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Sep 6, 2017 - In terms of the practical applications of water splitting, a .... (RHE); and (d) long-term chronoamperometry of mp-Ni2P under a static c...
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Large-Area Synthesis of a Ni2P Honeycomb Electrode for Highly Efficient Water Splitting Xu-Dong Wang, Yang Cao, Yuan Teng, Hong-Yan Chen,* Yang-Fan Xu, and Dai-Bin Kuang* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P. R. China S Supporting Information *

ABSTRACT: Transition metal phosphides have recently been regarded as robust, inexpensive electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Thus far, tremendous scientific efforts have been applied to improve the catalytic activity of the catalyst, whereas the scale-up fabrication of morphology-controlled catalysts while maintaining their desired performance remains a great challenge. Herein, we present a facile and scalable approach to fabricate the macroporous Ni2P/nickel foam electrode. The obtained electrocatalyst exhibits superior bifunctional catalytic activity and durability, as evidenced by a low overpotential of 205 and 300 mV required to achieve a high current density of 100 mA cm−2 for HER and OER, respectively. Such a spray-based strategy is believed to widely adapt for the preparation of electrodes with uniform macroporous structures over a large area (e.g., 100 cm2), which provides a universal strategy for the mass fabrication of high performance water-splitting electrodes. KEYWORDS: nickel phosphide, macroporous, large area, bifunctional electrocatalyst, water splitting

1. INTRODUCTION With the ongoing concerns regarding energy and environmental issues, increasing interest has been devoted to the development of a renewable energy resource and utilization technology. One of the fast-developing technologies for storing energy and further converting it to fuel is electrolysis of water to hydrogen.1 Pt/C and RuO2 are currently recognized as the state-of-the-art electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively.2 Nonetheless, the scarcity and the consequential high cost considerably impede their practical application on a large scale. Accordingly, substantial scientific communities have been driven to seek for alternative low-cost and efficient nanostructured water-splitting electrocatalysts, such as transition metal sulphides,3 nitrides, 4 phosphides, 5−12 and selenides.13,14 Recently, nickel phosphide has been found to act as an efficient alternative catalyst,15,16 by virtue of its unique electronic properties and superior corrosion resistance. Tailoring the surface morphology can facilitate to expose more active sites of catalysts, which is pivotal to develop materials with outstanding performances for water splitting.17−20 Therefore, a number of chemical transformation strategies have been carried out to prepare various nanostructured nickel phosphides, such as solvothermal method and phosphidation of nanostructured metal oxides/hydroxides.16,21−32 Among the diverse micro/nano architectures, the macroporous-structured catalyst is a good choice because the interior hollow space can not only provide a large specific surface area but also facilitate the infiltration and diffusion of the electrolyte.33−35 © 2017 American Chemical Society

In terms of the practical applications of water splitting, a high-quality large-area electrode with catalysts directly anchored on the current collector is required.36 Devices that combine flexibility with high mechanical stability can effectively reduce the disintegration tendency of the catalyst layer, remitting the impact of bubble accumulation. Although stunning progress has been made on metal phosphides, most of the previous research studies were focused mainly on the small-scale catalyst in the laboratory scale.37 Sandwiching the active particles between two current collector sheets (such as Ni foam and Ti foil) has been proposed to fabricate large-area electrodes,38,39 which, however, cannot satisfy the water splitting requirements such as high stability and good performance. Thus, there is a pressing need for a universal and facile fabrication strategy to prepare morphologycontrolled catalysts in a large area while maintaining the desired performance.37,40 Here, we report three-dimensional (3D) honeycomb monolithic catalysts consisting of macroporous Ni2P (mpNi2P) coated on nickel foam by a facile and easily scalable method, where Ni precursors and polystyrene spheres (PS) are spray-coated on nickel foam, followed by the annealing and phosphidation process at 300 °C. Such a hollow macroporous structure possesses many unique merits, including facilitating the infiltration and diffusion of the electrolyte,33−35 exposing more active sites and reducing the internal interfacial resistance. Received: July 24, 2017 Accepted: September 6, 2017 Published: September 6, 2017 32812

