1

Introduction

1.1

What Is a Fuel Cell?

1.2

Main Applications/The “Drivers”

Transportation

• Stationary Power • Portable Power

1.3

Low- and Medium-Temperature Fuel Cells

1.4

High-Temperature Fuel Cells

1.5

Liquid Fuel: The Direct Methanol Fuel Cell

The recent success of fuel-cell-powered demonstration vehicles using the proton exchange membrane
fuel cell developed by the Canadian company Ballard Power Systems, by DaimlerChrysler and many
others, suggests that fuel cells have finally come of age. Elsewhere, Plug Power and a number of other
developers are striving to bring their domestic grid-independent power supplies onto the market within
the next couple of years. What has driven these developments? And what exactly is a fuel cell?

1.1 What Is a Fuel Cell?

As early as 1839, William Grove discovered the basic operating principle of fuel cells by reversing water
electrolysis to generate electricity from hydrogen and oxygen. The principle that he discovered remains
unchanged today.

A fuel cell is an electrochemical “device” that continuously converts chemical energy into electric energy
(and some heat) for as long as fuel and oxidant are supplied.

Fuel cells therefore bear similarities both to batteries, with which they share the electrochemical nature

of the power generation process, and to engines which — unlike batteries — will work continuously
consuming a fuel of some sort. Here is where the analogies stop, though. Unlike engines or batteries, a
fuel cell does not need recharging, it operates quietly and efficiently, and — when hydrogen is used as
fuel — it generates only power and drinking water. Thus, it is a so-called zero emission engine. The
thermodynamics of the electrochemical power generation process are analyzed in Chapter 3, where fuel
cells are compared to thermal engines. Thermodynamically, the most striking difference is that thermal
engines are limited by the Carnot efficiency while fuel cells are not.

Grove’s fuel cell was a fragile apparatus filled with dilute sulfuric acid into which platinum electrodes

were dipped. From there to modern fuel cell technology has been an exciting but long and tortuous path,
as outlined in Chapter 2.

1.2 Main Applications/The “Drivers”

It was not until the beginnings of space travel that fuel cells saw their first practical application in
generating electric power (and drinking water) in the Gemini and Apollo programs. Extensive research

Gregor Hoogers

Trier University of Applied Sciences,
Umwelt-Campus Birkenfeld

© 2003 by CRC Press LLC

efforts were made in those days, and many results from that work are still perfectly valid and have been
incorporated into modern fuel cell systems; others continue to inspire modern-day researchers. The work
of the early fuel cell researchers has produced an awesome wealth of knowledge. Chapter 2 covers some
of the spirit of their pioneering work.

So, is there a road from messy bench experiments involving strong acids to clean, safe equipment

suitable for use in homes and vehicles, and from “rocket science” to practical applications in everyday
life? And why fuel cells in the first place?

It now looks as though fuel cells will eventually come into widespread commercial use through three main

applications: transportation, stationary power generation, and portable applications. We will see that the
reasons for having fuel cells are rather different, at least in relative importance, in each of these three sectors.

1.2.1 Transportation

In the transportation sector, fuel cells are probably the most serious contenders to compete with internal
combustion engines (ICEs). They are highly efficient because they are electrochemical rather than thermal
engines. Hence, they can help to reduce the consumption of primary energy and the emission of CO

2

.

What makes fuel cells most attractive for transport applications is the fact that they emit zero or ultra-

low emissions. And this is what mainly inspired automotive companies and other fuel cell developers in
the 1980s and 1990s to start developing fuel-cell-powered cars and buses. Leading developers realized that
although the introduction of the three-way catalytic converter had been a milestone, keeping up the pace
in cleaning up car emissions further was going to be very tough indeed. After legislation such as California’s
Zero Emission Mandate was passed, people initially saw battery-powered vehicles as the only solution to
the problem of building zero emission vehicles. However, the storage capacity of batteries has turned out
to be unacceptable for practical use because customers ask for the same drive range that they are accustomed
to with internal combustion engines. In addition, the battery solution is unsatisfactory for another reason:
With battery-powered cars the location where air pollution is generated is merely shifted back to the electric
power plant that provides the electricity for charging. Once this was understood, people began to see fuel
cells as the only viable technical solution to the problem of car-related pollution.

Unfortunately, public perception of fuel cells subsequently became blurred, and all sorts of miracles

were expected from this fledgling new motor. It was supposed to make us entirely independent of fossil
fuels (since “it only needs hydrogen”), and undoubtedly many still believe that fuel-cell-powered cars
will run on a tank full of water.

When the first fuel-cell-powered buses rolled out of the labs of Ballard Power Systems, it soon became

clear that buses would make the fastest entry into the market because the hydrogen storage problem
already had been solved (compare Chapter 5). The prospects of fuel-cell-powered vehicles are fully
discussed in Chapter 10; the fueling issue, particularly for cars, is covered in Chapter 5.

Clearly, the automotive market is by far the largest potential market for fuel cells. When developers

started doing their first cost calculations, they realized they were in for steep competition against improved
internal combustion engines, hybrid cars, and other possible contenders. The main competitors of fuel-
cell-powered cars are discussed in Chapter 11. Complete fuel chains, for both automotive and stationary
systems, are analyzed in Chapter 12.

