Carbon-Coated Nickel Phosphide Nanosheets as Efficient Dual

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Carbon-Coated Nickel Phosphide Nanosheets as Efficient Dual-Electrocatalyst for Overall Water Splitting Jing Yu, Qianqian Li, Na Chen, Cheng-Yan Xu, Liang Zhen, Jinsong Wu, and Vinayak P. Dravid ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10552 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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ACS Applied Materials & Interfaces

Carbon-Coated Nickel Phosphide Nanosheets as Efficient Dual-Electrocatalyst for Overall Water Splitting Jing Yu †, ‡, Qianqian Li †,, Na Chen ‡, Cheng-Yan Xu ‡,*, Liang Zhen ‡, Jinsong Wu †,*, Vinayak P. Dravid † †

Department of Materials Science and Engineering, NUANCE Center, Northwestern

University, Evanston, IL 60208, USA. ‡

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin

150001, China. Corresponding Authors: * [email protected], [email protected]

ABSTRACT Low-cost and efficient electrocatalysts for overall water splitting are in high demand for a wide range of applications across renewable and clean energy. Here, we report a simple one-step synthesis of three-dimensional (3D) carbon-coated Ni8P3 nanosheet array as bifunctional catalyst for both hydrogen evolution reactions (HER) and oxygen evolution reactions (OER). The nanosheet array possesses low overpotentials, high current densities and small Tafel slopes in both HER and OER, and shows high electrocatalytic activities and long-term stability. The carbon layer with high electric conductivity serves not only as a protective layer to prevent Ni8P3 dissolution, but also as an active layer to decrease the electrocatalysis overpotential. 1

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The nanosheet array has HER outstanding activity in both acid and alkaline media. Its superior performance in OER can be due to the synergistic interaction at the Ni8P3/NiOx heterojunction. Furthermore, cell voltage as low as 1.65 V can achieve 10 mA cm−2 current density for full water splitting in an alkaline water electrolyzer, indicating potential application of C@ Ni8P3 as bifunctional catalyst for clean and renewable energy utilization. Keywords: nickel phosphide, nanosheets array, carbon coating, overall water splitting, synergistic effect

1. INTRODUCTION Water electrolysis by splitting H2O into hydrogen and oxygen is currently recognized as attractive method for developing regenerative hydrogen energy and fuel cells, which could in turn help resolve the energy crisis and environmental pollution caused by overreliance on fossil fuels.1-6 This electricity-driven process can be divided into two half reactions, namely, a four-electron oxygen evolution reaction (OER) and a two-electron hydrogen evolution reaction (HER).7-9 Developing efficient electrocatalysts is critical for reducing the energy barrier and accelerating the reaction kinetic. Currently, noble-metal based catalysts perform best, for example, Pt-based materials for HER and Ir/Ru-based for OER; however, large-scale application is limited by the high cost and low reserves of these materials.10,11 Consequently, there is an urgent need to investigate potential non-precious electrocatalysts with high activity and long-term stability. Recently, the first row transition-metal-based materials, including oxides12-14, sulfides15-17, selenides18,19 and nitrides20, are found to perform 2


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well in the HER or OER processes. Generally, HER operates favorably in acid solution, while OER in basic media. However, for practical application, full water splitting including both HER and OER should be conducted in the same alkaline electrolyte. This remains a significant challenge, especially to sustain high activity for both cathode and anode.21 Thus, discovery and development of efficient bifunctional catalysts, able to be used in the same electrolyte, is highly desirable. Transition metal phosphides (TMPs), such as Ni-P22,23, Co-P24-26 and Fe-P27,28 as potentially novel alternative earth-abundant catalysts, have recently drawn intense attention because of their outstanding electrocatalytic performance as bifunctional catalysts for overall water splitting. The high activity of TMPs originates mainly from moderate bonding between phosphorus in the structure and reaction intermediates, leading to an appropriate surface structure as proton acceptor and hydride acceptor sites.22,29 During the electrocatalysis, zero-dimensional (0D) and one-dimensional (1D) structures normally resulted in reduced catalytic ability due to uncontrolled agglomeration under high current density or long-term measurement.30 Moreover, the electrical conductivity of catalyst plays a critical role in their electrochemical performance. The direct growth of 3D interconnected electrocatalysts nanoarrays on current collectors is an efficient approach to address these problems. For example, NiP2 nanosheet arrays on carbon cloth were constructed and exhibited superior HER activity with strong durability in both acid and alkaline solutions.31 This strategy endows good electrical contact and strong incorporation between the catalyst and support, as well as sufficient space for mass transfer and gas bubbles, resulting in high 3



