MaterialsLetters

Chemical synthesis of mesoporous CoFe2O4nanoparticles as promising bifunctional electrode materials for supercapacitors
Leilei Lv a,b,Qun Xu a,n,Rui Ding b,nn,Li Qi b,Hongyu Wang c
a College of Material Science and Engineering,Zhengzhou University,No.75University Road,Zhengzhou450052,China
b State Key Laboratory of Electroanalytical Chemistry,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,5625Renmin Street,
Changchun130022,China
c Changzhou Institute of Energy Storage Materials&Devices,No.9Hehai Eastern Road,Changzhou213000,China
a r t i c l e i n f o
Article history:
Received8May2013
Accepted12August2013
Available online22August2013
Keywords:
CoFe2O4
Nanoparticles
Pseudocapacitance
Mesoporous
Metallic composites
a b s t r a c t
A promising mesoporous cobalt iron oxide(CoFe2O4)electrode material for supercapacitors has been
synthesized via a chemical co-precipitation method using aluminum nitrate(Al(NO3)3)as a precursor of
aluminum oxide(Al2O3)hard template.The as-prepared CoFe2O4materials were spherical-like
nanoparticles with diameter of around25nm.Moreover,the as-prepared CoFe2O4materials exhibited
a high specific surface area(140.6m2gÀ1)and high porosity(0.23cm3gÀ1).The fabricated CoFe2O4
electrode showed typical pseudocapacitive behavior with a broad potential window(1.5V),a high
specific capacitance(142F gÀ1,2mV sÀ1)and a long cycling life(71.8%retention after1000cycles).
&2013Elsevier B.V.All rights reserved.
1.Introduction
With worldwide increasing warmth in the energy storagefield
of supercapacitors,3d transition metal oxides ranging from noble
metal oxides to inexpensive metal oxides,characterized by highly
reversible capacities,long cycle performance and high power
density,have been extensively studied[1].Among them,RuO2
could exhibit prominent performance with pseudocapacitance as
high as720F gÀ1[2],but the expensive cost and high toxicity
apparently hinder its commercial application,which made MnO2,
Co3O4,Fe3O4,V2O5and NiO,especially binary system materials
Co–Ni,Fe–Mn,Co–Mn,Mn–Ni oxides environmental and econom-
ical alternatives of choice for improved applicability[1].
Ferrosoferric oxide(Fe3O4)and cobalt oxide(Co3O4)of spinel
series are both attractive candidates for the application in super-
capacitors owning to their low-cost and environmental friendly
nature,as well as excellent electrochemical capacitive behavior
[3,4].Their binary compound cobalt iron oxide(CoFe2O4),as an
efficient magnetic material on demand infields of electronics,
photomagnetism,catalysis,has widely been studied[5].In2005,
Kuo and Wu[6]had a report of CoFe2O4with a specific capacitance
of7.1F gÀ1in neutral NaCl electrolyte,which shed light on its
electrochemical properties by improving inherent structure.
Herein,we employ a hard template of Al2O3derived from co-
precipitating Al(NO3)3precursor solution to fabricate the porous
capacitive CoFe2O4.And the artificial porous structures are proved
to largely enhance the electrochemical performance of the
CoFe2O4materials.
2.Experimental
In a typical procedure,first,2.25g Al(NO3)3Á9H2O,4.85g Fe
(NO3)3Á9H2O and1.75g Co(NO3)2Á6H2O were dissolved in120mL
deionized water to form a well-mixed solution.Excessive
NH3ÁH2O was subsequently added to the solution dropwise until
a pH level of10was reached.The obtained dark brown precipita-
tion was further vigorously stirred at501C for4h with a constant
speed in a water-jacketed reaction vessel using circulating ther-
mostatic bath.Then,the as-prepared sample was obtained by
centrifugalfiltration and dried at701C for10h,afterward
annealed in a muffle stove at4501C under air for2h at a heating
rate of21C minÀ1.Subsequently,the product was etched in2M
KOH solution at501C for24h to remove Al2O3template,after-
wardfiltrated by centrifugation and washed with distilled water
several times until a neutral pH level,andfinally dried at701C
for10h.For comparison,the experiment without Al(NO3)3Á9H2O
addition was also conducted.Thefinal products with and
without Al2O3template are named T-CoFe2O4and CoFe2O4,
respectively.
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Materials Letters
0167-577X/$-see front matter&2013Elsevier B.V.All rights reserved.
