Fe₂O₃ nanocluster-decorated graphene as O₂ electrode for high energy Li–O₂ batteries

Abstract

Fe₂O₃ nanocluster-decorated graphene (Fe₂O₃/graphene) hybrids with controlled contents of Fe₂O₃ were prepared by a facile electrochemical process. These Fe₂O₃/graphene hybrids were tested as O₂ electrodes for Li–O₂ batteries, which exhibited enhanced discharge capacities as compared to that of a pure graphene based O₂ electrode, e.g. the Fe₂O₃/graphene electrode with 29.0 wt% of Fe₂O₃ delivered a discharge capacity of 8290 mA h g⁻¹ and a round-trip efficiency of 65.9% as compared to 5100 mA h g⁻¹ and 57.5% for a pure graphene electrode. The excellent electrochemical properties of Fe₂O₃/graphene as an O₂ electrode is ascribed to the combination of the fast kinetics of electron transport provided by the graphene sheets and the high electrocatalytic activity for O₂ reduction provided by the Fe₂O₃.

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Introduction

Recently, lithium–O₂ batteries (LOBs) have attracted much attention due to their ultra-high theoretical specific energy of 11 680 W h kg⁻¹ (electrode level), which is an order of magnitude higher than that of Li ion batteries¹ currently used commercially.² At present, a prototype aprotic LOB includes Li metal, aqueous or non-aqueous electrolyte and an O₂ electrode.^3–7 In a typical discharge process of LOBs, two reduction reactions with O₂ have been suggested to take place at the O₂ electrode:^8,9

2Li + 2e⁻ + O₂ → Li₂O₂ (U₀ = 2.96 V) (1)

4Li + 4e⁻ + O₂ → 2Li₂O (U₀ = 2.91 V) (2)

The discharge products, e.g. Li₂O₂ or/and Li₂O, precipitate on the surface of the O₂ electrode.

High catalytic activity of the O₂ electrodes is a critical factor to facilitate the O₂ reduction processes, which affects the overall performance of LOBs.^2,5,9 Transition-metal oxides, e.g. MnO₂-based materials, have been investigated as O₂ electrodes in LOBs with carbonate-based electrolytes due to their low cost and low toxicity.^10–13 However, a recent report has indicated that the usage of carbonate-based electrolytes in LOBs involves complex decomposition chemistry of the electrolytes, which influences the proper characterization of the specific capacities of the LOBs.¹⁴ Ether-based electrolytes have been proved to be more suitable for LOBs as no obvious decomposition of the electrolytes takes place during the discharge process.¹⁴ However, there are limited reports on transition-metal oxide based O₂ electrodes with ether-based electrolytes.

Apart from catalytic activity, it is also important for the O₂ electrodes to possess high surface area and high electrical conductivity to achieve large specific capacities of the LOBs. Graphene-based materials have been extensively studied for applications in energy storage and conversion systems^15–27 due to their promising properties, e.g. high electrical conductivity, high surface area, mechanical strength and high thermal conductivity.²⁸ Hierarchically porous graphene prepared with the Hummer's method has shown exciting performance as LOB O₂ electrodes.²⁹ Attaching transition-metal oxides onto graphene could be a promising route to O₂ electrodes for LOBs. This approach combines the catalytic effects from transition-metal oxides with the high surface area and high electrical conductivity of graphene. As extensively studied electrode materials in Li ion batteries, the graphene based hybrid electrodes are typically prepared by Hummer's method,^27,30 which is a multi-step process. It should be noted that other methods also exist for graphene preparation, e.g. chemical vapor deposition,^31–33 mechanical exfoliation³⁴ and electrochemical exfoliation,^35,36 but some of these methods may not be easily adapted for large-scale sample preparation.

