chap66 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 66, the reader should
be able to:

•
•
•
•
•

Explain how a fuel cell generates electricity.

Discuss the advantages and disadvantages of
fuel cells.
List the types of fuel cells.
Explain how ultracapacitors work.
Discuss alternative energy sources.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
double-layer technology • electrolysis • electrolyte •
energy carrier • energy density
farads • fuel cell • fuel cell hybrid vehicle (FCHV) • fuelcell stack • fuel cell vehicle (FCV)
homogeneous charge compression ignition (HCCI) •

hydraulic power assist (HPA) • inverter
low-grade heat

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KEY TERMS:
membrane electrode assembly (MEA) • NEDRA
plug-in hybrid electric vehicle (PHEV) • polymer
electrolyte fuel cell (PEFC) • proton exchange
membrane (PEM) • range • specific energy
ultracapacitor • wheel motors • wind farms

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FUEL CELL TECHNOLOGY
What Is a Fuel Cell?  A fuel cell is an electrochemical device in which the 
chemical energy of hydrogen and oxygen is converted into electrical energy, 
discovered in 1839 by Sir William Grove, a Welsh physician.
In the 1950s, NASA put this principle to work in building devices for powering 
space exploration vehicles. In the present day, fuel cells are being developed to 
power homes and vehicles while producing low or zero emissions.

Figure 66–1
Ford Motor Company has produced
a number of demonstration fuel-cell

vehicles based on the Ford Focus.

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The chemical reaction in a fuel cell is the opposite of electrolysis, 
in which electrical current is passed through water in order to 
break it into its components, hydrogen and oxygen.
Energy can be retrieved by allowing hydrogen and oxygen to 
reunite in a fuel cell. It is important to note that while hydrogen 
can be used as a fuel, it is not an energy source. Instead, hydrogen 
is only an energy carrier, as energy must be expended to 
generate the hydrogen and store it so it can be used as a fuel.

Hydrogen is an excellent fuel because it has a very high specific 
energy when compared to an equivalent amount of fossil fuel. 
One kilogram (kg) of hydrogen has three times the energy content 
as one kilogram of gasoline. Hydrogen is the most abundant 
element on earth, but it does not exist by itself in nature. 
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Hydrogen is also found in many other compounds, most notably 
hydrocarbons, such as natural gas or crude oil. In order to store 
hydrogen for use as a fuel, processes must be undertaken to 
separate it from these materials. 
Figure 66–2 Hydrogen does not exist by itself in nature. Energy must be expended to separate

it from other, more complex materials.

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Benefits of a Fuel Cell  A fuel cell can be used to move a vehicle by generating 
electricity to power electric drive motors. Fuel cells by themselves do not 
generate carbon emissions such as CO 2. The only emissions are water vapor and 
heat, and this makes the fuel cell an ideal candidate for a ZEV (zero­emission 
vehicle).
Another benefit of fuel cells is they have very few moving parts and potential to 
be very reliable. A number of OEMs have spent many years and millions of 
dollars in order to develop a low­cost, durable, and compact fuel cell that will 

operate satisfactorily under all driving conditions. 

Figure 66–3 The Mercedes-Benz B-Class fuel-cell
car was introduced in 2005.

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A fuel­cell vehicle (FCV) uses the fuel cell as its only source of 
power, where a fuel­cell hybrid vehicle (FCHV) would also have 
an electrical storage device that can be used to power the vehicle.
Most new designs of fuel­cell vehicles are now based on a hybrid 
configuration due to the significant increase in efficiency and 

driveability that can be achieved with this approach. 

Figure 66–4 The Toyota FCHV is based
on the Highlander platform and uses
much of Toyota’s Hybrid Synergy Drive
(HSD) technology in its design.