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activity and structural stability.25 Nevertheless, the catalyst surfaces inevitably corroded after a prolonged electrocatalytic process. Here, we report a one-step synthesis of 3D carbon-coated nickel phosphides nanosheets array on the surface of Ni foam via a simple solvothermal procedure. The covered C shell can serve as a protective layer to prevent Ni8P3 dissolution and further enhance their electronic conductivity. Meanwhile, the synergic interaction between active catalyst and carbon help to reduce the adsorption energies of some catalytic species and decrease the required overpotential for the electrocatalysis, improving their catalytic activity.32,33 As expected, C@Ni8P3 exhibits excellent HER activity and extraordinary duration in both acid and basic media. In addition, the superior OER performance with an overpotential of 267 mV at j = 10 mA cm-2 and a Tafel slope of 51 mV dec-1 is superior to the most active non-precious OER catalysts. When employed in an alkaline water electrolyzer as both cathode and anode, C@Ni8P3 possesses high efficiency and long-term stability, indicating low-cost and high-performance electrocatalyst for full water splitting.

2. EXPERIMENTAL SECTION 2.1 Catalyst synthesis All reagents were used as received without any purification. Red phosphorus (≥ 99.99%), D-(+) glucose (≥ 99.5%), triethylene glycol (TEG, ≥ 99.0%) and polyvinyl pyrrolidone (PVP, Mw = 55000) were purchased from Sigma–Aldrich. Nickel foam was cleaned by ultrasonication treatment in 3 M HCl, ethanol and deionized water, respectively, to remove impurities and oxide layer. In a typical synthesis of C@Ni8P3 4


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nanosheet array, 0.05 g red phosphorus, 0.2 g glucose and 0.5 g PVP were dispersed into 30 mL TEG, followed by vigorously magnetic stirring for at least 0.5 h until homogeneous dispersion of red phosphorus. Then the suspension solution was transferred into a 50 mL Teflon-lined autoclave with a piece of cleaned Ni foam (1 cm × 3 cm) added. The autoclave was sealed and maintained at 220 °C for 12 h. The resulting Ni foam was washed with deionized water and ethanol several times, and then dried at room temperature, named as C@Ni8P3. Excessive C coated Ni8P3 was obtained with 0.5 g glucose added, which was recorded as C@Ni8P3-2. For comparison, Ni8P3 nanosheets were also prepared without the addition of glucose. The mass loadings were determined using microbalance and calculated to be about 1.7, 1.9 and 2.3 mg cm-2 for Ni8P3, C@Ni8P3 and C@Ni8P3-2, respectively. 2.2 Material characterization The powder X-ray diffraction (XRD) patterns were recorded on Rigaku D/max-IIIB X-ray diffractometer (Cu Kα irradiation, λ = 1.54178 Å). The morphology of as-obtained products was examined by scanning electron microscope (SEM, Hitachi SU8030 SEM) and transmission electron microscope (TEM, JEOL JEM-2100 TEM). The energy-dispersive spectroscopy (EDS) analysis was carried out with JEOL JEM-2100 TEM. X-ray photoelectron spectroscopy (XPS) measurement was determined on Thermo Fisher Scientific VG Kα Probe spectrometer. 2.3. Electrochemical measurement Hydrogen evolving reaction (HER) and oxygen evolution reaction (OER) All the HER and OER electrochemical tests were carried out with three-electrode 5



