Featherlike NiCoP Holey Nanoarrys for Efficient and Stable Seawater

May 6, 2019 - ... a promising electrocatalyst superior to platinum in a wide range of pH and may provide a new idea for electrocatalytic seawater spli...
0 downloads 0 Views 5MB Size
Download PDF
Article Cite This: ACS Appl. Energy Mater. 2019, 2, 3910−3917

www.acsaem.org

Featherlike NiCoP Holey Nanoarrys for Efficient and Stable Seawater Splitting Qingliang Lv,† Jianxin Han,† Xueling Tan,† Wei Wang,†,‡ Lixin Cao,*,† and Bohua Dong*,† †

School of Materials Science and Engineering, Ocean University of China, 238 Songling Road, Qingdao, 266100 P.R. China Aramco Research Center-Boston, Aramco Services Company, Cambridge, Massachusetts 02139, United States

‡

Downloaded via BUFFALO STATE on July 29, 2019 at 08:33:46 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Developing earth-rich highly efficient nonprecious electrocatalysts for hydrogen evolution reaction (HER) has become of great significance for sustainable energy technology. Herein, novel nickel foam (NF) supported porous featherlike NiCoP (PFNiCoP/NF) nanoarrys are constructed by a successive hydrothermal and phosphidation way. Simultaneously, their three-dimensional (3D) morphology, the holey structure, and the conductive substrate are favorable for the enhanced specific surface area, efficient electron and mass transfer, and exposure of more active sites, and also are beneficial for the release of generated H2. PF-NiCoP/NF demonstrates high activity and long-term durability in alkaline media (1 M KOH) and real seawater, reaching the current density of 10 mA cm−2 at overpotentials of 46 and 287 mV, respectively. Moreover, the faradaic efficiency of 3D PFNiCoP/NF is as high as 96.5% in real seawater. As expected, PF-NiCoP/NF exhibits superior performance in comparison to those of most of HER electrocatalysts in real seawater and alkaline media. This work may present a new strategy to design a promising electrocatalyst superior to platinum in a wide range of pH and may provide a new idea for electrocatalytic seawater splitting. KEYWORDS: seawater splitting, nanoarrys, electrocatalyst, porous, transition metal phosphide

1. INTRODUCTION Hydrogen, a promising alternative fuel, has gained increasing attention because there is no pollution, it is sustainable, and has high calorific value. It is an effective way to solve environmental pollution and energy crisis.1,2 Seawater occupies a primary status in the earth’s natural water resources. Considering the production of H2, electrolyzed seawater is a promising way to commercialize extensively thanks to its highpurity product, low energy consumption, and environmentally friendly process.3,4 Seawater is a natural electrolyte due to its salinity of 3.5 wt %. Although seawater splitting began to be studied in the 1970s, it always remains a huge challenge. The main problem hindering the development of seawater splitting is the release of chlorine (Cl2), the formation of insoluble precipitates such as calcium hydroxide (Ca(OH)2), and the corrosion of electrodes caused by local changes in acidity.5,6 Thus, development of highly active and steady, while seawaterworkable, electrocatalysts is a major challenge. At present, Pt-based catalyst is the most efficient electrocatalyst, which has high catalytic activity for hydrogen evolution reaction and has good electrolysis stability, but the scarcity and high cost limits its broad-scale utilization.7−9 The following criteria should be met for commercial HER catalysts: (i) similar or higher activity than Pt/C; (ii) composed of earthrich elements while obtaining more economic benefits; (iii) reducing the interface resistance without adhesive (attached to a conductive substrate), and preventing catalyst from shedding during formation of vigorous gas bubbles; (iv) stable, meaning © 2019 American Chemical Society

that the electrode will not be destroyed through the long-term HER reaction.3,6 Numerous studies have been devoted to exploration of the nonprecious metal alternatives. So far, many nonprecious metalsalloys,10−13 sulfides,14−19 selenides,20−23 oxides,24−28 and phosphides29−32 have been developed. Among them, transition metal phosphide (TMP) has a structural similarity to hydrogenase in which the metal and P sites on the surface serve as proton acceptors and hydride acceptor centers, respectively, and alter the electronic structure of the electrocatalyst, thus facilitating reaction kinetics.33 TMPs-based electrocatalyst has become a promising earth-rich electrocatalyst due to the ideal electrical conductivity, coordination effect of bimetal atoms, and structural stability. For instance, both Ni2P and CoP feature superior electrical conductivity, outstanding stability in a large pH range, and are highly active; due to the synergy effect between Ni and Co during HER reaction, and metal properties and strong capacity of water adsorption on NiCoP, it can be expected that the NiCoP exhibits more superior activity.34,35 Moreover, it is known that the performance can be affected by the size, shape, dimensionality, and morphology. Electrocatalysts with the morphologies of zero-dimensional (0D) nanoparticles,36 onedimensional (1D) nanowires,37 and two-dimensional (2D) nanosheets38 have been studied and show good activity in the Received: March 21, 2019 Accepted: May 6, 2019 Published: May 6, 2019 3910

DOI: 10.1021/acsaem.9b00599 ACS Appl. Energy Mater. 2019, 2, 3910−3917

Article

ACS Applied Energy Materials

Figure 1. Synthetic schematic illustration and structural characterization of NiCoP. (a) Schematic illustration of the synthetic. (b) SEM image of NiCo precursor nanoarrays. (c,d) SEM images of the NiCoP. (e) EDX elemental mapping of the NiCoP. (f) TEM images, and (g) HRTEM image and SAED pattern (inset) of the NiCoP. The scale bar of panels b, c, d, f, and g are 5 μm, 20 μm, 5 μm, 30 nm, and 2 nm, respectively.

