NiFe2O4 Imbedded in Carbon

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Phosphorus-Doped FeNi Alloys/NiFe2O4 Imbedded in Carbon Network Hollow Bipyramid as Efficient Electrocatalysts for Oxygen Evolution Reaction Aixin Fan,† Congli Qin,† Xin Zhang,*,† Xiaoping Dai,† Zhun Dong,† Chenglong Luan,† Lei Yu,† Jiaqi Ge,† and Fei Gao‡ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P. R. China Jiangsu Key Laboratory of Vehicle Emissions Control, 22 Hankou Road, Gulou District, Nanjing, Jiangsu Province 210093, China

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‡

S Supporting Information *

ABSTRACT: Ni/Fe-based bimetallic nanoarchitecture materials play an important role in the development of non-preciousmetal-based electrocatalysts toward water splitting, but the low activity and poor stability greatly hinder their commercial applications. It is significant to explore facile and effective methods to improve their electrocatalytic activity. A simple selftemplate strategy is demonstrated to fabricate a hollow bipyramid constructed by P-doped FeNi alloys/NiFe2O4 nanoparticles encapsulated in carbon network (P-Ni0.5Fe@C). Bimetallic analogous MIL-101 (Fe) precursor (Ni0.5Fe-BDC CP) with uniform morphology and stable structure was synthesized through a solvothermal reaction. By subsequent carbonization and phosphorization steps, P element was doped into the composite FeNi alloys/NiFe2O4 nanoparticles. Benefiting from the efficient mass and electron transfer of the hollow structure, the precise adjustment for the electron structure of P dopants, and carbon-encapsulated active components that could provide large numbers of active sites as well as prevent the aggregation and dissolution of active components, the optimal P-Ni0.5Fe@C catalyst exhibits a low overpotential of 256 mV to reach a current density of 10 mA cm−2, a small Tafel slope of 65 mV dec−1, and remarkable long-term stability toward oxygen evolution reaction in 1 M KOH, which is better than that of commercial IrO2 (318 mV at 10 mA cm−2 for overpotential and 120 mV dec−1 for Tafel slope, respectively). More remarkably, when it was employed in a two-electrode configuration based on PNi0.5Fe@C as anode and commercial Pt/C as cathode catalysts (P-Ni0.5Fe@C || Pt/C), a potential of only 1.49 V (corresponding overpotential of 260 mV) was required to achieve 10 mA·cm−2. This work provides insight into the rational composition and morphology design of an earth-abundant electrocatalyst with highly efficient electrocatalytic activities toward overall water splitting. KEYWORDS: Phosphorus doping, Hollow bipyramid, Morphology design, Oxygen evolution reaction, Overall water splitting

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nature, including metal alloys,12 metal oxides,13,14 and hydroxides.15−18 However, most of these electrocatalysts suffer from poor electronic conductivity, low activity, and long-term durability. Fortunately, this problem could be partly solved by doping heteroatoms (P, S, N, etc.) with metal to regulate the electronic structure such as metal chalcogenides19,20 and metal phosphides.21−24 Among these well-defined catalysts, Ni- or Fe-based metal phosphides (NFTMPs) have been extensively concerned as a new rich and highly active OER catalyst such as FeP,25 NiP,26,27 and NiFeP.28 Although significant progress has been made in promoting the OER performance of NFTMPs

INTRODUCTION In recent years, the oxygen evolution reaction (OER) has played an important role in energy conversion and storage such as efficient water splitting,1−4 rechargeable metal−air batteries,5,6 and regenerative fuel cells.7 In particular, electrocatalytic water splitting has been intensely investigated as one of the most environmentally friendly and promising ways for the production of hydrogen energy. As has been documented, the water splitting is severely hindered by OER, which is a kinetically sluggish reaction with 4e− transfer. RuO2 and IrO2 are well known to be efficient for OER; however, the scarcity and high cost have been restricting the large-scale application.8−11 Fortunately, many transition-metal-based catalysts show great potential as alternate catalysts for OER due to their good water oxidation performance and the earth-abundant © XXXX American Chemical Society

Received: September 28, 2018 Revised: November 19, 2018 Published: December 24, 2018 A

DOI: 10.1021/acssuschemeng.8b04997 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Ni0.5Fe@C catalyst presented unique hollow structures, large exposed surface, and kinetically favorable and carbonencapsulated active composition. As a result, the P-Ni0.5Fe@ C catalyst exhibited a low overpotential of 256 mV, a small Tafel slope of 65 mV dec−1, as well as remarkable long-term stability toward OER, which even surpassed the previous stateof-the-art transition-metal phosphide (TMP) catalysts to the best of our knowledge.

