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DNA-assisted assembly of carbon nanotubes and MnO2 nanospheres as electrodes for high-performance asymmetric supercapacitors† Chun Xian Guo, Amey Anil Chitre and Xianmao Lu* A DNA-assisted assembly approach is developed to fabricate a capacitor-type electrode material, DNAfunctionalized carbon nanotubes (CNTs@DNA), and a battery-type electrode material, DNA@CNTs-bridged MnO2 spheres (CNTs@DNA–MnO2), for asymmetric supercapacitors. An energy density of 11.6 W h kg
1
is
achieved at a power density of 185.5 W kg 1 with a high MnO2 mass loading of 4.2 mg cm 2. It is found that DNA assembly plays a critical role in the enhanced supercapacitor performance. This is because while DNA Received 1st November 2013, Accepted 6th December 2013 DOI: 10.1039/c3cp54911a
molecules functionalize carbon nanotubes (CNTs) via p–p stacking, their hydrophilic sugar-phosphate backbones also promote the dispersion of CNTs. The resultant CNTs@DNA chains can link multiple MnO2 spheres to form a networked architecture that facilitates charge transfer and effective MnO2 utilization. The improved performance of the asymmetric supercapacitors indicates that DNA-assisted assembly offers a
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promising approach to the fabrication of high-performance energy storage devices.
Introduction Supercapacitors have attracted great attention in recent years because they can provide a higher power density and longer cycle life than batteries.1–11 But supercapacitors suffer from low energy density.12–14 Based on the energy density E = 12CV2 for an electric energy storage device, E can be effectively improved by increasing the operation voltage (V). Supercapacitors with an asymmetric configuration, also called asymmetric supercapacitors, can provide relatively higher energy densities than symmetric ones.15–18 This is because an asymmetric supercapacitor consists of a battery-type Faradaic electrode and a capacitor-type electrode. Combined in an asymmetric configuration, these two types of electrodes in a supercapacitor would allow a high operation voltage for improved energy density without sacrificing much of its highpower performance.19,20 To date, various materials including transition metal oxides, metal hydroxides and conducting polymers have been investigated as Faradaic electrode materials for supercapacitors.21,22 Among them, MnO2 is considered as one of the most promising transition metal oxides for supercapacitors because of its relatively large theoretical capacitance, low cost, natural abundance and environmental friendliness.23–25 In principle, supercapacitors made from MnO2-based electrodes
Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore. E-mail: [email protected] † Electronic supplementary information (ESI) available: SEM images of MnO2 spheres, CNTs-MnO2, and CNTs@DNA-MnO2, and calculations for specific capacitance, energy density and power density. See DOI: 10.1039/c3cp54911a
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should provide a high specific capacitance with good rate capability.26 However, similar to many other metal oxides, MnO2 has a low electrical conductivity that limits its utilization and leads to poor performance in supercapacitors. To mitigate against this issue, nanocomposites consisting of both MnO2 and carbon materials such as carbon nanotubes (CNTs), graphene and active carbon have been investigated.27 Despite the fact that some performance enhancement has been achieved, typically these devices/electrodes have a relatively low MnO2 to carbon ratio and/or low mass loading of MnO2.28,29 On the other hand, practical applications of supercapacitors such as the case of hybrid electric vehicles require high mass loading of active materials.28 Unfortunately, an increase of MnO2 loading always causes poor electrical conductivity between the MnO2 and current collectors and also results in a dense morphology of electrodes, leading to reduced supercapacitor performance.30 To improve the utilization of MnO2 in supercapacitors with a high mass loading remains a great challenge. DNA has unique molecular recognition and self-assembly capabilities because of its complementary base pairs. It has been exploited by a growing number of researchers to build advanced materials for energy conversion/storage devices, electronic circuits, sensors and medical devices.31,32 For instance, using DNAs with different sequences and lengths, Alivisatos and co-workers have fabricated different silver nanostructures and quantum dots.33 Mirkin et al. have constructed well-organized nanoparticle superlattices through programmable DNA interactions.34 By designing tailored DNA molecules, Yan, Luo and others have built three-dimensional DNA nanostructures.35,36 In addition to assembling nanostructures based
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on the molecular recognition property, DNA has also been employed to functionalize nanostructures and mediate their growth.37,38 In particular, single-stranded DNA has been used to facilitate the dispersion and separation of bundled single-walled carbon nanotubes (CNTs) by forming p-stacking between DNA base pairs and the carbon rings of CNTs.39,40 Very recently, both single-stranded and double-stranded DNA molecules have been used to functionalize carbon nanomaterials and to guide the growth of uniform metal nanoparticles on the surface of the functionalized carbon materials as efficient catalysts for energy systems.41,42 Considering the unique properties of DNA derived from its base pairs as well as the abundant functional phosphate groups on its hydrophilic phosphate-sugar backbone, further explorations of using DNA in the synthesis and assembly of advanced nanostructured materials for a wide range of applications are expected. In this work, we employ a DNA-assisted assembly approach to rationally design and fabricate electrode materials for asymmetric supercapacitors with high mass loading of MnO2. As illustrated in Fig. 1, two types of electrode materials are prepared with this method: a capacitor-type electrode material, DNA-functionalized carbon nanotubes (CNTs@DNA), and a battery-type electrode material, MnO2 nanospheres bridged with DNA-functionalized carbon nanotubes (CNTs@DNA–MnO2). Firstly, single-stranded DNA with a length of B200 bp is used to functionalize CNTs via p–p stacking (non-covalent attractive interaction) between DNA base pairs and the CNT side walls. It is expected that the hydrophilic sugar-phosphate backbone of DNA in the CNTs@DNA can reduce the aggregation of CNTs and promote their dispersion in water. To fabricate CNTs@DNA–MnO2, MnO2 spheres self-assembled from MnO2 nanosheets are first synthesized via a redox reaction. The MnO2 spheres are then combined with CNTs@DNA to form a composite material. Phosphate groups on the DNA backbone in the CNTs@DNA can preferably bind to Mn sites on the surface of MnO2. Therefore, CNTs@DNA bridges MnO2 spheres to form a porous structure CNTs@DNA–MnO2. The DNA-assembled
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CNTs@DNA and CNTs@DNA–MnO2 as supercapacitor electrode materials could facilitate effective electrolyte transport, improve the accessibility of active sites, and promote charge transfer within and between MnO2 spheres.
