDH at a fixed current density of 1000 mA cm-2 and optical picture of the measured setup Hoffman apparatus. (f-h) SEM, XRD and STEM elemental mapping images of Ni2P/NF after the long-term electrolysis.

also be easily released (Figure S26). The schematic illustration reveals that the generated H2 bubbles adhered on the surface of the bare NF, induce deteriorated HER performance, due to reduced contact interface and consequent blocked electrolyte transfer (Figure 3c, left). Whereas, the “superaerophobic” structure of Ni2P/NF releases the formed H2 bubbles rapidly, retaining the original catalytic sites (Figure 3c, right). Therefore, the high-efficient electrocatalytic activity and excellent stability of the Ni2P/NF catalyst at high current densities should be ascribed to the unique “superaerophobic” property that expels H2 bubbles generated in-situ. Overall Water Splitting Using Ni2P||Ni-Fe LDH Catalysts. To demonstrate the advantages of the Ni2P/NF catalyst for large-scale H2 production, we conducted overall water splitting in a two-electrode system using 1 M KOH electrolyte. The Ni-Fe layered double hydroxide (LDH) on Ni foam was synthesized as an active catalyst for oxygen evolution reaction (OER), and the Ni2P/NF was used directly as the cathode. The electrolysis cell system is denoted as Ni2P||Ni-Fe LDH34. The Ni-Fe LDH catalyst has pure phase and outstanding OER property (Figure S27 to S29). For comparison, we also deposited commercial noble metal catalysts Ir/C (20 wt%) and Pt/C onto Ni foam and used these as anode and cathode, respectively (denoted as Pt/C||Ir/C).

Polarization curves and EIS Nyquist plots in Figure 4a reveal that Ni2P||Ni-Fe LDH exhibits an inferior performance, compared to Pt/C||Ir/C under low current density. However, it rapidly surpasses Pt/C||Ir/C at higher current densities (> 600 mA cm-2; also see Figure S30). These observations can be ascribed to the unique “superaerophobic” characteristics of Ni2P nanoarrays. By contrast, the polarization curve of Pt/C||Ir/C cell at high current densities becomes choppy due to the strong “bubble effect”. The superior overall water splitting performances of Ni2P||Ni-Fe LDH cell at high current densities are mainly ascribed to the special “superaerophobic” structure (Figure 4b and Table S3)35,45-48. We measured the long-term chronoamperometry for Ni2P||Ni-Fe LDH and Pt/C||Ir/C cells operated at a constant voltage of 2.1 V. The Ni2P||Ni-Fe LDH cell shows an excellent stability without decay of the current density during continuous electrolysis, However, the Pt/C||Ir/C cell exhibits an inferior property (Figure 4c and S31). We collected the generated H2 from the Ni2P||Ni-Fe LDH catalystdriving cell via a Hoffman apparatus setup. The amounts of produced H2 collected at current densities of 1000 mA cm-2 or 500 mA cm-2 matches well with the theoretically calculated value, corresponding to a Faradaic efficiency of ~100% (Figure 4d-e and Figure S32-33). Postmortem SEM, XRD and



STEM elemental mapping characterizations for Ni2P/NF catalyst show no obvious changes of the structure and phase after high-current electrolysis during overall water splitting, confirming the chemical and structural stability of the Ni2P/NF catalyst (Figure 4f-h and S34). CONCLUSION In summary, we report here a unique “superaerophobic” nickel phosphide nanoarray structure, which was synthesized on a Ni foam substrate via a facile hydrothermal-then-annealing strategy. The Ni2P/NF catalyst exhibits an exceptionally high electrocatalytic activity and stability for hydrogen evolution under high current density and basic electrolytes. A twoelectrode Lab-scale electrolyser composed of a Ni-Fe LDH anode and a Ni2P/NF cathode also demonstrated an excellent catalytic capability for water splitting, which surpassed the performance of the Pt/C||Ir/C cell at a higher current density (> 600 mA cm-2). This work uncovers the potential of the lowcost nickel phosphide nanoarrays as HER catalysts for H2 production at ultra-large current densities, owing to the unique “superaerophobic” structure, which offers a path to enable industrial-scale H2 production. EXPERIMENTAL SECTIONS Chemicals. All the chemicals were used as received without any further purification. Synthesis of Ni(OH)2/NF precursor. Before the synthetic procedures, commercial Ni foam (NF) was cut into 2.5 x 2.5 cm2 plates, then were compressed into thinner ones to improve the wettability. Ni foam were sequentially ultrasonically treated in 3 M hydrochloric acid (HCl) aqueous solution, deionized water and ethanol to remove the surface oxides and organic molecules, then dried in vacuum oven at 60 ºC. The Ni(OH)2/NF was obtained via a two-step hydrothermal method. First, the clean Ni foam plate was immersed in a 35 mL of uniform transparent aqueous solution containing (NH4)2HPO4 (1mM). Sequentially, the sealed 50 mL autoclave was retained at 180 ºC for different times to prepare the precursors. The achieved green precursors were washed several times with deionized water and dried in air. Then, another mild hydrothermal process was utilized to achieve the desired Ni(OH)2/NF. In a typical procedure, the green precursor was transformed into a 50 mL Teflon-lined stainless steel autoclave containing 30 mL of 0.1 M KOH aqueous solution. Subsequently, the autoclave was kept at 120 ºC for 5h. After cooling down to room temperature, the light grey Ni(OH)2/NF nanoarrays were rinsed with deionized water and dried in air. The temperature-control experiments were also operated while the reaction time was fixed and experiment temperatures were adjusted as needed to obtained the corresponding materials. Synthesis of Ni2P/NF. The Ni2P/NF was prepared via a lowtemperature calcination method. The fresh obtained Ni(OH)2/NF and NaH2PO2·H2O with a weight ratio of 1:10, were put into a same ceramic boat at two separated places, while NaH2PO2·H2O is at the upstream side. After excluding air for 30 min in a flowing Ar at room temperature, the samples were maintained at 300 °C for 30 min with a heating rate of 5 °C /min under a flowing Ar atmosphere. After naturally cooling down to room temperature, the achieved black samples were collected for further tests. Control experiments were also operated. Synthesis of Ni-Fe LDH. The Ni-Fe LDH was synthesized with a modified reported method. In a typical process, a clean Ni foam was put into 30 mL of a transparent light green aqueous solution containing 0.5 mM Fe(NO3)2.9H2O, 0.5 mM