Here, in this work, we carried out the study using Fe₂O₃ nanocluster-decorated graphene (Fe₂O₃/graphene) as the flexible O₂ electrode in LOBs. The Fe₂O₃/graphene samples are prepared by a facile electrochemical process, which combines the exfoliation of graphene and deposition of metal oxides in one step. It is shown in our study that the Fe₂O₃/graphene O₂ electrodes shows higher discharge capacities as compared to that of O₂ electrodes made of pure graphene prepared by a similar electrochemical process. For example, the Fe₂O₃/graphene electrode with 29.0 wt% of Fe₂O₃ exhibits a discharge capacity of 8290 mA h g⁻¹ and a round-trip efficiency of 65.9% as compared to 5100 mA h g⁻¹ and 57.5% for a pure graphene electrode. The discharge capacities of Fe₂O₃/graphene based O₂ electrodes with different Fe₂O₃ weight percentages (C_Fe2O3) were also examined. The increased discharge capacities for Fe₂O₃/graphene based O₂ electrodes are proposed to be related to the superior electron transporting properties of graphene and high electrocatalytic O₂ reduction activity of Fe₂O₃.

Experimental

Synthesis of Fe₂O₃/graphene

Fe₂O₃/graphene was produced in an electrochemical approach. In a typical process, the electrochemical cell comprises of a highly oriented pyrolytic graphite (HOPG) sheet as the working electrode and a Pt wire as the counter electrode. An electrolyte consisting of 0.1 M FeSO₄ and 0.9 M Na₂SO₄ in deionized water was used. Step voltages with 0 V (10 s) and 10 V (10 s) were applied between the working and counter electrodes. The precipitates were collected via vacuum filtration and washed with deionized water several times. The products were subsequently sintered in argon gas at 400 °C for 1 h.

Two further electrolyte solutions with different concentrations (0.03 M FeSO₄ + 0.97 M Na₂SO₄ and 0.15 M FeSO₄ + 0.85 M Na₂SO₄) were also used to prepare Fe₂O₃/graphene with different contents of Fe₂O₃.

Synthesis of graphene

Pure graphene was synthesized electrochemically using a previously reported method.³⁵ Graphene sheets were produced via electrochemical exfoliation. A highly oriented graphite sheet (HOPG) was used as the working electrode and a Pt wire was used as the counter electrode. The electrolyte was prepared by dissolving 1 M Na₂SO₄ in deionized water. A constant voltage of 10 V was applied for 5 min at room temperature. The exfoliated graphene sheets were collected via vacuum filtration and washed several times. The products were then sintered in argon gas at 400 °C for 1 h.

Materials characterization

Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) imaging of the samples were performed on a field-emission scanning electron microscope (JEOL, Model JSM-7600F) and transmission electron microscope (JEOL, Model JEM-2100). Atomic force microscopy (AFM) measurement was performed on a Dimension 3100 (Veeco, CA, USA). Raman microscopy was carried out on a WITec CRM 200 with a wavelength of 488 nm and a spot size of 0.5 mm. Thermogravimetric analyses were obtained on a TGA Q500 at a heating rate of 10 K min⁻¹. X-Ray photoelectron spectroscopy (XPS) was performed on Thermo Scientific Theta Probe XPS.

The X-ray diffraction (XRD) patterns were obtained on a powder X-ray diffraction (Shimadzu XRD-6000 X-ray diffractometer with Cu-Kα irradiation, λ = 1.5406 Å). Prior to XRD analysis of the discharged electrodes, the electrodes were carefully extracted from the cells and cleaned with 1,2-dimethoxyethane in a glove-box with contents of moisture and O₂ below 0.1 ppm. After drying in vacuum for several hours, the electrodes were sealed with parafilm in the glove box and tested by XRD with parafilm wrapped on.

