An evaporation-induced tri-constituent assembly approach to fabricate an ordered mesoporous carbon/graphene aerogel for high-performance supercapacitors

Abstract

Ordered mesoporous carbon/graphene aerogel (OMC/GA) is prepared by an evaporation-induced tri-constituent assembly route. Owing to the hierarchically porous structures and high specific surface area, the resultant OMC/GA exhibits enhanced electrochemical performance as the electrode in electrochemical capacitors.

---

Owing to the high power density and excellent cycling stability, the electrochemical capacitor (EC), also known as supercapacitor or ultracapacitor, has become one of the most appealing energy storage devices for high power delivery or uptake.^1,2 It is generally believed that the energy storage mechanism of an EC is based on the formation of an electric double layer at the interface between the electrodes and the electrolyte. Therefore, much effort has been devoted for making high-performance EC electrodes with high surface area and good conductivity. In this respect, porous carbons, including activated carbons, ordered mesoporous carbons, and carbon aerogels, have been the most commonly employed electrode candidates for EC due to their high surface areas and structural stability. However, the electrical performance of these porous carbons still suffers from poor electron conductivity caused by defects in their carbonaceous framework.^3–5 In general, improvement in the graphitization degree of the carbon framework can largely enhance the conductivity of carbon materials, but can also dramatically reduce the surface area.⁶ To improve the capacitive performance of the porous carbon based EC electrode, it is urgent to resolve the conflict between surface area and graphitization degree.

As an important class of porous carbons, three-dimensional (3D) graphene-based frameworks (GFs), such as aerogels, foams, and sponges, possess continuously interconnected macroporous structures, low mass density, large surface area, and high electrical conductivity.⁷ More importantly, GFs can serve as monolithic substrates for the decoration of diversified species such as metals,⁸ metal oxides,^9–12 polymers,^13,14 silica,^15,16 and carbons.^17–25 Moreover, the resulting composites exhibit much improved electrochemical performance for applications in ECs, secondary batteries,^26,27 and electrocatalysis. The introduction of well-defined micro/mesopores to the macroporous scaffold of GFs will efficiently increase the active sites for charge storage and enhance the penetration of electrolyte. As a result, the hierarchical porous composite is expected to have improved capacitance and rate capability as compared to the EC electrode. In addition, the high mechanical strength of GFs can also improve the structural stability of the electrode materials.

Herein, we report a novel evaporation-induced tri-constituent assembly strategy to fabricate composites of ordered mesoporous carbon and graphene aerogel (OMC/GA). During the self-assembly process, GA is used as a macroporous substrate, resol as a carbon precursor, pre-hydrolyzed TEOS as an inorganic precursor, and tri-block copolymer F127 as a soft template.²⁸ A thermal treatment of the resulting composite followed by etching of the SiO₂ component led to monolithic OMC/GA possessing 3D interconnected macropores, ordered mesopores with the size of ∼5 nm, and a very high surface area of 715 m² g⁻¹. Serving as the electrode in the EC, OMC/GA shows outstanding specific capacitance (177 F g⁻¹ at 5 mV s⁻¹), high rate capability (116 F g⁻¹ at 100 mV s⁻¹), and excellent cycling stability (only 7% capacitance loss after 10 000 cycles), outperforming pristine GA, OMC and previously reported EC electrode materials based on GA,²⁰ OMC²⁹ and GAOMC.³⁰ Such excellent electrochemical performance can be attributed to a synergistic effect of highly conductive macroporous GA and mesoporous carbons with high surface area.

The synthetic procedure for fabricating OMC/GA is illustrated in Scheme 1. First, monolithic GA was produced by the hydrothermal assembly of graphene oxide (GO)³¹ in an aqueous suspension (1.5 mg ml⁻¹) followed by a freeze-drying process.^31,32 The resulting GA was immersed into an ethanolic solution containing triblock copolymer F127, resols and pre-hydrolyzed TEOS followed by volatilization at room temperature. Driven by capillary force,³³ the ethanolic solution could effectively impregnate in the macropore spaces of GA. The continuously interconnected macroporous architecture of GA provided an abundance of interfaces for the tri-constituent self-assembly of resol, F127 and oligomer silicates. As the ethanol evaporated, the concentration of the inorganic–organic-F127 constituents increased and the triblock copolymers with silica oligomers and resols gradually deposited on the surface of the GA as an ordered liquid-crystalline mesophase. After the evaporation process, the resulting composite of GA containing the self-assembled F127/resols/SiO₂ was thermally treated at 100 °C in air to solidify the resol polymers, and then calcinated at 900 °C for 2 h under N₂. Finally, the etching of the silica component in the calcinated product with 10 wt% HF could generate OMC/GA as a black monolith (Fig. S1a†). It is worth noting that SiO₂ derived from TEOS is essential for the preparation of OMC/GA. As an effective reinforcing component, the rigid SiO₂ in the composite can greatly inhibit framework shrinkage during the thermal treatment, yielding ordered mesoporous carbon–silica nanocomposites on the surface of the GA. In addition, small pores can be generated in the carbon walls by the etching of silica frameworks, which will effectively improve the surface area of the OMC/GA.

