Growth of Manganese Oxide Nanoflowers on Vertically-Aligned Carbon Nanotube Arrays for High-Rate Electrochemical Capacitive Energy Storage

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1 Growth of Manganese Oxide Nanoflowers on Vertically-Aligned Carbon Nanotube Arrays for High-Rate Electrochemical Capacitive Energy Storage NANO LETTERS 2008 Vol. 8, No Hao Zhang, Gaoping Cao,*, Zhiyong Wang, Yusheng Yang, Zujin Shi, and Zhennan Gu Research Institute of Chemical Defense, West building, No. 35 Huayuanbei Road, Beijing , China, and College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing , China Received April 1, 2008; Revised Manuscript Received July 10, 2008 ABSTRACT Manganese oxide nanoflower/carbon nanotube array (CNTA) composite electrodes with hierarchical porous structure, large surface area, and superior conductivity was controllable prepared by combining electrodeposition technique and a vertically aligned CNTA framework. This binder-free manganese oxide/cnta electrode presents excellent rate capability (50.8% capacity retention at 77 A/g), high capacitance (199 F/g and 305 F/cm 3 ), and long cycle life (3% capacity loss after charge/discharge cycles), with strong promise for high-rate electrochemical capacitive energy storage applications. * Corresponding author. etwas-chang@sohu.com. Phone: Fax: Research Institute of Chemical Defense. Peking University. In development of energy storage devices, nanostructured electrode materials have attracted great interest, as they show not only higher capacities but also better rates than traditional materials. 1-6 Nanostructured electrode materials are key components in the advancement of future energy technologies; thus, strategies for preparing high-performance nanomaterials are required. 5 However, direct synthesis of complex nanostructures still remains a challenge in areas of materials science. 7,8 Nowadays, much research on electrochemical capacitors (ECs) is aimed at increasing power and energy densities as well as lowering fabrication costs while using environmentally friendly materials. Specifically, it was found that RuO 2 exhibits prominent capacitive properties as ECs electrode materials; 9 however, its high cost excludes it from wide application. Relatively low cost materials such as MnO x and NiO x can also be used as electrode materials, but their poor rate capability need to be enhanced. 10,11 Manganese oxides (MO) have been thoroughly investigated because of their importance in industrial applications, such as catalysis and energy storage Over the years various nanostructured manganese oxides, including dendritic clusters, nanocrystals with different shapes, nanowires, nanotubes, nanobelts, and nanoflowers, have been synthesized In order to greatly increase electrochemical performance of manganese oxides, a hierarchical porous structure 25 with high electronic conductivity 26 must be considered. Nevertheless, up to now, no method has been reported to fabricate hierarchical porous and binder-free manganese oxide composite electrodes with superior electronic conductive paths. In this paper, we realized this goal by a combination of a carbon nanotube array (CNTA) framework and an electrodeposition technique to fabricate a composite structure, that is, well-dispersed manganese oxide nanoflowers on a vertically aligned CNTA. CNTA is excellent for electrodepositing transition metal oxides because of its regular pore structure, high surface area, homogeneous property (binderfree), and excellent conductivity The experimental results show that the manganese oxide/cnta composite electrode exhibits superior rate performance (50.8% capacitance retention at 77 A/g) in an EC, thus, making it very promising for high-rate electrochemical capacitive energy storage applications. Our strategy is shown schematically in Figure 1. The preparation of manganese oxide/cnta composite mainly involves (1) growing a vertically aligned CNTA on a Ta foil directly by chemical vapor deposition at 800 C 31 and (2) electrodepositing manganese oxide on CNTA scaffold /nl800925j CCC: $40.75 Published on Web 08/21/ American Chemical Society

2 Figure 1. Procedure for the preparation of manganese oxide/cnta composite electrode. Figure 2. Morphology and microstructure of CNTA and the manganese oxide/cnta composite. (a,b) SEM images with different magnifications of the original CNTA and a photograph (inset of a) of the original (left) and CNTA-covered (right) Ta foils. (c) SEM image of the manganese oxide/cnta composite and its EDX pattern (inset). (d) TEM image of a manganese oxide nanoflower. Figure 3. (a) XRD pattern and (b) XPS spectrum of the manganese oxide/cnta composite. (c) Pore size distribution of the CNTA and the manganese oxide/cnta composite. (d) TEM images of the manganese oxide/cnta composite. prepared in step 1 by potentiodynamic method. The detailed preparation process can be found in Supporting Information. CNTA produced in the first step exhibits regular pore structure and large pore sizes (Figure 2a,b), enabling an