CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201301188
Fe3O4/Carbon Hybrid Nanoparticle Electrodes for HighCapacity Electrochemical Capacitors Jun Seop Lee, Dong Hoon Shin, Jaemoon Jun, Choonghyeon Lee, and Jyongsik Jang*[a] Fe3O4/carbon hybrid nanoparticles (FeCHNPs) were fabricated using dual-nozzle electrospraying, vapor deposition polymerization (VDP), and carbonization. FeOOH nanoneedles decorated with polypyrrole (PPy) nanoparticles (FePNPs) were fabricated by electrospraying pristine PPy mixed with FeCl3 solution, followed by heating stirring reaction. A PPy coating was then formed on the FeOOH nanoneedles through a VDP process. FeCHNPs were produced through carbonization of PPy and FeOOH phase transitions. These hybrid carbon nanoparticles (NPs) were used to build electrodes of electrochemical capaci-
tors. The specific capacitance of the FeCHNPs was 455 F g1, which is larger than that of pristine PPy NPs (105 F g1) or other hybrid PPy NPs. Furthermore, the FeCHNP-based capacitors exhibited better cycle stability during charge–discharge cycling than other hybrid NP capacitors. This is because the carbon layer on the Fe3O4 surface formed a protective coating, preventing damage to the electrode materials during the charge–discharge processes. This fabrication technique is an effective approach for forming stable carbon/metal oxide nanostructures for energy storage applications.
Introduction There has been growing interest in energy storage systems for applications including industrial power backup, hybrid electric vehicles, and personal electronic devices.[1–5] Much attention has been focused on electrochemical capacitors (ECs), also called supercapacitors, to satisfy these demands.[6–10] ECs are promising systems that can store energy through a simple mechanism and that have a potential for high power densities, short charging times, long cycle lifes, and favorable safety considerations.[11–15] Depending on the electrode materials and charge storage mechanism, ECs can be classified into electrical double-layer capacitors (EDLCs) or Faradaic redox reaction pseudocapacitors.[16–20] EDLCs are typically based on carbon materials; the energy storage mechanism arises from the accumulation of electronic and ionic charges at the interface between electrode materials and electrolyte.[21–23] Pseudocapacitors that use metal-oxide and conducting-polymer electrodes exhibit reversible Faradaic reactions of electroactive species or surface functional groups of the electrode to store energy.[24–26] In general, pseudocapacitors have a higher energy density than EDLCs; however, EDLCs are more stable with better cyclability. The fabrication of electrodes that provide a high energy density and maintain good stability is an important aspect of the development of ECs. Carbon-based nanomaterials, including carbon nanotubes (CNTs), carbon nanofibers (CNFs), fullerenes, and graphene, [a] J. S. Lee, D. H. Shin, J. Jun, C. Lee, Prof. Dr. J. Jang School of Chemical and Biological Engineering College of Engineering, Seoul National University 599 Gwanangno, Gwanakgu, Seoul, 151-742 (Korea) Fax: (+ 82) 2-888-1604 E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301188.
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have led to the development of devices such as EDLCs because of favorable cyclic stability and long service lifetime of the electrodes utilizing non-Faradaic processes.[27–30] CNTs and graphene have been investigated in an attempt to exploit the electronic, chemical, and structural properties of these nanoscale materials.[31–33] However, difficulties in obtaining singlespecies CNTs and uniform graphene layers that are free from defects have hindered their practical application. Carbon particles, however, are inexpensive and can be produced with good uniformity using large-scale processes.[34, 35] Despite these advantages, relatively little research has been conducted on the use of carbon nanoparticles as electrode materials for ECs due to difficulties in fabricating uniform small-diameter particles. Various transition metal oxides, including RuO2, NiO, and CoOx, have been studied for charge storage applications in pseudocapacitors.[36–38] However, the use of these materials has not been widespread, largely due to high costs and environmental concerns. Magnetite (Fe3O4) has emerged as a promising EC material due to its low cost and lack of pollution issues.[39, 40] However, there are two principle disadvantages of Fe3O4 for EC electrode applications. First, a large volume change occurs in the electrode matrix of Fe3O4 during charge– discharge cycling, which leads to pulverization of the electrodes. Second, the high resistivity of the metal oxide leads to a large internal resistance of devices. Herein, we report the fabrication of Fe3O4/carbon hybrid nanoparticles (FeCHNPs), which are carbon-coated porous Fe3O4 nanoneedles vertically aligned on the carbon nanoparticle surface fabricated using dual-nozzle electrospraying, vapor deposition polymerization (VDP), and carbonization. First, vertically aligned FeOOH nanoneedles were formed on the PPy surface using dual-nozzle electrospraying, followed by heat stirring reaction. Then, PPy-coated FeOOH nanoneedles (PFePNPs) ChemSusChem 2014, 7, 1676 – 1683
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CHEMSUSCHEM FULL PAPERS were fabricated using VDP to form a precursor to the Fe3O4/ carbon hybrid nanoparticles (FeCHNPs), which were formed through carbonization of PPy and the phase transition of PPy/ FeOOH to carbon/Fe3O4. The FeCHNPs were used as electrode materials for ECs, which exhibited a specific capacitance of 455 F g1, which is higher than that of pristine PPy NPs (105 F g1) or other hybrid PPy-based NPs. ECs based on FeCHNPs also exhibited better cycle stability than ECs prepared using other hybrid NP electrodes. The carbon coating inhibited degradation of the Fe3O4 structure and enhanced the specific capacitance due to its ability to form an EDL itself. To the best of our knowledge, there have been no previous reports of the production of carbon–metal oxide hybrid composites based on a dual-nozzle electrospraying process.
www.chemsuschem.org Fe3þ þ 3 OH ! FeOOH þ H2 O
ð1Þ
The electrosprayed PPy NPs were stirred in the FeCl3 aqueous solution at 70 8C to induce growth of FeOOH nanoneedles on the PPy surface. This resulted in the formation of FeOOH nanoneedles decorated with PPy nanoparticles (FePNPs), as shown in Figure 2 a. The high-resolution transmission electron microscopy (HRTEM) images indicate that the interplanar spacing between the needles was 0.29 and 0.26 nm in the (100)
Results and Discussion Fabrication of hybrid carbon nanoparticles Figure 1 shows an overview of the fabrication process for the FeCHNPs, which is based on the dual-nozzle electrospraying method and heat treatment. 60 nm-diameter PPy NPs were prepared uniformly and without aggregation using a monodis-
Figure 1. Illustrative diagram of the sequential fabrication steps for Fe3O4/ carbon hybrid nanoparticles.
perse method, as shown in Figure S1 in the Supporting Information.[41] The PPy NPs were stirred in an aqueous FeCl3 solution at room temperature to induce covalent bonding between the Fe3 + ions and the partial negative charge of the nitrogen atoms in the pyrrole structure. The mixed PPy solutions were electrosprayed from the outer part of the dual nozzle while compressed air flowed through the inner nozzle, as shown in Figure S2. During the electrospraying process, FeOOH particles were formed on the PPy surface. The high positive voltage applied allowed uniform dispersion of Fe3 + ions on the PPy surface, leading to the following reaction between the Fe3 + and hydroxide ions in the collector:[42, 43] 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 2. TEM and HRTEM images of (a, b) FeOOH nanoneedle-decorated PPy nanoparticles (FePNPs) and (c, d) PPy-coated-FeOOH nanoneedle-decorated PPy nanoparticles (PFePNPs). EELS dot mapping of PFePNP nanoneedle components of (e) iron, (f) oxygen, and (g) carbon atoms (scale bar: 10 nm).