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Fuel­Cell Challenges While major automobile manufacturers 
continue to build demonstration vehicles and work on improving 
fuel­cell system design, no vehicle powered by a fuel cell has been 

placed into mass production. There are a number of reasons:

High cost
Insufficient power density
Insufficient vehicle range; lack of refueling infrastructure
Safety perception; lack of durability
Freeze starting problem
All of these problems are being actively addressed by researchers, 
and significant improvements are being made. Once cost and 
performance levels meet that of current vehicles, fuel cells will
be adopted as a mainstream technology.
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Types of Fuel Cells  There are a number of different types of fuel 
cells, differentiated by type of electrolyte used in their design.
Some operate  at room temperature; others at up to 1800°F. 

The design best suited for automotive applications is the Proton 
Exchange Membrane (PEM). It must have hydrogen to operate, 
and this may be stored on the vehicle or generated as needed.

See the chart on Page 798 of your textbook. 
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PEM FUEL CELLS

Description and Operation  The Proton Exchange Membrane 
fuel cell is also known as a Polymer Electrolyte Fuel Cell 
(PEFC). It is known for lightweight and compact design, and 
ability to operate at ambient temperatures. 
The PEM is a simple design based on a membrane that is coated 
on both sides with a catalyst such as platinum or palladium. Two 
electrodes, one each side of the membrane, are responsible for 
distributing hydrogen and oxygen over the membrane surface, 
removing waste heat, and providing a path for electrical current.
The part of the PEM fuel cell that contains the membrane, catalyst 
coatings, and electrodes is known as the Membrane Electrode 
Assembly (MEA).
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The negative electrode (anode) has hydrogen gas directed to it, oxygen is 
sent to the positive electrode (cathode).
Hydrogen is sent to the negative electrode as H 2 molecules, which break 
apart into H+ ions (protons) in the presence of the catalyst. The electrons 
(e­) from the hydrogen atoms are sent through the external circuit, 
generating electricity to perform work.
These same electrons are then sent to the positive electrode where they 
rejoin the H+ ions that have passed through the membrane and have 
reacted with oxygen in the presence of the catalyst. This creates H 2O and 
waste heat, the only emissions from a PEM cell.

NOTE: Remember a fuel cell generates direct current (DC) electricity as 
electrons only flow in one direction (from the anode to the cathode).

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Figure 66–5 The polymer electrolyte membrane only allows H ions (protons) to pass through it.
This means electrons follow the external circuit and pass through the load to perform work.

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Fuel­Cell Stacks  A single fuel cell is not particularly useful, as it 

will generate less than 1 volt of electrical potential. It is common 
for hundreds of fuel cells to be built together in a fuel­cell stack. 

The fuel cells are connected in series so total voltage of the stack 
is the sum of the individual cell voltages. The fuel cells are placed 
end­to­end in the stack, much like slices in a loaf of bread. 
Automotive fuel­cell 
stacks contain upwards 
of 400 cells in their 
construction. 
Figure 66–6
A fuel-cell stack is made of
hundreds of individual cells
connected in series.
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Total voltage of the fuel­cell stack is determined by the number of 
individual cells incorporated into the assembly. Current­producing 
ability of the stack is dependent on the surface area of the 
electrodes.
Since output of the fuel­cell stack is related to both voltage and 
current (voltage x current = power), increasing the number of 
cells or increasing the surface area of the cells will increase
power output.
Some fuel­cell vehicles will use more than one stack, depending 
on power output requirements and space limitations.

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CO Poisons the PEM Fuel Cell Catalyst
Purity of the fuel gas is critical with PEM fuel cells. If more than 10 parts
per million (ppm) of carbon monoxide is present in the hydrogen stream
being fed to the PEM anode, the catalyst will be gradually poisoned and
the fuel cell will eventually be disabled.
This means that the purity must be “five nines” (99.999% pure).
This is a major concern in vehicles where hydrogen is generated by
reforming hydrocarbons such as gasoline, because it is difficult to
remove all CO from the hydrogen during the reforming process.
In these applications, some means of hydrogen purification must be
used to prevent CO poisoning of the catalyst.

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Direct Methanol Fuel Cells  High­pressure cylinders are one 
method simple and lightweight storage method onboard a vehicle 
for use in a fuel cell, but often does not provide sufficient range.

