MnO2 Nanosheets Grown on Internal Surface of Macroporous Carbon

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

MnO2 nanosheets grown on internal surface of macroporous carbon with enhanced electrochemical performance for supercapacitors Jie Yu, Jianpeng Li, Zhonghua Ren, Shuguang Wang, Yaqi Ren, and Yejun Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00092 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 8, 2016

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 free 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 accessible to all readers and 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.

ACS Sustainable Chemistry & Engineering 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 31

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 Sustainable Chemistry & Engineering

MnO2 nanosheets grown on internal surface of macroporous carbon with enhanced electrochemical performance for supercapacitors Jianpeng Li, Zhonghua Ren, Shuguang Wang, Yaqi Ren, Yejun Qiu*, Jie Yu* Shenzhen Engineering Lab for Supercapacitor Materials, Shenzhen Key Laboratory for Advanced Materials, Department of Material Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Shenzhen 518055, China. KEYWORDS: macroporous carbon; manganese oxide nanosheets; areal specific capacitance; supercapacitor

ABSTRACT:

Supercapacitor performance is strongly dependent on utilization rate of electrode materials. In this paper, MnO2 nanosheets (MONSs) have been grown on inner surface of macroporous carbon (MPC) for increasing the utilization rate. The MPC are prepared from luffa sponge fibers. The MPC possesses closely arranged straight channels at micrometer scale, which makes the MONSs be able to grow on the inner surface. Because of sufficient exposure towards electrolyte the MONSs exhibit high mass specific capacitance at different loadings such as 1332 F/g (150 µg/cm2) and 354 F/g (5690 µg/cm2). Due to presence of the large pores allowing the electrolyte

ACS Paragon Plus Environment

1


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

Page 2 of 31

solution to access easily the active materials are capable of working at high loadings, obtaining areal specific capacitance as high as 2.9 F/cm2. The assembled supercapacitors show a high specific energy of 194 µWh/cm2 at the specific power of 4.5 mW/cm2. As the luffa sponge is abundant and pollution-free in production the MONSs/MPC is of high promise for supercapacitor application. The present method to grow the active materials on inner surface to increase the utilization rate is also valuable for other applications, e.g. catalysis and Li-ion batteries.

INTRODUCTION Supercapacitors (SCs) have been attracting ever increasing interest due to the application potential as low-cost and green energy storage devices.1-3 Despite high specific power the SCs have a flaw of low specific energy comparing with Li-ion batteries,4,5 which hinders wide application of the SCs. The performance of SCs is determined by the type, morphology, and property of electrode materials. For the reported SC materials manganese dioxide (MnO2) attracts more interest because of its large specific capacitance (SPC),6,7 cheapness, and environment-friendly nature. MnO2 stores charges based on reversible reactions with the following general equation: MnO2+M++e-↔MnOOM,8,9 where M+ represents cations of hydrogen, lithium, sodium, and potassium. The above charge storage reactions proceed just near the surface of the MnO2 particles and the inner part can not be used. Therefore, the utilization rate of MnO2, which is dependent on the surface area exposed to electrolytes is of great importance for obtaining excellent SC performance. Various strategies have been used to increase the utilization rate of MnO2 materials by making them thinner, smaller, and better dispersed. Toupin et al. prepared scattered MnO2 particles on Pt foil with a loading of 13.3 µg/cm2, which possess a SPC of 1380 F/g (5 mV/s).7 Chen et al. deposited MnO2 nanoparticles


2

Page 3 of 31

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


on CNTs/sponge supports at a loading of 50 µg/cm2 using electrodeposition, obtaining a SPC of 1230 F/g (1 mV/s).10 By incorporating MnO2 nanocrystals into nanopores Lang et al. obtained a SPC of 1145 F/g (50 mV/s).11 Devaraj et al. prepared a MnO2 thin film by electrodeposition at a loading of 30 µg/cm2, which gives a large SPC of 1330 F/g at a discharge current density of 0.5 mA/cm2.12 It is noted that these high SPC were obtained at very low MnO2 loadings, which decease rapidly when increasing the mass loading, e.g. 930 F/g at 66.7 µg/cm2,7 250-200 F/g at 200-500 µg/cm2,10 and 310 F/g at 420 µg/cm2.12 However, increasing the utilization rate by sacrificing the loading amount of active materials on the electrodes is useless for enhancing the practical performance of SCs. At present, the most widely used method increasing the utilization rate is dispersing nanoscale MnO2 materials on various supports such as mesoporous carbon,13 graphene,14 carbon nanotubes (CNT),15 and carbon nanofibers.16,17 In the past reports the MnO2 materials are generally dispersed on the out surface of various supporters. When supported on the out surface the MnO2 materials will aggregate or contact with each other or with binder or conductive additive particles, which will reduce the effective surface of MnO2 and thus reduce the utilization rate. Consequently, we consider that growing MnO2 materials on the inner surface of macroporous carbon (MPC) may be a good measure to overcome this problem. For the growth of MnO2 materials on inner surface of the MPC appropriate pore structure is required. The pore size should be large enough so that reaction solution could enter freely and enough space could be provided to accommodate the grown nanostructures. As to the shape, straight and completely through pores ensuring efficient transfer of the reaction solution deep into the pore structure are advantageous to the full coverage growth on the entire pore surface. For the MPC reported previously the pores are generally spherical and not fully through.18-20 So it is necessary to find novel carbon substrates for the inner-surface growth of the MnO2


3


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

Page 4 of 31

nanomaterials. Recently, by carbonizing luffa sponge fibers we obtained the MPC materials, which have been investigated for SC and catalyst applications.21,22 This MPC possesses closely arranged parallel pores, which are below 20 µm in diameter and 2 µm in wall thickness. The porous structure of the MPC stems from that of the luffa sponge precursor. It is expected that this MPC is suitable to the inner-surface growth of the MnO2 nanomaterials due to its straight, through, and big diameter porous structure. Herein, we grew MnO2 nanosheets (MONSs) inside the macropores of the MPC and investigated their supercapacitive performance. The MONSs were produced using redox reaction between the MPC and potassium permanganate (KMnO4) solution. As the MONSs are located on the inner surface of the macropores the surface area loss due to coverage by other particles could be avoided, leading to excellent SC performance. The mass SPC of MnO2 for the MONSs reaches high values at different loadings such as 1332 F/g (1 A/g, 150 µg/cm2), 567 F/g (0.5 A/g, 2150 µg/cm2), and 354 F/g (0.5 A/g, 5690 µg/cm2). The areal SPC reaches 2.9 F/cm2 (0.5 A/g, 5690 µg/cm2). The assembled SCs based on the MONSs/MPC composite materials exhibit a large specific energy of 194 µWh/cm2 at the specific power of 4.5 mW/cm2. As the luffa sponge is abundant the MONSs/MPC is of high promise as electrode materials of SCs.

