Unique Fe2P Nanoparticles Enveloped in Sandwichlike Graphited

Nov 16, 2015 - CoP Nanoparticles Combined with WSe2 Nanosheets: An Efficient Hybrid Catalyst for Electrocatalytic Hydrogen Evolution Reaction. Jiahui ...
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Unique Fe2P Nanoparticles Enveloped in Sandwichlike Graphited Carbon Sheets as Excellent Hydrogen Evolution Reaction Catalyst and Lithium-Ion Battery Anode Yan Zhang, Huijuan Zhang, Yangyang Feng, Li Liu, and Yu Wang* The State Key Laboratory of Mechanical Transmissions and the School of Chemistry and Chemical Engineering, Chongqing University, 174 Shazheng Street, Shapingba District, Chongqing City 400044, PR China S Supporting Information *

ABSTRACT: The novel Fe2P nanoparticles encapsulated in sandwichlike graphited carbon envelope nanocomposite (Fe2P/ GCS) that can be first applied in hydrogen evolution reaction (HER) as well as lithium-ion batteries (LIBs) has been designed and fabricated. The unique sandwiched Fe2P/GCS is characterized with several prominent merits, including large specific surface area, nanoporous structure, excellent electronic conductivity, enhanced structural integrity and so on. All of these endow the Fe2P/GCS with brilliant electrochemical performance. When used as a HER electrocatalyst in acidic media, the harvested Fe2P/GCS demonstrates low onset overpotential and Tafel slope as well as particularly outstanding durability. Moreover, as an anode material for LIBs, the sandwiched Fe2P/GCS presents high specific capacity and excellent cyclability and rate capability. As a consequence, the acquired Fe2P/GCS is a promising material for energy applications, especially HER and LIBs. KEYWORDS: Fe2P, graphited carbon, envelope, hydrogen evolution reaction, lithium-ion battery

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INTRODUCTION Nowadays, the rapid depletion of nonrenewable fossil fuels and massive emission of greenhouse gas have led to fast development and vast demand of energy systems.1 It is worth mentioning that sophisticated nanomaterials are of great potential for sustainable energy applications because of their superb physicochemical properties endowed by reduced dimensions.2−5 As we know, nanosized materials with various structures and morphologies have been widely studied and reported, including core−shell or multishell hollow structures,6,7 peapodlike nanocomposites,8 sandwichlike architectures,9 mesoporous frameworks,10 and so on.11 Among these special nanostructured materials, sandwichlike nanocomposites have triggered dramatic interest owing to their combined properties of doubled graphited carbon sheets and functional nanoparticles, rendering them a promising candidate in a tremendous amount of significant fields, e.g., energy storage and conversion, electrocatalysis, and microelectronic devices.8,12 More importantly, a great effort has been made to seek active materials for hydrogen evolution reaction (HER) and lithium-ion batteries (LIBs). Transition metal phosphides, attractive as HER electrocatalysts as well as LIB anodes, have been extensively explored. Hydrogen, an ideal energy carrier, is regarded as a clean and renewable fossil fuel for suatainable energy fields.13−15 It is a feasible approach to generate numerous hydrogen by electro© XXXX American Chemical Society

chemical reduction of water. However, there is a need for an efficient HER electrocatalyst.16,17 At present, Pt-based catalysts are the most common catalysts for HER, but Pt is too expensive and scarce to apply in practice.17,18 To conquer this limitation, tremendous research has been studied on the transition metal phosphides, an interesting new class of catalysts for electrochemical HER with good electrical conductivity, including FeP,19,20 Ni2P,21,22 CoP,23 MoP,24 and WP.25 For the available electrical energy storage devices, rechargeable LIBs are the most promising medium on account of their abundant sources, long lifespan, environmental benignity, safety, and high energy density.26,27 Notably, there seems to be huge attention in study and investment of large-scale LIBs for electronic devices (EV) and hybrid electric vehicles (HEV).28,29 Because of low polarization and high specific capacities, transition metal phosphides, e.g., FeP30 and Ni2P,31−33 are accepted as alternative anode materials in place of graphite for the next-generation LIBs.34 So far, novel sandwichlike nanostructures with Ni, Ni2P, Co, and Co3O4 encapsulated nanoparticles have been successfully synthesized and applied in LIBs.9,12 From the previous literature, the natural superiorities of sandwiched nanoReceived: September 13, 2015 Accepted: November 16, 2015

