Fe3O4 Composite in Situ Grown on

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Fe2O3/RGO/Fe3O4 Composite in-situ Grown on Fe Foil for High performance Supercapacitors Chongjun Zhao, Xiaoxiao Shao, Yuxiao Zhang, and Xiuzhen Qian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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ACS Applied Materials & Interfaces

Fe2O3/RGO/Fe3O4 Composite in-situ Grown on Fe Foil for High performance Supercapacitors Chongjun Zhao*, Xiaoxiao Shao, Yuxiao Zhang, Xiuzhen Qian

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering,

East China University of Science and Technology, Shanghai 200237, P.R. China, Tel: +86-21-6425 0838; E-mail:

[email protected]

1

ACS Paragon Plus Environment


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Abstract: A Fe2O3/RGO/Fe3O4 nanocomposite in-situ grown on Fe foil was synthesized via a simple one-step hydrothermal growth process, where the iron foil served as support, reductant of GO, Fe source of Fe3O4 and also the current collector of electrode. When it was directly acted as the electrode of a supercapacitor, as-synthesized Fe2O3/RGO/Fe3O4@Fe exhibited excellent electrochemical performance with high capability of 337.5 mF/cm2 at 20 mA/cm2 and superior cyclability with 2.3 % of capacity loss from 600th cycle to 2000th cycle. Key words: Fe2O3/RGO/Fe3O4, Fe foil, Graphene oxide, Hydrothermal process, Supercapacitor

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1. Introduction As the environmental problems and energy crisis become increasingly serious, it is essential to develop the new energy resource and also energy storage devices. Supercapacitor (i.e., electrochemical capacitor), as a member of the most important energy storage devices, is thought to be a promising candidate for storing energy due to its distinctive merits, i.e., high power density, excellent cycle stability, low equivalent series resistance, and superior rate capacity.1-3 According to charge storage mechanisms, supercapacitors can be classified into pseudocapacitors and electrical double layer capacitors (EDLCs). Pseudocapacitors rely on the fast Faradaic reaction to store charge, while EDLCs store charge depending on the static electricity force on the basis of the electrical double layer.4 Compared with the EDLCs, the pseudocapacitor possesses higher specific capacitance and energy density to satisfy the practical power application. The electrode materials play an important role in developing excellent performance supercapacitors.5-6 Referring to pseudocapacitor material, it mainly includes conducting polymers and transition metal oxides (TMOs).7 TMOs are regarded as the popular electrode materials for pseudocapacitors because of their good redox reversibility and large theoretical specific capacitance.8-11 Many TMOs including RuO2,12 NiO,13 Co3O4,14 MnO2,15 SnO2,16 ZnO,17 and WO318, have been researched to increase the energy density and the specific capacitance of pseudocapacitors. Unfortunately, all these metal oxides are confined within positive electrode materials, while for comparison, a few efficient transitional metal oxides are

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paid attention, e.g., TiO2,19 V2O5,20 Fe2O3,21 which are used as the negative electrode of supercapacitors. Recently, Fe2O3 has been intensively researched for application of electrode materials in supercapacitors due to variable oxidation states, good stability, abundance in nature, non-toxicity, low cost and non-pollution.7,

22-23

Nevertheless, Fe2O3

generally suffers from poor conductivity (10-14 S cm-1) and low specific surface area, which leads the Fe2O3 electrode to exhibiting low specific capacitance, poor rate capability and cycling stability.24 For comparison, for another iron oxide, Fe3O4 shows excellent conductivity (102~103 S cm-1) which makes Fe3O4 become promising conductive support. To overcome poor electrical conductivity and optimize the electrochemical performance of Fe2O3 electrode, Fe3O4 have been introduced and combine with Fe2O3 to produce hybrid metal oxide composites. Chen et al.25 reported a α-Fe2O3/Fe3O4 heterostructure nanoparticle. Tang et al.26 fabricated hierarchical Fe2O3@Fe3O4 core shell nanorod arrays, in which Fe3O4 was used as a conductive support. These composites exhibited superior electrochemical performance to single component of Fe2O3 or Fe3O4, due to the synergistic effect of two kinds metal oxide.25-28 Graphene, a two-dimensional (2D) carbon material, is usually introduced and combined with other materials because of its high conductivity (16,000 S/m) and specific surface area of 2,630 m2/g, as well as superior mechanical properties.29-31 Also, due to its intrinsic electronic structure, the graphene can effectively prevent the agglomeration of nanoparticles, well-accommodate the volume changes of