due to auto-oxidation under a high oxidation potential, the active components are easy to agglomerate and dissolve, resulting in a decrease in electrochemical activity.29 Fortunately, recent researchers have shown that embedding active components into conductive porous carbons could effectively overcome these obstacles, which not only could prevent active components from corrosion and agglomeration with neighboring metal nanoparticles (NPs) during both materials synthesis and electrochemical reactions but also improve the conductivity of the electrocatalysts.30,31 However, conventional catalysts simply assembled from transition-metal/metal-phosphides NPs and carbon materials still lack sufficient activity for OER and therefore are not a substitute for precious metals for commercial applications. It is known that, in general, apart from the composition, catalytic properties of OER catalyst are strongly dependent on their morphology, and thus endowing them with various 3D hierarchical architectures is another efficient strategy for further enhancing OER activity. Among these architectures, hollow nanostructures are generally considered to be more advantageous than solid counterparts due to their unique properties such as large surface area, low density, and open structure for the electrocatalysts to contact the electrolyte, all of which are beneficial to improve the electrocatalytic performance.32−37 Numerous studies have been made to fabricate these nanostructures by template method including sacrificial, hard, and soft templates.38−42 However, it is very difficult to simultaneously implement P-atom doping, morphology modulation, and embedding active components into conductive porous carbons. In this context, metal−organic frameworks (MOFs) composed of well-organized metal ions and organic ligands are promising sacrificial templates or self templates to build hollow structures.43−47 Meanwhile, the carbon materials with different microporous, mesoporous, and porous structures including carbon nanotube,48 carbon spheres,49 carbon-inherited rhombic dodecahedral shapes,50 microboxes,51 and so on have been prepared by pretreating the MOFs at high temperature. In particular, MIL-101 (Fe), a well-studied Fe-containing octahedral MOF, has been regarded as a suitable precursor for obtaining high-performance nonprecious Fe−C catalyst. It should be noted that transition-metal Fe in such a structure can catalytic graphitization during the high-temperature carbonization process. However, only one kind of metal content is still lacking the sufficient OER activity of pure MIL-101 (Fe). Considering the good compatibility of metal ions and organic frameworks, homogeneous multimetallic MOFs could be easily synthesized. In contrast with monometallic MOFs, bimetallic MOFs may possess an even greater degree of catalytic activity and stability because more variables are available for tuning. Therefore, we believe that it is very interesting to engineer a novel architecture derived from multimetallic MOFs with an optimized multistage structure, which can provide abundant void space, fast mass and electron transfer, sufficient catalytic active sites, and positive synergy works as an ideal candidate for OER. Herein we first synthesized analogous MIL-101 (Fe) precursor (Ni0.5Fe-BDC CP) by introducing nickel acetylacetonate during the synthesis of MIL-101(Fe). Then, by subsequent one-step carbonization and phosphorization treatments with sodium hypophosphite, hollow bipyramid catalyst (P-Ni0.5Fe@C) was obtained with P element doped into the composite FeNi alloys/NiFe2O4 NPs. The resulting P-

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EXPERIMENTAL SECTION Material Preparation. Ferric chloride (FeCl3·6H2O), nickel(II) acetylacetonate (Ni(acac)2), terephthalic acid (H2BDC), N,N-dimethylformamide (DMF), potassium hydroxide (KOH), and sodium hypophosphite (NaH2PO2) were purchased from Sinopharm Chemical Reagent. Ethanol was purchased from Beijing Chemical Reagent Company. All chemicals were analysis reagent (A.R.) and used as received without further purification. Millipore water (≥18.5 MΩ) was used in all experiments. Synthesis of NixFe-BDC CP. In a typical synthesis, FeCl3· 6H2O (270 mg), Ni(acac)2 (0, 51, 128, 257, and 514 mg), and H2BDC (166 mg) were dissolved in 16 mL of DMF under ultrasonic and magnetic stirring. Subsequently, the resulting homogeneous solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave. The sealed vessel was then maintained at 120 °C for 8 h before it was cooled to room temperature. The brown powder was separated via centrifugation and further washed by an ethanol−water mixture solution several times. Finally, the brown products dried at 65 °C in oven overnight and are denoted as Ni xFe-terephthalic acid compounds precursors (NixFe-BDC CP, x = 0, 0.2, 0.5, 1, 2). Synthesis of NixFe@C. A quartz boat loaded with 0.1 g of NixFe-BDC CP was placed at the middle of the tube furnace. Subsequently, the furnace was maintained at 600 °C for 2 h under a N2 atmosphere at a heating rate of 5 °C min−1, then naturally cooled to room temperature. The final black products were harvested and are denoted as NixFe@C (x = 0, 0.2, 0.5, 1, 2). Synthesis of P-Ni0.5Fe@C. A quartz boat loaded with 1 g of NaH2PO2 was placed at the upstream side of the tube furnace, and the other quartz boat loaded with 0.1 g of Ni0.5FeBDC CP was placed at the downstream side. Subsequently, the furnace was elevated and maintained at 600 °C for 2 h under a N2 atmosphere at a heating rate of 5 °C min−1, then naturally cooled to room temperature. The final black products were harvested and are denoted as P-Ni0.5Fe@C. As a comparison, the amount of NaH2PO2 was increased to 3 g with the other conditions the same, and the product is denoted as 30PNi0.5Fe@C. Synthesis of Ni0.5FeP@C. A quartz boat loaded with 1 g of NaH2PO2 was placed at the upstream side of the tube furnace, and the other quartz boat loaded with 0.1 g of Ni0.5Fe@C was placed at the downstream side. Subsequently, the furnace was elevated and maintained at 600 °C for 2 h under a N2 atmosphere at a heating rate of 5 °C min−1 and then naturally cooled to room temperature. The final black products were harvested and are denoted as Ni0.5FeP@C. Characterization. Transmission electron microscopy (TEM) images were carried out on JEM-2100 at 200 kV. High-resolution transmission electron microscope (HRTEM) images of samples were observed on a Tecnai G2 F20 S-Twin at 200 kV. The high-angle annular dark-field scanning TEM (HAADF-STEM) was determined by a Tecnai G2 F20 S-Twin B