Experimental section Preparation of DNA-functionalized carbon nanotubes 100 mg DNA (200 base pairs) was dissolved in 90 mL of distilled water under sonication for 2 min, and then 5 mL of 2.0 M sodium hydroxide solution was added. After incubation for 5 min, the DNA solution was neutralized by adding 5 mL of 2.0 M hydrochloric acid. Then 100 mg of carbon nanotubes (CNTs) was added into the above DNA solution and the mixture was kept in an ice-water bath and sonicated for 2 hours, followed by centrifuging and washing to obtain DNAfunctionalized CNTs (CNTs@DNA). Synthesis of CNTs@DNA–MnO2 MnO2 spheres were prepared via a redox reaction. 0.6 g KMnO4 was dissolved in 50 mL pH = 2 aqueous solution (0.005 M H2SO4 solution). Meanwhile, 0.4 g MnSO4H2O were dispersed in 50 mL pH = 2 aqueous solution. Then the KMnO4 solution was added to the MnSO4 solution drop by drop under sonication. The mixture continued to sonicate for 2 hours. The resulting suspension was filtered and washed with DI water. The produce was dried at 100 1C under vacuum. Then, MnO2 spheres were mixed with freshly prepared CNTs@DNA with a weight ratio of 8 : 1 in aqueous solution. The mixture was mildly sonicated for 1 hour, and filtrated and dried at 80 1C under vacuum. CNTs–MnO2 was prepared with the same procedures as the CNTs@DNA–MnO2 except for the use of CNTs instead of CNTs@DNA. Material characterizations The structure and morphology of materials were characterized using transmission electron microscopy (TEM, JEM-2100F) and field emission scanning electron microscopy (JSM-6700F). The compositions of the products were determined using X-ray Photoelectron Spectroscopy (XPS). Electrochemical characterizations
Fig. 1 DNA-assisted assembly of CNTs and MnO2 nanospheres for the fabrication of CNTs@DNA and CNTs@DNA–MnO2 supercapacitor electrode materials.
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For the 3-electrode configuration measurements, a saturated calomel electrode (SCE) and Pt foil (purchased from Metrohm) were used as the reference and counter electrode, respectively. 1.0 M Na2SO4 was used as the electrolyte. The working electrode was fabricated using a glassy carbon electrode (diameter of 4 mm2) coated with paste of the active electrode materials/Nafion. To prepare the asymmetric supercapacitors, active materials were mixed with PTFE with a weight ratio of 9 : 1 via grinding and then pressed onto nickel foil with a sandwich structure to form an electrode, which was dried at 80 1C under vacuum. The asymmetric supercapacitors were constructed using Swagelok-type cells and filtration paper as the separator. The electrolyte was 1.0 M Na2SO4. The mass ratio between the positive electrode and negative electrode was determined using the method reported previously.20
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The final mass loading of MnO2 in the asymmetric supercapacitors was around 4.2 mg cm 2. All electrochemical characterizations were measured using Autolab PGSTAT128N with the software of NOVA1.9. Impedance analysis was recorded under open circuit conditions using a frequency range from 106 Hz to 100 mHz.
Results and discussion Materials morphology and composition characterizations Due to strong van der Waals attraction, plain CNTs are highly aggregated to form bundles as shown in the TEM image in Fig. 2a. However, after functionalization with DNA, the resultant CNTs@DNA exhibited a relatively uniform network structure with reduced aggregation (Fig. 2b). In addition, the dispersity of the CNTs@DNA in water is much better than that of the plain CNTs (insets of Fig. 2a and b). These results indicate that the use of DNA leads to the formation of a uniform structure of CNTs and promotes the dispersion of CNTs in aqueous solution. These effects can be attributed to the hydrophilic sugar-phosphate backbone of DNA molecules that wrap on the surface of CNTs.39,43 The structure of the CNTs@DNA was also examined using high-magnification TEM (Fig. 2c), which shows the good dispersity of DNA@CNT. Elemental analysis of the CNTs@DNA was obtained using XPS. The XPS survey scan in Fig. 2d shows four peaks at 133.5, 284.0, 400.0, and 530.6 eV that are attributed to P 2p, C 1s, N 1s and O 1s, respectively.44 Since CNTs are free of P and N elements, the peaks of P 2p and N 1s should be from the DNA molecules in the CNTs@DNA. A high-resolution P 2p spectrum of the CNTs@DNA reveals two peaks at 133.2 and 134.3 eV, respectively, corresponding to the P 2p3/2 and P 2p1/2
Fig. 2 TEM images of (a) CNTs and (b) CNTs@DNA, and a highmagnification TEM image of (c) CNTs@DNA. The insets of (a) and (b) are aqueous dispersions of CNTs and CNTs@DNA (1.0 mg ml 1), respectively. (d) XPS survey scan and (e) P 2p spectrum of CNTs@DNA.
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bands of phosphate groups in DNA. The XPS results further confirm that DNA molecules have successfully functionalized the CNTs. To prepare the CNTs@DNA–MnO2, MnO2 spheres were first synthesized based on the redox reaction between MnSO4 and KMnO4 (3Mn2+ + 2MnO4 + 2H2O - 5MnO2 + 4H+). The detailed procedure can be found in the Experimental Section. Fig. 3a shows that MnO2 spheres with an average diameter of B300 nm are self-assembled from the MnO2 nanosheets. When directly mixing these MnO2 spheres with CNTs without DNA functionalization, the resultant sample of CNTs–MnO2 shows that CNTs and MnO2 spheres remain separated from each other instead of forming a uniform mixture (Fig. 3b). In comparison, if the MnO2 spheres are mixed with CNTs@DNA, the CNTs@DNA and MnO2 spheres interact well with each other, forming a unique structure as shown in Fig. 3c. In this structure, the CNTs@DNA acts like chains to bridge the MnO2 spheres. Each individual MnO2 sphere has a few CNTs@DNA chains anchored on its surface (Fig. 3d), while each CNT@DNA chain also links to a number of MnO2 spheres (Fig. 3e). It is known that functional groups such as phosphate groups preferably bind to metal sites on metal and metal oxide surfaces.45–47 In the present case, the Mn sites on MnO2 surface should provide anchoring points for the CNTs@DNA to form bridges among the MnO2 spheres. These CNTs@DNA bridges should be useful in facilitating electron transfer not only within a MnO2 sphere but also among all linked MnO2 spheres when
Fig. 3 SEM images of (a) MnO2 spheres, (b) CNTs–MnO2, and (c) CNTs@DNA–MnO2. (d, e) High-magnification SEM images of CNTs@DNA– MnO2 showing the interfaces between CNTs@DNA and MnO2 spheres. (f) EDS spectrum of CNTs@DNA–MnO2. For both CNTs–MnO2 and CNTs@DNA–MnO2, the weight percentage of MnO2 is around 88.9 wt%.