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DOI: 10.1021/acsami.5b08620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of the Fabrication Process of the Fe2P Nanoparticles Enveloped in Sandwiched Graphited Carbon Sheets Nanocomposite

transformed into functional Fe2P nanoparticles upon hightemperature calcinations in a H2 atmosphere. Hence, the sandwichlike nanocomposites (Fe2P/GCS) are successfully attained. Figure 1 displays the general characterizations of as-prepared FeFe2(PO4)2(OH)2 nanosheets. As verified in Figure 1a, a low-

composite are clearly displayed. First, graphited carbon sheets possess excellent conductivity, which further accelerates the electron transfer rate. Second, graphited carbon sheets definitely isolate the nanoparticles and effectively prevent them from aggregating and agglomerating during the long-time electrochemical cycling; thus, the electrochemical performance of sandwichlike materials can be strikingly improved. Finally, graphited carbon sheets act as a reducing reagent in the reduction of precursors to final products evolving into CO or CO2, resulting in porous structure across graphited carbon sheets with a large specific surface area. Hence, the electrolyte can fluently shuttle through the graphited carbon layers, which is favorably beneficial for accelerating electrochemical kinetics. Giving these advantages, it is necessary to prepare sandwiched nanocomposites to apply in energy systems, particularly HER and LIBs. Although sandwichlike Ni-based (Fe)Ni2P/graphene using NiNH4PO4·H2O nanosheets as precursors have been reported as HER catalyst and LIB anodes,12 Ni is expensive, which hinders extensive applications of (Fe)Ni2P/graphene to a tremendous extent. As is well-known, in all transition metals, Fe is the most economical and resourceful element. Taking this into account, we employ a simple and general strategy to successfully fabricate a novel and hierarchical nanocomposite where Fe2P nanoparticles are enveloped in sandwichlike graphited carbon sheets (Fe2P/GCS). Notably, being quite different from Ni-based (Fe)Ni2P/graphene, Fe2P/GCS generated from low-cost FeFe2(PO4)2(OH)2 precursors has been facilely synthesized without cationic doping. Moreover, the unique Fe2P/GCS can be used as both HER catalysts and LIBs anodes for the first time, significantly manifesting improved catalytic activity and excellent electrochemical performance.

Figure 1. (a) Low-magnification SEM image, (b) XRD data, (c) TEM, and (d) HRTEM images of FeFe2(PO4)2(OH)2 nanosheets.

magnification scanning electron microscopy (SEM) image, an enormous quantity of sheetlike precursors for the synthesis of target product can be generated in a uniform way, and the sample’s size can reach up to tens of square micrometers. Meanwhile, a magnified SEM image (Supporting Information) is presented to further uncover the sheetlike morphology of FeFe2(PO4)2(OH)2. The corresponding X-ray diffraction (XRD) data is illustrated in Figure 1b, where all the peaks are in good agreement with FeFe2(PO4)2(OH)2 (Powder Diffraction File (PDF) No. 85-1728, International Centre for Diffraction Data (ICDD), 1959) and their sharpness implies that FeFe2(PO4)2(OH)2 nanosheets are highly crystallized. To our knowledge, transmission electron microscopy (TEM) is a useful instrument, which can unveil concrete information about microstructure for the precursor. As observed in Figure 1c, we can determine that the precursors are sheetlike, in accord with the SEM observation. To demonstrate further the crystal structure of FeFe2(PO4)2(OH)2 nanosheets in detail, a highresolution transmission electron microscopy (HRTEM) image is shown in Figure 1d, in which a clear interdistance of 0.48 nm

