Mesostructured carbon nanotube-on-MnO2 nanosheet composite for

Subscriber access provided by UNIV OF LOUISIANA

Energy, Environmental, and Catalysis Applications

Mesostructured carbon nanotube-on-MnO2 nanosheet composite for high-performance supercapacitors Henan Jia, Yifei Cai, Xiaohang Zheng, Jinghuang Lin, Haoyan Liang, Jun Lei Qi, Jian Cao, Jicai Feng, and Weidong Fei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14109 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

ACS Applied Materials & Interfaces

Mesostructured carbon nanotube-on-MnO2 nanosheet composite for highperformance supercapacitors Henan Jia†, Yifei Cai†, Xiaohang Zheng*, Jinghuang Lin, Haoyan Liang, Junlei Qi*, Jian Cao, Jicai Feng, Weidong Fei* State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China

*Corresponding authors: Tel. /fax: 86-451-86418146; 1

ACS Paragon Plus Environment

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

†These authors contributed equally. E-mail: [email protected] (J. L. Qi) Abstract Carbon nanomaterials have been widely used to enhance the performance of MnO2-based supercapacitors. However, it still remains a challenge to directly fabricate high combining strength, mesostructured and high-performance MnO2-carbon nanotube (CNT) nanostructured composite electrodes with a little weight percentage of carbon materials. Here, we report a novel mesostructured composite of CNT-on-MnO2 nanosheet with a high MnO2 percentage, which consists of vertical-aligned MnO2 nanosheets with nanopores and in-situ formed oriented CNTs on MnO2 nanosheets (tube-on-sheet). The optimized CNTs/MnO2 possesses favorable features, namely vertical-aligned nanosheets to shorted ion diffusion path, a hierarchical porous structure for increased specific surface areas and active sites, and in-situ formed CNTs for enhanced conductivity and robust structural stability. It is found that the unique tube-on-sheet CNTs/MnO2 nanocomposites with the high MnO2 percentage (>90 wt%) exhibit the high specific capacity of 1131 F g-1 based on total electrodes and 1229 F g-1 based on MnO2 at a current density of 1 A g-1, high rate capability, and ultrastable cycling life (94.4%@10000 cycles). This electrode design strategy in this paper demonstrates a new way for high-performance electrodes for supercapacitors with high active materials percentage. Keywords: supercapacitor, MnO2, CNTs, high MnO2 percentage, high performance

2


Page 2 of 30

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


1. Introduction Owing to the excellent cycling life, fast charge/discharge rate and high power density, supercapacitors

containing

electrical

double-layer

capacitors

(EDLCs)

and

pseudocapacitors have attracted intensive efforts for a promising eco-friendly energy storage system. However, conventional EDLCs used carbonaceous materials (specific capacitance: 100-250 F g-1) are limited by their low energy density.1,2 A promising approach to solve this problem is replacing conventional carbonaceous electrode materials with pseudocapacitive materials, such as MnO2, RuO2, Co3O4 and NiO, which can store and release more charges through redox reactions.3–6 Among the various pseudocapacitive materials, MnO2 is of particular interest for supercapacitors owing to its outstanding theoretical specific capacitance, environmentally friendly nature and natural abundance.7 Nevertheless, MnO2 does not currently satisfy the performance demands for practical use, mainly because of its poor electrical conductivity and limited specific capacitance.8 The specific capacitance can be further significantly enhanced by synthesis to the nanoscale level, such as nanowires, nanosheets and nanoparticles.7,9-10 Moreover, among all nanostructured electrodes, porous MnO2 nanosheets, especially vertical-aligned porous MnO2 nanosheets, have been recently developed. This unique nanostructure can not only provide large surface area and increase electro-active sites for electrochemical reaction, but also shorten the electrons transfer path and reduce contact resistance, which improves the overall electrochemical performance of 3



MnO2.11 Despite great advancements in this progress, the single MnO2 nanostructure is still limited by the inherent poor conductivity and structural instability of the MnO2 nanostructures. In such a context, a considerable attention has been devoted to integrating MnO2 nanostructures with conductive carbon materials (such as graphene, CNTs, amorphous carbon, etc). For example, Alshareef et al. reported that MnO2-CNTs on conductive sponge showed only 4% of degradation after 10000 cycles, the specific capacitance can be increased to 1230 F g-1 (based on the mass of MnO2) due to the much-improved conductivity, and the specific capacitance based on total active materials was still 600~700 F g-1.12 Bao et al. developed MnO2 nanoparticles inside in CNTs and benefited the composites to the specific capacitance to 1250 F g-1 based on the mass of MnO2 and 225 F g-1 based on overall electrodes (the loading of MnO2 is 15 wt%).13 Kaner et al. demonstrated a simple technique for fabricating MnO2/graphene composites, and the specific capacitance depended on the mass of MnO2 reaching a maximum value of 1145 F g-1 at a mass loading of 13% of MnO2 in composites with excellent stability of ~100% retention after 10000 cycles.14 These reports demonstrate that the introduction of conductive CNTs and graphene in MnO2-based electrodes considerably improves the cycling stability. Furthermore, the specific capacitances of MnO2 in these MnO2-based electrodes have been significantly enhanced almost to its theoretical value. However, in order to give full play to the outstanding theoretical value of MnO2, the final MnO2 percentages in these electrodes were not high ( 90 wt%), and demonstrate high capacity not only based on the mass of MnO2 (1229 F g-1) but also based on total electrodes (MnO2+CNTs, 1131 F g-1) with excellent cycling stability (94.4% retention after 10000 cycles) when directly utilized as electrodes for supercapacitors. 2. Results and discussions The synthesis and fabrication process of mesostructured tube-on-sheet CNTs/MnO2 5



