Hierarchical Nanostructures for Fuel Cells and Fuel Reforming - Hierarchical Nanostructures for Energy Devices - page 92

including Pt-Sn, 119-121 Pt-Pd, 122,123 Pt-Rh, 124 and Pt-Mo 125 have been in-

vestigated, as well as ternary catalysts. 126,127
Palladium is a very interesting alternative for Pt, since it is an excellent
catalyst for organic fuel electro-oxidation 128 and at least 50 times more
abundant on the Earth than Pt. 129 The release of hydrogen contained in Pd
can lead to a lower surface concentration of adsorbed CO. 130 Binary Pt-Pd
catalysts experience a strong resistance towards CO poisoning from the
oxidation of formic acid 131 and an enhancement of the catalytic activity of Pt-
Pd compared to pure Pt has been confirmed by cyclic voltammetry and
complete direct ethanol fuel cell measurements. 132 Carbon-supported Pt-Pd
catalysts also have a reduced ethanol adsorption compared to Pt, leading to
an oxygen reduction catalyst with higher ethanol tolerance and reducing the
effect of performance loss due to fuel crossover. 133 Uniformly distributed Pt-
Pd nanoparticles with a 2-4 nm diameter on a carbon support can be pre-
pared by co-reduction of mixed oxides by a single-step alcohol reduction
without stabilizers. 129 The oxidation current densities of both methanol and
ethanol on Pt-Pd/C increased 2 or 3 times compared to Pt/C electrodes.

d n 3 r 4 n g | 4

5.2.4 Solid Oxide Fuel Cells
Next to PEMFCs, solid oxide fuel cells (SOFCs) have attracted tremendous
attention, especially due to their potential for stationary power generation.
Applications of micro-SOFCs for portable applications have been investi-
gated as well. 6 With their high operating temperature, SOFCs achieve high
conversion eciencies due to increased reaction kinetics, avoid the use of
expensive materials, particularly for the catalyst, and can be directly oper-
ated with hydrocarbon fuels instead of hydrogen, reducing the need for fuel
reforming and increasing fuel flexibility. While SOFCs offer unique advan-
tages over PEMFCs, the high operating temperature and corresponding
lengthy heat-up process are major drawbacks of SOFC technology.

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5.2.4.1 Set-up of Solid Oxide Fuel Cells
The basic set-up of an SOFC MEA consists of the same three key components
as PEMFCs: anode, cathode, and electrolyte. 134 However, instead of a poly-
meric membrane, SOFCs utilize ceramic materials, thus reducing issues of
corrosion and water management. The electrolyte membrane is required to
be compact, dense, and exhibit very low electrical conductivity, but allow for
ecient ion transport. Comparable to the MEA of PEMFCs, the ceramic
membrane is sandwiched by two porous and catalytic electrodes, where the
fuel oxidation and the oxygen reduction take place.
The fundamental working principle of an SOFC is shown in Figure 5.2.
Oxygen is dissociated and ionized on the cathode and these oxygen ions
diffuse to the cathode/electrolyte interface and transfer through the elec-
trolyte to the anode/electrolyte interface. From there, they diffuse to
the catalytic sites of the anode, where they react with hydrogen from the fuel