Abstract :
A fuel
cell is a device that generates electricity by a chemical reaction. Every fuel
cell has two electrodes, one positive and one negative, called, respectively,
the anode and cathode. The reactions that produce electricity take place at the
electrodes.
Every fuel cell
also has an electrolyte, which carries electrically charged particles from one electrode
to the other, and a catalyst, which speeds the reactions at the electrodes.
Hydrogen is
the basic fuel, but fuel cells also require oxygen. One great appeal of fuel cells
is that they generate electricity with very little pollution–much of the hydrogen
and oxygen used in generating electricity ultimately combine to form a harmless
byproduct, namely water.
One detail
of terminology: a single fuel cell generates a tiny amount of direct current (DC)
electricity. In practice, many fuel cells are usually assembled into a stack. Cell
or stack, the principles are the same.
Parts of
Fuel Cell
Polymer electrolyte
membrane (PEM) fuel cells are the current focus of research for fuel cell vehicle
applications. PEM fuel cells are made from several layers of different materials.
The main parts of a PEM fuel cell are described below.
The heart
of a PEM fuel cell is the membrane electrode
assembly (MEA), which includes the membrane,
the catalyst layers,
and gas diffusion layers
(GDLs).
Hardware components
used to incorporate an MEA into a fuel cell include gaskets,
which provide a seal around the MEA to prevent leakage of gases, and bipolar plates,
which are used to assemble individual PEM fuel cells into a fuel cell stack.
Membrane Electrode
Assembly
The membrane,
catalyst layers (anode and cathode), and diffusion media together form the membrane
electrode assembly (MEA) of a PEM fuel cell.
Polymer
electrolyte membrane. The polymer electrolyte membrane, or PEM (also called a proton
exchange membrane)—a specially treated material that looks something like ordinary
kitchen plastic wrap—conducts only positively charged ions and blocks the electrons.
The PEM is the key to the fuel cell technology; it must permit only the necessary
ions to pass between the anode and cathode. Other substances passing through
the electrolyte would disrupt the chemical reaction. For transportation applications,
the membrane is very thin—in some cases under 10 microns.
Catalyst
layers. A layer of catalyst is added on both sides of the membrane—the anode layer
on one side and the cathode layer on the other. Conventional catalyst layers include
nanometer-sized particles of platinum dispersed on a high-surface-area carbon
support. This supported platinum catalyst is mixed with an ion-conducting polymer
(ionomer) and sandwiched between the membrane and the GDLs. On the anode side,
the platinum catalyst enables hydrogen molecules to be oxidized. On the cathode
side, the platinum catalyst enables oxygen by reacting with the hydrogen ions generated
by the anode, producing water. The ionomer mixed into the catalyst layers allows
the hydrogen ions to conduct through these layers.
Gas
diffusion layers. The GDLs sit outside the catalyst layers and facilitate transport
of reactants into the catalyst layer, as well as removal of product water. Each
GDL is typically composed of a sheet of carbon paper in which the carbon fibers
are partially coated with polytetrafluoroethylene (PTFE). Gases diffuse rapidly
through the pores in the GDL. These pores are kept open by the hydrophobic PTFE,
which prevents excessive water buildup. In many cases, the inner surface of the
GDL is coated with a thin layer of high-surface-area carbon mixed with PTFE, called
the microporous layer. The microporous layer can help adjust the balance between
water retention (needed to maintain membrane conductivity) and water release (needed
to keep the pores open so hydrogen and oxygen can diffuse into the electrodes).
Hardware
The MEA is
the part of the fuel cell where power is produced, but hardware components are
required to enable effective MEA operation.
Bipolar
plates. Each individual MEA produces less than 1 V under typical operating conditions,
but most applications require higher voltages. Therefore, multiple MEAs are
usually connected in series by stacking them on top of each other to provide a usable
output voltage. Each cell in the stack is sandwiched between two bipolar plates
to separate it from neighboring cells. These plates, which may be made of metal,
carbon, or composites, provide electrical conduction between cells, as well as
providing physical strength to
the stack. The surfaces of the plates typically contain a “flow field,” which
is a set of channels machined or stamped into the plate to allow gases to flow over
the MEA. Additional channels inside each plate may be used to circulate a liquid
coolant.
Gaskets.
Each MEA in a fuel cell stack is sandwiched between two bipolar plates, but gaskets
must be added around the edges of the MEA to make a gas-tight seal. These gaskets
are usually made of a rubbery polymer.
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