2. RESULTS AND DISCUSSIONS The NiCo precursor nanoarrays on Ni foam prepared by hydrothermal method were converted to the nickel foamsupported 3D featherlike NiCoP nanoarrys via phosphorization, as schematically demonstrated in Figure 1a. We first characterized the morphology and structure of NiCoP via the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM). The SEM image (Figure 1b) shows uniform growth of NiCo precursor nanoarrays on Ni foam. As provided in Figure 1c,d, the morphology was converted as featherlike after phosphorization of the precursor. For comparison, binary CoP and Ni2P were prepared by the same method. As shown in Figure S1, Ni2P nanoparticles and CoP nanoflowers uniformly are grown on Ni foam and Ti sheet, respectively. The 3D featherlike NiCoP perpendicularly grows on the surface of Ni foam and they are interlaced with each other, which form a great deal of nanoarrys with porous structure. There are many veins on the surface of the featherlike NiCoP nanoarry, which are interlaced and form a network. A unique structure with great specific surface area and a great number of active sites of 3D featherlike NiCoP will ensure rapid transfer of relational ions. In addition, the vertically distributed microstructure is proved to accelerate the release of gas, thereby effectively decreasing the interfacial resistance between the catalyst and the electrolyte.27 Moreover, the energy dispersive X-ray spectroscopy (EDX) of NiCoP elemental mapping (Figure 1e) proves that Ni, Co, and P elements are uniformly distributed and completely forming bimetallic phosphides. The phase purity of NiCoP, CoP, and Ni2P is identified by the X-ray diffraction (XRD) in which the pattern for the as-prepared sample has a number of peaks that can be well-indexed to NiCoP (JCPDS 71-2336), CoP (JCPDS 65-3544), and Ni2P (JCPDS 89-4862) (Figure S2). The diffraction data corresponds well to the NiCoP of the

HER, but these electrocatalysts generally go through lowly efficient proton transfer and slow diffusion of electrolytes and the injury of gas bubbling, which may cause the catalyst to peel off to further decline its long-term stability. Three dimensional (3D) nanocatalysts have attracted a tremendous amount of attention in light of sufficient active sites exposure, large specific surface area, and the rapid transfer of electron.39,40 It is foreseeable that 3D catalysts have excellent electrocatalytic performance. However, the construction of 3D structure still remains challenges owing to lack of simple and effective synthesis methods. Moreover, the structure−activity relationship between 3D structure and performance still needs to be explored in depth. In this work, we report the synthesis of novel 3D porous featherlike NiCoP (PF-NiCoP/NF) supported on Ni foam substrate for efficient electrocatalytic seawater splitting. The holey structures on the edge of the 3D PF-NiCoP not only expose more active sites but also provide a fast supply of related species and shorten the ion-diffusion distance. Moreover, the 3D featherlike nanoarrays grow on the surface of Ni foam and offer good conductivity and large specific surface area. Thanks to the unique 3D porous structure, the PFNiCoP/NF displayed superior electrocatalytic performance for HER, reaching a current density of 10 mA cm−2 at low overpotential of 46 mV in alkaline (1 M KOH) solution. Simultaneously, 3D PF-NiCoP could afford 10 mA cm−2 at overpotential of 287 mV in seawater (Yellow Sea, China) and exhibited durable long-term electrochemical stability for at least 20 h. These results mark a significant breakthrough in electrocatalytic seawater splitting for a noble-metal-free electrocatalyst system. 3911

DOI: 10.1021/acsaem.9b00599 ACS Appl. Energy Mater. 2019, 2, 3910−3917

Article

ACS Applied Energy Materials

atom possesses the ability of trapping positive charge H* during HER reaction. Moreover, the formation of phosphate species (PO43−) is caused by partial oxidation of the surface exposed to air, which is consistent with the previous researches.4 To summarize, Ni and Co have a partly positive charge (δ+), and P possesses a partly negative charge (δ−), indicating the low electron density transfer from Ni and Co to P.42 The metal and P sites on the surface serve as proton acceptors and hydride acceptor centers during HER process. The change of the binding energy of nickel, cobalt, and phosphorus means that altering the electronic structure of catalyst, which may reduce the energy barrier of the HER process and promote HER activity.34 The above series of characterizations fully demonstrate that we have successfully prepared porous featherlike NiCoP/NF catalysts. Next, electrocatalytic performance of NiCoP/NF is investigated. We first evaluated the HER activity of the porous featherlike NiCoP/NF catalysts in alkaline (1 M KOH) media, employed a standard three-electrode configuration. Figure 3a displays the linear sweep voltammetry (LSV) polarization curves of the NiCoP/NF catalyst toward HER performance. All tests carried out in this study were performed with 90% iR compensation. In order to explore the HER performance under real conditions, the LSV curves without iR compensation are also provided in Supporting Information (Figure S4). As a comparison, Ni2P/NF, CoP/Ti, Pt/C (20 wt %), and bare Ni Foam were also studied. In alkaline (1 M KOH) solution, Pt/ C shows outstanding HER activity with the low overpotential of 46 mV (j = 10 mA cm−2), while Ni foam demonstrates weak catalytic activity, which requires that overpotential of 216 mV reach the same current density. Compared to CoP/Ti (η10 = 76 mV), due to the limitation of intrinsic activity binary Ni2P/ NF shows weak HER activity, which requires an overpotential of 140 mV to reach the current density of 10 mA cm−2. Importantly, the porous featherlike NiCoP/NF displays superior electrocatalytic activity for HER with overpotential of 46 mV at a current density of 10 mA cm−2 in alkaline (1 M KOH) solution in which the activity is equivalent to commercial Pt/C and superior to NiCoP@Ru (about 52 mV), whisker-on-sheet NiCoP (about 59 mV), NiCoP holey nanosheets (about 58 mV), nest-like NiCoP (about 62 mV), and multishelled Ni2P (about 98 mV) (Figure 3b). It is one of the lowest value noble-metal-free HER electrocatalysts as indicated in Table S1. Meanwhile, Figure 3c shows that the Tafel slope of the 3D featherlike NiCoP/NF is 54 mV dec−1, which is close to Pt (about 49 mV dec−1) and is considerably lower than that of bare Ni Foam (about 134 mV dec−1), Ni2P/ NF (about 166 mV dec−1), and CoP/Ti (about 91 mV dec−1). Those results mean a more rapid HER kinetics for the NiCoP/ NF electrode. Moreover, the slopes of the NiCoP/NF belong to the range of 40−120 mV dec−1, revealing hydrogen production by the Volmer−Heyrovsky mechanism.43 Remarkably, the porous featherlike NiCoP/NF could serve as the superior catalyst for the HER over a wide range of pH. As shown in Figure 3d, the HER activity is also examined in acid and neutral solution. Compared to acidic and alkaline, neutral medium is a more environmentally friendly electrolyte. However, the application of neutral media is always affected by slow kinetics and low current density, which demands overpotential of 160 mV at the current density of 10 mA cm −2 in phosphate buffered electrolyte (1 M PBS). Continuous chronoamperometric response was conducted at a constant overpotential of 47 mV. As shown in (Figure 3e),

