DOI:10.1002/cssc.201301262
Construction of High-Energy-Density Supercapacitors from Pine-Cone-Derived High-Surface-Area Carbons
Kaliyappan Karthikeyan,*[a,b]Samuthirapandiyan Amaresh,[a]Sol Nip Lee,[a]Xueliang Sun,[b]Vanchiappan Aravindan,[c]Young-Gi Lee,[d]and Yun Sung Lee*[a]
Introduction
The search for new alternative electrochemical energy-storage devices for hybrid electric vehicle (HEV)and electric vehicle (EV)applications have been accelerated because of the deple-tion of oil resources/hike in oil prices and an increasing aware-ness of environmental concerns.[1]Among the electrochemical energy-storage systems,electrochemical double-layer capaci-tors (EDLCs)are expected to play a major role because of their high power capabilities with long life spans.One of the hot topics of research is to increase the energy density of such EDLCs.[2,3]In principle,electrical energy stored in an EDLC is based on the separation and pure electrostatic attraction of the charge carriers across the electrode and ionically conduc-tive electrolyte interface (non-Faradaic mechanism).Such a non-Faradaic reaction occurs predominantly on the surface of the carbonaceous
electrodes irrespective of the use of an
aqueous or organic medium.[2]So far,a variety of carbona-ceous materials has been investigated extensively for EDLC ap-plications.[4–6]Among them,highly porous activated carbons (ACs)with unique characteristics such as a high specific surface area,high electrical conductivity,chemical and thermal stabili-ty,and relatively low cost are promising electrode materials for such applications.[4,6,7]
pinesThe choice of raw material and the associated activation process (either chemical or physical activation)are the most important factors that influence the electrochemical profiles of AC.[2,6]Conventional ACs derived from,for example,coal,pe-troleum coke (PC),needle coke (NC),and biomass precursors,exhibit different electrochemical performances,which indicates the importance of the precursor.[7–10]Although ACs obtained from traditional precursors such as cokes and coal exhibit a high specific surface area >2000m 2g À1and good capaci-tance properties,difficulty in the activation of cokes and the associated environmental pollution is the main issue.[7,10]Therefore,a lot of research activity is focused on the develop-ment of biomass-derived AC with excellent textural proper-ties.[10,11]So far,few reports describe the use of biomass-de-rived ACs for supercapacitor applications with either aqueous,organic,or ionic liquid electrolytes,for instance,coconut shell,[12–14]
eucalyptus wood,[12]fir wood,[15,16]bamboo,[17]banana fiber,[18]corn grains,[19]sugar cane bagasse,[20]apricot shell,[21]sunflower seed shell,[22]potato starch,[12]coffee ground,[23,24]wheat straw,[25]fish scale,[26]chicken egg-shell membrane,[27]cherry stone,[28]rice husk,[29]recycled waste paper,[30]cassava peel waste,[31]pistachio shell,[16]argan seed shell,[32]seaweeds,[33]and dead neem leaves.[11]Generally,ACs derived from these sources exhibit a specific surface area be-tween 1000–3000m 2g À1and show good specific capacitance properties.[10]Haykırı-AÅma [34]first studied the synthesis
of
[a]Dr.K.Karthikeyan,S.Amaresh,S.N.Lee,Prof.Y.S.Lee
Faculty of Applied Chemical Engineering Chonnam National University
Gwang-ju 500-757(Republic of Korea)E-mail:leeys@chonnam.ac.kr
kkaliyap@uwo.ca [b]Dr.K.Karthikeyan,Prof.X.Sun
Department of Mechanical and Materials Engineering The University of Western Ontario London,Ontario,N6A 5B9(Canada)
[c]Dr.V.Aravindan
Energy Research Institute (ERI@N)Nanyang Technological University
Research Techno Plaza,50Nanyang Drive,Singapore 637553(Singapore)[d]Dr.Y.-G.Lee
Power Control Device Research Team
Electronics and Telecommunications Research Institute Daejeon 305-700(Republic of
Korea)
Supporting information for this article is available on the WWW under /10.1002/cssc.201301262.