and (021) planes, respectively, corresponding to an a-FeOOH lattice structure (see Figure 2 b). The FePNPs were dipped into a CuCl2 ethanol solution, leading to the adsorption of Cu2 + ions onto the surface of the FeOOH nanoneedles. This occurred because of the interaction between the Cu2 + ions and the partial negative charge of the oxygen atoms in the FeOOH structure, as shown in Figure S3. Following the adsorption of the Cu2 + ions onto the FeOOH needles, vaporized pyrrole monomers in contact with the Cu2 + ions were polymerized through chemical oxidation. As a result, PPy-coated FeOOH nanoneedles decorated with PPy nanoparticles (PFePNPs) were fabricated that had a uniform thickness of approximately 3 nm of PPy on the needle surface, as shown in Figure 2 c and d. No ChemSusChem 2014, 7, 1676 – 1683
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CHEMSUSCHEM FULL PAPERS aggregation of the coated particles was observed following polymerization, which was attributed to the dispersion of Cu2 + ions over the nanoneedle surface during adsorption. Additionally, electron energy loss spectroscopy (EELS) mapping analysis of iron, oxygen, and carbon atoms conducted to determine the atomic distribution of PFePNP nanoneedles (Figure 2 e–g). The iron and oxygen atoms were dispersed within the needle structure; on the other hand, the carbon atoms were dispersed on the surface of the needle structure, which confirmed that the PPy was coated onto the FeOOH needle surface through the VDP step. The as-prepared PFePNPs were then carbonized in an argon atmosphere to obtain the FeCHNPs. Figure 3 a and b show the Fe3O4 nanoneedle-decorated carbon nanoparticles (FeCNPs) formed from the FePNPs. The carbonization process was carried out at 400 8C; the Fe3O4 needle structures collapsed at higher temperatures (see Figure S4). HRTEM images of the needles indicate an interplanar spacing of 0.29 nm for the (220) plane of Fe3O4, which confirms that the FeOOH transformed into crystalline Fe3O4 following carbonization. In addition, small mesoporous structures (5 nm in diameter) were observed in the Fe3O4 nanoneedle structures owing to the formation of vapors during the heat treatment during the phase transition of FeOOH, as shown in Figure 3 c.[44] The PFePNPs were also changed from PPy/FeOOH to carbon/Fe3O4 during the carbonization step; thus, FeCHNPs were prepared. Figure 3 d and e show TEM and HRTEM images without aggregation of the particles and uniformly coated carbon layer on the Fe3O4 surface. The HRTEM image shows 2 nm-thick Fe3O4-coated carbon layers on the surface, with a mesoporous structure with no collapsed needles. In addition, the thickness of the coated layer decreased slightly as the PPy layer was converted into amorphous carbon during carbonization. EELS mapping of FeCHNPs was conducted to confirm carbon coating of the Fe3O4 needles (Figure 3 g–i). The iron and oxygen atoms are present inside the needles, whereas the carbon atoms are present on the surface of the needles. Therefore, carbon layer coats the needles on the surface, as the protecting layer.
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Characterization of hybrid carbon nanoparticles
Figure 3. TEM and HRTEM images of (a–c) Fe3O4 nanoneedle-decorated amorphous carbon nanoparticles (FeCNPs) and (d–f) Fe3O4/carbon hybrid nanoparticles (FeCHNPs). EELS dot mapping of FeCHNP needle components of (g) iron, (h) oxygen, and (i) carbon atoms (scale bar: 10 nm).
The crystal structure of the hybrid NPs was determined using powder X-ray diffraction (XRD) analysis, as shown in Figure 4 a. The broad peak at 2q = 248 corresponding to the pristine PPy NPs was consistent with scattering by the PPy chains at the interplanar spacing. The spectrum of the FePNPs matched the standard spectrum of a-FeOOH (JCPDS 29-0713), and the broad peak was characteristic of the PPy structure.[44] These peaks were also observed in the PFePNP samples, indicating that the structure of the FeOOH was maintained through the VDP process (see Figure S5). The spectrum of FeCHNPs showed a broad peak at 2q = 238, which indicates an amorphous carbon phase and sharp peaks of Fe3O4 crystal structures (JCPDS 19-0629).[44] However, the broad peak of the amorphous carbon was similar to that for PPy. Thus, the IG/ID ratio from Raman spectra was used to further characterize the samples, as shown in Figure 4 b. This ratio was higher for the
amorphous carbon NPs (~ 1.3) than for PPy NPs (~ 0.8), and the peaks for carbon were sharper than those for PPy. These findings confirm that the PPy NPs were successfully converted into amorphous carbon through the carbonization process. The chemical composition of the hybrid materials was characterized using X-ray photoelectron spectroscopy (XPS). Figure 5 a shows spectra over the range of 0–1200 eV, which reveal that carbon, nitrogen, oxygen, and iron atoms were present in the PFePNPs and FeCHNPs, whereas only carbon, oxygen, and nitrogen atoms were present in the pristine PPy NPs. The N 1s peak was attributed to nitrogen atoms from the pyrrole component. High-resolution XPS spectra for the C 1s region around 285 eV are shown in Figure 5 b; this peak was deconvoluted into four components for PPy-based particles
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FeOOH nanoneedles. However, the hybrid carbon-based particles displayed a large peak at 284.6 eV, which was attributed to graphitic sp2 hybridization, and two additional peaks at 285.5 and 287.9 eV, which were assigned to CO and CN, respectively. Figure 5 c shows the high-resolution XPS spectra for the Fe 2p peak. Spin-orbit components 2p3/2 and 2p1/2 were observed near 710.2 and 722.8 eV, indicating that the valence state of Fe was + 3. O 1s spectra of PFePNPs and FeCHNPs contained several components, as shown in Figure 5 d. For PFePNPs, the peak at 533.0 eV corresponded to CO bonds from the PPy structure component. The peaks at 531.7 and 529.6 eV were attributed to OH and O2 in the FeOOH structure. Otherwise, there were different peaks for FeCHNPs in comparison to PFePNPs. The peak at 533.0 eV was attributed to CO bonds from the carbon structure, and the peak at 529.6 eV to O2 in the Fe3O4 structure. Therefore, we can conclude that the FeOOH and Fe3O4 nanoneedles were composed of iron(III) and oxygen as confirmed by XRD results. Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) measurements were performed to measure the specific surface area and pore size distribution of the hybrid PPy and carbon NPs. The BET specific surface area of the pristine PPy Figure 4. (a) XRD patterns and (b) Raman spectra for the hybrid PPy and NPs was 50 m2 g1 and increased following the electrospraying, carbon nanoparticles. VPD, and carbonization steps, as shown in Figure 6 a. Electrospraying and stirring at elevated temperatures increased the and three components for carbon-based particles. The peaks surface area of the pristine PPy NPs due to the formation of of the PPy structure were attributed as follows: the 284.3 eV vertically aligned FeOOH nanoneedles on the surface, leading peak to C=C bonds, the 285.3 eV peak to CC bonds, the to a specific surface area of 227 m2 g1, as shown in Figure 6 b. 286.6 eV peak to CO bonds, and the 284.9 eV peak to CN VDP of pyrrole on the FeOOH surface further increased the bonds. The C 1s peaks for the PFePNPs were not shifted from specific surface area to 249 m2 g1 owing to the appearance of those of the pristine PPy NPs, confirming that the structure of pores due to the PPy layer on the FeOOH surface, as shown in the PPy NPs remained unchanged after the formation of the Figure 6 c. The pore size distribution of the hybrid PPy NPs exhibited a peak at 12 nm, which was attributed to the PPy structure. However, another sharp peak with a pore diameter of 3 nm was observed for the coating on the FeOOH surface. The carbonization process was another factor that increased the specific surface area of the hybrid particles by inducing the formation of a mesoporous layer through phase transition of the components, including 8 nm-diameter pores for the amorphous carbon particles, 2 nm-diameter pores for the coated carbon layer, and 5 nm-diameter pores for the porous Fe3O4 nanoneedles, as shown in Figure 6 d. As a result, the BET specific surface area of the FeCHNPs was 304 m2 g1, which is circa six times larger than that of the pristine PPy particles. Figure 5. XPS patterns of (a) fully scanned spectra and high resolution of (b) C 1s, (c) Fe 2p, and (d) O 1s of PPy, PFePNPs, and FeCHNPs.
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0.6 V to + 0.3 V using a Na2SO3 aqueous electrolyte. Figure 7 a shows the CV curves of FeCHNPs as well as those of the hybrid PPy NPs following each of the intermediate steps. The area under the CV curves increased following each of the fabrication steps, and the specific capacitance of the FeCHNPs was 455 F g1. The specific capacitance of the other NPs was 105 F g1 for pristine PPy, 265 F g1 for FePNPs, and 370 F g1 for PFePNPs. Additionally, the specific capacitance of the FeCHNPs was circa three times larger than that of CNTs (150 F g1) and rGO (164 F g1), which are well-known carbon nanomaterials (Figure S6). Figure 7 b shows the specific capacFigure 6. Nitrogen adsorption–desorption isotherm and pore size distribution curves (inset) of various hybrid itance as a function of the scan nanoparticles: (a) PPy, (b) FePNPs, (c) PFePNPs, and (d) FeCHNPs. rate. As the scan rate is increased from 5 to 50 mV s1, retention of the capacitance of the Electrochemical capacitance of hybrid carbon nanoparticles FeCHNPs was greater than 85 %, larger than that of other particles (67 % for pristine PPy, 70 % for FePNPs, 75 % for PFePNPs). To assess the potential of the hybrid carbon nanoparticles as This result shows that the FeCHNPs exhibits good rate capabilielectrode materials for ECs, the electrochemical properties ty, which is important for achieving a high power density. were investigated using a three-electrode cell. First, cyclic volGalvanostatic charge–discharge measurements were carried tammetry (CV) curves of the hybrid nanoparticle surfaces were out on the hybrid NP materials to obtain more detailed informeasured at a scan rate of 10 mV s1 in the voltage range
Figure 7. (a) CV curves (scan rate: 10 mV s1), (b) calculated specific capacitance with various voltage scan rates, (c) galvanostatic charge–discharge curves (current density: 2.0 A g1), and (d) IR drop at various current densities of hybrid nanoparticles. Cycling performance of (e) hybrid carbon nanoparticles and (f) hybrid PPy nanoparticles.