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system using the Gamry Instruments Reference 600 electrochemical station. The loaded Ni foam with geometric area of 1 cm × 1 cm, Pt wire and Ag/AgCl electrode were employed as the working, counter and reference electrodes, respectively. All the potentials involved in our work were calibrated to the reversible hydrogen electrode (RHE). Prior to all the electrochemical measurements, the catalysts were activated by 20 cyclic voltammetry scans between 0.1 V and −0.3 V with a scan rate of 100 mV s-1. The polarization curves were corrected with the iR compensation and the current densities were normalized to the geometrical surface area. Linear sweep voltammetry (LSV) curves of the samples were recorded in 0.5 M H2SO4 or 1M KOH at a scan rate of 2 mV s−1. Electrochemical impedance spectroscopy (EIS) was performed in 0.5 M H2SO4 from the frequency of 100 kHz to 100 mHz under different overpotentials with a 5 mV AC voltage. The electrical double layer capacitances (Cdl) were performed with double-layer charging cyclic voltammograms in a narrow potential window in order to estimate the effective electrode surface area of the materials. Long-term durability tests were carried out using the chronopotentiometric method. Overall water splitting The full water splitting tests were measured in a two-electrode system with loaded Ni foam as both cathode and anode using the Gamry Instruments Reference 600 electrochemical station. LSV curves were recorded in 1.0 M KOH with a scan rate of 2 mV s−1. The stability of the catalyst electrodes was assessed by chronopotentiometry with a constant current of 10 mA cm-2 and chronoamperometry with an external bias of 1.65 V, respectively. 6


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3. RESULTS AND DISCUSSION 3.1. Catalyst Synthesis and Characterization

Figure 1. Characterization of the morphology, compositions and crystallographic structure of C@ Ni8P3 nanosheets array. (a-c) SEM images with different magnifications; (d) TEM image; (e) HAADF image; (f) HRTEM image; (g) STEM image; (h, i) EDS elemental mappings. The red arrows in (e) and (f) index the uniform coated C layer with thicknesses of 3–4 nm.

The crystal structure of the as-obtained Ni8P3 was characterized by XRD and shown in Figure S1. Three strong diffraction peaks at 45.6°, 52.9° and 78.2° are assigned to the (111), (200) and (220) lattice planes of Ni substrate (PDF no. 7



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88–2326). Besides these, only quite weak peaks can be detected and indexed to hexagonal Ni8P3 (PDF no. 78–1183) due to relative low loading of Ni8P3. The formation process of 3D Ni8P3 nanosheets arrays were investigated based on time-dependent experiments (see Figure S2). In the initial stage, red P droplets were deposited randomly on the surface of Ni substrate. Ni–P particles formed with the prolonged solvothermal procedure. Then nanosheet structures began to appear with the induction of PVP as structure-directing agent. At the end of reaction, all the particles transformed into nanosheets structure that formed a dense nanosheet array (see Figure S2d). Meanwhile, the color of the Ni foam changed from silver-gray to black. After carbon coating, C@Ni8P3 presented a darker color as compared with Ni8P3, as observed in Figure S3. The morphology and fine structure of C@Ni8P3 were characterized by SEM and TEM, as shown in Figure 1. It is clearly observed that C@Ni8P3 nanosheets are homogeneously grown on the surface of Ni foam. Enlarged SEM image shows the surface is uniformly covered with a high-density and vertically grown nanosheet array. The unique structure formed by interconnected and ultrathin nanosheets as revealed by high-magnification SEM image creates a larger surface area and more active sites in the catalytic process. SEM-EDS analysis in Figure S4 confirms uniform distribution of Ni, P and C elements. SEM images of Ni8P3 and excessive C coated Ni8P3 (C@ Ni8P3-2) are shown in Figure S5. While the pure Ni8P3 nanosheet has quite a smooth surface, the surface of C coated Ni8P3 become rough. Meanwhile, the coated nanosheets become thicker as shown in Figure S5f. The morphology of the C@Ni8P3 8


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is further studied by TEM as shown in Figure 1d. From the high angle annular dark field (HAADF) image (Figure 1e), a thin surface layer formed mainly by a light element (which is carbon in this case) with thickness of 3–4 nm can be observed, as indicated by the arrow. High-resolution TEM (HRTEM) and Raman spectra further confirms the existence of an amorphous C covering layer as shown in Figure 1f and Figure S6. The well-resolved lattice fringes with interplanar spacing of 0.489 nm and 0.274 nm correspond to the (104) and (024) lattice planes of Ni8P3, respectively. The chemical compositions of C@Ni8P3 were measured by EDS elemental mapping, which verified the homogeneous distribution of Ni and P elements across the nanosheets structure (see Figure 1 g-i). The atomic ratio between Ni and P is calculated to be about 2.4, which is close to the stoichiometric ratio of Ni8P3. The HRTEM image of pure Ni8P3 shown in Figure S7 demonstrates there is no such a carbon layer on the surface. The surface chemistry of the synthesized Ni8P3 and C@Ni8P3 was further studied by X-ray photoelectron spectroscopy (XPS). As depicted in Figure S8a, the XPS survey spectra clearly indicate the existence of Ni and P on the surface of both Ni8P3 and C@Ni8P3 samples. In high-resolution Ni 2p spectra, two main peaks at 872.7 and 854.7 eV accompanied with two shake-up satellites are assigned to the Ni 2p1/2 and Ni 2p3/2 of Niδ+ in Ni8P3 and C@Ni8P334,35, respectively. The two satellites located at 879.8 and 860.6 eV correspond well to Ni-O species from surface oxidation of Ni8P3 and C@Ni8P3, which is in good agreement with previous results.31,36 With respect to the P 2p core level spectrum (Figure S8c), the dominant peak at 133.4 eV is attributed 9