nanosheets through a two-step method is illustrated in Figure 1. The vertical-aligned MnCo-O precursor nanosheets were directly grown on the surface of Ni foam with a facile hydrothermal process. As elucidated in Figure 2a, the Mn-Co-O precursor exhibits that nanosheets are highly ordered vertically grown on the surface of Ni form. Mn-Co-O precursor nanosheets have smooth surfaces and are intersected with each other to prevent the self-aggregation phenomenon. The transmission electron microscope (TEM) image (Figure S1) also confirms the nanosheet-like structure of Mn-Co-O precursor, and the lattice fringes with a distance of 0.215 nm and 0.211 nm can match the (113) plane of MnCO3 and CoCO3.18 Further, Figure 3a shows the XRD patterns of Mn-Co-O precursors. All the diffraction peaks can be indexed to the MnCO3 and CoCO3 (JCPDS card No. 421467 and No. 11-0692).18 As a result, Mn-Co-O precursors, with numerous wrinkled nanosheets, are vertically and densely growing on the Ni foam. The second step, the obtained Mn-Co-O precursor can be converted into mesostructured tube-on-sheet CNTs/MnO2 nanocomposites through PECVD process (details are provided in the experiment section in Supporting Information). Firstly, when through two step annealing, metal Co nanoparticles are first yielded from CoCO3, and MnO2 is yielded from MnCO3 (supported by XRD, as shown in Figure S2). As shown in Figure 2b, the vertical aligned and intersected nanosheet structures of the original Mn-Co-O precursors were well retained (Co-MnO2 nanosheets) in the annealing process. Furthermore, the surfaces of CoMnO2 nanosheets (inset in Figure 2b) become rough because of the formation of many nanoparticles on the surface. Moreover, it is worth noticing that the carbonate decomposition can cause the mismatching interfaces between Co and MnO2 and further generate in-plane nanopores on nanosheets. In order to further illustrate the architectural 6


Page 6 of 30

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


feature, we use diluted hydrochloric acid to dissolve the metal Co in Co-MnO2 (details in Supporting Information). It reveals that abundant in-plane pores which size ranges from 610 nm were uniformly distributed on the surface of MnO2 nanosheets, as shown in Figure 2c. Furthermore, this result can also be proved by Brunauer–Emmett–Teller (BET) surface area, as shown in Figure 3b and c, the BET surface areas of Mn-Co-O precursors and CoMnO2 are 20.4 m2 g-1 and 38.3 m2 g-1, respectively. The increase in BET surface area is mainly because of the in-plane nanopores on nanosheets. The generation of such architecture will basically increase the surface area and active site of active materials, which can accelerate the ion and electron transport to improve the electrochemical performance. Next, with the help of PECVD, the well-dispersed Co nanoparticles catalyze the growth of CNTs in the low temperature (450 oC) while maintaining the crystal structure of MnO2 (the crystal transition temperature is almost 500 oC, ref 19). As shown in Figure 2d, the vertical structure of nanosheets is not changed in the process of synthesizing CNTs by PECVD. The CNTs/MnO2 nanosheets become fluffy and rough and numerous CNTs can be clearly identified on the surface. These CNTs interlaced with each other increase the connection between the neighboring nanosheets and create the voids, which will benefit the electron transport among these MnO2 nanosheets(as shown in figure s3).20 Figure 2e shows the TEM image of a single mesostructured CNTs/MnO2 nanosheet. Many CNTs with a length of tens of nanometers are extruded from the nanosheets, which are in good agreement with the aforementioned SEM image. Based on the TEM and XRD result (Figure 2f and 3a), the CNTs/MnO2 can be indexed to metal Co and MnO2, which means the crystal structure of MnO2 still maintain during the growth of CNTs in PECVD process. 7