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CHEMSUSCHEM FULL PAPERS mation related to the specific capacitance, as shown in Figure 7 c. The applied potential was in the range 0.6 to + 0.3 V, and the current was fixed at 2.0 A g1 to be able to compare performances between the individual samples. All the curves displayed reversible charge–discharge behavior. The discharging time of the hybrid NPs increased with the number of fabrication steps: the discharging time was proportional to the specific capacitance. These times were as follows: 56 s for the pristine PPy; 147 s for FePNPs; 206 s for PFePNPs; and 252 s for FeCHNPs. Additionally, an IR drop was observed at the beginning of the discharging curve, which is associated with the internal resistance of electrode. The IR drop can be used as a direct measurement of the equivalent series resistance (ESR), which affects the performance of the capacitor. The IR drop of the NPs decreased following PPy/carbon coating on the FeOOH/Fe3O4 surfaces and increased with the current density, as shown in Figure 7 d. The coated layer served as a conductive channel on the hybrid NPs as well as a capacitive material. Excellent cycle stability is required for electrodes in practical implementation of ECs. Figure 7 e and f shows the cycle stability of the hybrid carbon and PPy NPs as a function of the cycle number at a current of 1.0 A g1 over a voltage range 0.6 to + 0.3 V. The FeCHNPs displayed a 9 % loss of capacitance following 200 cycles and retained an excellent cycle life, with 91 % capacitance retention over 3000 cycles. The FeCNPs, without a carbon coating, exhibited a 20 % drop in capacitance following 200 cycles; this became more serious after 400 cycles. The morphology of hybrid carbon nanoparticles with and without carbon coating after 3000 charge–discharge cycles is shown in Figure S7: the carbon layer on the Fe3O4 surface appeared to protect the Fe3O4 needles during charge–discharge cycling. This coating effect on the metal oxide was also displayed by the hybrid PPy NPs. Moreover, the carbon-coated particles were more stable than the PPy-coated particles because the carbon layer caused the devices to act as an EDLC rather than a pseudo-Faradaic capacitor. Electrochemical impedance spectroscopy (EIS) measurements were performed to further investigate the electrochemical and structural characteristics of the electrode material, as shown in Figure 8. All the Nyquist plots exhibited a semicircle at high frequencies and were nearly linear at low frequencies. The intercept of the real part at the beginning of the semicircle yields the equivalent series resistance (ESR), which is the combined series resistance of the electrolyte, electrode, current collectors, and electron/current collector contact resistance. The resistance of the electrolyte and current collectors was the same for all samples; therefore, the resistance of the electrodes was different between the samples. The intercept of the hybrid nanoparticle electrode was in the range 1.5–5.2 W and was 1.5 W for FeCHNPs, 2.0 W for PFePNPs, 3.1 W for FePNPs, and 5.2 W for the pristine PPy electrodes. The hybrid NPs exhibited a slight gradient at the start of the linear part of the Nyquist plots, which was attributed to pseudocapacitance. The diameter of the semicircle is related to the charge transfer resistance at the interface between the electrode and electrolyte (Rct). This diameter for the hybrid NPs exhibited behavior similar to that of the specific capacitance with regard to the processing 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 8. Electrochemical impedance spectra of hybrid nanoparticles in the frequency range of 100 kHz–10 mHz.
steps, indicating that the metal-oxide nanoneedles and coated conductive layers facilitated interfacial charge transfer between the electrolyte and electrode. In particular, in the FeCHNPs, the Fe3O4/carbon interfacial contact affected the Faradaic reactions at the electrode. Together with the conductive effect of the surface coating with carbon on the Fe3O4, the carbon layers facilitated charge transfer at the electrolyte–electrode interface. The electrode–electrolyte resistance of the hybrid NPs was as follows: Rct = 8.0 W for PPy, Rct = 5.1 W for FePNPs, Rct = 2.1 W for PFePNPs, and Rct = 1.5 W for the FeCHNPs.
Conclusions We fabricated Fe3O4/carbon hybrid nanoparticles (FeCHNPs) using dual-nozzle electrospraying, vapor deposition polymerization (VDP), and carbonization. To the best of our knowledge, this is the first demonstration of the fabrication of organic–inorganic composite nanoparticles based on a dual-nozzle electrospray process. Using these procedures, pristine polypyrrole (PPy) NPs were used as starting material and their morphology was transformed through the following steps. First, FeOOH nanoneedles were vertically aligned on the PPy surface using electrospraying. Then, a PPy layer was coated on the FeOOH nanoneedles using VDP, resulting in PFePNPs with enlarged surface areas. Finally, FeCHNPs with a mesoporous structure were formed through carbonization, resulting in a large specific surface area. The resulting carbon NPs were used as electrodes to prepare electrochemical capacitors (ECs) with enhanced specific capacitances and cyclability. The coated amorphous carbon layer was critical in improving the properties of the ECs because it prevented the collapse of the Fe3O4 needles during charge–discharge cycles, and non-Faradaic electrical double-layer capacitors (EDLCs) were realized. Thus, this study demonstrated an effective method to fabricate organic–inorganic composite hybrid nanomaterials for energy storage applications.
Experimental Section Materials: Poly(vinyl alcohol) (PVA) with a molecular weight of Mw = 9000, FeCl3 (97 %), and CuCl2 (97 %) were purchased from Aldrich Chemical Co. and used without further purification. Pyrrole ChemSusChem 2014, 7, 1676 – 1683