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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration of the Synthetic Strategy of the P-Ni0.5Fe@C

working electrode was made using the following method: 2.0 mg of the as-prepared catalyst was evenly dispersed in the mixture of DI−ethanol (1.0 mL, 4:1 v/v) and Nafion solution (40 μL, 5 wt %, Alfa Aesar) with ultrasonication for 30 min. Then, 5.0 μL of the suspension was dropped onto the clean surface of a GCE with a syringe and dried under the infrared lamp, corresponding to the mass loading of ∼0.14 mg·cm−2. For the alkaline electrolytic cell, Ni foam (NF, 1 × 1 cm) was utilized as the working electrode with an approximate catalyst loading of ∼3 mg·cm−2. All electrochemical tests, including cyclic voltammograms (CVs), linear sweep voltammograms (LSVs), chronoamperometry, and electrochemical impedance spectroscopy (EIS), were performed in an O2-saturated 1.0 M KOH aqueous solution at room temperature with 75% iR compensation. The CV tests were conducted, and the potential was scanned from 0 to 0.6 V (vs Hg/HgO) at a sweep rate of 50 mV·s−1. The CVs were obtained after 50 cycles. LSVs of the catalysts were carried out at a scan rate of 50 mV·s−1, and the potentials were scanned from 0 to 1.0 V (vs Hg/HgO). EIS measurements were conducted from 105 to 0.01 Hz. The stability tests were then performed by CVs scanning from 0 to 0.5 V versus Hg/HgO for 5000 cycles at a scan rate of 100 mV· s−1. The long-term durability of P-Ni0.5Fe@C was also investigated through chronoamperometry in 1.0 M KOH with a magnetic stirrer rotating at 40 rpm. The electrochemically active surface area (ECSA) was calculated based on the double-layer capacitance (Cdl) measurements. A series of CVs was performed at sweep rates of 20−120 mV·s−1 to collect the capacitance charging and discharging currents. The currents were plotted against the scan rates, whose slopes were the double-layer capacitances of the catalysts. All potentials measured were converted to the potential versus reversible hydrogen electrode (RHE) according to the following equations

HRTEM operating at 200 kV. The scanning electron microscope (SEM) images were determined by an ULTRA 55 SEM at 20 kV. Composition of the samples was determined by an energy-dispersive X-ray (EDX) spectroscope attached to the FESEM instrument. Particle-size distributions were all calculated according to a histogram considering more than 100 particles measured using multiple SEM or TEM micrographs. The X-ray diffraction (XRD) patterns of samples were recorded on a Bruker D8-advance X-ray powder diffractometer operated at voltage of 40 kV and current of 40 mA with CuK radiation (λ = 1.5406 Å). The X-ray photoelectron spectra (XPS) were obtained with an ESCALAB MARK II spherical analyzer using a magnesium anode (Mg 1253.6 eV) X-ray source. All XPS spectra were corrected using the C 1s line at 284.6 eV, and curve fitting and background subtraction were accomplished. The Raman spectra of the catalysts were recorded with a Renishaw Micro-Raman System 2000 spectrometer with spectral resolution of 2 cm−1. The laser line at 532 nm of a He/Cd laser was used as the exciting source with an output of 20 mW. The spectra were recorded at room temperature by the condition of 50 s integral time at a 1 cm−1 resolution. The XPS spectra of the catalyst material after longterm capacitance voltage (CV) test were obtained from a carbon paper electrode with a catalyst loading of ∼3 mg cm−2. N2 adsorption/desorption isotherms were obtained by a Kubo X10000 static volumetric gas adsorption analyzer at −196 °C. Before measurements, the samples were degassed at 200 °C for 3 h in vacuum. The specific surface area was calculated from the adsorption branches in the relative pressure range of 0.05 to 0.20 by the Brunauer−Emmett−Teller (BET) method. The mesopore size distribution was calculated from desorption branches by the Barrett−Joyner−Halenda (BJH) method, and the single-point adsorption total pore volume was taken at the relative pressure of 0.96. The thermal gravimetric analysis (TGA) was conducted on a Mettler TGA/DSC1 analyzer under N2 at a ramp rate of 10 °C·min−1. The inductively coupled plasma optical emission spectrometry (ICP-OES) of samples was performed on an iCAP7400 apparatus (Thermo Fisher Scientific). Working parameters: RF Power, 1150 W; auxiliary gas and nebulizer flow, 0.5 L/min; cooler flow, 12 L/ min. Electrochemical Measurements. All of the electrochemical experiments were performed on the CHI 760E electrochemical workstation (CHI Instrument, CHN). A conventional three-electrode system was used, including a mercuric oxide electrode (SME) as the reference electrode, a carbon rod as the counter electrode, and a glassy carbon electrode (GCE) (3 mm in diameter, 0.07065 cm2 in geometric area) as the working electrode. Before the experiments, the GC electrode was polished by Al2O3 powder and washed with an ethanol−water mixture solution. The