Moreover, it is worth noticing that small nanoparticles of ~10 nm are encapsulated into the top of CNTs (arrows in Figure 2e). As a result, it suggests that the Co nanoparticles leave from MnO2 surfaces lead to the in-plane nanopores on the MnO2 nanosheets, which further increase the porosity of MnO2 nanosheets. Furthermore, these CNTs in-situ grown on the surface of MnO2 nanosheets and guarantee the tight nanotube bonding to the MnO2 nanosheets, which are beneficial to the ion and electron transport.21 In addition, as shown in Figure 3d, the BET surface area of CNTs/MnO2 is 161.4 m2 g-1, which demonstrates that the introduction of CNTs significantly increases the surface area of composites. And this exposed surface area can facilitate access of ions to the electrodes and increase the charge storage.21 As shown in Figure S4, the Raman spectrum with the peak located at about 1350 and 1580 cm-1 are characteristic D and G peak of carbon materials. Moreover, the G peak shows a high intensity with a relatively narrow width, which suggests that the carbon materials is highly crystalline. According to previous reports,22,23 such high crystal carbon could enhance the conductivity and electrochemical performance of CNTs/MnO2 nanocomposites. To further study the composition of the electrode materials, Co-Mn-O precursors and CNTs/MnO2 are characterized by X-ray photoelectron spectroscopy(XPS). As shown in Figure S5, the survey spectrum confirms the presence of Co, Mn, O and C elements in the composites. As shown in Figure S6a, Co 2p XPS spectrum of Mn-Co-O precursor can be treated by multiple-peak fitting, where the two spin-orbit doublets at 797.8 and 782.2 eV are attributed to Co 2p1/2 and Co 2p3/2 signals of Co2+, and the weak peak at 786 eV is satellite signals (indicated as “Sat.”). This result indicated the existence of the Co2+ in Mn-Co-O precursor (CoCO3). As shown in Figure S6b, the Mn 2p spectrum shows two peaks at 641.2 and 653.6 eV corresponding to Mn2+ atoms in MnCO3, which are 8


Page 8 of 30

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


consistent with other characterization results.24,25 And the XPS results of Mn-Co-O precursor are in accordance with XRD results. Moreover, Co 2p spectrum of CNTs/MnO2 composites is shown in Figure S6c, the Co 2p spectrum can be deconvoluted into the metallic Co and Co2+,26 this result indicates that the CoCO3 in Mn-Co-O precursors can be reduced to metal Co, which agree with the XRD results of metal Co. And the Co2+ peaks may come from the mild oxidation of metal Co in atmosphere. Figure S6d shows the Mn 2p spectrum of CNTs/MnO2 composites with an energy separation of 11.8 eV between 2P1/2 (653.4eV) and 2p2/3 (641.6eV), in accordance with the results reported in the literature,27 suggesting a manganese valence of 4 (MnO2). XPS was employed to further confirm the average oxidation state of Mn in MnO2 before and after CNTs growth. Figure S7a and b show the Mn 2p core-level spectra of Co-MnO2 and CNTs/MnO2, and all curves consist of two peaks, corresponding to the spin-orbit doublet of Mn 2p3/2 and Mn 2p1/2, respectively. The Mn 2p3/2 peak of Co-MnO2 and CNTs/MnO2 can be deconvoluted into three peaks as Mn4+ (642.8 eV), Mn3+ (641.8 eV), and Mn2+ (640.7 eV). The composition for Co-MnO2, and CNTs/MnO2 was determined to be MnO1.86 and MnO1.82, respectively, demonstrating the phase stability after CNTs growth. Based on the above analysis, the vertical-aligned MnO2 nanosheets with in-situ formed CNTs on them have been realized to form tube-on-sheet structures. The structural advantages of tube-on-sheet CNTs/MnO2 nanocomposites can be summarized as follows: first, the vertical-aligned MnO2 nanosheets directly grow on Ni foam without binders shorten the electrons transfer path and reduce contact resistance. Second, in-plane nanopores caused by the carbonate decomposition increase the electrochemical active sites and surface area, which could benefit to enhance the 9



performance of MnO2. Finally, in-situ form CNTs on MnO2 nanosheets can increase the conductivity and form a strong bonding between CNTs and MnO2. Therefore, benefit from these hierarchical structural/compositional advantages, it is believed that these tube-onsheet CNTs/MnO2 nanocomposites have great potential for supercapacitors. The electrochemical behaviors of all samples were tested in three-electrode system using 1 M Na2SO4 in the operating window 0 to 1 V. Figure 4a shows the comparative cyclic voltammetry (CV) curves of Mn-Co-O precursors, Co-MnO2, and CNTs/MnO2 at a constant scan rate of 50 mV s-1. For the pristine Mn-Co-O precursors, the lower current response than that of others implies the capacitance of the precursors can be ruled out. The CV curve area of CNTs/MnO2 nanosheets obviously increases comparing with Co-MnO2, indicating the enhancement of capacitance and acceleration of ion transport. The CV curves of CNTs/MnO2 at different scan rates (5-50 mV s-1) show quasi-rectangular shapes, as shown in Figure 4b. This near mirror image symmetry suggests the ideal capacitive electrochemical behavior of CNTs/MnO2. With increasing the scan rate, the quasirectangular shape can still be observed, indicating the good rate capability.28 Typical galvanostatic charge/discharge (GCD) curves, with current densities from 2 A g-1 to 20 A g-1, are presented in Figure 4c. The potential-time curves at different current densities are almost with a triangular and symmetric sharp, demonstrating the highly reversible reactions of CNTs/MnO2 during charge/discharge process. The advantages of mesostructured character of CNTs/MnO2 should be demonstrated by comparing the electrochemical performance, and the CV curves and GCD curves of Co-MnO2 are shown in Figure S8. All GCD curves exhibited ideal bilateral symmetrical shapes. CNTs/MnO2 has longer charge– discharge time at the same current density than that of Co-MnO2. In addition, IR drop, 10


