A fuel cell is an electrochemical system in which the chemical energy of a fuel is converted directly into electrical energy. A fuel cell works in a similar
way to a primary battery with the difference
that the energy is not stored between the electrodes,
but is instead transferred from an external tank. Because there is no combustion,
fuel cells give off few emissions. Also, having no moving parts, they are
|Principle of a fuel cell
Fuel cell technology dates back to the 1800s, but it wasn't until the end
of the 20th century that it was used successfully in spacecraft to provide
electricity and water. The technology can be used to make electricity to
power vehicles, homes, and businesses. And if a renewable energy source
is used as the main source of hydrogen,
a fuel cell can be considered a renewable
Today the technology is used for the production of electric and thermal
energy in power-heat coupling systems (see block-type
thermal power station) and it is also the source of electricity in electric
automobiles. Unlike battery-powered electric automobiles, fuel cell powered
automobiles achieve similar ranges and load-carrying capacities to conventional
automobiles with combustion engines. In the past few decades, significant
advances in the materials sciences have helped spur the breakthrough of
fuel cell technology.
How fuel cells work
Unlike conventional technologies, fuel is not "burned" but is combined in
a chemical process. A fuel cell consists of two electrodes sandwiched around
an electrolyte. Oxygen passes over one electrode and hydrogen over the other,
generating electricity, water, and heat.
Hydrogen fuel is fed into the anode of the
fuel cell. Oxygen (or air) enters the fuel cell through the cathode.
Encouraged by a catalyst, the hydrogen atom splits into a proton and an
electron, which take different paths to the cathode. The proton passes through
the electrolyte. The electrons create
a separate current that can be utilized before they return to the cathode,
to be reunited with the hydrogen and oxygen in a molecule of water.
A fuel cell system that includes a fuel reformer can obtain
hydrogen from any hydrocarbon fuel – from natural gas, methanol, and
even gasoline. Other possible fuels include propane, hydrogen, anaerobic
digester gas from wastewater treatment facilities, and landfill gas.
Fuel cell types
Fuel cells are categorized according to the type of electrolyte used. Some
of the these include:
All three of the above fuel cell types operate at temperatures that require
that conversion of fuel to hydrogen occur outside of the fuel cell. This
approach introduces a level of complexity avoided by the following two fuel
- PEM fuel cells. Polymer
electrolyte membrane (or proton exchange membrane) fuel cells were originally
used in the Gemini spacecraft missions and designed by DuPont. A solid
polymer ion exchange membrane is used as an electrolyte. Platinum ruthenium
is used as the catalyst. PEM fuel cells are being tested in mobile sources
such as buses and smaller vehicles.
- Phosphoric acid
fuel cells. These use aqueous phosphoric acid as an electrolyte
and platinum as a catalyst, and are one of the most technologically
mature forms of fuel cell.
- Alkaline fuel cells.
These are one of the oldest types of fuel cell. They, too, rely upon
platinum (or palladium) as the catalyst for a potassium hydroxide electrolyte.
See also regenerative fuel
- Molten carbonate
fuel cells. These rely upon nickel-based catalysts (and molten
carbonates as electrolytes) and can operate at higher temperatures.
Reforming the fuel into hydrogen can occur inside the fuel cell. Most
of the larger fuel cells on the market today rely upon this approach.
- Solid oxide fuel cells.
These rely upon a coated zirconia ceramic as the electrolyte, which
translates into the ability to operate at even higher temperatures that
can support fuel formulation within the fuel cell. No catalyst at all
is required. This technology is the least mature of the fuel cell types
currently on the market. Nevertheless, it offers the promise of reduced
cost and greater quantities of thermal heat for use at the installation
Parts of a 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, as shown in the diagram. The three key layers
in a PEM fuel cell include
Other layers of materials are designed to help draw fuel and air into the
cell and to conduct electrical current through the cell.
- Membrane electrode assembly
The electrodes (anode and cathode), catalyst, and polymer electrolyte membrane
together form the membrane electrode assembly (MEA) of a PEM fuel cell.
The thickness of the membrane in a membrane electrode assembly can vary
with the type of membrane. The thickness of the catalyst layers depends
upon how much platinum (Pt) is used in each electrode. For catalyst layers
containing about 0.15 milligrams (mg) Pt/cm2, the thickness of the catalyst
layer is close to 10 micrometers (μm) – less than half the thickness
of a sheet of paper. This membrane/electrode assembly, with a total thickness
of about 200 µm (or 0.2 mm), can generate more than half an ampere of current
for every square centimeter of assembly area at a voltage of 0.7 volts,
but only when encased in well-engineered components – backing layers,
flow fields, and current collectors.
- Anode. The anode, the negative side of the fuel cell, has
several jobs. It conducts the electrons that are freed from the hydrogen
molecules so they can be used in an external circuit. Channels etched
into the anode disperse the hydrogen gas equally over the surface of
- Cathode. The cathode, the positive side of the fuel cell,
also contains channels that distribute the oxygen to the surface of
the catalyst. It conducts the electrons back from the external circuit
to the catalyst, where they can recombine with the hydrogen ions and
oxygen to form water.
- Polymer electrolyte membrane. The polymer electrolyte membrane
(PEM) – 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.
All electrochemical reactions in a fuel cell consist of two separate reactions:
an oxidation half-reaction at the anode and a reduction half-reaction at
the cathode. Normally, the two half-reactions would occur very slowly at
the low operating temperature of the PEM fuel cell. Each of the electrodes
is coated on one side with a catalyst layer that speeds up the reaction
of oxygen and hydrogen. It is usually made of platinum powder very thinly
coated onto carbon paper or cloth. The catalyst is rough and porous so the
maximum surface area of the platinum can be exposed to the hydrogen or oxygen.