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RESULTS AND DISCUSSION In this research, the whole synthesis process for novel Fe2P/ GCS is illustrated by Scheme 1. First, FeFe2(PO4)2(OH)2 nanosheets acting as the precursor were prepared by a facile hydrothermal treatment. Subsequently, owing to plentiful hydroxyl groups on the nanosheet surface, glucose molecules serving as the green carbon source were absorbed by virtue of hydrogen bonding and gradually polymerized into polymeric layers under hydrothermal conditions, thereby forming the intermediate products. Finally, the polymeric layers were converted into graphited carbon sheets, and at the same time, the precursors functioning as the sacrificial templates were B

DOI: 10.1021/acsami.5b08620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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In addition, X-ray energy-dispersive spectroscopy (EDS), Raman spectroscopy, nitrogen absorption/desorption analysis (Figure 3), and X-ray photoelectron spectroscopy (XPS) have

can be indexed as the (110) crystal plane for FeFe2(PO4)2(OH)2. In the synthesis, FeFe2(PO4)2(OH)2 is reduced to Fe2P under H2 atmosphere at an appropriate temperature. As a matter of fact, FeFe2(PO4)2(OH)2 nanosheets first generated function as templates. By means of a thermal annealing process, the sandwiched Fe2P/GCS can be easily harvested. As uncovered in the SEM image shown in Figure 2a, plenty of tiny Fe2P nanoparticles

Figure 3. (a) Energy-dispersive spectrum (EDS), (b) Raman spectrum, (c) N2 absorption/desorption isotherm curve, and (d) pore size distribution curve of Fe2P/GCS nanocomposite.

been implemented to investigate the sandwichlike Fe2P/GCS (Supporting Information). By the composition analysis of EDS, we can determine the presence of C, Fe and P elements in Fe2P/GCS because the exclusive signal of Si comes from silicon substrate utilized for supporting the sample. The characteristic of graphited carbon sheets encapsulating Fe2P nanoparticles is evaluated by Raman spectroscopy. In theory, two characteristic bands, D band at around 1340 cm−1 and G band at about 1590 cm−1, respectively, match well with amorphous and graphitized carbons.35 The graphitization degree can be relatively confirmed by the intensity ratio between the intensities of the D and G bands (ID/IG ratio). As expected, the ID/IG ratio of graphited carbon envelopes is low, which suggests good crystallization of sandwiched graphited carbon sheets. Brunauer−Emmett−Teller (BET) surface area and pore size distribution curves are taken on as well. The specific surface area is calculated as 132.2095 m2/g, and the pore size is centered at 3.9 nm, implying the nanoporous structure of the Fe2P/GCS. As is known to us, the large specific surface area and nanoporosity can sufficiently provide the extended contact area between the active materials and electrolyte.12,37 The bonding nature of Fe2P/GCS is captured by XPS so as to make sure the valence states of C, Fe, and P elements. The core-level photoelectron peaks of Fe2p and P2p are displayed after the standard C1s peak is used for correcting the shifts. The explicit C1s peak at 284.8 eV stands for graphited carbon.38 In the Fe2p XPS spectrum, two peaks accordingly appear at 723.6 and 720.3 eV in the Fe2p1/2 level, and yet two peaks are located at 710.2 and 707.1 eV in the Fe2p3/2 level, which demonstrates that elemental and oxidized Fe coexist. Meanwhile, the P2p XPS spectrum has two peaks emerge at 133.4 and 129.5 eV in the P2p level, resulting from the presence of both elemental and oxidized P. As shown in the XRD patterns (Figure 2b), no apparent oxidized Fe and P is observed; this may be attributed to the superficial oxidation of Fe2P exposed to air.35,39 The combination of the XPS spectra with XRD profiles reveals that the nanoparicles encapsulated in graphited carbon sheets are Fe2P. On the basis of the morphological, structural, and compositional characterizations, it is obvious that the final product is

Figure 2. (a) Typical SEM image (optical picture, inset of a), (b) XRD patterns, (c) magnified TEM, and (d) HRTEM images for obtained Fe2P/GCS nanocomposite.