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to oxidized phosphate-like structures which are bound to carbon lattices.37 The two peaks at 131.0 and 129.8 eV are assigned to P 2p1/2 and P 2p3/2 of Pδ- in Ni8P3, while those of C@Ni8P3 are located at 130.1 and 129.1 eV, lower than those of Ni8P3. It can be seen that Ni 2p3/2 (854.7 eV) and P 2p3/2 (129.8 eV) in Ni8P3 are positively and negatively shifted compared with the binding energies of metallic Ni (852.5–852.9 eV) and element P (130.2 eV), respectively.38 Moreover, upon coating with C shell, P 2p3/2 (129.1 eV) would be further shifted in a more negative direction, which might be ascribed to the interaction between P and introduced C atoms. This observation suggests that the Ni atom in C@Ni8P3 possesses a partial positive charge (δ+) and the P atom owns a partial negative charge (δ−).31,39 Consequently, electron transfer would take place from Ni to P, which is favorable for adsorption and desorption process of reactant and product molecules during electrocatalysis reaction.40 Based on density functional theory calculations (DFT), Rodriguez29 predicted that Ni2P is an excellent electrocatalyst for HER, in which P sites serve as proton-acceptor and Ni sites as hydride-acceptor would work in a cooperative pattern. The same mode is considered applicable in our work. The positive Ni centers and the negative P centers serve as hydride-acceptor and proton-acceptor sites, respectively. It was expected that more negative P in C@Ni8P3 would produce more beneficial proton-acceptor sites. Similarly, for OER, Ni centers would act as hydroxyl acceptors and P centers promote the adsorption of hydroxyl around Ni sites, which is favorable for oxygen evolution during discharging and desorption process.39 3.2. Hydrogen Evolution Activity 10


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We first evaluated the HER activity of C@Ni8P3 nanosheets in strong acid solution, and the linear sweep voltammogram (LSV) polarization curves are given in Figure 2a. All polarization curves are recorded with iR compensation to eliminate the effect of ohmic resistance. For comparison, the bare Ni foam is characterized as a controlled sample, which shows weak electrocatalytic activity toward HER. In sharp contrast, C@Ni8P3functions as an efficient HER cathode with a small onset potential. Moreover, cathodic current presents a dramatic increase along with increasing potential. The C@Ni8P3 nanosheet array cathode exhibits low overpotentials of 110, 133 and 188 mV at cathodic current density of 10, 20 and 100 mA cm-2, respectively, which is lower than that of Ni8P3 (152 mV at j = 10 mA cm-2) and C@ Ni8P3-2 (119 mV at j = 10 mA cm-2). The performance of C@Ni8P3 is favorably compared with previously reported non-noble HER catalysts measured in acidic solution (Table S1). The reaction kinetics for hydrogen generation can be investigated by the Tafel plots derived from the corresponding polarization curves. As shown in Figure 2b, the linear fitting of the Tafel plots reveals a small Tafel slope of 46 mV dec-1 for C@Ni8P3, much lower than that of Ni foam (162 mV dec-1), Ni8P3 (86 mV dec-1), and C@Ni8P3-2 (67 mV dec-1). The lower Tafel slope indicates more rapid reaction kinetics on C@Ni8P3, which is comparable to that of reported transition metal phosphides catalysts listed in Table S1. The apparent Tafel slope of 46 mV dec-1 for C@Ni8P3 in the region of 40 to 120 mV dec−1 suggests a Heyrovsky-dominated Volmer−Heyrovsky mechanism during hydrogen evolution behavior.41,42 To explore the origin of enhanced catalytic activity of C@Ni8P3, the 11



