Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into
electrical energy. The basic physical structure or building block of a fuel cell consists of an
electrolyte layer in contact with a porous anode and cathode on either side.
In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode)
compartment and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive
electrode) compartment; the electrochemical reactions take place at the electrodes to produce an
electric current. A fuel cell, although having components and characteristics similar to those of a
typical battery, differs in several respects. The battery is an energy storage device. The maximum energy available is determined by the amount of chemical reactant stored within the battery itself.
The battery will cease to produce electrical energy when the chemical reactants are consumed
(i.e., discharged). In a secondary battery, the reactants are regenerated by recharging, which
involves putting energy into the battery from an external source. The fuel cell, on the other hand,
is an energy conversion device that theoretically has the capability of producing electrical energy
for as long as the fuel and oxidant are supplied to the electrodes. In reality, degradation, primarily
corrosion, or malfunction of components limits the practical operating life of fuel cells.
Note that the ion specie and its transport direction can differ, influencing the site of water
production and removal, a system impact. The ion can be either a positive or a negative ion,
meaning that the ion carries either a positive or negative charge (surplus or deficit of electrons).
The fuel or oxidant gases flow past the surface of the anode or cathode opposite the electrolyte
and generate electrical energy by the electrochemical oxidation of fuel, usually hydrogen, and the
electrochemical reduction of the oxidant, usually oxygen. Appleby and Foulkes (1) have noted
that in theory, any substance capable of chemical oxidation that can be supplied continuously (as a
fluid) can be burned galvanically as the fuel at the anode of a fuel cell. Similarly, the oxidant can
be any fluid that can be reduced at a sufficient rate. Gaseous hydrogen has become the fuel of
choice for most applications, because of its high reactivity when suitable catalysts are used, its
ability to be produced from hydrocarbons for terrestrial applications, and its high energy density
when stored cryogenically for closed environment applications, such as in space. Similarly, the
most common oxidant is gaseous oxygen, which is readily and economically available from air for
terrestrial applications, and again easily stored in a closed environment. A three phase interface is
established among the reactants, electrolyte, and catalyst in the region of the porous electrode.
The nature of this interface plays a critical role in the electrochemical performance of a fuel cell,
particularly in those fuel cells with liquid electrolytes. In such fuel cells, the reactant gases diffuse
through a thin electrolyte film that wets portions of the porous electrode and react
electrochemically on their respective electrode surface. If the porous electrode contains an
excessive amount of electrolyte, the electrode may "flood" and restrict the transport of gaseous
species in the electrolyte phase to the reaction sites. The consequence is a reduction in the
electrochemical performance of the porous electrode. Thus, a delicate balance must be
maintained among the electrode, electrolyte, and gaseous phases in the porous electrode
structure. Much of the recent effort in the development of fuel cell technology has been devoted
to reducing the thickness of cell components while refining and improving the electrode structure
and the electrolyte phase, with the aim of obtaining a higher and more stable electrochemical
performance while lowering cost.
The functions of porous electrodes in fuel cells are: 1) to provide a surface site where gas/liquid
ionization or de-ionization reactions can take place, 2) to conduct ions away from or into the
three-phase interface once they are formed (so an electrode must be made of materials that have
good electrical conductance), and 3) to provide a physical barrier that separates the bulk gas
phase and the electrolyte. A corollary of Item 1 is that, in order to increase the rates of reactions,
the electrode material should be catalytic as well as conductive, porous rather than solid. The catalytic function of electrodes is more important in lower temperature fuel cells and less so in
high-temperature fuel cells because ionization reaction rates increase with temperature. It is also
a corollary that the porous electrodes must be permeable to both electrolyte and gases, but not
such that the media can be easily "flooded" by the electrolyte or "dried" by the gases in a
one-sided manner (see latter part of next section). A variety of fuel cells are in different stages of development. They can be classified by use of
diverse categories, depending on the combination of type of fuel and oxidant, whether the fuel is
processed outside (external reforming) or inside (internal reforming) the fuel cell, the type of
electrolyte, the temperature of operation, whether the reactants are fed to the cell by internal or
external manifolds, etc. The most common classification of fuel cells is by the type of electrolyte
used in the cells and includes 1) proton exchange membrane (polymer) electrolyte fuel cell
(PEFC), 2) alkaline fuel cell (AFC), 3) phosphoric acid fuel cell (PAFC), 4) molten carbonate
fuel cell (MCFC), and 5) solid oxide fuel cell (SOFC). These fuel cells are listed in the order of
approximate operating temperature, ranging from ~80° C for PEFC, ~100° C for AFC, ~200° C for
PAFC, ~650° C for MCFC, and 800° C to 1000° C for SOFC. The operating temperature and
useful life of a fuel cell dictate the physicochemical and thermomechanical properties of materials
used in the cell components (i.e., electrodes, electrolyte, interconnect, current collector, etc.).
Aqueous electrolytes are limited to temperatures of about 200° C or lower because of their high
water vapor pressure and/or rapid degradation at higher temperatures. The operating temperature
also plays an important role in dictating the type of fuel that can be utilized in a fuel cell. The
low-temperature fuel cells with aqueous electrolytes are, in most practical applications, restricted
to hydrogen as a fuel. In high-temperature fuel cells, CO and even CH4 can be used because of
the inherently rapid electrode kinetics and the lesser need for high catalytic activity at high
temperature. However, descriptions later in this section note that the higher temperature cells can
favor the conversion of CO and CH4 to hydrogen, then use the equivalent hydrogen as the actual
fuel.
electrical energy. The basic physical structure or building block of a fuel cell consists of an
electrolyte layer in contact with a porous anode and cathode on either side.