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used as electrode materials. In addition, compared with the CNTs–MnO2 formed by mixing MnO2 spheres with plain CNTs, the use of DNA resulted in CNTs@DNA–MnO2 with a more porous structure (Fig. 3c), which in turn may provide larger surface areas and more effective electrolyte transport.28
Electrochemical measurements The electrochemical properties of the CNTs and DNA@CNTs were first investigated in a three-electrode configuration. As shown in Fig. 4a, the cyclic voltammogram (CV) of CNTs@DNA in 1.0 M Na2SO4 exhibits a typical rectangular shape at a scan rate of 100 mV s 1. This rectangular shape is well maintained at scan rates in the range from 2 to 500 mV s 1 (Fig. 4b), indicating an excellent capacitive behavior and fast diffusion of electrolyte.1 The capacitive behavior of the CNTs@DNA electrode was further confirmed from nearly linear galvanostatic charge–discharge curves obtained at different current densities from 1 to 5 A g 1 (Fig. 4c). Based on the charge–discharge curves, the IR drop was also obtained. Generally, the IR drop for an electrode results from electrolyte potential drop and internal charge transfer resistance.20 For the CNTs@DNA electrode, relatively small IR drops of 3.5, 6.5 and 11.6 mV were measured at 1, 2 and 5 A g 1, respectively, indicating a low internal charge transfer resistance. Specific capacitances were calculated from the discharge curves of CNTs and [email protected] At current densities of 1, 2 and 5 A g 1, the specific capacitances of the CNTs@DNA electrode are 124.6, 102.3 and 88.1 F g 1, respectively (Fig. 4d). While for the CNT electrode at the same current densities, much lower specific capacitances of 67.9, 55.8 and 29.2 F g 1 are obtained. Compared to CNTs, CNTs@DNA electrode exhibits much higher specific capacitances at all current densities.
Fig. 4 (a) CVs of CNTs and CNTs@DNA electrodes in a three-electrode configuration (scan rate of 100 mV s 1, 1.0 M Na2SO4). (b) CVs of CNTs@DNA electrode with different scan rates (2 to 500 mV s 1). (c) Galvanostatic charge–discharge curves of CNTs@DNA electrodes at current densities of 1, 2 and 5 A g 1, and CNTs electrode at a current density of 1 A g 1 in a three-electrode configuration. (d) Comparison of specific capacitances for the CNTs and CNTs@DNA electrodes.
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The excellent electrochemical behavior of CNTs@DNA can be attributed to its networked structure with a reduced aggregation of CNTs as shown in Fig. 2b and c. Fig. 5a shows the CVs of MnO2, CNTs–MnO2 and CNTs@DNA–MnO2 electrodes in 1.0 M Na2SO4 at a scan rate of 100 mV s 1 in a three-electrode configuration. For MnO2, the charge storage mechanism is based on the Faradaic reaction with Na+ both on the surface and within the bulk of MnO2 via Na+ + MnO2 + e 2 MnOONa.26 As shown in Fig. 5a, the CNTs–MnO2 electrode exhibits a better capacitive behavior than that of MnO2, indicating that the addition of CNTs improves the utilization of MnO2. More importantly, compared with both MnO2 and CNTs–MnO2, the CNTs@DNA–MnO2 electrode shows much larger current densities over the whole potential window at the same scan rate, suggesting a higher specific capacitance. Furthermore, the more rectangular shape of the CV for CNTs@DNA–MnO2 indicates faster diffusion of the electrolyte to the electrode. Fig. 5b summarizes the specific capacitances calculated from CVs at different scan rates.9 At 100 mV s 1, CNTs@DNA–MnO2 provides a specific capacitance of 175 F g 1, which is 1.5 and 5.3 times higher than that of CNTs–MnO2 (116 F g 1) and MnO2 (33 F g 1), respectively. The CNTs@DNA–MnO2 electrode was further evaluated with galvanostatic charge–discharge measurements at current densities of 1, 2 and 5 A g 1; the corresponding specific capacitances are 265, 242 and 228 F g 1, respectively. A cycling test of the CNTs@DNA–MnO2 electrode was also performed (Fig. 5d). It is found that after 1000 cycles at 1 A g 1, 92.8% of the initial capacitance of CNTs@DNA–MnO2 can be retained. The improved electrochemical behavior of CNTs@DNA–MnO2 can be attributed to the use of DNA to assist the assembly of the
Fig. 5 (a) CVs of MnO2, CNTs@DNA and CNTs@DNA–MnO2 electrodes in a three-electrode configuration (scan rate of 100 mV s 1, 1.0 M Na2SO4). (b) Specific capacitances calculated from CVs for the three electrodes at scan rates ranging from 2 to 250 mV s 1. (c) Galvanostatic charge– discharge curves of CNTs@DNA–MnO2 electrode with current density of 1, 2 and 5 A g 1 in a three-electrode configuration. (d) Cycling test of the CNTs@DNA–MnO2 electrode in 1.0 M Na2SO4 at a charge–discharge current density of 1 A g 1.
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CNTs and MnO2 spheres. The resultant porous structure enables effective electrolyte transport and facile access to the active sites of the electrode material. In addition, highly dispersed CNTs anchored on the MnO2 surface facilitate both intra- and inter-particle electron transfer for the MnO2 spheres. Performance evaluation of the asymmetric supercapacitors Asymmetric supercapacitors were fabricated with CNTs@DNA– MnO2 as the battery-type positive electrode and CNTs@DNA as the capacitor-type negative electrode (denoted as CNTs@DNA– MnO2//CNTs@DNA) and 1.0 M Na2SO4 as the electrolyte. Fig. 6a shows the CVs of a CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitor at different voltage windows. It is found that a good capacitive behavior with quasi-rectangular CVs can be attained at voltages up to 1.50 V. Thus, an operation voltage window of 1.50 V was chosen to investigate the electrochemical performance of the asymmetric supercapacitors. For comparison, asymmetric supercapacitors without DNA assembly were also fabricated using CNTs–MnO2 and CNTs as the positive and negative electrodes, respectively (denoted as CNTs–MnO2//CNTs). Based on the CVs shown in Fig. 6b, the specific capacitance of the CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitor is 74.2 F g 1, 60% higher than that of the CNTs–MnO2//CNTs asymmetric supercapacitor (45.8 F g 1) under the same measurement conditions, indicating that the use of DNA improves the specific capacitance considerably. Fig. 7a shows galvanostatic charge–discharge curves of the CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitor at different current densities ranging from 0.5 to 25 A g 1. These curves have a nearly symmetric triangular shape, suggesting a
Fig. 6 (a) CVs of asymmetric supercapacitors with CNTs@DNA as the negative and CNTs@DNA–MnO2 as the positive electrode. (b) CVs of asymmetric supercapacitor devices with and without DNA assembly measured with the same condition at a scan rate of 100 mV s 1. For all measurements, the electrolyte is 1.0 M Na2SO4 solution and the scan rate is 100 mV s 1.