EXPERIMENTAL SECTION Preparation of MPC and MONSs/MPC composites. For preparing the MPC, the luffa sponge fibers were heated at 800 ºC in NH3 for 2 hours with the temperature increasing rate at 5 ºC/min. Afterwards, the above products were blended with potassium hydroxide in a weight ratio of 1:2 and activated in N2 to create pores. The activation temperature and time are 750 ºC and 1.5 hours, respectively. The above products are the so-called MPC. After activation, the MPC was


4

Page 5 of 31

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


rinsed in 1 M hydrochloric acid solution for removing the residue impurities resulted from potassium hydroxide. Finally, the MPC was further rinsed repeatedly using distilled water till the residue liquid is nearly neutral. For growing the MONSs, the MPC products (50 mg) were put into 4 mM KMnO4 solution (80 ml) to react for different times at room temperature (～25 °C), obtaining the MONSs/MPC. Material characterization. Scanning electron microscopy (SEM, HITACHI S-4700), X-ray diffraction (XRD, Rigaku D/Max 2500/PC), transmission electron microscopy (TEM, JEM2100HR), and Raman spectroscopy (Renishaw RM-1000) were used to characterize the materials structures. Bonding forms and composition of the obtained materials were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The composition of materials was further determined by an energy dispersive X-ray spectroscope (EDX) attached on SEM instrument. Thermogravimetric analysis (TGA, Netzsch STA 449F3) was performed in air (flowing rate 20 mL/min) up to 900 oC at 10 °C/min for determining the content of the MONSs in the MONSs/MPC composites. Specific surface area (SSA) was tested on Micromeritics Tristar 3000 analyzer by N2 sorption at -196 oC. Prior to testing, the samples were kept under vacuum for 6 hours to remove the sorbed molecules. The SSA was determined according to BET method. Electrochemical measurement. Electrochemical property of the samples was tested in 1 M Na2SO4 electrolyte using platinum wire (CHI115) as the counter electrode, Ag/AgCl (sat.) (CHI111) as the reference electrode, and the MONSs/MPC as the working electrode. For measurements the MONSs//MPC particles were made into paste by blending with carbon black (5%) in alcohol. Afterwards, a certain amount of the paste was coated on glassy carbon electrode with diameter of 3 mm or carbon paper (1×0.5 cm2) followed by drying in atmosphere. Finally, the dry coating was fixed using nafion solution (5 wt.%). During measurements when the mass


5


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

Page 6 of 31

loading is lower we used glassy carbon electrode as the supporting substrates while when the mass loading is higher we used carbon paper as the supporting substrates. The electrochemical performance of the MONSs/MPC was further evaluated by two-electrode symmetric SCs. During assembly of the SCs Al sheets act as current collectors and glass fiber cloths act as separators. The electrolyte is 1 M Na2SO4 solution. In order to prepare SC electrodes

the

MONSs/MPC

particles

were

made

into

paste

by

mixing

with

polytetrafluoroethylene dispersion (60 wt% in water) and carbon black in weight ratio of 90:5:5 in alcohol. The obtained paste was then spread on the Al sheets with an area of 2.27 cm2. In the following step the paste coatings were dried under vacuum at 80 oC for 12 hours. The loading amount of the active materials for the electrodes is 1130-2210 µg/cm2. Prior to assembling the glass fiber separators were put into 1 M Na2SO4 electrolyte and kept for 1 hour. After taking out from the electrolyte the separators were put in between two of the above electrodes and then put into the coin battery shells. Finally, the coin battery shells were sealed with polyvinylidene fluoride insulating gel. The cyclic voltammogram (CV), galvanostatic charge/discharge (GCD) curves, and electrochemical impedance spectra (EIS) were measured by a CHI 760D electrochemical work station (Shanghai CH Instruments Co., China). The operation stability was tested on a battery measuring system (CT2001A, Wuhan Land Electronics Co. Ltd.). The SPC, specific energy, and specific power were calculated in terms of the formulas below: C MOMPC =

C MO =

I × ∆t M MOMPC × ∆V

(1)

C MOMPC − C MPC × ( M MPC / M MOMPC ) ( M MO / M MOMPC )


(2)

6

Page 7 of 31

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


CA =

I × ∆t S × ∆V

(3)

1 E = × C A × ( ∆V ) 2 2 P=

(4)

E ∆t

(5)

In the above formulas the meaning of each term are explained as follows: CMOMPC (F/g)-SPC of the MONSs/MPC calculated using the total mass of MONSs and MPC, I (A)-discharge current, ∆V (V)-discharge voltage, ∆t (s)-discharge time, MMOMPC (g)-mass of the MONSs/MPC composites, CMO (F/g)-SPC of pure MnO2 in the MONSs/MPC composites, CMPC (F/g)-SPC of the MPC measured before growing the MONSs, MMPC-mass of the MPC in the MONSs/MPC composites, MMO (g)-mass of the MONSs in the MONSs/MPC composites, CA (mF/cm2)-areal SPC of the MONSs/MPC, S (cm2)-geometrical area of the electrodes, E (µWh/cm2)-areal specific energy of the SCs, P (mW/cm2)-areal specific power of the SCs. During calculation of the SPC of pure MnO2 in the MONSs/MPC composites the contribution of the MPC substrates was subtracted (formula 2).

