A

aETapp©

K

1

aacbcatrntcdiomb

0d

Available online at www.sciencedirect.com

Materials Chemistry and Physics 109 (2008) 241–248

Ammonium carboxylates assisted combustion process for thesynthesis of nanocrystalline LiCoO2 powders

S. Vivekanandhan a, M. Venkateswarlu b, N. Satyanarayana a,∗a Department of Physics, Pondicherry University, Pondicherry 605014, India

b HBL Power Systems Ltd., Hyderabad 500078, India

Received 26 July 2007; received in revised form 22 October 2007; accepted 10 November 2007

bstract

Nanocrystalline LiCoO2 powders were synthesized by combustion process using three different ammonium carboxylates named ammoniumcetate (AA), ammonium citrate (AC) and ammonium tartarate (AT) as fuels and metal nitrates as the source of metal ions as well as oxidants.ffect of three different ammonium carboxylates on the synthesis of nanocrystalline LiCoO2 powders was investigated through FTIR, XRD,G/DTA and SEM techniques. FTIR and XRD analysis confirmed that the LiCoO2 phase could obtain by calcining the polymeric intermediates

t 450 ◦C for 12 h. Among the three different fuels, ammonium citrate assisted combustion process exhibited the formation of organic free phaseure nanocrystalline LiCoO2 powders. The average crystallite size of the LiCoO2 powder prepared at 450 ◦C for 12 h by ammonium citrate assistedrocess is found to be 24 nm. 2007 Elsevier B.V. All rights reserved.

ctrosc

o[

iricromuthcfm

eywords: Nanostructures; Chemical synthesis; Fourier transform infrared spe

. Introduction

Layered LiCoO2 powders have been widely investigated ascathode material for rechargeable lithium batteries due to its

dvantages such as easy preparation, high voltage, better cycli-ality and high theoretical specific capacity [1–5]. So, it haseen used in commercial lithium cells [6]. The electrochemi-al properties of cathode materials like storage capacity, voltagend charging/discharging rates are considerably increase whenheir crystallite/particle size has reduced to nanosize [7–9]. Inecent years, significant effort has been made to synthesis theanostructured cathode materials in order to enhance their elec-rochemical properties [10–12]. Synthesis of nanocrystallineathode powders by conventional solid-state reaction is oftenifficult due to higher operating temperature and the result-ng powders exhibit poor homogeneity and wide distribution

f crystallite size [13,14]. Hence, a variety of wet chemicalethods such as sol–gel, co-precipitation, hydrothermal, com-

ustion, etc., were developed and investigated for the synthesis

∗ Corresponding author. Tel.: +91 413 2654404; fax: +91 413 2655265.E-mail address: nallanis2000@yahoo.com (N. Satyanarayana).

dpnlw

e

254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.11.027

opy (FTIR); Microstructure

f nanocrystalline cathode powders with desired properties15–20].

Among the various wet chemical processes, combustion routes found to be simple and cost effective for the synthesis of wideange of nanocrystalline powders [21–24]. Combustion processnvolves the exothermic decomposition of fuel and oxidant pre-ursors, which results in the formation of oxide nanoparticleselatively at lower temperatures. Physicochemical propertiesf the oxide powders synthesized by combustion process areainly depending on the nature and the amount of fuel that

sed [25,26]. In order to identify the suitable fuel for the syn-hesis of oxide ceramics in nanophase, wide range of researchas been made using the various organic fuels such as urea,itric acid, glycine, etc. [27–30]. Recently, we found that theormation of ammonium citrate with in the polymeric inter-ediates reduced the ignition temperature and also calcining

uration, which resulted the formation of organic free phaseure nanocrystalline oxide powders [31]. The role of ammo-ium citrate is to ignite the combustion reaction relatively at

ower temperatures and also caused the large gas evaluation,hich lead to the formation of ultra fine particles.Hence, in the present work, combustion process using differ-

nt ammonium carboxylates such as ammonium acetate (AA),

242 S. Vivekanandhan et al. / Materials Chemistry and Physics 109 (2008) 241–248

Ft

awpsX

Fa

2

bFatauMttf

ig. 1. Schematic of ammonium carboxylates assisted combustion process forhe synthesis of nanocrystalline LiCoO2 powders.

mmonium citrate (AC) and ammonium tartarate (AT) as fuelsas investigated for the synthesis of nanocrystalline LiCoO

2owders. The effect of various ammonium carboxylates on theynthesis of LiCoO2 powders was identified through FTIR,RD, TG/DTA and SEM techniques.

fuit

Fig. 2. Photograph of the polymeric intermediates prepared with

ig. 3. FTIR spectra of the polymeric intermediates prepared with three differentmmonium carboxylates (AA, AC and AT).