Electrochemical measurements

The electrochemical performance of the samples was examined in a Li–O₂ cell. The O₂ electrodes were fabricated by mixing the samples (90 wt%) and poly(vinylidene fluoride) (10 wt%) in N-methyl-2-pyrrolidone. Then the slurry was stirred overnight and cast onto a separator (Whatman GF/D microfiber filter paper), followed by vacuum-drying at 50 °C for 24 h. The Li–O₂ cell configuration was designed as proposed in a previous report.³⁷ Typically, a lithium metal foil as the counter and reference electrode was placed on a stainless-steel current collector. Then a separator (Whatman GF/D microfiber filter paper) was put on top of the lithium foil, followed by adding 20 μL electrolyte (0.1 M LiClO₄ in 1,2-dimethoxyethane). The O₂ electrode was then put on top with a stainless steel mesh and a stainless steel spring was used as the current collector. The cells were assembled in a glove box. After assembly, the cells were sealed and tested on a NEWARE system between 2 and 4.5 V under a pure O₂ flow of 1 mL min⁻¹.

Results and discussion

The Fe₂O₃/graphene hybrids were fabricated via an electrochemical method combining the deposition of metal oxides and the exfoliation of graphene sheets in a single process. Step voltages were applied in the process (Fig. S1, ESI†). The first step with no voltage applied was set to eliminate the ion concentration gradient so as to obtain homogeneous deposits. A high voltage of 10 V was used. This led to the electrochemical deposition of metal oxides onto the graphite surface while sulfate ions were driven by the electric field into the graphene layers, resulting in the expansion of graphene interspacing and the exfoliation of the outer graphene layers off the graphite surface. After cycling the above steps, the products were collected for further investigation. The amount of oxides in the hybrids can be easily tuned by varying the concentrations of FeSO₄ in the electrolyte.

The XRD pattern (Fig. 1a) of the sample prepared with a FeSO₄ concentration (I_FeSO4) of 0.1 M can be indexed to the tetragonal Fe₂O₃ phase (JCPDS 25-1402). There is no crystalline impurity detected in the XRD pattern. The grain size of Fe₂O₃ is estimated to be 6.9 nm according to the Scherrer equation. The FESEM and TEM images (Fig. 1b and c) show that Fe₂O₃ nanoclusters with diameters of 30–40 nm are uniformly anchored onto the graphene surface. The high-magnification image (Fig. 1d) reveals that these Fe₂O₃ nanoclusters are composed of many crystallites with size in the range of 30–40 nm. Varying I_FeSO4 values to 0.03 and 0.15 M resulted in hybrid samples with similar phases and morphology (Fig. S2 and S3, ESI†). Only the tetragonal Fe₂O₃ phase was detected in the XRD patterns while the size of the Fe₂O₃ nanoclusters is in the range of 20–50 nm for these hybrid samples.

Fig. 1 (a) XRD pattern, (b) SEM image, (c) TEM and (d) high magnification TEM image of the Fe₂O₃/graphene hybrid with I_FeSO4 = 0.1 M.

The detailed structure of the graphene in the as-prepared samples was studied by HRTEM (Fig. 2a). The interlayer spacing of the graphene sheet with a trilayer structure is estimated to be ∼0.49 nm, which is consistent with the AFM results (Fig. 2b).

Fig. 2 HRTEM image and AFM image of the Fe₂O₃/graphene hybrid with I_FeSO4 = 0.1 M.