EvsRHE = EvsSCE + ESME θ + 0.059pH

(1)

The turnover frequency (TOF) of the catalyst was calculated from the following equation TOF = J × A /(4 × F × n)

(2)

where J is the current density at a specific overpotential (A cm−2), A is the area of the GCE (0.07065 cm2), F is the Faraday constant (96 485 mol·C1−), and n is the total number of moles of the active metal sites (both Ni and Fe) of the catalyst that are deposited onto the GCE by assuming that every Ni and Fe atom is catalytically active in OER. Therefore, the TOF value is a conservative estimate.

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RESULTS AND DISCUSSION Characterizations of the Obtained Catalysts. A facile route to synthesize hollow P-Ni0.5Fe@C bipyramid is

C


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(Figure 1a inset). The HRTEM image reveals an interplanar distance of 0.255 and 0.176 nm that can be attributed to the (331) plane of NiFe2O4 and the (200) plane of Fe0.64Ni0.36 alloys phase, respectively (Figure 1b). The HAADF-STEM image of an individual Ni0.5Fe@C particle and EDX mapping verify the well-defined structure and uniform distribution of C, O, Ni, and Fe elements throughout the whole selected areas (Figure 1c−g). The Ni/Fe atomic ratio measured by EDX (1/ 1.75), SEM-EDS (1/2.02), and ICP-OES analyses (1/2.08) is close to 1/2 (Figures S6 and S7 and Table S1). By annealing Ni0.5Fe-BDC CP and NaH2PO2 under a N2 atmosphere while keeping other synthesis conditions the same with Ni0.5Fe@C, the P-Ni0.5Fe@C could be obtained. The SEM image shows that the as-formed products possess a hollow elongated bipyramid structure with an average length of ∼2.5 μm, which can be confirmed by some cracked samples (Figure 2a). The hollow structure might be created by the

schematically illustrated in Scheme 1. In a typical experiment, bimetallic analogous MIL-101 (Fe) precursors (NixFe-BDC CP) with different Ni and Fe ratios were first prepared through a solvothermal reaction. The XRD patterns of Ni0.5Fe-BDC CP and Fe-BDC CP (Figure S1) exhibit diffraction peaks between 5 and 40° that are similar to those of the reported precursor MIL-101 (Fe) and the simulated pattern.52,53 Then, the morphology of NixFe-BDC CP with different Ni/Fe molar ratios was examined by SEM (Figure S2a−e). The panoramic SEM images show that Fe-BDC CP is composed of a highly uniform spindle with an average apex-to-apex size of 2.1 μm, and Ni0.5Fe-BDC CP as well as Ni1Fe-BDC CP present uniform bipyramid with an average apex-to-apex size of 4.6 and 2.7 μm. However, as for Ni0.2Fe-BDC CP and Ni2Fe-BDC CP, the morphology is disorganized. Thus the appropriate addition of Ni(acac)2 is essential for uniform structure bimetallic NixFeBDC CP with a smooth and continuous surface. This is because appropriate Ni2+ ions are uniformly incorporated into the NixFe-BDC CP framework due to strong coordination between H2BDC and metal ions as well as the similar ionic radius in crystals.54,55 Before pyrolysis, the TGA of Ni0.5FeBDC CP was carried out under nitrogen, and the result is shown in Figure S3. Ni0.5Fe-BDC CP is nearly decomposed completely before 600 °C. By the carbonization under the same conditions, only the Ni0.5Fe@C well maintained the bipyramid morphology of the Ni0.5Fe-BDC CP precursors (Figure S4a−e). It also showed the lowest OER overpotential (285 mV vs RHE) at a current density of 10 mA·cm−2 and the smallest Tafel slope (115 mV dec−1) (Figure S5a,b). Therefore, the Ni0.5Fe-BDC CP is used as the optimal sample for further experiments. Finally, hollow P-Ni0.5Fe@C was made by annealing Ni0.5Fe-BDC CP and NaH2PO2 at 600 °C for 2 h under N2 flow, and the color of the samples changed from brown to black. As shown in Figure 1a, the TEM image reveals that the asformed Ni0.5Fe@C product possesses an average length of ∼2.5 μm and basically retained the bipyramid morphology of the precursors. The rough and discontinuous surface is assembled by many NPs with an average size of ∼50 nm