. Experimental procedure

Nanocrystalline LiCoO2 powders were synthesized using ammonium car-oxylates assisted combustion process by following the procedure shown inig. 1. Stoichiometric amount of lithium nitrate (AR grade, s-d fine, India)nd cobaltus(III) nitrate (AR grade, Merck, India) solutions were mixed withhe solutions of different ammonium carboxylates named ammonium acetate,mmonium citrate and ammonium tartarate anhydrous (AR grade, Merck, India)nder constant stirring. Total metal ion to ammonium carboxylate ratio (M:AA,:AC and M:AT) was kept constant as 1:1. The obtained pink colour solu-

ions were evaporated at 80 ◦C for 6 to 8 h. Continuous evaporation leads tohe formation of pink coloured polymeric resin. Obtained polymeric resign wasurther dried at 120 ◦C for 12 h to remove the excess water, which lead to the

ormation of solid mass. Photograph of as obtained polymeric intermediatessing ammonium acetate, ammonium citrate and ammonium tartarate is shownn Fig. 2. Further, these intermediates were ground and calcined at an optimumemperature to obtain nanocrystalline LiCoO2 powders.

three different ammonium carboxylates (AA, AC and AT).

S. Vivekanandhan et al. / Materials Chem

Fig. 4. XRD pattern of the polymeric intermediates prepared with three differentammonium carboxylates (AA, AC and AT).

spoeNfiwahpi

3

3

pnFt

Fig. 5. Scanning electron micrograph (two different magnifications) of the polymericand AT).

istry and Physics 109 (2008) 241–248 243

Fourier transform infrared (FTIR) spectra were recorded using FTIR-8000pectrometer of Shimadzu, Japan, to identify structural coordination of as pre-ared as well as calcined polymeric intermediates. The measurement was carriedut in the range of 400–4000 cm−1 with KBr diluter. X-ray diffraction (XRD)xperiments were carried out using a Panalytical, X-ray powder diffractometer,etherlands, with Cu K� radiation. The average crystallite size was calculated

rom the Scherrer’s formula. NBS standard silicon was used for estimation ofnstrumental broadening [32]. Thermal behavior of the polymeric intermediatesas investigated by simultaneous TG/DTA measurement using Labsys thermal

nalyzer, Setaram, France. Approximately, 3 mg of polymeric intermediate waseated at a rate of 10 ◦C min−1 between 30 and 600 ◦C. All thermal studies wereerformed in flowing oxygen. Microstructure of the polymeric intermediates wasdentified using scanning electron microscope, Hitachi, S-3400N, Japan.

. Results and discussion

.1. Characterization of polymeric intermediates

Fig. 3 shows the FTIR spectra of polymeric intermediates

repared with three different ammonium carboxylates (ammo-ium acetate, ammonium citrate and ammonium tartarate). Fromig. 3, the IR absorption at 3398–3425 cm−1 region is attributed

o stretching of O–H groups exist in the polymeric intermedi-

intermediates prepared with three different ammonium carboxylates (AA, AC

244 S. Vivekanandhan et al. / Materials Chemis

Ft

arwi1tobTsdipCmcc[

p

bpcpc

itsdimnAhh

3

pi2ttwTfwatdiwnbbsadttsttaabsa2af

ig. 6. TG/DTA thermograms of the polymeric intermediates prepared withhree different ammonium carboxylates (AA, AC and AT).

tes [33]. The strong broad peak observed at 3325–3030 cm−1

egion for all the intermediates is attributed to NH4+ groups,

hich shows the presence of ammonium ions in polymericntermediates [33]. IR peaks observed at 1608–1632 cm−1 and400–1415 cm−1 region for all the intermediates are, respec-ively due to the asymmetric and symmetric stretching vibrationf COO− groups, which confirms the chelation of metal ionsy COO− groups present in the ammonium carboxylates [34].he weak band observed at 1087–1098 cm−1 region for all theamples is assigned to CO3

2− functional groups, which may beue to the formation of metal carbonates [33]. The polymericntermediate prepared with citric acid shows an additional IReak at 1292 cm−1 corresponds to the asymmetric vibration of–O–C group (ester) (where the respective symmetric vibrationerged with the vibration of CO3

2− functional group), whichaused the formation of branched polymeric network between

itric acid derivatives, further it is confirmed by SEM analysis33].