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electrochemical double-layer capacitance (Cdl), which is considered proportional to the electrochemical active surface area, was measured by recording the cyclic voltammetry at different scan rates.43 From Figure 2c and Figure S11, one can see that carbon modified Ni8P3 nanosheet array electrode exhibits Cdl of 6.9 mF cm-2, much higher than that of Ni8P3 electrode (4.0 mF cm-2). The results indicate that carbon coating can create more active sites for electrochemical behavior. This agrees with SEM observation (Figure 1c) where a structure with high surface ratio formed by high-density vertical nanosheets and crimped ultrathin nanosheets are clear to see. The electrode kinetics and interface reactions during the HER process were further investigated by electrochemical impedance spectroscopy (EIS). Lower charge transfer resistance (Rct) usually means a faster reaction rate. As depicted in Figure 2d, only one semicircle can be seen in all EIS Nyquist plots, suggesting one time constant is characterized in the equivalent circuit of the electrocatalysis process. It can be seen that the Rct value of Ni8P3 decreased significantly from 0.98 to 0.24 Ω at overpotential of 220 mV after coated with C layer, indicating more favorable HER kinetics of C@Ni8P3 cathode. Furthermore, the EIS plots at different applied overpotentials were also carried out and provided in Figure S12. We can clearly observe that Rct values of the materials reduce tremendously with the increase of overpotentials, demonstrating the accelerated charge transfer kinetics upon increasing potential.44 Based on the above analysis, the outstanding HER activity of C@Ni8P3 can be ascribed to more exposed active sites and reduced resistance. On one hand, the unique morphology comprised of high-density upright nanosheets array with ultrathin nanosheets covered 12


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can maximize the number of exposed active edge sites, enhance the diffusion of charge and electrolyte; and desorb produced gas bubbles. On the other hand, the carbon layer can significantly reduce the electrochemical impedance of Ni8P3, accelerating the charge transfer and benefitting HER kinetics. Additionally, the synergistic chemical coupling effect between P and C (proved by XPS data with a more negative binding energy shift of P) may provide a superior proton-acceptor and contribute favorably to the excellent activity.

Figure 2. HER electrocatalytic performance of the samples. (a) LSV curves in 0.5 M H2SO4. (b) 13



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Corresponding Tafel plots derived from the polarization curves in (a). (c) Plots of the capacitive currents at 0.15 V vs RHE as a function of scan rate for Ni8P3 and C@Ni8P3. (d) EIS Nyquist plots of Ni8P3 and C@Ni8P3 at the overpotential of 220 mV in 0.5 M H2SO4. (e) LSV curves for Ni8P3 and C@Ni8P3 nanosheets in 1 M KOH. Inset: Corresponding Tafel plots. (f) LSV curves for C@Ni8P3 nanosheets before and after 1000 CV cycles (+0.1 to −0.3 V vs RHE, at a scan rate of 100 mV s-1) in 1 M KOH. The inset shows chronopotentiometric responses on Ni8P3 and C@Ni8P3 nanosheets at a constant current of 10 mA cm-2 after 80000 s of continuous operation, showing the superior stability of the C@Ni8P3 sample.

The existence of highly active oxygen evolution catalysis in alkaline medium has spurred the development of HER catalysts in alkaline condition. Hence, we also examined the HER activity of C@Ni8P3 in strong basic solution, and the results are shown in Figure 2e. The C@Ni8P3 electrode also exhibits superior HER performance in 1 M KOH, with an overpotential of 144 mV to produce 10 mA cm-2 current density, much smaller than that of Ni8P3 (194 mV). Moreover, the Tafel slope for C@Ni8P3 yielded from the linear portions of the Tafel plots is calculated to be 59 mV dec-1, while that of Ni8P3 is 102 mV dec-1, indicating hydrogen evolution is based on the Volmer–Heyrovsky mechanism. The small Tafel slope of C@Ni8P3 in alkaline solution is desirable to drive a large catalytic current at low overpotential. The electrochemical activity of C@Ni8P3 nanosheets array is considered excellent and comparable to most reported TMPs in basic solution (Table S2). Low overpotential and small Tafel slope suggest C@Ni8P3 possesses outstanding HER performance in 14