hexagonal space group and space group number, which are 189 and P6̅2m, respectively. Moreover, we further characterized chemical composition of the NiCo precursor, and the XRD patterns of the precursor match with the NiCo2(CO3)1.5(OH)3 phase (JCPDS 48-0083) (Figure S3). Further TEM image (Figure 1f) indicates that some pores are observed on the edge of the NiCoP heterostructure, providing a fast supply of related species and a shorting ion-diffusion distance. The highresolution transmission electron microscopy (HRTEM) image (Figure 1g) displays obvious lattice fringe with interplanar spacing of 2.2 Å, corresponding to the (111) planes of the hexagonal NiCoP (JCPDS No. 71-2336). It further states that the bimetallic phosphide is not a mixed phase, which is actually present in the XRD pattern. The selective region electron diffraction (SAED) pattern (inset of Figure 1g) shows an ordered lattice structure, which indicates single crystal nature of the as-prepared NiCoP. X-ray photoelectron spectroscopy (XPS) tests are further conducted to explore the surface composition of the NiCoP, the survey spectrum confirms the existence of Ni, Co, and P element (Figure 2a). In the Ni 2p spectrum (Figure 2b), the

Figure 2. (a) Overall XPS spectra of NiCoP, (b) Ni 2p3/2 spectra, (c) Co 2p3/2 spectra, (d) P 2p1/2 spectra of NiCoP.

two distinct peaks of binding energies of 853 and 856.9 eV can be consigned to Ni-P bond and Ni-POx bond,34 respectively, with its satellite peak at 861.6 eV. It is noted that the peak of 853 eV toward higher binding energy compared to metallic Ni (852.6 eV), which demonstrates that Ni has a partly positive charge (Niδ+). Similarly, In the Co 2p spectrum (Figure 2c), the peak of 778.5 eV is associated with the CoP bond and the binding energy is also observed that positively shifts from metallic Co (778.2 eV), which indicates the existence of partly positive charge Co species (Coδ+). The peak of 781.5 eV could be related to Co-POx bond.41 The P 2p spectrum shows two obvious peaks at 129.2 and 133.8 eV, corresponding to the metal phosphides bond and phosphate species (PO43−), respectively (Figure 2d). The binding energy of 129.2 eV negatively shifted from elemental P (130.0 eV), suggesting that P atom carries partly negative charge (Pδ−). Therefore, the P 3912

DOI: 10.1021/acsaem.9b00599 ACS Appl. Energy Mater. 2019, 2, 3910−3917

Article

ACS Applied Energy Materials

Figure 3. (a) Polarization curves per geometric area of related electrocatalysts for HER with 90% iR compensation recorded at a scan rate of 1 mV s−1 in 1 M KOH. (b) HER overpotentials of the NiCoP/NF and the reported electrocatalysts for comparison at 10 mA cm−2. (c) Tafel slopes of the related electrocatalysts. (d) HER performance of NiCoP/NF in acidic (0.5 M H2SO4), neutral (1.0 M PBS), and alkaline (1 M KOH) solution. (e) Chronoamperometric response under a constant overpotential of 47 mV. (f) Cycling stability of NiCoP/NF in 1.0 M KOH media. (g) Cdl values of related electrocatalysts. (h) Nyquist plots of different electrocatalysts. (i) Faraday efficiency of NiCoP/NF for HER.

the H2 production in the HER was measured (Figure 3i) and which is about 99% after 60 min. Electrocatalytic HER in seawater remains a challenging task. In the seawater environment, most electrocatalysts are poorly conductive (compared to 1 M KOH) and strongly corrosive, which results in weak activity and instability.5 Thus, it is of great significance to seek high activity and long-term stability HER catalysts, which can be used in real seawater instead of buffering or simulating seawater. In this part, we further measured the HER activity of the porous featherlike NiCoP/ NF electrode in real seawater (Yellow Sea, China; pH = 8.4). The LSV curves of the NiCoP/NF, Ni2P/NF, CoP/Ti, Pt/C, and bare Ni foam electrodes are represented in Figure 4a, respectively. In seawater, the featherlike NiCoP/NF exhibits excellent HER activity (η10 = 287 mV), which not only has higher activity than commercial Pt/C (20 wt %) but also far exceeds the recently reported CoMoP@C (η10 = 450 mV),3 cobalt selenide (η10 = 350 mV),43 and U-CNT-90054 composite materials (η10 = 670 mV) (Figure 4b).45 The superior HER activity is also attributed to porous featherlike NiCoP/NF, because of the high onset overpotential (η ∼ 460 mV) of bare Ni foam in natural seawater.