Scheme 1 The synthetic procedure for fabricating OMC/GA via the evaporation-induced tri-constituent assembly approach.

The morphology and microstructure of the OMC/GA composite were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. SEM images (Fig. 1a and S1b†) show that OMC/GA possesses a 3D interconnected macroporous framework with the pore diameters ranging from 3 to 8 μm. The similar macroporous structures of OMC/GA and GA indicate that the self-assembled mesophase of resol–F127–TEOS and the carbon layers converted from them have been successfully anchored on the surface of the GA without destroying the macroporous scaffold. The high resolution SEM image of OMC/GA (Fig. 1b) reveals that the graphene sheets in the composite are covered with mesoporous carbon layers. Remarkably, the TEM image (Fig. 1c) further verifies that the ordered arrays of mesopores with a narrow size distribution of ∼5 nm are homogeneously distributed on the surface of the graphene sheets.

Fig. 1 (a) and (b) SEM images, (c) TEM image and (d) XRD patterns of OMC/GA.

The small-angle XRD patterns (Fig. 1d) indicate that OMC/GA possesses a weak diffraction peak at 0.95°, corresponding to the d-spacing of 10.6 nm, which is similar to the pure OMC (Fig. S2†).²⁸ In the wide angle area, the major diffraction peak of OMC/GA around 26° is close to that of GA (Fig. S3a†), which can be indexed to the (002) diffraction plane of graphene.¹⁹ The similar graphitic frameworks of OMC/GA and GA can be further confirmed by their Raman spectra (Fig. S3b†), in which the two broad peaks around 1330 and 1580 cm⁻¹ are assigned to the D and G bands of carbon, respectively.^22,34,35 The intensity ratio of the D band to G band (I_D/I_G) of OMC/GA is calculated to be 1.47, which is close to that of GA (1.43).

To further investigate the porosity of OMC/GA, N₂ sorption isotherms of the samples were measured (Fig. 2). The adsorption–desorption curve of OMC/GA exhibits the prominent characteristic of type-IV isotherms with a distinct hysteresis loop of H₂ in the P/P₀ range of 0.4–0.9, implying the presence of mesopores in the composites.³⁶ Moreover, the pore sizes calculated by the DFT method are entered at 1.53, 2.84 and 5.10 nm with a narrow distribution. The mesopores with a diameter of 5.10 nm are in good agreement of the observation in the SEM and TEM images of OMC/GA. The small pores with diameters of 1.53 and 2.84 nm are supposed to be derived from the removal of silica from the carbon–silica framework in the calcined composite.^26,36,37 Brunauer–Emmett–Teller (BET) analysis reveals a specific surface area of 715 m² g⁻¹ and pore volume of 0.99 cm³ g⁻¹ for OMC/GA, which are considerably higher than those of GA (220 m² g⁻¹ and 0.45 cm³ g⁻¹), mesoporous graphene nanosheets^25,30 and mesoporous carbon/GA composites derived from a hard-template approach.³⁶

Fig. 2 (a) N₂ adsorption–desorption isotherm and (b) the pore size distribution of OMC/GA.

Given the unique features of OMC/GA, such as highly interconnected graphene framework, hierarchically porous structure and high surface area, we further evaluated its electrochemical performance as the electrode for an EC in a three-electrode system with 6.0 M KOH as the electrolyte. For comparison, pure GA and OMC were also tested under the same conditions. The cyclic voltammetry (CV) curves of OMC/GA and GA exhibit nearly symmetrical rectangular shapes and efficiently maintain them at scan rates up to 100 mV s⁻¹, indicative of good double-layer capacitive behaviors (Fig. 3a and S4a†), which can be further confirmed by their quasi triangular-shaped galvanostatic charge/discharge cycling curves (Fig. 3b and S4b†). In contrast, the CV profile of OMC manifests only a highly distorted rectangular shape at 100 mV s⁻¹ (Fig. 3c). The specific capacitance of OMC/GA is about 177 F g⁻¹ at a scan rate of 5 mV s⁻¹, much higher than that of the GA (∼99 F g⁻¹). Furthermore, OMC/GA displays a good capacitance retention at high scan rates and a specific capacitance of ∼115 F g⁻¹ can be achieved at 100 mV s⁻¹, which is better than those of GA (∼67 F g⁻¹) and OMC (∼102 F g⁻¹). Moreover, the OMC/GA based electrode delivers the best cycling stability among the three samples. Its specific capacitance remains at 94% of the initial value after 1000 consecutive cycles, while GA and OMC retain only 93% and 80% of their capacitance under the same conditions. With the charging rate at 100 mV s⁻¹, the electrode from OMC/GA still retains 93% of the initial capacitance (Fig. S5†) even after 10 000 consecutive cycles, which is better than the previously reported GAOMC.³⁰