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both acid and alkaline solutions. Note that C@Ni8P3 exhibits more superior HER activity in acid than that in alkali. This can be ascribed to different catalytic mechanisms and reaction species involved in electrolytes with different pH, where in acidic media reaction kinetics is more favorable. Long-term durability is a critical factor in evaluating the performance of electrocatalysts. To probe the stability of C@Ni8P3 electrode in alkaline environment, continuous cyclic voltammograms (CV) were carried out for 1000 cycles with an accelerated scanning speed of 100 mV s-1. As shown in Figure 2f, C@Ni8P3 presents an almost identical polarization curve after 1000 cycles along with a negligible loss of cathodic current compared to the initial one, suggesting high stability to undergo rapid degradation. To further verify the durability of C@Ni8P3, chronopotentiometry was performed on Ni8P3 and C@Ni8P3 nanosheets at a constant current of 10 mA cm-2 after 80000 s of continuous operation (the inset of Figure 2f). The C@Ni8P3 cathode exhibits a slight potential deviation of about 10% withstanding long-term electrocatalysis, while that of Ni8P3 has a major potential decrease of over 20%. In addition, we examined the morphology change of Ni8P3 and C@Ni8P3 nanosheets after the electrochemical measurement. As presented in Figure S14, the morphologies are well retained, indicating the good structural stability. This exceptional stability of C@Ni8P3 shows promise in practical applications to replace noble-metal-based catalyst for efficient HER. 3.3. Oxygen Evolution Activity We next assessed the OER activity of C@Ni8P3 nanosheets array in 1 M KOH 15



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solution with the identical electrode configuration to HER. Before measurements, the electrodes were activated electrochemically by repetitive 20 cyclic voltammetries from 1.0 to 1.6 V. Based on earlier reports, this pretreatment process could create oxygenated surface functional groups, increasing the hydrophilicity and wettability of electrode surface.45 Similar to HER, it can be expected that the bare Ni foam electrode exhibits small measurable contribution to produce oxygen. As depicted in Figure 3a, the polarization curve for C@Ni8P3nanosheet array exhibits early onset of catalytic current, followed by the dramatic increase of current density with increasing potential, which presents more obvious variation than that of Ni8P3 and C@Ni8P3-2. It is convenient and efficient to appraise the electrocatalyst performance by comparing the required overpotential at j = 10 mA cm-2, an expected metric for a 10% efficient solar water-splitting device46. Remarkably, the overpotential to deliver an anodic current density of 10 mA cm−2 for C@Ni8P3 electrode is only about 267 mV (correlated to the potential of 1.497 V). In comparison, Ni8P3 can afford the same current at a larger overpotential of 283 mV. The high activity of the C@Ni8P3 anode is competitive with that of the state-of-the-art non-noble OER electrocatalysts (see Table S3), such as Ni2P nanoparticles (290 mV)23, NiOOH/Ni5P4 (290 mV)22 and Co-P film (345 mV)24. Of note, apparent oxidation peaks could be detected prior to the onset of oxygen evolution procedure, centered at around 1.32 V for Ni8P3 and 1.34 V for C@Ni8P3, respectively. The oxidation peaks are characteristics of the oxidation of Ni (II) to Ni (III),

which

is

consistent

with

previous

reports

about

Ni-based

OER

electrocatalysts.23,47-49 It can be reasonably speculated that nickel oxides/hydroxides 16


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were probably formed in-situ on the surface of Ni8P3 during the OER process. The larger oxidation potential required for Ni oxidized in C@Ni8P3 anode could be ascribed to the protection of C layer over the Ni8P3 core. In addition, on C coating, the anodic peak was broadened, indicating more accessible active sites.50 We examined the OER kinetics based on the corresponding Tafel plots (Figure 3b). The enhanced OER property of C@Ni8P3 was further reflected with a Tafel slope of 51 mV dec-1, which is much smaller than that of Ni8P3 (78 mV dec-1) and C@Ni8P3-2 (64 mV dec-1), implying a more favorable reaction kinetics with a more rapid OER rate. Appropriate carbon coating has a positive effect on the catalytic performance, while excess C would lead to the decrease of electrocatalytic activity. Even after 1000 CV cycles, the J–V curve of C@Ni8P3 electrode was almost superimposed over the initial one (Figure 3c), while that of Ni8P3 exhibits a sharp decay at high potential (Figure S15). Furthermore, the long-term electrochemical stability was probed at a constant current density of 20 mA cm-2 over 90 000 s of continuous operation. Noticeably, only an imperceptible

potential variation

demonstrates excellent stability.