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nanoparticles and maintain the integrity of electrode.32-33 Following this strategy, iron oxides can also be integrated with graphene to improve the electrochemical performances. Wang et al.34 found that Fe2O3 mesocrystal/graphene nanohybrid had higher specific capacitance than pure reduced graphene oxide (RGO). Lee at al.35 reported that the specific capacitance of Fe2O3 nanotubes anchored on RGO was 8 times as much as that of original Fe2O3. It was found by Wang et al.36 that graphene/Fe2O3 composite had much better cyclability than Fe2O3 (75% after 200 cycles, vs. 49 % after 70 cycles). Yang et al.37 prepared a 3D Fe2O3/m-RGO film as the negative electrode of asymmetric supercapacitor which showed higher capacitance than the 3D m-RGO and RGO films. Moreover, in conventional method, powdery iron oxides as active material are often synthesized and thus further electrode preparation processes (e.g., grinding, slurry-coating) are necessary, which increases the cost and time. As far as we know, in supercapacitors, there is still no report on directly growth of iron oxide/graphene electrode materials on Fe foil or other conductive substrate, let alone through an in-situ redox reaction between reductive iron foil and oxidative graphene oxide (GO) in the absence of any other agents. In this work, a Fe2O3/RGO/Fe3O4 nanocomposite was in-situ grown on Fe foil via one-step hydrothermal growth process by means of the redox reaction among GO and Fe2+, as well as iron foil. Here, the iron foil acts as support, reductant of GO, iron source of Fe3O4 and also the current collector of electrode.38 To finally achieve hybrid metal oxide, the Fe2+ was added to use as the iron source of Fe2O3 because

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Fe2+ can be easily adsorbed on the surface of GO sheets because of the electrostatic force, in which Fe2+ was oxidized by GO.39 Benefited from the Fe-O bonding between FexOy particles and RGO, the agglomeration of FexOy particles is suppressed, and the electrons and ions transportation is also promoted.40 Thus, the as-prepared Fe2O3/RGO/Fe3O4@Fe

with

a

hierarchical

structure

exhibited

excellent

electrochemical performance when directly acted as supercapacitor electrode.

2. Experiment 2.1 Materials and reagents Pristine graphite powder, anhydrous ethanol, iron sulfate heptahydrate (FeSO4·7H2O) were purchased from Sinopharm Chemical Reagent Company. Sulfuric acid (H2SO4, 95-98 wt%), hydrochloric acid (HCl, 36.0-38.0 wt%), hydrogen peroxide (H2O2, 30 wt%), potassium permanganate (KMnO4), phosphorus pentoxide (P2O5), potassium persulfate (K2S2O8) and potassium hydroxide (KOH, 30 wt%) were obtained from Shanghai Ling Feng Chemical Reagent Company. 2.2 Synthesis of Fe2O3/RGO/Fe3O4 and RGO/Fe3O4 composites on iron foil Graphene oxide was synthesized through a modified Hummer’s method.41 Iron foil was cleaned with acetone and anhydrous ethanol under ultrasound irradiation to remove surface impurities, respectively. The synthesis of Fe2O3/RGO/Fe3O4 composites was conducted via a hydrothermal progress by immerging the iron foil in a mixture solution consisting of GO and iron salt. Typically, FeSO4·7H2O (2 mmol) was added in 38 ml DI water with

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stirring for 30 min. Then 2 ml of GO aqueous dispersion (5 mg·ml-1) was mixed homogeneously with the above solution under stirring. The above mixture was transferred into a Teflon-lined stainless steel autoclave (100 ml). Subsequently, the cleaned iron foil with a area of 1×1 cm2 was immerged in the solution, heating at 150˚C for 12 h. In experiments, the effect of FeSO4 content and temperature on Fe2O3/RGO/Fe3O4 was investigated, and the obtained samples were denoted as F1RF-150 (1 mmol FeSO4·7H2O, 150˚C), F2RF-150 (2 mmol FeSO4·7H2O), F3RF-150 (3 mmol FeSO4·7H2O), F2RF-120 (120˚C), F2RF-180 (180˚C), respectively. The final product was washed with DI water and dried in a vacuum oven at 80˚C. For comparison, RGO/Fe3O4 (RF-150) composite was synthesized at the same conditions in the absence of FeSO4·7H2O. 2.3 Characterization The X-ray diffraction (XRD) measurements were carried out using a X-ray diffractiometer with Cu Kα (λ = 0.15406 nm). Raman spectra were obtained on an INVIA Raman microprobe (Renishaw Instruments, England) with a 514 nm laser excitation. The X-ray photoelectron spectroscopy (XPS) spectra were performed on a PHI5300 instrument. The morphology and structure of products were investigated by a field-emission scanning electron microscope (FESEM, JEOL JSM-7800F), transmission electron microscopy (TEM, JEOL JEM-2100), respectively. 2.4 Electrochemical measurement The electrochemical performance of as-synthesized electrodes was measured by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical

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impedance spectroscopy (EIS). All electrochemical measurements were carried out on an electrochemical workstation (CHI 660e, Shanghai) with a typical three electrode system in 2 M KOH. The as-prepared electrode materials with an apparent area of 1×1 cm2, a platinum foil electrode, and a saturated calomel electrode (SCE) were acted as the working electrode, counter electrode, and reference electrode, respectively. When the electrochemical measurements finished, the loading amount of Fe2O3/RGO/Fe3O4 on iron foil was determined according to the weight difference.38 2.5 Fabrication and electrochemical performance of asymmetric supercapacitor (ASC) In a two electrode system, an ASC device was assembled by directly using the Fe2O3/RGO/Fe3O4@Fe foil as negative electrode and Ni(OH)2 (Synthetic method was given in Supporting Information)as positive electrode. Briefly, the positive electrode was fabricated by mixing Ni(OH)2 powder (80 %) with acetylene (10 %) and poly(tetrafluoroethylene) (10 %), and the above mixture was coated on a Nickel foam (1×1 cm2). Then the positive electrode was dried at 80˚C for 12 h. The weights of active materials for negative and positive electrodes were about 4.3 mg and 2.1 mg, respectively. The electrochemical performance of ASC was tested in 2 M KOH.

3. Results and discussion 3.1. XRD patterns, Raman and XPS spectra of Fe2O3/RGO/Fe3O4 (FRF) nanocomposites Fig.1 shows the XRD patterns of RF-150 and F2RF-150. There are two diffraction peaks at 44.7° and 65.0°, which are assigned to the (110) and (220) planes 8


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of Fe (JCPDS No. 06-0696) arising from the iron foil substrate. Diffraction peaks at 19.1°, 30.1°, 35.5°, 37.0°, 43.1°, 57.0°, and 62.6° for F2RF-150 and RF-150 are assigned to the (111), (220), (311), (222), (400), (511), and (440) planes of Fe3O4 (JCPDS No. 75-1609). For F2RF-150, the other diffraction peaks at 24.1°, 33.2°, 35.6°, 40.9°, 49.5°, 54.1°, 62.4° and 64.0° are assigned to the (012), (104), (110), (113), (024), (116), (214) and (300) planes of Fe2O3 (JCPDS No.33-0664), respectively. The existence of Fe2O3 and Fe3O4 can also be verified by the later Raman and XPS spectra of F2RF-150. In addition, the broad hump of RGO between 20°~30° is not obvious, which is due to its highly disordered structure and low content.42

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F2RF-150

♣ Fe O 2 3 ♥ Fe3O4 • Fe

♣ ♥ •

Intensity (a.u.)

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♣ ♥

♣

♥

♥ ♣♥

♣ ♣ ♥

♣ ♥ ♣ •

♥

♥ •

RF-150 •

♥ ♥

♥

♥

10

20

30

40

50

2θ (degree)

60

70

80

Fig. 1 XRD Patterns of F2RF-150 and RF-150.

The presence of RGO can be verified by Raman spectra of RF-150 and F2RF-150 composites, as displayed in Fig. 2 and there are two obvious broad peaks for these two composites. The G band of 1598 cm−1corresponds to the vibration of sp2-bonded carbon atoms, and the D band of 1354 cm−1 originates from the defects. The ratio of G band (~1598 cm-1) and the D band (~1354 cm-1) can be used to semi-quantitatively determine the reduction extent of GO.43 In Fig. 2, the intensity ratio of D to G band (ID/IG) for composites (RF-150: 1.18; F2RF-150: 1.44) are remarkably higher than that for GO (0.88) or pure RGO (0.98), confirming the reduction of GO. On the other hand, there are also other peaks appearing in the low wavenumber range in the Raman spectra of RF-150 and F2RF-150. For RF-150, it is

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clear to see an apparent peak at 664 cm−1 with two weak ones at 308 and 531 cm−1, which are attributed to the fundamental Raman vibration of Fe3O4, assigned to the Eg, T2g, and A1g vibrational mode, respectively.44 These results indicate the formation of the composite of RGO and Fe3O4 in RF-150. For F2RF-150, besides the peaks for GO and Fe3O4, there are five additional peaks in the low wavenumber range, which correspond to the fundamental Raman vibration of Fe2O3, that is, 217 cm−1 corresponds to A1g symmetry, 284, 395, 495, 592 cm−1 for Eg symmetry.45 These phenomena demonstrate the existence of Fe2O3 in the F2RF-150 composite. According to the results of XRD and Raman, the F2RF-150 is composed of Fe2O3, RGO and Fe3O4.

