Dual-Functional Starfish-like P-Doped CoâNiâS Nanosheets

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 7087−7095

www.acsami.org

Dual-Functional Starfish-like P‑Doped Co−Ni−S Nanosheets Supported on Nickel Foams with Enhanced Electrochemical Performance and Excellent Stability for Overall Water Splitting Fangfang Zhang,† Yuancai Ge,† Hang Chu,† Pei Dong,‡ Robert Baines,‡ Yu Pei,† Mingxin Ye,*,† and Jianfeng Shen*,† †

Institute of Special Materials and Technology, Fudan University, Shanghai 200433, P. R. China Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States

‡

S Supporting Information *

ABSTRACT: Dual-functional electrocatalysts have recently been reported to improve the conversion and storage of energy generated from overall water splitting in alkaline electrolytes. Herein, for the first time, a shape-controlled synthesis of starfish-like Co−Ni−S nanosheets on three-dimensional (3D) hierarchically porous nickel foams (Co−Ni−S/NF) via a one-step hydrothermal method was developed. The influence of reaction time on the nanosheet structure and properties was intensively studied. After 11 h reaction, the Co−Ni−S/ NF-11 sample displays the most regular structure of nanosheets and the most outstanding electrochemical properties. As to water splitting, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) required overpotentials of 284.3 and 296 mV, respectively, to provide a current density of 100 mA cm−2. The marvelous electrochemical performance can be attributed to the conductive networks of 3D layered porous nickel skeletons that are highly interconnected, which provided a large specific area and highly active sites. To further enhance the electrochemical performances of the electrocatalyst, the influence of the doping of the P element was also studied. The results proved that the P-doped Co−Ni−S/NF maintains the starfish structure and demonstrates outstanding properties, providing a current density of 100 mA cm−2 with only 187.4 and 292.2 mV overpotentials for HER and OER, respectively. It exhibited far more excellent properties than reported dual-functional electrocatalysts. Additionally, when used as an overall water-splitting catalyst, P−Co−Ni−S/NF can provide a 10 mA cm−2 current density at a given cell voltage of 1.60 V in 1 M KOH, which is competitive to the best-known electrocatalysts, with high long-term stability. KEYWORDS: Co−Ni−S/NF, P-doping, dual-functional, overall water splitting, high stability

■

INTRODUCTION Owing to the increased demand for energy and accelerated consumption of fossil fuels, renewable energy has attracted increasing attention. Meanwhile, global warming and pollution caused by fossil fuel have further expanded the demand for clean and renewable energy sources.1−5Therefore, inspired cooperative research studies have been conducted on environmentally friendly alternative sources of energy.6 As one of the cleanest fuels, hydrogen is predicted to play a significant role in the future energy landscape.7 Electrochemical hydrolysis is one of the most hopeful approaches for generating efficient renewable energy by converting electrical energy into chemical energy.8,9 Though the electrolytic hydrogen production and oxygen production have been identified as one of the simplest and cleanest methods of obtaining hydrogen, the preparation process still needs to be optimized.10 In general, water splitting consists of two distinct parts: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER).11 © 2018 American Chemical Society

Although no harmful byproducts are produced during the hydrolysis of water, the efficiency of electron fragmentation is hampered by the hysteresis kinetics of OER and HER. Currently, both HER and OER still require a high overpotential to achieve an appropriate reaction rate.12 Therefore, the development of an efficient and economical catalyst consisting of readily available materials becomes very important for water splitting.13−15 In view of the practical application of continuous integral water splitting, OER and HER electrocatalysts must be measured in the same electrolyte, particularly in alkaline solutions.16 It is a great challenge for most earth-rich catalysts to achieve this goal, in fact, many catalysts active at alkaline conditions may be inactive or unstable in acidic solutions. Typically, a noble metal catalyst, such as Pt or IrO2, is required during the hydrolysis of water to lower overpotential and speed Received: December 4, 2017 Accepted: February 5, 2018 Published: February 5, 2018 7087

DOI: 10.1021/acsami.7b18403 ACS Appl. Mater. Interfaces 2018, 10, 7087−7095

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a−c) Schematic illustration of the two-step strategy for preparing Co−Ni−S/NF and P-doped Co−Ni−S/NF electrodes.

■

up both HER and OER progress.17−21 Unfortunately, the scarcity of these precious metals makes it impractical and impossible for large-scale industrial applications. Therefore, scientists have turned to water-splitting catalysts composed of earth-rich elements.22,23 Transition metal chalcogenides,4,24−30 borides,31−33 and other metal-free materials34 have been reported for HER in strong acidic electrolytes, while many novel non-noble-metal OER catalysts on the basis of oxides/ hydroxides of cobalt,35−37 nickel,38 manganese,39,40 iron,41 and copper42,43 exhibit either ordinary or outstanding OER catalytic activities in alkaline environment. However, the dual-functional electrocatalysts for overall water splitting should have electrocatalytic activities for both HER and OER in the same electrolytes. Recently, Co−Ni−S compounds, exhibiting excellent electrochemical properties, have been extensively investigated as dualfunctional electrocatalysts for overall water splitting.44−46 The electrochemical properties can be improved and enhanced by controlling the morphology and incorporating other elements into the Co−Ni−S catalyst. On the other hand, transition-metal phosphides have drawn intense attention recently because of their excellent activity among the reported HER electrocatalysts.47−56 Although Co−Ni−S compounds and phosphides have been gradually studied, one-step hydrothermal synthesis of Co−Ni−S/NF and phosphides has rarely been reported before. Herein, for the first time, a novel one-step approach is introduced for the preparation of highly efficient and stable Co−Ni−S compounds on nickel foams (Co−Ni−S/NF) for overall water splitting with excellent properties. Furthermore, we doped the P element into Co−Ni−S/NF by phosphating through the hydrothermal method. The shape of the sample was adjusted by controlling the reaction time (7, 11, and 15 h), and Co−Ni−S/NF after reacting for 11 h (Co−Ni−S/NF-11) revealed the optimal starfish-like nanosheet structure and required only 284.3 and 296 mV of overpotentials to provide a current density of 100 mA cm−2 for HER and OER, respectively. Subsequently, nonmetallic element P has been successfully doped into Co−Ni−S/NF-11 by a hydrothermal method to obtain doped catalysts (P−Co−Ni−S/NF), which demonstrates enhanced electrochemical properties. The improved performances contribute to the lone pair electrons in the 3p orbitals and free 3d tracks of phosphorus and can be adapted to the surface charge states and induced local charge densities.57,58 The P−Co−Ni−S/NF electrocatalyst can provide a current density of 100 mA cm−2, requiring only 187.4 and 292.2 mV of overpotentials for HER and OER, respectively. It exhibits low Tafel slopes of 125.3 and 61.1 mV dec−1 for HER and OER in 1 M KOH, respectively, which are much better than the previously reported results. In addition, P−Co−Ni−S/NF, employed as both anode and cathode for full water splitting, only needs a cell voltage of 1.60 V to drive a current density of 10 mA cm−2 with excellent stability for over 20 h.