RESULTS AND DISCUSSION The preparation of the MPC was carried out by carbonization of the luffa sponge in NH3 and activation in N2 successively. The original fibrous shape and macropores observed for the luffa sponge can be well maintained after heat treatment (Figure S1).21,22 Before carbonization the luffa sponge fibers were cut short and the obtained MPC fibers are 400-1000 µm in length. The macropores of the obtained MPC are completely through from one end to another end (Figure S1). By soaking in KMnO4 solution for different times the MONSs were prepared on the


7


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

Page 8 of 31

macropore walls of the MPC with different loading amount. Figure 1a and b show the SEM images taken from the end face of a MPC fiber after growing the MONSs for 3 hours. It is observed that after growing the MONSs the original macroporous morphology remained and both the outer and inner surface were covered with the MONSs. Figure 1c was taken from the cross section of a broken MPC fiber after 3 hours growth reaction, where dense and continuous MONSs can be observed on the macropore walls. This demonstrates that the MONSs can be grown on all the inner surface of the MPC. It is believed that the unique porous structure including large size, completely through and straight shape, and smooth wall surface allows the reaction solution to penetrate deep into the macropores easily, resulting in the full coverage growth. The MONSs are vertically oriented on the wall surface with the length ranging from 50 to 130 nm (Figure 1d). It is considered that both the utilization rate and loading amount of MnO2 can be greatly increased in the electrode by growing on the inner surface as the MONSs can be fully exposed towards the electrolyte and the electrolyte is able to penetrate deep into the macropores freely. In this work, the dependence of the growth behavior of the MONSs on growth time was probed by soaking the MPC in KMnO4 solution for 0.5, 2, 3, and 4 hours, respectively. It is found that at all growth time the products are nanosheets. One visible change is that the nanosheet density increases with prolonging growth time (Figure S2). Cross section SEM images were taken for evaluating the thickness of the MONSs coatings (Figure S3). However, it is difficult to find a strictly horizontal cross section to accurately measure the thickness of the MONSs coatings. From the tilted cross section it is observed that the MONSs coatings are over 200 nm in thickness (Figure S3).


8

Page 9 of 31

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 1. SEM images of the MONSs/MPC after 3 hours reaction taken from end surface (a, b), cross section of a broken fiber (c), and the inner surface (pore walls) (d). Figure 2a presents XRD patterns of the MPC fibers and MONSs/MPC samples grown for different times. It is observed that after growth reaction some new peaks appear apart from the peaks of the pure carbon materials shown in pattern I. The new peaks appear at 12.2, 24.8, 36.6, _

__

42.1, and 65.6°, which correspond to the diffraction of (001), (002), (200)/(201), (112 ), and __

(020)/(312 ) planes of birnessite MnO2 (JCPDS 42-1317),17,23 respectively. The peak at 24.8° overlaps with the broad feature of amorphous carbon. At longer growth time the peak intensity of the MnO2 is higher, suggesting the continuous growth of the MONSs. Figure 2b presents Raman spectra of the pristine MPC fibers and MONSs/MPC samples grown for different times. The spectrum of the MPC fibers exhibits the well-known D and G peaks, which is typical of graphite


9


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

Page 10 of 31

structured material.24 For the spectrum of the MONSs/MPC sample besides the carbon peaks from the MPC substrates some new peaks locating at about 507, 574, and 626 cm-1 are observed, which originate from the MONSs.25,26 The peak at 574 cm-1 results from Mn-O stretching mode in basal plane of [MnO6] sheets while the 626 cm-1 peak is caused by symmetric stretching mode of Mn-O in the MnO6 groups.25,26 The composition and bond type of the MONSs was tested by XPS. Figure 2c shows the typical XPS spectrum of the MONSs/MPC. It is indicated that C, Mn, and O elements are present in the MONSs/MPC and the Mn:O atomic ratio is 1:2.25. On the Mn 2p spectrum shown in Figure 2d, the peaks at 642.4 eV and 654.2 eV correspond to Mn 2p3/2 and Mn 2p1/2 electrons, respectively. The spin-energy separation of the two peaks is 11.8 eV. This is well consistent with the previous reports about MnO2.27,28 The Mn:O atomic ratio tested by EDX spectroscopy is 1:2.05 (Figure S4), confirming the formation of MnO2. TEM was used to further characterize the structure of the MONSs. Figure 3a is a typical TEM image taken from a peeled MONSs fragment from the sample grown for 3 hours. It is found that the MONSs are interconnected firmly and difficult to be separated even subject to vigorous ultrasonic vibration. Figure 3b and c are the high resolution TEM images taken from a bended edge and a position on the plane of a MnO2 nanosheet, respectively, where the lattice fringes can be clearly observed. The interplanar distances measured from Figure 3b and c are 0.72 and 0.26 nm, approaching (001) and (200) interplanar distances of birnessite MnO2, respectively. This agrees with the XRD results, confirming that the obtained MONSs possess birnessite phase structure. The dark strips on the images are from the nanosheets in parallel with the TEM electron beam or folded edges of the nanosheets, from which thickness of the nanosheets can be measured. From Figure 3a and b we found that the nanosheets are mostly in the range of 3-5 nm in thickness. The TEM measurements were also made on the MONS sample grown for 0.5 hours,


10

Page 11 of 31

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


which indicates that the MONSs range from 2 to 4 nm mostly in thickness, a little thinner than the MONSs grown for 3 hours (Figure S5).

Figure 2. (a, b) XRD patterns (a) and Raman spectra (b) of the MPC and MONSs/MPC samples grown for different times: I-MPC, II-0.5 hours, III-2 hours, IV-3 hours, V-4 hours, (c) XPS survey spectrum of the MONSs/MPC, (d) Typical Mn 2p XPS spectrum of the MONSs/MPC.


