Thermal Synthesis of FeNi@Nitrogen-Doped Graphene Dispersed on

May 28, 2019 - This work may open a facile and low-cost way for large-scale synthesis of non-noble metals/nitrogen-doped carbon composite electrocatal...
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Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4075−4083

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Thermal Synthesis of FeNi@Nitrogen-Doped Graphene Dispersed on Nitrogen-Doped Carbon Matrix as an Excellent Electrocatalyst for Oxygen Evolution Reaction Sayyar Ali Shah,† Zhenyuan Ji,† Xiaoping Shen,*,† Xiaoyang Yue,† Guoxing Zhu,† Keqiang Xu,† Aihua Yuan,‡ Nabi Ullah,† Jun Zhu,† Peng Song,† and Xiaoyun Li†

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School of Material Science and Engineering, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China ‡ School of Environmental & Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, P. R. China S Supporting Information *

ABSTRACT: The fabrication of non-precious-metal and highly active electrocatalytic materials to drive oxygen evolution reaction (OER) at low overpotential from earthabundant elements by a low-cost and easy method is extremely important for many advanced energy technologies. Herein, a non-noble-metal catalyst with FeNi@nitrogendoped graphene dispersed on a nitrogen-doped carbon matrix (named FeNi@NGE/NC) is synthesized by a facile one-pot pyrolysis process. In this composite, FeNi alloy nanoparticles encapsulated with few-layer nitrogen-doped graphene are uniformly anchored on a nitrogen-doped carbon matrix. The FeNi@NGE/NC composite exhibits outstanding electrocatalytic performance for OER with a low overpotential of 275 mV at 10 mA cm−2, a small Tafel slope of 41.2 mV dec−1, and high stability in 1 M KOH solution. Even at the low concentration of basic solution (0.1 M KOH), it still displays good activity for OER with an overpotential of 372 mV at 10 mA cm−2. The outstanding OER performance of the FeNi@NGE/NC composite can be mainly attributed to the fast electron transfer from the FeNi alloy to nitrogen-doped graphene shells and the high structural stability due to the protection of graphene shells and the carbon matrix. This work may open a facile and low-cost way for large-scale synthesis of non-noble metals/nitrogen-doped carbon composite electrocatalysts for practical application. KEYWORDS: electrocatalytic, oxygen evolution reaction, FeNi@nitrogen-doped graphene, nitrogen-doped carbon matrix, pyrolysis process

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INTRODUCTION The oxygen evolution reaction (OER) plays a key role in many advanced energy technologies, such as electrochemical water splitting, rechargeable metal−air batteries, and carbon dioxide electrolysis.1−4 However, sluggish electron-transfer kinetics has been documented as a major problem for OER that decreases the overall efficiency of energy storage devices and water splitting.5−7 Traditionally, the famous OER catalysts with high catalytic activity are precious-metal-based materials such as Ru, RuO2, and IrO2.8,9 However, their high cost and scarcity are the main obstacles to their large-scale application. In recent decades, many efforts have been devoted to replace the precious-metal-based OER catalysts by cheap and earthabundant materials.10−17 Among them, FeNi-based catalysts are considered as potential candidates for OER.18−21 However, metals are easily oxidized and thus show poor OER kinetics due to lower conductivity and insufficient active sites.22 Currently, the most widespread approach for solving the issue is the growth of FeNi-based catalysts on conductive substrates such as carbonaceous materials and Ni foam,23−28 © 2019 American Chemical Society

but in these catalysts the metal sites are still often exposed to harsh environments and lose their activity. Recently, some research groups reported that carbonencapsulated nanoparticles of 3d transition metals or their alloys exhibit significantly enhanced electrocatalytic activity.29−35 In this kind of catalyst, electrons easily transfer from the metallic core to the surface of the carbon shell, facilitating the catalytic process.18,19,30 Moreover, the carbon shell can well protect the metallic core from harsh environments.18,19,30 It was found that the thickness of the carbon shell is crucial to the catalytic activity, and a too thick carbon shell will block the electron transfer from the core to the shell.19 In addition, the interaction between the metallic core and the carbon shell is also important to the catalytic activity,19 and the catalytic activity can be further enhanced by tuning the carbon layers via heteroatom doping.18,19 For instance, Xu et al. prepared the Ni Received: January 28, 2019 Accepted: May 28, 2019 Published: May 28, 2019 4075