EXPERIMENTAL SECTION

Materials Preparation. Thiourea, cobaltous chloride 6-hydrate (CoCl2·6H2O), sodium hypophosphite, sodium dihydrogen phosphate, disodium hydrogen phosphate, acetone, ethanol, and deionized (DI) water were obtained from Sinopharm Chemical Reagent Co., Ltd and directly used without further purification. Ni foams (2 cm × 3 cm) were cleaned by sonication successively in acetone, ethanol, and DI water (15 min each) before being used as the substrate. Preparation of Ni3S2/NF-11 h. Thiourea (17.5 mmol) was completely dissolved in 70 mL DI water through vigorous stirring for 45 min. The solution was subsequently transferred to a 100 mL Teflon-lined stainless-steel autoclave, and a piece of cleaned Ni foam (2 cm × 3 cm) was added into the solution. The autoclave was sealed in an electric oven at 150 °C for 11 h. After being cooled to room temperature, the uniformly grown Ni3S2/NF was taken out from the solution, cleaned with DI water and ethanol three times each to clear any unreacted residues, and then dried at 70 °C in an electric oven. Preparation of Co−Ni−S/NF. Thiourea (17.5 mmol) and CoCl2· 6H2O (3.5 mmol) were added into 70 mL DI water by vigorous stirring for 45 min. Then, the solution was poured into a 100 mL Teflon-lined stainless-steel autoclave with one piece of surface-cleaned Ni foam (2 cm × 3 cm) immersed in the solution. The autoclave was sealed in an electric oven at 150 °C for 11 h. After being cooled to room temperature, the uniformly grown Co−Ni−S/NF-11 was taken out from the solution, cleaned with DI water and ethanol three times each to get rid of any unreacted residues, and then dried at 70 °C in an electric oven. The Co−Ni−S/NF samples with reaction times of 7 and 15 h (Co−Ni−S/NF-7, Co−Ni−S/NF-15) were synthesized by the same method. Preparation of P-Doped Co−Ni−S/NF. The amount of different sodium hypophosphites (3.5, 7, and 10.5 mmol, respectively), used as the P source, was immersed into 70 mL DI water to synthesize doped samples with different ratios of P (marked as P3.5−Co−Ni−S/NF, P− Co−Ni−S/NF, and P10−Co−Ni−S/NF, respectively). After sonication for 15 min, a piece of as-prepared Co−Ni−S/NF-11 was put into the solution and was subsequently transferred into a Teflon-lined stainless-steel 100 mL autoclave and maintained at 150 °C for 11 h in an electric oven. After cooling down to room temperature, the Pdoped Co−Ni−S/NF-11 was taken out from the solution, washed with DI water and ethanol three times each to remove any unreacted remains, and then dried at 70 °C. Materials Characterization. X-ray diffraction (XRD) patterns were measured by a Rigaku D/max-γB diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed on PerkinElmer PHI 5000C to catalog the chemical elements. The fieldemission scanning electron microscopy (SEM, Tescan MAIA3 XMH) was used to characterize the morphologies of the catalysts, and Bruker Xflash 6160 was employed to test the energy-dispersive X-ray (EDX) spectra at an accelerating voltage of 15 kV. JEOL 2010 was used to measure the selected-area electron diffraction (SAED) patterns and transmission electron microscopy (TEM) images of the electrocatalysts. Evaluations of the Electrocatalytic Activities of the Prepared Catalysts. Electrochemical HER and OER catalytic activity measurements were conducted in a standard three-electrode system controlled by the Autolab PGSTAT128N electrochemical workstation using a platinum plate as the counter electrode, a saturated calomel electrode (SCE) used as the reference electrode, and the prepared materials as 7088


Research Article


Figure 2. (a) XRD patterns of the prepared electrocatalysts and XPS spectra of the (b) survey spectrum of P−Co−Ni−S/NF, (c) Ni 2p spectrum, (d) Co 2p spectrum, (e) S 2p spectrum, and (f) P 2p spectrum. working electrodes. The SCE reference electrode was adjusted against a reversible hydrogen electrode (RHE) by the equation: E(RHE) = E(SCE) + E(Hg2Cl2/KCl) + 0.059 × pH. All tests were processed in 1.0 M KOH solution. Linear sweep voltammetry (LSV) was measured at room temperature with a scan rate of 10 mV s−1, and the chronopotentiometry was used to test their stabilities. The faradic efficiency (FE) was determined as the ratio between the amount of experimental hydrogen production and theoretical hydrogen production. The amount of hydrogen gas production was collected through a water drainage method. A constant current density was applied to the electrode, and the volume of the escaped gas was recorded synchronously.