11


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

Page 12 of 31

Figure 3. TEM images of the MONSs grown for 3 hours: (a) low magnification, (b) taken from a bended edge, (c) taken from a position on the sheet plane. The SSA and pore diameter distribution of different samples were determined by nitrogen adsorption/desorption isotherms (Figure S6). The SSAs of the MPC and the MONSs/MPC


12

Page 13 of 31

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


grown for 0.5, 2, 3, and 4 hours are 1510, 968, 951, 850, and 828 m2/g (Table S1), respectively. It can be seen that the SSA of the samples decreases rapidly after growing the MONSs. This is because the surface of the MPC was covered by the MONSs and the pore structure of the MPC was destroyed after the growth reaction in KMnO4 solution. The SSA of the MONSs/MPC should be mainly from the surface of the nanoscale MONSs. The SSA decreases gradually with prolonging the reaction time, which is because more surface of the MPC was covered by the MONSs and the size of the MONSs increases correspondingly. In order to determine the content of the pure MnO2 in the MONSs/MPC composites TGA curves were measured on the different samples, which were shown in Figure 4a. In the TGA curves the weight decrease between 25 and 100 oC is due to desorption of physically sorbed water molecules and that between 100 and 250 o

C is caused by desorption of chemically bound water molecules. The major weight decrease

occurs in 250-420 oC for the MONSs/MPC and 500-643 oC for the MPC, which are all caused by combustion of carbon. XRD measurements indicate that the final product after calcination of the MONSs/MPC at 900 oC is Mn2O3 (Figure S7), consistent with the previous reports.29,30 It is noted that the carbon combustion temperature of the MONSs/MPC samples is lower than that of the MPC sample, which is obviously because of the effects of the MONSs. The MONSs (MnO2) is able to react with the MPC substrates directly in the MONSs/MPC composites at lower temperature due to its high oxidizability and transforms to a thermodynamically stable Mn2O3 with the release of oxygen.29 In addition, the MONSs may have some catalytic effect promoting carbon combustion and thus decrease the carbon combustion temperature in the MONSs/MPC composites. The TGA curves indicate that the pure MPC loses 96.0% of its initial weight at 900 o

C and the weight loss for the MONSs/MPC composites grown for 0.5, 2, 3, and 4 hours are

85.26%, 75.54%, 69.46%, and 66.65%, respectively. These data indicate that the content of


13


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

Page 14 of 31

MONSs increases with the reaction time. Assuming that the combustion ash content of the MPC in the MONSs/MPC composites is 4.0% also and all the MONSs were converted into Mn2O3 after calcinations at 900 oC under air atmosphere the contents of the MnO2 substance in the MONSs/MPC composites grown for 0.5, 2, 3, and 4 hours were calculated to be 10.5%, 21.2%, 27.9%, and 31.0% (Figure 4b), respectively.

Figure 4. (a) TGA curves of the MPC and MONSs/MPC samples grown for different time. (b) MnO2 content in the MONSs/MPC composites versus growth time. The supercapacitive properties of the MONSs/MPC were first tested by the three-electrode method. Figure 5a shows the CV curves of the MPC and MONSs/MPC grown for different times (20 mV/s), which are all roughly rectangular, suggesting good charge storage character.31,32 It was found that the current density increases with growth time first and decreases after 3 hours, reflecting the corresponding change of the capacitance. The CV curves keep stable when


14

Page 15 of 31

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


increasing the scan rate up to 100 mV/s for the different samples (Figure S8), indicating a fast charge transportation/transfer characteristic of the electrolyte, interfaces, and electrode structure of the MONSs/MPC. The GCD curves are roughly triangular, confirming the ideal capacitive properties of the obtained samples (Figure 5b and Figure S9). Similarly, the sample grown for 3 hours shows the longest discharge time, signifying the highest SPC in the different samples. The SPC of the samples was determined according to the discharge curves. Figure 5c shows the SPC of the MONSs/MPC composites (CMOMPC) grown for different times in 1-10 A/g. The SPC of the original MPC has been measured to be 171 to 132 F/g in 0.5-10 A/g (Table S2). The capacitance of the MPC mainly results from its large SSA generated by the activation reaction. After growth reaction the SPC of the MONSs/MPC composites increases greatly. The SPC increases with increasing growth time from 0.5 h to 3 h and then decreases. The MONSs/MPC grown for 3 hours possesses the largest SPC, which reaches 397 F/g at 1 A/g and mass loading of 1420 µg/cm2. For exploring the effect of inner-surface growth on utilization rate, the SPC of pure MnO2 was determined for different samples. The mass loading of MONSs/MPC composites is 1420 µg/cm2 on the glassy carbon electrode during the electrochemical measurements shown in Figure 5, corresponding to the MnO2 loadings of 150, 300, 400, and 440 µg/cm2 for the MONSs/MPC composite samples grown for 0.5, 2, 3, and 4 hours according to the above calculated MnO2 contents in the composites, respectively. Accordingly, the SPC of the pure MnO2 substance (CMO) can be calculated. As shown in Figure 5d, the MONSs show high SPC, e.g., 1332, 1027, 1000, and 819 F/g (1 A/g) for the MONSs/MPC samples grown for 0.5, 2, 3, and 4 hours, respectively. It can be seen that the SPC of the MONSs falls monotonously with increasing the growth time. This is because at longer growth time the MONS coating thickness and the density of the


15


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

Page 16 of 31

MONSs increase. Therefore, the amount of the active materials in the bottom part inaccessible to the electrolyte increases, resulting in the decrease of the SPC. It is clearly manifested that the utilization rate of the present MONSs is very high. At lower loading of 150 µg/cm2 the CMO is close to the theoretical value of MnO2.7 At higher loadings of 300-440 µg/cm2 the CMO of 1027, 1000, and 819 F/g outnumbers the top level in previous reports such as the electrodeposited MnO2 films (310 F/g, 420 µg/cm2, 0.5 mA/cm2),12 MnO2-CNT-sponge (about 235-205 F/g, 200500 µg/cm2, 1 mV/s),10 MnO2-CNT-textile (410 F/g, 60 µg/cm2, 5 mV/s),33 MnO2-graphene composites (about 780 F/g, 310 µg/cm2, 10 mV/s),34 MnO2-graphitic petals (580 F/g, 150 µg/cm2, 2 mV/s),35 MnO2-SnO2 (637 F/g, 80 µg/cm2, 2 mV/s),36 and MnO2-CNF ( 557 F/g, 330 µg/cm2, 1 A/g).37 It is worth noting that during calculation of the CMO the double-layer capacitance from the original MPC such as 164 F/g at 1 A/g has been subtracted (Equation 2). Nevertheless, the SPC of the MPC must decrease after growing the MONSs because of coverage of the surface by the MONSs and destruction of the pore structure by KMnO4 oxidation. Therefore, the calculated CMO should be smaller than the real values. It is noted that different from the monotonous decrease of the CMO with growth time (Figure 5d) the CMOMPC increases first and then decreases after 3 hours (Figure 5c). This is because initially the increasing rate of the capacitance is higher than the increasing rate of the mass during the growth of the MONSs as the capacitance of the MONSs/MPC composites is mainly from the MONSs. Because the CMO decreases monotonously with growth time the CMOMPC decreases with further increasing the growth time after growing for enough time such as 3 hours, when the capacitance increasing rate is lower that the mass increasing rate.


