DOI: 10.1002/celc.201400006

MnO2 Nanoflower Arrays with High Rate Capability forFlexible SupercapacitorsWenyao Li,[a, b] Kaibing Xu,[a] Bo Li,[a] Jianqing Sun,[a, c] Feiran Jiang,[a] Zhishui Yu,[b]

Rujia Zou,*[a] Zhigang Chen,[a] and Junqing Hu*[a]

Large-area a-MnO2 nanoflower arrays are synthesized by usinga facile and template-free electrochemical deposition on flexi-ble graphite paper. These nanoflower arrays exhibit a high spe-cific capacitance of 314 F g�1 at a current density of 1 A g�1

with remarkable rate capability (61.1 % retention at 40 A g�1)and excellent cycle stability. Furthermore, the fabricated flexi-ble symmetric supercapacitor also displays good performanceand flexibility.

Flexible energy-storage devices have attracted tremendous at-tentions in recent years because they show promise in integra-tion into stretchable and wearable electronics.[1–3] In particular,supercapacitors have attracted significant interest as energy-storage devices because of their high power density, long cy-cling life, and short charging–discharging time.[4, 5] Accordingto the mechanism of charge storage, supercapacitors can beclassified as either electrical double-layer capacitors (EDLCs) orpseudocapacitors (PCs). For EDLCs, the charges are adsorbedelectrostatically at the electrode/electrolyte interface, typicallybased on carbon materials, whereas PCs store energy throughredox reactions on electrode materials that are usually basedon conducting polymers and transition-metal oxides (TMOs).[6]

Until now, for PCs, there has been extensive interest in devel-oping commercially attractive TMO electrodes, because of theirhigher specific capacitance compared with conventional EDLCs, which can be attributed to their fast and reversible redox re-actions[7] involving, for example, MnO2,[8] Co3O4,[9] NiO,[10] VOx,

[11]

or Fe2O3.[12] Among these candidates, MnO2 has been consid-ered to be the most promising candidate, owing to its lowcost, natural abundance, and high theoretical capacity

(1370 F g�1).[13–15] More importantly, MnO2 nanomaterials aremore environmental friendly than other TMOs, as they are usu-ally applied in noncorrosive electrolytes; this can help to meetthe environmental requirements of green electrolytes in super-capacitors.

Over the past few years, several methodologies includingchemical vapor deposition (CVD), vapor–liquid–solid (VLS),template-assisted synthesis, and hydrothermal synthesis havebeen developed to prepare various MnO2 nanostructures.[16–18]

Compared with these synthetic methods, electrochemical dep-osition has great advantages over the others, because bysimply adjusting the applied current (or potential), bath pre-cursor solution, and/or temperature, certain properties such asmass, thickness, and morphology can be easily controlled.Hence, many efforts have been made to prepare MnO2 nano-structures by using the electrochemical deposition tech-nique.[19, 20] Nevertheless, most reported works have been car-ried out by using the template-assisted method, which is limit-ed by the template area, post-treatment process, and the un-avoidable impurities that are introduced. Meanwhile, many ofthe MnO2 nanostructures have been prepared on substrates offluorine-doped tin oxide, tin-doped indium oxide, stainlesssteel, or Ti plates that were rigid and could not be kinked,which are not suitable for modern, flexible electronics andpower systems.[21–24] Thus, the development of a convenientand reliable, template-free electrochemical method of prepar-ing MnO2 nanostructures on flexible substrates is of great sig-nificance.

Herein, we report a facile and template-free electrochemicalmethod for the growth of large-area a-MnO2 nanoflowerarrays (MONFAs) on flexible graphite paper (GP), and we dem-onstrate their application in supercapacitors. Electrochemicalmeasurements showed that MONFA deposited for 10 min ex-hibits an enhanced specific capacitance when employed as anelectrode. More importantly, this MONFA electrode demon-strated a remarkable rate capability with 61.1 % retention, evenwith current density increases to 40 A g�1, which is higher thanthe best values of other reported MONFAs.[25–27] Besides, thespecific capacitance of the MONFA electrode exhibits almostno decrease after 5000 cycles, showing excellent long-termcycle stability. Furthermore, a simple flexible symmetric super-capacitor based on the as-prepared MONFAs was fabricatedand showed good flexibility. These findings indicate that theMONFAs are a promising candidate as an electrode materialfor time-dependent flexible supercapacitors. The excellent per-formance of the MONFA electrodes can be attributed to theirunique structural features, including 1) the highly conductive