The quality of the as-prepared graphene sheets in the Fe₂O₃/graphene samples was examined by Raman spectroscopy (Fig. S4, ESI†). The intensity ratio I_2D:G of the 2D band (located at 2720 cm⁻¹) to the G band (located at 1580 cm⁻¹) was estimated to be ∼0.36 for the sample prepared with I_FeSO4 = 0.1 M. This I_2D:G value is higher than that of reduced graphene oxide (∼0.15).³⁸ The I_2D:G is proved to be relevant to the recovery for sp² C=C bonds in graphitic structures³⁹ and higher I_2D:G indicates fewer layers of graphene. In addition, the low intensity of the D band indicates the small amount of defects. As a comparison, the reduced graphene oxides normally show a D band with higher intensities than those of 2D and G bands.⁴⁰ Based on the results of the four-point probe measurements, the electrical conductivity of Fe₂O₃/graphene was calculated to be ∼250 S m⁻¹. Thermogravimetric analysis (TGA) of the Fe₂O₃/graphene hybrids (Fig. S5, ESI†) reveals that the weight percentages of Fe₂O₃ (C_Fe2O3), are 10.7, 29.0 and 52.1 wt% for samples prepared with I_FeSO4 = 0.03, 0.1 and 0.15 M, respectively. The Brunauer–Emmett–Teller (BET) analysis of the nitrogen absorption/desorption isotherm (Fig. S6, ESI†) suggests that the specific surface areas of pure graphene and Fe₂O₃/graphene electrodes with C_Fe2O3 = 10.7, 29.0 and 52.1 wt% are 218.1, 182.1, 145.2 and 165.7 m² g⁻¹, respectively.

The O₂ electrodes were fabricated by coating the Fe₂O₃/graphene hybrids on a pristine microfiber filter paper (MFFP). This type of filter paper is sponge-like (Fig. S7a, ESI†) with good wettability with most liquid electrolytes and high mechanical flexibility, which makes it suitable as a separator for LOBs. The Fe₂O₃/graphene-coated MFFP electrode remains robust and bendable (Fig. S7b, ESI†). The LOB configuration is similar to the commonly used coin cells of Li-ion batteries⁹ with some changes as shown in the scheme (Fig. S7c, ESI†). First, according to previous reports on LOBs,¹⁴ 1,2-dimethoxyethane (DME) was chosen as the electrolyte instead of using carbonate-based electrolytes (e.g. commonly used in Li ion batteries). The carbonate-based electrolytes are reported to be unstable in LOBs during discharge probably due to the presence of O₂ in the working environment. The DME remains stable without obvious decomposition during discharge due to its chemical stability in the presence of superoxide ions and high O₂ solubility.⁸ Secondly, LiClO₄ was chosen as the solute in the DME electrolyte instead of using LiPF₆ (e.g. commonly used in Li-ion batteries). LiPF₆ is highly sensitive to the ambient moisture while LiClO₄ is more stable while the average ion mobility and dissociation constant that determine the ionic conductivity of LiClO₄ is almost identical to that of LiPF₆.⁴¹ Additional information on the fabrication of the cells are provided as follows: (1) a stainless steel foil was used as the anode current collector with a lithium foil on top; (2) a MFFP was used as a separator and another MFFP with Fe₂O₃/graphene was placed on the topside; (3) a stainless steel mesh and a spring were adopted as a cathode current collector; (4) pure O₂ gas was purged through a channel on top of the spring.

The XRD patterns of the Fe₂O₃/graphene electrode (C_Fe2O3 = 29.0 wt%) and pure graphene electrode before and after discharge are shown in Fig. 3a and b. Results reveal that crystalline hexagonal Li₂O₂ (JCPDS 09-0355) forms during the discharge process. Due to the small amount of Fe₂O₃ in the electrode (∼0.5 mg cm⁻²), the peaks of Fe₂O₃ are not detectable in the XRD spectra. The observed peaks in the XRD pattern of the electrode before discharge are originated from the microfiber filter paper (MFFP). The grain size of Li₂O₂ was estimated to be 6.4 nm according to the full width at half maximum (FWHM) of the peaks using the Scherrer equation. Fe₂O₃/graphene electrodes with other C_Fe2O3 values show similar XRD patterns.

Fig. 3 The XRD patterns of (a) the Fe₂O₃/graphene electrode with C_Fe2O3 = 29.0 wt% and (b) pure graphene electrode before and after discharge. MFFP refers to microfiber filter paper.