/10.1016/j.matlet.2013.08.055
n Corresponding author.Tel.:þ8637167767827.
nn Corresponding author.Tel.:þ8643185262915.
E-mail addresses:*************.cn(Q.Xu),***************.cn(R.Ding).
Materials Letters111(2013)35–38
X-ray diffraction (XRD)patterns of the samples were recorded on a Rigaku D/max-2500diffractometer equipped with monochromated Cu K α(λ¼0.15406nm)radiation.Scanning electron microscopy (SEM)images were taken using Philips XL 30and a JEOL JSM-6700F microscope.N 2adsorption –desorption measurements were performed on a Micromeritics ASAP 2020apparatus.Electrochemical examinations were carried out with a CoFe 2O 4working electrode,a Pt mesh counter electrode and a Hg/HgO (2M KOH aqueous solution)reference electrode.The working electrodes were prepared by pressing the homogenous mixture of 70wt%CoFe 2O 4active materials,15wt%acetylene black,and 15wt%poly(tetra fluoroethy-lene)(Sigma Aldrich)onto a stainless steel mesh collector.Cyclic voltammetry (CV)and cyclic stability were collected on CHI700D electrochemical workstation and land CT2001A tester,respectively.
The gravimetric speci fic capacitance (C m )is calculated accord-ing to the following equation:
C m ¼1
2vm ðΔV ÞZ V b
V a I d V ð1Þ
where m ,νand (V a –V b )i.e.ΔV denote the mass of CoFe 2O 4or T-CoFe 2O 4active powders,scan rate and the potential window (1.5V),respectively.
3.Results and discussion
Fig.1a displays the XRD patterns of T-CoFe 2O 4and CoFe 2O 4materials.All the resultant peaks can be indexed as a face-centered-cubic spinel phase.The identi fied eight diffraction peaks at 2θvalue of 30.211,35.701,37.131,43.161,54.131,57.361,62.961and 74.321correspond to the (220),(311),(222),(400),(422),(511),(440)and (533)crystal planes,respectively,which is in well agreement with the standard patterns for CoFe 2O 4(JPCDS No.22-1086).No signals of Al 2O 3phase (JCPDS No.10-0425)are
detected in the patterns [7].Moreover,neither does Al emerge in XPS nor in EDAX spectra of T-CoFe 2O 4(seen in Fig.S1and S2).All suggested the successful removal of the template.Furthermore,it is clearly seen that diffraction peaks of T-CoFe 2O 4are duller indicating T-CoFe 2O 4comprises of smaller nanoparticles than CoFe 2O 4and this inference is further con firmed by their SEM images (Fig.1b and c).The CoFe 2O 4in Fig.1b,shows basically microsized agglomerate particles with few pores and voids,and a smooth surface,whereas the T-CoFe 2O 4exhibits basically dis-persed and uniform spherical-like particles of around 25nm size with rough surface and high porosity,as can be seen in the domain of Fig.1c.Thus by etching the hard template,T-CoFe 2O 4exhibits a porous framework with smaller granular size rather than bulk.To accommodate super ficial electroactive species as much as possible,it is essential for the electrode material to enrich its inner surface area and pores so as to ease the mass transfer of electrolytes [8].
The surface area and porosity of the T-CoFe 2O 4and CoFe 2O 4were further veri fied by nitrogen sorption measurements which are shown in Fig.1d.The N 2adsorption –desorption isotherms of CoFe 2O 4and T-CoFe 2O 4representative of type II and IV curves with distinct hyster-esis loops.The BET surface area of T-CoFe 2O 4is 140.6m 2g À1which is far larger than the value of 27.6m 2g À1for CoFe 2O 4.The main pore size distributes narrowly in the range of 4–8nm centered at 5.6nm and 2–4nm centered at 2.2nm for the T-CoFe 2O 4and CoFe 2O 4,respectively,(shown in the insets of in Fig.1d).Besides,average pore size and mesoporous volume of T-CoFe 2O 4are quantitatively eval-uated as 6.53nm and 0.23cm 3g À1,and the corresponding values of CoFe 2O 4are 2.2nm and 0.225cm 3g À1,respectively.High speci fic surface area and porosity is critical to enhance the electrochemical performances of electrode materials for supercapacitors [1].Thereby,improved electrochemical properties for the porous T-CoFe 2O 4,such as speci fic capacitance,high-rate capability,and longer cycling life,can be expected
accordingly.
Fig.1.XRD patterns (a)and N 2adsorption –desorption isotherms with insets of BJH pore size distribution (d)of CoFe 2O 4and T-CoFe 2O 4;SEM images of CoFe 2O 4(b)and T-CoFe 2O 4(c).