intimate contact between precursor solution and CNTA. Furthermore, CNTA presents good conductivity and homogeneous property. Accordingly, during the electrodeposition step, well-dispersed manganese oxide deposits form on the CNTA framework. This strategy can also enable straightforward integration of other transition metal oxide nanomaterials into energy storage devices or sensors. Although several methods, 32,33 such as sol-gel synthesis, template method, chemical vapor deposition, and layer-by-layer method, can be used to prepare nanostructures, electrodeposition is employed in our work because of its simplicity, onestep process, reliability, low cost, and versatility. The reason for using Ta foils is that Ta is very stable in acidic MnSO 4 precursor solutions, which does not influence the measure of mass load of manganese oxide deposits. Characterization methods include scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and N 2 adsorption/desorption. The electrochemical properties of the composite electrodes in ECs were evaluated systematically. All potentials reported here have been measured versus saturated calomel electrode (SCE). The detailed characterization can be found in Supporting Information. Figure 2 shows the morphology and microstructure of CNTA and the manganese oxide/cnta composite. The upper inset in panel a is a picture of original (left) and CNTA-covered (right) Ta foils. The thickness of CNTA is 35 µm (see Figure 2a). A close examination (Figure 2b) at the side of the CNTA illustrates that the CNTA is formed by numerous densely packed and aligned carbon nanotubes (CNTs). A SEM image of the manganese oxide/cnta composite is shown in Figure 2c. Interestingly, manganese oxide particles that are around 150 nm in diameter are welldispersed on CNTA. EDX pattern (inset of Figure 2c) of the composite demonstrates the existence of manganese oxide. TEM image (Figure 2d) reveals that nanostructured manganese oxide particle is composed of hundreds of surfboard-shaped nanosheets, and the nanosheets of each individual particle originate from the same core, forming a dandelion-like flower. The length and thickness of each surfboard-shaped petal are about 50 and 3 nm, respectively. XRD pattern of the manganese oxide/cnta composite (see Figure 3a) indicates an amorphous structure of manganese oxide in the composite. The composition of manganese oxide phase was characterized by XPS (Figure 3b). On the Nano Lett., Vol. 8, No. 9,

3 basis of the analysis of O 1s spectrum (see Supporting Information), the average manganese oxidation state is The specific surface area (SSA) and pore size distribution of the composite and CNTA were obtained from analysis of the desorption branch of N 2 gas isotherms using density function theory. The SSA of the composite is 234 m 2 /g. Thus, the SSA of the manganese oxide phase is as high as 236 m 2 /g, considering that CNTA is 6.5 wt % of the composite and that the SSA of CNTA is 201 m 2 /g. CNTA presents developed mesopore (2-50 nm) structure (see Figure 3c). Compared with CNTA, the manganese oxide/cnta composite exhibits more developed micropores (<2 nm), which can mainly be attributed to the numerous gaps between manganese oxide nanosheets (see Figure 2d). Besides the micropores in manganese oxide, there is a great deal of macropores (>50 nm) among CNTs (see Figure 2c). The micropores in manganese oxide and the macropores between CNTs compose a hierarchically porous structure. In addition, we claim that manganese oxide nanoflowers are apt to form at the junctions of CNTs, and our conclusion is based on an analysis of 97 TEM images (2 typical TEM images are shown in Figure 3d). This phenomenon can be attributed to the idea that manganese oxide tends to nucleate at the junctions of CNTs rather than at the curved surface of CNTs during electrodeposition. Generally, manganese oxide deposits readily form planar nanosheets on flat substrates. 24,34-36 The dandelion-like nanostructure can be due to the manganese oxide deposit nucleating at the junctions of CNTs and then growing radially from the junctions. On the basis of this point, the growth process of the manganese oxide/cnta composite is different from that of the manganese oxide/ nanocarbon composite, which is prepared by coating the surface of carbon aerogel with a thin manganese oxide through self-limiting reaction, reported by Fischer et al. 37 Details of the growth mechanism need further investigation. Electrochemical properties of the manganese oxide/cnta composite electrode in ECs are shown in Figure 4. The cyclic voltammetry (CV) curve (Figure 4a) of the manganese oxide/ CNTA composite shows rectangular shape at a high sweep rate of 200 mv/s, indicative of highly capacitive nature with good ion response. Electrochemical impedance spectroscopy (see Supporting Information) shows that the equivalent series resistance of manganese oxide/cnta electrode (1.66 Ω) is less than that of the manganese oxide/nanocarbon composite. 