the NiCoP/NF catalyst maintains steady HER activity during the stable hydrogen production for 20 h. Simultaneously, after 5000 cycles of cyclic voltammetry (CV) test, compared to the original CV of the NiCoP/NF (Figure 3f), there is no noticeable increase of the overpotential. A strong connection between featherlike NiCoP and Ni foam could also be favorable for improving their outstanding HER stability. To investigate the origin of the exceptional HER activity of the porous featherlike NiCoP/NF, the electrochemically active surface areas (ECSAs) (Figure S5) of NiCoP/NF, Ni2P/NF, CoP/Ti, and bare Ni foam was determined by measuring the double-layer capacitance (Cdl) at the potential window of −0.12 to 0.06 V, employing CV at different scan rates. As observed in Figure 3g, featherlike NiCoP/NF demonstrates the highest Cdl value, meaning a great number of active sites exposure and large specific surface area, which is one of the most important factors in improving HER activity. Besides, electrochemical impedance spectroscopy (EIS) exhibits the lowest charge transfer resistance (Rct) of the porous featherlike NiCoP/NF (Figure 3h), indicating that it has high conductivity and thus is contributing to the HER reaction kinetics, which is in agreement with the performances of studied electrocatalysts.44 Moreover, the faradaic efficiency of 3913

DOI: 10.1021/acsaem.9b00599 ACS Appl. Energy Mater. 2019, 2, 3910−3917

Article

ACS Applied Energy Materials

Figure 4. (a) Polarization curves per geometric area of related electrocatalysts for HER with 90% iR compensation recorded at a scan rate of 1 mV s−1 in real seawater. (b) HER overpotentials of the NiCoP/NF and the reported electrocatalysts for comparison at 10 mA cm−2. (c,d) Chronoamperometric response under a constant overpotential of 290 mV and photos of the NiCoP/NF (inset) before and after acid treatment, respectively. (e) HER performance comparison before acid treatment and after acid treatment. (f) Faraday efficiency of NiCoP/NF for HER.

water splitting in 1 M KOH. The electrode exhibits exceptional activity in this two-electrode system, which offers a current density of 10 mA cm−2 at 1.53 V (Figure S6b). Moreover, according to recent reports, the active site of metal phosphide in the OER reaction is a metal oxide or oxyhydroxide in situ generated during the OER process.47,48 Thus, XPS measures are further conducted to explore the surface composition of NiCoP after the OER test. Figure S7 shows that the P 2p disappears, which may be ascribed to transformation of P in NiCoP into phosphates during OER. Meanwhile, disappearance of the Ni-P and Co-P bond is ascribed to the formation of CoOOH and NiOOH species after the OER test, 49 respectively. The formation of the oxide layer provides a great deal of active site for OER. In addition, we further explored the morphology and composition of featherlike NiCoP after the OER test. As shown in Figures S8 and S9, there was no evident change in the morphology and composition of the 3D featherlike NiCoP after the OER test, which reveals wonderful structural stability and chemical state. The increased performance of HER and OER may be attributed to the special 3D morphology and holey structure of the featherlike NiCoP nanoarrys. Specifically, hierarchical featherlike structure with large specific surface area should be favorable so that the active site is fully exposed to the electrolyte. Moreover, the holey structure not only may facilitate facile release of generated H2 via the H-poisoned active sites but also may provide a fast supply of related species and a shorting ion-diffusion distance. Furthermore, the application of Ni foam as conductive substrate could further improve the conductivity and specific surface area to maximize the catalytic site density. Simultaneously, based on previous reports, density function theory (DFT) calculations illustrated that the Co substitution can modify the electronic structure of

Besides activity, stability is also an important factor, which hinders the development of HER electrolysis in real seawater.46 Interestingly, as shown in Figure 4c, continuous chronoamperometric response was conducted at the overpotential of 290 mV, whereas the current density of the porous featherlike NiCoP/NF remained almost unchanged during 20 h of continuous reaction. Additionally, a white precipitate can be observed on the surface of the electrode after the stable hydrogen production for 20 h (inset of Figure 4c), causing that the active site to eventually be sealed while decreasing HER performance. In order to restore the activity of electrodes in seawater, the electrode is treated by soaking in weak acid (0.05 M HCl) for 30 min. As provided in the inset of Figure 4d, the white precipitate has been removed. The electrode again performs continuous timing current response at a same overpotential of 290 mV. The current density is finally maintained at 6.5 mA cm−2 after continuous reaction for 12 h (Figure 4d). Simultaneously, compared to the result before acid treatment (Figure 4e), the overpotential of featherlike NiCoP/NF only increases slightly which indicates that the activity of the featherlike NiCoP/NF electrode can be restored via treatment of weak-acid and possesses continued hydrogen evolution in seawater for a long time.6 Meanwhile, the generation and release of obviously stable bubbles can be seen on the electrode surface (Video S1). Importantly, as offered in Figure 4f, the faradaic efficiency of featherlike NiCoP/NF is 96.5% in real seawater for the HER. Except excellent HER activity, the 3D featherlike NiCoP/ NF is featured by high OER activity, which was assessed in 1.0 M KOH electrolyte. Strikingly, the featherlike NiCoP requires only an overpotential of 250 mV to deliver a current density of 10 mA cm−2 (Figure S6a). Because the 3D featherlike catalyst exhibits both OER and HER superior activity, we further evaluate the NiCoP/NF as both anode and cathode for overall 3914