The cathodic scan is carried out to study the catalytic properties of our samples (Fig. 4a). The Fe₂O₃/graphene electrodes with C_Fe2O3 = 10.7, 29.0 and 52.1 wt% exhibit oxygen reduction reaction (ORR) onset potentials >2.9 V, which is higher than for pure graphene (∼2.75 V). Moreover, the typical discharge/charge voltage profiles (Fig. 4b) reveal that the round-trip efficiency, defined as discharge voltage/charge voltage, of Fe₂O₃/graphene with C_Fe2O3 = 29.0 wt% is ∼65.9% while pure graphene exhibits only 57.5%. These results indicate much improved catalytic properties of Fe₂O₃/graphene electrodes.

Fig. 4 (a) Cathodic scan of Fe₂O₃/graphene and graphene. (b) Typical voltage profiles of Fe₂O₃/graphene (29.0%) and graphene at a current density of 200 mA g_carbon⁻¹.

The discharge voltage profiles to 2 V (vs. Li) of Fe₂O₃/graphene and pure graphene electrodes at a current density of 100 mA g⁻¹ were evaluated (Fig. 5). The pure graphene electrode depicts a discharge capacity of 5100 mA h g⁻¹ at a voltage of 2.5 V. However, the Fe₂O₃/graphene electrode with C_Fe2O3 = 10.7, 29.0 and 52.1 wt% are able to deliver much higher discharge capacities of 6420, 8290 and 7920 mA h g⁻¹ at a voltage of ∼2.7 V. The voltage increase of ∼0.2 V with the addition of Fe₂O₃, indicates the high electrocatalytic activities of Fe₂O₃, which is comparable to that of gold.⁸ The specific energies are estimated to be 12 750 W h kg⁻¹ for pure graphene electrode and 17 334, 22 383 and 21 384 W h kg⁻¹ for Fe₂O₃/graphene electrodes with C_Fe2O3 = 10.7, 29.0 and 52.1 wt%, respectively. The energy capacities are significantly higher than that of today's Li-ion batteries (∼387 W h kg⁻¹)⁹ even when calculated based on the total weight of the electrodes including graphene, Fe₂O₃ and discharge products (∼2600 W h kg⁻¹ for C_Fe2O3 = 29.0 wt%). The discharge capacity does not increase continuously with C_Fe2O3 because excessive amounts of Fe₂O₃ impede the electron transfer in the electrodes, which compromises the electrochemical performance. The much improved properties enable the Fe₂O₃/graphene composites to be promising O₂ electrodes for LOBs.

Fig. 5 Discharge voltage profiles of Fe₂O₃/graphene and pure graphene electrodes at 100 mA g_carbon⁻¹.

We have noticed that the discharge curves of our samples are different from previous reports,^3,29,42 which is probably due to the different measuring protocols and electrolytes. For example, hybrid electrolytes,³ carbonate-based electrolytes⁴² and tri(ethylene glycol) dimethyl ether-based electrolytes²⁹ as well as confined discharge/charge capacities³ may result in different voltage curves.

To further examine the electrochemical performance of our samples, we have modified the electrochemical measurement protocols. The voltage range was changed to 2.4–4.5 V and the discharge/charge capacities are confined to 1000 mA h g⁻¹ (Fig. 6). All the Fe₂O₃/graphene electrodes with C_Fe2O3 = 10.7, 29.0 and 52.1 wt% depict a stable discharge voltage at ∼2.7 V for 30 cycles. However, the discharge voltage of pure graphene is ∼2.55 V at the 1st cycle and gradually descends to <2.4 V at the 30th cycle.

Fig. 6 Voltage profiles of selected cycles at 200 mA g_carbon⁻¹ of Fe₂O₃/graphene with C_Fe2O3 = (a) 10.7 wt%, (b) 29.0 wt%, (c) 52.1 wt% and (d) graphene at 1000 mA h g_carbon⁻¹.

