25-08-2017, 09:32 PM
Fuel cell materials and components
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Abstract
Fuel cells offer the possibility of zero-emissions electricity generation and increased energy security. We review
here the current status of solid oxide (SOFC) and polymer electrolyte membrane (PEMFC) fuel cells. Such solid
electrolyte systems obviate the need to contain corrosive liquids and are thus preferred by many developers over alkali,
phosphoric acid or molten carbonate fuel cells. Dramatic improvements in power densities have been achieved in both
SOFC and PEMFC systems through reduction of the electrolyte thickness and architectural control of the composite
electrodes. Current efforts are aimed at reducing SOFC costs by lowering operating temperatures to 500–800 °C, and
reducing PEMFC system complexity be developing ‘water-free’ membranes which can also be operated at temperatures
slightly above 100 °C.
Introduction
Because of their potential to reduce the environmental
impact and geopolitical consequences of the
use of fossil fuels, fuel cells have emerged as tantalizing
alternatives to combustion engines. Like a
combustion engine, a fuel cell uses some sort of
chemical fuel as its energy source; but like a battery,
the chemical energy is directly converted to
electrical energy, without an often messy and relatively
inefficient combustion step.
Solid oxide fuel cells: State-of-the-art
An excellent review of ceramic fuel cells and
the materials from which they are constructed has
been presented by Minh [9], and we only briefly
summarize here the technology status for what one
can term ‘conventional’ solid oxide fuel cells.
Somewhat more recent, but less comprehensive,
reviews have been published by Ormerod [10] and
by Singhal [11]. Today’s demonstration SOFCs
utilize yttria stabilized zirconia (YSZ), containing
typically 8 mol% Y, as the electrolyte; a ceramicmetal
composite (cermet) comprised of Ni + YSZ
as the anode; and La1xSrxMnO3-d, (lanthanum
strontium manganite or LSM) as the cathode. Specific
anode and cathode compositions are often
omitted from publications, but typically x is 0.15
to 0.25 in LSM cathodes.
Polymer electrolyte membrane fuel cells:
State-of-the-art
Polymer electrolyte membrane (PEM) fuel cells
have been reviewed by Costamagna and Srinivasan
[17,18] and readers are referred to that work for a
more comprehensive discussion than can be provided
here. The most widely implemented electrolyte
in PEM fuel cells is Nafion manufactured by
duPont. Nafion and related polymers are comprised
of perfluorinated back-bones, which provide
chemical stability, and of sulfonated side-groups
which aggregate and facilitate hydration (see discussion
below). It is these hydrated, acidic regions
which allow relatively facile transport of protons,
but also restrict PEMFCs to low temperatures of
operation. As a consequence, precious metals are
required for electrocatalysis. For hydrogen/air fuel
cells, Pt nano-particles supported on carbon are utilized
for both the anode and cathode.
Electrolytes
The most important property of a candidate electrolyte
material is, of course, the ionic conductivity.
Conductivity data of a broad range of
materials are summarized in Fig. 4 [20–25].
Material classes for electrolyte applications range
from ceramics, to polymers to acid salts, and the
mobile ion can be O2, H+, or (H2O)nH+. Solids
for which OH or CO3 are mobile are also
known, but the conductivities are not high enough
to be of technological relevance. It should be noted
that independent of the magnitude of the conductivity,
fuel cell design inherently leads to a preference
for a specific mobile species. In general, hydronium,
hydroxide and carbonate ion conductors
are unattractive because one must, by definition,
recycle an otherwise inert species: H2O in the case
of hydronium and hydroxide conductors or CO2 in
the case of carbonate conductors, to maintain ion
transport.
Conclusions
After almost a century of slow and at times
almost sputtering progress, fuel cell research has
exploded with activity over the past decade. The
results have been tremendous, with power densities
increasing by factors of two and catalyst utilization
by more than an order of magnitude. These
achievements have resulted from the development
of new materials (e.g. La1xSrxGa1yMgyO3d
oxide ion conductors) as well as new processing
techniques (e.g. electrocatalyst-layer deposition for
polymer electrolyte fuel cells). Reduction of cost
and system complexity remain significant challenges.
Current efforts in SOFC research are aimed
at (1) reducing operating temperatures to 500–800
°C to permit the use of low-cost ferritic alloys for
the interconnect component of the fuel cell stack
and (2) enabling the direct utilization of hydrocarbon
fuels.