DOI: 10.1021/acsaem.9b00599 ACS Appl. Energy Mater. 2019, 2, 3910−3917

Article

ACS Applied Energy Materials

Synthesis of CoP/Ti. The synthetic method was the same as that for NiCoP except for replacing foamed nickel with titanium sheet. The mass loading of CoP was determined to be ∼1.8 mg cm−2. 4.2. Electrochemical Measurements. All electrochemical tests were conducted at room temperature with a CHI750E electrochemical workstation (CH Instruments, Inc., Shanghai, China) via a three-electrode system. The 3D featherlike NiCoP/NF, Ni2P/NF, and CoP/Ti can be used directly as a working electrode with dimensions of 1 × 1 cm2. As for Pt/C and IrO2/C, the catalysts were first dispersed in 1 mL of a 4:1 (v/v) water/ethanol mixture and 10 μL of a 5% PVDF solution prior to a ≥30 min sonication to form a homogeneous ink and then loaded onto a preprocessed 1 cm2 Ni foam via drop-coating. Polarization curves were recorded by using linear sweep voltammetry (LSV) in 1 M KOH aqueous solution and real seawater with a scan rate of 1 mV s−1, using a saturated Hg/HgO electrode and saturated calomel electrode (SCE) as the reference electrode, respectively. All potentials measured were calibrated to RHE according to ERHE = EHg/HgO(V) + 0.098 V + 0.0591 × pH, ERHE(V) = ESCE (V) + 0.245 + 0.0591 × pH, and the polarization curves were corrected with 90% iR compensation within the electrolyte. The Nyquist plots of EIS were performed in the frequency range of 10 kHz to 0.1 Hz at the open circuit voltage of HER. The CV was carried out in 1 M KOH aqueous solution with scan rates of 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV s−1 to assess electrochemical double-layer capacitance (Cdl) with no faradic processes. The obtained current densities at the selected potential had a linear relationship with the scan rates and the corresponding slopes of fitting curves were considered as the Cdl. Herein, the specific capacitance (20−60 μF cm−2) of 40 μF cm−2 was used to calculate the ECSA values according to the following equation

Ni2P and reduce the surface adsorption energy of the reactants, and thus beneficial the HER activity.34 Interestingly, the HER process of the prepared the porous featherlike NiCoP/NF electrodes in alkaline media could be conducted via the following steps Volmer: H 2O + e− → Hads* + OH− (electrochemical) (1)

Heyrovsky: H 2Oads + e− + H* → H 2 + OH− (electrochemical)

(2)

The Volmer−Heyrovsky pathway involves a two-step process that adsorbed water molecule and the electrochemical reduction of trapped H2O into adsorbed H* and OH− species, followed by detachment of OH− and formation of H* to obtain H2. In the Volmer step, neutral solution requires much higher activation energy, which results in slow HER reaction dynamics compared to that conducted in alkaline and acidic solution.6 Pδ− serves NiCoP/NF as the hydride acceptor centers preferentially trap the formation of H* species via the Volmer step. The polarization-induced partial negative charge with an P-terminated surface at the center of NiCoP can significantly enhance the absorption and release of H*, and improve HER activity.43

3. CONCLUSION In summary, we constructed a novel porous featherlike NiCoP nanoarry on a Ni foam substrate for efficient stable seawater splitting. The PF-NiCoP/NF electrocatalyst shows remarkable activity and long-term durability in alkaline solution and seawater. Indeed, it only requires a very low overpotential of 46 mV to deliver a current density of 10 mA cm−2 in an alkaline (1 M KOH) solution. Furthermore, it also demonstrates superior HER activity in real seawater, affording current density of 10 mA cm−2 at low overpotential of 287 mV, which is better than commercial Pt/C (20 wt %) and other seawater electrocatalyst. The outstanding HER performance can be mainly ascribed to the synergistic effect of the 3D structure, enriched holes, and the conductive substrate. This work may present a new strategy to design a promising electrocatalyst that is superior to platinum over a wide range of pH, even in seawater, and this 3D structure can also be extended to other fields, (e.g., batteries, supercapacitors).

■

ECSA =

Cdl 40μF/cm 2

cm 2 ECSA

4.3. Materials Characterization. The TEM and HRTEM measurements were conducted utilizing a FEI Tecnai G2F20field emission electron microscope. SEM measurements were conducted on a SU8010 field scanning electron microscope at an accelerating voltage of 3 kV. High-resolution STEM-HAADF images were gained on an aberration corrected transmission electron microscope JEMARM200F equipped with cold field emission gun with acceleration voltage of 200 kV. XRD patterns were obtained on a Bruker D8 Advance X-ray diffractometer fitted with CuKα radiation. XPS measurements were carried out on a Kratos Amicus X-ray photoelectron spectrometer with an exciting source of Mg with the working power of 180 W.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00599. SEM images of the Ni2P and CoP; XRD patterns of the as-prepared sample; comparison of LSV polarization curves without correcting the iR drop; details for estimation of electrochemically active surface area; LSV polarization curves of OER and overall water splitting test; XPS spectrum of Ni 2p3/2, Co 2p3/2, and P 2p in NiCoP before and after OER tests; SEM and EDS of the porous featherlike NiCoP/NF after OER tests; and comparison of HER activities with recently documented materials (PDF) Video of generation and release of stable bubbles (MP4)