DOI: 10.1021/acsaem.9b00199 ACS Appl. Energy Mater. 2019, 2, 4075−4083

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ACS Applied Energy Materials

Figure 1. Schematic illustration of the synthesis of the FeNi@NGE@NC composite. photoelectron spectroscope (Thermo ESCALAB 250XI system) with an Al Kα X-ray as an excitation source, and the binding energy calibration is based on C 1s at 284.6 eV. Electrocatalytic Measurements. The electrocatalytic performance for OER was measured by a three-electrode system on an electrochemical workstation (CHI 760D, ChenHua Instruments Co. Ltd., Shanghai, China). A glassy carbon electrode (3 mm in diameter, ChenHua Instruments Co. Ltd., Shanghai, China) loaded with a catalyst, platinum foil, and a saturated calomel electrode (SCE) were used as working electrode, counter electrode, and reference electrode, respectively. For the working electrode, 4 mg of catalyst and 40 μL of Nafion solution (5 wt %) (Alfa Aesar) were dispersed in 960 μL of water and ethanol (≥99.7%, Changshu Hongsheng Fine Chemical Co., Ltd.) mixture (3:7) by sonication to obtain a homogeneous ink. Then, 5 μL of ink was loaded (ca. 0.28 mg cm−2) onto a glassy carbon electrode and dried in air. The electrolyte is saturated with air, and electrochemical tests were performed by a stagnant cell with stirring. Cyclic voltammograms (CVs) were measured from 0 to 0.4 V versus SCE in 1 or 0.1 M KOH (A.R) solution at a scan rate of 50 mV s−1 for 50 cycles. Then, the linear sweep voltammetry (LSV) with a scan rate of 5 mV s−1 was recorded in the same solution, and all the LSV values were iR-corrected. The potentials were converted to a reversible hydrogen electrode (RHE). The durability of catalysts was evaluated by taking continuous 5000 CV cycles at a scan rate of 100 mV s−1 between −0.1 and 0.5 V vs SCE in 1 M KOH solution. After 5000 cycles, the LSV measurement was again performed under the same conditions. The electrochemical active surface areas of the asprepared samples were estimated by measuring double-layer capacitance (Cdl) under the potential window of 0 to −0.1 V vs SCE with various scan rates from 10 to 110 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were performed at a frequency range of 100 kHz−100 mHz with an ac voltage of 5 mV in 1 M KOH solution at constant potential 0.1 vs SEC. The potentials were converted to those vs a reversible hydrogen electrode (RHE). In this work, ERHE = ESCE + 0.2458 V + 0.059pH.

nanoparticles encapsulated in few-layer N-doped graphene through the metal−organic framework (MOF) strategy.30 This material shows good OER performance for water splitting. Du’s group reported MOF-derived N-doped graphitic carbon encapsulated NiFe nanoparticles with highly durable and active OER performance.18 Despite this progress, it is highly desired to develop OER catalysts with N-doped carbon encapsulated metal/alloy nanoparticles on conductive substrates by more simple and cost-effective methods. Herein, we report the synthesis of few-layer N-doped graphene encapsulated FeNi alloy nanoparticles dispersed on nitrogen-doped carbon matrix by a facile and low-cost one-pot thermal process. The resulting composites show highly efficient electrocatalytic activity toward OER with a low overpotential of 275 and 372 mV at 10 mA cm−2 in 1 and 0.1 M KOH solutions, respectively, and high durability in an alkaline medium.