diffraction peaks of the electrocatalysts were well-matched with that of the orthorhombic Ni3S2 (JCPDS no. 44-1418) and cubic Co9S8 (JCPDS no. 02-1459) phases, besides the significant Ni diffraction peaks, marked with asterisk (at 44.4°, 51.8°, and 76.3°), confirming the coexisting phase of Ni3S2 and Co9S8 after the hydrothermal process without any residues and contaminants. Although these peaks confirm the existence of Ni3S2 and Co9S8, the diffraction peaks of cobalt and phosphor compounds are not obvious because the accentuated Ni diffraction peaks obscure them. Therefore, XPS spectrum and EDX mapping analyses were performed to further corroborate the successful preparation of cobalt and phosphor compounds in the obtained electrocatalysts after doping with Co and P. Figure 2b−f shows the XPS survey spectra for the P−Co− Ni−S/NF nanosheet, demonstrating the existence of Ni, Co, S, and P elements. Besides the peaks of C 1s and O 1s, the Co 2p, Ni 2p, S 2p, and P 2p peaks can be clearly observed (Figure 2c−f). Figure 2c−f displays the typical fitted Ni 2p, Co 2p, S 2p, and P 2p spectra, which are determined by the Gaussian fitting. As displayed in Figure 2c, the XPS spectrum of Ni 2p is well-fitted with two chiral spin−orbit. Binding energies (BEs) at 855.8 eV for Ni 2p3/2 and 873.5 eV for Ni 2p1/2 demonstrate the coexistence of Ni2+ and Ni3+.59,60 In the XPS spectrum of Co 2p (shown in Figure 2d), the BEs of Co 2p1/2 and Co 2p3/2 are 796.7 and 781.6 eV, respectively, corresponding to the results reported before.61 The spin−orbit splitting value of Co 2p1/2 and Co 2p3/2 is 15.1 eV, which can be ascribed to the coexistence of Co3+ and Co2+.62 The S 2p spectrum (Figure 2e) is divided into four peaks, the BE centered at 168.7 eV can be well-fitted with two peaks (a main peak and a shakeup satellite peak) and the peak centered at around 162.9 eV is consistent with the BEs of metal−sulfur bondings (Co−S and Ni−S bondings).63,64 The spectrum of P 2p (Figure 2f) exhibits a dominant phosphide signal peak at 133.8 eV.65 A wide feature peak at approximately 136.6 eV is distributed to phosphate. Therefore, the surface of P−Co−Ni−S/NF is composed of Co2+, Co3+, Ni2+, Ni3+, S2−, and P5−. The XPS of Co−Ni−S/NF is displayed in Figure S3, demonstrating the presence of Co, Ni,

■

RESULTS AND DISCUSSION Figure 1 illustrates the in situ growth process of threedimensional (3D) Co−Ni−S and P−Co−Ni−S supported on Ni foams through a simple two-step hydrothermal method. High-purity Ni foam (>99.99%) due to its superior conductivity and 3D microporous structure (Figure 1a) was selected as both the substrate and the Ni source. In the first hydrothermal process, Co2+ and S2− directly reacted with the Ni foam to form uniform assembled Co−Ni−S over the surface of the Ni foam. Clearly, these starfish-like Co−Ni−S arrays form close-knit structures on the Ni foam (the surface of the Ni foam becomes completely black after the reaction, as displayed in Figure S1). Co−Ni−S/NF-11 represents an excellent starfish-like nanosheet structure, furnished with numerous reaction active sites, contributing it to an exceptional favorable catalyst. Then, the Ni−Co−S starfish structure was converted into P−Co−Ni−S subsequently by the phosphating and hydrothermal anion-exchange reaction processes with wellretained morphology, and there are new little sheets appearing on the surface of the Co−Ni−S sheets. The XRD patterns and XPS shown in Figure 2 are conducted to corroborate the phase composition, identify elemental composition, and characterize the chemical valences of the elements of the prepared catalysts. As displayed in Figure 2a (the XRD pattern of Co−Ni−S/NF and P−Co−Ni−S/NF is shown in Figure S2, which shows a slight difference between the peaks of these two catalysts caused by the doping of P element, demonstrating the successful doping of P), the 7089


Research Article


Figure 3. SEM images of (a−c) Co−Ni−S/NF-7, (d−f) Co−Ni−S/NF-11, (g−i) Co−Ni−S/NF-15, (j−l) P−Co−Ni−S/NF, and (m−p) EDX mapping of P−Co−Ni−S/NF.

Figure 4. TEM images of (a,b) Co−Ni−S/NF-11, (d,e) P−Co−Ni−S/NF (clear nanosheet, inset 4d), and (c,f) HR-TEM images and SAED patterns (inset) of Co−Ni−S/NF-11 and P−Co−Ni−S/NF, respectively; (g−k) elemental mapping of cobalt, nickel, sulfur, and phosphorus of P− Co−Ni−S/NF.

and S elements, therefore proving the successful synthesis of the Co−Ni−S compound. Figure 3 shows the SEM images of the prepared electrocatalysts and the EDX mapping images of P−Co−Ni−S/NF. The SEM images of the Co−Ni−S/NF samples (Figure 3a−i) synthesized at different reaction times illustrate how different reaction times obtain different morphologies of the samples. Generally, Co−Ni−S/NF-7 presents irregular morphology (Figure 3a−c), while Co−Ni−S/NF-11 (Figure 3d−f) and Co−Ni−S/NF-15 (Figure 3g−i) present similar starfish-like morphologies. As for Co−Ni−S/NF-11, the entire surface of the Ni foam is covered evenly and densely by a layer of 3D porous starfish-like nanosheet framework, and its EDX mapping images (Figure S4) indicate that Ni, Co, and S elements distribute uniformly on the surface of the electrocatalyst. As for Co−Ni−S/NF-15, the morphology is mainly

composed of dense nanosheet architectures followed by aggregation. Furthermore, the SEM images of Co−Ni−S/NF7, Co−Ni−S/NF-11, and Co−Ni−S/NF-15 can also reflect the obvious relationship between morphology and different synthesis conditions. Figure 3j−p presents the SEM and EDX mapping images of P-doped Co−Ni−S based on Co−Ni−S/ NF-11. Compared to Co−Ni−S/NF-11, P−Co−Ni−S/NF (Figure 3j−l) only has a slight morphology change with the formation of small clumps (inset, Figure 3l). EDX mapping images of P−Co−Ni−S/NF (Figure 3m−p) show that all the elements homogeneously distribute in the catalysts, which demonstrate the successful doping of P source. To further explain the morphologies and structures of the Co−Ni−S/NF-11 and P−Co−Ni−S/NF nanosheet catalysts, these samples were separated from the Ni foam by ultrasonication and then characterized by transmission electron 7090


Research Article


Figure 5. (a−d) HER and (e−h) OER electrocatalytic performance of the samples. (a,e) LSV curves in 1 M KOH; (b,f) Corresponding Tafel plots derived from the HER (OER) polarization curves of Ni foam, Pt/C (RuO2), Co−Ni−S/NF-11, and P−Co−Ni−S/NF, respectively. (c,g) Required overpotentials of different electrocatalysts to obtain a 100 mA cm−2 current density for HER and OER, respectively. (d,h) Co−Ni−S/NF-11 and P− Co−Ni−S/NF nanosheets at a constant current of 10 mA cm−2 after more than 16 h of continuous operation, showing the superior HER and OER stability of the Co−Ni−S/NF-11 and P−Co−Ni−S/NF samples, respectively.