16

Page 17 of 31

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 5. (a-b) CV curves (a) at 20 mV/s and GCD curves (b) at 1 A/g of the MPC and MONSs/MPC samples grown for different times. (c) SPC of the MONSs/MPC composites (CMOMPC) versus growth time. (d) SPC of MnO2 contained in the MONSs/MPC composites (CMO) versus growth time. Loading amount of the active materials is a key factor influencing the actual SC performance. Loading more active materials at similar total weight of electrolyte, current collectors, and packaging, i.e. increasing the percentage of the active material in a single device, is required for obtaining excellent device performance. As the MONSs/MPC composite grown for 3 hours possess the highest SPC the effects of the loading amount was explored based on this sample, which is shown in Figure 6. In the figure the data points from 7700 to 22100 µg/cm2 were measured by coating the active materials on carbon paper because the coatings are too thick at these high loadings and those from 1130 to 5660 µg/cm2 were measured on glassy carbon electrode. It is observed that with increasing the mass loading the CMOMPC decreases monotonously (Figure 6a). This is obviously because with increasing the coating thickness of the active materials the resistance for the ion transport to bottom part of the coatings increases, decreasing the material utilization rate. It is indicated that the MONSs/MPC composites exhibit


17


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

Page 18 of 31

high SPC, which reaches 420 F/g at 1130 µg/cm2 (0.5 A/g) and 160 F/g even at 2040 µg/cm2 (0.5 A/g). For investigating the utilization rate of MnO2 at different loadings the CMO was calculated from the CMOMPC shown in Figure 6a (Table S2). Because the capacitance of the MPC in the MONSs/MPC decays also with increasing the mass loading we used the decay rates of the CMOMPC at different loadings to calculate the SPC of the MPC substrates. During calculation the data at the mass loading of 1130 µg/cm2 was set as the benchmark. For example, at 1 A/g and 1130 µg/cm2 the CMOMPC is 405 F/g and the contribution of the MPC is 164 F/g while at 2120 µg/cm2 the CMOMPC is 367 F/g, from which the decay rate was calculated to be 0.91 and thus the SPC of the MPC substrates is 149 F/g at 2120 µg/cm2. Then the CMO of 931 F/g was obtained using equation 2 (corresponding to the MnO2 loading of 590 µg/cm2). The calculated SPCs of MnO2 are shown in Figure 6b, which indicates that the MnO2 deposited on inner surface of the MPC possesses extraordinary SPC even at high MnO2 loading such as 551 F/g at 2150 µg/cm2 and 307 F/g at 5690 µg/cm2 at 1 A/g. It is manifested that the utilization rate of the MnO2 has been significantly enhanced by inner-surface growth comparing with the previously reported MnO2 at high loading such as MnO2-graphene composites (322.1 F/g, 1030 µg/cm2, 10 mV/s),34 MnO2-CNTs (290 F/g, 700 µg/cm2, 10 mV/s),38 graphene-MnO2 composites (155 F/g, 850 µg/cm2, 10 mV/s),18 MnO2-agarose gel (100 F/g, 750 µg/cm2, 10 mV/s),39 and MnO2-carbon (240 F/g, 1200 µg/cm2, 5 mV/s).40


18

Page 19 of 31

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 6. (a) Mass SPC CMOMPC at different MONSs/MPC loadings, (b) Mass SPC CMO at different MnO2 loadings, (c) Areal SPC CA at different MONSs/MPC loadings.


19


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

Page 20 of 31

One key criterion evaluating the electrode performance is areal SPC, which determines the actual capacitance of the final devices. Figure 6c shows the dependency relation of the areal SPC CA on the MONSs/MPC mass loading. It is observed that with increasing the mass loading the CA increases first and then decrease after reaching a maximum. With increasing the mass loading more active materials in unit area contribute to the charge storage but the mass SPC decreases simultaneously, resulting in the appearance of the maximum point. It is found that at higher current density the maximum shift towards lower mass loading, which is because the mass SPC decays more quickly with increasing the mass loading at higher current density (Table S2). It is indicated that the MONSs/MPC exhibit high areal SPC. For instance, at the loading of 2040 µg/cm2 (corresponding to MnO2 loading of 5690 µg/cm2) the CA reaches 1.0 to 2.9 F/cm2 in 100.5 A/g. The present values are among the top ones reported for other MnO2-related materials such as graphene-MnO2 composite networks (1.42 F/cm2, 9600 µg/cm2, 2 mV/s),18 MnO2-CNTtextile (2.8 F/cm2, 8300 µg/cm2, 0.05 mV/s),33 MnO2–graphene composites (3.2 F/cm2, MnO2 loading 13600 µg/cm2, 1 mV/s),34 and MnO2-polymer composites (1.1 F/cm2, 3100 µg/cm2, 5 mV/s).41 Rate capability is very important for SCs, especially at high loading. The MONSs/MPC composites show high rate capability at high loadings, as exemplified at the MONSs/MPC loading of 20400 µg/cm2 (MnO2 loading 5690 µg/cm2), where the capacitance retention reaches 36% when the discharge current density was increased from 0.5 A/g to 10 A/g and 48% from 0.5 A/g to 5 A/g. This rate capability is also excellent comparing with the reported high performance materials such as graphene-MnO2 composite networks (8.5%, 10-200 mV/s, 14900 µg/cm2),18 MnO2-CNT-textile (about 9.5%, 10-200 mV/s, 3800 µg/cm2),33 and MnO2–graphene composites (17%, 10-200 mV/s, 6100 µg/cm2).34 It has been clearly revealed that the utilization rate of MnO2 could be greatly increased by the inner-surface growth, leading to high mass SPC. We