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CHEMSUSCHEM FULL PAPERS (98 %) and NaOH were also obtained from Aldrich Chemical Co. and used as received. Fabrication of hybrid carbon nanoparticles: Uniform 60 nm-diameter PPy nanoparticles were prepared with PVA, FeCl3, and pyrrole in distilled water as described in Ref. [41]. The PPy particles were mixed with FeCl3 aqueous solution (10 wt %) and then stirred for 4 h at room temperature. The solution was loaded in a syringe pump (KD Scientific, USA) and pumped through the outer part of a dual nozzle (20 G nozzle with an inner diameter of 0.5 mm) while compressed air flowed through the inner nozzle (27 G nozzle with an inner diameter of 0.1 mm). A voltage of 15 kV was applied between the metal nozzle and a Petri dish containing the NaOH aqueous solution, maintaining a stable Taylor cone. The flow rate of the syringe pump was fixed a 0.1 mL h, and the distance between the nozzle and Petri dish was 15 cm. The materials obtained from the electrospray were dispersed in a FeCl3 aqueous solution (10 wt %) while stirring at 70 8C for 4 h. The resulting solution was cleaned with ethanol several times and dried at 60 8C for 12 h. The hybrid particles obtained were soaked in a CuCl2 (5 wt %)/ethanol dispersion and then exposed to the vaporized pyrrole for 5 min at room temperature in a vacuum chamber to induce polymerization on the surface. The pyrrole-coated hybrid particles were then carbonized at 400 8C for 1 h in an argon while heating at a rate of 5 8C min. Characterization of hybrid carbon nanoparticles: A JEOL 6700 was used to obtain field-emission scanning electron microscopy (FESEM) images. Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM-200CX and a JEOL JEM-3010, respectively. During sample preparation, nanomaterials diluted in ethanol were cast onto a copper grid. X-ray diffraction (XRD) and FTIR spectra were recorded using an M18XHF SRA (MAC Science Co.) and a Bomem MB 100 spectroscope (Quebec, Canada) in the absorption mode, respectively. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS-His (KRATOS). The BET surface area and BJH pore size distributions were measured by using an ASAP2010 (Micrometrics). Electron energy loss spectroscopy (EELS) mapping of the hybrid nanoparticles was performed with a JEOL JEM 2100F. Electrochemical measurements of hydrid carbon nanoparticles: The electrodes of the electrochemical capacitor were prepared by mixing the as-prepared hybrid particles (2 mg) and poly(vinylidene fluoride) (PVDF; 0.1 mg) binder with of N-methyl-2-pyrrolidone (NMP; 0.1 mg) to form a homogeneous paste. The homogeneous paste mixture was spread on a 1 1 cm2 stainless-steel mesh (SUS 304) and dried in an oven at 60 8C. The electrochemical properties of the hybrid particles were measured in a three-electrode cell with a platinum wire (CH Instruments, Inc.) as the counter electrode, Ag/AgCl as the reference electrode (0.01 m Ag + , + 0.682 V vs. a normal hydrogen electrode), and the hybrid particle paste as the working electrode in a Na2SO3 aqueous electrolyte (1 m). Cyclic voltammetry (CV) measurements were performed in the range from 0.6 to 0.3 V vs. Ag/Ag + at scan rate in the range 5– 50 mV s1. Galvanostatic charge–discharge experiments were also performed from 0.6 to 0.3 V vs. Ag/Ag + at currents in the range 1.0–5.0 A g1. The cycling performance was evaluated using galvanostatic charge-discharge testing at a current of 2.0 A g1 for 3000 cycles. Nyquist plots were obtained using 100 cycled electrode cells in the range of 100 kHz–10 mHz using a Zahner Electrik IM6 analyzer, where data were obtained from the plots using the regression package ZMAN 2.3. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org Keywords: carbon · capacitors · electrospray · nanoparticles · polypyrrole [1] N.-S. Choi, Z. Chen, S. A. Freunberger, X. Ji, Y.-K. Sun, K. Amine, G. Yushin, L. F. Nazar, J. Cho, P. G. Bruce, Angew. Chem. 2012, 124, 10134 – 10166; Angew. Chem. Int. Ed. 2012, 51, 9994 – 10024. [2] Z. Wang, D. Luan, S. Madhavi, Y. Hu, X. W. Lou, Energy Environ. Sci. 2012, 5, 5252 – 5256. [3] Z. Xiao, Y. Xia, Z. Ren, Z. Liu, G. Xu, C. Chao, X. Li, G. Shen, G. Han, J. Mater. Chem. 2012, 22, 20566 – 20573. [4] J. E. Lee, S.-H. Yu, D. J. Lee, D.-C. Lee, S. I. Han, Y.-E. Sung, T. Hyeon, Energy Environ. Sci. 2012, 5, 9528 – 9533. [5] E. Lee, C. Kim, J. Jang, Chem. Eur. J. 2013, 19, 10280 – 10286. [6] M. D. Stoller, R. S. Ruoff, Energy Environ. Sci. 2010, 3, 1294 – 1301. [7] Y. Gogotsi, P. Simon, Science 2011, 334, 917 – 918. [8] X. Zhao, L. Zhang, S. Murali, M. D. Stoller, Q. Zhang, Y. Zhu, R. S. Rouff, ACS Nano 2012, 6, 5404 – 5412. [9] M. B. Sassin, A. N. Mansour, K. A. Pettigrew, D. R. Rolison, J. W. Long, ACS Nano 2010, 4, 4505 – 4514. [10] L. Yuan, X. Xiao, T. Ding, J. Zhong, X. Zhang, Y. Shen, B. Hu, Y. Huang, J. Zhou, Z. L. Wang, Angew. Chem. 2012, 124, 5018 – 5022; Angew. Chem. Int. Ed. 2012, 51, 4934 – 4938. [11] H. Wang, H. S. Casalongue, Y. Liang, H. Dai, J. Am. Chem. Soc. 2010, 132, 7472 – 7477. [12] L. Zhang, F. Zhang, X. Yang, K. Leng, Y. Huang, Y. Chen, Small 2013, 9, 1342 – 1347. [13] C. Liu, F. Li, L.-P. Ma, H.-M. Cheng, Adv. Mater. 2010, 22, E28 – E62. [14] S. Cho, K.-H. Shin, J. Jang, ACS Appl. Mater. Interfaces 2013, 5, 9186 – 9193. [15] B. Liu, D. Tan, X. Wang, D. Chen, G. Shen, Small 2013, 9, 1998 – 2004. [16] M. Zhi, C. Xiang, J. Li, M. Li, N. Wu, Nanoscale 2013, 5, 72 – 88. [17] C. Wu, X. Wang, B. Ju, L. Jiang, H. Wu, Q. Zhao, L. Yi, J. Power Sources 2013, 227, 1 – 7. [18] M. Sathiya, A. S. Prakash, K. Ramesha, J- M. Tarascon, A. K. Shukla, J. Am. Chem. Soc. 2011, 133, 16291 – 16299. [19] A. Pendashteh, M. F. Mousavi, M. S. Rahmanifar, Electrochim. Acta 2013, 88, 347 – 357. [20] R. Liu, S. B. Lee, J. Am. Chem. Soc. 2008, 130, 2942 – 2943. [21] L. Dai, D. W. Chang, J.-B. Baek, W. Lu, Small 2012, 8, 1130 – 1166. [22] G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu, J. Lian, Small 2012, 8, 452 – 459. [23] W.-W. Liu, Y.-Q. Feng, X.-B. Yan, J.-T. Chen, Q.-J. Xue, Adv. Funct. Mater. 2013, 23, 4111 – 4122. [24] W. Yao, H. Zhou, Y. Lu, J. Power Sources 2013, 241, 359 – 366. [25] D. Sarkar, G. G. Khan, A. K. Singh, K. Mandal, J. Phys. Chem. C 2013, 117, 15523 – 15531. [26] M. Kim, S. Cho, J. Song, S. Son, J. Jang, ACS Appl. Mater. Interfaces 2012, 4, 4603 – 4609. [27] O. S. Kwon, T. Kim, J. S. Lee, S. J. Park, H.-W. Park, M. Kang, J. E. Lee, J. Jang, H. Yoon, Small 2013, 9, 248 – 254. [28] D. Mhamane, A. Suryawanshi, S. M. Unni, C. Rode, S. Kurungot, S. Ogale, Small 2013, 9, 2801 – 2809. [29] Y. Shim, H. J. Kim, ACS Nano 2010, 4, 2345 – 2355. [30] O. N. Kalugin, V. V. Chaban, V. V. Loskutov, O. V. Prezhdo, Nano Lett. 2008, 8, 2126 – 2130. [31] Y. Huang, J. Liang, Y. Chen, Small 2012, 8, 1805 – 1834. [32] A. L. Reddy, M. M. Shaijumon, S. R. Gowda, P. M. Ajayan, Nano Lett. 2009, 9, 1002 – 1006. [33] Y. Li, K. sheng, W. Yuan, G. Shi, Chem. Commun. 2013, 49, 291 – 293. [34] Z. Feng, R. Xue, X. Shao, Electrochim. Acta 2010, 55, 7334 – 7340. [35] B. Choi, H. Yoon, I.-S. Park, J. Jang, Y.-E. Sung, Carbon 2007, 45, 2496 – 2501. [36] J. Xu, Q. Wang, X. Wang, Q. Xiang, B. Liang, D. Chen, G. Shen, ACS Nano 2013, 7, 5453 – 5462. [37] M. Liu, J. Chang, H. Sun, L. Gao, RSC Adv. 2013, 3, 8003 – 8008. [38] S. Vijayakumar, A. K. Ponnalagi, S. Nagamuthu, G. Muralidharan, Electrochim. Acta 2013, 106, 500 – 505. [39] N.-L. Wu, S.-Y. Wang, C.-Y. Han, D.-S. Wu, L.-R. Shiue, J. Power Sources 2003, 113, 173 – 178.
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CHEMSUSCHEM FULL PAPERS [40] Y.-H. Kim, S.-J. Park, Curr. Appl. Phys. 2011, 11, 462 – 466. [41] J.-Y. Hong, H. Yoon, J. Jang, Small 2010, 6, 679 – 686. [42] B. Tang, G. Wang, L. Zhuo, J. Ge, L. Cui, Inorg. Chem. 2006, 45, 5196 – 5200. [43] H.-M. Xiao, W.-D. Zhang, S.-Y. Fu, Compos. Sci. Technol. 2010, 70, 909 – 915.