F2RF-150 284 217 395

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


495

ID/IG=1.44 592

664

RF-150 308

ID/IG=1.18

531

RGO

ID/IG=0.98

GO

ID/IG=0.88

200

400

600

800 1000 1200 1400 1600 1800

Raman shfit (cm-1) Fig. 2 Raman spectra of GO, RGO, RF-150 and F2RF-150.

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Fig. 3 shows the X-ray photoelectron spectroscopy (XPS) of F2RF-150 that can further identify the chemical composition of the nanocomposite. The survey spectrum (0-1000 eV) of F2RF-150 (Fig. 3a) mainly shows Fe 2p, O 1s and C 1speaks, which indicates the existence of Fe, O and C elements in the sample. As presented in Fig. 3b, it is appropriate to divide the C 1s spectrum of

F2RF-150 into three peaks at 284.6

eV, 285.8 eV and 288.5 eV, which are designated to C-C, C-O and C=O, respectively.39 In Fig. 3c, the peaks from F2RF-150 for Fe 2p1/2 and Fe 2p2/3 appear at 724.3 eV and 710.3 eV. For the Fe 2p1/2, it consists of two peaks, at 724.2 eV and 722.4 eV. Similarly, the Fe 2p3/2 is composed of two peaks at 710.8 eV and 709.3 eV, respectively. Peaks at 722.4 eV and 709.3 eV are ascribed to the Fe3+ from Fe2O3, while the two peaks at 724.2 eV and 710.8 eV are resulted from Fe2+ of Fe3O4.46-47 There is also a satellite peaks at 718 eV which correspond to the fingerprint of electronic structure of Fe2O3,48 confirming the existence of Fe2O3 in the F2RF-150 again. The results of XRD, Raman and XPS convincingly suggest that the Fe2O3/RGO/Fe3O4 composite is successfully synthesized.

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a

b

C 1s

Intensity (a.u.)

O KLL

Intensity (a.u.)

Fe 2p O 1s

C 1s

1000

800

600

400

200

0

Original C-C C-O C=O Background Fitted

294

292

290

Binding energy (eV)

288

286

284

282

280

Binding energy (eV)

Fe 2p

(c) 724.3 eV Fe 2p1/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


710.3 eV Fe 2p2/3 710.8 eV 718 eV

724.2 eV

709.3 eV

722.4 eV

Origianl Background Fitted

740

735

730

725

720

715

710

705

700

Binding energy (eV) Fig. 3 XPS spectra of F2RF-150: (a) survey spectrum, (b) C 1s spectrum, and (c) Fe 2p spectrum.

3.2. FESEM, TEM images and BET results of Fe2O3/RGO/Fe3O4(FRF) composites Fig. 4 presents the FESEM images of RF-150, F2RF-150, F2RF-120 and F2RF-180. In Fig.4a, it is found that the spherical Fe3O4 particles are anchored on the Fe substrate. Obviously, the number of these Fe3O4 spheres is not enough to cover the whole substrate surface, which leads to a low load amount and thus low specific capacitance. For comparison, the SEM image of F2RF-150 (Fig. 4b) is distinct difference: (1) No Fe substrate is observed, i.e., it is well decorated by the subsequent

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deposition of Fe2O3 from the Fe2+ ions. (2) Particles with interconnected 3D network structures are clearly seen, which was generated through the self-assembly of Fe2O3 nanosheets, and these hierarchical structures potentially facilitate fast electron transport and fast electrolyte ions diffusion. The formation process of Fe3O4/RGO and Fe2O3/RGO/Fe3O4@Fe composites is illustrated in Scheme 1. According to Fig. 4b-d, the morphology of Fe2O3/RGO/Fe3O4 nanocomposites is distinctly dependence of the hydrothermal synthesis temperatures (from 120 to 180ºC). At 120ºC (Fig. 4c), the space among Fe2O3 particles is bigger than 150ºC (Fig. 4b) because the reaction rate is slower so as only form a 2D coating layer of Fe2O3 nanoparticles. Therefore, the specific surface area of F2RF-120 is lower than that of F2RF-150, which is unfavorable to fully utilize the electrolyte ions. However, referring to F2RF-180 (Fig. 4d), ununiform and dense Fe2O3 particles are clearly seen due to the overgrowth at high temperature (i.e., 180ºC), which somewhat hinders the electrolyte ions from entering into the active materials easily. Therefore, the porous structure ensures F2RF-150 nanocomposite to exhibit the superior electrochemical performances. As shown in Fig. 5, the structure of as-synthesized Fe2O3/RGO/Fe3O4 nanocomposite is further investigated by transmission electron microscope (TEM). Compared with the TEM image of pure RGO in Fig. 5a, the interconnected Fe2O3 nanosheets are well decorated on the two-dimensional graphene sheets as seen in the Fig. 5b. A few Fe3O4 particles can be observed in Fig. 5b, because the Fe3O4 in-situ grows on Fe foil and hardly falls off from the Fe substrate even under ultrasonic irradiation condition. In the Fe2O3/RGO/Fe3O4 nanocomposite, graphene sheets not