DOI: 10.1021/acsami.7b10893 ACS Appl. Mater. Interfaces 2017, 9, 32812−32819

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Synthesis Procedure of the Ni2P Honeycomb Monolithic Electrode

Figure 1. Typical FE-SEM images of PS@NiCl2/Ni foam (a−c) and mp-NiO/Ni foam (d−f). foam as aforementioned. Next, the final products were washed thoroughly with 0.5 M H2SO4 and deionized water to remove the impurities. The average mass loading of mp-Ni2P is about 2 mg cm−2, which was determined by weighting Ni foam before and after catalyst loading using an analytical balance with a nominal precision of 0.1 mg. For comparison, approximately 4 mg cm−2 commercial Pt/C and RuO2 on Ni foam electrode was prepared using Nafion as a binder. 2.3. Structural and Electronic Property Characterization. The phase structure of samples was analyzed by an X-ray powder diffractometer [Rigaku Co.]. The morphology images were examined on the field emission scanning electron microscope [(SEM); Hitachi SU8010 SEM]. High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were obtained from TEM (Titan G2 60-300). Elemental mapping images were recorded by energy-dispersive X-ray spectroscopy (EDX) attached to the Titan G2 60-300 microscope. The surface chemical composition of samples was analyzed by X-ray photoelectron spectroscopy [(XPS), ESCALAB 250, Thermo Fisher Scientific]. 2.4. Electrochemical Measurements. The electrochemical measurements were conducted on a CHI660E potentiostat in a three-electrode cell using a graphite rod as a counter electrode and saturated Ag/AgCl (EAg/AgClθ = 0.1976 V) as a reference electrode. The polarization curves were obtained by linear sweep voltammetry (LSV) in the N2 saturated aqueous solution of 1.0 M KOH or 0.5 M H2SO4 with a scan rate of 2 mV s−1. Unless otherwise noted, all the potentials were iR-corrected. The determination of double layer capacitance (Cdl) of mp-Ni2P and bulk Ni2P was performed by cyclic voltammetry at different scan rates (10−100 mV s−1) in the potential range of −0.1 to 0.1 V versus a reversible hydrogen electrode (RHE). Electrochemical impedance spectroscopic (EIS) measurements were conducted at a fixed voltage −100 mV versus the RHE, the frequency range of which was 0.05 Hz to 100 kHz. The long-term durability test was conducted by potential cycling from +1.2 to +1.5 V and −0.2 to +0.1 V, versus

Thus, the obtained 3D mp-Ni2P honeycomb-based electrode delivers a superior bifunctional catalytic activity and durability as evidenced by the low overpotential of 205 and 300 mV required to achieve 100 mA cm−2 for the HER and the OER, respectively.

2. EXPERIMENTAL SECTION 2.1. Materials. Nickel chloride hexahydrate (NiCl2·6H2O) and citric acid monohydrate (C6H8O7·H2O) were all obtained from Tianjin Baishi Chemical Industry Co. Ltd. Ni foam was purchased from Shanxi Lizhiyuan Battery Materials Co. Ltd. 5 wt % Nafion solution was purchased from Sigma-Aldrich. Styrene was purchased from Tianjin Fu Chen Chemical Reagent Factory. Sodium hypophosphite (NaH2PO2) was purchased from Aladdin Industrial Corporation. All chemicals used in this experiment were of the analytical grade and used as received without further purification. 2.2. Catalyst Synthesis. Coating NiCl2 layers on the PS latex with a diameter of ∼600 nm to obtain the core−shell structure PS@NiCl2 particles was performed by a simple mixing and spray-coating process. Briefly, 10 mL of the PS34 latex was added to a 50 mL ethanol solution of 40 mM citric acid and 40 mM NiCl2. The resultant solution was deposited onto Ni foam (100 cm2) by the spray-coating method. During the coating process, the substrate was heated to 100 °C on a hot plate to accelerate the solvent evaporation. Subsequently, the asprepared sample was calcined at 300 °C for 30 min to obtain the macroporous NiO on Ni foam (mp-NiO/Ni foam). To synthesize the macroporous Ni2P/nickel (mp-Ni2P/Ni) foam sample, appropriate NaH2PO2 was placed at the front zone of the tube furnace, and the asprepared mp-NiO/Ni foam was put at the back zone. Subsequently, mp-NiO/Ni foam was heated up to 300 °C and the NaH2PO2 was heated up to 250 °C, holding at the temperatures for 90 min under a static N2 atmosphere. The similar phosphidation processes for bulk Ni2P were carried out by direct phosphorization of commercial nickel 32813