Page 10 of 30

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


associated with the internal resistance of the electrode, decreased with in-situ synthesized CNTs on the MnO2 surface. As shown in Figure S9, the IR drop of CNTs/MnO2 is 0.03 V and that of Co-MnO2 is 0.08 V. In this case, the in-situ synthesized CNTs contacted with each MnO2 nanosheet and served as a conductive channel on the MnO2 surface. The specific capacitances of Co-MnO2, and CNTs/MnO2 are shown in Figure 4d, and the values of specific capacitance for CNTs/MnO2 is 1131 F g-1 at 1 A g-1 which is higher than that of Co-MnO2. Furthermore, even at a high current density of 20 A g-1, a high specific capacitance of 633 F g-1 can be obtained, suggesting the good rate performance of CNTs/MnO2. The much better electrochemical performance of CNTs/MnO2 can be attributed to the unique mesostructured structure, which accelerates electrolyte to reach the surface of nanosheets and acts as the reservoir to afford the sufficient ions for electrochemical reactions even at high current densities.29 Moreover, based on our calculations (details in Supporting Information, Figure S10), the mass percentage of CNTs in CNTs/MnO2 is 8~9%, which is lower than traditional MnO2/carbon materials hybrid electrodes. As shown in Figure S11, the specific capacitance of CNTs/MnO2 based on MnO2 can obtain 1229 F g-1, which is close to the theoretical limit and further illustrates this mesostructured character takes full advantages of MnO2.30 As shown in Table S1, it is worth mentioning that the hierarchical CNTs/MnO2 electrodes have the dominant MnO2 content and high specific capacitance of total electrodes, which is propitious to actual applications. Based on the above analysis, the designed hierarchical tube-on-sheet CNTs/MnO2 nanocomposites are highly competitive for high performance supercapacitors. Furthermore, electrochemical impedance spectroscopy (EIS) analysis was also performed to provide the detailed electrochemical conductivity of Co-MnO2 and CNTs/MnO2 (Figure 11



4e). The EIS was carried in the frequency of 100 KHz to 0.01 Hz with a superimposed 5 mV sinusoidal voltage. As shown in Figure S12a, the impedance data are analyzed by fitting to an equivalent electrical circuit which consisting of the electrolyte resistance (Rs), charge transfer resistance (Rct), double layer capacitance (CDL) and Warburg diffusion resistance (Wo). In the high frequency region, both electrodes show the low equivalent series resistances, based on previous reports, it can be deduced that doping Co nanoparticles in MnO2 nanosheets can improve the conductivity and specific capacitance. Moreover, in order to further illustrate the effect of Co incorporation, we synthesize the sample without adding the Co source (Co(NO3)2) and finally obtain pure MnO2 samples. Figure S12b shows the EIS analysis of MnO2 and Co-MnO2, in the high frequency region, apparently, the Co-MnO2 electrode shows the lowest equivalent series resistance of around 2.5 Ω (that of MnO2 is 5.3 Ω), demonstrating its enhanced electrical conductivity. Moreover, the CNTs/MnO2 exhibits the smaller Rct (0.8 Ω) than that of Co-MnO2 (3.8 Ω). The smaller Rct is mainly due to the significantly improved conductivity by the orientedformed CNTs on MnO2 nanosheets, which indicates the lower electrolyte diffusion resistance and great ion transport speed in the electrode.31 In the low frequency region, the slope of the straight line for CNTs/MnO2 is much larger than Co-MnO2. This is mainly due to the hierarchical porous structure which is formed by the interconnected CNTs, these results remain the space to allow the electrolyte to reach the surface of nanosheets and increase the active sites for electrochemical reaction.32 To further elaborate the long-term sustainability of the electrodes, the cycling life was texted and shown in Figure 4f. For the cycling performance measurement, the CNTs/MnO2 electrode was charged and discharged between 0 V to 1 V at the current density of 10 A 12