Figure 66–7
A direct methanol
fuel cell uses a
methanol/water
solution for fuel
instead of
hydrogen gas.

Another approach has been to fuel a modified PEM fuel cell with 
liquid methanol instead of hydrogen gas. 
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Methanol, most often produced from natural gas, has a chemical 
symbol of CH3OH and a higher energy density than gaseous 
hydrogen. It exists in a liquid state at normal temperatures, and
no compressors or other high­pressure equipment is needed. 
Figure 66–8 A direct methanol fuel cell can be refueled similar to a gasoline-powered vehicle.

This means a fuel­cell 
vehicle can be refueled 
with a liquid instead of 
high­pressure gas. 
This makes refueling 
simpler and produces 
greater vehicle
driving range.
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Direct methanol fuel cells suffer from a number of problems, 
including the corrosive nature of methanol itself. Methanol cannot 
be stored in existing tanks and requires a separate infrastructure 
for handling and storage.
Another problem is “fuel crossover,” in which methanol makes its 
way across the membrane assembly and diminishes performance 
of the cell.
Direct methanol fuel cells also require much greater amounts of 
catalyst in their construction, which leads to higher costs.
These challenges are leading researchers to look for alternative 
electrolyte materials and catalysts to lower cost and improve cell 
performance.
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When is Methanol Considered to be a “Carbon-Neutral”
Fuel?
Most of the methanol in the world is produced by reforming natural gas. Natural
gas is a hydrocarbon, but does not increase the carbon content of our
atmosphere as long as it remains in reservoirs below the earth’s surface.
However, natural gas that is used as a fuel causes extra carbon to be released
into the atmosphere, which is said to contribute to global warming. Natural gas is
not a carbon-neutral fuel, and neither is methanol that is made from natural gas.
Fortunately, it is possible to generate methanol from biomass and wood waste.
Methanol made from renewable resources is carbon neutral, because no extra
carbon is being released into the earth’s atmosphere than what was originally
absorbed by the plants used to make methanol.

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FUEL CELL VEHICLE SYSTEMS
Humidifiers  Water management in PEM fuel cell is critical. Too 
much can prevent oxygen making contact with the positive 
electrode; too little allows the electrolyte to dry out and lower its 
conductivity.
Amount of water and where it resides is critical in determining at 
how low a temperature the fuel cell will start; water freezing in the 
fuel cell can prevent it from starting.
The humidifier achieves balance, providing sufficient moisture to 
the fuel cell by recycling water evaporating at the cathode. It is 
located in the air line leading to the cathode of the fuel­cell stack.

See Figure 66–9.
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Figure 66–9 Power train layout in a Honda FCX fuel-cell vehicle. Note the use of a humidifier
behind the fuel-cell stack to maintain moisture levels in the membrane electrode assemblies.

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What is the Role of the Humidifier in a PEM Fuel Cell?
The polymer electrolyte membrane assembly in a PEM fuel cell acts as
conductor of positive ions and as a gas separator. However, it can only
perform these functions effectively if it is kept moist.
A fuel-cell vehicle uses an air compressor to supply air to the positive
electrodes of each cell, and this air is sometimes sent through a humidifier
first to increase its moisture content. The humid air then comes in contact
with the membrane assembly and keeps the electrolyte damp and
functioning correctly.

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Fuel­Cell Cooling Systems   Excess heat generated by the fuel cell 
during normal operation can lead to a breakdown of the polymer 
electrolyte membrane. A liquid cooling system must be utilized to 
remove waste heat from the fuel­cell stack. 
The heat generated by a fuel cell 
is classified as low­grade heat. 
Figure 66–10 The Honda FCX uses one large
radiator for cooling the fuel cell, and two
smaller ones on either side for cooling
drive train components.

This means that there is only a 
small difference between the 
temperature of the coolant and 
that of the ambient air. 
Heat exchangers with a larger 
surface area must be utilized. 

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