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www.chemsuschem.org [44] J. Wang, L. Li, C. L. Wong, L. Sun, Z. Shen, S. Madhavi, RSC Adv. 2013, 3, 15316 – 15326. Received: November 6, 2013 Revised: March 3, 2014 Published online on April 6, 2014
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Fe3O4/Carbon Hybrid Nanoparticle Electrodes for HighCapacity Electrochemical Capacitors Jun Seop Lee, Dong Hoon Shin, Jaemoon Jun, Choonghyeon Lee, and Jyongsik Jang*[a] Fe3O4/carbon hybrid nanoparticles (FeCHNPs) were fabricated using dual-nozzle electrospraying, vapor deposition polymerization (VDP), and carbonization. FeOOH nanoneedles decorated with polypyrrole (PPy) nanoparticles (FePNPs) were fabricated by electrospraying pristine PPy mixed with FeCl3 solution, followed by heating stirring reaction. A PPy coating was then formed on the FeOOH nanoneedles through a VDP process. FeCHNPs were produced through carbonization of PPy and FeOOH phase transitions. These hybrid carbon nanoparticles (NPs) were used to build electrodes of electrochemical capaci-
tors. The specific capacitance of the FeCHNPs was 455 F g1, which is larger than that of pristine PPy NPs (105 F g1) or other hybrid PPy NPs. Furthermore, the FeCHNP-based capacitors exhibited better cycle stability during charge–discharge cycling than other hybrid NP capacitors. This is because the carbon layer on the Fe3O4 surface formed a protective coating, preventing damage to the electrode materials during the charge–discharge processes. This fabrication technique is an effective approach for forming stable carbon/metal oxide nanostructures for energy storage applications.
Introduction There has been growing interest in energy storage systems for applications including industrial power backup, hybrid electric vehicles, and personal electronic devices.[1–5] Much attention has been focused on electrochemical capacitors (ECs), also called supercapacitors, to satisfy these demands.[6–10] ECs are promising systems that can store energy through a simple mechanism and that have a potential for high power densities, short charging times, long cycle lifes, and favorable safety considerations.[11–15] Depending on the electrode materials and charge storage mechanism, ECs can be classified into electrical double-layer capacitors (EDLCs) or Faradaic redox reaction pseudocapacitors.[16–20] EDLCs are typically based on carbon materials; the energy storage mechanism arises from the accumulation of electronic and ionic charges at the interface between electrode materials and electrolyte.[21–23] Pseudocapacitors that use metal-oxide and conducting-polymer electrodes exhibit reversible Faradaic reactions of electroactive species or surface functional groups of the electrode to store energy.[24–26] In general, pseudocapacitors have a higher energy density than EDLCs; however, EDLCs are more stable with better cyclability. The fabrication of electrodes that provide a high energy density and maintain good stability is an important aspect of the development of ECs. Carbon-based nanomaterials, including carbon nanotubes (CNTs), carbon nanofibers (CNFs), fullerenes, and graphene, [a] J. S. Lee, D. H. Shin, J. Jun, C. Lee, Prof. Dr. J. Jang School of Chemical and Biological Engineering College of Engineering, Seoul National University 599 Gwanangno, Gwanakgu, Seoul, 151-742 (Korea) Fax: (+ 82) 2-888-1604 E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301188.
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have led to the development of devices such as EDLCs because of favorable cyclic stability and long service lifetime of the electrodes utilizing non-Faradaic processes.[27–30] CNTs and graphene have been investigated in an attempt to exploit the electronic, chemical, and structural properties of these nanoscale materials.[31–33] However, difficulties in obtaining singlespecies CNTs and uniform graphene layers that are free from defects have hindered their practical application. Carbon particles, however, are inexpensive and can be produced with good uniformity using large-scale processes.[34, 35] Despite these advantages, relatively little research has been conducted on the use of carbon nanoparticles as electrode materials for ECs due to difficulties in fabricating uniform small-diameter particles. Various transition metal oxides, including RuO2, NiO, and CoOx, have been studied for charge storage applications in pseudocapacitors.[36–38] However, the use of these materials has not been widespread, largely due to high costs and environmental concerns. Magnetite (Fe3O4) has emerged as a promising EC material due to its low cost and lack of pollution issues.[39, 40] However, there are two principle disadvantages of Fe3O4 for EC electrode applications. First, a large volume change occurs in the electrode matrix of Fe3O4 during charge– discharge cycling, which leads to pulverization of the electrodes. Second, the high resistivity of the metal oxide leads to a large internal resistance of devices. Herein, we report the fabrication of Fe3O4/carbon hybrid nanoparticles (FeCHNPs), which are carbon-coated porous Fe3O4 nanoneedles vertically aligned on the carbon nanoparticle surface fabricated using dual-nozzle electrospraying, vapor deposition polymerization (VDP), and carbonization. First, vertically aligned FeOOH nanoneedles were formed on the PPy surface using dual-nozzle electrospraying, followed by heat stirring reaction. Then, PPy-coated FeOOH nanoneedles (PFePNPs) ChemSusChem 2014, 7, 1676 – 1683
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CHEMSUSCHEM FULL PAPERS were fabricated using VDP to form a precursor to the Fe3O4/ carbon hybrid nanoparticles (FeCHNPs), which were formed through carbonization of PPy and the phase transition of PPy/ FeOOH to carbon/Fe3O4. The FeCHNPs were used as electrode materials for ECs, which exhibited a specific capacitance of 455 F g1, which is higher than that of pristine PPy NPs (105 F g1) or other hybrid PPy-based NPs. ECs based on FeCHNPs also exhibited better cycle stability than ECs prepared using other hybrid NP electrodes. The carbon coating inhibited degradation of the Fe3O4 structure and enhanced the specific capacitance due to its ability to form an EDL itself. To the best of our knowledge, there have been no previous reports of the production of carbon–metal oxide hybrid composites based on a dual-nozzle electrospraying process.
www.chemsuschem.org Fe3þ þ 3 OH ! FeOOH þ H2 O
ð1Þ
The electrosprayed PPy NPs were stirred in the FeCl3 aqueous solution at 70 8C to induce growth of FeOOH nanoneedles on the PPy surface. This resulted in the formation of FeOOH nanoneedles decorated with PPy nanoparticles (FePNPs), as shown in Figure 2 a. The high-resolution transmission electron microscopy (HRTEM) images indicate that the interplanar spacing between the needles was 0.29 and 0.26 nm in the (100)
Results and Discussion Fabrication of hybrid carbon nanoparticles Figure 1 shows an overview of the fabrication process for the FeCHNPs, which is based on the dual-nozzle electrospraying method and heat treatment. 60 nm-diameter PPy NPs were prepared uniformly and without aggregation using a monodis-
Figure 1. Illustrative diagram of the sequential fabrication steps for Fe3O4/ carbon hybrid nanoparticles.
perse method, as shown in Figure S1 in the Supporting Information.[41] The PPy NPs were stirred in an aqueous FeCl3 solution at room temperature to induce covalent bonding between the Fe3 + ions and the partial negative charge of the nitrogen atoms in the pyrrole structure. The mixed PPy solutions were electrosprayed from the outer part of the dual nozzle while compressed air flowed through the inner nozzle, as shown in Figure S2. During the electrospraying process, FeOOH particles were formed on the PPy surface. The high positive voltage applied allowed uniform dispersion of Fe3 + ions on the PPy surface, leading to the following reaction between the Fe3 + and hydroxide ions in the collector:[42, 43] 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 2. TEM and HRTEM images of (a, b) FeOOH nanoneedle-decorated PPy nanoparticles (FePNPs) and (c, d) PPy-coated-FeOOH nanoneedle-decorated PPy nanoparticles (PFePNPs). EELS dot mapping of PFePNP nanoneedle components of (e) iron, (f) oxygen, and (g) carbon atoms (scale bar: 10 nm).