Although

continuous gas was generated during the electrolysis, which could be dissipated rapidly out of the solution, no bubbles accumulated on the electrode surface, favoring long-term efficient operation.

17



(b) 0.7 600

Ni Foam Ni8P3 C@Ni8P3 C@Ni8P3-2

400

200

0.5 -1

78 mV dec

0.4 -1

64 mV dec

-1

51 mV dec

0.2

1.3

1.4

1.5

1.6

Potential (V vs. RHE)

Initial After 1000 cycles

400

200

0 1.2

1.3

1.4

1.5

1.0

1.6

1.7

-2

1.5

2.0

)

Log (j/mA cm

(d) 1.70

(c) 600

0.5

1.7


)

-1

148 mV dec

0.3

0 1.2

-2

Ni Foam Ni8P3 C@Ni8P3 C@Ni8P3-2

0.6

Overpotential (V)

Current Density (mA cm

-2

)

(a)

Current Density (mA cm

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1.65

-2

1.60

@ 20 mA cm

1.55

1.50


0

20000

40000

60000

80000

Time (s)

Figure 3. OER electrocatalytic performance of the samples in 1 M KOH. (a) LSV curves of the as-obtained samples along with that of bare Ni foam for comparison. (b) Corresponding Tafel plots derived from the polarization curves in (a). (c) LSV curves for C@Ni8P3 nanosheets before and after 1000 CV cycles. (d) Catalyst retention test on C@Ni8P3 nanosheets at a constant current of 20 mA cm-2 after 90000 s of continuous operation.

To better understand the origin of the excellent OER activity, we elucidated the structural and constitution modification of C@Ni8P3 nanosheets after OER by TEM and XPS analysis, as described in Figure 4 and Figure S17. From the TEM image in Figure 4a, C@Ni8P3 still remained in a nanosheet structure with slight corrosion of the surface. However, HRTEM image reveals that numerous small nanoparticles with sizes of several nanometers are formed surrounding Ni8P3 nanosheets after undergoing oxygen evolution. The ultrafine nanoparticles exhibit clear lattice fringes 18


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of 0.207 nm, which is referred to specific facets of nickel oxides/hydroxides species.22 The internal core is shown to be Ni8P3 still, with interplanar spacing of 0.178 nm, corresponding to (1,1,18) plane of hexagonal Ni8P3. It is worth mentioning that the outer C layer is clearly observed around metal compounds as before OER, suggesting good stability. The elemental mappings in Figure 4d-f further confirm the presence of an oxide shell, remaining in the inner Ni8P3 framework at the same time. Moreover, the relative content of O was found to increase significantly after OER, per the EDS analysis in Figure S16. The high-resolution P 2p XPS spectra in Figure S17b illustrate the reduced intensity of P 2p line, while that of oxidized phosphate species were sharply enhanced, confirming the occurrence of oxidation in Ni8P3. Furthermore, in O 1s core level shown in Figure S17d, two intense bands at 530.2 and 531.9 eV were detected after OER. Compared with that of initial C@Ni8P3, the emerging peak is attributed to Ni-O bond, corresponding to the abundant surface hydroxyls and oxygen from nickel oxides or hydroxides.51,52 All these results indicate the existence of nickel oxides/hydroxides shell on the surface of Ni8P3, consistent with previous reports.22,24,53 Moreover, from the inferior OER activity of nickel hydroxide shown in Figure S18, the exceptional OER activity is mainly derived from active Ni8P3, and the surface nickel oxides/hydroxides only serves as a synergist.

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Figure 4. (a) TEM image, (b) HRTEM image, (c) STEM image, (d-f) EDS elemental mappings of C@Ni8P3 nanosheets after OER test at 1.5 V for 1 h. The red arrow in (b) indexes the existence of C layer, indicating excellent structure stability.