DOI: 10.1021/acsami.7b10893 ACS Appl. Mater. Interfaces 2017, 9, 32812−32819

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ACS Applied Materials & Interfaces

Figure 2. General characterization of the mp-Ni2P nanostructures. (a−c) SEM images, (d−f) TEM and HRTEM images, (g) HAADF−STEM image of mp-Ni2P, and (h,i) element mapping images of Ni and P in the area shown in (g).

Figure 3. (a) HER polarization curves (iR-corrected) of mp-Ni2P, bulk Ni2P, Ni foam, and Pt/C in 0.50 M H2SO4 electrolytes at 2 mV s−1; (b) corresponding Tafel plots of different catalysts; (c) polarization curves of mp-Ni2P before and after 2500 CV cycles between −0.2 and +0.1 V (RHE); and (d) long-term chronoamperometry of mp-Ni2P under a static current density of −10 mA cm−2 for 24 h (none iR-corrected). the RHE at a scan rate of 100 mV s−1 for the OER and HER stability, respectively. Prior to the experiments, all electrodes were sealed by epoxy resin to define the electrode area (0.3 cm2). Chronopotentiometry experiments were conducted on an FL-9790 II instrument.

and Ti foil; herein, a typical sample of 100 cm2 using an Ni foam substrate was prepared as an illustration (Movie S1). A subsequent annealing treatment at 300 °C for 30 min in air was applied to convert the Ni-based precursor into NiO macropores (mp-NiO), after which an in situ P/O exchange process at 300 °C was carried out to obtain mp-Ni2P. As shown in Figure S1, the color of Ni foam turned brown from gray after NiO coating, and changed to black after the phosphidation process, indicating the formation of Ni2P. Such a simple procedure is highly desirable for the realization of a large-area catalyst electrode for hydrogen production. With the aid of the SEM characterization (Figure 1a−c), spheres of PS@NiCl2 with a diameter of about 600 nm were clearly observed, which were distributed uniformly on the

3. RESULTS AND DISCUSSION 3.1. Physical Structure and Morphological Characterizations. The Ni precursor ink was first prepared by mixing 40 mM NiCl2, 40 mM citric acid, and 10 mL PS latex (with diameter of ∼600 nm) in 50 mL ethanol solution, which was then deposited onto nickel foam by automatic spray coating, as illustrated in Scheme 1. Such a simple approach provides a valid strategy for large-area fabrication of the electrode on different conductive substrates such as Ni foam, fluorine-doped tin oxide, 32814

DOI: 10.1021/acsami.7b10893 ACS Appl. Mater. Interfaces 2017, 9, 32812−32819

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ACS Applied Materials & Interfaces

Figure 4. (a−c) Cyclic voltammograms at various scan rates to estimate the Cdl of (a) mp-Ni2P and (b) bulk Ni2P; (c) the capacitive current density at 0 V vs the RHE as a function of the scan rate. (d) Nyquist plots of mp-Ni2P and bulk Ni2P at −100 mV (RHE) in 0.5 M H2SO4.

