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Synthesis and characterization of manganese dioxide

spontaneously coated on carbon nanotubes

Sang-Bok Ma a ,Kyun-Young Ahn b ,Eun-Sung Lee b ,Ki-Hwan Oh c ,Kwang-Bum Kim

a,*

Department of Metallurgical Engineering,Yonsei University,134Shinchon-dong,Seodaemoon-gu,Seoul 120-749,Republic of Korea

Hyundai ECO Technology Research Institute,Yongin-Si,Gyeonggi-Do 449-912,Republic of Korea

NGV,Shinlim-dong,Kwanak-gu,Seoul 151-742,Republic of Korea

Received 8May 2006;accepted 6September 2006

Available online 27October 2006

Abstract

Manganese dioxide (MnO 2)was coated on carbon nanotubes (CNTs)by simple immersion of the CNTs into a KMnO 4aqueous solu-tion.The synthesis mechanism was investigated by in situ monitoring of the reduction potential and pH of the solution and supplemen-tary UV–VIS analysis of MnO à4ions in the https://www.wendangku.net/doc/0c2029724.html,Ts were found to act as a reducing agent and substrate for the heterogeneous nucleation of MnO 2in an aqueous KMnO 4solution.The morphology of the CNTs before and after MnO 2deposition was examined using scanning electron microscopy,which showed MnO 2deposited as a thin and uniform layer on the CNTs at an initial pH of 7,but as nano-rods of MnO 2at an initial pH 1.The MnO 2was shown to be a Birnessite-type MnO 2by X-ray powder di?raction and Raman spectroscopy.The thermal stability of the CNTs,examined by thermogravimetric analysis,was improved by the thin,uniform and continuous coating of MnO 2.

1.Introduction

Carbon nanotubes (CNTs)[1],including single-walled and multi-walled CNTs are conductive or semiconductive along tubules and quantized cross tubules.The unique physical and chemical properties of CNTs o?er unprece-dented opportunities of novel applications [2].Indeed,in the past several years,various potential applications of CNTs have been explored as ?eld-e?ect transistors [3],sen-sors [4]and templates [5].

Manganese dioxide (MnO 2)is one of the most attractive materials due to its ion exchange,molecular adsorption,catalytic,electrochemical,and magnetic properties.It is widely used as catalysts [6]and molecular-sieves [7],and especially as electrode materials in Li/MnO 2batteries [8]because of its energetic compatibility in a reversible lithium electrochemical system.

Recently,many studies have investigated MnO 2and CNTs for electrochemical capacitor applications.Among the various transition metal oxide materials for pseudoca-pacitors,amorphous and hydrated ruthenium oxide exhib-its remarkably high speci?c capacitance ($720F/g)compared with other oxides [9,10].However,its commer-cial use is limited by its high cost.Therefore,a great e?ort has been devoted to identifying alternative and inexpensive metal oxide electrode materials with acceptable electro-chemical properties.In line with these requirements,MnO 2is a promising electrode material for pseudocapaci-tors on account of its pseudocapacitive behavior,low cost and environmental compatibility [11–16].

CNTs have been continuously studied as electrode mate-rials (100–200F/g)for electrochemical capacitors [17,18],as additives to improve the electrode performance of metal oxide [19–25],and as deposition substrates for metal oxide for pseudocapacitors [26–29]because of their chemical sta-bility,good conductivity and large surface area.In addi-tion,CNTs are strongly entangled,providing a network of open mesopores.

Corresponding author.Tel.:+82221232839;fax:+8223125375.E-mail address:kbkim@yonsei.ac.kr (K.-B.Kim).

https://www.wendangku.net/doc/0c2029724.html,/locate/carbon

Carbon 45(2007)

375–382

Lately,several studies on the synthesis of MnO2/CNT nanocomposites[21–27]have been carried out with the aim of improving the electrochemical utilization of MnO2 and the high rate capability.Previous approaches to the synthesis of MnO2/carbon composite include various methods as physical mixing[24],thermal decomposition [27],ball milling[22],electrodeposition[26],sonochemical synthesis[30],sol–gel[31]and redox reaction[32,33].