The superior OER performance and stability of C@Ni8P3 nanosheet array could be due not only to the abundant exposed active sites and enhanced charge transfer capability. The one-step growth of C coated Ni8P3 nanosheets array also creates a robust integrated electrode, composed of powerful combination between Ni8P3 and Ni substrate, as well as between Ni8P3 core and amorphous carbon shell, which would provide an approach for fast charge and electrolyte transmission. The decorated C shell may serve as a protection layer to prevent dissolution and stabilize Ni8P3 electrode, to some degree. Furthermore, the in-situ produced oxidized nickel species act as primary active sites to promote the OER process. Meanwhile, the Ni8P3 core might provide a conducting support to the nickel oxides/hydroxides shell for effective electron diffusion.54,55 Taken together, the synergistic effect among these aspects 20


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makes contribution to the enhanced oxygen evolution capability of C@Ni8P3 nanosheets array. 3.4. Overall Water Splitting Ni Ni8P3 C@Ni8P3

1.8

40

) -2

20 10 0 1.0

-2

10 mA cm

1.6

30

1.2

1.4

1.6

1.8

2.0

Current (mA cm

-2

50

Potential (V)

(b) 2.0 60

)

(a) Current Density (mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

Ni8P3 C@Ni8P3

10

1.65 V

5 0 0

20000

Potential (V)

40000

60000

80000

Time (s)

Figure 5. (a) LSV curves of water electrolysis for Ni8P3 and C@Ni8P3 in a two-electrode configuration with a scan rate of 2 mV s-1 in 1 M KOH. b) Chronopotentiometric curves of water electrolysis for Ni8P3 and C@Ni8P3 in a two electrode configuration with a constant current density of 10 mA cm-2, and chronoamperometric curves with an external bias of 1.65 V after 90000 s of continuous operation.

Based on the aforementioned results, it is reasonable to predict that the C@Ni8P3 nanosheet array could act as a dual electrocatalyst for overall water splitting. For practical application, C@Ni8P3 was employed as both anode and cathode in a two-electrode electrolyzer. As illustrated in Figure 5a, C@Ni8P3 can afford a current density of 10 mA cm−2 at a cell voltage of 1.65 V, indicating a combined overpotential of about 420 mV required for full water oxidation and reduction. The potential is much lower than that of Ni8P3 electrode (1.79 V). It is also comparable to the behavior for the electrolyzers based on reported bifunctional catalysts (Table S4), 21



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such as Ni5P4 (1.69 V)22, Ni-P (1.68 V)56, NiFe LDH (1.70 V)57 and EG/Co0.85Se/NiFe-LDH (1.67)11. In addition, C@Ni8P3 maintained excellent stability with a negligible current change at 1.65 V and a sight increased voltage to deliver a 10 mA cm−2 current density after running for 90 000 s (Figure 5b). By comparison, the performance of Ni8P3 electrode presents considerable degradation after long-term operation. SEM images of C@Ni8P3 after overall water splitting (Figure S19) demonstrate that the nanosheets structure were well remained. These results support C@Ni8P3 electrode’s potential as an attractive candidate in low-cost and energy-efficient water electrolysis.

4. CONCLUSIONS In summary, a high-density C coated Ni8P3 nanosheet array has been synthesized via a simple, one-step solvothermal method. The resultant structure is suitable for both HER and OER due to its large number of exposed active sites and high electronic conductivity. The as-prepared C@Ni8P3 nanosheet array possesses low overpotential and robust durability for HER in both acidic and basic media. Functioned as anode material, it could afford the current density of 10 mA cm−2 at only 267 mV, compared favorably with most state of the art OER catalysts. The outstanding OER performance benefits from the formation of Ni8P3/NiOx heterojunction, producing the synergistic effect to promote oxygen evolution. In addition, the dual functionality enables us to construct an alkaline water electrolyzer with C@Ni8P3 as both cathode and anode. When assembled together, C@Ni8P3 exhibited promising overall water splitting ability with a cell voltage of 1.65 V to achieve 10 mA cm−2 current. Our findings 22


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would expand a desirable avenue to explore noble-metal-free overall water splitting catalysts for clean energy utilization.

ACKNOWLEDGMENTS Part of this work was supported by the Initiative for Sustainability and Energy at Northwestern (ISEN). This work made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. The work was also supported by China Scholarship Council.

Supporting Information Available: The details of chemical structure characterization and electrocatalytic performance of the as-prepared samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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Carbon-Coated Nickel Phosphide Nanosheets as Efficient Dual

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