CHEM SUS CHEM FULL PAPERS
pine-cone-derived carbonaceous materials with a high yield (>40%)and minimum ash content(0.7wt%).This minimum ash content and notable carbon yield makes pine cones attrac-tive for the synthesis of high-surface-area AC for manifold ap-plications compared to other biomass precursors reported.[34,35] Furthermore,pine cones or coulter pines and their seeds are a common form of biomass,which is highly abundant,low cost,and has existed for over200million years.Pine cones are used predominantly for decorative applications,food prepara-tion,and baking.Nevertheless,reports have described the syn-thesis of pine-cone-derived AC for the removal of Pb ions from water[36]and their adsorption properties for various dyes.[37–39] Recently,Duman et al.[35]evaluated the physical and chemical properties of such a pine-cone-derived AC and confirmed
the low ash content and high yield.The abovementioned appeal-ing properties of pine-cone-derived AC attracted us to evaluate its supercapacitance behavior in an EDLC configuration with the need to enhance the energy density in mind.To realize the higher energy density of EDLCs,the use of organic electrolytes is necessary as aqueous electrolytes experience oxygen evolu-tion at~1.23V.[6,40]In this regard,high-surface-area AC was prepared by activation of the charcoal obtained from the car-bonization of pine cones with KOH at two different concentra-tions.Among the samples prepared,the AC treated with a high concentration of activation agent(KOH)had an ultra-high surface area of~3950m2gÀ1along with enhanced struc-tural alignment and high electrical conductivity.The high-sur-face-area AC obtained was tested as an electrode material in an EDLC configuration in an organic medium.The EDLC cell that contained the AC with
5wt%KOH delivered a high ca-
pacitance of198F gÀ1and an
energy density of61Wh kgÀ1be-
tween0and3V at0.25A gÀ1
current density.Various studies
were conducted to corroborate
the results that were obtained
and are described in detail.
Results and Discussion
The XRD pattern of raw pine
cone petals(RPP)is presented in
Figure S1a.The XRD pattern of
RPP exhibits the main character-
istic peaks of cellulose I at2q=
15.9,22.1,and34.38and less or-
ganized peaks that correspond
to cellulose II at2q=~26,~36,
and~408,which indicates that
RPP is composed mainly of
highly organized crystalline cel-
lulose fibers.[41–43]The surface
morphology(Figure S1b)shows
clearly that RPP is composed of
large microfibers with an exter-
nal diameter of~45m m.More-over,the presence of macropores between the fibers(Fig-ure S1c)was also noted.The surface composition of RPP was examined by FTIR spectroscopy(Figure S1c),which suggests that RPP is composed of a mixture of functional groups.The peaks observed at n˜=1610and690cmÀ1are related to the ex-istence of lignin.[42]The stretching vibration of polysaccha
ride components such as the carbohydrate backbone(CÀC),sym-metric glycosidic groups(CÀOÀC),the cellulose backbone (CÀOH),and the OÀH and CÀH groups of cellulose are ob-served at n˜=1160,1100,1050,~1330,and2910cmÀ1,respec-tively.[43]The stretching vibration of C=O in the protonated car-boxylic acid because of the presence of uronic acids was con-firmed by the peak at n˜=~1720cmÀ1.[42,43]Notably,the texture of the raw material(RPP in this case)influences the surface area of the carbon.The existence of a large quantity of macro-pores on RPP is conducive to absorb more activation agent (KOH),which enables the complete oxidation of the reactive sites to form a more porous surface(Figure S1c).Hence,a very high surface area can be achieved with a high amount of acti-vation agent.[10]
The XRD patterns of PCC0,PCC3,and PCC5pyrolyzed at 7508C under an Ar flow are presented in Figure1a.The ob-tained patterns suggest clearly the highly disordered/amor-phous nature of the carbons derived from pine cones with dis-cernible characteristic(002)and(001)reflections.The broad reflections between2q=20and308(002)indicate the pres-ence of small domains of coherent graphene sheets,whereas the reflections around2q=438(001)correspond to the3D honeycomb structure formed by the sp2-hybridized carbon
Figure1.a)XRD patterns of PCC0,PCC3,and PCC5prepared at7508C and field-emission scanning el
ectron micro-graphs of b)PCC0,c)PCC3,and d)PCC5.