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only serve as a conductive network, but also prevent iron oxide particles from agglomeration, which is very important in promoting a better cyclability than pure iron oxides electrodes.49 The N2 adsorption-desorption isotherm of Fe2O3/RGO/Fe3O4 composite is shown in Fig. S1. The narrow hysteresis loop suggests the porosity of materials and the BET surface area of Fe2O3/RGO/Fe3O4 composite is about 16.93 m2/g.

Fig. 4 FESEM images of (a) RF-150, (b) F2RF-150, (c) F2RF-120, and (d) F2RF-180

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Fig. 5 TEM images of (a) RGO, and (b) F2RF-150 composite.

Scheme 1.Schematic diagram of preparation of the RF and FRF composites.

3.3. Electrochemical performance of Fe2O3/RGO/Fe3O4 (FRF) nanocomposites Fig. 6a presents the CV curves of F2RF-150 ranging from 5 to 50 mV/s and every curve has a reduction peak and two oxidation peaks in range from -1.2 to -0.4 V because of the conversion between Fe2+ and Fe3+, suggesting the pseudocapacitive characteristic of electrode materials. Specifically, the oxidation peak at -0.9 V 16


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corresponds to the formation of Fe(OH)2, while the other oxidation peak at -0.65 V is ascribed to the formation of FeOOH. The arising of reduction peak at -1.1 V results from the reduction from FeOOH to Fe(OH)2.21 In Fig. 6a, it is notable that these CV curves maintain a similar shape and the current response increases as the scan rate varies from 5 to 50 mV/s, indicating that the composite is beneficial for fast and reversible redox reactions.39 The CV curves of RF-150, F1RF-150, F2RF-150 and F3RF-150 at 10mV/s are shown in Fig. 6c. It is clear that F2RF-150 has higher current response than others, i.e., it has the highest specific capacitance, while the RF-150 composite has the lowest specific capacitance. This result is relative with the loading of Fe2O3 on the surface of RGO, which increases the content of active materials and improve capacitance. However, the specific capacitance decreases when the addition of Fe2+ is increased to 3 mmol (i.e., F3RF-150), because more Fe2O3 could not be utilized fully in the redox process and results in the growth of dead active materials.50 Fig. 6e presents the CV curves at 10 mV/s of composites synthesized at different temperatures (i.e., F2RF-120, F2RF-150, F2RF-180). The F2RF-150 also exhibits the best performance. Fig. 6b displays the GCD performance of F2RF-150 at different current density of 5, 10, 20, 30, 50, and 100 mA/cm2 (corresponding to 1.16, 2.33 4.65, 6.98, 11.63 and 23.26 A/g) with potential window from -1.2 to -0.4 V. All these GCD curves have the platforms which demonstrates that the capacitance is mainly governed by the redox reaction in according with the pseudocapacitive characteristic reflecting in the CV curve of F2RF-150 (Fig. 6a).51

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Similar to CV curves, Fig. 6c and 6f show the GCD curves at 20 mA/cm2 for those composites prepared at different conditions (Fe2+ content and reaction temperature). It can be seen that the F2RF-150 has the longest discharge time, so this nanocomposite holds the highest specific capacitance, corresponding to the results of CV curves. The specific capacitance (Cs) of all composites can be calculated using the following equation:52

CS =

C I * ∆t = S ∆V * S

(1)

Where I (A) is the charge-discharge current, ∆t (s) is the discharge time, ∆V (V) is the potential window and S (cm2) is the geometric surface area of working electrodes. Table 1 lists the specific capacitance at 20 mA/cm2 for all composites, and the F2RF-150 exhibits the best capacity (337.5 mF/cm2, 78.49 F/g). The F2RF-150 electrode also shows better rate capability than RF-150, F1RF-150 and F3RF-150, as shown in Fig. 6g.

