One-pot synthesized ultrathin MnO2 nanorods as advanced electrodes for supercapacitors--Min Lu

One-pot synthesized ultrathin MnO2nanorods as advanced electrodes

for high-performance supercapacitors

Min Lu a,b,Yang Lu a,b,Kangwen Qiu a,b,Jinbing Cheng a,b,Hailong Yan a,b,Yongsong Luo a,b,n

a School of Physics and Electronic Engineering,Xinyang Normal University,Xinyang464000,PR China

b Key Laboratory of Advanced Micro/Nano Functional Materials,Xinyang Normal University,Xinyang464000,PR China

a r t i c l e i n f o

Article history:

Received14October2015

Received in revised form

9December2015

Accepted23December2015

Available online23December2015

Keywords:

MnO2

Ultrathin nanorods

Nanocrystalline materials

Hydrothermal method

Energy storage and conversion

a b s t r a c t

Novel ultrathin MnO2nanorods are successfully prepared by a one-pot hydrothermal process,which

endows with porous one-dimensional(1D)nanostructures.As a result,the MnO2nanorods show good

electrochemical performance as electrode materials for supercapacitors,including high speci?c capaci-

tance of820.8F/g,long life over5000cycles,and good rate capability(562.5F/g at10A/g).The fabri-

cation strategy present here is facile,cost-effective,and can offer a way for energy storage device ap-

plications.

&2015Elsevier B.V.All rights reserved.

1.Introduction

Supercapacitors(SCs)are powerful energy storage systems that

?ll the gap between batteries and conventional capacitors in terms

of their speci?c energy and speci?c power[1,2].However,the

current commercial SCs are mainly based on carbonaceous anode

materials which deliver low theoretical speci?c capacities,and

their capabilities are far from meeting the growing demand for

electric vehicle and large energy-storage devices[3,4].With the

growing concerns over environmental pollution,metal chalcogen

elements have attracted increasing attention[5,6].For instance,

MnO2are considered as alternative electrode materials for su-

percapacitors due to their ultrahigh theoretical capacitance

(1370F/g),low cost,natural abundance,environmental friendli-

ness and wide potential window[7].Unfortunately,MnO2is

hampered by its lower electronic and ionic conductivities and

large volume variations during charge–discharge process,result-

ing in the destruction of the structure[8–10].

In order to overcome these challenges,lots of studies have been

carried out.After extensive literature review,the relevant solving

strategies can be summarized as following:the?rst one is to

fabricate nanostructured MnO2materials,such as nanowires,na-

no?akes,and nanotubes,have been comprehensively investigated

[11,12],which usually offer large surface area and free space for

accommodation of the volume change during cycling and short

path length for electrolyte diffusion;the second one is to grow

nanostructured MnO2material directly on a conducting substrate,

which greatly improve the electric and physical contact between

the active material and the current collector.Although these ap-

proaches have effective improved the speci?c capacitance and

cycling performance of the MnO2electrodes,they always involve

an excessive use of organic agents,complicated preparation pro-

cess,low productivity,etc.

In this work,ultrathin MnO2nanorods were synthesized by a

facile,low-cost hydrothermal method without using any surfac-

tant.The electrochemical measurements indicate MnO2nanorods

exhibit high speci?c capacitance and long stability.

2.Experimental section

2.5mmol KMnO4and1mL concentrated HCl were mixed with

45mL deionized water and stirred for30min to form a precursor

solution,which was then transferred into a50mL Te?on-lined

stainless steel autoclave.The autoclave was sealed and hydro-

thermally treated at140°C for12h.After the autoclave was cooled

naturally to room temperature,samples depositing at the bottom

were collected and washed by centrifugation for several cycles

using deionized water and absolute ethanol.The as-synthesized

samples were then dried in60°C.

Contents lists available at ScienceDirect

journal homepage:https://www.360docs.net/doc/28952932.html,/locate/matlet

Materials Letters

https://www.360docs.net/doc/28952932.html,/10.1016/j.matlet.2015.12.099

0167-577X/&2015Elsevier B.V.All rights

reserved.

n Corresponding author at:School of Physics and Electronic Engineering,Xinyang

Normal University,Xinyang464000,PR China.

E-mail address:ysluo@https://www.360docs.net/doc/28952932.html,(Y.Luo).

Materials Letters166(2016)255–258

Fig.1.(a)XRD patterns of α-MnO 2;(b)Raman spectra of α-MnO 2

.

Fig.2.SEM images (a –d)and TEM images (e,f)of the MnO 2nanorods at different magni ?cations.