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EXPERIMENTAL SECTION

Synthesis. Ni(CH3COO)2·4H2O (≥98%), Fe(NO3)3·9H2O (≥99%), urea (A.R), polyvinylpyrrolidone (PVP, A.R.), and absolute ethanol were purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and used without further purification. Nafion solution was purchased from Alfa Aesar. For the preparation of FeNi@NGE/NC, 0.4 g of Ni(CH3COO)2· 4H2O (1.6 mmol), 0.4 g of Fe(NO3)3·9H2O (0.99 mmol), 0.2 g of PVP, and 1 g of urea (16.65 mmol) were ground in a mortar for 15 min. They were again ground together for 30 min. The resulted powder was dissolved in 10 mL of ethanol to obtain a homogeneous mixture and subsequently dried by evaporating solvent at room temperature. The mixture was heated in a tube furnace at 800 °C for 30 min under Ar gas with a heating rate of 5 °C min−1. The product was collected after the furnace was cooled to room temperature naturally. The sample was denoted as FeNi@NGE/NC. For comparison, we also prepared other samples with the same precursor ratio and procedure, but heated at 700 and 900 °C. The samples prepared at 700 and 900 °C were named S-700 and S-900, respectively. In addition, the NGE/NC-800 composite was prepared by treating FeNi@NGE/NC with 1 mol L−1 H2SO4 solution for 24 h to remove Ni and Fe. Characterization. The morphologies and structures of the asprepared samples were investigated by transmission electron microscopy (TEM, JEM-2010). The elemental mapping was analyzed by an energy dispersive X-ray (EDX) spectroscope attached to a TEM (Tecnai G2 F30 S-Twin). Crystal structures of the prepared composites were characterized by powder X-ray diffraction (XRD, Bruker D-8 Advanced diffractometer) with a Cu Kα radiation source (λ = 1.5406 Å). The contents of Fe and Ni ions in the composites were determined by an inductively coupled plasma optical emission spectrometer (ICP-OES, Vista-MPX). Raman spectra were recorded on Raman spectroscope (JYHR800) by using a 532 nm laser source. The X-ray photoelectron spectrum (XPS) was measured on an X-ray

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RESULTS AND DISCUSSION Synthesis and Characterization. The preparation procedure for Ni@NGE@CM composite is illustrated in Figure 1. The precursors of Ni(CH3COO)2·4H2O, Fe(NO3)3· 9H2O, urea, and PVP were first dispersed in ethanol by simple sonication. The PVP could act as adsorption sites for Ni and Fe species and urea molecules due to N and O polar group on PVP chains.36,37 During heating process, the polar group of PVP and N atoms of urea may donate lone-pair electrons, forming a coordinative interaction with metals ions and controlling the size of metals particles.36−39 Meanwhile, PVP and urea transformed to N-doped carbon.38,39 As a result, the product of FeNi alloy nanoparticles encapsulated by few-layer nitrogen-doped graphene anchored on nitrogen-doped carbon matrix was obtained. For more details see the Experimental Section. 4076

DOI: 10.1021/acsaem.9b00199 ACS Appl. Energy Mater. 2019, 2, 4075−4083

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Figure 2. (a−d) TEM images of FeNi@NGE/NC composite. (e, f) HRTEM images of nanoparticles in FeNi@NGE/NC composite with carbon shell and crystalline core. (g) Dark-field TEM and the corresponding elemental mapping images of (h) Ni, (i) Fe, (j) C, and (k) N of FeNi@NGE/ NC.

Figure 3. (a) Normalized XRD patterns and (b) Raman spectra of the as-synthesized samples.

Figure 2a−d shows TEM images of the FeNi@NGE/NC composite. It is clearly observed that nanoparticles with an average size of about 10 nm are well dispersed on N-doped carbon matrix. The high-magnification images (Figure 2c,d) indicate that the nanoparticles are enveloped by single- or fewlayer graphene shells. High-resolution (HR) TEM images (Figure 2e,f) reveal that the nanoparticle encapsulated in graphene layers is highly crystalline. The lattice fringes with a spacing of 0.207 nm are consistent with the (111) crystal planes of FeNi alloy. In the catalyst prepared at 700 °C, the FeNi nanoparticles are also dispersed on a N-doped carbon matrix (Figure S1a, Supporting Information). However, the crystalline core of the nanoparticle is enveloped by more layers of graphene (Figure S1b−d) than that in FeNi@NGE/NC. In contrast, for the composite prepared at 900 °C, the FeNi nanoparticles are anchored on a