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Fig. 7 Galvanostatic charge–discharge curves for asymmetric supercapacitors (a) with DNA assembly and (b) without DNA assembly at current densities of 0.5, 1, 2, 5, 10 and 25 A g 1. (c) Cycling test for the asymmetric supercapacitors at a charge–discharge current density of 2 A g 1. (d) Nyquist plots for the asymmetric supercapacitors.
good Coulombic efficiency and electrochemical reversibility. Galvanostatic charge–discharge curves of the CNTs–MnO2// CNTs asymmetric supercapacitor at current densities ranging from 0.5 to 25 A g 1 are given in Fig. 7b. As seen from Fig. 7a and b, the specific capacitance of the CNTs@DNA–MnO2// CNTs@DNA asymmetric supercapacitor is higher than that of the CNTs–MnO2//CNTs asymmetric supercapacitor at all current densities. A cycling test was performed for both asymmetric supercapacitors at a current density of 2 A g 1 and the specific capacitances were calculated from the discharge curves (Fig. 7c). After 10 000 cycles, the capacitance retentions for the CNTs@DNA–MnO2//CNTs@DNA and CNTs–MnO2//CNTs asymmetric supercapacitors are 91.6% and 83.5%, respectively, suggesting an improved cycling stability of the asymmetric supercapacitors fabricated with the assistance of DNA assembly. The equivalent series resistances (ESRs) of the supercapacitors were obtained from impedance measurements as shown in Fig. 7d. For CNTs–MnO2//CNTs and CNTs@DNA–MnO2// CNTs@DNA asymmetric supercapacitors, the ESRs are 1.15 and 1.30 O, respectively. The lower ESR of the CNTs@DNA–MnO2// CNTs@DNA asymmetric supercapacitor indicates that the use of the DNA assembly improves charge transfer of the supercapacitor devices. Energy density (E) and power density (P) are important parameters to evaluate the performance of an electrochemical energy storage device. Fig. 8 shows the Ragone plots of the CNTs–MnO2//CNTs and CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitors. The specific energy density and power density of the asymmetric supercapacitors were calculated from the charge–discharge curves at different current densities using E = 12CspDV 2 and P = E/Dt where Csp is the specific capacitance, DV is the voltage window of the discharge process, and Dt is the discharge time of the asymmetric supercapacitor devices. For both devices, the specific energy density generally
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positive electrode and CNTs@DNA negative electrode show an energy density of 11.6 and 5.8 W h kg 1 at a power density of 185.5 W kg 1 and 4.5 kW kg 1, respectively. In addition, it has a capacitance retention of 91.6% after 10 000 charge– discharge cycles. The performance is superior to that of CNTs–MnO2//CNTs asymmetric supercapacitors without the use of DNA (7.3 W h kg [email protected] W kg 1 and 2.9 W h kg [email protected] kW kg 1, and 83.5% capacitance retention after 10 000 cycles). Considering the high energy and power densities of the asymmetric supercapacitors, and in particular, the high mass loading of the active material (4.2 vs. o0.5 mg cm 2), the DNA-assisted assembly can offer a promising approach to fabricate high-performance energy storage/conversion devices including supercapacitors, lithium ion batteries and solar cells. Fig. 8 Ragone plots for the asymmetric supercapacitors.
increases with the decrease of specific power density. The CNTs– MnO2//CNTs asymmetric supercapacitor without DNA assembly provides specific energy densities of 2.9 and 7.3 W h kg 1 at power densities of 4.5 kW kg 1 and 185.5 W kg 1, respectively. While for the CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitor, the corresponding specific energy densities are 5.8 and 11.6 W h kg 1, respectively, nearly double the energy densities of CNTs–MnO2//CNTs. Moreover, the maximum energy density (11.6 W h kg 1) of the CNTs@DNA–MnO2// CNTs@DNA asymmetric supercapacitor is higher than that ({10 W h kg 1) of symmetric supercapacitors made of activated carbon,48 CNTs49 and graphene.1 It is also higher than previously reported MnO2-based asymmetric supercapacitors including MnO2/active carbon (10.4 W h kg 1).50 Although the energy and power densities are not the highest among reported values,53–57 it should be noted that this performance is achieved with a mass loading of MnO2 at 4.2 mg cm 2, higher than those reported MnO2-based supercapacitors where the mass loadings of MnO2 are typically less than 0.5 mg cm 2.28,30,51–59 These results demonstrate that DNA-assisted assembly approach improves the performance of MnO2/CNT asymmetric supercapacitors.
Conclusion In summary, we have developed a DNA-assisted assembly approach to fabricate a capacitor-type electrode material of CNTs@DNA and a battery-type electrode material of CNTs@DNA–MnO2. Material characterizations show that DNA functionalization improves the dispersion of CNTs in aqueous solution. The use of DNA also promotes the interaction between CNTs@DNA and MnO2 spheres and leads to the formation of a unique network structure. Within this structure, CNTs@DNA is anchored on the MnO2 surface and acts like chains to bridge the MnO2 spheres. The resultant network structure consisting of CNTs@DNA and MnO2 facilitates both inter- and intra-particle charge transfers. In addition, compared to CNTs–MnO2, CNTs@DNA–MnO2 has a more porous structure, which is useful in promoting electrolyte transport. Asymmetric supercapacitors with CNTs@DNA–MnO2
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Acknowledgements We are grateful for the financial support of Ministry of Education, Singapore (Grant# R279-000-391-112) and National Research Foundation CRP program (Grant# R279-000-337-281).