M.Lu et al./Materials Letters 166(2016)255–258

256

The phase structures of the products were characterized by powder X-ray diffraction (XRD)on a Bruker D8Advance X-ray diffractometer.The morphologies were examined by ?eld emis-sion scanning electron microscopy (FESEM,Hitachi S-4800)and transmission electron microscopy (TEM,JEOL JEM-2010).Raman spectra were recorded on an INVIA Raman microprobe (Renishaw Instruments,England).Nitrogen adsorption/desorption isotherms were measured at 77K with micrometrics ASAP 2020analyzer.The speci ?c surface area was calculated from the adsorption curve by the Barrett-Joyner-Halenda (BJH)method.

Electrochemical measurements were conducted on a CHI660E electrochemical workstation by using a three-electrode cell.Car-bon black was used as conductor,and polytetra ?uoroethene was used as adhesion agent.Active material (MnO 2nanorods,$3mg),carbon black and polytetra ?uoroethene with a weight ratio of 80:10:10were mixed.The polyvinylidene ?uoride was added to the solid mixture to form mash.Then the mash was pressed onto a nickel foam (ca.1cm 2)under 10MPa for 10min,and dried at 80°C overnight.The nickel foam with MnO 2?lm,a Pt foil and Ag/AgCl electrode were used as the working electrode,the counter elec-trode and the reference electrode respectively.1M KOH aqueous solution was used as an electrolyte.

3.Result and discussion

Fig.1a shows the XRD pattern of the synthesized nanorods where the diffraction peaks corresponded to pure tetragonal phases of α-MnO 2(JCPDS 44-0141).The structural features of the

MnO 2are further con ?rmed by Raman measurements.As shown in Fig.1b,the three Raman bands located at 501,575,and 645cm à1are in good agreement with the three major vibrational features of the birnessite-type MnO 2compounds [13].

Fig.2a –d shows the SEM images of the resultant MnO 2sample at different magni ?cations.According to the SEM observations,it displays that the as-prepared MnO 2product is composed of countless rod-like aggregates,which have ?at and smooth surfaces and their typical size is in the range of 1–2m m in length and 25nm in diameter,nearly all of them hold the same morphology.The MnO 2nanorods form an ultrathin surface layer,which is capable of contributing to interfacial electrochemical reactions and provides a large electrode surface area [14].The structure and morphology of the MnO 2nanorods were further investigated by TEM.Fig.2e and f show TEM images of the MnO 2nanorods in different mag-ni ?cations.From Fig.2f,the obvious contrast between the center and the edge indicates the ultrathin feature and porous structure.Another factor inducing the rod growth is suggested by Hartanto et al.that the gas pressure in the synthesis environment of the main matrix can cause growth of nanorods [15].When the mate-rial is used as an electrode of SCs,such a porous structure could effectively endure the volume change during discharging and charging and promote the diffusion rate of the electrolyte.The ultrathin nanorods are future investigated by BET measurements (Fig.S1).The BET speci ?c surface area is calculated to be ca.89.2m 2g à1.The mesoporous structure shows a narrow pore diameter (7nm)and a large BET speci ?c surface area (89.2m 2g à1).

Cyclic voltammetry (CV)measurement is a suitable tool

to

Fig.3.Electrochemical characterization of MnO 2nanorods as supercapacitors material:(a)CV curves at different scan rates;(b)charge/discharge voltage curves at different current densities;(c)the capacitance of the nanorods at different current densities;and (d)cycling performance of the nanorods with current density of 5A/g (corre-sponding charge/discharge curves of the ?rst 8and last 8cycles of the MnO 2electrode in inset).

M.Lu et al./Materials Letters 166(2016)255–258257

characterize the capacitive behavior of electrode materials.Fig.3a shows the CV curves of the MnO2electrode conducted in the po-tential region of0–0.8V with scan rates ranging from5,10,20,30 and50mV/s,respectively.No peaks are presented,indicating that the electrode is charged and discharged at a pseudo-constant rate over the complete voltammetric cycle.The rectangular CV shape remains very well even at a high scan rate of50mV/s,suggesting the electrode possesses excellent rate capability desirable for high-power supercapacitors.Fig.3b shows the galvanostatic charge–discharge(CD)curves of the MnO2electrode at current densities of 1,2,3,5and10A/g,respectively.A linear variation of the potential during both charging and discharging processes is observed for the MnO2nanorods electrode,and the CD curves are almost linear and symmetrical,which is a typical characteristic of an ideal capacitor behavior.The speci?c capacitances calculated from the discharge curves are820.8,757.5,718.1,587.5,562.5F/g at the current densities of1,2,3,5,10A/g,respectively.The results present a trend that the speci?c capacitance gradually decreases with in-crease of the current density.The decrease in capacitance suggests that parts of the surface of the electrode are inaccessible at high charge/discharge rates[16,17].Remarkably,Over this current density range,the speci?c capacitance remains68%of its initial value(Fig.3c),implying the good ion diffusion and electron transport at a high current density.