N-doped carbon matrix but without graphene layer around them (Figure S2). This result indicates that the pyrolysis temperature has an important effect on the formation of graphene shells around the FeNi nanoparticles, and single- or few-layer graphene shells can be obtained at 800 °C. The elemental composition and distribution of FeNi@NGE/NC were examined by an EDX spectroscope attached to a TEM (Figure 2g). The element mapping (Figure 2h−k) images reveal that the product contains Ni, Fe, C, and N elements, and both Ni and Fe exhibit a uniform distribution in the nanoparticles, which are well consistent with the composition of FeNi@NGE/NC (Figure 2a). XRD measurement was performed to clarify the crystalline phases of the as-synthesized electrocatalysts. The normalized XRD patterns of all the synthesized samples are shown in Figure 3a. These XRD patterns display diffraction peaks at 4077

DOI: 10.1021/acsaem.9b00199 ACS Appl. Energy Mater. 2019, 2, 4075−4083

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Figure 4. High-resolution XPS spectra of FeNi@NGE/NC: (a) C 1s (b) N 1s, (c) Ni 2p, and (d) Fe 2p.

Figure 5. Electrochemical OER catalytic activity of as-prepared products, RuO2, and 20% Pt/C in 1 M KOH solution: (a) polarization curves and (b) the corresponding Tafel plots derived from the polarization curves.

to less graphitic carbon shells around FeNi particles and the lower graphitization degree of carbon matrix. The intensity of 2D peaks in S-900 is much weaker than the other two samples, possibly because of its nearly disappearing graphitic carbon shells. Furthermore, XPS was employed to examine the composition and valence state of the elementals in FeNi@NGE/NC. The survey spectrum displays the existence of Ni, Fe, C, N, and O elements in the composite (Figure S3), and the atomic percentages of C, N, O, Ni, and Fe are 83.4, 2.7, 8.3, 3.6, and 2.0%, respectively The high-resolution XPS spectrum of C 1s shows a main peak at 284.7 eV, which can be referenced to C− C/CC bonds (Figure 4a).42,43 The subpeaks at 285.6 and 287.6 eV correspond to C−N and CO bonding species, respectively.42−45 The N 1s spectrum can be deconvoluted into pyridinic (398.4 eV), pyrrolic (399.8 eV), and graphitic (401.1 eV) nitrogen (Figure 4b).44 The N species could be doped in

43.6°, 50.8°, and 75.7°, corresponding to (111), (200), and (220) crystal planes of the cubic phase FeNi alloy (JCPDS No. 47-1405). These peaks are comparable with those of the cubic FeNi alloys nanoparticles, but different from Fe, Ni, and their oxides.22,29,30,40,41 The peak at around 26° for all the three samples can be indexed to (002) crystal plane of graphitic carbon. It is obvious that the intensity of (002) peaks decreases gradually with the increase of the temperature from 700 to 900 °C, suggesting that the sample prepared at relatively low temperature contains more graphitic carbon. Raman spectroscopy is a very useful technique to characterize carbon materials. As shown in Figure 3b, the normalized Raman spectra of all these samples display peaks at around 1348, 1580, and 2698 cm−1, corresponding to the D, G, and 2D bands of carbon materials, respectively.42 The ID/IG values of S-700, FeNi@NDE/NC, and S-900 are 0.89, 0.78, and 0.95, respectively. The higher ID/IG value of S-900 can be ascribed 4078

DOI: 10.1021/acsaem.9b00199 ACS Appl. Energy Mater. 2019, 2, 4075−4083

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Figure 6. OER performances of FeNi@NGE/NC, RuO2, and 20% Pt/C catalysts in 1 and 0.1 M KOH solutions: (a) polarization curves and (b) the corresponding Tafel plots.