Page 12 of 30

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


g-1 for 10000 cycles. Only 5.6 % of the initial capacitance of CNTs/MnO2 was lost, which is much higher than that of Co-MnO2 (37 %), demonstrating excellent cycling stability. Moreover, as shown in Figure S13a, the morphology of vertical-aligned nanosheets preserved well and still adhered to Ni foam. And the slight destruction of in-situ formed CNTs and still maintain the tube-on-sheet structure, suggesting a high cycling stability. This result is further confirmed by the EIS curves before and after cycles, as shown in Figure S13b, which show similar curves and no obvious change in the Rct from 0.8 Ω to 1.4 Ω after the cycling test. The high stability further demonstrates the in-situ formed CNTs on the MnO2 nanosheets prevent the aggregation and collapse and remit the volume changes resulting from charge/discharge process. To further illustrate the superior electrochemical performance of CNTs/MnO2 composites toward practical applications, an asymmetric supercapacitor (ASC) was fabricated by employing the CNTs/MnO2 composites as the positive electrode and active carbon (AC) as negative electrode in 1 M Na2SO4 electrolyte. Figurre 5a shows the CV curves of CNTs/MnO2 composites and AC electrodes at the scan rate of 50 mV s-1 with their corresponding voltage windows. And it can be seen that the potential window of CNTs/MnO2 is 0-1 V and that of AC is -1-0 V. As shown in Figure 5b, from the CV curves of CNTs/MnO2//AC recorded at different voltage windows, this ASC can stably work at 2 V. Figure 5c exhibits the CV curves of the CNTs/MnO2//AC devices in the voltage window of 0-2 V at the scan rate from 5-50 mV s-1. Due to the fact both CNTs/MnO2 and AC have nearly ideal capacitive behaviors, the CV curves of the CNTs/MnO2//AC also show quasirectangular shapes, indicating fast charge transport and capacitive behavior. Figure 5d shows the GCD curves of CNTs/MnO2//AC at different densities, which also shows the 13



symmetric features and suggesting the nearly ideal capacitive characteristic. The specific capacitance of CNTs/MnO2//AC is calculated from the GCD curves as the function of current densities, as shown in Figure 5e. The high specific capacitance of 152 F g-1 can be obtained at the current density of 0.3 A g-1. With the current density increased than 20 times (7.5 A g-1), the high capacitance retention of 61% can still be obtained, demonstrating good rate performance. Due to the structural features of our CNTs/MnO2 nanocomposites, the cycling stability of our CNTs/MnO2//AC ASC exhibited excellent electrochemical stability with 91% capacitance retention after 10000 cycles, as shown in Figure S14. Moreover, Figure 5f exhibits the Ragone plot relating the energy density to the power density of MnO2-based ASCs. Most importantly, the CNTs/MnO2//AC ASC demonstrates a maximum energy density of 84.6 Wh kg-1 at a power density of 190 W kg-1. Remarkably, the capacitance and energy densities of CNTs/MnO2//AC ASC are higher than those of other MnO2-based ASCs, indicating our ASC is useful in practical application, as shown in Table S2. Even at the high power density of 4748 W kg-1, the energy density can still reach 51.4 Wh kg-1. These values make them quite promising for application in future energy storage systems. The hierarchical CNTs/MnO2 nanocomposites reserve structural features from both subunits. (1) The vertical-directional nanosheets can provide a large surface area and reduce the interfacial impedance; (2) The in-plane nanopores caused by mismatched interfaces at nanoscale on nanosheets increase the surface area and active sites; (3) The insitu-formed CNTs on the MnO2 nanosheets as bridges not only enhance electron transfer, but also prevent the aggregation and collapse of MnO2 nanosheets; (4) The interconnected CNTs networks possess hierarchical and well-defined pores and stable frameworks. 14


Page 14 of 30

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


Moreover, this hierarchical structure guarantees the mass percentage of MnO2 and maintained highly competitive in practical applications. Consequently, these hierarchical CNTs/MnO2 nanocomposites would open doors toward future supercapacitor devices. 3. Conclusions In summary, we develop an advanced strategy for enhancing the electrochemical performance of MnO2 by synthesizing mesostructured tube-on-sheet CNTs/MnO2 nanocomposites, which consist of in-plane nanopores on nanosheets caused by mismatched contraction between Co with MnO2 and 3D porous architectures caused by the in-situ formation of CNTs on MnO2 nanosheets. The CNTs/MnO2 electrode with low CNTs content (8 wt%) exhibits the high specific capacitance, high rate capability, and ultrastable cycling life. Typically, the CNTs/MnO2 obtained the specific capacitance of 1229 F g-1 based on MnO2, and it can still achieve 1131 F g-1 based on total active materials. Moreover, the CNTs/MnO2 electrode exhibits superior cycling life of 94.4% capacitance retention after 10000 cycles. The enhanced performance is correlated with the synergy effect from much enhanced conductivity, high specific surface area, hierarchical porous structure, short ion diffusion length, and the robust structure stability of CNTs. Our research provides a potential strategy for high-performance supercapacitors, which may open a new space in advanced energy storage materials.

Supporting Information Electronic supplementary information (ESI) available. See DOI: Additional experiment section, TEM, XRD, Raman, XPS characterizations, and electrochemical results of Mn-Co-O precursors, Co-MnO2 nanosheets and CNTs/MnO2 15



nanocomposites.

Acknowledgements The support from the National Natural Science Foundation of China (Grant Nos. 51575135, 51622503, U1537206 and 51621091), China, is highly appreciated.