L.Lv et al./Materials Letters 111(2013)35–38
36
The CV plots of CoFe 2O 4and T-CoFe 2O 4electrodes tested in the potential range of À1.0–0.5V were plotted in Fig.2a and b.Two obvious pseudocapacitive blocks are observed in the positive (À0.1–0.5V)and negative (À0.5–À1.0V)potential regions which contribute to most of the capacitance.While the middle intervals (À0.1–À0.5V)contribute to the minor EDL capacitance [9].Here,the observed redox couples well elucidate the pseudocapacitive properties of CoFe 2O 4electrodes.The three couples of redox peaks located at around 0.28/0.05V,0.5/0.38V,À0.75/À0.85V indicate the reversible redox processes of Co 3þ/Co 2þ,Co 4þ/Co 3þ,Fe 3þ/Fe 2þredox couples in alkaline electrolytes [4,9,10],which can be expressed as the following equations:1/3Co 3O 4þ1/3OH Àþ1/3H 2O 2CoOOH þ1/3e À(2)CoOOH þOH À2CoO 2þH 2O þe À
(3)2/3Fe 3O 4þ2/3OH ÀþH 2O 22FeOOH þ2/3e À
(4)
Even though a series of reports have studied the capacitive properties of M Fe 2O 4(M ¼Mn,Fe,Co,Ni),CoFe 2O 4in our work may draw fresh attention as it can both act as anode and cathode electrode materials by virtue of the broad potential window area.
The speci fic capacitance values of CoFe 2O 4and T-CoFe 2O 4electrodes are shown in Fig.2c,both speci fic capacitance decreases with increasing scan rate because of insuf ficient active material involved in the redox reactions under higher scan rate.The CoFe 2O 4electrode exhibits a low speci fic capacitance range of 44–13F g À1while the T-CoFe 2O 4electrode exhibits a much higher and considerable speci fic capacitance range of 142–23F g À1in the scan rate range of 2–50mV s À1.Obviously,the porous T-CoFe 2O 4has much higher electrochemical activity than the bulk CoFe 2O 4thanks to suf ficient electroactive sites for electrochemical reac-tions and easy ion diffusion pathways for electrolyte ions transfer process.
Long cycle life is a crucial parameter for electrode materials used for supercapacitors.The cyclic performances of CoFe 2O 4and T-CoFe 2O 4electrodes are shown in Fig.2d.The CoFe 2O 4electrode
showed a capacitance ′s decay in the first 200cycles,then gradually went up with the increasing cycle numbers and finally a retention rate of 68.7%was obtained.The speci fic capacitance of T-CoFe 2O 4,which is nearly three times of CoFe 2O 4,slightly decreased before 200cycles and remained almost stable in the subsequent cycles.A retention rate of 71.8%was obtained after 1000cycles for the T-CoFe 2O 4electrode.Here,the decline in the speci fic capacitance with cycle number may be ascribable to the loss of active material caused by the dissolution and/or detach-ment during early cycle number [11].Further work on searching the most appropriate ratio of template to improve the cycling behavior is currently under progress.
4.Conclusions
In summary,the mesoporous CoFe 2O 4materials for super-capacitors have been successfully synthesized with assistance of co-precipitating Al 2O 3template.High BET speci fic surface and porosity of 140.6m 2g À1and 0.23cm 3g À1were obtained which facilitate the Faradaic pseudocapacitive performance by virtue of suf ficient electroactive sites and easy ions pathways.The unique mesoporous CoFe 2O 4electrode delivered a wide potential window of 1.5V and a high speci fic capacitance of 142F g À1at 2mV s À1,which can be expected to take important roles in both anode and cathode materials for supercapacitors.
Acknowledgments
We gratefully acknowledge the financial support of this research by National Basic Research Program of China (2012CB932800),Scienti fic Research Foundation for the Returned Overseas Chinese Scholars and State Education Ministry (SRF for ROCS,
SEM).
Fig.2.CV plots of CoFe 2O 4electrode (a)and T-CoFe 2O 4electrode (b);speci fic capacitance values under different scan rates (c)and cycling performances (d)of CoFe 2O 4and T-CoFe 2O 4electrodes.
L.Lv et al./Materials Letters 111(2013)35–3837
Appendix A.Supporting information
Supplementary data associated with this article can be found in the online version at /10.1016/j.matlet.2013.08.055. References
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