37 The charge-transfer resistance of the manganese oxide/ CNTA electrode is 0.16 Ω, which is comparable to that of the original CNTA electrode (0.11 Ω) and is a reflection of the developed conductive network in the manganese oxide/ CNTA electrode. The manganese oxide/cnta composite delivers a C sp of 199 F/g (Figure 4b, based on the mass of the composite) at low current density, which is much higher than the C sp (27 F/g, see Figure 4b) of the original CNTA substrate. Surprisingly, this composite still retains 101 F/g (50.8% retention) at a current density of as high as 77 A/g, indicating that such high C sp can be maintained under very high power operation. To the best of our knowledge, this rate capability is the best reported for manganese oxide composites. In addition, the density of the composite is 1.53 Figure 4. Electrochemical properties of the manganese oxide/cnta composite electrode in ECs: (a) CV curves at mv/s, (b) specific capacitance of manganese oxide/cnta, manganese oxide/ ECNT, manganese oxide/ac, and original CNTA versus discharge current density, and (c) charge-discharge cycle test. (d) SEM and TEM images (inset) of the manganese oxide/ecnt composite. g/cm 3, leading to a high volumetric specific capacitance (C v ) of 305 F/cm 3, which is much higher than the C v value of the manganese oxide/carbon nanofoam composite reported previously. 37 The density of high surface area nanoporous carbon is around 0.5 g/cm 3, with a result that their C v are around 120 F/cm 3, 38,39 which are much lower than the C v of manganese oxide/cnta composite. The electrochemical stability of the composite electrode was examined by charge-discharge cycling at a current of 5 ma. The capacity loss after consecutive cycles is only 3%, indicative of long-term electrochemical stability. Good electrochemical stability is an advantage for manganese oxide/cnta composite compared with conducting polymers whose cycle lives are less than 5000 cycles. 33,40 We electrodeposited manganese oxide on an entangled CNT (ECNT) and activated carbon (AC) electrode, which have high surface areas of 231 and 1033 m 2 /g, respectively, to prepare manganese oxide/ecnt and manganese oxide/ AC composites for comparison. Preparation procedure of ECNT and AC electrode and electrodepositing condition can be found in Supporting Information. Compared with manganese oxide/ecnt composite, the manganese oxide/cnta composite shows not only higher C sp but also better rate capability (Figure 4b). Figure 4d shows the SEM and TEM (inset of Figure 4d) images of the manganese oxide/ecnt composite. Unlike CNTA electrode, binder is required for fabrication of ECNT electrode and then leads to a heterogeneous property; therefore, the manganese oxide deposited on ECNT is not uniform and readily forms manganese oxide/ CNT agglomerates (some of them are larger than 1 µm), which results in worse electrochemical accessibility and lower ionic conductivity for the manganese oxide/ecnt composite than for the manganese oxide/cnta composite. Furthermore, the conductive paths in ECNT electrode are longer and more irregular than that in CNTA electrode, 26 leading to a lower electronic conductivity. Although the C sp 2666 Nano Lett., Vol. 8, No. 9, 2008

4 Figure 5. Schematic representation of the microstructure and energy storage characteristics of the manganese oxide/cnta composite. of manganese oxide/ac is 201 F/g, the rate performance of this composite is poor, that is, 24% (49 F/g) capacitance retention at 10 A/g, which is attributed to the low conductivity of AC substrate. On the basis of the results above, the energy storage characteristics of the manganese oxide/cnta composite are illustrated in Figure 5. Manganese oxide nanoflowers are grown directly on nanostructured current collector (CNTA). This geometry has several advantages. First, each manganese oxide nanoflower is connected directly with the current collector (Ta foil) by two or more electron superhighways (CNTs); thus, this superior conducting network allows for efficient charge transport and enhances the electronic conductivity of composite significantly. Second, the high SSA and the nanometer size, which reduces the diffusion length of ions within manganese oxide phase during the charge/ discharge process, 33 ensure a high utilization of electrode materials, and then a high specific capacitance. Third, a hierarchically porous structure enhances the ionic conductivity of the composite greatly. The physicochemical properties of the electrolyte in macropores are similar to those of the bulk electrolyte with the lowest resistance. 