Because the pseudocapacitive reaction of MnO2is known to be a surface reaction,only the surface or a very thin surface layer of the oxide can participate in the pseud-ocapacitive reaction.Therefore,in the synthesis of MnO2/ CNT nanocomposites,it is ideal to deposit a very thin layer ($nm)of MnO2onto CNTs with a control of the coating layer thickness,surface coverage and phase in order to improve the electrochemical utilization of MnO2.An increase in the e?ective interfacial area between MnO2 and an electrolyte can increase the electrochemical utiliza-tion of MnO2in the MnO2/CNT nanocomposites.Greater chemical contact and increased contact area between MnO2and CNT can improve the electric conductivity of the electrode due to the high electric conductivity of CNT[28,29].

In this study,in order to make MnO2more attractive for pseudocapacitor electrode materials,MnO2was coated spontaneously on CNTs by the simple immersion of the CNTs into a KMnO4aqueous solution.The synthesis mechanism of the heterogeneous nucleation of MnO2on the CNTs was investigated by in situ monitoring of the solution chemistry.

2.Experimental

MnO2was spontaneously deposited onto the CNTs(multi-walled CNTs,ILJIN Nanotech,S=200m2/g)by a direct redox reaction between

the CNTs and MnOà

4.First,200ml of0.01M and0.1M KMnO4(99+%,

Aldrich)solutions were heated to70°C using a circulator(FP40,Julabo) and then1.0g of the as-received CNTs was added to the solutions of dif-ferent pHs.The pH of the solution was controlled by0.01M HCl.During the synthesis,temperature of the solution was maintained at70°C by the circulator.The reduction potential of the solution was monitored in situ by measuring the electrode potential(E)with a platinum electrode and a saturated calomel electrode using a potentiostat/galvanostat(VMP2, PRINSTON APPLIED RESEARCH).The pH variation of the solution was also in situ measured with a pH meter(720A,Thermo Electron Cor-poration)and a pH electrode(9107BN,Thermo Electron Corporation). The suspension was?ltered and washed several times using distilled water, and then dried at100°C for12h in preparation for further analysis.

Thermogravimetric analysis(TGA)was performed at a heating rate of 10°C/min in air using a thermal gravimetric analyzer(TGA2050,TA instrument)in order to study the structural transformation of MnO2 and the thermal stability of the CNTs.The crystalline phase of the MnO2on the CNTs was determined by X-ray powder di?raction(XRPD) using a di?ractometer(D/MAX-IIIC,Rigaku)equipped with a vertical goniometer.Di?raction patterns were taken at room temperature in the range of10°<2H<80°at intervals of0.01°.The Raman spectra were measured with a Jobin-Yvon LabRam HR with LN2cooled CCD multi-channel detector at room temperature using conventional backscattering geometry.The laser light source was the emission of the argon-ion laser at wavelength514.5nm.Scanning electron microscopy(SEM)(Sirion,FEI)was used to observe the morphology of the composite powder. UV–VIS(UV-2401PC,SHIMADZU)was used to measure changes in the concentration of MnOà

ions during the synthesis process.

3.Results and discussion

3.1.In situ monitoring of E and pH

Fig.1shows the change in the E and pH of the solution with time during the synthesis of MnO2/CNT nanocom-posites in the200ml aqueous solution of0.1M KMnO4 containing1.0g CNT at70°C.This change in E and pH of the aqueous KMnO4solution containing CNTs is very similar to that of the solution containing acetylene black reported in our previous work[33].

Considering the changes of E and pH during the synthe-sis,Fig.1can be divided into four stages.When CNTs were added to the solution,a rapid drop in the electrode poten-tial and an increase in pH were observed in the?rst stage (stage I).CNTs can be envisaged as roll-up graphene layer sheet,regarding their molecular structure[34].The basic sites,which consist of delocalized p electrons,on the sur-face of the CNTs are responsible for the rise in pH in the ?rst stage since they can act as Lewis bases capable of com-plexing protons to its p structure[35].

C p+2H2O!C p H3Ot+OHàe1T

where C p is the idealized graphized surface having maxi-mum p electrons.

The E of the solution for the reduction of MnOà

ions to MnO2is a function of pH,as shown in the following reac-

tion[36].According to Eq.(2),the MnOà

ion needs both protons and electrons for its reduction to MnO2.

MnOà

t4Htt3eà!MnO2t2H2Oe2T

E?E0t

a MnOà

áa4

a MnO

áa2

H2O

Fig.1.Electrode potential(E)and pH vs.time curves measured during

the reduction of MnOà

ions in the200ml aqueous solution of0.1M KMnO4containing1.0g CNT at70°C.