References 1

L. Liu, Y. Yin, J. Li, N. Li, X. Zeng, H. Ye, Y. Guo, L. Wan, Free-standing hollow carbon fibers as high-capacity containers for stable lithium metal anodes. Joule 2017, 1, 563-575.

2

L. Liu, Y. Yin, J. Li, S. Wang, Y. Guo, L, Wan, Uniform Lithium Nucleation/Growth Induced by Light-Weight Nitrogen-Doped Graphitic Carbon Foams for HighPerformance Lithium Metal Anodes. Adv. Mater. 2018, 30, 1706216.

3

C. Chen, Y. Zhang, Y. Li, J. Dai, J. Song, Y. Yao, Y. Gong, I. Kierzewski, J. Xie, L. Hu, All-wood, Low Tortuosity, Aqueous, Biodegradable Supercapacitors with Ultrahigh Capacitance. Energy Environ. Sci. 2017, 10, 538-545.

4

Y. Zhu, X. Ji, C. Pan, Q. Sun, W. Song, L. Fang, Q. Chen, C. E. Banks, A Carbon Quantum Dot Decorated RuO2 Network: Outstanding Supercapacitances under Ultrafast Charge and Discharge. Energy Environ. Sci. 2013, 6, 3665-3675.

5

Q. Ren, S. Mo, R. Peng, Z. Feng, M. Zhang, L. Chen, M. Fu, J. Wu, D. Ye, Controllable Synthesis of 3D Hierarchical Co3O4 Nanocatalysts with Various Morphologies for the Catalytic Oxidation of Toluene. J. Mater. Chem. A 2018, 6, 498-509. 16


Page 16 of 30

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


6

H. Lai, Q. Wu, J. Zhao, L. Shang, H. Li, R. Che, Z. Lyu, J. Xiong, L. Yang, X. Wang, Z. Hu, Mesostructured NiO/Ni composites for High-performance Electrochemical Energy Storage. Energy Environ. Sci. 2016, 9, 2053-2060.

7

Z. Wu, W. Ren, D. Wang, F. Li, B. Liu, H. Cheng, High-Energy MnO2 Nanowire/Graphene and Graphene Asymmetric Electrochemical Capacitors. ACS Nano 2010, 4, 5835-5842.

8

Y. Fu, X. Gao, D. Zha, J. Zhu, X. Ouyang, X. Wang, Yolk–shell-structured MnO2 Microspheres with Oxygen Vacancies for High-performance Supercapacitors. J. Mater. Chem. A 2018, 6, 1601-1611.

9

N. Yu, H. Yin, W. Zhang, Y. Liu, Z. Tang, M. Zhu, High-Performance Fiber-Shaped All-Solid-State

Asymmetric

Supercapacitors

Based

on

Ultrathin

MnO2

Nanosheet/Carbon Fiber Cathodes for Wearable Electronics. Adv. Energy Mater. 2016, 6, 1501458. 10 S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, Graphene Oxide−MnO2 Nanocomposites for Supercapacitors. ACS Nano 2010, 4, 2822-2830. 11 G. Zhang, L. Ren, Z. Yan, L. Kang, Z. Lei, H. Xu, F. Shi, Z. Liu, Rational Design and Controllable Preparation of Holey MnO2 Nanosheets. Chem. Commun. 2017, 53, 29502953. 12 W. Chen, R. B. Rakhi, L. Hu, X. Xie, Y. Cui, H. N. Alshareef, High-Performance Nanostructured Supercapacitors on a Sponge. Nano Lett. 2011, 11, 5165-5172. 13 W. Chen, Z. Fan, L. Gu, X. Bao, C. Wang, Enhanced Capacitance of Manganese Oxide via Confinement inside Carbon Nanotubes. Chem. Commun. 2010, 46, 3905. 14 M. F. El-Kady, M. Ihns, M. Li, J. Y. Hwang, M. F. Mousavi, L. Chaney, A. T. Lech, 17