[a] Dr. W. Li, K. Xu, B. Li, J. Sun, F. Jiang, Dr. R. Zou, Prof. Z. Chen, Prof. J. HuState Key Laboratory for Modification of Chemical Fibers andPolymer Materials, College of Materials Science and EngineeringInstitution Donghua University, Shanghai 201620 (PR China)E-mail : rjzou@dhu.edu.cn

hu.junqing@dhu.edu.cn

[b] Dr. W. Li, Prof. Z. YuSchool of Material EngineeringShanghai University of Engineering ScienceShanghai 201620 (PR China)

[c] J. SunKey Laboratory of Organofluorine ChemistryShanghai Institute of Organic ChemistryChinese Academy of Sciences, 345 Ling-Ling RoadShanghai 200032 (PR China)

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/celc.201400006.

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GP substrate that enables efficient charge transport and acces-sible diffusion of the electrolyte, 2) the 3D structure that offersa large contact area for surface reactions as well as enablesfast intercalation/deintercalation of ions, and 3) the MNOFAsnaturally grown on the GP, which ensures good mechanical ad-hesion and electrical connection to the current collector, andno polymer binder or conductive additives are involved, thussubstantially reducing the dead volume.

The phase of the product was characterized by using X-raydiffraction (XRD), and the results are shown in Figure 1 a. In ad-dition to the C atom peaks (blue rectangle, marked with #)coming from the GP substrate, there are some peaks (marked

with *) that can be observed in the MONFAs that are consis-tent with the pattern reported for the tetragonal phase of a-MnO2 (JCPDS: 44-0141). In order to better understand thechemical composition of the products, X-ray photoelectronspectroscopy (XPS) was also conducted. The survey XPS spec-trum of the MnO2 flowers in Figure 1 b shows that there areonly three elements (Mn, O, and C) existing on the surface ofthe sample. The C signal is attributed to the GP substrate. Fig-ure 1 c gives the Mn 2p spectra of the deposited MnO2 flowers.Two peaks are observed located at approximately 642.2 and654.0 eV, which can be attributed to the Mn 2p3/2 and Mn 2p1/2

binding energies, respectively.[28] This indicates that the Mn4 +

ions are dominant in the product. In addition, elemental analy-sis by using XPS reveals that the atomic ratio of Mn/O is closeto 1:2. Thus, based on the above results, it can be concludedthe composition of the manganese oxide is a-MnO2. The opti-cal image of the MONFAs prepared through electrodepositionis shown in Figure 1 d. It can be seen that the surface of theGP substrate (around 17 cm2) has changed from gray to black,implying a highly uniform thin film was obtained on the GPsubstrate. It also demonstrates that this method can producea good-quality thin film on a large scale. In particular, the GP

coated with MONFAs is flexible and it can even be bent with-out destroying its construction, as shown in the inset of Fig-ure 2 a.

The morphology and microstructure of the MONFAs werestudied by using scanning electron microscopy (SEM) andtransmission electron microscopy (TEM). Figure 2 a presentsa typical SEM image of MONFAs, indicating that the MONFAsare grown uniformly on the GP substrate. These MONFAs havean average diameter of about 4–5 mm, as shown in Figure 2 b.Figure 2 c shows the SEM image of a single MnO2 nanoflower,which presents a perfect flower morphology, demonstratingthe uniform growth of the MnO2 during the electrodepositionprocess ; the inset is a TEM image of petals, with a width of ap-proximately 20–200 nm. Figure 2 d shows a high-resolutionTEM (HRTEM) image of a petal, revealing that it is a structurallyuniform single crystal, and no amorphous sheathed phasesform on the surface. The lattice fringes give an interplanarspacing of 0.28 nm, corresponding to that of the {001} latticeplanes of the tetragonal MnO2 phase. The corresponding fastFourier transform (FFT) diffraction pattern can be indexed tothe [100] zone axis of a MnO2 crystal (Figure 2 d inset).