376S.-B.Ma et al./Carbon45(2007)375–382

where E is the electrode potential,E0the standard poten-tial,R the gas constant,T the absolute temperature,F the Faraday constant,n the moles of electron participating a redox reaction and a x the activity of x species.The sharp decrease in E can be attributed to the sharp rise in pH of the KMnO4solution caused by the CNT addition.

Right after the rapid drop in the E and the increase in pH in the?rst stage,protons,adsorbed at basic sites on the CNT surface during the?rst stage,are expected to be

desorbed as MnOà

4ions were absorbed,thereby producing

a heterogeneous MnO2?lm on the CNTs.Protons des-orbed from the surface of the CNTs contributed to the slight decrease in pH and the slight increase in E seen dur-ing the initial second stage.It should be noted that the pH stayed relatively constant after the initial decrease in the second stage.This is possibly related to the reduction reac-

tion of MnOà

4ions,since this reaction consumes protons

from the CNT surface.

The second stage,which is characterized by a potential plateau with respect to time,represents the reduction of

MnOà

4ions to MnO2by the CNTs.The potential plateau

in the second stage corresponds to the reduction potential

of MnOà

4ions in the aqueous solution containing the

CNTs.At the end of the plateau,i.e.,when all the MnOà

4 ions are reduced,E shifts downward signi?cantly during the third stage(stage III)and is indicative of the complete

reduction of MnOà

4ions.

In the fourth stage(stage IV),a slight decrease in E and small increase in pH is observed.The measured E corre-sponds to the equilibrium in an aqueous solution after the reaction for given electrochemical cells.No further reaction followed.

3.2.UV–VIS analysis

Fig.2shows the changes in the concentration of MnOà

4 ions after the addition of1.0g CNT into0.01M KMnO4 200ml solution at50°C.After the addition of CNT into KMnO4solution,a small amount of the solution contain-

ing MnOà

4ions,MnO2and CNT was sampled at regular

intervals and cooled rapidly to4°C to prevent any further reaction.UV–VIS spectroscopy was used to measure

absorption spectrum for the MnOà

4ions in order to inves-

tigate the change in the concentration of MnOà

4ions in the

sample solutions.For the UV–VIS spectroscopy measure-ments,it should be noted that the reduction reaction of

MnOà

4ions to MnO2by the CNTs was performed at the

low KMnO4concentration of0.01M and the low reaction temperature of50°C to quench the reduction reaction e?ectively in the sampled solution during the preparation step for the UV–VIS spectroscopy.

Following the calibration of UV–VIS spectra with stan-

dard KMnO4solutions,the concentration of MnOà

4ions in

the sampled solutions was calculated and plotted against the reaction time in Fig.2.The change in E of the solution is presented in Fig.2for ease of comparison.The concen-

tration of MnOà

4ions decreased monotonously with the

reaction time.It should be noted that the time for the com-

plete removal of MnOà

ions from the solution coincides

exactly with the time for the negative shift of E in the third

stage at around150min in Fig.2.This con?rms that the E

plateau in the second stage in Fig.2represents the reduc-

tion of MnOà

ions to MnO2by the CNTs.

3.3.E?ect of pH

Fig.3shows the changes of E and pH in200ml solu-

tions of0.1M KMnO4at di?erent initial pHs containing

1.0g CNT at70°C.Before the CNTs were added to the

solution,the solution pH was adjusted by0.01M HCl

and the corresponding initial solution E was0.66,0.96

and1.12for pH7,2.5and1,respectively,as shown in

the inset of Fig.3(a)and(b).The aqueous KMnO4solu-

tions with the di?erent pHs have di?erent E values because

E is a function of the solution pH according to Eq.(3).

After the CNTs were added to the solution,as shown in

the inset of Fig.3(a)and(b),pH increased in all the solu-

tions due to the adsorption of protons on the surface of the

CNTs which caused the rapid drop in E.For the solutions

of initial pH2.5and7,the solution pH instantly jumped to

pH9and8immediately after the addition of CNT,respec-

tively,and then gradually decreased to pH around7over

time.For the solutions of the initial pH1,however,their

pH increased gradually to pH7over time without an immi-

nent sharp jump in pH after the addition of CNT because

of the very high proton concentration in the initial solu-

tion.As expected from the pH change in Fig.3(b)and

Eq.(3),the change in the solution E of the initial pH2.5,

7and1over time can be divided into four stages during

the synthesis of MnO2/CNT nanocomposites.For the solu-

tion of the initial pH1,however,the solution pH increased

without a peak and gradually approached pH around7in

the later stage,while correspondingly,E sharply decreased

immediately after the addition of CNT without a dip in the

?rst stage and approached the reduction potential in the

second stage.Di?erent initial pHs of the solutions

did

Fig.2.Concentration of MnOà

ions and electrode potential(E)vs.time

during the synthesis of MnO2/CNT nanocomposite in the200ml aqueous

solution of0.01M KMnO4containing1.0g CNT at50°C.

S.-B.Ma et al./Carbon45(2007)375–382377

not a?ect the overall change in the solution E and pH with time during the reduction of MnO à4ions to MnO 2by CNT.It should be pointed out that the reduction reaction time between the moment of CNT addition and the time for the negative shift of E in the third stage,i.e.,the time required for the complete removal of MnO à4ions from the solutions,decreased with decreasing pH.This indicates that the reduction reaction rate increases with decreasing solution pH.Since more protons in the solution become available for the given concentration of MnO à4ions with decreasing solution pH,an increase in the reaction rate for the reduc-tion of MnO à4ions to MnO 2on CNT is expected with a decrease in pH of the initial https://www.wendangku.net/doc/0c2029724.html,T as reducing agent

Choi et al.[37]reported the spontaneous reduction of metal ions to metallic form on the sidewalls of the CNTs in aqueous solutions of noble metal ions.They reported that Au and Pt could be deposited spontaneously on the sidewall of the CNTs by a direct redox reaction between the CNTs and metal ions via the immersion of the CNTs in HAuCl 4(Au 3+)and Na 2PtCl 4(Pt 2+)solutions,respec-tively.They explained that the principle for the spontane-ous reduction of the metal ions on the sidewalls of the

CNTs is the di?erence in the reduction potential between the CNTs and the metal ions.Similarly,the spontaneous formation of MnO 2from MnO à4ions on the CNT sidewall may also be explained by the di?erence in the reduction potential between the CNT and MnO à4ions.The work function of the CNTs was determined to be $5eV.The Fermi level of the CNTs is approximately +0.5V above the potential of the standard hydrogen electrode (SHE),which is well above the reduction potential of MnO à4ions,which is +1.692V (vs.SHE).The relative potential levels may explain the spontaneous electron transfer from the CNTs to the MnO à4ions.

MnO 2is known to be thermodynamically the most sta-ble form of manganese species in an aqueous solution con-taining MnO à4ions.MnO à

4ions,therefore,tend to oxidize water with the concurrent evolution of oxygen and sponta-neous precipitation of MnO 2[36].Although spontaneous,the reduction reaction of MnO à4ions by water is very slow kinetically.Two identical solutions of 0.01M KMnO 4,one with CNTs and the other without CNTs,were prepared and allowed to sit at room temperature without further treatment and observed for the formation of MnO 2.A few days later,the solution with the CNTs became clearer from the original deep purple which indicates that MnO à4ions were reduced to MnO 2by the CNTs.However,no vis-ible change in color was observed in the solution without the CNTs.It is clear that the CNTs dispersed in an aque-ous solution containing MnO à4ions act as a reducing agent for MnO à

4ions and promote the reduction process even at room temperature.

Previous approaches to the synthesis of metal and metal oxide nanoparticles on the CNTs have included various methods such as physical evaporation [5],electrodeposition [29],solid state reactions [38],polyol process [39],use of interlinkers [40],spontaneous reduction by CNTs [37]and other chemical routes.In our approach,MnO 2was deposited on the CNTs via simple immersion of the CNTs into an aqueous KMnO 4solution without any addition of chemicals such as reducing agent,surfactant or alcohol that could reduce MnO à4ions to MnO 2.In the present study,the CNTs acted as a reducing agent as well as a sub-strate for the spontaneous deposition of MnO 2in an aque-ous KMnO 4solution.Hence,this process is the simplest method for synthesizing MnO 2on CNTs.It also provides a useful approach for coating other potential metal oxide materials on CNTs using the relative potential di?erence.3.5.Morphology of MnO 2on CNT

Fig.4shows SEM images of the CNTs before and after MnO 2deposition in the 200ml aqueous solution of 0.1M KMnO 4with di?erent initial pHs containing 1.0g CNT at 70°C.Each CNT particle is around 20–30nm in size,as shown in Fig.4(a).After MnO 2deposition,the CNT par-ticles became a little thicker with size increased to approx-imately 30–50nm.This indicates that the CNTs acted as a substrate for the heterogeneous precipitation of a very

thin

Fig.3.(a)E vs.time curves and (b)pH vs.time curves measured during the reduction of MnO à4ions by 1.0g CNT in the 200ml aqueous solution of 0.1M KMnO 4at 70°C under di?erent pH conditions of the initial solution.

378S.-B.Ma et al./Carbon 45(2007)375–382

layer of MnO 2,as shown in Fig.4(b)–(d).The roughness of the MnO 2deposit on the CNTs increased with decreasing pH of the initial solution.MnO 2deposit on the CNTs

formed in the solution of the initial solution pH of 1in Fig.4(d)consisted of nano-bar MnO 2while that from the solution of the initial solution pH of 7in Fig.4(b)was of a thin ?lm form.This suggests that the reduction rate of MnO à4ions to MnO 2by CNT might a?ect the mor-phology of the MnO 2deposit on the CNTs.As shown in Fig.3(a),the time required for the complete removal of MnO à4ions from the solutions decreased with decreasing pH,i.e.,the reduction reaction rate increased with decreas-ing solution pH.Therefore,it was observed that the increased reduction rate of MnO à4ions to MnO 2by CNT with decreasing initial pH of the solution roughened the morphology of MnO 2.

3.6.Phase and structure of MnO 2on CNT

Fig.5shows the XRPD patterns for the CNTs and for MnO 2on the CNTs prepared in the 200ml aqueous solu-tion of 0.1M KMnO 4containing 1.0g CNT at 70°C with di?erent initial pHs adjusted by 0.01M HCl,as well as the XRPD peaks of the reference oxide,birnessite-type MnO 2(JCPDS 42-1317).As shown in Fig.5(b),there are three broad peaks at 2h around 12°,37°and 66°,in addition to those of the CNTs.These three peaks can be indexed to birnessite-type MnO 2including an amorphous phase [41,42].This is consistent with the report by McKenzie [43]on the birnessite-type MnO 2synthesized by the drop-wise addition of hydrochloric acid to a boiling solution of KMnO 4.Previously,we reported the very similar XRPD patterns for the nanocomposites of birnessite-type MnO 2and acetyleneblack [33].However,it is di?cult to analyze the structure of MnO 2in detail using XRPD patterns due to their broad and weak

peaks.

Fig.4.SEM images of (a)pristine CNT and MnO 2/CNT nanocomposite prepared in the 200ml aqueous solution of 0.1M KMnO 4containing 1.0g CNT at 70°C under (b)pH 7;(c)pH 2.5;(d)pH 1of the initial

solution.

Fig.5.XRPD patterns of (a)CNT and (b)MnO 2/CNT nanocomposite prepared in the 200ml aqueous solution of 0.1M KMnO 4containing 1.0g CNT at 70°C,and (c)XRPD peaks of birnessite-type MnO 2(JCPDS No.42-1317).

S.-B.Ma et al./Carbon 45(2007)375–382379

Raman techniques are useful for analyzing the local structure of MnO2,especially for samples with poor crystallinity,to which it is di?cult to apply the Rietveld re?nement of XRPD data.Raman spectra for many kinds of MnO2have been well investigated by Julien et al.[41,42]. Many MnO2materials have similar gross structural fea-tures,namely the MnO6octahedral units building the MnO2frame work.MnO2compound classi?cations are based on the nature of the polymerization of the MnO6 units,in which six oxygen atoms surround a central manga-nese cation in approximately octahedral coordination. Raman shifts are correlated with the M–O bond order and bond lengths.

Fig.6shows the Raman spectra of MnO2/CNT nano-

composites synthesized by the reduction of MnOà

4ions to

MnO2in the200ml aqueous solution of0.1M KMnO4 containing1.0g CNT at70°C corresponding to the di?er-ent initial solution pHs of7,2.5and1.Three major fea-tures for MnO2can be recognized at500,575and 640cmà1.The two high wave number bands are dominant in all of the spectra,while the bands in the low-frequency region appear with a signi?cantly weaker intensity.The Raman band at640cmà1can be recognized as the symmet-ric stretching vibration(Mn–O)of the MnO6groups.The band located at575cmà1is usually attributed to the (Mn–O)stretching vibration in the basal plane of MnO6 sheet[41,42].Fig.6shows that all the reaction products have the Raman spectra of the birnessite-type https://www.wendangku.net/doc/0c2029724.html,-parison of the Raman spectra of MnO2synthesized under various pH conditions in this study indicates that the pH of the initial solution had no in?uence on the phase and the short range ordering of MnO2.

Suib et al.[44]have reported the synthesis of di?erent phases of MnO2by a hydrothermal method,in which ini-tial pH played an important role in determining the crystal-lographic form of the?nal products of MnO2.In this study,the pH of the solutions in the second stage was around7during the reduction of MnOà

ions to MnO2 by CNT,which may explain the synthesis of the birnes-site-type MnO2regardless of the di?erent initial pHs.

3.7.Thermal stability of CNT coated with MnO2

Fig.7shows the TGA and DTA of as-received CNT and MnO2/CNT nanocomposites synthesized by the reduction

of MnOà

ions to MnO2with the CNTs in this study.As shown in Fig.7(b)–(d),the weight loss of MnO2/CNT nanocomposites was observed from25to800°C.From the DTA data of MnO2/CNT nanocomposites,two broad peaks were observed at temperatures near100and340°C. It is believed that the10%weight loss observed under 150°C was mainly due to the liberation of adsorbed and interlayer water[45].This suggests that the MnO2sponta-neously deposited on the CNTs is hydrous and that it is dehydrated when heated.The additional weight loss of 25%between250and400°C corresponds to the loss of oxygen atoms from the octahedral layer framework in rela-tion to the partial reduction of Mn4+to Mn3+[45].

CNTs are oxidized to CO2gas over550°C with a large amount of weight loss as shown in Fig7(a)[46].However, for MnO2coated CNT,weight loss of only2%was observed over400°C,as shown in Fig.7(b)–(d).It can be inferred that a thin,uniform and continuous

coating Fig.6.Raman spectra of MnO2/CNT nanocomposite synthesized by the

reduction of MnOà

4ions to MnO2in the200ml aqueous solution of0.1M

KMnO4containing1.0g CNT at70°C under di?erent pH conditions of the initial

solution.Fig.7.Thermogravimetric analysis(TGA)and di?erential thermal analysis(DTA)of(a)as-received CNT and MnO2/CNT nanocomposite synthesized by the reduction of MnOà

ions to MnO2in the200ml aqueous solution of0.1M KMnO4containing1.0g CNT at70°C under (b)pH7;(c)pH2.5;(d)pH1of the initial solution.

380S.-B.Ma et al./Carbon45(2007)375–382

layer of MnO2might protect the CNTs against oxidation.

A similar protection against oxygen and air oxidation by coating a thin,uniform and continuous carbide?lms has been reported for carbon?ber[47].

4.Conclusion

Birnessite-type MnO2was spontaneously deposited on CNTs via simple immersion of the CNTs into an aqueous KMnO4solution.Through in situ monitoring of the reduc-tion potential and pH of the solution and supplementary UV–VIS analysis,the CNTs were found to act as a reduc-ing agent and substrate for the heterogeneous nucleation of MnO2in an aqueous KMnO4solution.This process is the simplest method for synthesizing MnO2on CNTs for electrochemical capacitor applications.Reduction times were measured from the potential plateau in a plot of the reduction potential vs.reaction time.With decreasing ini-tial solution pH,the reaction time for complete reduction

of MnOà

4ions to MnO2was shortened and the morphol-

ogy of MnO2roughened,while the phase of MnO2on the CNTs was?xed to birnessite.Furthermore,the increased thermal stability of the CNTs indicates that a thin,uniform and continuous layer of MnO2was coated on the CNTs.

Acknowledgments

This work was?nancially supported by the Ministry of Education and Human Resources Development(MOE), the Ministry of Commerce,Industry and Energy(MO-CIE),and the Ministry of Labor(MOLAB)through the fostering project of the Lab of Excellency and Hyundai-Kia Motors,and NGV.

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