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DNA-assisted assembly of carbon nanotubes and MnO2 nanospheres as electrodes for high-performance asymmetric supercapacitors† Chun Xian Guo, Amey Anil Chitre and Xianmao Lu* A DNA-assisted assembly approach is developed to fabricate a capacitor-type electrode material, DNAfunctionalized carbon nanotubes (CNTs@DNA), and a battery-type electrode material, DNA@CNTs-bridged MnO2 spheres (CNTs@DNA–MnO2), for asymmetric supercapacitors. An energy density of 11.6 W h kg
1
is
achieved at a power density of 185.5 W kg 1 with a high MnO2 mass loading of 4.2 mg cm 2. It is found that DNA assembly plays a critical role in the enhanced supercapacitor performance. This is because while DNA Received 1st November 2013, Accepted 6th December 2013 DOI: 10.1039/c3cp54911a
molecules functionalize carbon nanotubes (CNTs) via p–p stacking, their hydrophilic sugar-phosphate backbones also promote the dispersion of CNTs. The resultant CNTs@DNA chains can link multiple MnO2 spheres to form a networked architecture that facilitates charge transfer and effective MnO2 utilization. The improved performance of the asymmetric supercapacitors indicates that DNA-assisted assembly offers a
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promising approach to the fabrication of high-performance energy storage devices.
Introduction Supercapacitors have attracted great attention in recent years because they can provide a higher power density and longer cycle life than batteries.1–11 But supercapacitors suffer from low energy density.12–14 Based on the energy density E = 12CV2 for an electric energy storage device, E can be effectively improved by increasing the operation voltage (V). Supercapacitors with an asymmetric configuration, also called asymmetric supercapacitors, can provide relatively higher energy densities than symmetric ones.15–18 This is because an asymmetric supercapacitor consists of a battery-type Faradaic electrode and a capacitor-type electrode. Combined in an asymmetric configuration, these two types of electrodes in a supercapacitor would allow a high operation voltage for improved energy density without sacrificing much of its highpower performance.19,20 To date, various materials including transition metal oxides, metal hydroxides and conducting polymers have been investigated as Faradaic electrode materials for supercapacitors.21,22 Among them, MnO2 is considered as one of the most promising transition metal oxides for supercapacitors because of its relatively large theoretical capacitance, low cost, natural abundance and environmental friendliness.23–25 In principle, supercapacitors made from MnO2-based electrodes
Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore. E-mail: [email protected] † Electronic supplementary information (ESI) available: SEM images of MnO2 spheres, CNTs-MnO2, and CNTs@DNA-MnO2, and calculations for specific capacitance, energy density and power density. See DOI: 10.1039/c3cp54911a
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should provide a high specific capacitance with good rate capability.26 However, similar to many other metal oxides, MnO2 has a low electrical conductivity that limits its utilization and leads to poor performance in supercapacitors. To mitigate against this issue, nanocomposites consisting of both MnO2 and carbon materials such as carbon nanotubes (CNTs), graphene and active carbon have been investigated.27 Despite the fact that some performance enhancement has been achieved, typically these devices/electrodes have a relatively low MnO2 to carbon ratio and/or low mass loading of MnO2.28,29 On the other hand, practical applications of supercapacitors such as the case of hybrid electric vehicles require high mass loading of active materials.28 Unfortunately, an increase of MnO2 loading always causes poor electrical conductivity between the MnO2 and current collectors and also results in a dense morphology of electrodes, leading to reduced supercapacitor performance.30 To improve the utilization of MnO2 in supercapacitors with a high mass loading remains a great challenge. DNA has unique molecular recognition and self-assembly capabilities because of its complementary base pairs. It has been exploited by a growing number of researchers to build advanced materials for energy conversion/storage devices, electronic circuits, sensors and medical devices.31,32 For instance, using DNAs with different sequences and lengths, Alivisatos and co-workers have fabricated different silver nanostructures and quantum dots.33 Mirkin et al. have constructed well-organized nanoparticle superlattices through programmable DNA interactions.34 By designing tailored DNA molecules, Yan, Luo and others have built three-dimensional DNA nanostructures.35,36 In addition to assembling nanostructures based
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on the molecular recognition property, DNA has also been employed to functionalize nanostructures and mediate their growth.37,38 In particular, single-stranded DNA has been used to facilitate the dispersion and separation of bundled single-walled carbon nanotubes (CNTs) by forming p-stacking between DNA base pairs and the carbon rings of CNTs.39,40 Very recently, both single-stranded and double-stranded DNA molecules have been used to functionalize carbon nanomaterials and to guide the growth of uniform metal nanoparticles on the surface of the functionalized carbon materials as efficient catalysts for energy systems.41,42 Considering the unique properties of DNA derived from its base pairs as well as the abundant functional phosphate groups on its hydrophilic phosphate-sugar backbone, further explorations of using DNA in the synthesis and assembly of advanced nanostructured materials for a wide range of applications are expected. In this work, we employ a DNA-assisted assembly approach to rationally design and fabricate electrode materials for asymmetric supercapacitors with high mass loading of MnO2. As illustrated in Fig. 1, two types of electrode materials are prepared with this method: a capacitor-type electrode material, DNA-functionalized carbon nanotubes (CNTs@DNA), and a battery-type electrode material, MnO2 nanospheres bridged with DNA-functionalized carbon nanotubes (CNTs@DNA–MnO2). Firstly, single-stranded DNA with a length of B200 bp is used to functionalize CNTs via p–p stacking (non-covalent attractive interaction) between DNA base pairs and the CNT side walls. It is expected that the hydrophilic sugar-phosphate backbone of DNA in the CNTs@DNA can reduce the aggregation of CNTs and promote their dispersion in water. To fabricate CNTs@DNA–MnO2, MnO2 spheres self-assembled from MnO2 nanosheets are first synthesized via a redox reaction. The MnO2 spheres are then combined with CNTs@DNA to form a composite material. Phosphate groups on the DNA backbone in the CNTs@DNA can preferably bind to Mn sites on the surface of MnO2. Therefore, CNTs@DNA bridges MnO2 spheres to form a porous structure CNTs@DNA–MnO2. The DNA-assembled
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CNTs@DNA and CNTs@DNA–MnO2 as supercapacitor electrode materials could facilitate effective electrolyte transport, improve the accessibility of active sites, and promote charge transfer within and between MnO2 spheres.