20

Page 21 of 31

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


consider that the three factors of inner-surface growth, macropore structure, and nanosheet structure work together to account for the high SPC. The inner-surface growth makes the MONSs fully exposed, the macropores provide free channels for the electrolyte penetrating, and the small thickness of the MONSs makes the ion diffusion path short. Additionally, the macropores at micrometer scale allowing the electrolyte solution to access easily enables the active materials work at high loadings, leading to the high areal SPC. In addition, we found that carbonization in NH3 is beneficial to the enhancement of the SC performance of MONSs/MPC comparing with carbonization in N2 and Ar. It is considered that carbonization in NH3 can generate nitrogen doped and rough surface due to the high chemical reactivity of NH3,4,22 which may improve the interfacial bonding structure between the MONSs and MPC and thus enhances the performance. Therefore, we selected NH3 as the carbonization atmosphere. For clarifying the practicability of the MONSs/MPC materials their electrochemical performance was further evaluated in a two-electrode SC system. We assembled symmetric SCs using the MONSs/MPC composites grown for 3 hours as the electrode material since this sample has the largest SPC. Figure 7a presents the CV curves of the fabricated SCs with the MONSs/MPC loading of 12010 µg/cm2, which exhibits quasi-rectangle shape. However, the CV curves become inclined at higher scan rates, which is due to the high diffusion resistance at high mass loadings. The GCD curves of the SCs in Figure 7b also exhibit nearly triangular shape. Similarly, the deviation from linearity mainly results from the increased diffusion resistance of the electrodes at high mass loading. The areal specific energy of the SCs with different mass loading was obtained from the discharge curves (Figure 7c). The specific energy increases with increasing the mass loading from 5100 to 12010 µg/cm2 and then decreases with further increasing the mass loading to 15820 µg/cm2, which is in agreement with the changing tendency


21


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

Page 22 of 31

of the three-electrode measurements. The maximum specific energy reaches 194 µWh/cm2 at a specific power of 4.5 mW/cm2. As far as we know, this areal specific energy is higher than the symmetric SCs based on MnO2 in previous reports,42,43 demonstrating the superiority of the present method. It is worth noting that the present macropores are too big for space utilization. The electrochemical performance of the SCs may be improved further greatly if using the MPC with smaller pore size as the substrates. It is also expected that the present inner-surface growth is applicable to other SC materials including Ni(OH)2, V2O5, NiCo2O4, PANI, PPy, etc. Cycling stability was tested at 5 mA/cm2, which indicates that the capacitance retention reaches 80% after 5000 cycles (about 17 days) and the capacitance is very stable after 2000 cycles (Figure 7d 6d). This cycling stability is excellent considering the inherent poorer stability of MnO2 than carbon materials. The EIS spectra were measured for the SC before and after 5000 cycles (Figure 7e 6e). It is found that the internal resistance of the SC is as small as 0.58 Ω before cycling test and only increases slightly to 1.03 Ω after 5000 cycles, suggesting the high structure stability of the MONSs. By charging two series SCs for 13 s (at 50 mA/cm2) the LEDs with different colors has been successfully lightened for 192 (red), 136 (yellow), 118 (green), 80 (blue), and 42 (white) seconds, respectively (Figure 7f), showing the high application value of the MONSs/MPC. Because of the process compatibility between our method and industrial production and the abundance, greenness, and renewability of the luffa sponge the MONSs/MPC may be of high potential for future application.


22

Page 23 of 31

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 7. Electrochemical performance of the symmetric SCs assembled using the MONSs/MPC electrode materials: (a) CV curves; (b) GCD curves; (c) Ragone plot at different MONSs/MPC loadings; (d) Operation stability of the device at 5 mA/cm2; (e) Nyquist plots before and after cycling test; (f) Optical images of the lightened LED lights. CONCLUSIONS In conclusion, the utilization rate of MnO2 for SC application has been greatly improved by inner-surface growth. By using the MPC prepared from luffa sponge fibers as the supporting substrates the MONSs were successfully grown on the inner surface of macropores. The macropores of the MPC are at micrometer-scale in diameter, parallel with each other, closely arranged, fully through, and smooth in wall surface, enabling the reaction solution to penetrate deep into the pores and thus inner-surface growth of the MONSs. Because the MONSs are fully exposed towards electrolyte the MONSs exhibit high utilization rate for SC application, resulting in extraordinarily high mass SPC of MnO2. As the large pores at micrometer scale allow the electrolyte solution to access easily the active materials are capable of working at high loadings, generating high areal SPC. The SCs based on the MONSs/MPC exhibit excellent supercapacitive performance with high areal specific energy and long-term stability. The present method to


23


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

Page 24 of 31

increase the utilization rate of active materials by inner-surface growth is also valuable for other applications, e.g. catalysis and Li-ion batteries.

ACKNOWLEDGEMENTS This work is supported by the National Basic Research Program of China (2012CB933003), National Natural Science Foundation of China (No. 51272057), and Shenzhen Basic Research Program (JCYJ20130329150737027). ASSOCIATED CONTENT Supporting Information. SEM images, TEM images, EDX spectra, TGA curves, XRD patterns, CV curves, GCD curves, SPC at different loadings, and BET data. AUTHOR INFORMATION Corresponding Author E-mail: [email protected], [email protected]. (Jie Yu). Tel: 86-755-26033478. Fax: 86-755-26033504 Funding Sources The authors declare no competing financial interest.


24

Page 25 of 31

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


REFERENCES (1) Hall, P. J.; Mirzaeian, M.; Fletcher, S. I.; Sillars, F. B.; Rennie, A. J. R.; Shitta-Bey, G. O.; Wilson, G.; Cruden, A.; Carter, R. Energy storage in electrochemical capacitors: designing functional materials to improve performance. Energy Environ. Sci. 2010, 3 (9), 1238-1251. (2) Pandolfo, A. G.; Hollenkamp, A. F. Carbon properties and their role in supercapacitors. J. Power Sources 2006, 157 (1), 11-27.

