(419c) Influence of Chemistry and Structure of Nitrogen Doped Mesoporous Carbon Support On the ORR Activity of Platinum for Fuel Cell Applications

            It
is well known that the bonding between Pt and graphitic carbon surface is weak
[1]. This weak interaction has important implications on the mass activity,
dispersion and stability of Pt nanoparticles supported on these materials.  One
way that all three can be concomitantly improved is by supporting the Pt
electrocatalyst on a support material that interacts more strongly.  This can
be accomplished on carbon materials by introducing an adatom to the surface
that disrupts the π-stabilized sp² bonding, such as nitrogen
[2]. However, the physicochemical properties of nitrogen doped carbon are
multivariate and complex. Here, we investigate the effect of both the nitrogen
content and the level of disorder in the support microstructure on the catalyst
properties and stability of supported Pt.

            Nitrogen-doped
mesoporous carbons were prepared using a hard templating synthesis procedure.
The template used was SBA-15, which has a hexagonally ordered mesoporous
structure [3].One of the unique properties of SBA-15 is that the carbon casted
on it is its exact inverse replica. This allows for rational control of the
pore structure of the carbon. Here, carbons with different pore structure were
made by modifying the template with various secondary sol-gel treatments. Pt
was deposited on such carbons using a modified polyol process. The physical
structure and surface chemistry of the carbons were characterized using BET,
XPS, XRD, Raman and TEM techniques. The activity of the supported Pt toward the
oxygen reduction reaction (ORR) was analyzed by thin-film RDE method and
stabilities were investigated by AC polarography accelerated degradation
testing and cyclic voltammetry.

            The
SBA-15 was prepared by self-assembly of triblock copolymer Pluronic® P123 (E₂₀PO₇₀EO₂₀,
BASF) and tetraethyl orthosilicate (TEOS) catalyzed by 2 M HCl [3]. As
synthesized SBA-15 was modified further with 0, 4 and 8 mL of TEOS in 2 M HCl
which resulted in SBA-15 with decreasing pore diameter, evidenced by the pore
size distribution plot from nitrogen adsorption isotherm.  Then, pyrrole was
vacuum infiltrated into the templates, polymerized to polypyrrole, and
carbonized at high temperature for 3 h in N₂ atmosphere. Finally,
the template was removed in hot 10 M KOH. Pt was deposited on these carbon
supports with hexachloroplatinic acid as a Pt precursor and ethylene glycol as
the stabilizing and reducing reagent. The ORR activities of Pt on these carbons
support were investigated in O₂ saturated 0.1 M HClO₄ at
1600 RPM and 25°C.

            In
Figure 1, the templates are denoted as SBA for as prepared and SBA-3.5/4 and
SBA-3.5/8 for SBA-15 modified with 4 and 8 mL of TEOS respectively. The
resulting nitrogen doped mesoporous carbon supports are referenced as
CPPy-3.5/X and are shown below their respective templates.

Preliminary results suggest that we have prepared
nitrogen doped mesoporous carbons with varying degree of structural order. The
ORR polarization curves for Pt deposited on CPPy, CPPy-3.5/4 and CPPy-3.5/8 are
shown in Figure 2. A slight enhancement in ORR activity of Pt on CPPy-3.5/8 can
be observed. Further characterization and ORR activity of Pt on such carbons
will be presented and discussed.

Figure
1. TEM images of nitrogen doped
mesoporous carbons and their respective templates. The 3.5 g of original
template with ordered mesopores (SBA) was modified with 4 and 8 mL of
tetraethyl orthosilicate (TEOS) to give SBA-3.5/4 and SBA-3.5/8. The carbons
synthesized on these templates are labeled CPPy, CPPy-3.5/4 and CPPy-3.5/8.

Figure
2. ORR polarization curves of
Pt supported on nitrogen doped mesoporous carbon. The curves were measured in
0.1 M HClO₄ at 1600 RPM with a scan rate of 5 mV/s.

References

1.
S. Shrestha, Y. Liu, W. E.
Mustain, Catal. Rev. Sci. Eng., 53 (2011) 256-336.

2.
Y. Zhou, K. Neyerlin, T. S. Olson,
S. Plypenko, J. Bult, H. N. Dinh, T. Gennett, Z. Shao, R. O'Hayre, Energy
Environ. Sci. 3 (2010) 1437-1446.

3.
D. Zhao, J. Feng, Q. Huo, N.
Melosh, G. H. Fredrickson, B. F. Chmelka, and G. D. Stucky. Science, 279
(1998) 548-552.