In this study, as the electrodeposition time increased, themass of MONFAs deposited on the GP substrate also increased.Their relationships are illustrated in Figure 3 a, which showsthe curves of the loading mass as a function of electrodeposi-tion time, indicating that the mass of MONFAs can be well-controlled by the deposition time (the corresponding SEMimage is shown in Figure S1 of the Supporting Information).The weight ratios of the deposited MnO2 to GP are 6.0, 8.4,12.2, and 24.2 % under the electrodeposition times of 5, 10, 20,and 40 min, respectively. Although the different time-depositedMONFAs are composed of the same components, the charge-storage efficiency is different, owing to their microstructure dif-ferences. In order to identify which is favorable for high-per-formance energy storage, the specific capacitance values ofthe as-fabricated electrode materials were determined byusing cyclic voltammetry (CV) and galvanostatic charge–dis-

Figure 1. a) XRD pattern, b) survey XPS spectrum, and c) Mn 2p XPS spectrafor the MnO2 nanoflowers, and d) optical image of deposited MONFAs ona large scale.

Figure 2. a) Low- and b) high-magnification SEM images of MONFAs grownon GP substrate, c) SEM image of a single MnO2 nanoflower, inset shows theTEM image of the petals, and d) HRTEM image of a petal ; inset shows thecorresponding FFT image.

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charge (CD) measurements in three-electrode configurations.Firstly, the specific capacitances of MONFAs with different load-ing masses were obtained from their CV curves (Figure S2)under the same scan rate (50 mV s�1), see Figure 3 a. As can beseen, the specific capacitance of the MONFAs with 10 min dep-osition reaches 216.5 F g�1, which is better than the specific ca-pacitance for the MONFAs with 5, 20, and 40 min deposition,which are 180.1, 151.6, and 130.4 F g�1, respectively.

We then subjected the MONFAs with 10 min deposition tofurther CV investigation under different scan rates, see Fig-ure 3 b. All curves show a rectangular shape and symmetric CVcurves, indicating that the Faraday redox reactions are electro-chemically reversible and present ideal supercapacitor behav-ior. Notably, as the scan rates increases from 1 to 50 mV s�1,the shapes of the CV curves of the MONFAs remain almostsquare, indicating a relatively high rate capability. In addition,the GP substrate may influence the final CV curves witha small capacitance contribution, so we compared it with theMONFAs (Figure S2) and found that its effect to the results wasnegligible. These results demonstrate that the MONFAs area good candidate for supercapacitor electrode materials.

To further evaluate the capacitive performance of the as-syn-thesized products, we carried out galvanostatic CD testing.Galvanostatic CD curves for all the MONFAs electrodes mea-sured between �0.1 and 0.9 V at a current density of 1 A g�1

are shown in Figure 3 c. A high symmetric nature is observedin all the charging–discharging curves, indicating ideal electro-chemical capacitive characteristics and a superior reversibleredox reaction in the entire potential region. It is noted thatthe specific capacitance of the MONFAs deposit in 10 min canreach 314 F g�1, which is significantly higher than that of the

MONFAs deposited in 5, 20, and40 min, with corresponding spe-cific capacitances of 281.6, 235.4,and 202.4 F g�1, respectively. Inparticular, the smaller mass load-ing of 5 min-deposited MONFAsexhibited lower capacitance than10 min-deposited MONFAs couldbe attributed to the half-grownMnO2 nanoparticles accumulatedon the petals (Figure S1), whichblocked the way for charge trans-fer and reversible faradic process-es.