The S-700, S-900, and NGE/NC-800 catalysts show higher Tafel slopes values, suggesting comparatively slower kinetics for OER. The oxygen evolution is more facile at a high concentration of OH− and thus commonly is performed in strong alkaline solution. However, it is important for some devices, especially the metal−air battery, in which oxygen evolution is often performed in a lower concentration of KOH solution (0.1 M).50,51 Therefore, the OER performance of FeNi@NGE/NC was also investigated in 0.1 M KOH solution. For comparison, the commercial RuO2 and 20% Pt/C were also measured. The overpotential of the FeNi@NGE/NC product is 372 mV at a current density of 10 mA cm−2 (Figure 6a), which is much better than the state-of-art catalysts of RuO2 (465 mV) and 20% Pt/C (550 mV). At present, the mostly focused on nonprecious-metal catalysts for OER in diluted alkaline solution (0.1 M) are transition-metal-based composites, which are suggested to have better OER performance, for example, Co3O4/porous carbon (η10= 360 mV),52 amorphous FeCoOx material (η10 = 400 mV),53 cobalt carbonate hydroxide/C (η10 = 470 mV),54 and especially MOF-derived NiIIFeIII@NC (η10 = 360 mV)18 and NiIIFeII@NC catalyst (η10 = 394 mV).18 Interestingly, the FeNi@NGE/NC product shows superior or comparable catalytic performance to these reported transitionmetal-based catalysts. The Tafel slopes of FeNi@NGE/NC, RuO2, and 20% Pt/C in 0.1 KOH solution are 55.2, 71.5, and 93.8 mV dec−1, respectively (Figure 6b). The small Tafel slope of FeNi@NGE/NC implies that it still exhibits highly efficient kinetics for water oxidation even in low concentration alkaline solution (0.1 M KOH). The overpotential depends on the concentration of OH− ions in alkaline solution.55 Usually, the catalyst in 1 M KOH shows a higher OER activity than in low concentration alkaline solution (0.1 M KOH).10,16 The OER activity depends on the solution pH because pH is one of important parameters for rate-determining step. The different pH may affect the formation kinetics of intermediates during the OER process, and consequently, the Tafel slope and reaction order are changed in different concentration alkaline solution.55 The changes in overpotential and Tafel slops in different concentration alkaline solution have also been reported by many other groups.10,16,55,56 We also evaluate the effective surface area of the as-prepared electrocatalysts by measuring the Cdl values (Figure S5a−d). The Cdl values decrease in the order of 3.1 mF cm−2 (FeNi@

both the graphene shell and the carbon matrix. The highresolution XPS spectrum of Ni shows (Figure 4c) a pair of doublet peaks at around 869.8 and 852.6 eV for Ni 2p1/2 and Ni 2p3/2, which indicates the zero valence of Ni in the product.30,46 Besides, the weak broad peaks centered at 860.2 and 876 eV may be attributed to the satellites of Ni0 or slight oxidation of Ni in the composite.42,47,48 Similarly, the strong peaks at 707.3 and 720.7 eV in the Fe 2p spectrum (Figure 4d) clearly indicate the zero valence of Fe.46,47,49 The weak peaks centered at 709.9 and 723.2 eV can be ascribed to the satellites or slight oxidation of Fe.46,47 The dominant peaks of Ni0 and Fe0 in the XPS spectra demonstrate that the FeNi alloy nanoparticles are well protected by graphene shells and hardly oxidized in air. The contents of Fe and Ni in the as-prepared composites were also estimated by ICP-OES (Table S1), and the ratio of Fe to Ni is consistent with that of XPS results. Electrocatalytic Performance for OER. OER performances were measured in 1 and 0.1 M KOH aqueous solution with a three-electrode setup by loading the same amount (0.28 mg cm−2) of the as-synthesized samples, commercial RuO2 (A.R., Macklin), and 20% Pt/C (A.R., Alfa Aesar) on glassy carbon electrodes. Figure 5a shows the LSV curves in 1 M KOH solution. It can be seen that the FeNi@NGE/NC exhibits the highest catalytic activity with a lowest overpotential of 275 mV at the current density of 10 mA cm−2. The overpotentials of S-700 and S-900 and NGE/NC-800 at the current density of 10 mA cm−2 are 301, 395, and 614 mV, respectively. It is worth noting that the OER performances of FeNi@NGE/NC and S-700 are even better than the commercial RuO2 catalyst (Table S2). The overpotential of FeNi@NGE/NC at current density of 10 mA cm−2 is also superior or comparable to the reported FeNi−carbon electrocatalysts (Table S3). When the OER performance was measured by using carbon rod and Hg/HgO as counter electrode and reference electrode, respectively, almost negligible difference was observed in overpotentials at current density of 10 mA cm−2 (see Figure S4). The Tafel slopes were obtained by fitting the linear portions using the Tafel equation (η = a + b log J, where η, J, b, and a are the overpotential, current density, Tafel slope, and constant, respectively),43 and the corresponding Tafel plots of the electrocatalysts are shown in Figure 5b. The FeNi@ NGE/NC catalyst shows the smallest Tafel slope (41.2 mV dec−1) among these catalysts, demonstrating the most effective kinetics of water oxidation on the FeNi@NGE/NC catalyst. 4079