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2 Current density (A/cm )

0.03 0.02

b

-0.4

5mV/s 10mV/s 20mV/s 30mV/s 50mV/s

a

0.04

2

5mA/cm 2 10mA/cm 2 20mA/cm 2 30mA/cm 2 50mA/cm 2 100mA/cm

-0.5

Potentital vs. SCE (V)

0.05

0.01 0.00 -0.01 -0.02 -0.03 -0.04

-0.6 -0.7 -0.8 -0.9 -1.0 -1.1

-0.05 -1.2 -1.2

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

0

10

20

30

40

Potentital vs. SCE (V) RF-150 F1RF-150 F2RF-150 F3RF-150

60

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80

0.01

0.00

-0.01

-0.5

100

110

-0.6 -0.7 -0.8 -0.9 -1.0 -1.1

-0.02 -1.2

-1.1

-1.0

-0.9

-0.8

-0.7

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-0.5

-1.2

-0.4

0

2

4

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Potential vs. SCE (V)

12

14

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20

-0.4

24

26

F2RF-120 F2RF-150 F2RF-180

f

-0.5


F2RF-120 F2RF-150 F2RF-180

0.01

22

Time (s)

e

0.02

2 Current density(A/cm )

90

RF-150 F1RF-150 F2RF-150 F3RF-150

d

-0.4


Current density (A/cm2)

50

Time (s)

c

0.02

0.00

-0.01

-0.6 -0.7 -0.8 -0.9 -1.0 -1.1

-0.02

-1.2 -1.2

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

-0.5

0

-0.4

2

4

6

8

400

g

RF-150 F1RF-150 F2RF-150 F3RF-150

350 300 250 200 150 100 50 0 0

10

20

30

10

12

14

Time (s)


Specific capacitance (mF/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

50

Current density (mA/cm2)

19


16

18

20

22

24

26


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

Fig. 6 (a) Cyclic voltammetry (CV) curves at different scan rates, and (b) Galvanostatic charge/discharge (GCD) curves under various current densities for F2RF-150. (c) CV curves at scan rate of 10 mV/s,and (d) GCD discharge curves at current density of 20 mA/cm2 for RF-150, F1RF-150, F2RF-150 and F3RF-150. (e) CV curves at scan rate of 10 mV/s and (f) GCD discharge curves at 20 mA/cm2 for F2RF-120, F2RF-150, and F2RF-180. (g) The specific capacitance of RF-150, F1RF-150, F2RF-150 and F3RF-150 electrodes under different current density.

Table.1 Specific capacitances for various composites at 20 mA/cm2.

Samples

RF-150

F1RF-150

F2RF-150

F3RF-150

F1RF-120

F1RF-180

Area capacitance (mF/cm2)

55

95

337.5

130

87.5

170

The F2RF-150 exhibits excellent cycle stability and Fig. 7 shows the cycling performance of F2RF-150 electrode after 2000 cycles of charge/discharge at 20 mA/cm2. It can be observed that the specific capacitance of F2RF-150 electrode increases obviously in the first 600 cycles (from 337.5 to 524.5 mF/cm2) and then remain stable until 2000 cycles (512.5 mF/cm2). There is only 2.3 % capacity loss from 600th cycle to 2000th cycle. The increase of specific capacitance may be attributed to the activation process which enable the trapped ions to diffuse out, while the expansion of interlayer spacing of RGO sheet facilitates the intercalation of counter ion.35 For comparison, the capacity decay of RF-150 electrode is up to 42.11% after 2000 cycles at 20 mA/cm2, which is much worse than F2RF-150. Furthermore, 20


Page 21 of 33

the F2RF-150 electrode still remains 95 % retention of the maximum capacity even after 5000 cycles (Fig. 7 inset). Compared with the SEM image of F2RF-150 before cycling test (Fig. 4b), the Fe2O3 particles after cycling become smaller and distribute more uniformly, as shown in Fig. S2. Also, the Coulombic efficiency always keeps above 90 % during the whole cycling process, which suggests the good reversibility of F2RF-150 electrode material.53

600

F2RF-150 RF-150

500 1400

400 300 200 100

100

1200 80 1000 60

800 600

40

400 20 200 0

0

1000

2000

3000

4000

Coulombic Efficiency (%)

Specific Capacitance (mF cm-2)

Specific Capacitance (mF cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 5000

Cycle number

0

0

200 400 600 800 1000 1200 1400 1600 1800 2000

Cycle number Fig. 7 Cyclability of the RF-150 and F2RF-150 electrodes at 20 mA/cm2. Inset: the coulombic efficiency and 5000 cycle curve of F2RF-150 at 20 mA/cm2.

