Synthesis and Characterization of Carbon Incorporated Fe-N/carbon for Methanol-tolerant Oxygen Reduction Reaction of Polymer Electrolyte Fuel Cells

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Synthesis and characterization of carbon incorporated Fe

eN/carbons

for methanol-tolerant oxygen reduction reaction of polymer

electrolyte fuel cells

Shou-Heng Liu

a

,

*

_{, Jyun-Ren Wu}

a

_{, Chun-Jern Pan}

b

_{, Bing-Joe Hwang}

b

a_{Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan} b_{Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10617, Taiwan}

h i g h l i g h t s

Carbon incorporated non-noble FeNxcatalysts were prepared via a facile route.

ORR activity of the FeNC/C-3.1 catalysts was superior among the catalysts. The higher surface atomic N/C, Fe/C, and FeeN4centers result in this enhancement.

a r t i c l e i n f o

Article history:

Received 20 September 2013 Received in revised form 4 November 2013 Accepted 9 November 2013 Available online 19 November 2013 Keywords:

Non-noble N-doped Carbide

Oxygen reduction reaction X-ray absorption spectroscopy

a b s t r a c t

A simple method has been developed for synthesis of carbons incorporating FeNxelectrocatalysts (FeNC/ C-z) based on heat treatment of nitrogen-rich species (pentaethylenehexamine) and iron precursors (FeCl3) on carbon blacks (Vulcan XC-72) under high temperature in a nitrogen atmosphere. These resulting catalysts have been fully characterized by various spectroscopic and analytical techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). Results obtained from the polarization curves show that FeNC/C-3.1 possesses the surpassing electrocatalytic activity and the tolerance to methanol crossover during the oxygen reduction reaction (ORR) among all the FeNC/C-z catalysts, which may be due to their higher surface Fe/C and N/C atomic ratios as well as a dominant carbon incorporated FeNx (x z 4), as revealed from XPS and XAS spectroscopies.

1. Introduction

Fuel cells, in particular for direct methanol fuel cells (DMFCs)

and proton-exchange membrane fuel cells (PEMFCs), have been

recognized as one of the potential power sources for stationary and

mobile applications. However, two major challenges, namely their

costs and durability

[1

e5]

, limit the future large-scale

commer-cialization applications of DMFCs/PEMFCs. In general, Pt-based

catalysts have been intensively investigated because they are the

most active and ef

ﬁcient electrocatalysts for oxygen reduction

re-action (ORR) which is a sluggish and complicated four-electron

reaction. Thus, it is crucial to develop alternative catalysts which

possess reduced amounts of Pt

[6

e12]

and even are Pt-free

[13

e19]

,

meanwhile, increase their durability during long-term operation.

In the recent reports, pyrolyzed transition metal together with

nitrogen containing compounds in an inert or N-rich atmosphere,

such as Fe based ethylenediamine, phthalocyanines, porphyrins,

phenanthroline and tripyridyl triazine, etc.

[20

e26]

, has been

extensively investigated and shown promising catalytic activity.

However, the catalytic performance of aforementioned

electro-catalysts is still incomparable to Pt based electro-catalysts. Consequently, a

more active non-noble electrocatalyst is extremely desirable to

realize the cost-effective and industrial applications. Most recently,

in order to enhance ORR performance, incorporation of carbons

into iron nitride

[21,27,28]

or PdFe alloy

[29]

has been proposed,

which may be a prospective method to create newly and effectively

active sites for development of fuel cells.

The key scienti

ﬁc issues relating to the chemical forms of

cata-lysts ultimately depend on their molecular-scale structures. Basic

studies at this scale are necessary in the newly fabricated materials

that may also facilitate the development of valuable tools for

designing ef

ﬁciency of devices. X-ray absorption near-edge

* Corresponding author. Tel.: þ886 7 381 4526x5152; fax: þ886 7 3830674. E-mail address:[email protected](S.-H. Liu).

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Journal of Power Sources

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http://dx.doi.org/10.1016/j.jpowsour.2013.11.011

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(see

Table 4

), respectively, implying that FeNC/C-3.1

electro-catalysts are dominantly proceeded ORR with four-electron

trans-ferred pathway. Unlike two-electron reduction reactions for FeNC/

C-z (z

¼ 5.6, 8.2 and 10.6), FeNC/C-3.1 electrocatalysts can

circum-vent the production of hydrogen peroxide radicals which lead to an

undesirable degradation of the membrane-electrolyte-assembly

(MEA) of the fuel cells. Based on the XPS data in

Table 1

, it can be

observed that the N/C and Fe/C atomic ratios of FeNC/C-3.1 are the

highest among the FeNC/C-z. The high N/C and Fe/C atomic ratios,

and the moderate FeN

x

coordination number (x

z 4, in

Table 3

)

which were characterized by XPS and EXAFS show that the surface

iron concentration and the number of iron coordinated with proper

pyridinic-N should be responsible for the superior ORR

perfor-mance of FeNC/C-3.1.

The electrocatalytic durability and tolerance to the methanol

crossover at the cathode have been considered as two of the most

critical issues for fuel cells

[54

e56]

. To further study the

methanol-tolerant properties of the synthesized FeNC/C-z electrocatalysts, a

series of LSV tests were also performed to reveal the effect of

different concentrations of methanol (0.5

e2 M) on electrocatalytic

ORR performance of FeNC/C-3.1 catalysts. As shown in

Fig. 7

, little

increase in overpotential is observed for FeNC/C-3.1 electrocatalysts

even in the presence of 2 M methanol as compared to ORR in pure

H

2

SO

4

solution. The signi

ﬁcant inhibition in ORR overpotential on

the FeNC/C-3.1 catalysts is due to the weak competition reaction

between oxygen reduction and methanol oxidation, which could be

attributed to the unique properties of carbon-incorporated FeN

x

catalysts (x

z 4). Transition metal N

4

chelates

[57,58]

were

re-ported to have highly methanol-tolerant ability during ORR.

4. Conclusions

In summary, we report a facile method to prepare carbons

incorporated FeN

x

electrocatalysts (FeNC/C-z) with various Fe

loadings in this study. Electrocatalytic ORR polarization curves

were used to evaluate their corresponding activities. Among all the

FeNC/C-z electrocatalysts, FeNC/C-3.1 was found to have the

superior ORR properties. In addition, transferred electron number

obtained from Koutecky

eLevich equation suggests that the FeNC/

C-3.1 is mostly four-electron transferred ORR process. From XPS

data, the FeNC/C-3.1 was also found to possess the notable

in-creases in the atomic surface Fe/C and N/C ratios and more

pyridinic-N sites. Moreover, combining the results from XRD and

XAS spectroscopies, it can be observed that carbons may

incorpo-rate into the FeN

4

matrix to form Fe

eNeC active centers which are

attributed to the signi

ﬁcant ORR enhancement with methanol

tolerance. Thus, these FeNC/C-3.1 nanocomposites may render

future cost-effective applications in hydrogen-energy related areas,

for instance, as electrocatalysts for PEMFCs and DMFCs.

Acknowledgments

Financial support of this work from the National Science

Council, Taiwan (Contract No.: NSC99-2221-E-151-044-MY2) is

gratefully acknowledged.

Appendix A. Supplementary material

Supplementary data related to this article can be found at

http://

dx.doi.org/10.1016/j.jpowsour.2013.11.011

.

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