1-s2.0-S0378775313011713-main

Metal free nitrogen doped hollow mesoporous graphene-analogous spheres as effective electrocatalyst for oxygen reduction reaction

Jing Yan a ,Hui Meng b ,Fangyan Xie c ,Xiaoli Yuan a ,Wendan Yu a ,Worong Lin a ,Wenpeng Ouyang a ,Dingsheng Yuan a ,*

a

Department of Chemistry,Jinan University,Guangzhou 510632,PR China

b

Department of Physics and Siyuan Laboratory,College of Science and Engineering,Jinan University,Guangzhou 510632,PR China c

Instrumental Analysis &Research Center,Sun Yat-sen University,Guangzhou 510275,PR China

g r a p h i c a l a b s t r a c t

Nitrogen-doped hollow mesoporous carbon sphere (N-doped HMCS)composed by broken graphene was synthesized,the carbon spheres showed effective activity as electrocatalyst for the oxygen reduction reaction (ORR)in alkaline solution with unique methanol-tolerant

property.

a r t i c l e i n f o

Article history:

Received 19April 2013Received in revised form 7June 2013

Accepted 1July 2013

Available online 9July 2013Keywords:

Hollow mesoporous carbon sphere Nitrogen-doped carbon Graphene

Oxygen reduction reaction Fuel cell

a b s t r a c t

Nitrogen-doped hollow mesoporous carbon spheres has been synthesized from mesoporous silica spheres using glycine as carbon and nitrogen precursor.The wall of the spheres is composed by broken graphene.The metal free nitrogen-doped hollow mesoporous carbon spheres are proven to be active electrocatalyst for the oxygen reduction reaction in alkaline solution.A unique advantage of the nitrogen-doped hollow mesoporous carbon sphere is its methanol-tolerant property because of the absence of active metal.The catalytic activity is ascribed to the pyridinic-nitrogen formed during py-rolysis and the graphene-like structure.To the best of our knowledge this is the ?rst report on the nitrogen-doped hollow mesoporous carbon sphere as a metal-free electrocatalyst for the oxygen reduction reaction which is an important reaction in fuel cell.The prepared mesoporous carbon material can also be used as catalyst support and ?nd application both in the anode and cathode of fuel cell.

ó2013Published by Elsevier B.V.

1.Introduction

Fuel cells convert chemical energy directly into electrical cur-rent without combustion and they are attractive power sources for

portable,automotive and stationary applications due to high en-ergy density and ef ?ciency.However the low kinetics of cathodic oxygen reduction reaction (ORR)causes a big loss in the ef ?ciency [1].Pt is a typical catalyst for ORR due to its high activity and sta-bility.However,the high price and limited supply of Pt poses serious problems to the commercialization of fuel cell [2].In addition,Pt-based catalyst is susceptible to time-dependent drift

*Corresponding author.

E-mail addresses:tmh@https://www.sodocs.net/doc/4c1025174.html, (H.Meng),tydsh@https://www.sodocs.net/doc/4c1025174.html, (D.

Yuan).Contents lists available at SciVerse ScienceDirect

Journal of Power Sources

journal ho mep age:www.elsevi https://www.sodocs.net/doc/4c1025174.html,/locate/jpo

wsour

0378-7753/$e see front matter ó2013Published by Elsevier B.V.https://www.sodocs.net/doc/4c1025174.html,/10.1016/j.jpowsour.2013.07.003

Journal of Power Sources 245(2014)772e 778

and the crossover of methanol causes mixed potential in the cathode[3e5].Thus,research efforts in the development of cath-ode catalyst have been focused on decreasing Pt content or replacing Pt with less expensive materials.Noble metal free catalyst for the ORR became a hot topic and lots of progress has been made [4,6].

Recently nitrogen-doped carbon nanostructures(NCNS)were found to be active for ORR[7e13].The origin of the activity was explained by the presence of pyridinic nitrogen[8,9].Various synthesis methods for NCNS have been reported in literature,such as post synthesis modi?cation[14e16],chemical vapor deposition [17,18],and liquid impregnation[19].The structure of the nitrogen-doped carbon materials were usually ordered mesoporous carbon, carbon nanotubes,carbon nanowires,graphene,and so on.These materials exhibited high ORR activity,however,their mass pro-duction was limited by the complexity of the synthesis process.For practical application in fuel cells,it is required to develop cheap material that is possible for mass production.

In this study,nitrogen-doped hollow mesoporous carbon spheres(HMCS)was synthesized via a simple liquid impregnation method using mesoporous silica spheres as template and cheap glycine as carbon and nitrogen precursor.The HMCS was used as metal-free ORR catalyst and exhibited excellent activity with direct four electron process and methanol-tolerant behavior.The cathode ORR reaction is a bottleneck for the mass commercialization of fuel cell because the low kinetics of ORR requires much more catalyst than anode.The activity of HMCS was inferior to the commercial Pt/ C catalyst,there are several strategies to improve the activity which is still going on,but considering the cost and the potential improvement the HMCS was promising candidate as ORR catalyst.

2.Experimental

2.1.Synthesis of submicrometer-sized mesoporous silica spheres (MSS)

In a typical synthesis,3.5ml aqueous ammonia(30wt.%)was added into a solution containing75ml ethanol and10ml ultra-pure water.After stirring for15min,5ml tetraethylorthosilicate (TEOS)(98wt.%)was added at30 C under vigorous stirring,and the mixture was stirred for6h to yield uniform silica spheres.

A mixture containing5ml TEOS and2ml trimethoxy(octadecyl) silane(C18TMS)(90wt.%)was added into the colloidal solution containing silica spheres and further reacted for1h.The?nal material was retrieved by centrifugation and further calcined at 550 C for6h under oxygen atmosphere.

2.2.Synthesis of nitrogen-doped hollow mesoporous carbon sphere (HMCS)

0.1g MSS was added to a mixture of0.5g glycine and50g H2O. The mixture was ultrasonicated at30 C for30min to get pale yellow mixture,which was transferred into a Te?on-lined auto-clave,the autoclave was sealed and kept at180 C overnight in an electric oven.After the autoclave was cooled down to room tem-perature naturally,the pale yellow emulsion was transferred into a beaker and stirred at70 C,until the emulsion became dry and an orange solid was obtained,in this process the internal pores of MSS were completely in?ltrated with carbon source.The as-prepared dry mixtures was heat treated at300 C for20min,and then increased to900 C and maintained for another3h for carbon-ization.This process was under pure N2atmosphere with the heating rate of2 C minà1and the cooling rate of5 C minà1to room temperature.Then the carbon-silica composite was washed in10wt.%HF solution overnight to completely remove the silica template.The?nal product was labeled as HMCS-1.

In parallel experiments glycine was changed into L-lysine or glucose and the products were labeled as HMCS-2and HMCS-3. HMCS-2and HMCS-1were of different nitrogen source and HMCS-3had no nitrogen.

2.3.Physical characterization

The samples were characterized by a MSAL-XD2X-ray diffrac-tometer(XRD,Cu Ka,40kV,20mA,l?1.54056 A).The fourier-transformed infrared spectra(FT-IR)measurements were con-ducted on a Nicolet6700FT-IR spectrometer by using pressed KBr pellets.The morphology was observed via a Philips TECNAI-10 transmission electron microscope(TEM)using an accelerating voltage of100kV and JEM2010high-resolution transmission elec-tron microscopy(HRTEM)operating at200kV.The crystallization degree of HMCS was investigated by Raman spectroscopy,which was recorded in a backscattering con?guration using the514.5nm Artion laser and a Renishaw Raman spectrometer.The X-ray photoelectronic spectroscopy(XPS)was carried out using an ESCALab250spectrometer with Alumina Ka(1486.6eV)source. Nitrogen sorption isotherms of as-prepared materials were studied by a Micromertics TriStar3000analyzer at77K.The plot of speci?c surface area was deduced from the isotherm analysis of adsorption data at the relative pressure(P/P0)of0e1.0and the average pore diameters were collected from the peak value on the pore diameter distribution.

2.4.Electrochemical characterization

The electrochemical measurements were carried out on poten-tiostat(CHI660D,CH Instruments,Inc.)at25C in a conventional three-electrode system.Platinum foil(1.0?1.0cm2)and Ag/AgCl (saturated KCl)were used as counter electrode and reference electrode,respectively.The potential was converted to versus reversal hydrogen electrode(RHE)for parallel comparison with literature values.A rotating disk electrode(RDE)system was used as working electrode,the glassy carbon electrode(5mm diameter) was polished before each experiment with1,0.3and0.05mm alumina power,respectively,and then washed in a mixture of ethanol and water before each experiment.The ORR activity of HMCS was evaluated in0.1M KOH solution and recorded with the potentiostat at a scan rate of10mV sà1.To make KOH solution oxygen or nitrogen saturated,pure O2or pure N2gas was bubbled directly into the solution for at least30min before measurements and was?ushed over the cell solution during the tests.

All the working electrodes were prepared with the same pro-cedure as follows:2mg catalyst was put into1ml ethanol solution containing100m l Na?on(5wt.%).The mixture was sonicated for 30min,and5m l or10m l of the dispersion was deposited on the glassy carbon electrode and dried under ambient conditions.

3.Results and discussion

3.1.Formation mechanism of the HMCS

The HMCS was designed to be hollow carbon spheres according to the preparation procedure shown in Fig.1.Silica spheres were used as template,the surface of which was covered by a layer of mesoporous structure(MSS).Under controlled reaction conditions the mesoporous layer of MSS was?lled with nitrogen and carbon precursor.The precursor was carbonized at high temperature, forming a carbon layer over the silica core.The nitrogen element in the precursor was left in the carbon cover.The carbon layer was

J.Yan et al./Journal of Power Sources245(2014)772e778773

mesoporous with lots of breach,which made the removal of inner silica possible.The ?nal product was hollow mesoporous carbon spheres (HMCS)with breach inside the wall.The hollow structure was favorable because the active sites were expected to be dispersed uniformly on both the inner and outer wall of the carbon spheres,which would make the best use of the active elements and lead to high performance.

3.2.Morphology and crystalline characterization of HMCS

Following the mechanism described above,the HMCS was designed to be hollow carbon spheres.However TEM micrographs revealed different structures for the two different precursors.As shown in Fig.2a and b,when glycine was used as precursor a kind of hollow carbon sphere structure was prepared,while with L -lysine as precursor all the spheres were broken.Considering that all other conditions and procedures were the same,the difference could only be explained by the precursor.In the preparation pro-cedure a key step was the carbonization of the nitrogen precursor ?lled MSS,in this step the macromolecular turned into carbon and nitrogen was left.At the high temperature of carbonization glycine and L -lysine would melt ?rst,the difference of viscosity and the interaction with the carbon surface of the melted precursor determined the dispersion.L -lysine has longer carbon chain than glycine,which makes melted L -lysine less hydrophilic to the surface of silica.The melted glycine remained where it was while the melted L -lysine ?owed down along the wall of the carbon sphere and accumulated at the bottom,leaving the upper side of the MSS no precursor.In the carbonization process carbon was formed only at the bottom,forming the hemi-spheres and broken spheres as shown in Fig.2b.While the melted glycine was ?lled uniformly inside the surface of MSS,after carbonization a perfect carbon layer was formed on the MSS.After the removal of the inner silica the hollow carbon sphere was formed as shown in Fig.2a.As catalyst for electro-catalytic reaction the hollow carbon structure was more favorable because in the hollow sphere the active sites were dispersed uniformly while in the broken spheres some of the active sites were covered inside the wall and not exposed.The predicted performance in the ORR was con ?rmed by following

electrochemical characterizations.Enlarged TEM in Fig.2c gave more details of the hollow carbon sphere HMCS-1,most of the carbon spheres were spherical and uniform with an average par-ticle size of about 180nm and shell thickness of 20nm.No obvious lattice-fringe images could be observed in the HRTEM micrograph of HMCS-1in Fig.2d,which meant the carbon was not graphitized.Fragment of graphene could be observed in Fig.2d,implying the wall of the HMCS was composed by broken graphene.The selected area electron diffraction (SAED)inserted in Fig.2d also proved above conclusion because the appearance of circles of confusion instead of lattice diagram.The X-ray diffraction (XRD)patterns in Fig.S1(see support materials)also support above conclusion.Two broad diffraction peaks appeared at 2q of 24and 44,corresponding to the (002)facet of disordered carbon phase and (101)facet of graphitized carbon [20].The intensity of the graphite peak was negligible compared with the disordered peak,suggesting that the sample was amorphous carbon.

3.3.Speci ?c surface area and pore size of HMCS

Nitrogen adsorption e desorption isotherms and the derived pore size distribution of HMCS-1and MSS were plotted in Fig.3.The prepared material was abundant with mesopores and micro-pores,which was proven by the pseudo-type-I isotherm with H1hysteresis loop at high relative pressure [20].The adsorption of micropores took place at low relative pressure where the adsorp-tion isotherm of the sample became rapidly saturated.The platform at P /P 0?0.20e 0.70originated from the outer surface adsorption of the shell of the sphere.There was a clear increase from a relative pressure of 0.8in the adsorption branch of HMCS-1and a hysteresis loop at P/P 0?0.80e 0.99with a pronounced desorption step,demonstrating macroporous adsorption of the carbon sphere.The macropores could be observed in the HRTEM image.Brunauer e Emmett e Teller (BET)measurements were used to get the speci ?c surface area of HMCS-1and MSS.High speci ?c surface area (451m 2g à1)of HMCS-1was get compared with 335m 2g à1for MSS.The increased speci ?c surface area was contributed by the mesoporous structure of the hollow carbon shell.With the same method the speci ?c surface area of HMCS-2and HMCS-3

were

Fig.1.Schematic illustration of the formation of HMCS.

J.Yan et al./Journal of Power Sources 245(2014)772e 778

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calculated to be469and381m2gà1as shown in Fig.S2.The higher speci?c surface area of HMCS-2was explained by the structure of broken spheres.The pore size distribution plots in Fig.3b.It was observed that uniform pore size distribution centered at around 3nm for HMCS-1and4nm of MSS.The total pore volumes of HMCS-1and MSS were1.16and0.303cm3gà1,respectively.

3.4.Raman characterization of HMCS

Raman spectroscopy is a convenient tool for characterizing car-bon materials.Raman spectroscopy of HMCS was given in Fig.4.Both HMCS-1and HMCS-2exhibited two peaks at around1340and 1590cmà1.The D peak(1340cmà1)corresponds to modes associated with transverse optical phonons around the edge of the Brillouin zone.In the molecular picture,it is associated with the breathing mode of the sp[2]aromatic rings.The D peak is usually very intense in amorphous carbon samples,while it is absent in perfect graphitic samples.The G peak(1580cmà1)corresponds to doubly degenerate E2g mode at the Brillouin zone center.In the molecular picture of carbon materials,the G peak is due to the bond stretching of all pairs of https://www.sodocs.net/doc/4c1025174.html,ually the D peak was assigned to the disordered carbon and the G band was referred to graphite band of carbon.The I D/I G ratio is a factor for judging the degree of graphitization of car-bon.The I D/I G ratios determined from integrated peak area of

HMCS-Fig.3.N2adsorption e desorption isotherms of HMCS-1(a)and pore size distribution

(b).

Fig.2.TEM micrographs of HMCS-1(a)and HMCS-2(b);HRTEM micrographs(c and d)and SAED(inset in d)of HMCS-1.

J.Yan et al./Journal of Power Sources245(2014)772e778775

1and HMCS-2were 1.24and 1.23,which was the same of the bi-layer graphene [21,22].Going back to HRTEM in Fig.2d,interrupted gra-phene layers could be observed in the wall of the hollow carbon spheres.It was concluded that the synthesized carbon material was analogous graphene.Other characterizations such as TEM,SAED and XRD all proved the amorphous nature of the material.This was because the graphene was broken and macroscopically the material did not show the graphitic nature of graphene.3.5.Elemental analysis of HMCS

Since the synthesized carbon material was nitrogen-doped,it was vital to determine the chemical state and content of nitrogen

in the material which would directly in ?uence the electrochemical performance [23,24].X-ray photoelectron spectroscopy (XPS)was employed to identify the surface nitrogen content and the chem-ical states of nitrogen.As shown in Fig.5a was the XPS survey spectrum of HMCS-1and HMCS-2,showing C1s,N1s and O1s peaks.The N1s peak in the survey spectrum proved the existence of the nitrogen element in both HMCS-1and HMCS-2.Detailed analysis of C1s and N1s peaks were shown in Fig.5b and c.The overlapped C1s peaks were decomposed into four components arising from C e C (284.5eV),C e N (285.6eV),C e O (286.3eV)and C ]O (287.2eV).The ?rst peak was usually attributed to the sp 2hybridized carbon.The second was originated from the interaction between carbon and nitrogen,implying the existence of interac-tion between carbon and nitrogen.The peaks centered around 286.3eV and 287.2eV were interpreted as the sp 2and sp 3hy-bridized carbon bonded to sp 2and sp 3hybridized oxygen,respectively [17,25].Four types of nitrogen were recognized from N1s spectrum in Fig.5c which were located at 398.4eV,400.1eV,401.2eV and 403.4eV,corresponding to the pyridinic-N(N-6),pyrrolic-N(N-5),quaternary-N(N e Q)and pyridinic-N-oxide (N-oxide)groups [26,27].

The total nitrogen contents of HMCS-1and HMCS-2were calculated from XPS to be 3.82and 3.05wt%.Detailed analysis from the XPS spectrum in Fig.5c indicated high percentage of pyridinic-N and quaternary-N.The content of the pyridinic-N in HMCS-1was 33.4wt%,which was higher than HMCS-2(25.0wt%).The content of pyrrolic-N in HMCS-1was 12.3wt%,also higher than HMCS-2(7.6wt%).While the content quaternary-N was opposite:41.8wt%for HMCS-1and 59.8wt%for HMCS-2.It was commonly accepted that the ORR activity of nitrogen-doped carbon came from the pyridinic-N and pyrrolic-N [12,28,29].Since HMCS-1had more

8001000120014001600

1800

2000

G

D

I n t e n s i t y / a .u .

Raman shift / cm

-1

HMCS-1 HMCS-2

Fig.4.Raman spectra of HMCS-1and HMCS-2.

Fig.5.XPS survey spectra (a),High-resolution XPS spectra of C1s (b)and N1s (c)of the HMCS-1and HMCS-2.

J.Yan et al./Journal of Power Sources 245(2014)772e 778

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pyridinic-N than HMCS-2,it was predicted to have better perfor-mance in ORR.

3.6.Electrochemical characterization of HMCS

The activities of HMCS for oxygen electroreduction in O 2-satu-rated 0.1M KOH electrolyte were evaluated as shown in https://www.sodocs.net/doc/4c1025174.html,pared with nitrogen-free HMCS-3,HMCS-1and HMCS-2showed obvious activity for ORR.The limit current of HMCS-3only reached 0.44mA and the onset potential of ORR was 0V versus reversal hydrogen electrode (RHE)which was typical per-formance of carbon powder for ORR [30,31].When the carbon material was doped with nitrogen there appeared obvious im-provements in ORR https://www.sodocs.net/doc/4c1025174.html,paring HMCS-1and HMCS-2,the onset potential of HMCS-1was 50mV positive than HMCS-2(0.14V and 0.08V versus RHE for HMCS-1and HMCS-2,respectively),the limit current of HMCS-1reached 0.87mA.The improved perfor-mance of HMCS-1compared with HMCS-2was explained by two factors:more pyridinic-N and the hollow structure made more nitrogen available in the reaction.Fig.6b compared the perfor-mance of HMCS-1,commercial Vulcan XC-72carbon powder and commercial 50%Pt/C catalyst.It was noteworthy to ?nd similar performance of HMCS-1with Pt/C catalyst.Both catalysts had similar limit current and the onset potential of Pt/C catalyst was 60mV positive than HMCS-1which proves the present system still cannot compete with Pt/C.But considering the totally free of Pt in HMCS-1,these values were among the best reported in literature.The performance of Vulcan XC72carbon was similar with that of HMCS-3,which was common for carbon materials without nitro-gen.It was concluded that doping carbon with nitrogen could make

the material active for ORR in alkaline solution,the activity was to some degree determined by the content of the pyridinic-N,and the graphene like structure of the carbon sphere would also contribute to the high performance.

There were two possible mechanism of ORR on different cata-lysts:the two electron pathway and the direct four electron pathway.The four electron pathway was favorable in fuel cells because it offered higher kinetics of ORR.It was recognized that ORR took place via the four electron pathway on Pt/C catalyst.The number of electrons transferred on the HMCS-1catalyst was calculated according to the Kouteckye Levich equation as shown in Fig.7[32,33].

1=j lim ?1=j lev t1=j k ?1=

B u 1=2 t1=j k (1)

B ?0:62nF

C O e

D O T2=3n à1=6(2)j k ?nFkC O

(3)

where the j lim (mA cm à2)is the measured current density,which is related to Levich current (j lev )and kinetic current (j k ).F is the Faraday constant (96486.4C mol à1),D O is the diffusion coef ?cient of oxygen in 0.1M KOH (1.9?10à5cm 2s à1),n is the kinematic viscosity of the water (0.01cm 2s à1),C O is the bulk concentration of oxygen in air-saturated 0.1M KOH (1.21?10à6mol cm à3).And u is the rotation rate of RDE,n is the electron transfer numbers of

ORR.The linear plot of j lim à1

versus u

à1/2has a slope of 1/0.62nFCD 2/3n à1/6

(Fig.7b).The constant 0.62is adopted when the rotation speed is expressed in rpm.On the basis of the line

slopes,

Fig.6.ORR polarization curves of HMCS in O 2saturated 0.1M KOH solution with RDE,sweep rate:10mV s à1,rotation speed:1600

rpm.

Fig.7.(a)ORR polarization curves of HMCS-1at different rotating speeds in 0.1M KOH solution saturated with O 2.Scan rate:10mV s à1.(b)Kouteckye Levich plot of j lim à1

vs.u à1/2obtained at à0.3,à0.4,à0.5,à0.6and à0.7V.

J.Yan et al./Journal of Power Sources 245(2014)772e 778777

a n value of 3.9was obtained in the potential range of à0.3to à0.7V.The results indicated that the ORR on HMCS-1was a four electron reaction,which was the same with Pt/C catalyst,proving HMCS-1a promising metal-free ef ?cient ORR catalyst in alkaline solution.

The crossover of methanol from anode to cathode in direct methanol fuel cells was one of the challenges to overcome in direct methanol fuel cells.The usually used cathode catalyst,such as the Pt group metal,was also active in the oxidation of methanol,thus causing mixed cathode potential and greatly reduced the ef ?-ciency of fuel cell.So the cathode catalyst was required to have no activity in methanol oxidation while active in ORR.The Pt group catalyst could not meet this requirement.As shown in Fig.8b,the addition of methanol into the electrolyte caused obvious mixed reaction.While methanol had no in ?uence on the ORR of HMCS-1,there was almost no change in the polarization curves with and without methanol in Fig.8a.The methanol-tolerant property of HMCS-1showed its potential application in direct methanol fuel cells.4.Conclusions

A kind of metal free nitrogen-doped carbon material (HMCS-1)was synthesized via a liquid impregnation method,using glycine as carbon and nitrogen source.The prepared material was composed by hollow spheres of high mesoporosity and large pore volume.HMCS-1displayed comparable although inferior ORR activity with commercial Pt/C catalyst and excellent methanol tolerant ability.The ORR activity of the nitrogen-doped carbon was proved to originate from the pyridinic-N.The content and the availability of pyridinic-N and the graphene-like structure of the carbon material were vital factors determining the ORR activity.Subsequent work is the application of HMCS-1in a membrane electrode assembly (MEA)with alkaline membrane and full cell test,which is underway.

Acknowledgments

This work was supported by National Natural Science Founda-tion of China (21031001and 21106190),Program of the Pearl River Young Talents of Science and Technology in Guangzhou,China (2013055),Scienti ?c Research Foundation for the Returned Over-seas Chinese Scholars,State Education Ministry and Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang Univer-sity),Ministry of Education.

Appendix A.Supplementary data

Supplementary data related to this article can be found at https://www.sodocs.net/doc/4c1025174.html,/10.1016/j.jpowsour.2013.07.003.References

[1] D.Su,G.Sun,Angew.Chem.Int.Ed.50(2011)11570.[2] C.Yang,Energy Policy 37(2009)1805.

[3]K.Gong,F.Du,Z.Xia,M.Durstock,L.Dai,Science 323(2009)760.[4]M.Lefevre,E.Proietti,F.Jaouen,J.P.Dodelet,Science 324(2009)71.[5]G.Wu,K.L.More,C.M.Johnston,P.Zelenay,Science 332(2011)443.

[6] E.Proietti,F.Jaouen,M.Lefèvre,https://www.sodocs.net/doc/4c1025174.html,rouche,J.Tian,J.Herranz,J.P.Dodelet,https://www.sodocs.net/doc/4c1025174.html,mun.2(2011)416.

[7]R.Bashyam,P.Zelenay,Nature 443(2006)63.

[8]J.Ozaki,S.Tanifuji,A.Furuichi,K.Yabutsuka,Electrochim.Acta 55(2010)1864.[9]V.Nallathambi,J.W.Lee,S.P.Kumaraguru,G.Wu,B.N.Popov,J.Power Sources 183(2008)34.

[10]G.Liu,X.Li,B.N.Popov,ECS Trans.25(2009)1251.

[11]S.M.Lyth,Y.Nabae,S.Moriya,S.Kuroki,M.Kakimoto,J.Ozaki,S.Miyata,J.Phys.Chem.C 113(2009)20148.

[12]J.I.Ozaki,S.I.Tanifuji,N.Kimura,A.Furuichi,A.Oya,Carbon 44(2006)1324.[13] A.Janosevic,I.Pasti,N.Gavrilov,S.Mentus,G.Ciric-Marjanovic,J.Krstic,J.Stejskal,Synth.Met.161(2011)2179.

[14]J.Li,M.J.Vergne,E.D.Mowles,W.Zhong,D.M.Hercules,C.M.Lukehart,Carbon 43(2005)2883.

[15] E.N.Konyushenko,J.Stejskal,M.Trchová,J.Hradil,J.Kovárová,J.Prokes,M.Cieslar,J.Hwang,K.Chen,I.Sapurina,Polymer 47(2006)5715.

[16]T.Ramanathan,F.T.Fisher,R.S.Ruoff,L.C.Brinson,Chem.Mater.17(2005)1290.[17]L.Qu,Y.Liu,J.-B.Baek,L.Dai,ACS Nano 4(2010)1321.

[18]K.S.Kim,Y.Zhao,H.Jang,S.Y.Lee,J.M.Kim,K.S.Kim,J.H.Ahn,P.Kim,J.Y.Choi,B.H.Hong,Nature 457(2009)706.

[19]J.Zeng,C.Francia,C.Gerbaldi,M.A.Dumitresca,S.Specchia,P.J.Spinelli,Solid State Electrochem.16(2012)3087.

[20] D.Yuan,T.Zhou,S.Zhou,W.Zou,S.Mo,N.Xia,https://www.sodocs.net/doc/4c1025174.html,mun.13(2011)242.

[21] C.S.Lee,L.Baraton,Z.He,J.L.Maurice,M.Chaigneau,D.Pribat,C.S.Cojocaru,Proc.SPIE 7761(2010)77610.

[22] A.Jorio,M.M.Lucchese, F.Stavale, E.H.M.Ferreira,M.V.O.Moutinho,R.B.Capaz,C.A.Achete,J.Phys.Condens.Matter 22(2010)334204.[23]S.Maldonado,K.J.Stevenson,J.Phys.Chem.B 108(2004)11375.

[24]Y.Tang,B.L.Allen,D.R.Kauffman,A.Star,J.Am.Chem.Soc.131(2009)13200.[25]N.Gavrilov,I.A.Pasti,M.Mitric,J.Travas-Sejdic,G.Ciric-Marjanovic,S.V.Mentus,J.Power Sources 220(2012)306.

[26]S.Chen,J.Bi,Y.Zhao,L.Yang,C.Zhang,Y.Ma,Q.Wu,X.Wang,Z.Hu,Adv.Mater.24(2012)5593.

[27]T.Shari ?,G.Hu,X.Jia,T.Wagberg,ACS Nano 10(2012)8904.

[28]T.C.Nagaiah,S.Kundu,M.Bron,M.Muhler,W.Schuhmann,https://www.sodocs.net/doc/4c1025174.html,mun.12(2010)338.

[29]N.Gavrilov,M.Vujkovic,I.A.Pasti,G.Ciric-Marjanovic,S.V.Mentus,Electro-chim.Acta 56(2011)9197.

[30]S.Yang,K.Chang,Y.Huang,Y.F.Lee,H.W.Tien,S.Li,Y.H.Lee,C.Liu,C.Ma,C.Hu,https://www.sodocs.net/doc/4c1025174.html,mun.14(2012)39.

[31] D.S.Yu,E.Nagelli,F.Du,L.M.Dai,J.Phys.Chem.Lett.1(2010)2165.

[32]I.Y.Jeon,D.Yu,S.Y.Bae,H.J.Choi,D.W.Chang,L.Dai,J.-B.Baek,Chem.Mater.23(2011)3987.

[33]

H.Meng,P.K.Shen,https://www.sodocs.net/doc/4c1025174.html,mun.8(2006)588

.

Fig.8.Steady state polarization curves of ORR in O 2saturated 0.1M KOH with different concentration of methanol on HMCS-1(a)and commercial 50%Pt/C catalyst (b).

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