The cycle stability of the electrode is another important para-meter for supercapacitor.The performance of the electrode was measured with a current density of5A/g for5000cycles.After-ward,the capacitance was kept at98.5%of the initial capacitance, indicating excellent long-term stability from the fabricated ultra-thin nanorods electrode.Importantly,the charge–discharge curves are still keeping quite symmetric after5,000cycles test(inset in Fig.3d is the charge/discharge curves of the?rst8cycles and last 8cycles for the MnO2electrode).

The cycling performance of the MnO2electrode at progressively increased current density was recorded in Fig.S2.During the?rst 300cycles with a charge/discharge density of1A/g,the MnO2 show a capacitance of820.8F/g.In the following1,500cycles,the charge/discharge rate changes successively.This porous1D na-nostructured electrode always demonstrates stable capacitance. After1,800cycles,with the current rate being again decreased back to1A/g for another300cycles,a capacitance of818.5F/g can be recovered without noticeable decrease,which demonstrates that the nanostructure has excellent rate performance and cyclability.

4.Conclusions

Porous ultrathin MnO2nanorods were successfully prepared by a facile hydrothermal method.TEM and BET analysis on the pro-ducts demonstrated their ultrathin and porous characteristics.The products exhibit excellent electrochemical performance as super-capacitor with high speci?c capacitance and long stability,which result from its special1D porous structure.

Acknowledgments

This work is?nancially supported by the National Natural Science Foundation of China(Nos.U1304108,U1204501and 21373107).

Appendix A.Supplementary material

Supplementary data associated with this article can be found in the online version at https://www.360docs.net/doc/28952932.html,/10.1016/j.matlet.2015.12. 099.

References

[1]L.Y.Gong,X.H.Liu,L.H.Lu,Mater.Lett.67(2012)226–228.

[2]S.X.Liu,L.F.Hu,X.J.Xu,A.A.Al-Ghamdi,X.S.Fang,Small11(2015)4267–4283.

[3]D.Villers,D.Jobin,C.Soucy,D.Cossement,R.Chahine,L.Breau,D.Bélanger,J.

Electrochem.Soc.150(2003)A747–A752.

[4]F.Fusalba,H.A.Ho,L.Breau,D.Be,Chem.Mater.12(2000)2581–2589.

[5]X.J.Xu,L.F.Hu,N.Gao,S.X.Liu,S.Wageh,A.A.Al-Ghamdi,A.Alshahrie,X.

S.Fang,Adv.Funct.Mater.25(2015)445–454.

[6]W.Wei,X.Cui,W.Chen,D.G.Ivey,Chem.Soc.Rev.40(2011)1697–1721.

[7]S.W.Zhang,G.Z.Chen,Adv.Energy Mater.3(2008)186–200.

[8]C.Y.Guo,H.Li,X.Zhang,H.H.Huo,C.L.Xu,Sens.Actuators B:Chem.206(2015)

407–414.

[9]T.Zhai,S.L.Xie,M.H.Yu,P.P.Fang,C.L.Liang,X.H.Lu,Y.X.Tong,Nano Energy8

(2014)255–263.

[10]Y.Y.Li,Q.W.Zhang,J.L.Zhu,X.L.Wei,P.K.Shen,J.Mater.Chem.A2(2014)

3163–3168.

[11]W.X.Tang,X.Shan,S.D.Li,H.D.Liu,X.F.Wu,Y.F.Chen,Mater.Lett.132(2014)

317–321.

[12]H.L.Li,L.X.Jiang,Q.L.Cheng,Y.He,V.Pavlinek,P.Saha,C.Z.Li,Electrochim.

Acta164(2015)252–259.

[13]D.Z.Kong,J.S.Luo,Y.L.Wang,W.N.Ren,T.Yu,Y.S.Luo,Y.P.Yang,C.W.Cheng,

Adv.Funct.Mater.24(2014)3815–3826.

[14]Y.S.Luo,J.Jiang,W.W.Zhou,H.P.Yang,J.S.Luo,X.Y.Qi,H.Zhang,Y.W.Y.Denis,

C.M.Li,T.Yu,J.Mater.Chem.22(2012)8634–8640.

[15]S.Demirel,E.Oz,E.Altin,S.Altin,A.Bayri,P.Kaya,S.Turan,S.Avci,Mater.

Charact.105(2015)104–112.

[16]K.W.Qiu,Y.Lu,D.Y.Zhang,J.B.Cheng,H.L.Yan,J.Y.Xu,X.M.Liu,J.K.Kim,Y.

S.Luo,Nano Energy11(2015)687–696.

[17]C.J.Hung,P.Lin,T.Y.Tseng,J.Power Sources259(2014)145–153.

M.Lu et al./Materials Letters166(2016)255–258 258