X-ray diffraction patterns of the polymeric intermediatesrepared by combustion process, using different ammonium car-

acpe

try and Physics 109 (2008) 241–248

oxylates are shown in Fig. 4. The observed peaks in the XRDatterns indicate the formation of metal carboxylates (as pre-ipitate) as well as metal carbonates, which may be due to theoor polymerization of ammonium carboxylates. Further, it isonfirmed by SEM analysis.

Fig. 5 shows the scanning electron micrographs of polymericntermediates. SEM images of the polymeric intermediates syn-hesized using ammonium acetate and ammonium tartaratehowed porous microstructure, where the voids are uniformlyistributed through out the surface. Where as, the polymericntermediates prepared with ammonium citrate, exhibits poly-

eric microstructure with big voids. The observed polymericature of the intermediate is well consistent with FTIR results.mmonium tartarate assisted polymeric intermediate exhibitsigh porosity than other two intermediates, which are shown inigher magnification.

.2. Thermal behavior of the polymeric intermediates

TG/DTA thermograms of the polymeric intermediates pre-ared using three different ammonium carboxylates are shownn Fig. 6. From Fig. 6 the observed endothermic peak around00 ◦C for all the intermediates indicates the deformation ofhe polymeric structure. Endothermicity of the denaturing reac-ion is more for the ammonium citrate assisted intermediates,hich indicates the strong bond between the citrate derivatives.hermogram of all the polymeric intermediates showed two dif-

erent exothermic peaks DTA curve and respective weight lossas observed in the TG curve. The exothermic peak observed

t low-temperature region (∼240–300 ◦C) is might be due tohe decomposition of ammonium carboxylate present in theried intermediates. Weight loss is more for the polymericntermediate prepared using ammonium tartarate (about 53%);here as the weight loss for ammonium acetate and ammo-ium citrate assisted intermediates are respectively found toe 28 and 30%. The second exothermic reaction, which maye due to the decomposition of metal carboxylate derivatives,tarts at 281, 348 and 287 ◦C, respectively for the intermedi-tes prepared with AA, AC and AT. The exothermicity of aboveecomposition reaction defers with respect to the microstruc-ures obtained from SEM. More exothermicity is observed forhe AT assisted intermediate, which is due to the high-poroustructure (Fig. 5c), also the observed lower exothermicity forhe intermediate prepared with ammonium acetate is may be dueo the lower porosity (Fig. 5a). Though the ammonium citratessisted polymeric intermediates exhibits poor porosity in SEMnalysis, it exhibits high exothermicity during the combustion,ecause of its combustion mechanism as shown in Fig. 7. Fig. 7hows the photographs of the AC assisted polymeric intermedi-te at different temperatures. The intermediate begins to melt at00 ◦C (respective exothermic peak is observed in DTA curve)nd the volume of the intermediate has increased, which resultsoam type product. Further heating, caused the decomposition

t 250 ◦C with large gas evaluation and it is observed in DTAurve as exothermic peak between 250–280 ◦C. And the com-lete decomposition has observed at 400 ◦C and the respectivexothermic peak was observed in DTA curve. Occurrence of

S. Vivekanandhan et al. / Materials Chemistry and Physics 109 (2008) 241–248 245

F red wi ) and

tbwaTbp

wFtitaas

ipbaFaisl

3

daa

ioFa1that indicates the presence of undecomposed organic derivativesin the intermediates due to poor combustion. It is attributed to itsmicrostructure as observed in scanning electron micrograph andthermal behavior as observed in TG/DTA. Organic free LiCoO2

ig. 7. Photographs of the combustion reaction of polymeric intermediate prepantermediate (200 ◦C), (C) combustion of polymeric foam intermediate (250 ◦C

he foam intermediate during the calcining process caused theetter combustion and lead to the organic free final products,hich is not observed for the intermediates prepared using AA

nd AT. Further it is confirmed from FTIR results. From theG/DTA analysis, all the combustion reactions are completedefore 450 ◦C and hence, it is optimized for the calcinations ofolymeric intermediates in order to obtain the final product.

