2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO)

The International Conference Centre Birmingham

20-23 August 20112, Birmingham, United Kingdom

Electrochemical Oxygen Reduction on Nitrogen-Containing Graphene

Stephen M. Lyth, Jianfeng Liu, and Kazunari Sasaki

Abstract- Graphene is ideally suited to electrochemistry by virtue of its high surface area and impressive electronic properties. Nitrogen incorporation can be used to tailor the properties of graphene. Here we present a simple solvothermal technique to produce a nitrogen-containing foam-like macroporous graphene powder doped with up to 15 wt% nitrogen. This is applied as an effective non-precious, metal-free electrochemical catalyst for oxygen reduction in acid media.

I. INTRODUCTION

GRAPHENE is a monolayer of Sp2 -bonded carbon atoms,

famously peeled from graphite using sticky tape, in

2004.[1] It is ideally suited to electrochemistry by virtue of its

high surface area and electronic conductivity. A controllable

method of tailoring the properties is to dope with heteroatoms

such as nitrogen to form nitrogen-containing graphene (NG).

NG has been applied to the electrochemical oxygen reduction

reaction (ORR) in several instances, mainly in alkaline

solution.[2]

The excellent ORR activity of nitrogen-doped carbons in

alkaline media is well-known.[3] However, alkaline fuel cells

are not practical due to the limited availability/performance

of negative ion conduction membranes, and the poor stability

of fuel cell materials in alkaline media. [4] It is currently more

desirable to measure ORR in acid media,[S] since fuel cells

based on Nafion® are well-established. For example, NG has

been applied as a catalyst for ORR in HCI04.[6] Additionally,

NG may provide a simple Pt catalyst support in acid-based

fuel cells, as a replacement for carbon black.[7] Here we

present a simple solvothermal, scalable chemical

synthesis,[8] of a nitrogen-containing macroporous graphene

foam. This is applied as a non-precious, metal-free

electrochemical catalyst for oxygen reduction in acid media.

II. EXPERIMENTAL

A nitrogen-containing alcohol (2-aminoethanol) reacts with

sodium in a sealed PTFE vessel, at high temperature (180'C)

and pressure « S MPa) for three days, yielding a

clathrate-like alkoxide precursor powder. This is ignited in

air, and the carbonized product is collected, sonified, washed,

dried, and pyrolysed at 1000°C for one hour, yielding a black

powder. The product was characterized using scanning

electron microscopy (SEM), atomic force microscopy

(AFM), CRN elemental analysis, X-ray photoelectron

spectroscopy (XPS), and BET nitrogen adsorption. The ORR activity was measured via rotating ring-disk electrode

(RRDE) voltammetry at room temperature in a O.S M H2S04

electrolyte solution.

1Il. RESULTS

Error! Reference source not found. shows SEM of NG,

comparing favorably with the literature. [8] A

three-dimensional macroporous foam-type structure is

observed, made up of graphene-like walls encapsulating cells

of �200 nm diameter. The thickness of the graphene walls

was � 1.7 nm as investigated by AFM. BET nitrogen

adsorption reveals a surface area greater than 170 m2/g.

Fig. I Scanning electron microscopy of graphene-like foam.

CRN elemental analysis gave a nitrogen content of 4.8

wt.%, after pyrolysis at 1000'C. Pyrolysis at 600'C resulted

in IS. 1 wt.% nitrogen, demonstrating tunability. The XPS

Nls signal was used to glean information about nitrogen

species. Pyridinic and graphite-like bonds are major

components, with roughly equal intensities. There is a large

proportion of graphite-like nitrogen in all the samples,

suggesting doping throughout the basal-plane and not just at

edges and defects (as is often observed in nitrogen containing

graphenes), reflecting the bottom-up nature of this synthesis

technique.

Figure 2 shows linear sweep voltammograms of the NG

product and an undoped reference. Un doped graphene has an

onset potential (at -2 /-lA/cm2) of 0.78 V and a current density

at 0 V of -O.S mA/cm2. After pyrolysis at 1000'C, the onset

potential is 0.84 V and the current density is -2.3 mA/cm2.

The current density is much improved by pyrolysis at

elevated temperature, possibly due to the increase in surface

area, and/or the increase in electronic conductivity associated

with the decrease in nitrogen and other impurities.