atomic ratio of Ni/P was lower than 2, revealing that a P-rich surface was formed on the as-prepared mp-Ni2P electrocatalyst.43 All these elaborated that a 3D honeycomb electrode consisting of macroporous Ni2P coated on nickel foam has been successfully prepared. 3.2. Electrochemical Performance. The Ni2P honeycomb can be directly employed as a catalytic electrode for water splitting. The electrocatalytic performance of the “reference” Pt/C, bulk Ni2P, mp-Ni2P, and Ni foam substrate in 0.5 M H2SO4 was compared by polarization curves and Tafel plots (Figure 3a,b). It is obvious that Pt/C possesses an excellent HER activity with a negligible onset overpotential. As determined from polarization curves, large overpotentials of 355 and 468 mV for the N were required to reach 20 and 100 mA cm−2, respectively. By contrast, the mp-Ni2P electrode exhibited a higher activity than bulk Ni2P at current densities of 20 and 100 mA cm−2 (η20, 140 mV vs 198 mV; η100, 205 mV vs 265 mV), which are comparable or even superior to many transition metal sulfides and phosphides reported in the literature (Table S1). The resulted impressive current densities with mp-Ni2P highlighted that the macroporous structure is of critical importance with regard to obtaining a high HER performance. It is noteworthy that the mp-Ni2P electrode performed steadily (suggested by the smooth J−V curve) while the reduction current of the bulk Ni2P and Ni foam electrode fluctuated obviously, suggesting that the hollow architecture of mp-Ni2P/Ni foam can facilitate a rapid release of gas bubbles and provide a stable working area. The electrical resistance of the whole system was significantly increased when gas bubbles formed on the surface of the electrode because of the reduced contact area between the electrolyte and the electrode, thus blocking the interfacial electron transfer.22 The Tafel slope (Figure 3b) of mp-Ni2P fitted from the polarization curve was 68.9 mV dec−1, which was designated to the Volmer reaction. This value was smaller than that of bulk Ni2P and commercial Ni foam (83.3 and 118.2 mV dec−1, respectively). The durability of mp-Ni2P was subsequently assessed using the accelerated durability test protocol by cycling the catalysts

surface of Ni foam. After removing the PS template via an annealing process, macroporous-structured NiO (mp-NiO) with a smooth surface was obtained (Figure 1d−f). The diameter of the open pore from the top view of the SEM image is ∼400 nm; however, the real pore size of the same micropore would be larger than 400 nm, which has also been observed by other reports.41 Interestingly, the macroporous structure of NiO was wellmaintained after phosphidation, as confirmed by SEM and TEM images (Figure 2a−d). The X-ray powder diffraction (XRD) pattern (Figure S2) confirmed that the resulted sample was the Ni2P phase with a hexagonal structure (JCPDS no. 030953). From the HRTEM image (Figure 2e), interconnected nanoparticles (∼16 nm, Figure S3) were observed on the macroporous skeleton of mp-Ni2P. Comparing with the porefree bulk Ni2P electrode (bulk Ni2P/Ni foam, Figure S4) fabricated by directly phosphorizing the commercial nickel foam current collector, such a rough and loose structure would greatly enlarge the active surface area. The high crystallinity of the Ni2P particle was confirmed with the HRTEM image (Figure 2f), and the observed interplanar distance of 0.209 nm was consistent with the (201) plane of Ni2P. Energy-dispersive X-ray (EDX) mappings (Figure 2g−i) verified that Ni and P distributed homogeneously across the macroporous structure. The chemical composition and elemental chemical states of mp-Ni2P was investigated by XPS. As plotted in Figure S5, the XPS peaks at 853.0 and 870.4 eV suggested the presence of Niδ+ (0 < δ < 2) in Ni2P. In addition, the peak at 856.9 eV for Ni 2p3/2 indicated that Ni(OH)2 was formed as a result of the surface passivation in air. The O 1s band in mp-Ni2P was deconvoluted into two peaks. The low energy one (at 531.9 eV) was ascribed to the −OH species adsorbed on the surface by surface hydroxides, whereas the peak at a higher binding energy (at 533.4 eV) was assigned to the chemisorbed oxygen.42 Moreover, the P 2p3/2 peak at 129.5 eV indicated that P was bonded to Ni in the form of a metal phosphide. The other peak at 134.4 eV reflected the oxidized phosphorus species. The quantitative analysis of XPS data showed that the 32815