The 10 min-deposited MONFAsalso exhibit an enhanced specificcapacitance compared with thevalues of other reported MnO2

electrode materials with of differ-ent morphologies and phases,such as the ultrafine MnO2 nano-wire network (279 F g�1 at1 A g�1),[29] urchin-structured a-MnO2 (167 F g�1 at 2.5 mA g�1),[30]

and birnessite-type MnO2 beltbundles (268 F g�1 at 5 mV s�1).[31]

The high specific capacitance performance of 10 min-deposit-ed MONFAs can be attributed to three points: 1) the nanoflow-er structures could provide large, accessible surface areas thatenable effective electrolyte ion transport during supercapacitoroperations; 2) owing to the uniform growth and rational distri-bution of 10 min-deposited MONFAs on the electrical conduc-tivity substrate, the Na+ from the electrolyte can easily diffuseinto almost all available spaces of the electrode, leading toa complete insertion reaction, and showing almost ideal capac-itive behavior; and 3) the electronic resistance of the 10 min-deposited MONFAs possessed the lowest equivalent series re-sistance (ESR) value in the electrode/electrolyte system, im-proving the electrical conductivity of the electrode, as detailedlater.

Rate capability is another important factor for the use of su-percapacitors in power applications. A good electrochemicalenergy-storage device is required to provide its high specificcapacitance at high CD rate. The variations of specific capaci-tances of different MONFAs electrodes with increased currentdensities are shown in Figure 3 d. As can be seen, the 10 min-deposited MONFAs not only exhibit high specific-capacitancevalues, but also maintain them better at high current densitiescompared to other electrodes. The 10 min-deposited MONFAspreserved 61.1 % of its initial specific capacitance (from 314 to192 F g�1) as the current density increased from 1 to 40 A g�1

(the corresponding CD curves are shown in Figure S3). The su-perior rate capability can be attributed to the nanoflower ar-chitectures of as-electrodeposited MnO2, which offer large elec-trochemically active surface areas for charge transfer andreduce the ion-diffusion distance during the CD process, andthe conductive GP current collector is directly attached to

Figure 3. a) Mass loading and specific capacitance at 50 mV s�1 of the MONFAs as a function of electrodepositiontime. b) CV curves of the MONFAs with 10 min electrodeposition. c) Galvanostatic CD curves of MONFAs with dif-ferent electrodeposition time at a current density of 1 A g�1. d) Specific capacitance of the MONFAs obtainedwith different electrodeposition times as a function of the current densities.

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active materials, which provides fast electron transport andfast surface electrosorption of Na+ as well as the fast and re-versible faradic process that occurs between Na+ andMONFAs.[32] The specific capacitance value of the 5 min-depos-ited MONFAs was not only lower than the 10 min-depositedMONFAs, but also decreased significantly with increasing cur-rent densities (from 281.6 to 144 F g�1 at current densities of1 to 40 A g�1, with 51.1 % retention). Similar results were ob-served in 20- and 40 min-deposited MONFAs (decrease from235.4 to 140 F g�1 and 202.4 to 64 F g�1, with 59.5 and 31.6 %retention, respectively). Besides, such remarkable rate capabili-ty of 10 min-deposited MONFAs is also better than the previ-ously reported MnO2 materials, such as porous MnO2 multilay-er-nanosheet clusters (521.5 F g�1 at 5 mV s�1 but only 42 F g�1

at 50 mV s�1),[33] meso-nanowire MnO2 arrays (493 F g�1 at4 A g�1 but only 84 F g�1 at 12 A g�1),[34] further confirming theremarkable rate capability of 10 min-deposited MONFAs.

Cycle performances are of great importance for electro-chemical supercapacitors. The electrochemical stability ofMONFAs is shown in Figure 4 a. After 3000 cycles of CV testingat a scan rate of 50 mV s�1 the 10 min-deposited MONFAs didnot show any loss, compared with the specific capacitance ofinitial cycle, instead the specific capacitance increased to104.4 % (ca. 212.59 F g�1) of the initial value (ca. 203.55 F g�1),see Figure S4; this may be ascribed to an activation processthat occurs at the beginning of the CV cycling test.[35] For com-parison, we also tested the stability of 40 min-depositedMONFAs and a similar phenomenon was observed, that is, itincreases to 100.4 % (ca. 135.6 F g�1) of the initial value (ca.130.4 F g�1). These results demonstrate that all of the MONFAsshow promising results for long-term cycle ability, and the cy-