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Ni(NO3)2.6H2O, 3 mM urea and 7.5 mM NH4F. Then the 50 mL autoclave was transformed into an oven, kept at 120 ºC for 6 h. After cooling down to room temperature, the obtained Ni-Fe LDH were rinsed with deionized water and dried in air for further use. Characterization. X-ray powder diffraction (XRD) pattern was examined via a Japan Rigaku DMax-γA X-ray diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). Scanning electron microscope (SEM) images were investigated on a Zeiss Supra 40 at 5 kV. Transmission electron microscope (TEM) images were measured through a JEOL 2010F(s) at 120 kV. High-resolution TEM (HRTEM) images, selected-area electron diffraction (SAED), energy dispersive X-ray (EDX) spectra and elemental mappings were all obtained on an Atomic Resolution Analytical Microscope JEM-ARM 200F, operating at 200 kV. Raman spectra were obtained via a Raman microscope (Renishaw®), excited with a 514 nm excitation laser. X-ray photoelectron spectra (XPS) spectra were performed on an X-ray photoelectron spectrometer of ESCALab MKII, equipped with an X-ray source (Mg Kα) at hυ = 1253.6 eV. Contact angles were analyzed via an OCA20 machine (Data-Physics, Germany) at room temperature. Electrocatalytic measurements. Electrochemical activities were operated via a Multipotentiostat (IM6ex, ZAHNER elektrik, Germany) in a traditional three-electrode system at room temperature, using a graphite rod and a saturated calomel electrode as the counter electrode and the reference electrode, respectively. The prepared samples were used as the working electrodes and the electrolyte was 1 M KOH aqueous solution. During the controlled experiments, the homo-disperse commercial Pt/C and Ir/C (5 mg catalysts in 1 mL ethanol and 40 µL 5% Nafion) were loaded onto the clean Ni foam as the working electrodes, and the loading amount is corresponding with that of the fore-mentioned samples. Potentials described during this work without any special statement were normalized versus the reversible hydrogen electrode (RHE), according to the equation: ERHE = Eaoolied + 0.241 + 0.059pH. Where the Eapplied is the applied potential. Electrochemical performances for overall water splitting were tested in a two-electrode system. The prepared Ni2P/NF and NiFe LDH samples were worked as cathode and anode, respectively. For comparison, noble metal catalysts Pt/C and Ir/C were also used as cathode and anode under the same conditions. The amount of produced H2 and O2 was collected via a commercial two-electrode water splitting setting using drainage method. The Faradic efficiency was calculated by comparing the measured and theoretical produced amounts of H2. To simulate industrial application, the fresh prepared electrolyte was used directly without any further degassing period during the electrochemical tests. Polarization curves were performed via sweeping the potentials at a rate of 10 mV s-1. The corresponding Tafel plots were obtained by fitting the linear portion between the overpotential (η) and log current (log j), using the equation: η = b log (j) + a, where b is the Tafel slope. Electrochemical impedance spectroscopy (EIS) test were acquired ranging from 100 kHz to 0.1 Hz at the amplitude of 5 mV. Doule-layer capacitance (Cdl) data were collected via the cyclic voltammetry (CV) curves measured, the potentials were between 0.067 V and 0.167 V and the scanning rates were from 20 mV s-1 to100 mV s-1. The stability performances were examined through various electrochemical measurements including accelerated cycling polarization curves with a rate of 100 mV s-1, multi-potential steps tests, multi-current steps tests, current density-time curves at a constant potential and potential-time curves at a constant current density. All the