The platinum-coated side of the catalyst faces the PEM. Platinum-group metals
are critical to catalyzing reactions in the fuel cell, but they are very
expensive. DOE's goal is to reduce the use of platinum in fuel cell cathodes
by at least a factor of 20 or eliminate it altogether to decrease the cost
of fuel cells to consumers.
The backing layers, flow fields, and current collectors are designed to
maximize the current from a membrane/electrode assembly. The backing layers
– one next to the anode, the other next to the cathode – are
usually made of a porous carbon paper or carbon cloth, about as thick as
4 to 12 sheets of paper. The backing layers have to be made of a material
(like carbon) that can conduct the electrons that leave the anode and enter
the cathode. The porous nature of the backing material ensures effective
diffusion (flow of gas molecules from a region of high concentration to
a region of low concentration) of each reactant gas to the catalyst on the
membrane/electrode assembly. The gas spreads out as it diffuses so that
when it penetrates the backing, it will be in contact with the entire surface
area of the catalyzed membrane.
|Magnified image of catalyst in contact with the solid
polymer electrolyte membrane of a fuel cell. The catalyst is rough
and porous so that the maximum surface area of the platinum can be
exposed to the hydrogen or oxygen. The platinum-coated side of the
catalyst faces the PEM.
The backing layers also help in managing water in the fuel cell; too little
or too much water can cause the cell to stop operating. Water can build
up in the flow channels of the plates or can clog the pores in the carbon
cloth (or carbon paper), preventing reactive gases from reaching the electrodes.
The correct backing material allows the right amount of water vapor to reach
the membrane/electrode assembly and keep the membrane humidified. The backing
layers are often coated with Teflon to ensure that at least some, and preferably
most, of the pores in the carbon cloth (or carbon paper) do not become clogged
with water, which would prevent the rapid gas diffusion necessary for a
good rate of reaction at the electrodes.
Pressed against the outer surface of each backing layer is a piece of hardware
called a bipolar plate that typically serves as both flow field and current
collector. In a single fuel cell, these two plates are the last of the components
making up the cell. The plates are made of a lightweight, strong, gas-impermeable,
electron-conducting material – graphite or metals are commonly used
even though composite plates are now being developed.
The first task served by each plate is to provide a gas "flow field." Channels
are etched into the side of the plate next to the backing layer. The channels
carry the reactant gas from the place where it enters the fuel cell to the
place where it exits. The pattern of the flow field in the plate (as well
as the width and depth of the channels) has a large impact on how evenly
the reactant gases are spread across the active area of the membrane/electrode
assembly. Flow field design also affects water supply to the membrane and
water removal from the cathode.
|Photograph of the bipolar plates that serve as both
flow field and current collectors in PEM fuel cells. The plates are
made of a lightweight, strong, gas-impermeable, electron-conducting
material-graphite or metals are commonly used, although composite
plates are now being developed.
Each plate also acts as a current collector. Electrons produced by the oxidation
of hydrogen must (1) be conducted through the anode, through the backing
layer, along the length of the stack, and through the plate before they
can exit the cell; (2) travel through an external circuit, and (3) re-enter
the cell at the cathode plate. With the addition of the flow fields and
current collectors, the PEM fuel cell is complete; only a load-containing
external circuit, such as an electric motor, is required for electric current
Fuel cell technology challenges
Cost and durability are the major challenges to fuel cell commercialization.
However, hurdles vary according to the application in which the technology
is employed. Size, weight, and thermal and water management are barriers
to the commercialization of fuel cell technology. In transportation applications,
these technologies face more stringent cost and durability hurdles. In stationary
power applications, where cogeneration of heat and power is desired, use
of PEM fuel cells would benefit from raising operating temperatures to increase
performance. The key challenges include:
- Cost. The cost of fuel cell power systems must be
reduced before they can be competitive with conventional technologies.
Currently, the costs for automotive internal-combustion engine power
plants are about $25–$35/kW; for transportation applications,
a fuel cell system needs to cost $30/kW for the technology to be competitive.
For stationary systems, the acceptable price point is considerably higher
($400–$750/kW for widespread commercialization and as much as $1000/kW
for initial applications).
- Durability and reliability. The durability of fuel
cell systems has not been established. For transportation applications,
fuel cell power systems will be required to achieve the same level of
durability and reliability of current automotive engines (i.e., 5,000-hour
lifespan (150,000 miles)) and the ability to function over the full
range of vehicle operating conditions (40°C to 80°C). For stationary
applications, more than 40,000 hours of reliable operation in a temperature
at -35°C to 40°C will be required for market acceptance.
- System size. The size and weight of current fuel
cell systems must be further reduced to meet the packaging requirements
for automobiles. This applies not only to the fuel cell stack, but also
to the ancillary components and major subsystems (i.e., fuel processor,
compressor/expander, and sensors) making up the balance of power system.
- Air, thermal, and water management. Air management
for fuel cell systems is a challenge because today's compressor technologies
are not suitable for automotive fuel cell applications. In addition,
thermal and water management for fuel cells are issues because the small
difference between the operating and ambient temperatures necessitates
large heat exchangers.
- Improved heat recovery systems. The low operating
temperature of PEM fuel cells limits the amount of heat that can be
effectively utilized in combined heat and power (CHP) applications.
Technologies need to be developed that will allow higher operating temperatures
and/or more-effective heat recovery systems and improved system designs
that will enable CHP efficiencies exceeding 80%. Technologies that allow
cooling to be provided from the low heat rejected from stationary fuel
cell systems (such as through regenerating desiccants in a desiccant
cooling cycle) also need to be evaluated.