In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode)
compartment and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive
electrode) compartment; the electrochemical reactions take place at the electrodes to produce an
electric current. A fuel cell, although having components and characteristics similar to those of a
typical battery, differs in several respects. The battery is an energy storage device. The maximum energy available is determined by the amount of chemical reactant stored within the battery itself.
The battery will cease to produce electrical energy when the chemical reactants are consumed
(i.e., discharged). In a secondary battery, the reactants are regenerated by recharging, which
involves putting energy into the battery from an external source. The fuel cell, on the other hand,
is an energy conversion device that theoretically has the capability of producing electrical energy
for as long as the fuel and oxidant are supplied to the electrodes. In reality, degradation, primarily
corrosion, or malfunction of components limits the practical operating life of fuel cells.
Note that the ion specie and its transport direction can differ, influencing the site of water
production and removal, a system impact. The ion can be either a positive or a negative ion,
meaning that the ion carries either a positive or negative charge (surplus or deficit of electrons).
The fuel or oxidant gases flow past the surface of the anode or cathode opposite the electrolyte
and generate electrical energy by the electrochemical oxidation of fuel, usually hydrogen, and the
electrochemical reduction of the oxidant, usually oxygen. Appleby and Foulkes (1) have noted
that in theory, any substance capable of chemical oxidation that can be supplied continuously (as a
fluid) can be burned galvanically as the fuel at the anode of a fuel cell. Similarly, the oxidant can
be any fluid that can be reduced at a sufficient rate. Gaseous hydrogen has become the fuel of
choice for most applications, because of its high reactivity when suitable catalysts are used, its
ability to be produced from hydrocarbons for terrestrial applications, and its high energy density
when stored cryogenically for closed environment applications, such as in space. Similarly, the
most common oxidant is gaseous oxygen, which is readily and economically available from air for
terrestrial applications, and again easily stored in a closed environment. A three phase interface is
established among the reactants, electrolyte, and catalyst in the region of the porous electrode.
The nature of this interface plays a critical role in the electrochemical performance of a fuel cell,
particularly in those fuel cells with liquid electrolytes. In such fuel cells, the reactant gases diffuse
through a thin electrolyte film that wets portions of the porous electrode and react
electrochemically on their respective electrode surface. If the porous electrode contains an
excessive amount of electrolyte, the electrode may "flood" and restrict the transport of gaseous
species in the electrolyte phase to the reaction sites. The consequence is a reduction in the
electrochemical performance of the porous electrode. Thus, a delicate balance must be
maintained among the electrode, electrolyte, and gaseous phases in the porous electrode
structure. Much of the recent effort in the development of fuel cell technology has been devoted
to reducing the thickness of cell components while refining and improving the electrode structure
and the electrolyte phase, with the aim of obtaining a higher and more stable electrochemical
performance while lowering cost.
The functions of porous electrodes in fuel cells are: 1) to provide a surface site where gas/liquid
ionization or de-ionization reactions can take place, 2) to conduct ions away from or into the
three-phase interface once they are formed (so an electrode must be made of materials that have
good electrical conductance), and 3) to provide a physical barrier that separates the bulk gas
phase and the electrolyte. A corollary of Item 1 is that, in order to increase the rates of reactions,
the electrode material should be catalytic as well as conductive, porous rather than solid. The catalytic function of electrodes is more important in lower temperature fuel cells and less so in
high-temperature fuel cells because ionization reaction rates increase with temperature. It is also
a corollary that the porous electrodes must be permeable to both electrolyte and gases, but not
such that the media can be easily "flooded" by the electrolyte or "dried" by the gases in a
one-sided manner (see latter part of next section). A variety of fuel cells are in different stages of development. They can be classified by use of
diverse categories, depending on the combination of type of fuel and oxidant, whether the fuel is
processed outside (external reforming) or inside (internal reforming) the fuel cell, the type of
electrolyte, the temperature of operation, whether the reactants are fed to the cell by internal or
external manifolds, etc. The most common classification of fuel cells is by the type of electrolyte
used in the cells and includes 1) proton exchange membrane (polymer) electrolyte fuel cell
(PEFC), 2) alkaline fuel cell (AFC), 3) phosphoric acid fuel cell (PAFC), 4) molten carbonate
fuel cell (MCFC), and 5) solid oxide fuel cell (SOFC). These fuel cells are listed in the order of
approximate operating temperature, ranging from ~80° C for PEFC, ~100° C for AFC, ~200° C for
PAFC, ~650° C for MCFC, and 800° C to 1000° C for SOFC. The operating temperature and
useful life of a fuel cell dictate the physicochemical and thermomechanical properties of materials
used in the cell components (i.e., electrodes, electrolyte, interconnect, current collector, etc.).
Aqueous electrolytes are limited to temperatures of about 200° C or lower because of their high
water vapor pressure and/or rapid degradation at higher temperatures. The operating temperature
also plays an important role in dictating the type of fuel that can be utilized in a fuel cell. The
low-temperature fuel cells with aqueous electrolytes are, in most practical applications, restricted
to hydrogen as a fuel. In high-temperature fuel cells, CO and even CH4 can be used because of
the inherently rapid electrode kinetics and the lesser need for high catalytic activity at high
temperature. However, descriptions later in this section note that the higher temperature cells can
favor the conversion of CO and CH4 to hydrogen, then use the equivalent hydrogen as the actual
fuel.
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