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Figure 7. (a) Nyquist plots of as-synthesized products; the inset displays the equivalent circuit used to fit the experimental data. (b) LSV polarization curves of FeNi@NGE/NC at the initial and after 5000 CV cycles; the inset shows its amperometric test curve.

NGE/NC) > 2.6 mF cm−2 (S-700) > 1.2 mF cm−2 (S-900) > 0.6 mF cm−2 (NGE/NC-800) (Figure S5e). The highest Cdl value of FeNi@NGE/NC implies that the composite prepared at 800 °C possesses the most electrochemical active sites for OER. The turnover frequency (TOF) of all as-prepared samples was calculated on the basis of metal (Fe and Ni) ion contents in the composites. It is observed that the TOF values increase in the order of S-900 (0.00112 s−1), FeNi@NGE/NC (0.074 s−1), and S-700 (0.078 s−1). The much lower TOF value of S-900 suggests that the OER catalytic activity mostly arises from N-doped graphene active sites located on the surface of FeNi nanoparticles instead of the FeNi nanoparticle itself.19 The OER kinetics at the electrode/electrolyte interface was investigated by EIS in 1 M KOH solution (Figure 7a) at constant potential of 0.1 V (vs SCE). The Nyquist plots were fitted with equivalent circuit as shown in the inset of Figure 7a. The charge-transfer resistances (Rct) of S-700, FeNi@NGE/ NC, S-900, and NGE/NC are 1.5, 5.5, 90, and 113 Ω, respectively. The S-700 and FeNi@NGE/NC show the lower Rct values because of their graphitic carbon encapsulated FeNi nanoparticles in the composites, suggesting higher electron conductivity and easier charge transfer at the electrode/ electrolyte interface as compared to other two samples. This is favorable for the electrocatalytic reaction. The stability is important to the long-term utilization of catalysts. The stability of the as-prepared FeNi@NGE/NC electrocatalyst was evaluated by taking continuous CV cycling at a scan rate of 100 mV s−1 from −0.1 to 0.5 V (vs SCE) as well as amperometric test (i−t) in 1 M KOH aqueous solution. The polarization curve of FeNi@NGE/NC catalyst obtained after 5000 CV cycles is almost the same as the initial one (Figure 7b) and suggests that the performance decay of FeNi@NGE/NC catalyst is negligible. It is notable that no oxidation peak appeared in the LSV curve after 5000 CV cycles, which is usually observed in Ni-based electrocatalysts.19 The TEM images of FeNi@NGE/CM after stability test show that nanoparticles are well dispersed in the carbon matrix (Figure S6a), and the carbon shells are still visible around nanoparticle (Figure S6b). This indicates that the particles were well protected by carbon shell and maintained the original morphology in the catalyst. The XPS spectrum of FeNi@NGE/CM after the stability test (Figure S7a) exhibits that the percent atomic contents of C, N, O, Ni, and Fe elements are 84.0, 1.4, 10.4, 2.9, and 1.3%, respectively. The high-resolution XPS spectrum of Fe 2p displays both metallic and oxidized Fe atoms (Figure S7b). Similarly, the high-