DOI: 10.1021/acsami.7b10893 ACS Appl. Mater. Interfaces 2017, 9, 32812−32819

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ACS Applied Materials & Interfaces

Figure 5. (a) OER polarization curves (iR-corrected) of mp-Ni2P, bulk Ni2P, commercial Ni foam, and RuO2 in 1.0 M KOH electrolytes. (b) Corresponding Tafel plots of different catalysts; (c) polarization curves of mp-Ni2P before and after 2500 CV cycles between +1.2 and +1.5 V (RHE); and (d) long-term chronoamperometry of mp-Ni2P under a static current density of 10 mA cm−2 for 12 h in 1 M KOH (none iR-corrected).

between +100 and −200 mV at 100 mV s−1 under a N2 atmosphere. After 2500 continuous cycles, a negligible difference was observed in the polarization curve compared with the initial one (Figure 3c). In addition, the long-term stability of mp-Ni2P was also tested at 10 mA cm−2 for 24 h, the HER catalytic activity of mp-Ni2P/Ni foam decays gradually because of the disintegration of the catalyst during the gas production process. A slight potential increase of 67 mV (at −10 mA cm−2) within 24 h was observed (Figure 3d). After the long-term stability electrochemical HER process, the crystallization structure of mp-Ni2P was analyzed by XRD. As illustrated in Figure S6, the XRD peaks remained the same, indicating its chemical stability. The surface morphology and microstructure of the mp-Ni2P electrode were detected with SEM (Figure S7a,b), where the macroporous structure was retained well after the long duration test for 24 h. Clear lattice fringes with an interplanar distance of 0.220 nm appeared in the high-resolution TEM (HRTEM) images (Figure S7c,d), which corresponded to the (111) plane of Ni2P. Furthermore, XPS (Figure S8) results of Ni2P after the 24 h HER confirmed the retention of its chemical composition. The above results unambiguously demonstrated that as-prepared mp-Ni2P possesses an impressively high chemical stability under the current condition. It is worthwhile to note that the alkaline electrolysis is a commercially used method for large-scale hydrogen production44,45 because the corrosion process is accelerated in the acidic electrolysis technique. However, most electrocatalysts currently delivered barely satisfactory HER performances in alkaline media, compared with those in acid media.46 Accordingly, the catalyst activity of mp-Ni2P was further evaluated in basic media. As shown in Figure S9, mp-Ni2P exhibited a remarkable HER performance with a small Tafel slope of 71.6 mV dec−1. An overpotential of 255 mV was needed for the mp-Ni2P electrode to achieve a current density of 100 mA cm−2, which was superior to that of bulk Ni2P. The above-mentioned results suggest that the honeycomblike hollow architecture of the mp-Ni2P/Ni foam electrode was

the key parameter that contributes to the excellent HER catalytic performance which provided an excellent electric interconnection with improved mass transport and abundant catalytic sites. To estimate the electrochemical active area of different electrodes, the Cdl measurements were conducted (Figure 4a−c). The Cdl of 32.5 mF cm−2 for mp-Ni2P is much higher than that of bulk Ni2P (17.0 mF cm−2), revealing a higher electrochemical active area in mp-Ni2P. The calculated turnover frequency according to the previous method18 is 0.10 s−1 at η = 200 mV, close to that of NiMoP2,19 indicating that Ni2P has a good intrinsic activity. Moreover, to confirm the positive effect of the macroporous structure on the catalytic kinetics of the Ni2P samples, EIS was further performed (Figure 4d). The mp-Ni2P/Ni electrode presented the smaller semicircle compared to that of bulk Ni2P, indicating its excellent active HER kinetics. These results indicated that the 3D mp-Ni2P honeycomb electrode could function as an efficient HER catalyst in both acidic and alkaline electrolytes. Recently, metal phosphides as emerging OER catalysts have been exhibiting excellent electrochemical activity.47−51 An oxide/hydroxide overlayer can in situ form on their surfaces during the catalytic process while metal phosphides with good electrical conductivity remain in the core. Such an interface between the phosphide and oxide/hydroxide helps to provide better carrier transportation from the core MPx to outer MOx. Herein, the electrocatalytic activity of mp-Ni2P/Ni foam, bulk Ni2P/Ni foam, and Ni foam for the OER was also investigated. As shown in Figure 5, commercial RuO2 exhibits an excellent OER activity, whereas Ni foam displays poor activity. Surprisingly, mp-Ni2P/Ni foam demanded an overpotential of 230 and 300 mV to achieve a catalytic current density of 20 and 100 mA cm−2 (Figure 5a), which are much lower than those required for bulk Ni2P (η20, 276 m; η100, 354 mV), commercial RuO2, and other noble metal free electrocatalysts (Table S1). The derived Tafel slope (Figure 5b) for mp-Ni2P was 84.6 mV dec−1, which was lower than those of the RuO2 catalyst (90.0 mV dec−1) and bulk Ni2P (104.7 mV dec−1). Aside from the 32816