and (021) planes, respectively, corresponding to an a-FeOOH lattice structure (see Figure 2 b). The FePNPs were dipped into a CuCl2 ethanol solution, leading to the adsorption of Cu2 + ions onto the surface of the FeOOH nanoneedles. This occurred because of the interaction between the Cu2 + ions and the partial negative charge of the oxygen atoms in the FeOOH structure, as shown in Figure S3. Following the adsorption of the Cu2 + ions onto the FeOOH needles, vaporized pyrrole monomers in contact with the Cu2 + ions were polymerized through chemical oxidation. As a result, PPy-coated FeOOH nanoneedles decorated with PPy nanoparticles (PFePNPs) were fabricated that had a uniform thickness of approximately 3 nm of PPy on the needle surface, as shown in Figure 2 c and d. No ChemSusChem 2014, 7, 1676 – 1683
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CHEMSUSCHEM FULL PAPERS aggregation of the coated particles was observed following polymerization, which was attributed to the dispersion of Cu2 + ions over the nanoneedle surface during adsorption. Additionally, electron energy loss spectroscopy (EELS) mapping analysis of iron, oxygen, and carbon atoms conducted to determine the atomic distribution of PFePNP nanoneedles (Figure 2 e–g). The iron and oxygen atoms were dispersed within the needle structure; on the other hand, the carbon atoms were dispersed on the surface of the needle structure, which confirmed that the PPy was coated onto the FeOOH needle surface through the VDP step. The as-prepared PFePNPs were then carbonized in an argon atmosphere to obtain the FeCHNPs. Figure 3 a and b show the Fe3O4 nanoneedle-decorated carbon nanoparticles (FeCNPs) formed from the FePNPs. The carbonization process was carried out at 400 8C; the Fe3O4 needle structures collapsed at higher temperatures (see Figure S4). HRTEM images of the needles indicate an interplanar spacing of 0.29 nm for the (220) plane of Fe3O4, which confirms that the FeOOH transformed into crystalline Fe3O4 following carbonization. In addition, small mesoporous structures (5 nm in diameter) were observed in the Fe3O4 nanoneedle structures owing to the formation of vapors during the heat treatment during the phase transition of FeOOH, as shown in Figure 3 c.[44] The PFePNPs were also changed from PPy/FeOOH to carbon/Fe3O4 during the carbonization step; thus, FeCHNPs were prepared. Figure 3 d and e show TEM and HRTEM images without aggregation of the particles and uniformly coated carbon layer on the Fe3O4 surface. The HRTEM image shows 2 nm-thick Fe3O4-coated carbon layers on the surface, with a mesoporous structure with no collapsed needles. In addition, the thickness of the coated layer decreased slightly as the PPy layer was converted into amorphous carbon during carbonization. EELS mapping of FeCHNPs was conducted to confirm carbon coating of the Fe3O4 needles (Figure 3 g–i). The iron and oxygen atoms are present inside the needles, whereas the carbon atoms are present on the surface of the needles. Therefore, carbon layer coats the needles on the surface, as the protecting layer.
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Characterization of hybrid carbon nanoparticles
Figure 3. TEM and HRTEM images of (a–c) Fe3O4 nanoneedle-decorated amorphous carbon nanoparticles (FeCNPs) and (d–f) Fe3O4/carbon hybrid nanoparticles (FeCHNPs). EELS dot mapping of FeCHNP needle components of (g) iron, (h) oxygen, and (i) carbon atoms (scale bar: 10 nm).
The crystal structure of the hybrid NPs was determined using powder X-ray diffraction (XRD) analysis, as shown in Figure 4 a. The broad peak at 2q = 248 corresponding to the pristine PPy NPs was consistent with scattering by the PPy chains at the interplanar spacing. The spectrum of the FePNPs matched the standard spectrum of a-FeOOH (JCPDS 29-0713), and the broad peak was characteristic of the PPy structure.[44] These peaks were also observed in the PFePNP samples, indicating that the structure of the FeOOH was maintained through the VDP process (see Figure S5). The spectrum of FeCHNPs showed a broad peak at 2q = 238, which indicates an amorphous carbon phase and sharp peaks of Fe3O4 crystal structures (JCPDS 19-0629).[44] However, the broad peak of the amorphous carbon was similar to that for PPy. Thus, the IG/ID ratio from Raman spectra was used to further characterize the samples, as shown in Figure 4 b. This ratio was higher for the
amorphous carbon NPs (~ 1.3) than for PPy NPs (~ 0.8), and the peaks for carbon were sharper than those for PPy. These findings confirm that the PPy NPs were successfully converted into amorphous carbon through the carbonization process. The chemical composition of the hybrid materials was characterized using X-ray photoelectron spectroscopy (XPS). Figure 5 a shows spectra over the range of 0–1200 eV, which reveal that carbon, nitrogen, oxygen, and iron atoms were present in the PFePNPs and FeCHNPs, whereas only carbon, oxygen, and nitrogen atoms were present in the pristine PPy NPs. The N 1s peak was attributed to nitrogen atoms from the pyrrole component. High-resolution XPS spectra for the C 1s region around 285 eV are shown in Figure 5 b; this peak was deconvoluted into four components for PPy-based particles
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FeOOH nanoneedles. However, the hybrid carbon-based particles displayed a large peak at 284.6 eV, which was attributed to graphitic sp2 hybridization, and two additional peaks at 285.5 and 287.9 eV, which were assigned to CO and CN, respectively. Figure 5 c shows the high-resolution XPS spectra for the Fe 2p peak. Spin-orbit components 2p3/2 and 2p1/2 were observed near 710.2 and 722.8 eV, indicating that the valence state of Fe was + 3. O 1s spectra of PFePNPs and FeCHNPs contained several components, as shown in Figure 5 d. For PFePNPs, the peak at 533.0 eV corresponded to CO bonds from the PPy structure component. The peaks at 531.7 and 529.6 eV were attributed to OH and O2 in the FeOOH structure. Otherwise, there were different peaks for FeCHNPs in comparison to PFePNPs. The peak at 533.0 eV was attributed to CO bonds from the carbon structure, and the peak at 529.6 eV to O2 in the Fe3O4 structure. Therefore, we can conclude that the FeOOH and Fe3O4 nanoneedles were composed of iron(III) and oxygen as confirmed by XRD results. Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) measurements were performed to measure the specific surface area and pore size distribution of the hybrid PPy and carbon NPs. The BET specific surface area of the pristine PPy Figure 4. (a) XRD patterns and (b) Raman spectra for the hybrid PPy and NPs was 50 m2 g1 and increased following the electrospraying, carbon nanoparticles. VPD, and carbonization steps, as shown in Figure 6 a. Electrospraying and stirring at elevated temperatures increased the and three components for carbon-based particles. The peaks surface area of the pristine PPy NPs due to the formation of of the PPy structure were attributed as follows: the 284.3 eV vertically aligned FeOOH nanoneedles on the surface, leading peak to C=C bonds, the 285.3 eV peak to CC bonds, the to a specific surface area of 227 m2 g1, as shown in Figure 6 b. 286.6 eV peak to CO bonds, and the 284.9 eV peak to CN VDP of pyrrole on the FeOOH surface further increased the bonds. The C 1s peaks for the PFePNPs were not shifted from specific surface area to 249 m2 g1 owing to the appearance of those of the pristine PPy NPs, confirming that the structure of pores due to the PPy layer on the FeOOH surface, as shown in the PPy NPs remained unchanged after the formation of the Figure 6 c. The pore size distribution of the hybrid PPy NPs exhibited a peak at 12 nm, which was attributed to the PPy structure. However, another sharp peak with a pore diameter of 3 nm was observed for the coating on the FeOOH surface. The carbonization process was another factor that increased the specific surface area of the hybrid particles by inducing the formation of a mesoporous layer through phase transition of the components, including 8 nm-diameter pores for the amorphous carbon particles, 2 nm-diameter pores for the coated carbon layer, and 5 nm-diameter pores for the porous Fe3O4 nanoneedles, as shown in Figure 6 d. As a result, the BET specific surface area of the FeCHNPs was 304 m2 g1, which is circa six times larger than that of the pristine PPy particles. Figure 5. XPS patterns of (a) fully scanned spectra and high resolution of (b) C 1s, (c) Fe 2p, and (d) O 1s of PPy, PFePNPs, and FeCHNPs.
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0.6 V to + 0.3 V using a Na2SO3 aqueous electrolyte. Figure 7 a shows the CV curves of FeCHNPs as well as those of the hybrid PPy NPs following each of the intermediate steps. The area under the CV curves increased following each of the fabrication steps, and the specific capacitance of the FeCHNPs was 455 F g1. The specific capacitance of the other NPs was 105 F g1 for pristine PPy, 265 F g1 for FePNPs, and 370 F g1 for PFePNPs. Additionally, the specific capacitance of the FeCHNPs was circa three times larger than that of CNTs (150 F g1) and rGO (164 F g1), which are well-known carbon nanomaterials (Figure S6). Figure 7 b shows the specific capacFigure 6. Nitrogen adsorption–desorption isotherm and pore size distribution curves (inset) of various hybrid itance as a function of the scan nanoparticles: (a) PPy, (b) FePNPs, (c) PFePNPs, and (d) FeCHNPs. rate. As the scan rate is increased from 5 to 50 mV s1, retention of the capacitance of the Electrochemical capacitance of hybrid carbon nanoparticles FeCHNPs was greater than 85 %, larger than that of other particles (67 % for pristine PPy, 70 % for FePNPs, 75 % for PFePNPs). To assess the potential of the hybrid carbon nanoparticles as This result shows that the FeCHNPs exhibits good rate capabilielectrode materials for ECs, the electrochemical properties ty, which is important for achieving a high power density. were investigated using a three-electrode cell. First, cyclic volGalvanostatic charge–discharge measurements were carried tammetry (CV) curves of the hybrid nanoparticle surfaces were out on the hybrid NP materials to obtain more detailed informeasured at a scan rate of 10 mV s1 in the voltage range
Figure 7. (a) CV curves (scan rate: 10 mV s1), (b) calculated specific capacitance with various voltage scan rates, (c) galvanostatic charge–discharge curves (current density: 2.0 A g1), and (d) IR drop at various current densities of hybrid nanoparticles. Cycling performance of (e) hybrid carbon nanoparticles and (f) hybrid PPy nanoparticles.