(3) AricÒ, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Schalkwijk, W. V. Nanostructured materials for advanced energy conversion and storage devices. Nat. Mater. 2005, 4 (5), 366-377. (4) Zhao, L.; Qiu, Y. J.; Yu, J.; Deng, X. Y.; Dai, C. L.; Bai, X. D. Carbon nanofibers with radially grown graphene sheets derived from electrospinning for aqueous supercapacitors with high working voltage and energy density. Nanoscale 2013, 5 (11), 4902-4909. (5) Zhang, H. G.; Yu, X. D.; Braun, P. V. Three-dimensional bicontinuous ultrafast-charge and -discharge bulk battery electrodes. Nat. Nanotechnol. 2011, 6, 277-281. (6) Toupin, M.; Brousse T.; Bélanger, D. Influence of microstucture on the charge storage properties of chemically synthesized manganese dioxide. Chem. Mater. 2002, 14 (9), 3946– 3952. (7) Toupin, M.; Brousse, T.; Bélanger, D. Charge storage mechanism of MnO2 electrode used in aqueous electrochemical capacitor. Chem. Mater. 2004, 16 (16), 3184-3190. (8) Kuo, S. L.; Wu, N. L. Investigation of pseudocapacitive charge-storage reaction of MnO2 · nH2O supercapacitors in aqueous electrolytes. J. Electrochem. Soc. 2006, 53 (7), A1317A1324.


25


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

Page 26 of 31

(9) Pang, S. C.; Anderson, M. A.; Chapman, T. W. Novel electrode materials for thin-film ultracapacitors: comparison of electrochemical properties of sol-gel-derived and electrodeposited manganese dioxide. J. Electrochem. Soc. 2000, 147 (2), 444-450. (10) Chen, W.; Rakhi, R. B.; Hu. L. B.; Xie, X.; Cui, Y.; Alshareef, H. N. High performance nanostructured supercapacitors on a sponge. Nano Lett. 2011, 11 (12), 5165-5172. (11) Lang, X. Y.; Hirata, A.; Fujita, T.; Chen, M. W. Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors. Nat. Nanotechnol. 2011, 6, 232-236. (12) Devaraj, S.; Munichandraiah, N. High capacitance of electrodeposited MnO2 by the effect of a surface-active agent. Electrochem. Solid ST. 2005, 8 (7), A373-A377. (13) Patel, M. N.; Wang, X. Q.; Wilson, B.; Ferrer, D. A.; Dai, S.; Stevenson, K. J.; Johnston, K. P. High pseudocapacitance of MnO2 nanoparticles in graphitic disordered mesoporous carbon at high scan rates. J. Mater. Chem. 2010, 20 (7), 390-398. (14) Yan, J.; Fan, Z. J.; Wei, T.; Qian, W. Z.; Zhang, M. L.; Wei, F. Fast and reversible surface redox reaction of graphene–MnO2 composites as supercapacitor electrodes. Carbon 2010, 48 (13), 3825-3833.

(15) Raymundo-piñero, E.; Khomenko, V.; Frackowiak, E.; Béguin, F. Performance of manganese oxide/CNTs composites as electrode materials for electrochemical capacitors. J. Electrochem. Soc. 2005, 152 (1), A229-A235.

(16) Zhi, M. J.; Manivannan, A.; Meng, F. K.; Wu, N. Q. Highly conductive electrospun carbon nanofiber/MnO2 coaxial nano-cables for high energy and power density supercapacitors. J. Power Sources 2012, 208, 345-353.


26

Page 27 of 31

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


(17) Zhao, L.; Yu, J.; Li, W. J.; Wang, S. G.; Dai, C. L.; Wu, J. W.; Bai, X. D.; Zhi, C.Y. Honeycomb porous MnO2 nanofibers assembled from radially grown nanosheets for aqueous supercapacitors with high working voltage and energy density. Nano Energy 2014, 4, 39-48. (18) He, Y. M.; Chen, W. J.; Li, X. D.; Zhang, Z. X.; Fu, J. C.; Zhao, C. H.; Xie, E.Q. Freestanding three-dimensional graphene/MnO2 composite networks as ultralight and flexible supercapacitor electrodes. ACS Nano 2013, 7 (1), 174-182. (19) Choi, B. G.; Yang, M. H.; Hong, W. H.; Choi, J. W.; Huh, Y. S. 3D macroporous graphene frameworks for supercapacitors with high energy and power densities. ACS Nano 2012, 6 (5), 4020-4028.

(20) Worsley, M. A.; Pauzauskie, P. J.; Olson, T. Y.; Biener, J.; Satcher Jr, J. H. Baumann, T. F. Synthesis of graphene aerogel with high electrical conductivity. J. Am. Chem. Soc. 2010, 132 (40), 14067-14069.

(21) Li, J. P.; Ren, Z. H.; Ren, Y. Q.; Zhao, L.; Wang, S. G.; Yu, J. Activated carbon with micrometer-scale channels prepared from luffa sponge fibers and their application for supercapacitors. RSC Adv. 2014, 4 (67), 35789-35796. (22) Li, J. P.; Wang, S. G.; Ren, Y. Q.; Ren, Z. H.; Qiu, Y. J.; Yu, J. Nitrogen-doped activated carbon with micrometer-scale channels derived from luffa sponge fibers as electrocatalysts for oxygen reduction reaction with high stability in acidic media. Electrochim. Acta 2014, 149, 5664. (23) Ma, S. B.; Ahn, K. Y.; Lee, E. S.; Oh, K. H.; Kim, K. B. Synthesis and characterization of manganese dioxide spontaneously coated on carbon nanotubes. Carbon 2007, 45 (2), 375-382.


27


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

Page 28 of 31

(24) Qiu, Y. J.; Yu, J.; Shi, T. N.; Zhou, X. S.; Bai, X. D.; Huang, J. Y. J. Nitrogen-doped ultrathin carbon nanofibers derived from electrospinning: Large-scale production, unique structure, and application as electrocatalysts for oxygen reduction. J. Power Sources 2011, 196 (23), 9862-9867.