To study the electrochemical stability of Fe₂O₃ in the discharge process of LOBs, XPS was carried out on the pristine and discharged electrodes with C_Fe2O3 = 29.0 wt% (Fig. 7). Before measurements, the discharged electrode was immersed in deionized water to remove Li₂O₂ that was formed on the electrode. The Fe 2p_3/2 and Fe 2p_1/2 bands show peaks at 711.4 and 724.8 eV, respectively, with the satellite peak at ∼720 eV, which confirms the presence of Fe³⁺.^43,44 This indicates that Fe₂O₃ remains stable during the discharge process without change in the valence state.

Fig. 7 The XPS spectra of the pristine and discharged Fe₂O₃/graphene electrode with C_Fe2O3 = 29.0 wt%.

The superior performance of Fe₂O₃/graphene electrode can be explained in terms of two aspects as shown in the scheme in Fig. 8. First, the incorporation of graphene with high surface area favors the electron transport. Second, Fe-based catalysts can exhibit efficient electrocatalytic activities towards O₂ reduction as demonstrated in polymer electrolyte fuel cells ⁴⁵ and solid oxide fuel cells.⁴⁴ The iron oxide-based catalysts can easily adsorb O₂ on their surface by forming a bond between Fe and O and thereby weakens the bonding within the O₂ molecule, which leads to lower activation energy of O₂. Therefore, the electrochemical process during discharge can be described as follows (Fig. S8a and b, ESI†): the graphene facilitates the transport of charge electrons to the catalytic sites of Fe₂O₃, on which O₂⁻ is easily adsorbed. The O₂⁻ reacts with Li⁺ to form a layer of LiO₂ on top of the Fe₂O₃/graphene electrode. LiO₂ is then quickly converted to Li₂O₂via disproportionation and further reduction to generate a layer of Li₂O₂.⁹ Meanwhile, a new layer of LiO₂ forms between the Fe₂O₃/graphene electrode and the Li₂O₂ layer and quickly decomposes upon formation. Therefore, at the end of discharge, the electrode is coated with a layer of Li₂O₂ (Fig. S8b, ESI†). According to previous study,⁴⁶ the O₂ reduction process by lithium on discharge can be summarized as three steps:

O₂ + e⁻ → O₂⁻ (i)

Li⁺ + O₂⁻ → LiO₂ (ii)

2LiO₂ → Li₂O₂ + O₂ (iii)

or/and

LiO₂ + Li⁺ + e⁻ → Li₂O₂

Fig. 8 Proposed working mechanism of the Fe₂O₃/graphene electrode.

The working mechanism of catalysts in Li–air and Li–O₂ batteries has been bewildering on account of the buildup of insoluble discharge products. The model proposed here may provide a reasonable explanation and support for further investigation on the role of catalysts in Li–air and Li–O₂ batteries.

Conclusions

Fe₂O₃/graphene composites with different contents of Fe₂O₃ are prepared electrochemically and investigated as O₂ electrodes for Li–O₂ batteries. The Fe₂O₃/graphene electrode with C_Fe2O3 = 29.0 wt% exhibits a discharge capacity of 8290 mA h g⁻¹ and corresponds to a specific energy of 22 383 W h kg⁻¹ while a pure graphene electrode delivers only a discharge capacity of 5100 mA h g⁻¹ and a specific energy of 12 750 W h kg⁻¹. Moreover, at confined discharge/charge capacities of 1000 mA h g⁻¹, the Fe₂O₃/graphene electrodes exhibits a stable discharge voltage at ∼2.7 V for 30 cycles while pure graphene exhibits a 1st-cycle voltage at ∼2.55 V and 30th-cycle voltage at <2.4 V. The improved performance is ascribed to the fast electron transport provided by the graphene sheets and the high electrocatalytic activities for O₂ reduction provided by the Fe₂O₃.

Acknowledgements

The authors gratefully acknowledge, NRF2009EWT-CERP001-026 (Singapore), Singapore National Research Foundation under CREATE program: EMobility, A*STAR SERC grant 1021700144 and Singapore MPA 23/04.15.03 RDP 009/10/102 and MPA 23/04.15.03 RDP 020/10/113 grant.