0.0

�

"'E -0.5

� S -1.0

z:. '(jj c

C!S -1.5

C Q)

t:::: -2.0 :::J

() -- Graphene

-- NG 1000°C

-2.5 L-�_L-�---'_�---'_�---'-_�---' 0.0 0.2 0.4 0.6 0.8

P oten tial (V vs RHE) 1.0

-' Fig. 2 Linear sweep voltammograms of graphene and nitrogen-containing graphene foams. [9]

The current measured by the ring electrode in RRDE

measurements can be used to estimate the proportion of

H202 produced during ORR. This in turn can be used to gain

information about the electron pathway, specifically, the

number of electrons transferred per ORR event (n). For a

Ptlcarbon electrode, n "" 4.0, and this is the target for fuel cell

applications. The expected value for pure carbon is n "" 2.0.

The value measured here is n "" 2.9. This indicates an

approximately equal mix of 2- and 4- electron pathways in

NG, providing the possibility that iron does not necessarily

form part of the 4- electron oxygen reduction active site in

Fe/C/N-based catalysts. This has also previously been shown

in pyrolysed carbon nitride and poly imide systems. [5]

However, this value still falls short of that required for fuel

cell applications. With further improvements, the high onset

potential and current density show that NG materials could

potentially be used as non-precious catalysts for ORR in fuel

cells.

IV. CONCLUSION

In conclusion, nitrogen doping offers a facile method of

tailoring of the properties of graphene. In this work, a

macroporous NG powder with up to 15% nitrogen was

synthesized using a solvothermal method with simple

precursors. The nitrogen content and surface area were

tailored by pyrolysis. The electrochemical behavior of the

material was tested, and good ORR activity in acid media was

observed for pyrolysed samples. This work shows that

nitrogen-doped graphene powder has potential for use as a

non-precious catalyst in fuel cells, even in acid media,

although further work clearly needs to be done.

V. ACKNOWLEDGMENT

This work was supported by the World Premier

International Research Center Initiative (WPI), MEXT,

Japan.

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M. Chokai, M. Taniguchi, S. Moriya, K. Matsubayashi, T. Shinoda, Y. Nabae, S. Kuroki, T. Hayakawa, M.-a. Kakimoto, and J.-i. Ozaki, Journal of Power Sources 195,5947 (2010).

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[1] S. M. Lyth, Y. Nabae, N. M. Islam, T. Hayakawa, S. Kuroki, M. Kakimoto, S. Miyata, e-J. SUlj Sci. Nanotech. Vol. 10,29-32 (2012).

Manuscript received May 15, 2012. Authors are with the International Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka, 819-0395 Japan (Phone: +81-92-802-3094; fax: +81-92-802-3094; e-mail: lyth@i2cner.kyushu-u.ac.jp).

[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 (2004)

[2] D. Geng, Y. Chen, Y. Chen, Y. Li, R. Li, X. Sun, S. Ye, and S. Knights, Energy & Environmental Science 4,760+ (2011); L. Qu, Y. Liu, J.-B. Baek, and L. Dai, ACS Nano 4, 1321 (2010); S. Yang, X. Feng, X. Wang, and K. Miillen, Angewandte Chemie International Edition 50,

5339 (2011); Y. Shao, S. Zhang, M. H. Engelhard, G. Li, G. Shao, Y. Wang, 1. Liu, I. A. Aksay, and Y. Lin, J. Mater. Chem.20, 7491

(20 I 0).

[3] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, 1. M. Carlsson, K. Domen, and M. Antonietti, Nature Materials 8, 76 (2008).

[4] J. R. Varcoe and R. C. T. Slade, Fuel Cells 5,187 (2005).

[ 5 ] S. M. Lyth, Y. Nabae, N. M. Islam, S. Kuroki, M. Kakimoto, and S. Miyata, Journal of The Electrochemical Society 158, B 194 (20 I I);

M. Chokai, M. Taniguchi, S. Moriya, K. Matsubayashi, T. Shinoda, Y. Nabae, S. Kuroki, T. Hayakawa, M.-a. Kakimoto, and J.-i. Ozaki, Journal of Power Sources 195,5947 (2010).

[ 6 ] K. R. Lee, K. U. Lee, J. W. Lee, B. T. Ahn, and S. 1. Woo, ElectrochemistlY Communications 12, 1052 (2010).

[7] R. 1. Jafri, N. Rajalakshmi, and S. Ramaprabhu,J. Mater. Chem. 20,7114

(2010).

[8] M. Choucair, P. Thordarson, and J. A. Stride, Nature Nanotechnology 4,

30 (2008).

[9] S. M. Lyth, Y. Nabae, N. M. Islam, T. Hayakawa, S. Kuroki, M. Kakimoto, S. Miyata, e-J. SUlj Sci. Nanotech. Vol. 10,29-32 (2012).

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