Fig. 3 (a) CV curves of OMC/GA at different scan rates in 6 M KOH electrolyte; (b) galvanostatic charging/discharging curves of OMC/GA; (c) CV curves of GA, OMC and OMC/GA at 100 mV s⁻¹; (d) specific capacitance as a function of scan rates for GA, OMC and OMC/GA electrodes; (e) cycling stability of GA, OMC and OMC/GA. (f) Nyquist plots of the experimental impedance data for OMC/GA, GA and OMC.

These excellent electrochemical properties of OMC/GA can be attributed to the successful combination of OMC with ordered mesostructures and GA with highly conductive 3D framework. First, the OMC layer over the composite can provide a highly ionic accessible surface area for the penetration of electrolytes, and thus leads to the high capacitance for the storage of charges. Second, the synergistic effect of the opened mesoporous architecture and interconnected macroporous framework allows the fast transportation of charge carriers in the electrodes and efficiently enhances the conductivity of the composite accordingly. This can be verified by the electrochemical impedance spectra for GA, OMC and OMC/GA (Fig. 3f). All the samples display a depressed semicircle in the high frequency regions and a sloped straight line at low frequencies. Compared with GA and OMC, the shorter radius of OMC/GA in the high frequency range reflects the lower resistance to the mass transfer/diffusion rate of ions through the hierarchically porous architecture.³⁸ A more vertical straight line lying in the low frequency range is more evident for OMC/GA than for GA and OMC, which indicates the faster ion diffusion behavior of the sample.³⁰ Moreover, the outstanding cycling stability of OMC/GA would have been derived from the existence of the macroporous graphene scaffold with its good mechanical stability, which can efficiently tolerate the structural variation of the electrode during the charging/discharging process.^10,36,39

Conclusions

In summary, we have successfully fabricated a composite of OMC/GA with macroporous graphene frameworks enwrapped in ordered mesoporous carbon shells via an evaporation-induced tri-constituent co-assembly route. The obtained OMC/GA possesses hierarchically porous structures with both macro- and mesopores and a high specific surface area of 715 m² g⁻¹. As the electrode in EC, the composite shows enhanced electrochemical performance such as high specific capacitance, good rate capability, and excellent cycling stability. More importantly, the fabrication strategy provides a facile way to synthesize various hierarchically porous materials for a broad range of applications in ECs, secondary batteries, sensors, catalytic reactions, adsorbents, and fuel cells.

Acknowledgements

This work was financially supported by 973 Program of China (2013CB328804 and 2014CB239701), the National Natural Science Foundation of China (21343002, 61235007 and 21372155), Program for Professor of Special Appointment (Eastern Scholar) and Program for Innovative Research Team in University (no. IRT13078). The authors also thank Lab for Microstructure, Instrumental Analysis and Research Center, Shanghai University, for materials characterizations.