The EIS measurements were conducted at open circuit potential (OCP). Fig. 8 gives the Nyquist plots of F1RF-150, F2RF-150 and F3RF-150 electrodes collected with an sinusoidal signal of 5 mV, in the frequency vary from 100 kHz to 0.01 Hz. In Nyquist plots, the X-intercept of semicircle represents the equivalent series

21



resistances (Rs), which are 0.9, 1.1, 2.1 Ω for the F1RF-150, F2RF-150, and F3RF-150, respectively. With the increase of Fe2+, Rs values increase because the content of poor electrical conductivity Fe2O3 in the nanocomposites grows. In Fig. 8, the diameter of semicircle is a characteristic of charge transfer resistance (Rct) at electrolyte/electrode interface, and the order of Rct value is: F1RF-150 > F2RF-150 > F3RF-150. Comprehensively, the F2RF-150 holds more moderate resistance than other two materials and exhibits the best electrochemical performance. After 5000 cycles, there is a little change for the Nyquist plot of F2RF-150 (Fig. S2) and the internal resistance even become smaller than 1st cycle, which can further confirm the exceptional cyclability of F2RF-150 electrode.

50

40

30 12

F1RF-150 F2RF-150 F3RF-150

20

10

-Z"/ohm

-Z"/ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

F1RF-150 F2RF-150 F3RF-150

8 6 4

10

2 0 0

0

2

4

6

8

10

12

Z'/ohm

0

10

20

30

40

50

Z'/ohm Fig. 8 Nyquist plots of F1RF-150, F2RF-150, F3RF-150. Inset: the date of high frequency region and equivalent circuit.

22


Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.4 Electrochemical performances of asymmetric supercapacitors In an ASC device, the Ni(OH)2 and F2RF-150 were employed as positive and negative

electrodes,

respectively

(denoted

as

Ni(OH)2//F2RF-150).

The

electrochemical performances of Ni(OH)2 electrode measured in 2 M KOH using a three electrode system

are given in Fig. 9. Fig. 9a displays the CV curves of

Ni(OH)2 electrode and every curve has a pair of redox peaks, suggesting that the capacitance of Ni(OH)2 is mainly contributed by Faradic reaction.54 Non-linear GCD curves (Fig. 9b) further confirm the pseudocapacitance of Ni(OH)2. In Fig. 9c, the mass specific capacitance of Ni(OH)2 is 260.4, 203.1, 153.6 and 62.6 F/g at a current density of 5.2, 7.8, 13 and 26 A/g, respectively.

23



a

5 mV/s 10 mV/s 20 mV/s 30 mV/s 50 mV/s

0.04 0.02

b

0.5


0.06

0.00 -0.02 -0.04

20 mA/cm2 30 mA/cm2 50 mA/cm2

0.4

100 mA/cm2 0.3

0.2

0.1

-0.06 0.0

0.0

0.1

0.2

0.3

0.4

0.5

0

10

20

250

30

40

50

60

Time (s)


Mass specific capacitance (F/g)

2 Current density (A/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

c

200

150

100

50

0 1

2

3

4

Current density (A/g)

Fig. 9 (a) CV curves at different scan rates, (b) GCD curves under various current densities and (c) The mass specific capacitance under different current density of Ni(OH)2 electrode.

To achieve the optimal performance of asymmetric supercapacitor, the charge should be balanced between the two electrodes according to Q+= Q −. The mass loading of Ni(OH)2 is calculated according to the following equation:

Cୱି ∆Vି = C୫ା ∆Vା mା

(2)

Where CS— (mF/cm2) and Cm+ (F/g) represent the specific capacitance of negative electrode and positive electrode. ∆V— (V) and ∆V+ (V) are the potential window of negative and positive electrodes. S (cm2) is the area of Fe2O3/RGO/Fe3O4 electrode,

24


Page 25 of 33

m+ (mg) is the loading mass of Ni(OH)2.55 According to the above equation, the optimal loading mass of Ni(OH)2 is determined to be 2.1 mg in the asymmetric supercapacitor. The CV curves of F2RF-150 and Ni(OH)2 electrodes at scan rate of 20 mV/s in three electrodes system are displayed in Fig. 10a. The potential window of F2RF-150 and Ni(OH)2 electrodes are -1.2~-0.4 V and 0~0.5 V, respectively. This indicates that the cell voltage of Ni(OH)2//F2RF-150 can be extended to 1.7 V in 2 M KOH. The CV curves of Ni(OH)2//F2RF-150 in different cell voltage at 50 mV/s are given in Fig. 10b, in which the potential window is 0~1.7 V for the ASC.

0.02

a

b

F2RF-150 Ni(OH)2

0.03


0.04


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.02 0.01 0.00 -0.01 -0.02 -0.03

0.01

0.00

-0.01

-0.04 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.0

0.6

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Voltage (V)

Potential vs. SCE (V)

Fig. 10 (a) CV curves of F2RF-150 and Ni(OH)2 electrodes at 20 mV/s in three electrodes system. (b) CV curves of Ni(OH)2// F2RF-150 in different cell voltage at 50 mV/s.