References

1. C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2007, 3, 31–35.
2. G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson and W. Wilcke, J. Phys. Chem. Lett., 2010, 1, 2193–2203.
3. E. Yoo and H. S. Zhou, ACS Nano, 2011, 5, 3020–3026.
4. Y. G. Wang and H. S. Zhou, J. Power Sources, 2010, 195, 358–361.
5. Y. C. Lu, Z. C. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli and Y. Shao-Horn, J. Am. Chem. Soc., 2010, 132, 12170–12171.
6. T. Ogasawara, A. Débart, M. Holzapfel, P. Novák and P. G. Bruce, J. Am. Chem. Soc., 2006, 128, 1390–1393.
7. R. Black, S. H. Oh, J. H. Lee, T. Yim, B. Adams and L. F. Nazar, J. Am. Chem. Soc., 2012, 134, 2902–2905.
8. Y.-C. Lu, D. G. Kwabi, K. P. C. Yao, J. R. Harding, J. Zhou, L. Zuin and Y. Shao-Horn, Energy Environ. Sci., 2011, 4, 2999–3007.
9. P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J. M. Tarascon, Nat. Mater., 2011, 11, 19–29.
10. L. Jin, L. P. Xu, C. Morein, C. H. Chen, M. Lai, S. Dharmarathna, A. Dobley and S. L. Suib, Adv. Funct. Mater., 2010, 20, 3373–3382.
11. A. Debart, A. J. Paterson, J. Bao and P. G. Bruce, Angew. Chem., Int. Ed., 2008, 47, 4521–4524.
12. A. K. Thapa, K. Saimen and T. Ishihara, Electrochem. Solid-State Lett., 2010, 13, A165–A167.
13. G. Q. Zhang, J. P. Zheng, R. Liang, C. Zhang, B. Wang, M. Au, M. Hendrickson and E. J. Plichta, J. Electrochem. Soc., 2011, 158, A822–A827.
14. B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar and A. C. Luntz, J. Phys. Chem. Lett., 2011, 2, 1161–1166.
15. Z. S. Wu, W. C. Ren, D. W. Wang, F. Li, B. L. Liu and H. M. Cheng, ACS Nano, 2010, 4, 5835–5842.
16. N. Li, G. Liu, C. Zhen, F. Li, L. L. Zhang and H. M. Cheng, Adv. Funct. Mater., 2011, 21, 1717–1722.
17. Y. Shi, L. Wen, F. Li and H. M. Cheng, J. Power Sources, 2011, 196, 8610–8617.
18. S. P. Pang, Y. Hernandez, X. L. Feng and K. Mullen, Adv. Mater., 2011, 23, 2779–2795.
19. S. B. Yang, X. L. Feng and K. Mullen, Adv. Mater., 2011, 23, 3575–3579.
20. J. Zhu, K. Sun, D. Sim, C. Xu, H. H. Hng and Q. Yan, Chem. Commun., 2011, 47, 10383–10385.
21. X. Rui, J. Zhu, D. Sim, C. Xu, Y. Zeng, H. H. Hng, T. M. Lim and Q. Yan, Nanoscale, 2011, 3, 4752–4758.
22. N. Xiao, X. C. Dong, L. Song, D. Y. Liu, Y. Tay, S. X. Wu, L. J. Li, Y. Zhao, T. Yu, H. Zhang, W. Huang, H. H. Hng, P. M. Ajayan and Q. Y. Yan, ACS Nano, 2011, 5, 2749–2755.
23. S. Evers and L. F. Nazar, Chem. Commun., 2012, 48, 1233–1235.
24. H. Wang, H. S. Casalongue, Y. Liang and H. Dai, J. Am. Chem. Soc., 2010, 132, 7472–7477.
25. H. Wang, Y. Liang, T. Mirfakhrai, Z. Chen, H. Casalongue and H. Dai, Nano Res., 2011, 4, 729–736.