with the diameter of about 40 nm are uniformly dotted in the sheets, and it is clear that the thin envelopes and graphited carbon sheets wonderfully inherit the sheet morphology of precursors. The inset image of an optical picture affirms that grams of Fe2P/GCS can be achieved and that the production can be feasibly scaled up. Furthermore, a low-magnification SEM image shows that numerous sandwichlike Fe2P/GCS can be facilely prepared (Supporting Information). As distinctly demonstrated in the XRD pattern shown in Figure 2b, all the diffraction peaks correspond well to pure and well-crystallized Fe2P (PDF No. 88-1803, ICDD, 1973).35 To reveal the morphology information on the obtained Fe2P/GCS, the TEM image in Figure 2c directly discloses that the strictly monodispersed Fe2P nanoparticles are evenly distributed in the sandwichlike graphited carbon sheets even after strong ultrasonication, which provides powerful evidence to support the declaration of nanoparticles being enveloped in coupled graphited carbon sheets. As is known to us, the easy aggregation and agglomeration of active nanoparticles have a serious impact on the electrochemical performance of materials. The novel sandwiched structure remarkably improves the cyclability because of its keeping the active nanoparticles available from aggregation during a long-term cycling. Moreover, HRTEM techniques are utilized to analyze the detailed crystal structures for harvested samples composed of Fe2P nanoparticles and sandwichlike graphited carbon sheets. In Figure 2d, the lattice fringe with a spacing of 0.293 nm is consistent with the (110) crystal plane of Fe2P, indicating the existence of Fe2P nanoparticles. At the same time, graphited carbon sheets are confirmed by HRTEM image (Supporting Information), wherein the hexagonal lattices of graphene can be clearly detected. C

DOI: 10.1021/acsami.5b08620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces sandwichlike Fe2P/GCS with unique morphology. To study the mechanism of HER, the HER catalytic activity of harvested Fe2P/GCS was measured in 0.5 M H2SO4 solution using a typical three-electrode system. Fe2P/GCS was deposited on a glassy-carbon electrode (GCE, 0.07 cm2) as the working electrode. For comparison, the pure Fe2P particles and Pt/C were also tested under the same condition. Figure 4a displays

Table 1. Comparison of HER Performance in Acidic Media for Fe2P/GCS with Other Transition Metal Phosphide Electrocatalysts catalyst nanoporous FeP nanosheets FeP-GS CoP/CNT bulk MoP Ni2P nanoparticles Fe2P/GCS

electrolyte 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 1M H2SO4 0.5 M H2SO4

onset overpotential (mV)

Tafel slope (mV dec−1)

reference

100

67

19

50

20

40

54

23

50

54

24

50 40

22 49

this work

retains low and stable for over 10 h. In contrast, Fe2P particles need higher potential increased by 33% under the same condition, indicating that the as-obtained Fe2P/GCS is a stable catalyst for HER in acid environment. Meanwhile, the stability of Fe2P/GCS electrode was explored under the long-term cyclic voltammetric (CV) cycling in 0.5 M H2SO4. As revealed in Figure 4d, after 2000 consecutive cycles, Fe2P/GCS only exhibits a slight increase of potential. This result manifests the admirable acidic stability of Fe2P/GCS in electrochemical process. The superior catalytic activity and especially exceptional stability can be attributed to the additional active sites by uniformly dispersed nanoparticles, large specific surface area, and rich nanopores.8,12 More importantly, the cycle number for Fe2P/GCS is twice that for (Fe)Ni2P/graphene,12 clearly manifesting the enhanced stability of Fe2P/GCS. It is obvious that Fe2P/GCS makes a major breakthrough in electrochemical HER reaction. The electrochemical properties of harvested Fe2P/GCS as LIB anodes were estimated by a series of measurements for the first time. CV was used to trace the oxidation and reduction properties during the Li-storage process. On the basis of previously reported results, it is believed that the electrochemical mechanism for Fe2P, similar to other transition metal phosphides, stems from an alloying/dealloying reaction, which can be identified as Fe2P + 3Li+ + 3e− ↔ Li3P + 2Fe. Figure 5a shows the first three cycles at a scan rate of 0.5 mV s−1 in the voltage range of 0.01−3.0 V (vs Li/Li+). The CV tendency is in the line with those of transition metal phosphide anode materials reported in the literature,12,33 and the curves scarcely display variation except for that of the first cycle, implying that the electrochemical process is reversible. The galvanostatic charge−discharge profiles at a rate of 0.1 A g−1 over 200 cycles are described in Figure 5b. The initial discharge and charge capacities of Fe2P/GCS are around 945 and 605 mA h g−1, respectively, corresponding to a Coulombic efficiency of ∼64%. It should be noted that the initial discharge capacity of Fe2P/GCS far exceeds the theoretical capacity of Fe2P (∼564 mA h g−1), which is associated with the formation of a solidelectrolyte interface (SEI) layer on the electrode surface arising from electrolyte degradation, a typical phenomenon widespread in many other reported anode materials.8,9 Cycling stability and Coulombic efficiency are crucial criteria for LIB anodes. As shown in Figure 5c, starting from the second cycle, the discharge specific capacity of Fe2P/GCS trends to be stable, and the Coulombic efficiency is up to 93%. Even after 200