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Journal of the American Chemical Society measured potentials were calibrated with iR compensation if not explicitly specified, where R is the solution resistance. H2 amount measurements. H2 amount for Ni2P||Ni-Fe LDH was tested via a Hoffman apparatus setup in a two-electrode system at room temperature. The electrolyte was 1 M KOH aqueous solution. In order to detect the H2 amount more accurately, methyl orange was added into the electrolyte (0.1 mg/mL) to observe a more obvious colour.

(5) Zhang, B.; Zheng, X. L.; Voznyy, O.; Comin, R.; Bajdich, M.;

ASSOCIATED CONTENT

(6) Chen, G. B.; Wang, T.; Zhang, J.; Liu, P.; Sun, H. J.; Zhuang, X.

Supporting Information. The Supporting Information including the additional SEM, TEM, EDX, Mapping and HRTEM images, XRD and Raman patterns, XPS spectra, and additional electrochemical data is available free of charge via the Internet at http://pubs.acs.org.

D.; Chen, M. W.; Feng, X. L. Accelerated hydrogen evolution kinetics on NiFe-layered double hydroxide electrocatalysts by tailoring water dissociation active sites. Adv. Mater. 2018, 30, 1706279.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] * [email protected]

Garcia-Melchor, M.; Han, L. L.; Xu, J. X.; Liu, M.; Zheng, L. R.; de Arquer, F. P. G.; Dinh, C. T.; Fan, F. J.; Yuan, M. J.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P. F.; Li, Y. H.; De Luna, P.; Janmohamed, A.; Xin, H. L. L.; Yang, H. G.; Vojvodic, A.; Sargent, E. H. Homogeneously dispersed multimetal oxygen-evolving catalysts.

Science 2016, 352, 333.

(7) Masa, J.; Sinev, I.; Mistry, H.; Ventosa, E.; de la Mata, M.; Arbiol, J.; Muhler, M.; Roldan Cuenya, B.; Schuhmann, W. Ultrathin high surface area nickel boride (NixB) nanosheets as highly efficient electrocatalyst for oxygen evolution. Adv. Energy Mater. 2017, 7, 1700381. (8) Yin, J.; Li, Y. X.; Lv, F.; Lu, M.; Sun, K.; Wang, W.; Wang, L.;

Notes

Cheng, F. Y.; Li, Y. F.; Xi, P. X.; Guo, S. J. Oxygen vacancies dominated NiS2/CoS2 interface porous nanowires for portable Zn-Air batteries driven water splitting devices. Adv. Mater. 2017, 29,

The authors declare no competing financial interests.

1704681.

ACKNOWLEDGMENT

(9) Chang, K.; Hai, X.; Pang, H.; Zhang, H. B.; Shi, L.; Liu, G. G.;

This work was funded by National Natural Science Foundation of China (Grants 21521001, 21431006, 21225315, 21321002, 91645202), Chinese Academy of Sciences (Grants KGZD-EW-T05, XDA090301001), Key Research Program of Frontier Sciences, CAS(Grant QYZDJ-SSW-SLH036), Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS(2015HSCUE007), Fundamental Research Funds for the Central Universities (WK2060190045, WK2340000076), Recruitment Program of Global Youth Experts, Australian Research Council (ARC), ARC DECRA project (DE180100036) and Discovery Projects (DP160104340, DP170100436, DP180100077).

Liu, H. M.; Zhao, G. X.; Li, M.; Ye, J. H. Targeted synthesis of 2H-and 1T-phase MoS2 monolayers for catalytic hydrogen evolution. Adv. Mater. 2016, 28, 10033. (10) Zheng, Y. R.; Wu, P.; Gao, M. R.; Zhang, X. L.; Gao, F. Y.; Ju, H. X.; Wu, R.; Gao, Q.; You, R.; Huang, W. X.; Liu, S. J.; Hu, S. W.; Zhu, J. F.; Li, Z. Y.; Yu, S. H. Doping-induced structural phase transition in cobalt diselenide enables enhanced hydrogen evolution catalysis. Nat. Commun. 2018, 9, 2533. (11) Jiang, N.; You, B.; Sheng, M. L.; Sun, Y. J. Electrodeposited cobalt-phosphorous-derived films as competent bifunctional catalysts for overall water splitting. Angew. Chem. Int. Ed. 2015, 54, 6251. (12) Xie, L. S.; Qu, F. L.; Liu, Z.; Ren, X.; Hao, S.; Ge, R. X.; Du,

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