Typical FTIR spectra of the polymeric intermediate (preparedith citric acid) calcined at different temperatures are shown inig. 8. The FTIR peaks related to organic derivatives are begins

o disappear at 300 ◦C and completely disappeared when thentermediate is calcined at 450 ◦C and above, which is consis-ence with TG/DTA results. The newly observed FTIR peakst 510–520 cm−1 and 580–610 cm−1 regions are attributed tosymmetric stretching modes of [CoO6] octahedral in LiCoO2tructure [35,36].

Fig. 9 shows the typical XRD patterns of calcined polymericntermediate prepared with ammonium citrate at different tem-eratures. From Fig. 9, it is clear that the layered LiCoO2 phaseegins to form at 300 ◦C and the complete phase was obtainedt 450 ◦C and above which is consistence with TG/DTA andTIR results. Also, the observed intensification of the peakss a function of calcining temperature indicates the incrementn crystallinity. XRD pattern for the sample calcined at 450 ◦Chowed all the peaks, which indicates the completion of crystal-ization process.

.3. Characterization of LiCoO2 powders

Fig. 10 shows the FTIR spectra of synthesized LiCoO2 pow-ers prepared by AA, AC and AT assisted combustion processt 450 ◦C for 12 h. The observed FTIR peaks at 510–520 cm−1

nd 580–610 cm−1 region are attributed to asymmetric stretch-Ft

ith ammonium citrate (A) dried polymeric intermediate, (B) melt of polymeric(D) polymeric intermediate after the combustion (400 ◦C).

ng modes of [CoO6] octahedral, which confirmed the formationf LiCoO2 structure for all the samples. From Fig. 10, theTIR spectra for the intermediates prepared with ammoniumcetate and ammonium tartarate showed low intense peaks at455–1553 cm−1 region, which is attributed to organic residuals

ig. 8. Typical FTIR Spectra of the polymeric intermediate calcined at differentemperature prepared with ammonium citrate.

246 S. Vivekanandhan et al. / Materials Chemistry and Physics 109 (2008) 241–248

Ft

spc

at(tFpob

Fw

Fig. 11. XRD pattern of polymeric intermediate calcined at 450 ◦C preparedwith three different ammonium carboxylates (AA, AC and AT).

Table 1Comparison of lattice parameter values of synthesized LiCoO2 powders by ACassisted combustion process with literatures

S. No. a (A) c (A) c/a References

1 2.81 13.99 4.97 Present work2 2.83 13.85 4.90 [39]3 2.82 14.06 4.99 [40]4

ig. 9. Typical XRD pattern of the polymeric intermediate calcined at differentemperature prepared with ammonium citrate.

tructure is observed only for the ammonium citrate assistedrocess, which confirms the better combustion and lead to theomplete decomposition of organic derivatives.

The XRD patterns of synthesized LiCoO2 powders at 450 ◦Clong with JCPDS standard are shown in Fig. 11. The observedhree strong peaks at 19.00◦ (0 0 3), 37.38◦ (1 0 1) and 45.51◦1 0 4) regions for all the samples indicate the formation of crys-alline LiCoO2 phase of �-NaFeO2 type structure [37]. From

◦

ig. 11, the observed small peak at 31.5 for LiCoO2 samplesrepared by AA and AT assisted process indicates the presencef Co2O3 impurity phase, which may be due to the poor com-ustion [38]. The XRD pattern for LiCoO2 sample prepared

ig. 10. FTIR spectra of polymeric intermediates calcined at 450 ◦C preparedith three different ammonium carboxylates (AA, AC and AT).

5

btiac

Fw

2.83 14.00 4.95 [41]2.83 13.87 4.9 [42]

y AC assisted process, exhibits not only pure phase but alsohe hexagonal doublets (0 0 6)/(0 1 2) and (1 0 8)/(1 1 0), which

ndicates the good hexagonal ordering and great layered char-cteristics and the obtained peaks are indexed to the hexagonalell [39]. Cell parameters calculated for the LiCoO2 powders

ig. 12. Typical SEM image of the nanocrystalline LiCoO2 powder preparedith ammonium citrate.