resolution XPS spectrum of Ni 2p shows Ni0 and Ni2+ valence states (Figure S7c). This is reasonable since the FeNi species involved in OER will be in situ oxidized during the process under the high potential. The slightly high O content in the composite also reveals the oxidized Fe and Ni in the composites. In i−t measurements, the current densities of FeNi@NGE/NC decreased from 11.4 to 10.5 mA cm−2 after continuous operation for 12 h (inset of Figure 7b), further confirming the high stability of the catalyst. However, the i−t test of the S-900 catalyst shows a faster decrease in the current density and exhibits relatively low stability compared to FeNi@ NGE/NC (Figure S8). Therefore, the FeNi@NGE/NC catalyst with excellent catalytic activity and high stability could be considered as a promising candidate for OER catalyst in practical application. Based on the above results and previous reports, the catalytic mechanism of FeNi@NGE/NC for OER is proposed as follows. The catalytic active sites for OER mainly locate on surfaces of N-doped graphene layer surrounding metals particles.18,19,30 The thickness of the graphene shell and the alloy in the core play important roles for the excellent catalytic activity.19,57−59 The sample with the few-layer graphene shell (FeNi@NGE/NC) shows better OER performance than that with the thick one (S-700), while both these samples display much better OER catalytic activity than that without the graphene shell (S-900). Thus, the layer number of graphene shell around FeNi particles has an important effect on the catalytic performance. Such phenomena were also observed by other authors for catalytic activity hydrogen evolution reaction49 and oxygen reduction reaction.58 When the FeNi alloy was removed from FeNi@NGE/NC by acid, the residual product (NGE/NC-800) shows poor OER performance with a much higher overpotential (Table S2) than FeNi@NGE/NC. This indicates that the outstanding OER performance of the FeNi@NGE/NC composite mainly results from the synergetic effect between FeNi particles and N-doped graphene shell, where electrons transfer from the metallic core to the N-doped graphene shell, and the active sites for OER mainly arise from N-doped graphene on the surface of FeNi nanoparticles, especially for single- and few-layer graphene.19,20 Meanwhile, the graphene shell can protect FeNi nanoparticles and maintain the high durability of the catalyst. In addition, the N-doped carbon matrix can prevent FeNi nanoparticles from aggregation and provide conductive matrix, leading to overall high OER catalytic performance. 4080

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CONCLUSION In summary, the FeNi@NGE/NC electrocatalyst is successfully prepared by a facile and low-cost one-pot pyrolysis method. The FeNi alloy nanoparticles with a size of ca. 10 nm are enveloped by N-doped single- or few-layer graphene and are evenly anchored on the N-doped carbon matrix in FeNi@ NGE/NC. The FeNi@NGE/NC catalyst exhibits excellent OER performance in 1 and 0.1 M KOH solutions with low overpotentials of 275 and 372 mV at 10 mA cm−2 and small Tafel slopes of 41.2 and 55.2 mV dec−1, respectively. The synergistic effect between few-layer graphene and FeNi nanoparticles is responsible for the excellent OER performance. This synthesis strategy provides a facile approach to constructing various graphene-enveloped metals/alloys nanoparticles on a carbon matrix with enhanced catalytic performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00199.

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TEM and HRTEM images of S-700 and S-900 samples; XPS survey spectrum of FeNi@NGE/NC and LSV curves and discussions of as-synthesized sample using carbon rod and Hg/HgO as counter electrode and reference electrode; CV curves and Cdl values of samples; TEM and HRTEM images and XPS spectra of FeNi@NGE/NC after 5000 CV cycles; amperometric test curve of the S-900 product and contents of Fe and Ni in composites measured by ICP-OES; table for electrochemical performance and table for comparison (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel +86 511 88791800; Fax +86 511 88791800; e-mail [email protected], [email protected]. ORCID

Xiaoping Shen: 0000-0003-0366-6433 Guoxing Zhu: 0000-0002-0756-7451 Aihua Yuan: 0000-0001-9246-8142 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (Nos. 21875091, 21776115, and 51602129) and the Natural Science Foundation of Jiangsu Province (Nos. BK20171295, BK20150507, and BK20161343).

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