EXPERIMENTAL SECTION

4.1. Materials Preparation. Synthesis of 3D featherlike NiCoP/ NF. A piece of Ni foam (typically 3 × 3.5 cm) was precleaned by 3 M HCl, ethanol, and water. In a typical synthesis, 3 mmol of Co (NO3)2· 6H2O, 15 mmol of urea, and 8 mmol of NH4F were dissolved in 60 mL of deionized water. Then, the aqueous solution and Ni foam was transferred to Teflon-lined stainless autoclave (100 mL) sealed and maintained at 120 °C for 6 h. The product was washed with ethanol and ultrapure water several times, and then dried in an oven at 60 °C to obtain NiCo nanowires arrays on Ni foam as precursor. The obtained NiCo precursor and 1 g of NaH2PO2·H2O (molar ratio: ∼1:5) were placed at two positions of the porcelain boat in a tube furnace and then heated at 300 °C for 2 h with a heating speed of 2 °C min−1 in N2 atmosphere. After the reaction, the mass loading was ∼2 mg cm−2. Synthesis of Ni2P/NF. The Ni foam and 1 g of NaH2PO2·H2O (molar ratio: ∼1:5) were placed at two positions of the porcelain boat in tube furnace and then heated at 300 °C for 2 h with a heating speed of 2 °C min−1 in N2 atmosphere. After the reaction, the mass loading was ∼1.6 mg cm−2.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.D.). *E-mail: [email protected] (L.C.). 3915

DOI: 10.1021/acsaem.9b00599 ACS Appl. Energy Mater. 2019, 2, 3910−3917

Article

ACS Applied Energy Materials ORCID

(14) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661−4672. (15) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R.; Liu, S.; Zhuang, X. D.; Feng, X. L. Interface Engineering of MoS2/Ni3S2 Heterostructures for Highly Enhanced Electrochemical OverallWater-Splitting Activity. Angew. Chem., Int. Ed. 2016, 55, 6702−6707. (16) Yin, J.; Li, Y.; Lv, F.; Lu, M.; Sun, K.; Wang, W.; Wang, L.; Cheng, F. Y.; Li, Y. F.; Xi, P. X.; Guo, S. J. Oxygen Vacancies Dominated NiS2/CoS2 Interface Porous Nanowires for Portable ZnAir Batteries Driven Water Splitting Devices. Adv. Mater. 2017, 29, 1704681. (17) Chen, F.; Ji, S.; Liu, Q.; Wang, H.; Liu, H.; Brett, D. J. L.; Wang, G.; Wang, R. Rational Design of Hierarchically Core-Shell Structured Ni3S2@NiMoO4 Nanowires for Electrochemical Energy Storage. Small 2018, 14, 1800791. (18) Xiong, Q. Z.; Wang, Y.; Liu, P. F.; Zheng, L. R.; Wang, G. Z.; Yang, H. G.; Wong, P. K.; Zhang, H. M.; Zhao, H. J. Cobalt Covalent Doping in MoS2 to Induce Bifunctionality of Overall Water Splitting. Adv. Mater. 2018, 30, 1801450. (19) Zhang, J. J.; Zhang, C. H.; Wang, Z. Y.; Zhu, J.; Wen, Z. W.; Zhao, X. Z.; Zhang, X. X.; Xu, J.; Lu, Z. G. Synergistic Interlayer and Defect Engineering in VS2 Nanosheets toward Efficient Electrocatalytic Hydrogen Evolution Reaction. Small 2018, 14, 1703098. (20) Fang, L.; Li, W. X.; Guan, Y. X.; Feng, Y. Y.; Zhang, H. J.; Wang, S. L.; Wang, Y. Tuning Unique Peapod-Like Co(SxSe1‑x)2 Nanoparticles for Efficient Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1701008. (21) Fang, Z. W.; Peng, L. L.; Lv, H. F.; Zhu, Y.; Yan, C. S.; Wang, S. Q.; Kalyani, P.; Wu, X. J.; Yu, G. H. Metallic Transition Metal Selenide Holey Nanosheets for Efficient Oxygen Evolution Electrocatalysis. ACS Nano 2017, 11, 9550−9557. (22) Hou, Y.; Qiu, M.; Nam, G.; Kim, M. G.; Zhang, T.; Liu, K.; Zhuang, X.; Cho, J.; Yuan, C.; Feng, X. Integrated Hierarchical Cobalt Sulfide/Nickel Selenide Hybrid Nanosheets as an Efficient Threedimensional Electrode for Electrochemical and Photoelectrochemical Water Splitting. Nano Lett. 2017, 17, 4202−4209. (23) Zhang, Y. J.; Gong, Q. F.; Li, L.; Yang, H. C.; Li, Y. G.; Wang, Q. B. MoSe2Porous Microspheres Comprising Monolayer Flakes with High Electrocatalytic Activity. Nano Res. 2015, 8, 1108−1115. (24) Jin, Y.; Wang, H.; Li, J.; Yue, X.; Han, Y.; Shen, P. K.; Cui, Y. Porous MoO2 Nanosheets as Non-noble Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Mater. 2016, 28, 3785−90. (25) Hwang, J.; Rao, R. R.; Giordano, L.; Katayama, Y.; Yu, Y.; Shao-Horn, Y. Perovskites inCatalysis and Electrocatalysis. Science 2017, 358, 751−756. (26) Zhu, Y. L.; Zhou, W.; Zhong, Y. J.; Bu, Y. F.; Chen, X. Y.; Zhong, Q.; Liu, M. L.; Shao, Z. P. Manipulating Adsorption-Insertion Mechanisms in Nanostructured Carbon Materials for High-Efficiency Sodium Ion Storage. Adv. Energy Mater. 2017, 7, 1602122. (27) Liu, Y. R.; Du, Y. M.; Gao, W. K.; Dong, B.; Han, Y.; Wang, L. Surface PhosphorSulfurization of NiCo2O4Nanoneedles Supported on Carbon Cloth with Enhanced Electrocatalytic Activity for Hydrogen Evolution. Electrochim. Acta 2018, 290, 339−346. (28) Lv, Q. L.; Yang, L.; Wang, W.; Lu, S. Q.; Wang, T. E.; Cao, L. X.; Dong, B. H. One-step Construction of Core/Shell Nanoarrays with Holey shell and Exposed Interfaces for Overall Water Splitting. J. Mater. Chem. A 2019, 7, 1196−1205. (29) Liu, Y. R.; Hu, W. H.; Han, G. Q.; Dong, B.; Li, X.; Shang, X.; Chai, Y. M.; Liu, Y. Q.; Liu, C. G. Novel CoP Hollow Prisms as Bifunctional Electrocatalysts for Hydrogen Evolution Reaction in Acid Media and Overall Water-splitting in Basic Media. Electrochim. Acta 2016, 220, 98−106. (30) Ma, B.; Yang, Z.; Chen, Y.; Yuan, Z. Nickel Cobalt Phosphide with Three-Dimensional Nanostructure as aHighly Efficient Electrocatalyst for Hydrogen Evolution Reaction in both Acidic and Alkaline Electrolytes. Nano Res. 2019, 12, 375−380.