41 Ion-buffering reservoirs can be formed in macropores to minimize the diffusion distances to interior surfaces of manganese oxide. Fourth, the use of CNTs with exceptional mechanical properties as a support and the geometry of the manganese oxide nanoflower can release the cycle degradation problems caused by mechanical problems or volume changes 42 and can overcome nanoparticle aggregation. In addition, as every manganese oxide particle is connected to the conducting framework; the need for binders or conducting additives, which add extra contact resistance or weight, is eliminated. Thus, the manganese oxide/cnta composite electrode presents the best electrochemical capacitive performance (see Figure 4a-c). The manganese oxide particle size and distribution can be influenced by electrodeposition parameters, such as CV cycle number and potential range. In brief, more CV cycles Figure 6. SEM and TEM (insets) images of the manganese oxide/ CNTA composite prepared by (a) 50 and (b) 200 CV cycles. (c) C sp and (d) C v of the manganese oxide/cnta composite prepared by cycles. and higher upper limit of potential range result in larger manganese oxide particles and denser particle distribution, respectively. Morphology and microstructure of the manganese oxide/cnta composite prepared by 50 and 200 CV cycles (details for preparation can be found in Supporting Information) are shown in Figure 6a,b. Compared with the composite prepared by 100 CV cycles (Figure 2c,d, 5.0 mg/ cm 2 mass load), the composite prepared by 50 CV cycles presents similar manganese oxide particle distribution but smaller particle sizes (see Figure 6a and its inset) and smaller manganese oxide mass load (2.1 mg/cm 2 ). The space between nanotubes is stuffed by manganese oxide deposits when too much manganese oxide is deposited onto CNTA (6.7 mg/ cm 2 mass load, prepared by 200 CV cycles, see Figure 6b and its inset). These results exhibit that more CV cycles lead to larger mass load of manganese oxide, which has a profound effect on the morphology and then the electrochemical properties of manganese oxide/cnta composites. Figure 6c shows the C sp of the manganese oxide/cnta composite prepared by cycles (named MO/CNTA- 10, 50, 100, and 200) at different current densities. MO/ CNTA-10, 50, and 100 have similar C sp values at low current density; however, their rate capability is different. MO/ CNTA-10 presents the best rate capability, that is, 144 F/g at a very high current density of 500 A/g. MO/CNTA-50 has 103 F/g at a 144 A/g, which is better than the rate performance of MO/CNTA-100. MO/CNTA-200 has the lowest C sp and worst rate capability. It is concluded that more mass load of manganese oxide leads to lower C sp and worse rate performance. The reason is that less mass load of manganese oxide results in smaller scale of manganese oxide phase (see Figure 6a), and thus the distance over which the electrolyte ions must transport is shorter, leading to higher utilization of electrode materials and then a higher C sp and better rate performance. Furthermore, the macropores between CNTs in MO/CNTA-200 are stuffed by deposits, Nano Lett., Vol. 8, No. 9,

5 leading to a larger ion diffusion resistance at high discharge rates. Figure 6d shows the C v of manganese oxide/cnta composites at different discharge current densities. Because MO/CNTA-200 has the largest mass load of manganese oxide (6.7 mg/cm 2 ) and then the largest density (2.0 g/cm 3 ), MO/CNTA-200 presents the high C v (299 F/cm 3 )atlow current density. MO/CNTA-10 presents the lowest C v because of its low density (0.20 g/cm 3 ). MO/CNTA-100 has the highest C v at all current densities. On the basis of the results above, it is concluded MO/CNTA-100 has the best electrochemical capacitive performance, indicating that 100 CV cycles is the optimal electrodeposition condition. In summary, we have subtly realized the ordered growth of manganese oxide nanostructure on an excellent conductive support (CNTA) through a simple and appropriate synthesis process (electrodeposition), preparing a composite electrode with hierarchical porous structure, high SSA, and excellent conductivity. The binder-free manganese oxide/cnta composite electrode presents high capacitance, long cycle life, and superior rate capability, making it very promising in ECs. The mass load of manganese oxide deposits as well as the morphology and electrochemical capacitive properties of manganese oxide/cnta composite can be simply controlled by the changing of CV cycle number of electrodeposition. Furthermore, other deposition conditions such as solution precursor concentration and additives (such as acetate), deposition time, or subsequent heat treatment also influence the structural and electrochemical properties of the manganese oxide phase. Strategies to tune the structure of manganese oxide/cnta composites may further improve the achievable capacitive performance for such electrode structures. In addition, this work reveals that CNTA is an ideal substrate for depositing transition metal oxides to synthesize novel nanomaterials and opens up a novel route for the direct synthesis of advanced functional materials with hierarchical porous structure and superior conductivity. These materials can find applications in not only energy storage but also sensors, catalysis, and microelectronics. Acknowledgment. This research was financial supported by National Natural Science Foundation of China ( ) and National 863 Project (2006AA03Z342, 2006AA11A163). 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