26. X. Xie, G. Yu, N. Liu, Z. Bao, C. S. Criddle and Y. Cui, Energy Environ. Sci., 2012, 5, 6862.
27. H. L. Wang, L. F. Cui, Y. A. Yang, H. S. Casalongue, J. T. Robinson, Y. Y. Liang, Y. Cui and H. J. Dai, J. Am. Chem. Soc., 2010, 132, 13978–13980.
28. A. K. Geim, Science, 2009, 324, 1530–1534.
29. J. Xiao, D. Mei, X. Li, W. Xu, D. Wang, G. L. Graff, W. D. Bennett, Z. Nie, L. V. Saraf, I. A. Aksay, J. Liu and J.-G. Zhang, Nano Lett., 2011, 11, 5071–5078.
30. J. Zhu, Z. Yin, H. T. Hai Li, C. L. Chow, H. Zhang, H. H. Hng, J. Ma and Q. Yan, Small, 2011, 7, 3163–3168.
31. A. Reina, X. T. Jia, J. Ho, D. Nezich, H. B. Son, V. Bulovic, M. S. Dresselhaus and J. Kong, Nano Lett., 2009, 9, 30–35.
32. L. Ci, L. Song, C. H. Jin, D. Jariwala, D. X. Wu, Y. J. Li, A. Srivastava, Z. F. Wang, K. Storr, L. Balicas, F. Liu and P. M. Ajayan, Nat. Mater., 2010, 9, 430–435.
33. Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei and H.-M. Cheng, Nat. Mater., 2011, 10, 424–428.
34. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669.
35. C. Y. Su, A. Y. Lu, Y. P. Xu, F. R. Chen, A. N. Khlobystov and L. J. Li, ACS Nano, 2011, 5, 2332–2339.
36. N. Liu, F. Luo, H. X. Wu, Y. H. Liu, C. Zhang and J. Chen, Adv. Funct. Mater., 2008, 18, 1518–1525.
37. Y. C. Lu, H. A. Gasteiger, M. C. Parent, V. Chiloyan and Y. Shao-Horn, Electrochem. Solid-State Lett., 2010, 13, A69–A72.
38. J. X. Zhu, T. Zhu, X. Z. Zhou, Y. Y. Zhang, X. W. Lou, X. D. Chen, H. Zhang, H. H. Hng and Q. Y. Yan, Nanoscale, 2011, 3, 1084–1089.
39. B. Krauss, T. Lohmann, D. H. Chae, M. Haluska, K. von Klitzing and J. H. Smet, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 79, 165428.
40. J. X. Zhu, Y. K. Sharma, Z. Y. Zeng, X. J. Zhang, M. Srinivasan, S. Mhaisalkar, H. Zhang, H. H. Hng and Q. Y. Yan, J. Phys. Chem. C, 2011, 115, 8400–8406.
41. K. Xu, Chem. Rev., 2004, 104, 4303–4418.
42. Y. Li, J. Wang, X. Li, D. Geng, R. Li and X. Sun, Chem. Commun., 2011, 47, 9438–9440.
43. T. Yamashita and P. Hayes, Appl. Surf. Sci., 2008, 254, 2441–2449.
44. W. Zhou, L. Ge, Z. G. Chen, F. Liang, H.-Y. Xu, J. Motuzas, A. Julbe and Z. Zhu, Chem. Mater., 2011, 23, 4193–4198.
45. M. Lefevre, E. Proietti, F. Jaouen and J. P. Dodelet, Science, 2009, 324, 71–74.
46. C. O. Laoire, S. Mukerjee, K. M. Abraham, E. J. Plichta and M. A. Hendrickson, J. Phys. Chem. C, 2009, 113, 20127–20134.

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Footnote

† Electronic supplementary information (ESI) available: Supplementary Raman and TGA results. See DOI: 10.1039/c2ra20757e/

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