Figure 4. (a) Polarization curves and (b) corresponding Tafel plots of Pt/C, sandwichlike Fe2P/GCS, and Fe2P particles in 0.5 M H2SO4 at a scan rate of 5 mV s−1. (c) Potential change of Fe2P/GCS and Fe2P particle at a constant current density of 20 mA cm−2. (d) Stability measurement for Fe2P/GCS at 50 mV s−1after 2000 cycles.

the polarization curves of Pt/C, Fe2P/GCS, and Fe2P particles. As anticipated, the Pt/C exhibits the superior HER activity with a near-zero overpotential. The Fe2P/GCS discloses an onset overpotential of ∼40 mV, notably lower than that of Fe2P particles. This positively indicates that the catalytic activity of Fe2P/GCS for HER has been improved because of the increased electrical conductivity by graphene envelopes and enriched active sites by designed structure. In contrast with the value of 50 mV for (Fe)Ni2P/graphene,12 the overpotential for Fe2P/GCS is lower, directly suggesting the advantage of lowcost Fe2P/GCS. More importantly, Fe2P/GCS only requires 88 and 107 mV to afford 10 and 20 mA cm−2, respectively. The Tafel plots are presented in Figure 4b. According to the Tafel equation (η = b log j + a, where η is the overpotential, j is the current density, and b is the Tafel slope), Pt/C exhibits a Tafel slope of 30 mV dec−1, which is in conformity with the reported value.39 Notably, a Tafel slope of 49 mV dec−1 for Fe2P/GCS is lower than many values reported for transition metal phosphide HER catalysts (Table 1). By contrast, Fe2P particles display a Tafel slope of 80 mV dec−1, much higher than that of Fe2P/ GCS, which further suggests the high intrinsic catalytic activity of Fe2P/GCS. Moreover, the temperature-dependent catalytic measurement has also been implemented at a freezing temperature of 0 °C, room temperature of 25 °C, and elevated temperature of 50 °C (Supporting Information). As the temperature ranges from 0 to 25 to 50 °C, the catalytic activity is gradually increased. We have investigated the durability of the catalysts during continuous HER at a constant current density in acidic solution as well. Figure 4c shows the potential change at a constant current of 20 mA cm−2. For Fe2P/GCS, the overpotential D

DOI: 10.1021/acsami.5b08620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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templates. With large specific surface area, high nanoporous feature, and preeminent structural stabilization, the sandwiched Fe2P/GCS serves as both non-noble-metal HER catalyst and LIB anode for the first time. As a HER electrocatalyst, it exhibits excellent electrochemical properties in acidic environment, including an overpotential as low as 40 mV, a low Tafel slope of 49 mV dec−1, and attractive stability over 2000 cycles. Moreover, when utilized as anodes for LIBs, the special Fe2P/ GCS displays high specific capacity of 602 mA h g−1 at 0.1 A g−1, outstanding cyclability over 200 cycles, and intriguing rate capability, e.g., 362 mA h g−1 at a rate of 10 A g−1. The excellent electrochemical performance indicates that the sandwichlike Fe2P/GCS is a promising candidate in energy systems. Furthermore, it is believed that other sandwiched nanocomposite can be applied in plenty of important fields, such as electrocatalysis and LIBs.