HER properties of various electrocatalysts showing LSV polarization curves. The Tafel slope (log j−η) and the required overpotential of reaching a 100 mA cm−2 current density were discussed to compare and evaluate the HER properties of the electrocatalysts. As shown in Figure 5a, P−Co−Ni−S/NF displays an extremely small overpotential of 187.4 mV to achieve a current density of 100 mA cm−2 lower than many other Co and Nibased catalysts (Table S1) and P3.5−Co−Ni−S/NF (266.5 mV) and P10.5−Co−Ni−S/NF (196.6 mV) (Figure S5a); meanwhile, it has a low Tafel slope of 125.3 mV dec−1. Figure 5b shows the Tafel plot of the electrocatalysts obtained by fitting the linear region in the LSV curve to the Tafel equation. The Tafel slopes of other electrocatalysts are as follows: 201.5, 112.3, and 139.2 mV dec−1 for bare Ni foam, Pt/C, and Co− Ni−S/NF-11, respectively. Figure 5c clearly describes the overpotentials required for the electrocatalysts to achieve a current density of 100 mA cm−2 in 1 M KOH. It is obvious that the overpotentials of Co−Ni−S/NF-7, Co−Ni−S/NF-11, and Co−Ni−S/NF-15 were also relatively low (375.6, 284.3, and 350.3 mV). Hence, they also appear to be efficient electrocatalysts for HER in alkaline solution. However, Ni3S2/NF requires a higher overpotential of 441.2 mV to attain a 100 mA cm−2 current density. Though bare Ni foam has electrochemical activity on HER, it needs a very high overpotential of 477.3 mV to reach a 100 mA cm−2 current density. Therefore, the overpotential of Ni3S2/NF decreased with the presence of Co and P sources to different degrees. As for exorbitant Pt/C catalyst, a much lower overpotential of 168.5 mV was needed to afford a 100 mA cm−2 current density with a Tafel slope of 112.3 mV dec−1. Even more surprisingly, the current density of P−Co−Ni−S/NF is higher than that of the noble metal catalyst Pt/C after −0.2 V. The electrochemical LSV results demonstrate that 11 h is the optimal reaction time and that the doping of P source indeed significantly enhanced the HER performance of the Co−Ni−S compound. To calculate the turnover frequency (TOF) of the P−Co−Ni−S/NF catalyst, the surface area of the electrocatalyst was measured to be ∼44.8 m2 g−1 by Brunauer−Emmett−Teller (BET). The TOF was

microscopy (TEM) and high-resolution (HR) TEM. TEM images of Co−Ni−S-11 and P−Co−Ni−S catalysts are shown in Figure 4. Figure 4a,b displays the representative TEM images of Co−Ni−S-11, indicating that the starfish-like nanomaterials are indeed nanosheets. The corresponding HR-TEM image is shown in Figure 4c, in which the lattice fringes of Co−Ni−S-11 were calculated to be around 0.203, 0.220, 0.280, and 0.409 nm, which matched well with the (422), (420), and (222) lattice planes of Co9S8 and the (101) lattice plane of Ni3S2, respectively. Figure 4d,e reveals the morphology of P−Co− Ni−S, showing a uniform lamellar morphology, and has a small change compared with that of Co−Ni−S-11. As shown in Figure 4f, the lattice fringes of P−Co−Ni−S were calculated to be around 0.202, 0.228, and 0.287 nm indexed to the (422) and (331) lattice planes of Co9S8 and the (287) lattice plane of Ni3S2, respectively. These results confirmed that Co−Ni−S nanosheets were synthesized successfully and existed still after being doped with P source. The SEAD images (shown in the inset of Figure 4c,f) of Co−Ni−S-11 and P−Co−Ni−S demonstrate that the prepared electrocatalysts are polycrystalline, which are consistent with XRD results. The compositional distributions of the P−Co−Ni−S nanosheet are confirmed by elemental mapping analyses through high-angle annular darkfield scanning TEM−EDS, in which the uniform distributions of Ni, Co, S, and P are clearly observed throughout the whole sheet (Figure 4g−k). The HER performances of the prepared electrocatalysts were tested in 1 M KOH alkaline solution (Figures 5a−c and S5a) and in 1 M phosphate-buffered saline (PBS) neutral media (Figure S6a) with a standard three-electrode system. As shown in Figure 5a, among the varying samples prepared with different reaction times, the Co−Ni−S/NF-11 catalyst displays the best electrochemical performance, indicating that 11 h is the most suitable time for the synthesis of Co−Ni−S/NF, which is also better than those of bare Ni foam and Ni3S2/NF. To further examine the effect of the doping of P source, the HER activity of P−Co−Ni−S/NF was tested under the same conditions and the 40% commercial Pt/C loaded on Ni foam was used for contrast experiments. There are significant differences in the 7091


Research Article


Figure 6. (a) CVs for Co−Ni−S/NF-11. (b) Capacitive currents of the catalysts at 0.868 V vs RHE with a scan rate of 20, 40, 80, 100, and 120 mV s−1 in 1 M KOH. (c) EIS tests of Co−Ni−S/Ni-7, Co−Ni−S/Ni-11, Co−Ni−S/Ni-15, and P−Co−Ni−S/Ni recorded at a given potential of −1.2 V with a frequency range of 10 mHz to 10 kHz in 1 M KOH, and (d) two-electrode overall water splitting of Co−Ni−S/Ni-11//Co−Ni−S/Ni-11 and P−Co−Ni−S/Ni//P−Co−Ni−S/Ni in 1 M KOH: chronopotentiometry durability measurement (without iR-compensated) at an invariable current density of 10 mA cm−2.