Experimental section Preparation of DNA-functionalized carbon nanotubes 100 mg DNA (200 base pairs) was dissolved in 90 mL of distilled water under sonication for 2 min, and then 5 mL of 2.0 M sodium hydroxide solution was added. After incubation for 5 min, the DNA solution was neutralized by adding 5 mL of 2.0 M hydrochloric acid. Then 100 mg of carbon nanotubes (CNTs) was added into the above DNA solution and the mixture was kept in an ice-water bath and sonicated for 2 hours, followed by centrifuging and washing to obtain DNAfunctionalized CNTs (CNTs@DNA). Synthesis of CNTs@DNA–MnO2 MnO2 spheres were prepared via a redox reaction. 0.6 g KMnO4 was dissolved in 50 mL pH = 2 aqueous solution (0.005 M H2SO4 solution). Meanwhile, 0.4 g MnSO4H2O were dispersed in 50 mL pH = 2 aqueous solution. Then the KMnO4 solution was added to the MnSO4 solution drop by drop under sonication. The mixture continued to sonicate for 2 hours. The resulting suspension was filtered and washed with DI water. The produce was dried at 100 1C under vacuum. Then, MnO2 spheres were mixed with freshly prepared CNTs@DNA with a weight ratio of 8 : 1 in aqueous solution. The mixture was mildly sonicated for 1 hour, and filtrated and dried at 80 1C under vacuum. CNTs–MnO2 was prepared with the same procedures as the CNTs@DNA–MnO2 except for the use of CNTs instead of CNTs@DNA. Material characterizations The structure and morphology of materials were characterized using transmission electron microscopy (TEM, JEM-2100F) and field emission scanning electron microscopy (JSM-6700F). The compositions of the products were determined using X-ray Photoelectron Spectroscopy (XPS). Electrochemical characterizations
Fig. 1 DNA-assisted assembly of CNTs and MnO2 nanospheres for the fabrication of CNTs@DNA and CNTs@DNA–MnO2 supercapacitor electrode materials.
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For the 3-electrode configuration measurements, a saturated calomel electrode (SCE) and Pt foil (purchased from Metrohm) were used as the reference and counter electrode, respectively. 1.0 M Na2SO4 was used as the electrolyte. The working electrode was fabricated using a glassy carbon electrode (diameter of 4 mm2) coated with paste of the active electrode materials/Nafion. To prepare the asymmetric supercapacitors, active materials were mixed with PTFE with a weight ratio of 9 : 1 via grinding and then pressed onto nickel foil with a sandwich structure to form an electrode, which was dried at 80 1C under vacuum. The asymmetric supercapacitors were constructed using Swagelok-type cells and filtration paper as the separator. The electrolyte was 1.0 M Na2SO4. The mass ratio between the positive electrode and negative electrode was determined using the method reported previously.20
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The final mass loading of MnO2 in the asymmetric supercapacitors was around 4.2 mg cm 2. All electrochemical characterizations were measured using Autolab PGSTAT128N with the software of NOVA1.9. Impedance analysis was recorded under open circuit conditions using a frequency range from 106 Hz to 100 mHz.
Results and discussion Materials morphology and composition characterizations Due to strong van der Waals attraction, plain CNTs are highly aggregated to form bundles as shown in the TEM image in Fig. 2a. However, after functionalization with DNA, the resultant CNTs@DNA exhibited a relatively uniform network structure with reduced aggregation (Fig. 2b). In addition, the dispersity of the CNTs@DNA in water is much better than that of the plain CNTs (insets of Fig. 2a and b). These results indicate that the use of DNA leads to the formation of a uniform structure of CNTs and promotes the dispersion of CNTs in aqueous solution. These effects can be attributed to the hydrophilic sugar-phosphate backbone of DNA molecules that wrap on the surface of CNTs.39,43 The structure of the CNTs@DNA was also examined using high-magnification TEM (Fig. 2c), which shows the good dispersity of DNA@CNT. Elemental analysis of the CNTs@DNA was obtained using XPS. The XPS survey scan in Fig. 2d shows four peaks at 133.5, 284.0, 400.0, and 530.6 eV that are attributed to P 2p, C 1s, N 1s and O 1s, respectively.44 Since CNTs are free of P and N elements, the peaks of P 2p and N 1s should be from the DNA molecules in the CNTs@DNA. A high-resolution P 2p spectrum of the CNTs@DNA reveals two peaks at 133.2 and 134.3 eV, respectively, corresponding to the P 2p3/2 and P 2p1/2
Fig. 2 TEM images of (a) CNTs and (b) CNTs@DNA, and a highmagnification TEM image of (c) CNTs@DNA. The insets of (a) and (b) are aqueous dispersions of CNTs and CNTs@DNA (1.0 mg ml 1), respectively. (d) XPS survey scan and (e) P 2p spectrum of CNTs@DNA.
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bands of phosphate groups in DNA. The XPS results further confirm that DNA molecules have successfully functionalized the CNTs. To prepare the CNTs@DNA–MnO2, MnO2 spheres were first synthesized based on the redox reaction between MnSO4 and KMnO4 (3Mn2+ + 2MnO4 + 2H2O - 5MnO2 + 4H+). The detailed procedure can be found in the Experimental Section. Fig. 3a shows that MnO2 spheres with an average diameter of B300 nm are self-assembled from the MnO2 nanosheets. When directly mixing these MnO2 spheres with CNTs without DNA functionalization, the resultant sample of CNTs–MnO2 shows that CNTs and MnO2 spheres remain separated from each other instead of forming a uniform mixture (Fig. 3b). In comparison, if the MnO2 spheres are mixed with CNTs@DNA, the CNTs@DNA and MnO2 spheres interact well with each other, forming a unique structure as shown in Fig. 3c. In this structure, the CNTs@DNA acts like chains to bridge the MnO2 spheres. Each individual MnO2 sphere has a few CNTs@DNA chains anchored on its surface (Fig. 3d), while each CNT@DNA chain also links to a number of MnO2 spheres (Fig. 3e). It is known that functional groups such as phosphate groups preferably bind to metal sites on metal and metal oxide surfaces.45–47 In the present case, the Mn sites on MnO2 surface should provide anchoring points for the CNTs@DNA to form bridges among the MnO2 spheres. These CNTs@DNA bridges should be useful in facilitating electron transfer not only within a MnO2 sphere but also among all linked MnO2 spheres when
Fig. 3 SEM images of (a) MnO2 spheres, (b) CNTs–MnO2, and (c) CNTs@DNA–MnO2. (d, e) High-magnification SEM images of CNTs@DNA– MnO2 showing the interfaces between CNTs@DNA and MnO2 spheres. (f) EDS spectrum of CNTs@DNA–MnO2. For both CNTs–MnO2 and CNTs@DNA–MnO2, the weight percentage of MnO2 is around 88.9 wt%.