Wei Wang: 0000-0003-3185-4570 Bohua Dong: 0000-0002-0917-0204 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work is supported by National Natural Science Foundation of China (Grant 21301187), Fundamental Research Funds for the Central Universities (Grant 201564001), and Shandong Provincial Natural Science Foundation, China (Grant ZR2018QB001)

(1) Jin, H.; Guo, C.; Liu, X.; Liu, J.; Vasileff, A.; Jiao, Y.; Zheng, Y.; Qiao, S. Z. Emerging Two Dimensional Nanomaterials for Electrocatalysis. Chem. Rev. 2018, 118, 6337−6408. (2) Han, G. Q.; Li, X.; Liu, Y. R.; Dong, B.; Hu, W. H.; Shang, X.; Zhao, X.; Chai, Y. M.; Liu, Y. Q.; Liu, C. G. Controllable Synthesis of ThreeDimensionalElectrodeposited Co-P Nanosphere Arrays as Efficient Electrocatalysts for Overall Water Splitting. RSC Adv. 2016, 6, 52761−52771. (3) Ma, Y. Y.; Wu, C. X.; Feng, X. J.; Tan, H. Q.; Yan, L. K.; Liu, Y.; Kang, Z. H.; Wang, E. B.; Li, Y. G. Highly Efficient Hydrogen Evolution from Seawater by a Low-Cost and Stable CoMoP@C Electrocatalyst Superior to Pt/C. Energy Environ. Sci. 2017, 10, 788− 798. (4) Li, Y. P.; Liu, J. D.; Chen, C.; Zhang, X. H.; Chen, J. H. Preparation of NiCoP hollow quasi-polyhedra and their electrocatalytic properties for hydrogen evolution in alkaline solution. ACS Appl. Mater. Interfaces 2017, 9, 5982−5991. (5) Hsu, S. H.; Miao, J.; Zhang, L.; Gao, J.; Wang, H.; Tao, H.; Hung, S. F.; Vasileff, A.; Qiao, S. Z.; Liu, B. An Earth-Abundant Catalyst-Based Seawater Photoelectrolysis System with 17.9% Solarto-Hydrogen Efficiency. Adv. Mater. 2018, 30, 1707261. (6) Lu, X.; Pan, J.; Lovell, E.; Tan, T. H.; Ng, Y. H.; Amal, R. A SeaChange: Manganese Doped Nickel/Nickel Oxide Electrocatalysts for Hydrogen Generation from Seawater. Energy Environ. Sci. 2018, 11, 1898−1910. (7) Yang, H.; Zhang, Y.; Hu, F.; Wang, Q. Urchin-like CoP Nanocrystals as Hydrogen Evolution Reaction and Oxygen Reduction Reaction Dual-Electrocatalyst with Superior Stability. Nano Lett. 2015, 15, 7616−7620. (8) Wang, C. H.; Yang, H. C.; Zhang, Y. J.; Wang, Q. B. NiFe Alloy Nanoparticle with Hexagonal Close-Packed Crystal Structure Stimulates Superior Oxygen Evolution Reaction Electrocatalytic Activity. Angew. Chem., Int. Ed. 2019, 58, 6099. (9) Yang, L.; Guo, Z. L.; Huang, J.; Xi, Y. N.; Gao, R. J.; Su, G.; Wang, W.; Cao, L. X.; Dong, B. H. Vertical Growth of 2D Amorphous FePO4 Nanosheet on Ni Foam: Outer and Inner Structural Design for Superior Water Splitting. Adv. Mater. 2017, 29, 1704574. (10) McKone, J. R.; Sadtler, B. F.; Werlang, C. A.; Lewis, N. S.; Gray, H. B. Ni-Mo Nanopowders for Efficient Electrochemical Hydrogen Evolution. ACS Catal. 2013, 3, 166−169. (11) Deng, J.; Ren, P. J.; Deng, D. H.; Yu, L.; Yang, F.; Bao, X. H. Highly Active and Durable Non-Precious-Metal Catalysts Encapsulated in Carbon Nanotubes for Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 1919−1923. (12) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting devices. J. Am. Chem. Soc. 2015, 137, 4347−4357. (13) Liu, Y. R.; Hu, W. H.; Li, X.; Dong, B.; Shang, X.; Han, G. Q.; Chai, Y. M.; Liu, Y. Q.; Liu, C. G. One-pot Synthesis of Hierarchical Ni2P/MoS2Hybrid Electrocatalysts with Enhanced Activity for Hydrogen Evolution Reaction. Appl. Surf. Sci. 2016, 383, 276−282. 3916