calculated to be 1.872 × 10−3 s−1 at an overpotential of 190 mV on the basis of the BET surface area, assuming that all of the sulfur sites the surface participated in the HER catalysis (see detailed calculations in the Supporting Information). The FE of hydrogen production was also calculated through a drainage method (the experimental device is shown in the inset of Figure S7). The FE was almost over 99.6% during 20 min of electrolysis (Figure S7a). Additionally, when tested in PBS solution, P−Co−Ni−S/NF requires 296.8 mV overpotential (Figure S6a) to obtain a current density of 100 mA cm−2 lower than Co−Ni−S/NF-11 (373.4 mV) but higher than that of the overpotential in 1 M KOH. This result reveals that the catalyst has a better catalytically performance alkaline solution than in neutral media. The long-term durable test displayed in Figure 5d indicates that the P−Co−Ni−S/NF catalyst has a better HER stability than Co−Ni−S/NF-11 and requires an overpotential around 190, 90 mV smaller than that of Co−Ni−S/ NF-11, to drive a stable current density of 10 mA cm−2 for over 16 h. In addition, the cycling stability test for the HER of P− Co−Ni−S/NF (Figure S8) shows that negligible HER current is lost after 1000 cycles, demonstrating its great stability. To certify the applicability of these electrocatalysts in full water-splitting course, the electrocatalytic OER properties of all the catalysts were then evaluated using the same method in 1 M KOH solution and in neutral media. Figure 5e presents the polarization curves of bare Ni foam, Ni3S2/NF, RuO2, Co−Ni− S/NF-7, Co−Ni−S/NF-11, Co−Ni−S/NF-15, and P−Co− Ni−S/NF. Figure 5f shows the Tafel slopes of P−Co−Ni−S/ NF (61.1 mV dec−1), which is lower than that of the bare Ni foam (283.4 mV dec−1), RuO2 (142.7 mV dec−1), and Co−Ni− S/NF-11 (81.6 mV dec−1). It can be observed (Figure 5g) that the bare Ni foam demands an overpotential of 656 mV to acquire a current density of 100 mA cm−2. On the other hand,

P−Co−Ni−S/NF can provide a current density of 100 mA cm−2 at a relatively low overpotential of 292.2 mV, which is much lower than many other reported catalysts (Table S2) and P3.5−Co−Ni−S/NF and P10.5−Co−Ni−S/NF (when the potential is bigger than 1.65 V) (Figure S5b), superior to RuO2 (428.7 mV), Co−Ni−S/NF-7 (499.7 mV), Co−Ni−S/ NF-11 (296.0 mV), and Co−Ni−S/NF-15 (321.5 mV). As for the PBS solution, P−Co−Ni−S/NF needs an overpotential of 58.2 mV (Figure S6b) to gain a current density of 100 mA cm−2, which is lower than that of Co−Ni−S/NF-11 (197.8 mV) and the results in alkaline solution. The FE of oxygen production was calculated to be 99.4% for more than 20 min of electrolysis (Figure S7b). To assess the long-term electrochemical OER stability of Co−Ni−S/NF-11 and P−Co−Ni−S/NF, we conducted a three-electrode system for long-term OER durability in 1 M KOH electrolyte at a current density of 10 mA cm−2. Figure 5h describes how Co−Ni−S/NF-11 and P−Co−Ni−S/NF suffer from stable OER activity, and no appreciable potential rise is observed after oxygen release exceeding 16 h. Additionally, P− Co−Ni−S/NF requires a low overpotential of 310 mV to afford a 10 mA cm−2 OER current density, which is 60 mV lower than that of Co−Ni−S/NF-11 (370 mV). Comparison of XRD patterns and XPS spectra between P−Co−Ni−S/NF and post long-term OER durability sample (Figures S9 and S10) shows slight changes, further proving its excellent stability. The SEM images and corresponding EDX mapping of Co−Ni−S/ NF (Figure S11) and P−Co−Ni−S/NF (Figure S12) after long-term test demonstrate the high stability of P−Co−Ni−S/ NF, in which the morphology and elemental distribution of P− Co−Ni−S/NF have not been destroyed. Furthermore, the TEM images of Co−Ni−S/NF-11 and P−Co−Ni−S/NF before and after long-term OER tests (Figure S13) demonstrate 7092


Research Article


demonstrate that 11 h is the optimal reaction time, which displays the best electrochemical properties, requiring 284.3 and 296 mV of overpotentials to provide a current density of 100 mA cm−2 for HER and OER, respectively. Second, elemental doping by a hydrothermal method indeed has a significant effect on improving the properties of catalysts, and P−Co−Ni−S/NF reveals the best properties among the doped catalysts. Specifically, for P−Co−Ni−S/NF, it requires only 187.4 and 292.2 mV to obtain a 100 mA cm−2 current density for HER and OER, respectively. Meanwhile, the Tafel slopes for HER and OER in 1 M KOH are as low as 125.3 and 61.1 mV dec−1, respectively. Actually, P−Co−Ni−S/NF is much better than many other catalysts reported previously. In addition, when applied in an alkaline electrolyte, the precious metal-free P−Co−Ni−S/NF catalyst can afford vigorous and continuous evolution of H2 at 10 mA cm−2 current density, requiring a low 174 mV overpotential. In merits of above advantages, P−Co−Ni−S/NF can be a promising effective electrocatalyst for overall water splitting. Finally, P−Co−Ni−S/ NF, employed as both the cathode and the anode for full water splitting, only needs a cell voltage of 1.60 V to drive a 10 mA cm−2 current density with excellent stability for over 20 h.