S. Vivekanandhan et al. / Materials Chemistry and Physics 109 (2008) 241–248 247

nocry

pcievpcfmfdt

p1aaSmsdLoaFnoo

4

cbaFaTmridpXaeLp2

A

Fig. 13. SEM–EDS spectrum and elemental mapping of na

repared by AC assisted process are a = 2.81 A, c = 13.99 A and/a = 4.97, which are very much comparable with those reportedn the literature. Table 1 gives the comparison of lattice param-ters values of synthesized LiCoO2 powders with the reportedalues [40–42]. The crystallite size of the synthesized LiCoO2owders at 450 ◦C by ammonium citrate assisted process wasalculated using the broadening data of the 100% peak at 19.03◦or the (0 0 3) plane (recorded at the scanning rate of 1/2◦ perin) obtained through Lorentz fitting method and the Scherrer’s

ormula [32]. The calculated crystallite size of the LiCoO2 pow-ers synthesized by ammonium citrate assisted process is foundo be 24 nm.

Fig. 12 shows the typical SEM micrographs of the LiCoO2owders prepared by ammonium citrate process at 450 ◦C for2 h. SEM images of the LiCoO2 powders exhibit the looselygglomerated ultra fine LiCoO2 particles. All the agglomer-ted LiCoO2 particles exhibit uniform size of ∼40–70 nm.EM–EDS (energy dispersive spectroscopy) spectrum with ele-ental mapping of LiCoO2 powders synthesized using AC is

hown in Fig. 13. EDS elemental mappings show the uniformistribution of Co and O, which indicates the formation ofiCoO2 structures. No evidence was identified for the presencef carbon in LiCoO2 powders from SEM–EDS analysis, whichlso confirmed the formation of organic free LiCoO2 structure.

TIR, XRD and SEM–EDS analysis confirmed that the ammo-ium citrate assisted combustion process results the phase purerganic free nanocrystalline LiCoO2 powders and hence it isptimized for the synthesis of LiCoO2 powders.

Dmm(

stalline LiCoO2 powder prepared with ammonium citrate.

. Conclusion

Nanocrystalline LiCoO2 powders were synthesized usingombustion process with three different ammonium car-oxylates named ammonium acetate, ammonium citrate andmmonium tartarate as the fuels at 450 ◦C. TG/DTA, XRD andTIR studies showed that the LiCoO2 phase begun to formt 300 ◦C and the complete phase was obtained at 450 ◦C.here were remarkable differences in the process in terms oficrostructure of the intermediates, combustion mechanism,

esidual carbon and the phase nature of the final products. It wasdentified that the microstructures of the intermediates were wellepend on the precursor chemical and plays major role in theirhysicochemical properties of the final products. From FTIR,RD, TG/DTA and SEM–EDS analysis, it is concluded that

mmonium citrate assisted process is more favorable than oth-rs for the synthesis of phase pure organic free nanocrystallineiCoO2 powders. The crystallite size of LiCoO2 powders pre-ared by ammonium citrate assisted process was found to be4 nm.

cknowledgements

Dr. N. Satyanarayana gratefully acknowledges DST, CSIR,

RDO, Government of India, for the financial support throughajor research projects. SV acknowledges the CSIR, Govern-ent of India, for the award of Senior Research Fellowship

SRF).

2 emis

R

[[

[

[[

[

[

[[

[

[

[

[

[[

[[

[[[

[

[

[

[

[

[

[

[

[

[

48 S. Vivekanandhan et al. / Materials Ch

eferences

[1] M.S. Whittingham, Chem. Rev. 104 (2004) 4271–4281.[2] J.M. Tarascon, M. Armand, Nature 414 (2001) 359–367.[3] D. Linden, T.B. Reddy, Handbook of Batteries, Third ed., McGraw-Hill

Inc., New York, 2002.[4] P.G. Bruce, Chem. Commun. 19 (1997) 1817–1824.[5] R. Koksbang, J. Barker, H. Shi, M.Y. Saidi, Solid State Ionics 84 (1996)

1–21.[6] L.F. Nazar, G. Goward, F. Leroux, M. Duncan, H. Huang, T. Kerr, J.

Gaubicher, Int. J. Inorg. Mater. 3 (2001) 191–200.[7] E. Stura, C. Nicolini, Anal. Chim. Acta 568 (2006) 57–64.[8] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W.V. Schalkwijk, Nat.

Mater. 4 (2005) 366–376.[9] J. Jamink, J. Maier, Phys. Chem. Chem. Phys. 5 (2003) 5215–5219.10] F. Jiao, K.M. Shaju, P.G. Bruce, Angew. Chem. Int. Ed. 44 (2005) 2–6.11] H.M. Wu, J.P. Tu, Y.F. Yuan, Y. Li, W.K. Zhang, H. Huang, Physica B 369

(2005) 221–226.12] X. Li, F. Cheng, B. Guo, J. Chen, J. Phys. Chem. B 109 (2005)

14017–14024.13] A. Lundblad, B. Bergman, Solid State Ionics 96 (1997) 173–181.14] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, Mater. Res.