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CHEMSUSCHEM FULL PAPERS mation related to the specific capacitance, as shown in Figure 7 c. The applied potential was in the range 0.6 to + 0.3 V, and the current was fixed at 2.0 A g1 to be able to compare performances between the individual samples. All the curves displayed reversible charge–discharge behavior. The discharging time of the hybrid NPs increased with the number of fabrication steps: the discharging time was proportional to the specific capacitance. These times were as follows: 56 s for the pristine PPy; 147 s for FePNPs; 206 s for PFePNPs; and 252 s for FeCHNPs. Additionally, an IR drop was observed at the beginning of the discharging curve, which is associated with the internal resistance of electrode. The IR drop can be used as a direct measurement of the equivalent series resistance (ESR), which affects the performance of the capacitor. The IR drop of the NPs decreased following PPy/carbon coating on the FeOOH/Fe3O4 surfaces and increased with the current density, as shown in Figure 7 d. The coated layer served as a conductive channel on the hybrid NPs as well as a capacitive material. Excellent cycle stability is required for electrodes in practical implementation of ECs. Figure 7 e and f shows the cycle stability of the hybrid carbon and PPy NPs as a function of the cycle number at a current of 1.0 A g1 over a voltage range 0.6 to + 0.3 V. The FeCHNPs displayed a 9 % loss of capacitance following 200 cycles and retained an excellent cycle life, with 91 % capacitance retention over 3000 cycles. The FeCNPs, without a carbon coating, exhibited a 20 % drop in capacitance following 200 cycles; this became more serious after 400 cycles. The morphology of hybrid carbon nanoparticles with and without carbon coating after 3000 charge–discharge cycles is shown in Figure S7: the carbon layer on the Fe3O4 surface appeared to protect the Fe3O4 needles during charge–discharge cycling. This coating effect on the metal oxide was also displayed by the hybrid PPy NPs. Moreover, the carbon-coated particles were more stable than the PPy-coated particles because the carbon layer caused the devices to act as an EDLC rather than a pseudo-Faradaic capacitor. Electrochemical impedance spectroscopy (EIS) measurements were performed to further investigate the electrochemical and structural characteristics of the electrode material, as shown in Figure 8. All the Nyquist plots exhibited a semicircle at high frequencies and were nearly linear at low frequencies. The intercept of the real part at the beginning of the semicircle yields the equivalent series resistance (ESR), which is the combined series resistance of the electrolyte, electrode, current collectors, and electron/current collector contact resistance. The resistance of the electrolyte and current collectors was the same for all samples; therefore, the resistance of the electrodes was different between the samples. The intercept of the hybrid nanoparticle electrode was in the range 1.5–5.2 W and was 1.5 W for FeCHNPs, 2.0 W for PFePNPs, 3.1 W for FePNPs, and 5.2 W for the pristine PPy electrodes. The hybrid NPs exhibited a slight gradient at the start of the linear part of the Nyquist plots, which was attributed to pseudocapacitance. The diameter of the semicircle is related to the charge transfer resistance at the interface between the electrode and electrolyte (Rct). This diameter for the hybrid NPs exhibited behavior similar to that of the specific capacitance with regard to the processing 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 8. Electrochemical impedance spectra of hybrid nanoparticles in the frequency range of 100 kHz–10 mHz.
steps, indicating that the metal-oxide nanoneedles and coated conductive layers facilitated interfacial charge transfer between the electrolyte and electrode. In particular, in the FeCHNPs, the Fe3O4/carbon interfacial contact affected the Faradaic reactions at the electrode. Together with the conductive effect of the surface coating with carbon on the Fe3O4, the carbon layers facilitated charge transfer at the electrolyte–electrode interface. The electrode–electrolyte resistance of the hybrid NPs was as follows: Rct = 8.0 W for PPy, Rct = 5.1 W for FePNPs, Rct = 2.1 W for PFePNPs, and Rct = 1.5 W for the FeCHNPs.
Conclusions We fabricated Fe3O4/carbon hybrid nanoparticles (FeCHNPs) using dual-nozzle electrospraying, vapor deposition polymerization (VDP), and carbonization. To the best of our knowledge, this is the first demonstration of the fabrication of organic–inorganic composite nanoparticles based on a dual-nozzle electrospray process. Using these procedures, pristine polypyrrole (PPy) NPs were used as starting material and their morphology was transformed through the following steps. First, FeOOH nanoneedles were vertically aligned on the PPy surface using electrospraying. Then, a PPy layer was coated on the FeOOH nanoneedles using VDP, resulting in PFePNPs with enlarged surface areas. Finally, FeCHNPs with a mesoporous structure were formed through carbonization, resulting in a large specific surface area. The resulting carbon NPs were used as electrodes to prepare electrochemical capacitors (ECs) with enhanced specific capacitances and cyclability. The coated amorphous carbon layer was critical in improving the properties of the ECs because it prevented the collapse of the Fe3O4 needles during charge–discharge cycles, and non-Faradaic electrical double-layer capacitors (EDLCs) were realized. Thus, this study demonstrated an effective method to fabricate organic–inorganic composite hybrid nanomaterials for energy storage applications.
Experimental Section Materials: Poly(vinyl alcohol) (PVA) with a molecular weight of Mw = 9000, FeCl3 (97 %), and CuCl2 (97 %) were purchased from Aldrich Chemical Co. and used without further purification. Pyrrole ChemSusChem 2014, 7, 1676 – 1683
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CHEMSUSCHEM FULL PAPERS (98 %) and NaOH were also obtained from Aldrich Chemical Co. and used as received. Fabrication of hybrid carbon nanoparticles: Uniform 60 nm-diameter PPy nanoparticles were prepared with PVA, FeCl3, and pyrrole in distilled water as described in Ref. [41]. The PPy particles were mixed with FeCl3 aqueous solution (10 wt %) and then stirred for 4 h at room temperature. The solution was loaded in a syringe pump (KD Scientific, USA) and pumped through the outer part of a dual nozzle (20 G nozzle with an inner diameter of 0.5 mm) while compressed air flowed through the inner nozzle (27 G nozzle with an inner diameter of 0.1 mm). A voltage of 15 kV was applied between the metal nozzle and a Petri dish containing the NaOH aqueous solution, maintaining a stable Taylor cone. The flow rate of the syringe pump was fixed a 0.1 mL h, and the distance between the nozzle and Petri dish was 15 cm. The materials obtained from the electrospray were dispersed in a FeCl3 aqueous solution (10 wt %) while stirring at 70 8C for 4 h. The resulting solution was cleaned with ethanol several times and dried at 60 8C for 12 h. The hybrid particles obtained were soaked in a CuCl2 (5 wt %)/ethanol dispersion and then exposed to the vaporized pyrrole for 5 min at room temperature in a vacuum chamber to induce polymerization on the surface. The pyrrole-coated hybrid particles were then carbonized at 400 8C for 1 h in an argon while heating at a rate of 5 8C min. Characterization of hybrid carbon nanoparticles: A JEOL 6700 was used to obtain field-emission scanning electron microscopy (FESEM) images. Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM-200CX and a JEOL JEM-3010, respectively. During sample preparation, nanomaterials diluted in ethanol were cast onto a copper grid. X-ray diffraction (XRD) and FTIR spectra were recorded using an M18XHF SRA (MAC Science Co.) and a Bomem MB 100 spectroscope (Quebec, Canada) in the absorption mode, respectively. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS-His (KRATOS). The BET surface area and BJH pore size distributions were measured by using an ASAP2010 (Micrometrics). Electron energy loss spectroscopy (EELS) mapping of the hybrid nanoparticles was performed with a JEOL JEM 2100F. Electrochemical measurements of hydrid carbon nanoparticles: The electrodes of the electrochemical capacitor were prepared by mixing the as-prepared hybrid particles (2 mg) and poly(vinylidene fluoride) (PVDF; 0.1 mg) binder with of N-methyl-2-pyrrolidone (NMP; 0.1 mg) to form a homogeneous paste. The homogeneous paste mixture was spread on a 1 1 cm2 stainless-steel mesh (SUS 304) and dried in an oven at 60 8C. The electrochemical properties of the hybrid particles were measured in a three-electrode cell with a platinum wire (CH Instruments, Inc.) as the counter electrode, Ag/AgCl as the reference electrode (0.01 m Ag + , + 0.682 V vs. a normal hydrogen electrode), and the hybrid particle paste as the working electrode in a Na2SO3 aqueous electrolyte (1 m). Cyclic voltammetry (CV) measurements were performed in the range from 0.6 to 0.3 V vs. Ag/Ag + at scan rate in the range 5– 50 mV s1. Galvanostatic charge–discharge experiments were also performed from 0.6 to 0.3 V vs. Ag/Ag + at currents in the range 1.0–5.0 A g1. The cycling performance was evaluated using galvanostatic charge-discharge testing at a current of 2.0 A g1 for 3000 cycles. Nyquist plots were obtained using 100 cycled electrode cells in the range of 100 kHz–10 mHz using a Zahner Electrik IM6 analyzer, where data were obtained from the plots using the regression package ZMAN 2.3. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org Keywords: carbon · capacitors · electrospray · nanoparticles · polypyrrole [1] N.-S. Choi, Z. Chen, S. A. Freunberger, X. Ji, Y.-K. Sun, K. Amine, G. Yushin, L. F. Nazar, J. Cho, P. G. Bruce, Angew. Chem. 2012, 124, 10134 – 10166; Angew. Chem. Int. Ed. 2012, 51, 9994 – 10024. [2] Z. Wang, D. Luan, S. Madhavi, Y. Hu, X. W. Lou, Energy Environ. Sci. 2012, 5, 5252 – 5256. [3] Z. Xiao, Y. Xia, Z. Ren, Z. Liu, G. Xu, C. Chao, X. Li, G. Shen, G. Han, J. Mater. Chem. 2012, 22, 20566 – 20573. [4] J. E. Lee, S.-H. Yu, D. J. Lee, D.-C. Lee, S. I. Han, Y.-E. Sung, T. Hyeon, Energy Environ. Sci. 2012, 5, 9528 – 9533. [5] E. Lee, C. Kim, J. Jang, Chem. Eur. J. 2013, 19, 10280 – 10286. [6] M. D. Stoller, R. S. Ruoff, Energy Environ. Sci. 2010, 3, 1294 – 1301. [7] Y. Gogotsi, P. Simon, Science 2011, 334, 917 – 918. [8] X. Zhao, L. Zhang, S. Murali, M. D. Stoller, Q. Zhang, Y. Zhu, R. S. Rouff, ACS Nano 2012, 6, 5404 – 5412. [9] M. B. Sassin, A. N. Mansour, K. A. Pettigrew, D. R. Rolison, J. W. Long, ACS Nano 2010, 4, 4505 – 4514. [10] L. Yuan, X. Xiao, T. Ding, J. Zhong, X. Zhang, Y. Shen, B. Hu, Y. Huang, J. Zhou, Z. L. Wang, Angew. Chem. 2012, 124, 5018 – 5022; Angew. Chem. Int. Ed. 2012, 51, 4934 – 4938. [11] H. Wang, H. S. Casalongue, Y. Liang, H. Dai, J. Am. Chem. Soc. 2010, 132, 7472 – 7477. [12] L. Zhang, F. Zhang, X. Yang, K. Leng, Y. Huang, Y. Chen, Small 2013, 9, 1342 – 1347. [13] C. Liu, F. Li, L.-P. Ma, H.-M. Cheng, Adv. Mater. 2010, 22, E28 – E62. [14] S. Cho, K.-H. Shin, J. Jang, ACS Appl. Mater. Interfaces 2013, 5, 9186 – 9193. [15] B. Liu, D. Tan, X. Wang, D. Chen, G. Shen, Small 2013, 9, 1998 – 2004. [16] M. Zhi, C. Xiang, J. Li, M. Li, N. Wu, Nanoscale 2013, 5, 72 – 88. [17] C. Wu, X. Wang, B. Ju, L. Jiang, H. Wu, Q. Zhao, L. Yi, J. Power Sources 2013, 227, 1 – 7. [18] M. Sathiya, A. S. Prakash, K. Ramesha, J- M. Tarascon, A. K. Shukla, J. Am. Chem. Soc. 2011, 133, 16291 – 16299. [19] A. Pendashteh, M. F. Mousavi, M. S. Rahmanifar, Electrochim. Acta 2013, 88, 347 – 357. [20] R. Liu, S. B. Lee, J. Am. Chem. Soc. 2008, 130, 2942 – 2943. [21] L. Dai, D. W. Chang, J.-B. Baek, W. Lu, Small 2012, 8, 1130 – 1166. [22] G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu, J. Lian, Small 2012, 8, 452 – 459. [23] W.-W. Liu, Y.-Q. Feng, X.-B. Yan, J.-T. Chen, Q.-J. Xue, Adv. Funct. Mater. 2013, 23, 4111 – 4122. [24] W. Yao, H. Zhou, Y. Lu, J. Power Sources 2013, 241, 359 – 366. [25] D. Sarkar, G. G. Khan, A. K. Singh, K. Mandal, J. Phys. Chem. C 2013, 117, 15523 – 15531. [26] M. Kim, S. Cho, J. Song, S. Son, J. Jang, ACS Appl. Mater. Interfaces 2012, 4, 4603 – 4609. [27] O. S. Kwon, T. Kim, J. S. Lee, S. J. Park, H.-W. Park, M. Kang, J. E. Lee, J. Jang, H. Yoon, Small 2013, 9, 248 – 254. [28] D. Mhamane, A. Suryawanshi, S. M. Unni, C. Rode, S. Kurungot, S. Ogale, Small 2013, 9, 2801 – 2809. [29] Y. Shim, H. J. Kim, ACS Nano 2010, 4, 2345 – 2355. [30] O. N. Kalugin, V. V. Chaban, V. V. Loskutov, O. V. Prezhdo, Nano Lett. 2008, 8, 2126 – 2130. [31] Y. Huang, J. Liang, Y. Chen, Small 2012, 8, 1805 – 1834. [32] A. L. Reddy, M. M. Shaijumon, S. R. Gowda, P. M. Ajayan, Nano Lett. 2009, 9, 1002 – 1006. [33] Y. Li, K. sheng, W. Yuan, G. Shi, Chem. Commun. 2013, 49, 291 – 293. [34] Z. Feng, R. Xue, X. Shao, Electrochim. Acta 2010, 55, 7334 – 7340. [35] B. Choi, H. Yoon, I.-S. Park, J. Jang, Y.-E. Sung, Carbon 2007, 45, 2496 – 2501. [36] J. Xu, Q. Wang, X. Wang, Q. Xiang, B. Liang, D. Chen, G. Shen, ACS Nano 2013, 7, 5453 – 5462. [37] M. Liu, J. Chang, H. Sun, L. Gao, RSC Adv. 2013, 3, 8003 – 8008. [38] S. Vijayakumar, A. K. Ponnalagi, S. Nagamuthu, G. Muralidharan, Electrochim. Acta 2013, 106, 500 – 505. [39] N.-L. Wu, S.-Y. Wang, C.-Y. Han, D.-S. Wu, L.-R. Shiue, J. Power Sources 2003, 113, 173 – 178.
ChemSusChem 2014, 7, 1676 – 1683
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CHEMSUSCHEM FULL PAPERS [40] Y.-H. Kim, S.-J. Park, Curr. Appl. Phys. 2011, 11, 462 – 466. [41] J.-Y. Hong, H. Yoon, J. Jang, Small 2010, 6, 679 – 686. [42] B. Tang, G. Wang, L. Zhuo, J. Ge, L. Cui, Inorg. Chem. 2006, 45, 5196 – 5200. [43] H.-M. Xiao, W.-D. Zhang, S.-Y. Fu, Compos. Sci. Technol. 2010, 70, 909 – 915.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org [44] J. Wang, L. Li, C. L. Wong, L. Sun, Z. Shen, S. Madhavi, RSC Adv. 2013, 3, 15316 – 15326. Received: November 6, 2013 Revised: March 3, 2014 Published online on April 6, 2014
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