(25) Julien, C.; Massot, M.; Baddour-Hadjean, R.; Franger, S.; Bach, S.; Pereira-Ramos, J. P. Raman spectra of birnessite manganese dioxides. Solid State Ionics 2003, 159 (3-4), 345-356. (26) Julien, C. M.; Massot, M.; Poinsignon, C. Lattice vibrations of manganese oxides: Part I. Periodic structures. Spectrochim. Acta A 2004, 60 (3), 689-700. (27) Lee, S. W.; Kim, J.; Chen, S.; Hammond, P. T.; Shao-Horn Y. Carbon nanotube/manganese oxide ultrathin film electrodes for electrochemical capacitors. ACS Nano 2010, 4 (7), 3889-3896. (28) Tan, B. J.; Klabunde, K. J.; Sherwood, P. M. A.; XPS studies of solvated metal atom dispersed (SMAD) catalysts. Evidence for layered cobalt-manganese particles on alumina and silica. J. Am. Chem. Soc. 1991, 113 (3), 855-861. (29) Li, H. H.; Zhang, X. D.; Ding, R.; Qi, L.; Wang, H. Y. Facile synthesis of mesoporous MnO2 microspheres for high performance AC//MnO2 aqueous hybrid supercapacitors. Electrochim. Acta 2013, 108, 497-505.

(30) Sharma, R. K.; Oh, H. S.; Shul, Y. G.; Kim, H. Growth and characterization of carbonsupported MnO2 nanorods for supercapacitor electrode. Phy. Rev. B: Condens Matter. 2008, 403 (10-11), 1763-1769.


28

Page 29 of 31

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


(31) Guo, W. H.; Liu, T. J.; Jiang, P.; Zhang, Z. J. Free-standing porous Manganese dioxide/graphene composite films for high performance supercapacitors. J. Colloid. Interface Sci. 2015, 437, 304-310.

(32) Devaraj, S.; Gabriel, D. S.; Gajjela, S. R.; Balaya, P. Mesoporous MnO2 and Its Capacitive Behavior. Electrochem. Solid ST. 2012, 15 (4), A57-A59. (33) Hu, L. B.; Chen, W.; Xie, X.; Liu, N.; Yang, Y.; Wu, H.; Yao, Y.; Pasta, M.; Alshareef, H.N.; Cui, Y. Symmetrical MnO2–carbon nanotube–textile nanostructures for wearable pseudocapacitors with high mass loading. ACS Nano 2011, 5 (11), 8904-8913. (34) Zhai, T.; Wang, F. X.; Yu, M. H.; Xie, S. L.; Liang, C. L.; Li, C.; Xiao, F. M.; Tang, R. H.; Wu, Q. X.; Lu, X. H.; Tong, Y. X. 3D MnO2–graphene composites with large areal capacitance for high-performance asymmetric supercapacitors. Nanoscale 2013, 5 (15), 67906796. (35) Xiong, G. P.; Hembram, K. P. S. S.; Reifenberger, R. G.; Fisher, T. S. MnO2-coated graphitic petals for supercapacitor electrodes. J. Power Sources 2013, 227, 254-259. (36) Yan, J.; Khoo, E.; Sumboja, A.; Lee, P. S. Facile coating of manganese oxide on tin oxide nanowires with high-performance capacitive behavior. ACS Nano 2010, 4 (7), 4247-4255. (37) Wang, J. G.; Yang, Y.; Huang, Z. H.; Kang, F. Y. Coaxial carbon nanofibers/MnO2 nanocomposites as freestanding electrodes for high-performance electrochemical capacitors. Electrochim. Acta 2011, 56 (25), 9240-9247.


29


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

Page 30 of 31

(38) Lin, Y. P.; Tsai, C. B.; Ho. W. H.; Wu, N. L. Comparative study on nanostructured MnO2/carbon composites synthesized by spontaneous reduction for supercapacitor application. Mater. Chem. Phys. 2011, 130 (1-2), 367–372.

(39) Park, S.; Nam, I.; Kim, G. P.; Han, J. W.; Yi, J. Hybrid MnO2 film with agarose gel for enhancing the structural integrity of thin film supercapacitor electrodes. ACS Appl. Mater. Interfaces 2013, 5 (20), 9908−9912.

(40) Chang, J. K.; Tsai, W. T. Material characterization and electrochemical performance of hydrous manganese oxide electrodes for use in electrochemical pseudocapacitors. J. Electrochem. Soc. 2003, 150 (10), A1333-A1338.

(41)

Lv,

P.;

Feng,

Y.

Y.;

Li,

Y.;

Feng,

W.

Carbon

fabric-aligned

carbon

nanotube/MnO2/conducting polymers ternary composite electrodes with high utilization and mass loading of MnO2 for super-capacitors. J. Power Sources 2012, 120, 160–168. (42) Wang, X.; Myers, B. D.; Yan, J.; Shekhawat, G.; Dravid, V.; Lee, P. S. Manganese oxide micro-supercapacitors with ultra-high areal capacitance. Nanoscale 2013, 5 (10), 4119-4122. (43) Wang, C.; Zhan, Y.; Wu, L. X.; Li, Y. Y.; Liu, J. P. High-voltage and high-rate symmetric supercapacitor based on MnO2-polypyrrole hybrid nanofilm. Nanotechnology 2014, 25 (30), 305401-305409.

Abstract graphics


30

Page 31 of 31

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


MnO2 nanosheets grown on internal surface of macroporous carbon with enhanced electrochemical performance for supercapacitors Jianpeng Li, Zhonghua Ren, Shuguang Wang, Yaqi Ren, Yejun Qiu*, Jie Yu*

Shenzhen Engineering Lab for Supercapacitor Materials, Shenzhen Key Laboratory for Advanced Materials, Department of Material Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Shenzhen 518055, China. *Corresponding author. E-mail: [email protected], [email protected]

The table of contents entry: The utilization efficiency of MnO2 nanosheets can be greatly enhanced by growing on the internal wall surface of macroporous carbon, resulting in high mass and areal specific capacitance at high loadings.


31

MnO2 Nanosheets Grown on Internal Surface of Macroporous Carbon

Recommend Documents