Bull. 15 (1980) 783–789.15] L.J. Fu, H. Liu, C. Li, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Prog. Mater.

Sci. 50 (2005) 881–928.16] A. Burukhin, O. Brylev, P. Hany, B.R. Churagulov, Solid State Ionics 151

(2002) 259–263.17] S.H. Wu, H.J. Su, Mater. Chem. Phys. 78 (2002) 189–195.18] L. Li, W.H. Mayer, G. Wegner, M.W. Mehrens, Adv. Mater. 17 (2005)

984–988.19] S. Vivekanandhan, M. Venkateswarlu, N. Satyanarayana, Mater. Chem.

Phys. 91 (2005) 54–59.20] W.L. Guo, L.Q. Mai, W. Chen, Q. Xu, Q.Y. Zhu, J. Mater. Sci. Lett. 22

(2003) 1035–1037.21] C.C. Hwang, T.Y. Wu, J. Wan, J.S. Tsai, Mater. Sci. Eng. B 111 (2004)

49–56.

[[

[

try and Physics 109 (2008) 241–248

22] S.W. Kwon, S.B. Park, G. Seo, S.T. Hwang, J. Nucl. Mater. 257 (1998)172–179.

23] F. Li, K. Hu, J. Li, D. Zhang, G. Chen, J. Nucl. Mater. 300 (2002) 82–88.24] H.S. Potdar, S.B. Deshpande, Y.B. Khollam, A.S. Despande, S.K. Date,

Mater. Lett. 57 (2003) 1066–1071.25] R.D. Purohit, A.K. Tyagi, J. Mater. Chem. 12 (2002) 312–316.26] R. Ganesan, S. Vivekanandhan, T. Gnanasekaran, G. Periaswami, S.S.

Raman, J. Nucl. Mater. 325 (2004) 134–140.27] S. Roy, L. Wang, W. Sigmund, Mater. Lett. 39 (1999) 138–141.28] Y. Zhang, H.C. Shin, J. Dong, M. Liu, Solid State Ionics 171 (2004) 25–31.29] S. Rodrigues, N. Munichandraiah, A.K. Shukla, J. Power Sources 102

(2001) 322–325.30] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J.

Exarhos, Mater. Lett. 10 (1990) 6–12.31] S. Vivekanandhan, M. Venkateswarlu, N. Satyanarayana, J. Alloy Compd.

441 (2007) 284–290.32] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline

and Amorphous Materials, Wiley, New York, 1954.33] G. Socrates, Infrared and Raman Characteristic Group Frequencies, John

Wiley and Sons, New York, 2001.34] G.V. Rama Rao, D.S. Surya Narayana, U.V. Varadaraju, G.V.N. Rao, S.

Venkadesan, J. Alloy Compd. 217 (1995) 200–208.35] K.J. Rao, H. Benqlilou-Moudden, B. Desnat, P. Vinatier, A. Levasseur, J.

Solid State Chem. 165 (2002) 42–47.36] C. Julien, M.A. Camacho-Lopez, T. Mohan, S. Chitra, P. Kalyani, S.

Gopukumar, Solid State Ionics 135 (2000) 241–248.37] K.I. Gnanasekar, H.A. Cathrino, J.C. Jiang, A.A. Mrse, G. Nagasubrahma-

nian, D.H. Doughty, B. Rambabu, Solid State Ionics 148 (2002) 299–309.38] P. Periasamy, H.S. Kim, S.H. Na, S.I. Moon, J.C. Lee, J. Power Sources

132 (2004) 213–218.39] B. Gracia, P. Barboux, F. Ribot, A. Kahn-Harari, L. Mazerolles, N. Baffier,

Solid State Ionics 80 (1995) 111.40] Y. Gu, D. Chen, X. Jiao, J. Phys. Chem. B 109 (2005) 17901–17906.41] G.T.K. Fey, C.Z. Lu, T. Prem Kumar, Y.C. Chang, Surf. Coat. Technol. 199

(2005) 22.42] E. Antolini, Solid State Ionics 170 (2004) 159.