Figure 2. (a) SEM (inset: size distribution histogram) of P-Ni0.5Fe@ C. (b) TEM (inset: size distribution histogram) of the small particles in P-Ni0.5Fe@C. (c) HR-TEM image and (d) HAADF-STEM images of P-Ni0.5Fe@C. (e−i) Elemental mapping analysis for the spatial distribution of C, O, Ni, Fe, and P.

release of gas during the phosphorization process. Furthermore, the TEM image indicates that the P-Ni0.5Fe@C possesses a rough and discontinuous surface consisting of numerous small NPs with an average size of 100 nm (Figure 2b). The HRTEM image also shows that the NPs are surrounded by the carbon network and the interplanar distances of 0.255 and 0.176 nm that can be attributed to the (331) plane of NiFe2O4 and (200) plane of Fe0.64Ni0.36 alloys phase, respectively (Figure 2c). Corresponding to the HAADF-STEM image (Figure 2d), elemental mapping analysis (EDX) images (Figure 2e−i) reveal that C, O, Ni, Fe, and P elements are distributed in a homogeneous and overlapped manner throughout the selected areas. The atomic ratio of Ni/Fe is 1/2.18, analyzed by EDX (Figure S8), which

Figure 1. (a) TEM (inset: size distribution histogram), (b) HR-TEM image, (c) HAADF-STEM image of Ni0.5Fe@C, and (d−g) elemental mapping analysis for the spatial distribution of C, O, Ni, and Fe. D


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Figure 3. (a) XRD patterns of P-Ni0.5Fe@C, Ni0.5Fe@C. and Fe@C. (b) High-resolution XPS scans of Fe 2p spectrum. (c) High-resolution XPS scans of Ni 2p spectrum. (d) High-resolution XPS scans of P 2p spectrum, respectively.

Figure 4. (a) Raman spectrum, (b) nitrogen adsorption−desorption isotherm diagram, and (c) the corresponding pore-size distribution of PNi0.5Fe@C, Ni0.5Fe@C, and Fe@C.

S10a−c). By increasing the amount of NaH2PO2, the crystal structure of 30P-Ni0.5Fe@C is nearly the same as that of PNi0.5Fe@C. Ni2P and Fe2P could be formed by calcining Ni0.5Fe@C with NaH2PO2; however, the structure of the obtained Ni0.5FeP@C collapsed and severely agglomerated. In addition, the P-Ni0.5Fe@C exhibits the best electrochemical performance among those of 30P-Ni0.5Fe@C and Ni0.5FeP@C (Figure S10d). Therefore, the P-Ni0.5Fe@C is used as the optimal sample for subsequent experiments. Furthermore, surface chemical compositions and valence states of Ni, Fe, P, C, and O elements in Fe@C, Ni0.5Fe@C, and P-Ni0.5Fe@C were monitored by XPS survey spectra. The Fe, C, and O elements are detected in all catalysts, and the Ni element exists in Ni0.5Fe@C and P-Ni0.5Fe@C, whereas the P element is only observed in P-Ni0.5Fe@C compounds (Figure S11). Figure 3b shows high-resolution XPS spectra of Fe 2p. All of the catalysts show two main peaks of Fe3+ and Fe2+ in Fe 2p3/2 at 712.9 and 710.7 eV and a satellite at 718.6 eV. As for

is close to the results of SEM-EDS (1/2.01) and ICP-OES (1/ 2.3) analyses (Figure S9 and Table S1). The crystal structures of the obtained catalysts were examined by XRD. Figure 3a illustrates the XRD patterns of P-Ni0.5Fe@C, Ni0.5Fe@C, and Fe@C. As for Fe@C catalyst, two phases of Fe (JCPDS No. 06-0696) and Fe3O4 (JCPDS No. 65-3107) are exclusively found without the introduction of Ni and P. By the introduction of the Ni element, Fe0.64Ni0.36 alloy (JCPDS No. 47-1405) and cubic NiFe2O4 (JCPDS No. 10-0325) appear for NiFe@C. For instance, the diffraction peaks at 43.5 and 50.7° as well as those at 30.6 and 35.7° are indexed to NiFe2O4 and Fe0.64Ni0.36 alloys, respectively. After subsequent carbonization and phosphorization treatments, the diffraction peak of P-Ni0.5Fe@C has not changed, which means that no NiP, FeP, or NiFeP is generated, and the P element may enter the composite by element doping. As a comparison, the morphology and crystal structure of 30P-Ni0.5Fe@C and Ni0.5FeP@C were also investigated by SEM and XRD (Figure E