atoms.[17]Although no discernible peaks are observed,a de-crease in the peak intensity is noted as the KOH loading is in-creased during activation.This reveals the increase in d spac-ing,which leads to ACs with a disordered nature to result in a larger specific surface area.It is well known that the mea-surement of the empirical parameter(R factor)is an effective way to evaluate the number of graphene sheets arranged as a single layer.This R factor can be defined as the ratio of the intensity of the(002)reflection with respect to the back-ground.[44,45]The more prominent(002)reflection for PCC0 than that of the KOH-treated samples indicates a higher degree of carbonization.As expected(Table1),a decrease in the R factor on increasing the KOH ratio suggests that the acti-vation process leads to a breakdown of aligned structural do-mains,that is,long-range order and allows the random distri-bution of graphene nanosheets.[45]However,the sample with a lower empirical value exhibits a large number of single layers,which results in better
textural characteristics.There-
fore,better capacitive properties
are expected for PCC5than the
other materials prepared.The
morphological features of pyro-
lyzed carbon from pine cones
under an Ar flow are shown in
Figure1b–d.As expected,there
is no porosity generated in the
case of pyrolyzed carbon,that is,
if the PCC/KOH ratio is1:0.Many
open cavities are noted in PCC
treated with the minimum
amount of KOH(1:3ratio),
whereas a3D architecture and
interconnected network-like
morphology are observed for
the material prepared with a1:5
ratio.The formation mechanism
of the cavities on the surface
was attributed to the intercala-
tion of K+ions into the lamellae
of the crystallites,which broke
the lamellae arrangement to
result in the expansion of the in-
terlayer voids,elimination of
crosslinking,and the stabilization
of carbon atoms in the crystalli-tes.[20,31]Although the K+ions were removed by washing with HCl,the reorganization of lamellae is not possible.Thus,it pro-vides free interlayer voids,which is considered to be the key factor to achieve a very high surface area.Such a morphology is beneficial for facile electron transport and to enable more active sites for the double-layer formation across the interface, which results in more favorable electrochemical characteristics irrespective of a symmetric or asymmetric assembly.[6,7]
The nitrogen adsorption–desorption isotherms of AC derived from pine cones are presented in Figure2a.Notably,shape of the isotherms varied remarkably with KOH treatment.The iso-therm of PCC0indicates type I behavior according to the IUPAC classification.In contrast,the curves of PCC3and PCC5
exhibit an intermediate isotherm
between type I and II with
a knee and increasing plateau,
which suggests a combination
of micro-and mesoporous struc-
tures in the samples.The early
stage of the isotherms corre-
sponds to micropore filling,and
the sloppy region at a high rela-
tive pressure represents the mul-
tilayer adsorption on the meso-pores.In addition,if the amount of activation agent is in-creased,the nitrogen adsorption properties of the AC also in-creased with relative pressure until P/P0%0.5.It is well known that an isotherm with a slight hysteresis loop and wide
knees
Figure2.a)Nitrogen adsorption–desorption isotherms,b)pore width versus volume adsorption,and c)pore size distribution of the prepared carbon samples.
at P/P0of around0.35–0.5indi-cates the presence of a consider-able amount of small meso-pores,which can be observed on the PCC5surface.The porous textural parameters of ACs de-rived from pine cones,such as BET surface area,micropore volume,and the average pore diameter,are summarized in Table1.It is evident that the BET surface area and total pore volume of the samples increase remarkably after the introduc-tion of the KOH activation agent. Among the samples,PCC5 shows the highest surface area (3950m2gÀ1)and pore volume, which demonstrate the well-de-veloped porous structure.How-ever,PCC0exhibited poor tex-tural properties with a surface area and pore volume of 569m2gÀ1and0.25cm3gÀ1,re-spectively.Notably,the BET sur-face area obtained for PCC5is comparable to the highest value ever reported for AC obtained from a biomass precursor.[10,11] PCC5showed a larger absorp-tion volume than PCC0and PCC3(Figure2b),which indi-cates a higher surface area and larger pore volume with a favora-ble pore size distribution.The pore size distributions of all the samples measured by N2adsorp-tion are illustrated in Figure2C. These indicate that the addition of KOH significantly affects the
pore size distribution,and an increased KOH weight ratio can enlarge the pore size(Figure2c).Both P
CC3and PCC5have a pore size around3–4nm,whereas the pore size of PCC0is less than2nm.Moreover,PCC3and PCC5also showed micro-porosity.This could be because pine cone petals localize the carbon rods inside the structures during carbonization.Fur-thermore,with the increased KOH concentration,more KOH is believed to take part in the activation process to result in a more porous structure,which leads to an increase in the sur-face area of the samples.Moreover,the addition of a larger amount of activation agent(KOH)provides extra surface oxygen complexes that release more carbonaceous gases and allow a slower and controlled carbonization reaction,which is responsible for the enhancement of the porosity as well as the surface area of the resultant material.The ultrahigh surface area and narrow pore size distribution of PCC5is beneficial for charge-storage applications.[40]
The surface features of the pine-cone-derived ACs were ana-lyzed by X-ray photoelectron spectroscopy(XPS;Figure3).The core-level C1s and O1s spectra show the presence of function-al groups over the AC surface clearly.In the core-level C1s spectra,the sp2CÀC binding energy is centered around 284.6eV,and the appearance of peaks at binding energies of 287.1and288.5eV correspond to carbonyl(C=O)and carboxyl (O=CÀOH)groups,respectively.The peaks positioned around 285.9and290.2eV are assigned to the hydroxyl groups (CÀOH)and p–p*,respectively.[46,47]
An increased intensity of all the peaks is noted with increased KOH concentration, which suggests clearly that the activating agent plays a vital role to develop these active functional groups.[46–48]Similarly,in the core-level O1s spectra,the appearance of peaks at binding energies of534.7,533.2,530.8,and529.9eV also reflects the presence of the functional groups noted above.[47]Further-more,oxygen contents of~12.66,~19.02,and~14.41wt
% Figure3.Core-level(C1s and O1s)XPS spectra of a,b)PCC0,c,d)PCC3,and e,f)PCC5.