that the nanosheets are well-preserved and the lattice fringes prove that the composition of the catalysts have not changed, which indirectly reflect its excellent stability. The electrochemical active surface areas (ECSAs) of the prepared catalysts are determined through their electrical double-layer capacitance tests in alkaline electrolyte (1 M KOH) (Figure 6a,b). To interpret better and compare the ECSAs of different catalysts, cyclic voltammograms (CVs) are recorded at a window from −0.25 to −0.15 V with different scan rates (from 20 to 200 mV s−1) (Figures 6a and S14). The capacitive currents of all the catalysts are collected at the same potential of −0.2 V, and the plot with respect to the scan rate is shown in Figure 6b. We calculated the capacitance values of Ni foam, Ni3S2/NF, Co−Ni−S/NF-7, Co−Ni−S/NF-11, Co− Ni−S/NF-15, and P−Co−Ni−S/NF from these linear plots. As we expected, P−Co−Ni−S/NF shows the highest specific capacitance of 131.1 mF cm−2 than those of Ni foam, Ni3S2/ NF, Co−Ni−S/NF-7, Co−Ni−S/NF-11, and Co−Ni−S/NF15 of 1.15, 73.1, 108.0, and 40.3 mF cm−2, suggesting that it has the largest electrochemical surface area and the corresponding highest surface roughness.66 Therefore, we attribute the great ECSA to the doping of P source, resulting in the superior electrochemical activities. Electrochemical impedance spectroscopy (EIS) displayed that the change of the reaction time had a great effect on the electrocatalysts and that the introduction of P source caused a significant decrease in the radius of arc compared with Co−Ni−S/NF-7, Co−Ni−S/NF-11, and Co− Ni−S/NF-15 (Figure 6c). The radius of P−Co−Ni−S/NF was the smallest among all above electrocatalysts. In the Nyquist diagram, the smaller radius is the lower charge-transfer resistance at the electrode/electrolyte interface.67 These results revealed that the presence of P source remarkably enhanced electron transfer from the sulfide surface to the electrolyte with the concomitant evolution of H2 by suppressing the charge recombination. All of the experimental results described above indicated that P−Co−Ni−S/NF could be used as an active and stable electrocatalyst for both HER and OER in 1 M KOH. Therefore, we fabricated a two-electrode system employing P−Co−Ni−S/ NF as both the cathode and the anode (P−Co−Ni−S/NF// P−Co−Ni−S/NF) to simulate a real full water-splitting process in 1 M KOH electrolyzer. Meanwhile, a Co−Ni−S/NF-11// Co−Ni−S/NF-11 electrolyzer structure was prepared as a control. As shown in Figure S15, polarization curves indicate that the P−Co−Ni−S/NF//P−Co−Ni−S/NF system needs 1.60 V to obtain 10 mA cm−2 current density, while Co−Ni−S/ NF-11//Co−Ni−S/NF-11 requires 1.64 V. The overall watersplitting properties of P−Co−Ni−S/NF//P−Co−Ni−S/NF and Co−Ni−S/NF-11//Co−Ni−S/NF-11 are shown in Figure 6d. The P−Co−Ni−S/NF//P−Co−Ni−S/NF pair exhibits the highest catalytic activity for water splitting (Table S3), requiring a relative low cell voltage of 1.60 V to obtain a 10 mA cm−2 overall water-splitting current density with continuous gas evolution on both electrodes with an excellent long-term stability for over 20 h. To this end, the P-doped sulfide nanosheets are expected to become low-cost, high-activity, and high-stability electrocatalysts for overall water splitting.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18403. Detailed structural and electrochemical performances of starfish-like P-doped Co−Ni−S nanosheets and their counterparts (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.Y.). *E-mail: [email protected] (J.S.). ORCID

Mingxin Ye: 0000-0002-4532-2594 Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474−6502. (2) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253−278. (3) Liang, Y.; Li, Y.; Wang, H.; Dai, H. Strongly Coupled Inorganic/ Nanocarbon Hybrid Materials for Advanced Electrocatalysis. J. Am. Chem. Soc. 2013, 135, 2013−2036. (4) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (5) Wang, H.; Dai, H. Strongly coupled inorganic−nano-carbon hybrid materials for energy storage. Chem. Soc. Rev. 2013, 42, 3088− 3113. (6) Merki, D.; Hu, X. Recent developments of molybdenum and tungsten sulfides as hydrogen evolution catalysts. Energy Environ. Sci. 2011, 4, 3878−3888. (7) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141−145. (8) Chow, J.; Kopp, R. J.; Portney, P. R. Energy Resources and Global Development. Science 2003, 302, 1528−1531.

■

CONCLUSIONS In summary, first, starfish-like Co−Ni−S/NF nanosheets were synthesized through a simple one-step hydrothermal method, whose shape of Co−Ni−S/NF can be controlled by changing the reaction time. The analysis results of synthesized catalysts 7093


Research Article

ACS Applied Materials & Interfaces (9) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060−2086. (10) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Gratzel, M. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 2014, 345, 1593−1596. (11) Zhu, W.; Zhang, R.; Qu, F.; Asiri, A. M.; Sun, X. Design and Application of Foams for Electrocatalysis. ChemCatChem 2017, 9, 1721−1743. (12) 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 Dual-functional Electrocatalyst for Full Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 12361−12365. (13) Gerken, J. B.; McAlpin, J. G.; Chen, J. Y. C.; Rigsby, M. L.; Casey, W. H.; Britt, R. D.; Stahl, S. S. Electrochemical Water Oxidation with Cobalt-Based Electrocatalysts from pH 0-14: The Thermodynamic Basis for Catalyst Structure, Stability, and Activity. J. Am. Chem. Soc. 2011, 133, 14431−14442. (14) Gorlin, Y.; Jaramillo, T. F. A Dual-functional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132, 13612−13614. (15) Yeo, B. S.; Bell, A. T. Enhanced Activity of Gold-Supported Cobalt Oxide for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2011, 133, 5587−5593. (16) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmeková, E.; Asefa, T. Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values. Angew. Chem. 2014, 126, 4461−4465. (17) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Nørskov, J. K. Computational high-throughput screening of electrocatalytic materials for hydrogen evolution. Nat. Mater. 2006, 5, 909− 913. (18) Petrykin, V.; Macounova, K.; Shlyakhtin, O. A.; Krtil, P. Tailoring the Selectivity for Electrocatalytic Oxygen Evolution on Ruthenium Oxides by Zinc Substitution. Angew. Chem., Int. Ed. 2010, 49, 4813−4815. (19) Wang, T.; Kou, Z.; Mu, S.; Liu, J.; He, D.; Amiinu, I. S.; Meng, W.; Zhou, K.; Luo, Z.; Chaemchuen, S.; Verpoort, F. 2D Dual-Metal Zeolitic-Imidazolate-Framework-(ZIF)-Derived Bifunctional Air Electrodes with Ultrahigh Electrochemical Properties for Rechargeable Zinc−Air Batteries. Adv. Funct. Mater. 2017, 28, 1705048. (20) Amiinu, I. S.; Liu, X.; Pu, Z.; Li, W.; Li, Q.; Zhang, J.; Tang, H.; Zhang, H.; Mu, S. From 3D ZIF Nanocrystals to Co−Nx/C Nanorod Array Electrocatalysts for ORR, OER, and Zn−Air Batteries. Adv. Funct. Mater. 2017, 28, 1704638. (21) Amiinu, I. S.; Pu, Z. H.; Liu, X. B.; Owusu, K. A.; Monestel, H. G. R.; Boakye, F. O.; Zhang, H. N.; Mu, S. C. Multifunctional Mo−N/ C@MoS2 Electrocatalysts for HER, OER, ORR, and Zn−Air Batteries. Adv. Funct. Mater. 2017, 27, 1702300. (22) Gao, M.-R.; Xu, Y.-F.; Jiang, J.; Zheng, Y.-R.; Yu, S.-H. Water Oxidation Electrocatalyzed by an Efficient Mn3O4/CoSe2 Nanocomposite. J. Am. Chem. Soc. 2012, 134, 2930−2933. (23) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-doped carbon nanomaterials as non-metal electrocatalysts for water oxidation. Nat. Commun. 2013, 4, 2390. (24) Benck, J. D.; Chen, Z.; Kuritzky, L. Y.; Forman, A. J.; Jaramillo, T. F. Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production: Insights into the Origin of their Catalytic Activity. ACS Catal. 2012, 2, 1916−1923. (25) Du, P.; Eisenberg, R. Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: Recent progress and future challenges. Energy Environ. Sci. 2012, 5, 6012−6021. (26) Kong, D.; Cha, J. J.; Wang, H.; Lee, H. R.; Cui, Y. First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction. Energy Environ. Sci. 2013, 6, 3553−3558. (27) Di Giovanni, C.; Wang, W.-A.; Nowak, S.; Grenèche, J.-M.; Lecoq, H.; Mouton, L.; Giraud, M.; Tard, C. Bioinspired Iron Sulfide