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Figure 5. (a) OER LSV curves of P-Ni0.5Fe@C, Ni0.5Fe@C, Fe@C, and IrO2. (b) Corresponding comparison of overpotentials at current density of 10 mA cm−2. (c) Corresponding Tafel plots. (d) Linear fitting of the capacitive currents versus CVs scan rates. (e) TOFs of P-Ni0.5Fe@C, Ni0.5Fe@C, and Fe@C at different overpotentials from 300 to 400 mV by assuming that every Ni and Fe atom is catalytically active. (f) Durability tests for P-Ni0.5Fe@C after 5000 potential cycles at a rate of 100 mV and chronoamperometry measurement of P-Ni0.5Fe@C (inset).

P-Ni0.5Fe@C and Ni0.5Fe@C catalysts, the typical Ni3+ 2p3/2 and Ni2+ 2p3/2 peaks and a satellite (Figure 3c) are obtained at 856.9, 854.9, and 862.1 eV, respectively, and a weak feature at 852.6 eV is assigned to metallic Ni 2p3/2.56,57 Furthermore, the high-resolution XPS spectra of the P element in Figure 3d reveal only one peak at ∼133.6 eV, reflecting the binding energies (BEs) of P−O species.58,59 However, the elemental P with the BE of ∼130.2 eV does not appear. This means that P element is mainly doped into the FeNi alloys/NiFe2O4 composite rather than forming metal phosphide.60 Raman analysis was also used to characterize the carbon structure of P-Ni0.5Fe@C, Ni0.5Fe@C, and Fe@C (Figure 4a). For all samples, the specific D band (∼1334.6 cm−1) and G band (∼1588.7 cm−1) of carbon are clearly observed.61 The intensity ratio of D band to G band (denoted as ID/IG) indicates the degree of the graphitic crystalline structure. As a result, the intensity ratios of ID/IG are found to be about 0.92, 0.90, and 0.83 for P-Ni0.5Fe@C, Ni0.5Fe@C, and Fe@C, respectively. The higher ID/IG value of P-Ni0.5Fe@C compared with Ni0.5Fe@C and Fe@C exhibits an increase in disordered graphitic nanostructures due to the incorporation of P heteroatoms into the carbon skeleton structure of carbon layers. Furthermore, the higher wavelength position of the G

band at P-Ni0.5Fe@C compared with Ni0.5Fe@C and Fe@C suggests a charge transfer between the P and the materials,62 matching well with the XPS analysis presented above. The porous texture of P-Ni0.5Fe@C, Ni0.5Fe@C, and Fe@C was further studied by the N2 adsorption−desorption isotherm and BJH pore-size distribution curves. All of the samples exhibit a type-IV isotherm (Figure 4b), suggesting the existence of relative uniform mesoporous structures with narrow distribution, which is further demonstrated by pore-size distribution (Figure 4c). In addition, BET surface areas (pore volumes) are 72.9 m2 g−1 (0.10 cm3 g−1) for P-Ni0.5Fe@C, 394.3 m2 g−1 (0.35 cm3 g−1) for Ni0.5Fe@C, and 189.5 m2 g−1 (0.24 cm3 g−1) for Fe@C, respectively (Table S2). It is well known that mesopores play a critical role in electrochemical processes due to their capability of facilitating mass diffusion/ transport (e.g., electrolyte penetration and ion transport) and ensuring a high electroactive surface area. Therefore, the different porous structures of these three catalysts may affect their electrocatalytic properties.63−65 All of the above observations show that uniform hollow P-Ni0.5Fe@C has been successfully prepared. Because of its large surface area, open structure, and carbon-encapsulated active components, F


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Figure 6. Polarization curves of P-Ni0.5Fe@C/Ni foam electrode and bare Ni foam for (a) oxygen evolution reaction and (b) hydrogen evolution reaction. (c) Polarization curves for water electrolysis of P-Ni0.5Fe@C || Pt/C, P-Ni0.5Fe@C || P-Ni0.5Fe@C, commercial IrO2 || Pt/C, and Ni foam || Ni foam in 1 M KOH solution at a scan rate of 10 mV s−1. The inset is the corresponding optical photograph of H2 and O2 bubbles. (d) Chronopotentiometry curve of the P-Ni0.5Fe@C || P-Ni0.5Fe@C at current density of 20 mA cm−2.