R. B. Kaner, Engineering Three-dimensional Hybrid Supercapacitors and Microsupercapacitors for High-performance Integrated Energy Storage. Proc. Natl. Acad. Sci. USA 2015, 112, 4233-4238. 15 Z. Li, J. T. Zhang, Y. M. Chen, J. Li, X. W. Lou, Pie-like Electrode Design for HighEnergy Density Lithium–sulfur Batteries. Nat. Commun. 2015, 6, 8850. 16 J. Yang, M. Ma, C. Sun, Y. Zhang, W. Huang, X. Dong, Hybrid NiCo2S4@MnO2 Heterostructures for High-performance Supercapacitor Electrodes. J. Mater. Chem. A 2014, 3, 1258-1264. 17 L. Bao, J. Zang, X. Li, Flexible Zn2SnO4/MnO2 Core/Shell Nanocable−Carbon Microfiber Hybrid Composites for High-Performance Supercapacitor Electrodes. Nano Lett. 2011, 11, 1215-1220. 18 M. Jana, P. Samanta, N. Chandra Murmu, T. Kuila, Morphology Controlled Synthesis of MnCO3–RGO Materials and Their Supercapacitor Applications. J. Mater. Chem. A 2017, 5, 12863-12872. 19 S. Rehman, T. Tang, Z. Ali, X. Huang, Y. Hou, Integrated Design of MnO2@Carbon Hollow Nanoboxes to Synergistically Encapsulate Polysulfides for Empowering Lithium Sulfur Batteries. Small 2017, 13, 1700087. 20 X. Xia, D. Chao, Y. Zhang, J. Zhan, Y. Zhong, X. Wang, Y. Wang, Z. X. Shen, J. Tu, H. J. Fan, Generic Synthesis of Carbon Nanotube Branches on Metal Oxide Arrays Exhibiting Stable High-Rate and Long-Cycle Sodium-Ion Storage. Small 2016, 12, 3048-3058. 21 G. Xiong, P. He, Z. Lyu, T. Chen, B. Huang, L. Chen, T. S. Fisher, Bioinspired Leaveson-branchlet Hybrid Carbon Nanostructure for Supercapacitors. Nat. Commun. 2018, 18


Page 18 of 30

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


9, 790. 22 Y. Li, K. Yan, H. Lee, Z. Lu, N. Liu, Y. Cui, Growth of Conformal Graphene Cages on Micrometre-sized Silicon Particles as Stable Battery Anodes. Nat. Energy 2016, 1, 15029. 23 N. Li, H. Song, H. Cui, C. Wang, Sn@Graphene Grown on Vertically Aligned Graphene for High-capacity, High-rate, and Long-life Lithium Storage. Nano Energy 2014, 3, 102-112. 24 Tang, Y.; Chen, S.; Chen, T.; Guo, W.; Li, Y.; Mu, S.; Yu, S.; Zhao, Y.; Wen, F.; Gao, F. Synthesis of Peanut-like Hierarchical Manganese Carbonate Microcrystals via Magnetically

Driven

Self-Assembly

for

High

Performance

Asymmetric

Supercapacitors. J. Mater. Chem. A 2017, 5, 3923–3931. 25 Devaraj, S.; Liu, H.; Balaya, P. MnCO3: a novel electrode material for supercapacitors. J. Mater. Chem. A 2014, 2, 4276–4281. 26 Liu, Y.; Fu, N.; Zhang, G.; Xu, M.; Lu, W.; Zhou, L.; Huang, H. Design of Hierarchical Ni-Co@Ni-Co Layered Double Hydroxide Core–Shell Structured Nanotube Array for High-Performance Flexible All-Solid-State Battery-Type Supercapacitors. Adv. Funct. Mater. 2017, 27, 1–11. 27 Huang, Z.-H.; Song, Y.; Feng, D.-Y.; Sun, Z.; Sun, X.; Liu, X.-X. High Mass Loading MnO2 with Hierarchical Nanostructures for Supercapacitors. ACS Nano 2018, 12, 3557–3567. 28 W. He, C. Wang, H. Li, X. Deng, X. Xu, T. Zhai, Ultrathin and Porous Ni3S2/CoNi2S4 3D-Network Structure for Superhigh Energy Density Asymmetric Supercapacitors. 19



Adv. Energy Mater. 2017, 7, 1700983. 29 D. Wang, F. Li, M. Liu, G. Q. Lu, H. Cheng, 3D Aperiodic Hierarchical Porous Graphitic Carbon Material for High-Rate Electrochemical Capacitive Energy Storage. Angew. Chem. 2008, 120, 379-382. 30 C. Yuan, L. Yang, L. Hou, L. Shen, X. Zhang, X. W. Lou, Growth of Ultrathin Mesoporous Co3O4 Nanosheet Arrays on Ni Foam for High-performance Electrochemical Capacitors. Energy Environ. Sci. 2012, 5, 7883-7887. 31 W. Jiang, S. Zhai, Q. Qian, Y. Yuan, H. E. Karahan, L. Wei, K. Goh, A. K. Ng, J. Wei, Y. Chen, Space-confined Assembly of All-carbon Hybrid Fibers for Capacitive Energy Storage: Realizing a Built-to-order Concept for Micro-supercapacitors. Energy Environ. Sci. 2016, 9, 611-622. 32 S. Y. Kim, H. M. Jeong, J. H. Kwon, I. W. Ock, W. H. Suh, G. D. Stucky, J. K. Kang, Nickel Oxide Encapsulated Nitrogen-rich Carbon Hollow Spheres with Multiporosity for High-performance Pseudocapacitors having Extremely Robust Cycle Life. Energy Environ. Sci. 2015, 8, 188-194. 33 Zhao, Y.; Ran, W.; He, J.; Huang, Y.; Liu, Z.; Liu, W.; Tang, Y.; Zhang, L.; Gao, D.; Gao, F. High-Performance Asymmetric Supercapacitors Based on Multilayer MnO2/Graphene Oxide Nanoflakes and Hierarchical Porous Carbon with Enhanced Cycling Stability. Small 2015, 11, 1310–1319. 34 Ma, Z.; Shao, G.; Fan, Y.; Wang, G.; Song, J.; Shen, D. Construction of Hierarchical α-MnO2 Nanowires@Ultrathin δ-MnO2 Nanosheets Core–Shell Nanostructure with Excellent Cycling Stability for High-Power Asymmetric Supercapacitor Electrodes. ACS Appl. Mater. Interfaces 2016, 8, 9050–9058. 20