Nanoparticles for Cheap and Long-Lived Electrocatalytic Molecular Hydrogen Evolution in Neutral Water. ACS Catal. 2014, 4, 681−687. (28) Sun, Y.; Liu, C.; Grauer, D. C.; Yano, J.; Long, J. R.; Yang, P.; Chang, C. J. Electrodeposited Cobalt-Sulfide Catalyst for Electrochemical and Photoelectrochemical Hydrogen Generation from Water. J. Am. Chem. Soc. 2013, 135, 17699−17702. (29) Zhu, W.; Yue, X.; Zhang, W.; Yu, S.; Zhang, Y.; Wang, J.; Wang, J. Nickel sulfide microsphere film on Ni foam as an efficient bifunctional electrocatalyst for overall water splitting. Chem. Commun. 2016, 52, 1486−1489. (30) Pu, Z.; Amiinu, I. S.; Liu, X.; Wang, M.; Mu, S. Ultrastable nitrogen-doped carbon encapsulating molybdenum phosphide nanoparticles as highly efficient electrocatalyst for hydrogen generation. Nanoscale 2016, 8, 17256−17261. (31) Vrubel, H.; Hu, X. Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in both Acidic and Basic Solutions. Angew. Chem. 2012, 124, 12875−12878. (32) Pan, L. F.; Li, Y. H.; Yang, S.; Liu, P. F.; Yu, M. Q.; Yang, H. G. Molybdenum carbide stabilized on graphene with high electrocatalytic activity for hydrogen evolution reaction. Chem. Commun. 2014, 50, 13135−13137. (33) Liao, L.; Wang, S.; Xiao, J.; Bian, X.; Zhang, Y.; Scanlon, M. D.; Hu, X.; Tang, Y.; Liu, B.; Girault, H. H. A nanoporous molybdenum carbide nanowire as an electrocatalyst for hydrogen evolution reaction. Energy Environ. Sci. 2014, 7, 387−392. (34) Zhao, Y.; Zhao, F.; Wang, X.; Xu, C.; Zhang, Z.; Shi, G.; Qu, L. Graphitic Carbon Nitride Nanoribbons: Graphene-Assisted Formation and Synergic Function for Highly Efficient Hydrogen Evolution. Angew. Chem., Int. Ed. 2014, 53, 13934−13939. (35) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (36) Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science 2010, 328, 342−345. (37) Jiao, F.; Frei, H. Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts. Energy Environ. Sci. 2010, 3, 1018−1027. (38) Oliver-Tolentino, M. A.; Vázquez-Samperio, J.; Manzo-Robledo, A.; González-Huerta, R. d. G.; Flores-Moreno, J. L.; Ramírez-Rosales, D.; Guzmán-Vargas, A. An Approach to Understanding the Electrocatalytic Activity Enhancement by Superexchange Interaction toward OER in Alkaline Media of Ni−Fe LDH. J. Phys. Chem. C 2014, 118, 22432−22438. (39) Zaharieva, I.; Najafpour, M. M.; Wiechen, M.; Haumann, M.; Kurz, P.; Dau, H. Synthetic manganese−calcium oxides mimic the water-oxidizing complex of photosynthesis functionally and structurally. Energy Environ. Sci. 2011, 4, 2400−2408. (40) Song, F.; Hu, X. Ultrathin Cobalt−Manganese Layered Double Hydroxide Is an Efficient Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2014, 136, 16481−16484. (41) Landon, J.; Demeter, E.; Iṅ oğlu, N.; Keturakis, C.; Wachs, I. E.; Vasić, R.; Frenkel, A. I.; Kitchin, J. R. Spectroscopic Characterization of Mixed Fe−Ni Oxide Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Electrolytes. ACS Catal. 2012, 2, 1793−1801. (42) Yu, F.; Li, F.; Zhang, B.; Li, H.; Sun, L. Efficient Electrocatalytic Water Oxidation by a Copper Oxide Thin Film in Borate Buffer. ACS Catal. 2015, 5, 627−630. (43) Du, J.; Chen, Z.; Ye, S.; Wiley, B. J.; Meyer, T. J. Copper as a Robust and Transparent Electrocatalyst for Water Oxidation. Angew. Chem., Int. Ed. 2015, 54, 2073−2078. (44) Wang, T.; Zhao, B.; Jiang, H.; Yang, H.-P.; Zhang, K.; Yuen, M. M. F.; Fu, X.-Z.; Sun, R.; Wongde, C.-P. Electro-deposition of CoNi2S4 flower-like nanosheets on 3D hierarchically porous nickel skeletons with high electrochemical capacitive performance. J. Mater. Chem. A 2015, 3, 23035−23041. (45) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and 7094