(vs RHE) against the scan rate (Figure 5d).68 Obviously, the Cdl value of the P-Ni0.5Fe@C elongated bipyramid is 7.9 mF cm−2, considerably larger than that of Ni0.5Fe@C (6.1 mF cm−2) and Fe@C (5.7 mF cm−2), implying that the highest OER activity of P-Ni0.5Fe@C could be attributed to the largest ECSA, which benefits from the high electrocatalyst−electrolyte contact interface area provided by its hollow and mesoporous structure. Furthermore, the intrinsic activities of the catalysts were explored through calculating the TOFs by assuming that all of the Ni and Fe ions were available from ICP-OES characterization. It is noteworthy that the TOF value is a conservative estimate because not all of the metal sites are accessible for OER. The TOF value of P-Ni0.5Fe@C is larger than that of Ni0.5Fe@C and Fe@C at the same overpotential, revealing the better intrinsic OER activity of P-Ni0.5Fe@C (Figure 5e). Encouraged by the excellent catalytic activity, the OER stability of P-Ni0.5Fe@C was also investigated in 1.0 M KOH. As shown in Figure 5f, the decrease in the current density is negligible after 5000 cycles of successive CVs scanning at a scan rate of 100 mV·s−1. Furthermore, a long-term test of i−t stability for P-Ni0.5Fe@C at a current density of 10 mA cm−2 was also carried out (Figure 5f inset). It shows that P-Ni0.5Fe@ C remains stable over 15 h without obvious potential shift, which further demonstrates the super stability of the catalyst. In addition, the TEM images show that the P-Ni0.5Fe@C nearly retain their original morphology after the long-term CV test (Figure S15). However, some differences between the XPS spectra before and after the long-term CV test can be identified according the narrow scan spectra of Ni 2p, Fe 2p, and P 2p displayed in Figure S16a−d. The disappearance of the metallic Ni and the increases with the ratio of Ni3+/Ni2+ (from 1.05/1 to 1.57/1) as well as Fe3+/Fe2+ (from 0.94/1 to 0.96/1) indicate the surface oxidation of [email protected]−72 On the

the P-Ni0.5Fe@C catalyst is expected to have good OER activity. OER Performances of the Obtained Catalysts. The OER activity of the as-synthesized catalysts was examined by LSVs with a scan rate of 50 mV·s−1 in 1.0 M KOH solution at room temperature with 75% iR compensation using a typical three-electrode system. As revealed in Figure 5a,b, the PNi0.5Fe@C exhibits obviously superior OER activity with an overpotential of 256 mV at a current density of 10 mA cm−2, which is lower than those of Ni0.5Fe@C (285 mV), Fe@C (395 mV), commercial IrO2 (318 mV), and most of the reported TMP catalysts under alkaline conditions (Table S3). For comparison, the LSV polarization curves of P-Ni0.5Fe@C with and without infrared (IR) correction were also compared in Figure S12. Besides, the Tafel slope of the P-Ni0.5Fe@C elongated bipyramid is 65 mV dec−1, as observed in Figure 5c, considerably lower than those of the Ni0.5Fe@C (115 mV dec−1), Fe@C (74 mV dec−1), and commercial IrO2 (120 mV dec−1), indicating the more favorable OER kinetics of PNi0.5Fe@C. It can be concluded from the activity data that the catalytic activity of P-Ni0.5Fe@C is much better than that of Ni0.5Fe@C, which is probably due to the fact that the intrinsic properties of the metal phosphides lead to better electronic conductivity.66,67 The EIS data further prove that P-Ni0.5Fe@C has much smaller charge-transfer resistance than Ni0.5Fe@C and Fe@C (Figure S13). Furthermore, the ECSA of catalysts is estimated based on their Cdl because the ECSA is generally proportional to the Cdl of the electrocatalysts. Specifically, the scan-rate dependence of CVs is performed in the potential range of 1.074 to 1.174 V (vs Hg/HgO) without redox processes to obtain the capacitive current related to double-layer charging of P-Ni0.5Fe@C, Ni0.5Fe@C, and Fe@C (Figure S14). Then, the Cdl can be obtained by plotting the Δj = ja − jc at 1.124 V G


Research Article


overpotential, which indicates the better intrinsic OER activity of P-Ni0.5Fe@C.