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used as electrode materials. In addition, compared with the CNTs–MnO2 formed by mixing MnO2 spheres with plain CNTs, the use of DNA resulted in CNTs@DNA–MnO2 with a more porous structure (Fig. 3c), which in turn may provide larger surface areas and more effective electrolyte transport.28
Electrochemical measurements The electrochemical properties of the CNTs and DNA@CNTs were first investigated in a three-electrode configuration. As shown in Fig. 4a, the cyclic voltammogram (CV) of CNTs@DNA in 1.0 M Na2SO4 exhibits a typical rectangular shape at a scan rate of 100 mV s 1. This rectangular shape is well maintained at scan rates in the range from 2 to 500 mV s 1 (Fig. 4b), indicating an excellent capacitive behavior and fast diffusion of electrolyte.1 The capacitive behavior of the CNTs@DNA electrode was further confirmed from nearly linear galvanostatic charge–discharge curves obtained at different current densities from 1 to 5 A g 1 (Fig. 4c). Based on the charge–discharge curves, the IR drop was also obtained. Generally, the IR drop for an electrode results from electrolyte potential drop and internal charge transfer resistance.20 For the CNTs@DNA electrode, relatively small IR drops of 3.5, 6.5 and 11.6 mV were measured at 1, 2 and 5 A g 1, respectively, indicating a low internal charge transfer resistance. Specific capacitances were calculated from the discharge curves of CNTs and [email protected] At current densities of 1, 2 and 5 A g 1, the specific capacitances of the CNTs@DNA electrode are 124.6, 102.3 and 88.1 F g 1, respectively (Fig. 4d). While for the CNT electrode at the same current densities, much lower specific capacitances of 67.9, 55.8 and 29.2 F g 1 are obtained. Compared to CNTs, CNTs@DNA electrode exhibits much higher specific capacitances at all current densities.
Fig. 4 (a) CVs of CNTs and CNTs@DNA electrodes in a three-electrode configuration (scan rate of 100 mV s 1, 1.0 M Na2SO4). (b) CVs of CNTs@DNA electrode with different scan rates (2 to 500 mV s 1). (c) Galvanostatic charge–discharge curves of CNTs@DNA electrodes at current densities of 1, 2 and 5 A g 1, and CNTs electrode at a current density of 1 A g 1 in a three-electrode configuration. (d) Comparison of specific capacitances for the CNTs and CNTs@DNA electrodes.
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The excellent electrochemical behavior of CNTs@DNA can be attributed to its networked structure with a reduced aggregation of CNTs as shown in Fig. 2b and c. Fig. 5a shows the CVs of MnO2, CNTs–MnO2 and CNTs@DNA–MnO2 electrodes in 1.0 M Na2SO4 at a scan rate of 100 mV s 1 in a three-electrode configuration. For MnO2, the charge storage mechanism is based on the Faradaic reaction with Na+ both on the surface and within the bulk of MnO2 via Na+ + MnO2 + e 2 MnOONa.26 As shown in Fig. 5a, the CNTs–MnO2 electrode exhibits a better capacitive behavior than that of MnO2, indicating that the addition of CNTs improves the utilization of MnO2. More importantly, compared with both MnO2 and CNTs–MnO2, the CNTs@DNA–MnO2 electrode shows much larger current densities over the whole potential window at the same scan rate, suggesting a higher specific capacitance. Furthermore, the more rectangular shape of the CV for CNTs@DNA–MnO2 indicates faster diffusion of the electrolyte to the electrode. Fig. 5b summarizes the specific capacitances calculated from CVs at different scan rates.9 At 100 mV s 1, CNTs@DNA–MnO2 provides a specific capacitance of 175 F g 1, which is 1.5 and 5.3 times higher than that of CNTs–MnO2 (116 F g 1) and MnO2 (33 F g 1), respectively. The CNTs@DNA–MnO2 electrode was further evaluated with galvanostatic charge–discharge measurements at current densities of 1, 2 and 5 A g 1; the corresponding specific capacitances are 265, 242 and 228 F g 1, respectively. A cycling test of the CNTs@DNA–MnO2 electrode was also performed (Fig. 5d). It is found that after 1000 cycles at 1 A g 1, 92.8% of the initial capacitance of CNTs@DNA–MnO2 can be retained. The improved electrochemical behavior of CNTs@DNA–MnO2 can be attributed to the use of DNA to assist the assembly of the
Fig. 5 (a) CVs of MnO2, CNTs@DNA and CNTs@DNA–MnO2 electrodes in a three-electrode configuration (scan rate of 100 mV s 1, 1.0 M Na2SO4). (b) Specific capacitances calculated from CVs for the three electrodes at scan rates ranging from 2 to 250 mV s 1. (c) Galvanostatic charge– discharge curves of CNTs@DNA–MnO2 electrode with current density of 1, 2 and 5 A g 1 in a three-electrode configuration. (d) Cycling test of the CNTs@DNA–MnO2 electrode in 1.0 M Na2SO4 at a charge–discharge current density of 1 A g 1.
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CNTs and MnO2 spheres. The resultant porous structure enables effective electrolyte transport and facile access to the active sites of the electrode material. In addition, highly dispersed CNTs anchored on the MnO2 surface facilitate both intra- and inter-particle electron transfer for the MnO2 spheres. Performance evaluation of the asymmetric supercapacitors Asymmetric supercapacitors were fabricated with CNTs@DNA– MnO2 as the battery-type positive electrode and CNTs@DNA as the capacitor-type negative electrode (denoted as CNTs@DNA– MnO2//CNTs@DNA) and 1.0 M Na2SO4 as the electrolyte. Fig. 6a shows the CVs of a CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitor at different voltage windows. It is found that a good capacitive behavior with quasi-rectangular CVs can be attained at voltages up to 1.50 V. Thus, an operation voltage window of 1.50 V was chosen to investigate the electrochemical performance of the asymmetric supercapacitors. For comparison, asymmetric supercapacitors without DNA assembly were also fabricated using CNTs–MnO2 and CNTs as the positive and negative electrodes, respectively (denoted as CNTs–MnO2//CNTs). Based on the CVs shown in Fig. 6b, the specific capacitance of the CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitor is 74.2 F g 1, 60% higher than that of the CNTs–MnO2//CNTs asymmetric supercapacitor (45.8 F g 1) under the same measurement conditions, indicating that the use of DNA improves the specific capacitance considerably. Fig. 7a shows galvanostatic charge–discharge curves of the CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitor at different current densities ranging from 0.5 to 25 A g 1. These curves have a nearly symmetric triangular shape, suggesting a
Fig. 6 (a) CVs of asymmetric supercapacitors with CNTs@DNA as the negative and CNTs@DNA–MnO2 as the positive electrode. (b) CVs of asymmetric supercapacitor devices with and without DNA assembly measured with the same condition at a scan rate of 100 mV s 1. For all measurements, the electrolyte is 1.0 M Na2SO4 solution and the scan rate is 100 mV s 1.
