World Journal of Engineering
FUNCTIONALIZED CARBON NANOTUBES AND ANODE DEGRADATION IN
HIGH TEMPERATURE PEM FUEL CELLS
A. Orfanidi, M. K. Daletou, S. G. Neophytides
Foundation of Research and Technology Hellas-Institute of Chemical Engineering and High Temperature
Processes
Stadiou Str, Platani Rion, Patras 26504, Greece
Fax: +302610965223; Tel: +302610965265, E-mail:neoph@iceht.forth.gr
Introduction
OH
The present work deals with a study towards the
development of new Pt electrocatalysts supported on
CNT with increased catalyst utilization for application in
PEMFCs.
Carbon nanotubes (CNTs) [1], single
(SWCNTs) or multi walled (MWCNTs) have attracted
academic and industrial interest due to their
extraordinary mechanical, electrical, and thermal
properties. Their surface modification to increase their
dissolution and processability properties has been the
top priority of material scientists during the past decade.
Several methodologies have been reported using
covalent modifications of CNTs with polymers, organic
or inorganic molecules [2].
Toward the development of an optimized
electrocatalytic system, there are two important
considerations; depositing fine Pt particles on the carbon
support and bringing them into the three-phase region. A
series of high temperature polymer electrolytes
developed by our group consist of imbibed H3PO4
aromatic polyethers bearing pyridine units in the main
chain [3]. The general structure is depicted in Figure 1.
In order to increase the catalyst utilization, the uniform
distribution of pyridine moieties that bind H3PO4 must
be achieved throughout the catalytic layer. In this
respect a series of chemically modified carbon supports
bearing pyridine based molecules were prepared.
O
O
R
co O
X
O
N
n
N
n
Figure 2. Functionalization of carbon nanotubes with
pyridine moieties (ox.MWCNT)-Py.
Catalyst preparation. Platinum was introduced on
the different MWCNT based supports by the use of
modified polyol synthesis [5]. The
Dihydrogen
hexachloroplatinate(IV) hexahydrate is being reduced
into Pt by ethylenglycol. The modification of the method
refers to the fact that the deposition of Pt takes place at
low pH so that the fast reduction and deposition results
in smaller Pt particle size. The detaile description of the
synthesis method can be found in [5].
Instrumentation. X-ray photoelectron spectroscopy
(XPS) was incorporated for determination of the
elemental composition and the chemical state of the
elements that exist within the various CNT supports
prepared in this work. Thermogravimetric analysis
(TGA) was used in order to quantify the
functionalization of CNTs. The study was performed on
a TGA Q50 (TA instruments) under argon atmosphere at
10oC/min within the temperature range from 30 to
800oC. The crystalline structure, the particle size
distribution and the active Pt surface area was
determined by means of XRD, TEM and H2
chemisorption meassurements.
MEA Preparation. The prepared electrocatalysts
were formulated into electrodes to be used for the
assembly of high temperature Membrane Electrode
Assemblies. The electrode was made of carbon cloth as
current collector and the gas diffusion layer on which
the electrocatalyst layer was supported. The
Electrocatalyst’s ink was air sprayed on to the gas
diffusion layer. The electrodes were hot pressed on the
two sides of a H3PO4 impregnated ADVENT TPS®
polymer electrolyte membrane.
Electrochemical meassurements comprised steady
state potentiostatic fuel cell experiments, galvanostatic
AC impedance meassurements and CO strip
potentiodynamic meassuremnts for the determination of
the Electrochemical
Surface Area (ECSA). The
meassurements were focused on the performance and
R
N
Figure 1: General structure of the HT PEM,
Experimental
Materials
Solvent-free functionalization of MWCNT.
Functionalization of MWCNT or the as received
MWCNTs took place using the solvent-free
functionalization method [5]. The aim of this study was
to achieve a uniform distribution of polar pyridine
moieties on the surface of the carbon nanotubes and
therefore throughout the catalytic layer (Figure 2). The
preparation method is described in detail in [5].
893
World Journal of Engineering
degradation issues of the anode operating under various
compositions of reformate H2 at 180oC.
Under these operating conditions the conventional Pt/C
electrocatalysts show degradation rates as high as
100μV/h. Fig. 5 shows the stable operation of the anode
made of the new 30% Pt/(ox.MWCNT)-Py catalyst fed
with reformate gas with very low H2 content (25%). As
it is explicitly shown in the AC impedance spectra the
anodic
polarization
resistance
of
the
30%
Pt/(ox.MWCNT)-Py catalyst is lower than the
corresponding anodic polarization of the conventional
Pt/C.
Results and Discussion
The modified MWCNT supports were characterized
using thermogravimetric analysis (TGA), X-ray
photoelectron spectroscopy (XPS) and Raman, in order
to confirm and quantify the NT functionalization. The
fine distribution of deposited Pt nanoparticles on the
modified support (ox.MWCNT)-Py is shown in Fig. 3.
Fig. 3: TEM micrographs of 30% Pt/(ox.MWCNT)-Py
Figure 5. AC impedance spectra of MEAs prepared by
conventional Pt/C and Pt/(ox.MWCNT)-Py electrocatalysts
As it is clearly shown, the aggregation of the Pt
nanoparticles is minimal and the sizes obtained (2-3nm)
from image analysis have a narrow size distribution.
The pyridine moieties attached on the sidewalls of the
nanotubes result in better distribution of nanotubes in
the reaction solution. It is also plausible that these
groups interact with Pt ions from the Pt precursor,
serving as molecular templates for ion adsorption and
leading to the creation of nucleation centers for the
reduction reaction by ethylene glycol. At the same time
they could act as barriers causing steric hindrance for the
direct deposition of Pt precursor on the nanotube
surface.
Conclusions
The
stability
of
the
Pt/(ox.MWCNT)-Py
electrocatalysts under low H2 content reformate can be
attributed to the stability of the Pt nanoparticles on the
modified MWCNT support with pyridine units. The
pyridine moieties act as a template that define the
uniform distribution of the supported Pt particles, and
the distribution of the phosphoric acid for the formation
a well distributed electrochemical interface.
Acknowledgement
The authors acknowledge the financial support of the
FCH-JU within the framework of the DEMMEA project
Contract Nr: 245156
References
1.
2.
3.
Figure 4. Long term stability 30% Pt/(ox.MWCNT)-Py
fed with synthetic reformate gas.
One of the most significant factors in HT-PEM MEAs
stability is the stable operation of the anode under
reformate gas especially under low H2 content (ca 50%).
4.
5.
894
(a) Iijima S., Nature 354 (1991), 56; (b) Iijima, S.; Ichihashi, T.,
Nature 363 (1993), 603.
(a) Holzinger M., Vostrowsky O., Hirsch A., Hennrich F.,
Kappes M., Weiss R., Jellen F., Angew Chem Int Ed 40 (2001),
4002; (b) Bahr J.L, Yang J.b., Kosynkin D.V., Bronikowski
M.J., Smalley R.E., J.M. Tour, J Am Chem Soc 123 (2001),
6536
(a) Pefkianakis E.K., Deimede V., Daletou M.K., Gourdoupi N.,
Kallitsis J.K., Macromol. Rapid Commun. 26 (2005), 1724. (b)
Gourdoupi N., Triantafyllopoulos N., Deimede V., Pefkianakis
E.K.,. Daletou M.K, Neophytides S., Kallitsis J., US patent,
WO/2008/038162, 2008.
Christopher, A. D.; Tour, J.M. J Am Chem Soc 125 (2003),
1156.
Orfanidi A., et al., Appl. Catal. B: Environ. (2011),
doi:10.1016/j.apcatb.2011.05.043