cling performance of the 10 min-deposited MONFAs is superiorto the 40 min-deposited MONFAs. We further reveal the cur-rent-density dependence of the cycling performance for10 min-deposited MONFAs, which were tested for 3000 CVcycles, see Figure 4 b. As can be seen, during the first 300cycles with a CD density of 1 A g�1, the MONFAs show a slightincrease and maintain a nearly stable specific capacitance of282 F g�1. In the following 1700 cycles, although the CD ratechanges successively, the MONFAs always demonstrate stablecapacitance in each situation. When the current returns to1 A g�1, a fully recovered specific capacitance of 283 F g�1 is ob-served after the following 300 cycles, which confirms that theMONFAs have remarkable rate capability ; the inset imageshows the last ten CD cycles of the last stage; after CD cyclingfor 858 min, the charge curves are still very symmetric to theircorresponding discharge counterparts, showing no significantstructural change of the MONFAs electrode during the CD pro-cesses and excellent cycling stability. In addition, electrochemi-cal impedance spectroscopy (EIS) was also employed to char-acterize the MONFAs electrodes. As shown in Figure 4 c, theESR values of the 5-, 10-, 20-, and 40 min-deposited MONFAsare 3.35, 2.11, 3.67, and 3.61 W, respectively, indicating thebetter charge-transport properties of the 10 min-depositedMONFAs electrode.

Power density and energy density are the two key parame-ters for evaluating the power applications of electrochemicalsupercapacitors. The Ragone plot of the 10 min-depositedMONFAs is shown in Figure 4 d. At a power density of20 kW kg�1, the electrode delivers an energy density as high as26.7 Wh kg�1. Even at a low power density of 500 W kg�1, theenergy density of the MONFAs can reach 43.6 Wh kg�1. It sug-

gests that the MONFAs representa promising electrode materialfor fabricating supercapacitors.

To test the feasibility ofMONFAs for flexible capacitanceapplications, a simple symmetricsupercapacitor device based onMONFAs was presented for dem-onstration purposes. The flexibledevice was fabricated by attach-ing two pieces of GP modifiedwith MONFAs, and a separatorsandwiched between them. Asshown in Figure 5 a, the as-pre-pared supercapacitor representsgood flexibility and can endurebending several times withoutdestroying its construction. Fig-ure 5 b depicts the CV curves ofthe as-fabricated symmetric su-percapacitor device under differ-ent scan rates, and the curveswere found to be nearly rectan-gular in shape, even at scan rateas high as 500 mV s�1, withina selected potential range, indi-

Figure 4. a) CV cycling performance of the MONFAs. b) CD cycling stability of the MONFAs at progressively variedcurrent densities; inset shows CD curves of the MONFAs in the potential range from �0.1 to 0.9 V at a currentdensity of 1 A g�1. c) EIS spectra comparison of the MONFAs deposited for different times. d) Ragone plot (powerdensity vs. energy density) of the 10 min-deposited MONFAs.

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cating that the device maintained good capacitive behavior athigh rates. We calculated the specific capacitances of thewhole cells to be 68, 63.6, 58.3, 50.7, 33.4, 22.9, and 14.1 F g�1

at scan rates of 5, 10, 20, 50, 100, 200, and 500 mV s�1, respec-tively. In order to evaluate the feasibility of the MONFAs super-capacitor for flexible energy-storage devices, we tested theelectrochemical properties under different bending conditions.Figure 5 c shows that the changes in the CV curves (tested at50 mV s�1) collected under different bending angles are subtle,confirming that the device has a good mechanical flexibility.Also, we measured the long-term cycling performance by cy-cling for 5000 times at 50 mV s�1 under different bending con-ditions over the entire device, as shown in Figure 5 d. As canbe seen, the device was tested for 1000 cycles in each bendingcondition, and there is a slight capacitance loss between theplane and different bending degrees, which can be ascribed tophysical damage caused by bending the device that results inthe MnO2 material detaching from the substrate. Nevertheless,there is only an 8.1 % decay in specific capacitance observedafter 5000 cycles, showing good cycling performance anddemonstrating the potential to be used for the flexible super-capacitors.