Fig. 11a and Fig. 11b show the CV and GCD curves of Ni(OH)2//F2RF-150 ASC device. According to GCD curves (Fig. 11b), the specific capacitance of the ASC device is 66.2 mF/cm2 (10.3 F/g), 44.1 mF/cm2 (6.9 F/g), 36.5 mF/cm2 (5.7 F/g), 35.29 mF/cm2 (5.5 F/g) and 35.29 mF/cm2 (5.5 F/g) at 5, 10 , 20, 30, 50 mA/cm2,

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

respectively, as shown in Fig. 11c. Besides, in Fig. 11d, the cyclability of Ni(OH)2//F2RF-150 was conducted for 2000 cycles under 20 mA/cm2 and the specific capacitance increases from 45.9 mF/cm2 to 152.9 mF/cm2. Fig. 11e is the Nyquist plot of

ASC device, where the equivalent series resistance is only 1.5 Ω. To evaluate the

electrochemical performances of supercapacitors, the energy density (E) and power density (P) are two key factors, which can be calculated by the following equations:

E = (C × ∆V ଶ )/2

(3)

P = E/∆t

(4)

Where E (Wh/kg) is energy density, C (F/g) is mass specific capacitance, ∆V (V) is cell voltage of ASC, P (W/kg) is power density, and ∆t (s) is discharging time.55 The Ragone plot of Ni(OH)2//F2RF-150 is displayed in Fig. 11f, in which the energy density is 4.1, 2.8, 2.3, 2.2, 2.2 Wh/kg at the power density of 661.5, 1329.4, 2656.9, 3973.8, 6600 W/kg, correspondingly.

26


Page 27 of 33

a

0.010

b

1.8

5 mA/cm2 10 mA/cm2 20 mA/cm2 30 mA/cm2 50 mA/cm2

1.4

Voltage (V)

0.005

0.000

5 mV/s 10 mV/s 20 mV/s 30 mV/s 50 mV/s

-0.005

-0.010 0.0

0.2

0.4

1.2 1.0 0.8 0.6 0.4 0.2

0.6

0.8

1.0

1.2

1.4

1.6

0.0

1.8

0

10

20

70

30

40

50

Time (s)

Voltage (V) 160

12

c

d

10 60 8 50 6 40 4

30 10

20

30

40

120 100 80 60 40 20 0 -20

2 0



140

0

50

500

1000

Current density (mA/cm2) 800

e

10

f 4

Energy density (Wh/kg)

2 0 0

2

4

6

8

10

200

0 0

2000

8

4

400

1500

Cycle number

6

600

-Z"/ohm


1.6

Mass specific capaciance (F/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

400

600

3

2

0

800

2000

4000

6000

Power density (W/kg)

Z'/ohm

Fig. 11 (a) CV curves at different scan rates, (b) GCD curves under different current densities, (c) Cyclability at 20 mA/cm2, (d) The specific capacitance and mass specific capacitance at different current densities, (e) Nyquist plot (Inset: the data of high-frequency region) and (f) Ragone plot related to energy and power density of Ni(OH)2//F2RF-150 ASC device.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

4. Conclusions In summary, a Fe2O3/RGO/Fe3O4 nanocomposite in-situ grown on Fe foil was successfully synthesized through a one-step hydrothermal synthesis process. Both Fe2+ content and temperature affected the nanostructures and electrochemical performances of Fe2O3/RGO/Fe3O4 nanocomposite. F2RF-150 electrode exhibits the best electrochemical performance based on the synergistic effect of Fe2O3, Fe3O4 and RGO. The specific capacitance of this electrode can achieve 337.5 mF/cm2 at 20 mA/cm2 in 2 M KOH solution, and only 2.3 % capacity loss was observed from 600th cycle to 2000th cycle. An asymmetric supercapacitor of Ni(OH)2//F2RF-150 was assembled and delivered energy density of 4.1, 2.8, 2.3, 2.2, 2.2 Wh/kg at the power density of 661.5, 1329.4, 2656.9, 3973.8, 6600 W/kg, respectively.

Acknowledgement: We are grateful for the support of Shanghai Natural Science Foundation

(No.

13ZR1411900),

and

Shanghai

Key

Laboratory

Project

(08DZ2230500).

SUPPORTING INFORMATION Synthesis of Ni(OH)2; the N2 adsorption-desorption isotherm of Fe2O3/RGO/Fe3O4 composite; SEM image of F2RF-150 after 5000 cycles; Nyquist plots of F2RF-150 electrode after 1st and 5000th cycle; CV curves for the F2RF-150 electrode in the KOH solution with different concentrations.

28


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Graphical abstract


Fe3O4 Composite in Situ Grown on

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