(atomic)are calculated for PCC/KOH ratios of1:5,1:3,and1:0, respectively.According to Simon and Gogotsi,[40]the presence of a small amount of oxygen-containing functional groups is crucial to ensure the long-term cycleability of the carbona-ceous materials.The presence of higher surface functionalities such as surface oxygen affects the electrical conductivity pro-file and is involved in an unwanted side reaction with the elec-trolyte counterpart.As a result,poor electrochemical profiles are noted for such oxygen-rich carbonaceous electrodes.[40] Therefore,a good electrochemical profile is noted for PCC5. The Raman spectra of RPP,PCC0,PCC3,and PCC5are illus-trated in Figure S2.RPP did not show any peaks within the re-corded area,whereas all other samples showed characteristic carbon peaks centered at n˜=~1350and1595cmÀ1,which in-dicate the successful carbonization of the pine cone petals.[49] The peaks at n˜=1350and1595cmÀ1could be assigned to dis-ordered carbon(D band)and the symmetric E2g vibration mode of graphite-like materials(G band),respectively.[49]In ad-dition,the intensity ratio of the D and G bands(I D/I G)for PCC0, PCC3,and PCC5was1.17,0.94,and0.99,respectively.It is clear that the sample prepared with the highest KOH ratio showed a high structural alignment,which suggests that PCC5contains predominantly sp2-type carbon.However,the high structural alignment of PCC5could effectively decre
ase the presence of heteroatoms(O,S,and N)on the surface,which would reduce the risk of side reactions and thus affect the charge-storage behavior.This correlates well with the result obtained from XPS analysis.In addition,the high structural alignment further confirmed the high degree of intralayer condensation of PCC5, which improved its electrical conductivity remarkably and en-hanced the ionic/electronic transfer to lead to an excellent im-provement in the electrochemical performance of the PCC5 electrodes.
The cyclic voltammetry(CV)traces of the symmetric superca-pacitors fabricated with pine-cone-derived AC cycled between 0–3V at scan rate of50mV sÀ1are shown in Figure4a.The CV curves revealed clearly the more or less rectangular behavior for all the three electrodes with variation in the current re-sponse that is characteristic of typical EDLCs.[2]At higher scan rates,the distortion of the CV signatures noted for all three electrodes suggests a diffusion-limited capacitance because of the polarization effect(Figure S3).From the CV studies,the energy-storage mechanism of such symmetric supercapacitors could be explained by electrical double-layer theory,which is mainly based on the accumulation of charge carriers across the electrode–electrolyte interface,which subsequently forms the double layer.[2,7]In this case,the symmetric cells delivered specific capacitances of~141,~100,and~17F gÀ1at a slow
scan rate of5mV sÀ1for PCC5,PCC3,and PCC0,respectively. This increase in specific capacitance is consistent with the linear variation of the specific surface area of the ACs derived from pine cones and in the literature.[4]As expected,a decrease in the specific capacitance profiles are noted for all the three ACs with an increase in scan rate(Figure4b).
Galvanostatic charge–discharge studies were performed for all the three symmetric supercapacitor cells between0–3V at a current density of1A gÀ1(Figure5a).The charge–discharge curves of all the cells show a linear relationship with time, which is indicative of perfect EDLC behavior.However,a sharp decrease in voltage is noted at the beginning of the dis-charge–charge that results from the equivalent series resist-ance(ESR)of the cell.The ESRs of symmetrical cells that con-tained PCC0,PCC3,and PCC5electrodes were calculated as50, 39,and29W,respectively.A longer charge–discharge time is noted for PCC5compared to PCC3and PCC0,which is consis-tent with the CV analysis.[4]The symmetric cells delivered a
spe-Figure4.a)CV traces of PCC0,PCC3,and PCC5electrodes in a symmetric su-percapacitor configuration at a scan rate of50mV sÀ1between0and3V and b)the dependence of capacitance versus scan
rate.
Figure5.a)Typical charge–discharge of the symmetric supercapacitor cells at a current density of1A gÀ1,and b)plot of specific discharge capacitance versus cycle number.
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