DOI: 10.1021/acsami.7b10893 ACS Appl. Mater. Interfaces 2017, 9, 32812−32819

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ACS Applied Materials & Interfaces

effectively reduce the diffusion length of electrons and ions but also provide more catalytic active sites, thus leading to high current density for the HER and the OER, low Tafel slope, and long stability. Additionally, the spray-based strategy is facile and reproducible to fabricate large-area electrodes with uniform macroporous structures, which may stimulate future development of various materials in a large scale for water electrolysis and other catalytic applications.

catalytic activity, the long-term stability is another important criterion for evaluating an advanced electrocatalyst. The cycling stability of the mp-Ni2P catalyst, cyclic voltammetry (CV) measurement from 1.2 to 1.5 V versus the RHE was conducted continuously for 2500 cycles. As can be seen in Figure 5c, the polarization data showed no significant change compared to the initial one. Meanwhile, the long-term electrochemical stability of the mp-Ni2P electrode was further confirmed by the continuous oxygen evolution for 12 h at a constant current density of 10 mA cm−2 (Figure 5d). After the 12 h test, main XRD diffraction peaks of Ni2P were retained, whereas the diffraction peaks were obviously broadened (Figure S6), indicating a decrease in the catalyst crystallite size during the OER process. As evidenced by their SEM and TEM images (Figure S10a−c), the macroporous feature of the sample was still maintained while the original nanoparticles (16 nm) were splintered into smaller ones. Note that the XPS spectrum of mp-Ni2P after the OER electrolysis showed two new peaks at 856.8 and 874.7 eV for NiOOH (Figure S11), which is the most probable active species for the OER.42 An apparent oxidation peak at 1.35 V versus the RHE in the polarization curve of mp-Ni2P and bulk Ni2P (Figure 5a) ascribed to the process of Ni2+ to Ni3+52 can also prove the formation of NiOOH. To confirm the formation of NiOOH overlayer in situ on the surfaces of mp-Ni2P and bulk Ni2P during the catalytic process, HRTEM was conducted. As shown in Figure S10d, lattice spacing of 0.219 nm ascribing to the (111) crystallographic plane of Ni 2P (JCPDS no. 03-0953) can be distinguished. Meanwhile, an ultrathin noncrystalline layer can be clearly observed, indicating the formation of an amorphous NiOOH layer on the surface of Ni2P. As a result, mp-Ni2P exhibited excellent electrochemical performance and a long-term stability for the OER process, which was ascribed to abundant active NiOOH at the surface and good conductivity from the Ni2P core. In view of the good catalytic activity of mp-Ni2P/Ni foam for both the OER and the HER, the LSV profile in the twoelectrode system was investigated, in which both the anode and the cathode were composed of mp-Ni2P. As shown in Figure S12, an overpotential required to reach a current density of 10 mA cm−2 was 440 mV (none iR-correction). A single battery (1.5 V) can drive a two-electrode electrolyzer, where H2 bubbles along with O2 bubbles can be observed at the cathode and the anode (Figure S13). Meanwhile, from the gas chromatography spectra analysis and the i−t curve, the experimentally generated amount of H2 and O2 was very close to the theoretical values, indicating that faradaic efficiency is almost 100% for the bifunctional Ni2P electrodes in alkaline electrolytes (Figure S14). The achieved electrocatalytic performance of mp-Ni2P here has outperformed that of many Ni-based bifunctional water-splitting electrocatalysts (Table S1). Such excellent performances are attributed to the increased active sites of Ni2P supplied by the 3D macroporous architecture and the strong interaction between the conductive scaffold of Ni foam and the catalyst. Based on the above results, it is obvious that the mp-Ni2P/Ni foam electrode holds a great promise for its practical application in alkaline water electrolysis.