In summary, large-area and highly crystalline MONFAs canbe successfully synthesized on flexible GP substrates by usinga facial and template-free electrochemical deposition. As-syn-thesized 10 min-deposited MONFAs display a high specific ca-pacitance of 314 F g�1 at a current density of 1 A g�1. More im-portantly, this electrode demonstrates a remarkable rate capa-bility with 61.1 % retention, even though the current densityincreases 40 times (40 A g�1), and excellent cycling stability.Furthermore, the fabricated simple symmetric supercapacitorpresents good electrochemical performance and flexibility.These findings show the MONFAs are a promising electrode

material and open up great op-portunities to make flexible su-percapacitors.

Experimental Section

Firstly, a piece of GP (ca. 4 � 1 cm2)was carefully cleaned with deion-ized water and absolute ethanolseveral times. Secondly, the elec-trochemical deposition was per-formed in a standard three-elec-trode glass cell, consisting of theclean graphite paper working elec-trode, a platinum plate (ca. 1.5 �1.5 cm2) counter electrode, anda saturated calomel reference elec-trode (SCE). MONFAs were electro-deposited on the GP at a currentdensity of 0.5 mA cm�2, at 70�2 8C, in a solution containing 0.1 m

maganese acetate (MnAc2) and0.02 m ammonium acetate (NH4Ac)with 10 % dimethylsulfoxide(DMSO), by using an Autolabelectrochemical workstation

(PGSTAT302N). After deposition, the as-prepared MnO2 electrodematerials were washed with deionized water and absolute ethanolseveral times with the assistance of ultrasonication, and thenplaced in a vacuum oven at 60 8C for 2 h. Finally, the as-preparedMnO2 nanoflowers were annealed at 200 8C for 3 h in air. The massof the as-grown MnO2 nanomaterials was obtained by its weightdifference before and after electrochemical deposition.

As-prepared products were characterized with a D/max-2550 PC X-ray diffractometer (Rigaku, Japan; CuKa radiation) and a scanningelectron microscope (S-4800, Hitachi, Japan), a transmission elec-tron microscope (JEM-2100F, JEOL, Japan) equipped with anenergy dispersive X-ray spectrometer (EDS), and XPS (ESCALab250,Thermo-VG Scientific, USA). The mass of electrode materials wasweighed on an XS analytical balance (Mettler Toledo, Switzerland;d= 0.01 mg).

Acknowledgements

This work was financially supported by the National Natural Sci-ence Foundation of China (Grant Nos. 21171035, 51302035, and51375294), the Key Grant Project of Chinese Ministry of Education(Grant No. 313015), the PhD Programs Foundation of the Minis-try of Education of China (Grant Nos. 20110075110008 and20130075120001), the National 863 Program of China (Grant No.2013AA031903), the Science and Technology Commission ofShanghai Municipality (Grant No. 13ZR1451200), the Fundamen-tal Research Funds for the Central Universities, the ShanghaiLeading Academic Discipline Project (Grant No. B603), and theProgram of Introducing Talents of Discipline to Universities (No.111–2–04).

Keywords: electrochemical deposition · flexiblesupercapacitors · manganese · nanostructures · rate capability

Figure 5. a) Optical photograph of the as-fabricated symmetric supercapacitor device; inset demonstrates thehigh flexibility of the as-prepared device. b) CV curves of the MONFAs device with scan rates from 5 to500 mV s�1. c) CV curves and d) cycling performance of the MONFAs device under different bending conditions.

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Received: January 9, 2014Published online on && &&, 2014

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COMMUNICATIONS

W. Li, K. Xu, B. Li, J. Sun, F. Jiang, Z. Yu,R. Zou,* Z. Chen, J. Hu*

&& –&&

MnO2 Nanoflower Arrays with HighRate Capability for FlexibleSupercapacitors

Great things in small packages: Large-area a-MnO2 nanoflower arrays on flexi-ble substrates are synthesized througha facial template-free electrochemicaldeposition process. These nanoflowerarrays produce an enhanced electro-chemical performance in the form ofa high specific capacitance with remark-able rate capability and excellent cy-cling stability.

� 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemElectroChem 0000, 00, 1 – 6 &7&

These are not the final page numbers! ��