Notes and references

1. M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245–4269.
2. L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38, 2520–2531.
3. D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu and T. J. Bandosz, Adv. Funct. Mater., 2009, 19, 438–447.
4. M. F. El-Kady, V. Strong, S. Dubin and R. B. Kaner, Science, 2012, 335, 1326–1330.
5. L. Estevez, R. Dua, N. Bhandari, A. Ramanujapuram, P. Wang and E. P. Giannelis, Energy Environ. Sci., 2013, 6, 1785–1790.
6. L. Sun, C. G. Tian, L. Wang, J. L. Zou, G. Mu and H. G. Fu, J. Mater. Chem., 2011, 21, 7232–7239.
7. M. A. Worsley, P. J. Pauzauskie, T. Y. Olson, J. Biener, J. H. Satcher and T. F. Baumann, J. Am. Chem. Soc., 2010, 132, 14067–14069.
8. C. Chang, P. Gao, D. Bao, L. Q. Wang, Y. Wang, Y. J. Chen, X. M. Zhou, S. C. Sun, G. B. Li and P. P. Yang, J. Power Sources, 2014, 255, 318–324.
9. S. B. Yang, X. L. Feng and K. Mullen, Adv. Mater., 2011, 23, 3575–3579.
10. D. Wu, F. Zhang, H. Liang and X. Feng, Chem. Soc. Rev., 2012, 41, 6160–6177.
11. X. Li, W. Qi, D. Mei, M. L. Sushko, I. Aksay and J. Liu, Adv. Mater., 2012, 24, 5136–5141.
12. G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J. R. McDonough, X. Cui, Y. Cui and Z. Bao, Nano Lett., 2011, 11, 2905–2911.
13. Y. Zhao, J. Liu, Y. Hu, H. Cheng, C. Hu, C. Jiang, L. Jiang, A. Cao and L. Qu, Adv. Mater., 2013, 25, 591–595.
14. K. Zhang, L. L. Zhang, X. S. Zhao and J. Wu, Chem. Mater., 2010, 22, 1392–1401.
15. Z.-M. Wang, W. Wang, N. Coombs, N. Soheilnia and G. A. Ozin, ACS Nano, 2010, 4, 7437–7450.
16. S. B. Yang, X. L. Feng, L. Wang, K. Tang, J. Maier and K. Mullen, Angew. Chem., Int. Ed. Engl., 2010, 49, 4795–4799.
17. M. Li, J. Ding and J. Xue, J. Mater. Chem. A, 2013, 1, 7469–7476.
18. K. H. Kim, Y. Oh and M. F. Islam, Nat. Nanotechnol., 2012, 7, 562–566.
19. Y. Q. Sun, C. Li and G. Q. Shi, J. Mater. Chem., 2012, 22, 12810–12816.
20. X. Li, X. Zang, Z. Li, X. Li, P. Li, P. Sun, X. Lee, R. Zhang, Z. Huang, K. Wang, D. Wu, F. Kang and H. Zhu, Adv. Funct. Mater., 2013, 23, 4862–4869.
21. Z. S. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, X. Feng and K. Mullen, Adv Mater., 2012, 24, 5130–5135.
22. J. Liang, X. Du, C. Gibson, X. W. Du and S. Z. Qiao, Adv Mater., 2013, 25, 6226–6231.
23. S. B. Yang, L. J. Zhi, K. Tang, X. L. Feng, J. Maier and K. Mullen, Adv. Funct. Mater., 2012, 22, 3634–3640.
24. Y. Zheng, Y. Jiao, J. Chen, J. Liu, J. Liang, A. Du, W. M. Zhang, Z. H. Zhu, S. C. Smith, M. Jaroniec, G. Q. Lu and S. Z. Qiao, J. Am. Chem. Soc., 2011, 133, 20116–20119.
25. L. Wang, L. Sun, C. G. Tian, T. X. Tan, G. Mu, H. X. Zhang and H. G. Fu, RSC Adv., 2012, 2, 8359–8367.
26. X. Li, X. Meng, J. Liu, D. Geng, Y. Zhang, M. N. Banis, Y. Li, J. Yang, R. Li, X. Sun, M. Cai and M. W. Verbrugge, Adv. Funct. Mater., 2012, 22, 1647–1654.
27. Y. Zhao, X. Li, B. Yan, D. Li, S. Lawes and X. Sun, J. Power Sources, 2015, 274, 869–884.
28. R. L. Liu, Y. F. Shi, Y. Wan, Y. Meng, F. Q. Zhang, D. Gu, Z. X. Chen, B. Tu and D. Y. Zhao, J. Am. Chem. Soc., 2006, 128, 11652–11662.
29. D. Feng, Y. Lv, Z. Wu, Y. Dou, L. Han, Z. Sun, Y. Xia, G. Zheng and D. Zhao, J. Am. Chem. Soc., 2011, 133, 15148–15156.
30. F. Yang, M. W. Xu, S. J. Bao and H. Wei, RSC Adv., 2013, 3, 25317–25322.
31. Y. Y. Liang, D. Q. Wu, X. L. Feng and K. Mullen, Adv. Mater., 2009, 21, 1679–1683.
32. Y. X. Xu, K. X. Sheng, C. Li and G. Q. Shi, ACS Nano, 2010, 4, 4324–4330.
33. C. Xue, J. Wang, B. Tu and D. Zhao, Chem. Mater., 2010, 22, 494–503.
34. Y. P. Zhai, Y. Q. Dou, D. Y. Zhao, P. F. Fulvio, R. T. Mayes and S. Dai, Adv. Mater., 2011, 23, 4828–4850.
35. M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, L. G. Cancado, A. Jorio and R. Saito, Phys. Chem. Chem. Phys., 2007, 9, 1276–1291.
36. Z. S. Wu, Y. Sun, Y. Z. Tan, S. Yang, X. Feng and K. Mullen, J. Am. Chem. Soc., 2012, 134, 19532–19535.
37. C. Xue, J. Wang, B. Tu and D. Zhao, Adv. Funct. Mater., 2008, 18, 3914–3921.
38. W. Li, F. Zhang, Y. Q. Dou, Z. X. Wu, H. J. Liu, X. F. Qian, D. Gu, Y. Y. Xia, B. Tu and D. Y. Zhao, Adv. Energy Mater., 2011, 1, 382–386.
39. M. X. Wang, Q. Liu, H. F. Sun, E. A. Stach, H. Y. Zhang, L. Stanciu and J. Xie, Carbon, 2012, 50, 3845–3853.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13720e

---