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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/acsami.7b10893. Details of XRD, XPS, SEM, TEM, and the electrochemical performances of the devices (PDF) Deposition of Ni precursor ink onto the nickel foam (100 cm2) substrate by automatic spray coating (AVI)

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AUTHOR INFORMATION

Corresponding Authors

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

Hong-Yan Chen: 0000-0001-6704-1665 Yang-Fan Xu: 0000-0002-4479-6157 Dai-Bin Kuang: 0000-0001-6773-2319 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial supports from the National Natural Science Foundation of China (21673302), GDUPS (2016), the Pearl River S&T Nova Program of Guangzhou (2014J2200025), the Program of Guangzhou Science and Technology (201504010031), and the Fundamental Research Funds for the Central Universities.

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REFERENCES

(1) Mallouk, T. E. Water Electrolysis: Divide and Conquer. Nat. Chem. 2013, 5, 362−363. (2) Wang, J.; Yang, W.; Liu, J. CoP2 Nanoparticles on Reduced Graphene Oxide Sheets as a Super-efficient Bifunctional Electrocatalyst for Full Water Splitting. J. Mater. Chem. A 2016, 4, 4686− 4690. (3) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R.; Liu, S.; Zhuang, X.; Feng, X. Interface Engineering of MoS 2 /Ni 3 S 2 Heterostructures for Highly Enhanced Electrochemical OverallWater-Splitting Activity. Angew. Chem., Int. Ed. 2016, 55, 6702−6707. (4) Jia, X.; Zhao, Y.; Chen, G.; Shang, L.; Shi, R.; Kang, X.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. Ni3FeN Nanoparticles Derived from Ultrathin NiFe-Layered Double Hydroxide Nanosheets: An Efficient Overall Water Splitting Electrocatalyst. Adv. Energy Mater. 2016, 6, 1502585. (5) Ledendecker, M.; Calderón, S. K.; Papp, C.; Steinrück, H.-P.; Antonietti, M.; Shalom, M. The Synthesis of Nanostructured Ni5P4 Films and their Use as a Non-Noble Bifunctional Electrocatalyst for Full Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 12361−12365. (6) Yu, X.; Zhang, S.; Li, C.; Zhu, C.; Chen, Y.; Gao, P.; Qi, L.; Zhang, X. Hollow CoP Nanopaticle/N-doped Graphene Hybrids as Highly Active and Stable Bifunctional Catalysts for Full Water Splitting. Nanoscale 2016, 8, 10902−10907. (7) Wu, T.; Chen, S.; Zhang, D.; Hou, J. Facile Preparation of Semimetallic MoP2 as a Novel Visible Light Driven Photocatalyst with High Photocatalytic Activity. J. Mater. Chem. A 2015, 3, 10360−10367.

4. CONCLUSIONS A honeycomb catalyst, macroporous Ni2P coated on 3D conductive Ni foam, is successfully synthesized by a facile and reproducible method. Especially, porous Ni2P can not only 32817

DOI: 10.1021/acsami.7b10893 ACS Appl. Mater. Interfaces 2017, 9, 32812−32819

Research Article

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DOI: 10.1021/acsami.7b10893 ACS Appl. Mater. Interfaces 2017, 9, 32812−32819