Research Article

ACS Applied Materials & Interfaces Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661−4672. (46) Liu, D.; Lu, Q.; Luo, Y.; Sun, X.; Abdullah, M. A. NiCo2S4 nanowires array as an efficient dual-functional electrocatalyst for full water splitting with superior activity. Nanoscale 2015, 7, 15122−15126. (47) Jiang, N.; Bogoev, L.; Popova, M.; Gul, S.; Yano, J.; Sun, Y. Electrodeposited nickel-sulfide films as competent hydrogen evolution catalysts in neutral water. J. Mater. Chem. A 2014, 2, 19407−19414. (48) McEnaney, J. M.; Chance Crompton, J.; Callejas, J. F.; Popczun, E. J.; Read, C. G.; Lewis, N. S.; Schaak, R. E. Electrocatalytic hydrogen evolution using amorphous tungsten phosphide nanoparticles. Chem. Commun. 2014, 50, 11026−11028. (49) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X. High-Efficiency Electrochemical Hydrogen Evolution Catalyzed by Tungsten Phosphide Submicroparticles. ACS Catal. 2015, 5, 145−149. (50) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem. 2014, 126, 5531−5534. (51) Feng, L.; Vrubel, H.; Bensimon, M.; Hu, X. Easily-prepared dinickel phosphide (Ni2P) nanoparticles as an efficient and robust electrocatalyst for hydrogen evolution. Phys. Chem. Chem. Phys. 2014, 16, 5917−5921. (52) Xu, Y.; Wu, R.; Zhang, J.; Shi, Y.; Zhang, B. Anion-exchange synthesis of nanoporous FeP nanosheets as electrocatalysts for hydrogen evolution reaction. Chem. Commun. 2013, 49, 6656−6658. (53) Jiang, P.; Liu, Q.; Liang, Y.; Tian, J.; Asiri, A. M.; Sun, X. A CostEffective 3D Hydrogen Evolution Cathode with High Catalytic Activity: FeP Nanowire Array as the Active Phase. Angew. Chem., Int. Ed. 2014, 53, 12855−12859. (54) Pu, Z.; Zhang, C.; Amiinu, I. S.; Li, W.; Wu, L.; Mu, S. General Strategy for the Synthesis of Transition-Metal Phosphide/N-Doped Carbon Frameworks for Hydrogen and Oxygen Evolution. ACS Appl. Mater. Interfaces 2017, 9, 16187−16193. (55) Wang, P.; Pu, Z.; Li, Y.; Wu, L.; Tu, Z.; Jiang, M.; Kou, Z.; Amiinu, I. S.; Mu, S. Iron-Doped Nickel Phosphide Nanosheet Arrays: An Efficient Bifunctional Electrocatalyst for Water Splitting. ACS Appl. Mater. Interfaces 2017, 9, 26001−26007. (56) Pu, Z.; Amiinu, I. S.; Kou, Z.; Li, W.; Mu, S. RuP2-Based Catalysts with Platinum-like Activity and Higher Durability for the Hydrogen Evolution Reaction at All pH Values. Angew. Chem., Int. Ed. 2017, 56, 11559−11564. (57) Yan, H.; Tian, C.; Wang, L.; Wu, A.; Meng, M.; Zhao, L.; Fu, H. Phosphorus-Modified Tungsten Nitride/Reduced Graphene Oxide as a High-Performance, Non-Noble-Metal Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2015, 54, 6325−6329. (58) Yang, D.-S.; Bhattacharjya, D.; Inamdar, S.; Park, J.; Yu, J.-S. Phosphorus-Doped Ordered Mesoporous Carbons with Different Lengths as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media. J. Am. Chem. Soc. 2012, 134, 16127− 16130. (59) Pu, J.; Cui, F.; Chu, S.; Wang, T.; Sheng, E.; Wang, Z. Preparation and Electrochemical Characterization of Hollow Hexagonal NiCo2S4 Nanoplates as Pseudocapacitor Materials. ACS Sustainable Chem. Eng. 2014, 2, 809−815. (60) Kong, W.; Lu, C.; Zhang, W.; Pu, J.; Wang, Z. Homogeneous core−shell NiCo2S4 nanostructures supported on nickel foam for supercapacitors. J. Mater. Chem. A 2015, 3, 12452−12460. (61) Yang, B.; Yu, L.; Yan, H.; Sun, Y.; Liu, Q.; Liu, J.; Song, D.; Hu, S.; Yuan, Y.; Liu, L.; Wang, J. Fabrication of urchin-like NiCo2(CO3)1.5(OH)3@NiCo2S4 on Ni foam by an ionexchange route and application to asymmetrical supercapacitors. J. Mater. Chem. A 2015, 3, 13308−13316. (62) Gong, X. F.; Cheng, J. P.; Ma, K. Y.; Liu, F.; Zhang, L.; Zhang, X. Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance. Mater. Chem. Phys. 2016, 173, 317−324.

(63) Chen, H.; Jiang, J.; Zhang, L.; Wan, H.; Qi, T.; Xia, D. Highly conductive NiCo2S4 urchin-like nanostructures for high-rate pseudocapacitors. Nanoscale 2013, 5, 8879−8883. (64) Du, W.; Wang, Z.; Zhu, Z.; Hu, S.; Zhu, X.; Shi, Y.; Pang, H.; Qian, X. Facile synthesis and superior electrochemical performances of CoNi2S4/graphene nanocomposite suitable for supercapacitor electrodes. J. Mater. Chem. A 2014, 2, 9613−9619. (65) Grosvenor, A. P.; Wik, S. D.; Cavell, R. G.; Mar, A. Examination of the Bonding in Binary Transition-Metal Monophosphides MP (M = Cr, Mn, Fe, Co) by X-Ray Photoelectron Spectroscopy. Inorg. Chem. 2005, 44, 8988−8998. (66) Dai, X.; Du, K.; Li, Z.; Liu, M.; Ma, Y.; Sun, H.; Zhang, X.; Yang, Y. Co-Doped MoS2 Nanosheets with the Dominant CoMoS Phase Coated on Carbon as an Excellent Electrocatalyst for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2015, 7, 27242−27253. (67) Hou, Y.; Wen, Z.; Cui, S.; Guo, X.; Chen, J. Constructing 2D Porous Graphitic C3N4 Nanosheets/Nitrogen-Doped Graphene/ Layered MoS2 Ternary Nanojunction with Enhanced Photoelectrochemical Activity. Adv. Mater. 2013, 25, 6291−6297.

7095


Dual-Functional Starfish-like P-Doped CoâNiâS Nanosheets

Recommend Documents

Dual-Functional Starfish-like P-Doped CoâNiâS Nanosheets