basis of these results, we can conclude that the bipyramid PNi0.5Fe@C catalyst has a strong stability of structure; however, the outermost is oxidized during electrocatalysis. Recently, TMPs have shown desirable electrocatalytic activity for alkaline HER. According to the above evaluations, the high OER activity and stability of P-Ni0.5Fe@C suggest that P-Ni0.5Fe@C may serve as the overall water splitting catalyst in the alkaline electrolytic cell. We first investigated the OER and HER activities of P-Ni0.5Fe@C samples applied on Ni foam substrates with a loading of 3 mg cm−2 (based on the mass of P-Ni0.5Fe@C). Impressively, P-Ni0.5Fe@C exhibited a prominently high OER activity with an overpotential of 195 mV to achieve 20 mA·cm−2; however, the overpotential for HER was only 312 mV at 20 mA·cm−2 (Figure 6a,b). Then, the overall water splitting measurement was carried out in a two-electrode cell based on the P-Ni0.5Fe@C as both anode and cathode catalysts applied on Ni foam. For comparison, the integration of P-Ni0.5Fe@C and commercial Pt/C, commercial IrO2 particles, and Pt/C, which are the state-of-the-art catalysts for OER and HER, Ni foam, and Ni foam couples were also prepared and examined. According to the polarization curves (Figure 6c), it is clear that the P-Ni0.5Fe@C || P-Ni0.5Fe@C system delivers a current density of 10 mA cm−2 at the cell voltage of 1.54 V, representing an overpotential of only 310 mV, which is comparable to or even superior to other transition-metal-based bifunctional catalysts (Table S4). Surprisingly, when the cathode catalyst was replaced by commercial Pt/C, the P-Ni0.5Fe@C || Pt/C couple presented the smallest overpotential of 260 mV among those of the IrO2 || Pt/C (300 mV) and Ni foam || Ni foam systems (521 mV). More importantly, the P-Ni0.5Fe@C || Pt/C couple could give a higher current density than the IrO2 || Pt/C system when a large potential (>1.8 V) was applied, which is critical for practical application. Furthermore, stability is another vital parameter for evaluating the catalytic performance of electrocatalysts. Then, we conducted the long-term durability test for overall water splitting by a chronoamperometry measurement at a current density of 20 mA cm−2, as presented in Figure 6d; the cell exhibits considerable stability at least 33 h without trivial activity losses. These results and excellent overall water splitting activities of P-Ni0.5Fe@C || Pt/C reveal that our prepared P-Ni0.5Fe@C materials may serve as a highly efficient large-scale oxygen production catalyst in the future. The superior electrocatalytic performance of P-Ni0.5Fe@C could be attributed to the following facts: (1) According to the XRD and XPS results, P dopants can effectively adjust the electron structure, thus optimizing the BEs of the OER intermediates. (2) BJH pore-size distribution curves reveal that the P-Ni0.5Fe@C possesses mesoporous structure, which can accelerate electron transfer and ion diffusion in electrochemical processes. (3) Unique hollow structure could provide large surface area and open structure for the electrocatalysts to contact the electrolyte. (4) In addition, the carbon network coated around the P-doped FeNi Alloys/NiFe2O4 NPs can effectively prevent the possible aggregation and dissolution of the active composition. (5) The P-Ni0.5Fe@C gives a smaller Tafel slope (56 mV dec−1) than those of Ni0.5Fe@C (115 mV dec−1), Fe@C (74 mV dec−1), and commercial IrO2 (120 mV dec−1), indicating the favorable OER kinetics. (6) The PNi0.5Fe@C has the largest ECSA value among the three catalysts, indicating that it can provide high electroactive surface area and active sites. (7) The TOF of P-Ni0.5Fe@C is larger than that of Ni0.5Fe@C and Fe@C at the same

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CONCLUSIONS The uniform hollow bipyramid P-Ni0.5Fe@C catalyst was synthesized through a simple self-templating strategy by onestep annealing bimetallic Ni0.5Fe-BDC CP with sodium hypophosphite. The in situ encapsulation of P-doped NiFebased active composition into carbon network can prevent agglomeration with neighboring NPs, and the hollow structure contributes to the smaller charge-transfer resistance and fast electrode kinetics. Benefitting from these synergistic features, the P-Ni0.5Fe@C catalyst exhibits superior OER electrocatalytic activity with a low overpotential of 256 mV to reach 10 mA cm−2 and excellent stability. More remarkably, when it was employed as an overall water splitting catalyst, only a potential of 1.54 V was required at current density of 10 mA cm−2. More remarkably, when it was employed in a twoelectrode configuration based on P-Ni0.5Fe@C as anode and commercial Pt/C as cathode catalysts (P-Ni0.5Fe@C || Pt/C), only a potential of 1.49 V (corresponding overpotential of 260 mV) was required to achieve 10 mA cm−2 as well as excellent durability in alkaline solution. The facile synthesis of hybrid nanostructures with controllable morphologies and functionalities could be expanded to the development of other highperformance and cost-efficient OER catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04997.

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ICP-OES, SEM images, SEM−EDS spectrum, TEM images, EDX spectrum, XRD patterns, TGA thermograms, nitrogen adsorption−desorption isotherms, XPS survey spectra, polarization curves, and EIS Nyquist plots of P-Ni0.5Fe@C, Ni0.5Fe@C, and Fe@C (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xin Zhang: 0000-0003-0033-5233 Xiaoping Dai: 0000-0003-2289-8133 Fei Gao: 0000-0001-8626-5509 Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the NSFC (nos. 21573286, 21173269, 91127040, 21576288), Ministry of Science and Technology of China (nos. 2011BAK15B05, 2015AA034603), Specialized Research Fund for the Doctoral Program of Higher Education (20130007110003), and Science Foundation of China University of Petroleum, Beijing (no. 2462015YQ0304). H


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