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Fig. 7 Galvanostatic charge–discharge curves for asymmetric supercapacitors (a) with DNA assembly and (b) without DNA assembly at current densities of 0.5, 1, 2, 5, 10 and 25 A g 1. (c) Cycling test for the asymmetric supercapacitors at a charge–discharge current density of 2 A g 1. (d) Nyquist plots for the asymmetric supercapacitors.
good Coulombic efficiency and electrochemical reversibility. Galvanostatic charge–discharge curves of the CNTs–MnO2// CNTs asymmetric supercapacitor at current densities ranging from 0.5 to 25 A g 1 are given in Fig. 7b. As seen from Fig. 7a and b, the specific capacitance of the CNTs@DNA–MnO2// CNTs@DNA asymmetric supercapacitor is higher than that of the CNTs–MnO2//CNTs asymmetric supercapacitor at all current densities. A cycling test was performed for both asymmetric supercapacitors at a current density of 2 A g 1 and the specific capacitances were calculated from the discharge curves (Fig. 7c). After 10 000 cycles, the capacitance retentions for the CNTs@DNA–MnO2//CNTs@DNA and CNTs–MnO2//CNTs asymmetric supercapacitors are 91.6% and 83.5%, respectively, suggesting an improved cycling stability of the asymmetric supercapacitors fabricated with the assistance of DNA assembly. The equivalent series resistances (ESRs) of the supercapacitors were obtained from impedance measurements as shown in Fig. 7d. For CNTs–MnO2//CNTs and CNTs@DNA–MnO2// CNTs@DNA asymmetric supercapacitors, the ESRs are 1.15 and 1.30 O, respectively. The lower ESR of the CNTs@DNA–MnO2// CNTs@DNA asymmetric supercapacitor indicates that the use of the DNA assembly improves charge transfer of the supercapacitor devices. Energy density (E) and power density (P) are important parameters to evaluate the performance of an electrochemical energy storage device. Fig. 8 shows the Ragone plots of the CNTs–MnO2//CNTs and CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitors. The specific energy density and power density of the asymmetric supercapacitors were calculated from the charge–discharge curves at different current densities using E = 12CspDV 2 and P = E/Dt where Csp is the specific capacitance, DV is the voltage window of the discharge process, and Dt is the discharge time of the asymmetric supercapacitor devices. For both devices, the specific energy density generally
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positive electrode and CNTs@DNA negative electrode show an energy density of 11.6 and 5.8 W h kg 1 at a power density of 185.5 W kg 1 and 4.5 kW kg 1, respectively. In addition, it has a capacitance retention of 91.6% after 10 000 charge– discharge cycles. The performance is superior to that of CNTs–MnO2//CNTs asymmetric supercapacitors without the use of DNA (7.3 W h kg [email protected] W kg 1 and 2.9 W h kg [email protected] kW kg 1, and 83.5% capacitance retention after 10 000 cycles). Considering the high energy and power densities of the asymmetric supercapacitors, and in particular, the high mass loading of the active material (4.2 vs. o0.5 mg cm 2), the DNA-assisted assembly can offer a promising approach to fabricate high-performance energy storage/conversion devices including supercapacitors, lithium ion batteries and solar cells. Fig. 8 Ragone plots for the asymmetric supercapacitors.
increases with the decrease of specific power density. The CNTs– MnO2//CNTs asymmetric supercapacitor without DNA assembly provides specific energy densities of 2.9 and 7.3 W h kg 1 at power densities of 4.5 kW kg 1 and 185.5 W kg 1, respectively. While for the CNTs@DNA–MnO2//CNTs@DNA asymmetric supercapacitor, the corresponding specific energy densities are 5.8 and 11.6 W h kg 1, respectively, nearly double the energy densities of CNTs–MnO2//CNTs. Moreover, the maximum energy density (11.6 W h kg 1) of the CNTs@DNA–MnO2// CNTs@DNA asymmetric supercapacitor is higher than that ({10 W h kg 1) of symmetric supercapacitors made of activated carbon,48 CNTs49 and graphene.1 It is also higher than previously reported MnO2-based asymmetric supercapacitors including MnO2/active carbon (10.4 W h kg 1).50 Although the energy and power densities are not the highest among reported values,53–57 it should be noted that this performance is achieved with a mass loading of MnO2 at 4.2 mg cm 2, higher than those reported MnO2-based supercapacitors where the mass loadings of MnO2 are typically less than 0.5 mg cm 2.28,30,51–59 These results demonstrate that DNA-assisted assembly approach improves the performance of MnO2/CNT asymmetric supercapacitors.
Conclusion In summary, we have developed a DNA-assisted assembly approach to fabricate a capacitor-type electrode material of CNTs@DNA and a battery-type electrode material of CNTs@DNA–MnO2. Material characterizations show that DNA functionalization improves the dispersion of CNTs in aqueous solution. The use of DNA also promotes the interaction between CNTs@DNA and MnO2 spheres and leads to the formation of a unique network structure. Within this structure, CNTs@DNA is anchored on the MnO2 surface and acts like chains to bridge the MnO2 spheres. The resultant network structure consisting of CNTs@DNA and MnO2 facilitates both inter- and intra-particle charge transfers. In addition, compared to CNTs–MnO2, CNTs@DNA–MnO2 has a more porous structure, which is useful in promoting electrolyte transport. Asymmetric supercapacitors with CNTs@DNA–MnO2
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Acknowledgements We are grateful for the financial support of Ministry of Education, Singapore (Grant# R279-000-391-112) and National Research Foundation CRP program (Grant# R279-000-337-281).
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