Page 20 of 30

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


35 Fan, Z.; Yan, J.; Wei, T.; Zhi, L.; Ning, G.; Li, T.; Wei, F. Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated Carbon Nanofiber Electrodes with High Power and Energy Density. Adv. Funct. Mater. 2011, 21, 2366–2375. 36 Jin, Y.; Chen, H.; Chen, M.; Liu, N.; Li, Q. Graphene-Patched CNT/MnO2 Nanocomposite Papers for the Electrode of High-Performance Flexible Asymmetric Supercapacitors. ACS Appl. Mater. Interfaces 2013, 5, 3408–3416. 37 Li, L.; Hu, Z. A.; An, N.; Yang, Y. Y.; Li, Z. M.; Wu, H. Y. Facile Synthesis of MnO2/CNTs Composite for Supercapacitor Electrodes with Long Cycle Stability. J. Phys. Chem. C 2014, 118, 22865–22872. 38 Tang, P.; Han, L.; Zhang, L. Facile Synthesis of Graphite/PEDOT/MnO2 Composites on Commercial Supercapacitor Separator Membranes as Flexible and HighPerformance Supercapacitor Electrodes. ACS Appl. Mater. Interfaces 2014, 6, 10506– 10515. 39 Xiong, C.; Li, T.; Dang, A.; Zhao, T.; Li, H.; Lv, H. Two-Step Approach of Fabrication of Three-Dimensional MnO2-Graphene-Carbon Nanotube Hybrid as a Binder-Free Supercapacitor Electrode. J. Power Sources 2016, 306, 602–610. 40 Kong, S.; Cheng, K.; Ouyang, T.; Gao, Y.; Ye, K.; Wang, G.; Cao, D. Facile Dip Coating Processed 3D MnO2-Graphene Nanosheets/MWNT-Ni Foam Composites for Electrochemical Supercapacitors. Electrochim. Acta 2017, 226, 29–39. 41 Kong, S.; Cheng, K.; Ouyang, T.; Ye, K.; Wang, G.; Cao, D. Freestanding MnO2 nanoflakes on Carbon Nanotube Covered Nickel Foam as a 3D Binder-Free Supercapacitor Electrode with High Performance. J. Electroanal. Chem. 2017, 786, 35–42. 21



Figure captions:

Figure 1. Fabrication process of tube-on-sheet CNTs/MnO2 nanocomposites.

Figure 2. SEM images of a) Mn-Co-O precursors, b) Co-MnO2 nanosheets; TEM image of c) Co-MnO2 nanosheets dissolve Co nanoparticles; SEM image d), TEM image e) and HRTEM image f) of CNTs/MnO2 nanocomposites.

Figure 3. a) The representative XRD patterns for Mn-Co-O precursor and CNTs/MnO2 nanocomposites. Nitrogen adsorption and desorption isotherms of b) Mn-Co-O precursor, c) Co-MnO2 nanosheets and d) CNTs/MnO2 nanocomposites. BJH adsorption pore size distribution inset in each image.

Figure 4. CV curves of Mn-Co-O precursor, Co-MnO2 nanosheets and CNTs/MnO2 nanocomposites at a scan rate of 50 mV s-1. b) CV curves of CNTs/MnO2 at different scan rates (5-50 mV s-1). c) GCD curves of CNTs/MnO2 at different current density (2-20 A g1).

d) Specific capacitances of Co-MnO2 nanosheets and CNTs/MnO2 as a function of the

current density. e) Nyquist plots in a frequency range from 0.01 Hz to 100 kHz for CoMnO2 nanosheets and CNTs/MnO2. f) cycling stability tests over 10000 cycles for CoMnO2 nanosheets and CNTs/MnO2.

Figure 5. a) CV curves of the CNTs/MnO2 and AC at 50 mV s-1. b) CV curves of the CNTs/MnO2//AC device at different voltage window; c) CV curves of the 22


Page 22 of 30

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


CNTs/MnO2//AC device at different scan rates; d) GCD curves of the ASC device measured at different current densities; e) specific capacitances of CNTs/MnO2//AC at different current densities. f) The Ragone plot related to energy and power densities of the CNTs/MnO2//AC asymmetric supercapacitors.

23



Figure 1

24


Page 24 of 30

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


Figure 2

25



Figure 3

26


Page 26 of 30

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


Figure 4

27



Figure 5

28


Page 28 of 30

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


TOC figure

29



30


Page 30 of 30

Mesostructured carbon nanotube-on-MnO2 nanosheet composite for

Recommend Documents