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Figure 5. (a) Cyclic voltammetry (CV) curves, (b) galvanostatic charge−discharge profiles, (c) cycling stability and Coulombic efficiency at 0.1 A g−1 over 200 cycles, and (d) rate capability for Fe2P/GCS.

EXPERIMENTAL SECTION

Materials. Ethylene glycol (Fisher Chemical, 99.99%), ferric nitrate (FeNO3, 99.9%, Aldrich), sodium dihydrogen phosphate (NaH2PO4, 99.9%), sodium carbonate (Na2CO3, 99.9%, Aldrich), D(+)-glucose (Cica-Reagent, Kanto Chemical), H2SO4 (99.999%), and metallic Cu foil (99.9%, Aldrich) were used as received. Preparation of FeFe2(PO4)2(OH)2 Nanosheets. First, ethylene glycol (12.5 mL), 1 M FeNO3 aqueous solution (2.5 mL), 1 M NaH2PO4 aqueous solution (2.5 mL), 1 M Na2CO3 aqueous solution (0.5 mL), and 1 M glucose aqueous solution (1.5 mL) were mixed stage by stage with continued stirring. Then, the precursor solution was magnetically stirred for another 15 min. The color of the homogeneous solution becomes bright yellow at last. After that, the solution mixture was transferred into a Teflon-lined stainless-steel autoclave (45 mL volume), and the hydrothermal reaction was carried out at 170 °C for 24 h in an electric oven. The resulting product was collected and washed by centrifugation (three cycles with deionized water and one cycle with ethanol), followed by drying at 60 °C overnight. Preparation of Fe2P Nanoparticles Encapsulated by Sandwiched Graphited Carbon Sheets. FeFe2(PO4)2(OH)2 nanosheets (50 mg) were dissolved in 30 mL of a solution of 1 M glucose aqueous solution (1.5 mL) and deionized water (28.5 mL) by ultrasonication treatment for 5 min to form a homogeneous solution. The above solution was introduced into a Teflon-lined autoclave (45 mL volume). Afterward, the autoclave was heated at 180 °C for 8 h in an electric oven. Then, the samples were washed by centrifugation with deionized water three times and ethanol one time and dried at 60 °C in an oven. Finally, the intermediate products were converted into sandwiched Fe2P/GCS under H2 atmosphere at 850 °C in the muffle furnace. Characterization. A field-emission scanning electron microscope (JEOL, JSM-7800F, 15 kV), a transmission electron microscope (Philips, Tecnai, F30, 300 kV) coupled with an energy-dispersive spectrometer (EDS) analyzer, an X-ray diffractometer with Co Kα (λ = 1.78897 Å) radiation (XRD, Bruker D8 Advance), a BET surfacearea and pore-size analyzer (Quantachrome Autosorb-6B), and a RENISHAW Invia Raman microscope (voltage (ac) 100−240 V, Power 150 W, UK) were utilized. Carbon Content Measurements of Fe2P Nanoparticles Encapsulated by Sandwiched Graphited Carbon Sheets. Final samples (200.00 mg of Fe2P/GCS) were added to concentrated hydrochloric acid. The Fe2P nanoparticles were completely dissolved after stirring for 24 h. The reserved carbon was collected and washed with deionized water and ethanol several times and then dried at 60 °C overnight. The weight tests of carbon were conducted using a MettlerToledo analytical balance. The carbon content of Fe2P/GCS was calculated by the following formula:

cycles, the high discharge capacity maintains at 560 mA h g−1, corresponding to 93% of that of the second cycle. The value is much higher than that of the previously reported Ni2P (Supporting Information). Comparing the similar results reveals the high specific capacity and excellent cyclability of Fe2P/GCS, which could be put down to the novel sandwiched structure. On the one hand, the graphene envelopes with large specific surface area can isolate the functional nanoparticles from aggregation as well as pulverization during long-term electrochemical reactions, and the inner space between encapsulated nanoparticles would buffer the massive volume expansion so as to well preserve the architectural integration. On the other hand, the nanoporous feature is extremely beneficial for the diffusion of the electrolyte, thus accelerating the Li ion exchange rate during the charge−discharge process. Furthermore, the outstanding rate capability of Fe2P/GCS is observed in Figure 5d. The steady capacity of 580 mA h g−1 is delivered over 100 cycles run at a rate of 0.1 A g−1. When the current rate increases to 0.2, 1, 2, and 10 A g−1 for 20 cycles, the corresponding high values of 540, 465, 403, and 362 mA h g−1, respectively, are achieved. Then, the current density is reduced back to 0.1 A g−1 for another 20 cycles; the delivered discharge capacity is restored to 577 mA h g−1. As is known to all, there are few reports about Fe2P materials in LIBs. The sandwiched Fe2P/GCS first utilized as LIB anode exhibits an enhanced rate performance relative to that of (Fe)Ni2P/graphene at high rate.12 To certify the improved charge transport by graphene envelopes and the shortened particle size, the electrochemical impedance spectroscopy (EIS) was implemented on Fe2P/GCS and Fe2P particles (Supporting Information). In light of the Nyquist plots for two different morphological materials, the charge-transfer resistance of sandwichlike Fe2P/GCS is much lower than that of Fe2P particles, suggesting electron can transport more quickly on Fe2P/electrolyte interface than on Fe2P particles.8,12 All the results mentioned above indicate the remarkable Li-storage performance, particularly rate capability for Fe2P/GCS.

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CONCLUSIONS A facile and simple method is introduced for synthesizing novel Fe2P/GCS using sheetlike FeFe2(PO4)2(OH)2 precursors as

C% = G(C)/G(Fe2P/C) × 100% E

DOI: 10.1021/acsami.5b08620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Herein, G(C) and G(Fe2P/C) are the weight of carbon and Fe2P/ GCS, respectively. Results reveal that the carbon content of composite is about 8.7 wt %. Electrochemical Measurements. For HER, electrochemical measurement was implemented on a CHI660E electrochemical workstation. The electrochemical performance was studied in a three-electrode setup containing a modified GCE (geometric area = 0.07 cm2) as working electrode, a saturated calomel electrode (SCE) as reference electrode, and a Pt foil as counter electrode. A 3 mg portion of catalyst, 100 μL of Nafion solution (0.5 wt %), and 300 μL of ethanol solvent were mixed by ultrasonication for 15 min to prepare the catalyst suspension. The working electrode was prepared by dropping 4 μL of suspension on the GCE (mass loading: 0.36 mg cm−2) and dried at room temperature. Linear sweep voltammetry (LSV) was taken from 0 to −0.5 V at a scan rate of 50 mV s−1 in 0.5 M H2SO4 solution. The durability test was carried out by cyclic voltammetry (CV) scanning from 0 to −0.5 V at 50 mV s−1. In 0.5 M H2SO4 solution, all the potentials were referenced to a reversible hydrogen electrode (RHE) according to the equation E(RHE) = E(SCE) + 0.281 V. For LIB tests, a mixture made up of 75 wt % of composite, 15 wt % of carbon black, and 15 wt % of polyvinyl difluoride (PVDF) using 1methyl-2-pyrrolidinone (NMP) as solvent was well mixed and then spread on Cu foils. The Cu foils covered with samples were then used as working electrodes. The electrolyte was 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1/1 by volume). Celgard 2400 was used as the separator film to isolate the two electrodes. Pure Li foil was used as counter electrode and reference electrode. The cell was assembled in a glovebox filled with pure argon. Charge−discharge curves were recorded using LAND battery program−control test system (CT2001A, Jinnuo electronic Co.).CV curves were recorded using a CHI660D electrochemistry workstation (Chenhua Instrument Co.). EIS tests were conducted on this apparatus over a frequency range of 0.01 Hz to 100 kHz.

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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/acsami.5b08620. Additional SEM, HRTEM, XPS, and electrochemical test data. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Fundamental Research Funds for the Central Universities (0301005202017), Thousand Young Talents Program of the Chinese Central Government (grant no. 0220002102003), National Natural Science Foundation of China (NSFC, grant no. 21373280, 21403019), the Beijing National Laboratory for Molecular Sciences (BNLMS), and Hundred Talents Program at Chongqing University (grant no. 0903005203205).

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DOI: 10.1021/acsami.5b08620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX