C A R B O N 5 1 ( 2 0 1 3 ) 2 7 4 – 2 8 1

.sc iencedi rect .com

Avai lab le at www

journal homepage: www.elsev ier .com/ locate /carbon

The role of nitrogen in a carbon support on the increasedactivity and stability of a Pt catalyst in electrochemicalhydrogen oxidation

Ganghong Bae, Duck Hyun Youn, Suenghoon Han, Jae Sung Lee *

Department of Chemical Engineering, Division of Advanced Nuclear Engineering, Pohang University of Science and Technology (POSTECH),

San 31, Hyoja-dong, Pohang 790-784, Republic of Korea

A R T I C L E I N F O

Article history:

Received 10 June 2012

Accepted 21 August 2012

Available online 31 August 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.08.054

* Corresponding author: Fax: +82 542795528.E-mail address: jlee@postech.ac.kr (J.S. Le

A B S T R A C T

Nitrogen-containing carbon materials were prepared by acetonitrile pyrolysis on carbon

black and used as a support for a Pt catalyst. The Pt particles on N-containing carbon exhib-

ited increased activity and stability in electrochemical hydrogen oxidation relative to Pt on

pristine carbon black. The N-doped carbon had a graphitic structure and contained pyrid-

inic and quaternary nitrogen species. The Pt nanoparticles were better-dispersed because

of increased hydrophilicity induced by the nitrogen species. The Pt/N-containing carbon

showed higher stability in a potential cycling test than Pt/C, because of an increased

metal-support interaction. Using XPS and EELS mapping, we demonstrated that the

metal-support interaction became stronger and more specific by adding nitrogen into

carbon.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Fuel cells are electrochemical conversion devices that convert

chemical energy into electrical energy through electrochemi-

cal reactions. Among them, proton exchange membrane fuel

cell (PEMFC) is an attractive energy device because of high

efficiency, low emission and rapid start-up [1,2]. It finds appli-

cations in various areas including transportation, residential

power generation and portable systems.

In order to convert H2 and O2 electrochemically into water

and electricity, highly dispersed Pt metal particles supported

on carbon black are usually utilized as electrocatalysts for

both cathode and anode sides of PEMFC. However, insufficient

catalytic activity and durability delay the large scale, practical

applications of PEMFC [3,4]. To obtain enhanced electrocata-

lytic activity, Pt metals should be well dispersed on the sup-

port material to form small nano-size particles. Carbon

black (e.g. Vulcan XC-72R) has been used widely as a catalyst

support for Pt in PEMFC, because it has a high electron con-

er Ltd. All rights reservede).

ductivity, corrosion resistance, large surface areas and low

cost [5,6]. However, carbon black is not necessarily an ideal

support because it has several drawbacks. In particular, car-

bon is oxidized electrochemically to surface oxides, and even-

tually to CO2 under PEMFC reaction conditions [7]. The

corrosion of carbon could decrease the electronic continuity

between carbon and Pt, and make isolated Pt particles that

could not participate in the electrochemical reactions. This

would cause the performance of PEMFC decreases continually

[8]. Hence, the durability is the most serious concern of the

current carbon black support for Pt catalysts.

Recently, nitrogen-containing carbons have received much

attention by increasing catalytic activity and durability in

PEMFC reactions, when Pt is loaded onto them [9–11]. But

the origin of these interesting nitrogen effects has not been

well established. In the present work, we prepared N-contain-

ing carbon by acetonitrile pyrolysis on the carbon black and

verified the nitrogen effects in increased activity and

improved durability of Pt/C in PEMFC conditions. Through

.

C A R B O N 5 1 ( 2 0 1 3 ) 2 7 4 – 2 8 1 275

various characterization methods and electrochemical tests,

we attempted to elaborate experimentally on the origin of

the improved performance by nitrogen and find direct evi-

dence relating catalyst activity/durability and nitrogen.

2. Experimental

2.1. Preparation of N-containing carbon and Pt loading

N-containing carbon (N–C) was prepared by using acetonitrile

as a nitrogen source [12]. First, 150 mg of commercial carbon

black (Vulcan XC-72R) was put into an alumina boat and

placed in a tube furnace. The temperature was then ramped

at a rate of 10 �C/min to the final treatment temperature in

N2 flow (100 cc(STP)/min). When the temperature reached

900 �C, Ar carrier gas (100 cc(STP)/min) flew through a bubbler

containing an acetonitrile solution into the tube side of the

furnace. Treatment time was varied (acetonitrile pyrolysis

for 1 h denoted as N–C-1, and for 2 h denoted as N–C-2). After

the pyrolysis was complete at 900 �C, the carrier gas was

changed from Ar to N2 to cool down.

Supported Pt nanoparticles on carbon were prepared by

the conventional sodium borohydride reduction method with

a constant Pt loading of 20 wt% [13]. Commercial carbon black

(Vulcan XC-72R) and acetonitrile-pyrolyzed carbon blacks

were dispersed in distilled water for 1 h with stirring. Then

H2PtCl6 (Aldrich) was added into the solution. After mixing,

NaBH4 solution was poured into the solution while stirring

and after stirring for 1 h, the mixtures were filtered, washed

with water and ethanol, and then dried at 100 �C.

2.2. Electrochemical tests

The catalyst slurry for electrochemical tests was prepared by

mixing catalysts, distilled water and 5 wt% Nafion. The 1 ml

of distilled water and 10 ll of Nafion solution per 20 mg cata-

lysts were mixed, and then sonicated until the ink became

homogeneous. Then 5 ll of catalyst slurry was loaded on

the glassy carbon electrode, and dried. Then 5 ll of Nafion

was dropped on the glassy electrode and dried again.

All electrochemical tests were carried out using a three

electrode cell with a Pt wire and Ag/AgCl electrode serving

as counter and reference electrodes, respectively. All poten-

tials are given relative to reversible hydrogen electrode

(RHE). Cyclic voltammetry was performed in a range of 0–

1.2 V at a scan rate of 50 mV/s. To analyze the stability of cat-

alysts, potentiostatic tests were performed by applying differ-

ent potentials to the electrode (1.2, 1.3 and 1.4 V) at room

temperature for 12 h. In addition, an accelerated durability

test was performed by applying linear potential from 0 to

1.3 V repeatedly in N2-purged 1 M H2SO4.

2.3. Catalyst characterization

X-ray photoelectron spectroscopy (XPS) was used to analyze

the functional groups of nitrogen in N-containing carbon.

The morphology of the synthesized catalysts was studied by

Cs-corrected high-resolution scanning transmission electron

microscopy (HRTEM, JEOL, JEM-2100FS). The crystalline struc-

ture was analyzed by using powder X-ray diffraction (PANa-

lytical, PW 3040/60 X’pert, Cu Ka radiation). Contact angle

measurements (FACE contact angle meter, Kyowa Kaimen Ka-

gaku Co.) were conducted using a carbon paper. Carbon black

or acetonitrile-pyrolyzed carbon black was mixed with a 2-

propanol solution, and sprayed on a 2.5 · 2.5 cm carbon pa-

per, and hot-pressed at 130 �C for 3 min under 2 ton pressure.

The total carbon loading was 2 mg/cm2, and measurements

were taken at six different points on the carbon paper.

3. Results and discussion

3.1. Physical properties of N-containing carbon

As mentioned, N-containing carbon was prepared by using

acetonitrile as a nitrogen source and pyrolysis at high temper-

atures up to 900 oC [12]. XPS was employed to determine the

state of nitrogen in carbon as shown in Fig. 1 and the quanti-

tative results are summarized in Table 1. While acetonitrile

was pyrolyzed on the carbon black surface, the nitrogen

atoms could be incorporated into the graphite layers to re-

place carbon atoms. The surface state of nitrogen in carbon

observed by XPS is usually classified into four groups, i.e.

pyridinic (398.6 eV ± 0.3 eV), pyrrolic (400.5 eV ± 0.3 eV), qua-

ternary (401.3 eV ± 0.3 eV) and pyridinic N+–O� (402–

405 eV ± 0.3 eV) [12,14–16]. In the present case, four peaks

were dominant. The first peak is pyridinic nitrogen, the sec-

ond is quaternary (nitrogen substituted in graphitic carbon),

and third and fourth are pyridinic N+–O� bonded to oxygen

species. It has been reported that these nitrogen species are

the most stable in carbon at high temperatures (>600 �C)

[17]. Thus, XPS identified that pyridinic and quaternary nitro-

gen species were formed in our N-containing carbon. This

represents the evidence of successful substitutional nitrogen

doping into carbon lattice. The prolonged pyrolysis (N–C-2)

more than doubled the nitrogen level, and population of the

quaternary form of nitrogen increased mainly at the expense

of oxidized pyridinic species, indicating that the graphitic or-

der of carbon increased further.

The morphological change could be clearly seen in the

HRTEM images in Fig. 2, which were taken after Pt loading.

The acetonitrile-pyrolyzed carbon blacks had more compact

and layered graphitic structures compared to pristine carbon

black. This kind of change was also reported for the CNx films

and N-doped multiwalled CNT grown by chemical vapor

deposition [18,19]. It was proposed that N-incorporation could

lead to the more graphite-like structure formation in carbon

structure [20]. The electron spin resonance (ESR) and Raman

investigation also indicated that the CNx films became more

graphitic after N-doping [21]. Our TEM images are consistent

with the previous results, indicating that the N atoms have

been successfully incorporated into the carbon structures by

replacing the carbon atoms located at the edges and inside

of the graphitic carbon layers.

In the XRD patterns of these samples, the N-containing

carbons showed positive shifts of the C (002) peak compared

with the carbon black. This indicates a decrease of the d-spac-

ing of the (002) crystal plane in the graphitic structure of the

N-doped carbons. The peak also became sharper, especially in

Fig. 1 – N 1 s spectra of N-containing carbon black, (a) N–C-1 and (b) N–C-2. Inset shows the pyridinic and quaternary forms of

nitrogen.

(a) (b) (c)

Fig. 2 – TEM images of (a) pristine carbon black, (b) N–C-1 and (c) N–C-2. Denser phases represent Pt particles.

Table 1 – Surface concentration of N (atomic%) and the relative concentration of each N speciesa obtained by deconvolution ofN 1 s XPS spectra.

Total N (at.%) Quaternary-N (%) Pyridinic-N (%) Pyridinic N+–O� (%)

N–C-1 2.7 44 23 26.5N–C-2 6.6 60.4 20.7 18.8

a The balance from 100% is the contribution of the other nitrogen species.

276 C A R B O N 5 1 ( 2 0 1 3 ) 2 7 4 – 2 8 1

the case of the N–C-2, indicating that the N-containing carbon

had more graphitic structure. As discussed later, this en-

Fig. 3 – XRD patterns of (a) pristine carbon black, (b) N–C-1

and (c) N–C-2.

hanced crystallinity of N-containing carbon could give it a

better resistance to electrochemical oxidation (carbon corro-

sion) than amorphous carbon black (See Fig. 3).

3.2. Dispersion of Pt particles on the surface of N-containing carbon black

TEM images of Pt catalyst particles loaded on the pristine and

N-containing carbon are shown in Fig. 4. The TEM images

show that the Pt nanoparticles supported on the pristine car-

bon black are more agglomerated on its surface, whereas Pt

particles are distributed more uniformly over the whole N-

containing carbon supports. Also, Pt nanoparticles supported

on the N-containing carbons were smaller than those on pris-

tine carbon black. Thus, the average Pt particle sizes deter-

mined from randomly selected 50 particles in the images

were 4.51, 4.15 and 4.23 nm on pristine carbon black, N–C-1

and N–C-2, respectively. Thus the difference in particle sizes

Fig. 4 – TEM images of (a) Pt/C, (b) Pt/N–C-1 and (c) Pt/N–C-2.

0.0 0.2 0.4 0.6 0.8 1.0 1.2-5

-4

-3

-2

-1

0

1

2

3

4

Cur

rent

den

sity

(mA

/cm

2 )

Potential V (vs RHE)

Pt/C Pt/N-C-1 Pt/N-C-2

Fig. 6 – Cyclic voltammograms of Pt/C, Pt/N–C-1 and Pt/N–C-

2 in N2-purged 1 M H2SO4.

C A R B O N 5 1 ( 2 0 1 3 ) 2 7 4 – 2 8 1 277

is not that great, but more significant is their dispersion.

Thus, Pt particles on pristine carbon black tend to agglomer-

ate each other locally forming large clusters, and some sur-

faces are free of Pt particle coverage. In contrast, Pt

nanoparticles tend to disperse more homogeneously over

the entire surface of N-containing carbon support without

agglomeration. The results indicate that nitrogen doping

can affect particle size as well as particle distribution during

the reduction of H2PtCl6 by borohydride.

As an origin of the higher dispersion of Pt particles on N-

containing carbon supports, it has been reported that N-func-

tional groups increase the hydrophilicity and wettability of

the carbon supports, which makes easy the access of solvated

and charged ions onto the N-containing carbon surface

[10,22–24]. During the metal loading process, the Pt precursor

is dissolved in water. Hence, if the surface of the N-containing

carbon is more hydrophilic than that of the carbon black, the

solvated Pt precursor would be easier to access to the whole

surface of the support. Therefore, we measured the contact

angles of pristine and N-containing carbon black supports

to check the hydrophilicity. As shown in Fig. 5, the contact an-

gles were about 50� and 18� for carbon black and N–C-2,

respectively. During the measurements, water droplets are

gradually disappeared by wetting the surface. In case of the

N-containing carbon, the wetting time was much faster than

the carbon black, another indication that N-containing car-

bon was more hydrophilic than pristine carbon black. In addi-

tion, there is a report that incorporated N-dopant promotes

the nucleation of metal nanoparticles [25]. Since we used

water as a solvent and H2PtCl6 as a Pt precursor during the

loading process, the increased hydrophilicity and improved

nucleation ability of N-containing carbon black would make

(a)

Fig. 5 – Measurements of water contact angle

Pt better dispersed on the support as observed in Fig. 4. Thus,

N-containing carbon should be a better support than pristine

carbon black to obtain higher dispersion of Pt particles.

3.3. Electrochemical activity and stability of Pt/C and Pt/N–C

The cyclic voltammograms (CVs) of Pt/C and Pt/N–C were ob-

tained in N2-purged 1 M H2SO4 solution. The electrochemi-

cally active surface area (EAS) was calculated as a measure

of electrochemical hydrogen oxidation activity by integrating

the total charge corresponding to the hydrogen desorption

(b)

for, (a) pristine carbon black and (b) N–C-2.

Table 2 – Electrochemical activity of Pt/C and Pt/N–C.

Electrocatalyst EASa/m2g�1

Pt/C 44.8Pt/N–C-1 63.0Pt/N–C-2 68.9

a The electrochemically active surface area was calculated by QH/

[Pt] · CML, where QH = the charge exchanged during the electro-

adsorption of hydrogen on Pt (mC cm�2), [Pt] = the Pt loading

(mg cm�2), and CML = the charge needed for the oxidation of single

layer of hydrogen on smooth Pt surface (0.21 mC cm�2).

Table 3 – Accelerated durability tests of Pt/C and Pt/N–C-2.

EASa/m2g�1 Remaining

Electrocatalyst Before After 500 cyclesfrom 0 to 1.3 V

activity (%)

Pt/C 49.2 31.3 63.6Pt/N–C-2 65.8 52.1 79.2

a EAS was calculated by QH/[Pt] · CML as in Table 2.

278 C A R B O N 5 1 ( 2 0 1 3 ) 2 7 4 – 2 8 1

peak [26]. Fig. 6 and Table 2 show that EAS values of the two

Pt/N–C catalysts are higher than that of Pt/C by factors of

1.40 and 1.56. The Pt/N–C-2 catalyst was slightly more active

than Pt/N–C-1. Thus, Pt/N–C catalysts are more active toward

electrochemical hydrogen oxidation reaction compared to Pt/

C. This result is believed to originate from the fact that Pt on

N-doped carbon is better-dispersed than Pt on undoped car-

bon black. As discussed above, the better dispersion may be

related with the improved hydrophilicity or wettability of

the N-containing carbon, which spreads the Pt precursor

solution uniformly all over the surface of the support. An ef-

fect that might have contributed to the improved activity is

the increased conductivity by N-doping. There have been sev-

eral reports that the electric conductivity of graphene and

Fig. 7 – Cyclic voltammograms of, (a) Pt/C and (b) Pt/N–C-2 after o

Fig. 8 – Cyclic voltammograms of, (a) Pt/C and (b) Pt/N–C-2 dur

CNT increases by N-doping [27–29]. Likewise, it is believed

that the electric conductivity of N-doped carbon black has

also increased.

The electrochemical stability of Pt was determined by

potentiostatic hold and potential cycling tests for Pt/C and

Pt/N–C-2. First, CVs were compared before and after the 1.2,

1.3 and 1.4 V hold for 12 h. In Fig. 7, CV did not change signif-

icantly after oxidation at 1.2 and 1.3 V for both Pt/C and Pt/N–

C-2. But at 1.4 V, both catalysts showed marked changes in CV,

indicating a large decrease in EAS values. Thus, we were not

able to differentiate the stability between the two samples

under this test conditions. Through the hydroquinone/qui-

none redox peaks of around 0.6 V of carbon black [7], however,

we could just deduce that pristine carbon black is oxidized

more easily than N-containing carbon. The better corrosion

resistance of N-containing carbons may originate from their

xidation at 1.2, 1.3 and 1.4 V for 12 h in N2-purged 1 M H2SO4.

ing the repeated potential cycles in N2-purged 1 M H2SO4.

Fig. 9 – XPS spectra of Pt 4f in Pt/C and Pt/N–C-2.

C A R B O N 5 1 ( 2 0 1 3 ) 2 7 4 – 2 8 1 279

higher degrees of graphitic order than pristine carbon black as

we discussed in relation to Fig. 2. Next, an accelerated dura-

bility test was performed by repeating CV for 500 cycles from

0 to 1.3 V. As shown in Fig. 8 and Table 3, deactivation of Pt/N–

Fig. 10 – EELS mapping images for, (a) C element (dark spots rep

image (d) is a combined image of (a), (b) and (c) superimposed o

nanoparticles are near N sites.

C-2 was less serious than Pt/C. Thus, ca. 80% of the activity

was retained for Pt/N–C-2, whereas only ca. 64% of the initial

activity remained for Pt/C after the potential cycling. This re-

sult indicates that Pt on the N–C-2 is more stable than Pt on

pristine carbon black during the repeated potential cycles.

3.4. The interaction between Pt particles and N-containingcarbon support

To find out why Pt loaded on N-doped carbon is more stable

than Pt on undoped carbon black, XPS analysis was used to

probe the electronic states of Pt on two types of carbon sup-

ports. In Fig. 9, the intense two XPS peaks represent Pt 4f7/2

and 4f5/2 electrons. The binding energies of Pt 4f peaks for

Pt on the N-containing carbon were 71.4 eV (4f7/2) and

74.8 eV (4f5/2), which were shifted from 70.9 eV and 74.3 eV

of Pt on pristine carbon black. These binding energies are typ-

ical values for zero-valent Pt, indicating that metallic Pt has

been deposited. In addition, the positive shift of 0.5 eV upon

N-doping into carbon indicates that Pt becomes electron-defi-

cient on N-containing carbon support. Smaller Pt particles

can show a higher binding energy, and the average sizes of

Pt particles were 4.51 and 4.23 nm on pristine carbon black

and N–C-2, respectively. This small difference in particle sizes

resent Pt), (b) N element, and (c) Pt element of Pt/N–C-2. The

n a single frame. Blue circles indicate the regions where Pt

280 C A R B O N 5 1 ( 2 0 1 3 ) 2 7 4 – 2 8 1

cannot account for the substantial binding energy shift of

0.5 eV. Thus, we believe that this shift originates from an elec-

tron transfer from Pt to the N–C-2 support, which induces a

stronger metal-support interaction than with the pristine car-

bon black support. Because of their stronger electronegativity

than carbon, nitrogen atoms doped in carbon matrix can at-

tract electrons of the neighboring carbon atoms and Pt metal

deposited on them, making both carbon and Pt electron-

deficient.

In line with what we observed here, there were several pre-

vious theoretical and experimental investigations, which re-

ported that incorporation of N into carbon improved the

durability of the resultant Pt/C catalysts [11,22,27–31]. This

may be due to a particular interaction between nitrogen spe-

cies and Pt particles on the carbon surface as XPS results sug-

gested. In an attempt to directly observe this interaction, we

first obtained a TEM image of Pt/N–C-2 and electron energy

loss spectroscopy (EELS) mapping was conducted for this im-

age in Fig. 10. After identifying the distribution of C, N and Pt

elements, we combined the distributions of these three ele-

ments by superimposing them in a single frame in Fig. 10d.

As noted by circles, we could observe that Pt nanoparticles

(red) are clustering around N sites (green) of N-containing car-

bon, rather than showing a random distribution on carbon

(white). This could be taken as a direct evidence of increased

interaction between Pt particles and the surface of N-contain-

ing carbon black.

Then how can N-doping bring Pt precursors near N-sites.

As discussed, the enhanced hydrophilicity could spread the

precursor solution better over the entire surface of the sup-

port rendering better Pt particles dispersion, but it cannot ac-

count for the selective Pt clustering around the N-sites. There

are several suggested mechanisms of metal loading on the

carbon support. Rodriguez–Reinoso [32] summarized the

interactions taking place between the carbon surface and

the metal cation or anion. Maximum catalyst dispersion

and resistance to sintering can be obtained by several origins,

but most relevant to our case is following: Increasing the ba-

sic Cp sites on the basal plane surface of oxygen-free carbon

will maximize the electrostatic attraction with the metal an-

ion (e.g. Cp–H3O+–PtCl62�) and also will minimize the electro-

static repulsion (e.g. COO�PtCl62�), thus increasing the Pt

dispersion as well as their interaction.

Nitrogen has one more electron compared to carbon. If N

incorporates into the structure of carbon surface, the carbon

surface will become more basic by the excess electron of N.

Most of the N functional groups formed during the doping

process are pyridinic and quaternary nitrogen species, and

the graphitic order of that carbon increased than carbon

black. Therefore, p electron delocalization will occur easily

because of their structure, and the electron donor effect of

N can also make the p bonding stronger. Therefore, the in-

creased basicity of N-doped carbon surface will attract the

metal anion strongly through the mechanism discussed

above (Cp–H3O+–PtCl62�). This interaction will make Pt nano-

particles located around N sites. This is consistent with XPS

and EELS mapping results discussed above and appears to

be the origin of improved durability of Pt loaded on N-con-

taining carbon. This interaction could also improve the dis-

persion of Pt particles more uniformly over the support

surface and increase activity. Thus we have confirmed that

one of the most important effects of N-doping is the stronger

metal-support interaction.

4. Conclusions

By acetonitrile pyrolysis on carbon black, we successfully

incorporated nitrogen into the carbon lattice, forming graph-

ite-like structures with pyridinic and quaternary nitrogen

species. Decreased contact angle indicated that N-doping in-

creased the hydrophilicity of carbon black surface, which

could facilitate the access of solvated and charged precursor

ions onto N-containing carbon surface. Thus, Pt nanoparticles

were better dispersed on the N-containing carbon black sur-

face compared to pristine carbon black, and showed higher

activity toward electrochemical hydrogen oxidation reaction.

The Pt particles on N-doped carbon were also more stable

than Pt on pristine carbon black. These results appear to

come from the increased metal-support interaction caused

by the N-doping. Through XPS, we found that the binding en-

ergy of Pt on the N-containing carbon was larger than the Pt

on the carbon black because of the charge transfer from Pt

to N-doped carbon support. We directly observed the strong

and specific interaction by EELS mapping images, which

showed that Pt metals tended to cluster around nitrogen

sites. This selective interaction was explained by increased

p electron delocalization of N-doped carbon that could make

carbon black surface more basic, which attract the metal an-

ion near N-sites of the surface. Thus, the surface of the car-

bon black became more hydrophilic, graphitic and basic by

the N-doping, which increased the interaction of the support

surface with the Pt precursor in water as well as formed Pt

particles. As a result, Pt on the N-containing carbon became

more active and more stable than the Pt on the pristine car-

bon black.

Acknowledgement

This work was supported by the Hydrogen Energy R & D

center and the Brain Korea 21 and WCU (R31-30005) Programs

and Basic Science Research Program through the National

Research Foundation of Korea(NRF) funded by the Ministry

of Education, Science and Technology (No. 2009-0083512).

R E F E R E N C E S

[1] Hamnett A. Mechanism and electrocatalysis in the directmethanol fuel cell. Catal Today 1997;38(4):445–57.

[2] Tseung ACC, Chen KY. Hydrogen spill-over effect on Pt/WO3

anode catalysts. Catal Today 1997;38(4):439–43.[3] Huang HX, Chen SX, Ce Yuan. Platinum nanoparticles

supported on activated carbon fiber as catalyst for methanoloxidation. J Power Sources 2008;175(1):166–74.

[4] Bindra P, Clouser SJ, Yeager E. Platinum dissolution inconcentrated phosphoric acid. J Electrochem Soc1979;126(9):1631–2.

[5] Dicks AL. The role of carbon in fuel cells. J Power Sources2006;156(2):128–41.

C A R B O N 5 1 ( 2 0 1 3 ) 2 7 4 – 2 8 1 281

[6] Tang H, Qi Z, Ramani M, Elter JF. PEM fuel cell cathode carboncorrosion due to the formation of air/fuel boundary at theanode. J Power Sources 2006;158(2):1306–12.

[7] Kangasniemi KH, Condit DA, Jarvi TD. Characterization ofvulcan electrochemically oxidized under simulated pem fuelcell conditions. J Electrochem Soc 2004;151(4):E125–32.

[8] Kinoshita K, Lundquist JT, Stonehart P. Potential cyclingeffects on platinum electrocatalyst surfaces. J ElectroanalChem 1973;48(2):157–66.

[9] Shukla AK, Ravikumar MK, Roy A, Barman SR, Sarma DD,Arico AS, et al. Electro-oxidation of methanol in sulfuric acidelectrolyte on platinized-carbon electrodes with severalfunctional-group characteristics. J Electrochem Soc1994;141(6):1517–22.

[10] Maiyalagan T, Viswanathan B, Varadaraju UV. Nitrogencontaining carbon nanotubes as supports for Pt – Alternateanodes for fuel cell applications. Electrochem Commun2005;7(9):905–12.

[11] Shao Y, Yin G, Gao Y, Shi P. Durability study of Pt/C and Pt/CNTs catalysts under simulated PEM fuel cell conditions. JElectrochem Soc 2006;153(6):A1093–7.

[12] Matter PH, Zhang L, Ozkan US. The role of nanostructure innitrogen-containing carbon catalysts for the oxygenreduction reaction. J Catal 2006;239(1):83–96.

[13] Glavee GN, Klabunde KJ, Sorensen CM, Hadjipanayis GC.Borohydride reduction of cobalt ions in water. Chemistryleading to nanoscale metal, boride, or borate particles.Langmuir 1993;9(1):162–9.

[14] Choi B, Yoon H, Park I-S, Jang J, Sung Y-E. Highly dispersed Ptnanoparticles on nitrogen-doped magnetic carbonnanoparticles and their enhanced activity for methanoloxidation. Carbon 2007;45(13):2496–501.

[15] Kang KY, Lee BI, Lee JS. Hydrogen adsorption on nitrogen-doped carbon xerogels. Carbon 2009;4:1171–80.

[16] Kang KY, Hong SJ, Lee BI, Lee JS. Enhanced electrochemicalcapacities of nitrogen-doped carbon gels synthesized bymicrowave-assisted polymerization of resorcinol andformaldehyde. Electrochem Commun 2008;10:1105–8.

[17] Pels JR, Kapteijn F, Moulijn JA, Zhu Q, Thomas KM. Evolutionof nitrogen functionalities in carbonaceous materials duringpyrolysis. Carbon 1995;33(11):1641–53.

[18] Jang JW, Lee CE, Lyu SC, Lee TJ, Lee CJ. Structural study ofnitrogen-doping effects in bamboo-shaped multiwalledcarbon nanotubes. Appl Phys Lett 2004;84(15):2877–9.

[19] Wu G, Swaidan R, Li D, Li N. Enhanced methanol electro-oxidation activity of PtRu catalysts supported on

heteroatom-doped carbon. Electrochim Acta2008;53(26):7622–9.

[20] Zheng B, Zheng WT, Zhang K, Wen QB, Zhu JQ, Meng SH,et al. First-principle study of nitrogen incorporation inamorphous carbon. Carbon 2006;44(5):962–8.

[21] Fuge GM, Rennick CJ, Pearce SRJ, May PW, Ashfold MNR.Structural characterisation of CNx thin films deposited bypulsed laser ablation. Diamond Relat Mater 2003;12(3–7):1049–54.

[22] Maldonado S, Stevenson KJ. Influence of nitrogen doping onoxygen reduction electrocatalysis at carbon nanofiberelectrodes. J Phys Chem B 2005;109(10):4707–16.

[23] Ye S, Vijh AK, Dao LH. A new fuel cell electrocatalyst based oncarbonized polyacrylonitrile foam. J Electrochem Soc1997;144(1):90–5.

[24] Sun C-L, Chen L-C, Su M-C, Hong L-S, Chyan O, Hsu C-Y, et al.Ultrafine platinum nanoparticles uniformly dispersed onarrayed CNx nanotubes with high electrochemical activity.Chem Mater 2005;17(14):3749–53.

[25] Zhou Y, Pasquarelli R, Holme T, Berry J, Ginley D, O’Hayre R.Improving PEM fuel cell catalyst activity and durability usingnitrogen-doped carbon supports: observations from modelPt/HOPG systems. J Mater Chem 2009;19(42):7830–8.

[26] Pozio A, De Francesco M, Cemmi A, Cardellini F, Giorgi L.Comparison of high surface Pt/C catalysts by cyclicvoltammetry. J Power Sources 2002;105(1):13–9.

[27] van Dommele S, de Jong KP, Bitter JH. Nitrogen-containingcarbon nanotubes as solid base catalysts. Chem Commun2006;46:4859–61.

[28] Strelko VV, Kuts VS, Thrower PA. On the mechanism ofpossible influence of heteroatoms of nitrogen, boron andphosphorus in a carbon matrix on the catalytic activity ofcarbons in electron transfer reactions. Carbon2000;38(10):1499–503.

[29] Czerw R, Terrones M, Charlier JC, Blase X, Foley B,Kamalakaran R, et al. Identification of electron donor statesin n-doped carbon nanotubes. Nano Lett 2001;1(9):457–60.

[30] Leon y Leon CA, Solar JM, Calemma V, Radovic LR. Evidencefor the protonation of basal plane sites on carbon. Carbon1992;30(5):797–811.

[31] Li Y-H, Hung T-H, Chen C-W. A first-principle study ofnitrogen- and boron-assisted platinum adsorption on carbonnanotubes. Carbon 2009;47(3):850–5.

[32] Rodrı́guez-Reinoso F. The role of carbon materials inheterogeneous catalysis. Carbon 1998;36(3):159–75.