DOI: 10.1021/acsaem.9b00599 ACS Appl. Energy Mater. 2019, 2, 3910−3917

Article

ACS Applied Energy Materials (31) Li, Y.; Zhang, H.; Jiang, M.; Kuang, Y.; Sun, X.; Duan, X. Ternary NiCoP Nanosheet Arrays: An Excellent Bifunctional Catalyst for Alkaline Overall Water Splitting. Nano Res. 2016, 9, 2251−2259. (32) Surendran, S.; Shanmugapriya, S.; Sivanantham, A.; Shanmugam, S.; Selvan, K. R. Electrospun Carbon Nanofibers Encapsulated with NiCoP: A Multifunctional Electrode for Supercapattery and Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution Reactions. Adv. Energy Mater. 2018, 8, 1800555. (33) Wu, R.; Xiao, B.; Gao, Q.; Zheng, Y. R.; Zheng, X. S.; Zhu, J. F.; Gao, M. R.; Yu, S. H. A Janus Nickel Cobalt Phosphide Catalyst for High-Efficiency Neutral-pH Water Splitting. Angew. Chem., Int. Ed. 2018, 57, 15445−15449. (34) Liang, H.; Gandi, A. N.; Anjum, D. H.; Wang, X.; Schwingenschlogl, U.; Alshareef, H. N. Plasma-Assisted Synthesis of NiCoP for Efficient Overall Water Splitting. Nano Lett. 2016, 16, 7718−7725. (35) Sun, H. M.; Xu, X. B.; Yan, Z. H.; Chen, X.; Cheng, F. Y.; Weiss, P. S.; Chen, J. Porous Multishelled Ni2P Hollow Microspheres as an Active Electrocatalyst for Hydrogen and Oxygen Evolution. Chem. Mater. 2017, 29, 8539−8547. (36) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267−9270. (37) Cheng, L.; Huang, W. J.; Gong, Q. F.; Liu, C. H.; Liu, Z.; Li, Y. G.; Dai, H. J. Ultrathin WS2 Nanoflakes as a High-Performance Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 7860−7863. (38) He, W. D.; Wang, C. G.; Li, H. Q.; Deng, X. L.; Xu, X.; Zhai, T. Y. Ultrathin and Porous Ni3S2/CoNi2S4 3D-Network Structure for Superhigh Energy Density Asymmetric Supercapacitors. Adv. Energy Mater. 2017, 7, 1700983. (39) Nardecchia, S.; Carriazo, D.; Ferrer, M. L.; Gutierrez, M. C.; Del Monte, F. Three Dimensional Macroporous Architectures and Aerogels Built of Carbon Nanotubes and/or Graphene: Synthesis and Applications. Chem. Soc. Rev. 2013, 42, 794−830. (40) Du, C.; Yang, L.; Yang, F.; Cheng, G. Z.; Luo, W. Nest-like NiCoP for Highly Efficient Overall Water Splitting. ACS Catal. 2017, 7, 4131−4137. (41) Fang, Z.; Peng, L.; Qian, Y.; Zhang, X.; Xie, Y.; Cha, J. J.; Yu, G. H. Dual Tuning of Ni-Co-A (A= P, Se, O) Nanosheets by Anion Substitution and Holey Engineering for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2018, 140, 5241−5247. (42) Liu, S.; Liu, Q.; Lv, Y.; Chen, B. Y.; Zhou, Q.; Wang, L.; Zheng, Q. H.; Che, C. J.; Chen, C. Y. Ru Decorated with NiCoP: An Efficient and Durable Hydrogen Evolution Reaction Electrocatalyst in both Acidic and Alkaline Conditions. Chem. Commun. 2017, 53, 13153− 13156. (43) Zhao, Y. Q.; Jin, B.; Zheng, Y.; Jin, H. Y.; Jiao, Y.; Qiao, S. Z. Charge State Manipulation of Cobalt Selenide Catalyst for Overall Seawater Electrolysis. Adv. Energy Mater. 2018, 8, 1801926. (44) Mishra, I. K.; Zhou, H.; Sun, J.; Qin, F.; Dahal, K.; Bao, J.; Chen, S.; Ren, Z. F. Hierarchical CoP/Ni5P4/CoP Microsheet Arrays as a Robust pH-universal Electrocatalyst for Efficient Hydrogen Generation. Energy Environ. Sci. 2018, 11, 2246−2252. (45) Gao, S.; Li, G. D.; Liu, Y.; Chen, H.; Feng, L. L.; Wang, Y.; Yang, M.; Wang, D.; Wang, S.; Zou, X. X. Electrocatalytic H2Production from Seawater over Co, N-Codoped Nanocarbons. Nanoscale 2015, 7, 2306−2316. (46) Song, L. J.; Meng, H. M. Effect of Carbon Content on Ni-Fe-C Electrodes for Hydrogen Evolution Reaction in Seawater. Int. J. Hydrogen Energy 2010, 35, 10060−10066. (47) Ryu, J.; Jung, N.; Jang, J. H.; Kim, H. J.; Yoo, S. J. In Situ Transformation of Hydrogen-Evolving CoP Nanoparticles: Toward Efficient Oxygen Evolution Catalysts Bearing Dispersed Morphologies with Co-oxo/hydroxo Molecular Units. ACS Catal. 2015, 5, 4066− 4074.

(48) Stern, L. A.; Feng, L. G.; Song, F.; Hu, X. L. Ni2P as a Janus Catalyst for Water Splitting: the Oxygen Evolution Activity of Ni2P Nanoparticles. Energy Environ. Sci. 2015, 8, 2347−2351. (49) Wang, X. G.; Li, W.; Xiong, D. H.; Petrovykh, D. Y.; Liu, L. F. Bifunctional Nickel Phosphide Nanocatalysts Supported on Carbon Fiber Paper for Highly Efficient and Stable Overall Water Splitting. Adv. Funct. Mater. 2016, 26, 4067−40.

3917

DOI: 10.1021/acsaem.9b00599 ACS Appl. Energy Mater. 2019, 2, 3910−3917