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Abstract:
Increasing energy demand and depletion of fossil resources urge the search for alternative
and sustainable energy systems. One promising
alternative
technology for mobile
applications is the polymer electrolyte membrane fuel cell (PEMFC).
A major restriction
in successful commercialization and implementation of the technology is the availability
and high over potential of
commonly employed
platinum
catalysts
as well as their lack in
long-term stability.
In particular, latter is the result of an interplay between different degradation
mechanisms. The essential high stability, demanded for real applications, requires the
synthesis of advanced electrocatalysts that withstand the harsh operation conditions. In
the first part of this work, the synthesis of oxygen
reduction electrocatalysts consisting of
Pt-Co (i.e. Pt5Co, Pt3Co, and, PtCo) alloyed nanoparticles encapsulated in the
mesoporous shell of hollow graphitic spheres (HGS) is reported. The mass activities of
the activated catalysts depend on the initial alloy composition and an activity increase on
the order of 2 to 3 fold, compared to pure Pt@HGS, was achieved. The key point of the
investigation is the degradation of PtCo@HGS (showing the highest activity). The
impact of dissolution/dealloying and carbon corrosion can be studied without the
interplay of other degradation mechanisms that would induce a substantial change in the
particle size distribution. Therefore, impact of the upper potential limit (UPL) and the
scan rates on the dealloying and ECSA evolution are examined in detail.
Besides the incorporation of Pt-Co catalysts, also shape-controlled Pt-Ni catalysts were
finely dispersed within the mesoporous shell of the HGS. Pre-impregnated
Pt seeds,
acting as anchor points for the shell growth, are demonstrated to be a prerequisite for the
successful precious metal catalyst dispersion,
as omitting the Pt seeds led
to
severe
agglomeration and inhomogeneous particle distribution.
A strong increase
in specific
activity could be obtained, whereas stability enhancement,
originated from the
encapsulation,
was superimposed by agglomerated
particles on the graphitic domains of
the HGS.
Alternative synthesis strategies are reported,
enabling large scale synthesis of hollow
mesoporous carbon structures. Chemical vapor deposition (CVD) of ferrocene is
employed for the synthesis of hollow graphitic spheres (HGScvd). Thanks to the
precursor, iron particles are embedded into the mesoporous template during the CVD,
facilitating the synthesis of highly ordered graphite structures.
The
effect of the
subsequent annealing temperatures
on
the graphitization degree and on textural properties
are highlighted. Ultimately, the HGScvd
is employed as a support material,
significantly
improving the stability of finely dispersed nanoparticles in comparison to commercial
high surface area carbon materials.
Alternatively, the synthesis of hollow mesoporous
carbon, following a soft templating method, is reported in a single step.
The textural
properties
can be fine-tuned
by subsequent hydrothermal treatments, allowing the control
over particle porosity in the range of 3–12nm.