1.2.2 Stationary Power

Cost targets were first seen as an opportunity. The reasoning was that when fuel cells met automotive
cost targets, other applications, including stationary power, would benefit from this development, and a
cheap multipurpose power source would become available.

Stationary power generation is viewed as the leading market for fuel cell technology other than buses. The

reduction of CO

2

emissions is an important argument for the use of fuel cells in small stationary power

systems, particularly in combined heat and power generation (CHP). In fact, fuel cells are currently the only
practical engines for micro-CHP systems in the domestic environment (5–10 kW). The higher capital

© 2003 by CRC Press LLC

investment for a CHP system would be offset against savings in domestic energy supplies and — in more
remote locations — against power distribution cost and complexity. In the 50- to 500-kW range, CHP systems
will have to compete with spark or compression ignition engines modified to run on natural gas. So far,
several hundred 200-kW phosphoric acid fuel cell plants manufactured by ONSI (IFC) now have UTC fuel
cells been installed worldwide. The current range of stationary power systems is presented in Chapter 8.

1.2.3 Portable Power

The portable market is less well defined, but a potential for quiet fuel cell power generation is seen in
the 1-kW portable range and possibly, as ancillary supply in cars, so-called auxiliary power units (APUs).
The term “portable fuel cells” often includes grid-independent applications such as camping, yachting,
and traffic monitoring. The fuels under consideration vary from one application to another. In addition,
the choice of fuel is not the only way in which these applications vary. Different fuel cells may be needed
for each sub-sector in the portable market. Portable fuel cells are discussed in Chapter 9.

1.3 Low- and Medium-Temperature Fuel Cells

A whole family of fuel cells now exists that can be characterized by the electrolyte used — and by a related
acronym as listed in

Table 1.1

. All of these fuel cells function in the same basic way. At the anode, a fuel

(usually hydrogen) is oxidized into electrons and protons, and at the cathode, oxygen is reduced to oxide
species. Depending on the electrolyte, either protons or oxide ions are transported through the ion-con-
ducting but electronically insulating electrolyte to combine with oxide or protons to generate water and
electric power. A more detailed analysis of the power generation process is presented in Chapters 3 and 4.

Table 1.1 lists the fuel cells that are currently undergoing active development. Phosphoric acid fuel cells

(PAFCs) operate at temperatures of 200°C, using molten H

3

PO

4

as an electrolyte. The PAFC has been

developed mainly for the medium-scale power generation market, and 200 kW demonstration units have
now clocked up many thousands of hours of operation. However, in comparison with the two low-
temperature fuel cells, alkaline and proton exchange membrane fuel cells (AFCs, PEMFCs), PAFCs achieve
only moderate current densities.

The alkaline fuel cell, AFC, has one of the longest histories of all fuel cell types, as it was first developed

as a working system by fuel cell pioneer F.T. Bacon since the 1930s (compare Chapter 2). This technology
was further developed for the Apollo space program and was key in getting people to the moon. The
AFC suffers from one major problem in that the strongly alkaline electrolytes used (NaOH, KOH) adsorb
CO

2

, which eventually reduces electrolyte conductivity. This means that impure H

2

containing CO

2

(reformate) cannot be used as a fuel, and air has to “scrubbed” free of CO

2

prior to use as an oxidant in

an AFC. Therefore, the AFC has so far only conquered niche markets, for example space applications
(the electric power on board the space shuttle still comes from AFCs).

Some commercial attempts has been made to change this. Most notably, ZETEK/ZEVCO started in

the mid-1990s to reexamine the AFC technology developed by ELENCO, a Belgian fuel cell developer
that had previously gone into bankruptcy. A number of ZETEK’s activities attracted extensive publicity.
In the late 1990s, ZETEK presented a so-called fuel-cell-powered London taxi. Little is known about the
technology of the engine in this vehicle. However, the AFC employed had a power range of only 5 kW,
which means it cannot be the main source of power and merely served as a range extender to some on-
board battery. Other recent activities based on AFC technology include the construction of trucks (by
ZEVCO) and boats (etaing GmbH). A big advantage of the AFC is that it can be produced rather cheaply.
This may help this technology penetrate the highly specialized market for indoor propulsion systems,
such as airport carrier vehicles, and possibly a number of segments in the portable sector.

The proton exchange membrane fuel cell, PEMFC, takes its name from the special plastic mem-

brane

1

that it uses as its electrolyte. Robust cation exchange membranes were originally developed

1

Therefore, it is also known as a solid polymer fuel cell (SPFC).

© 2003 by CRC Press LLC

for the chlor-alkali industry by DuPont and have proved instrumental in combining all the key parts
of a fuel cell, anode and cathode electrodes and the electrolyte, in a very compact unit. This membrane
electrode assembly (MEA), not thicker than a few hundred microns, is the heart of a PEMFC and,
when supplied with fuel and air, generates electric power at cell voltages up to 1 V and power densities
of up to about 1 Wcm

–2

.

The membrane relies on the presence of liquid water to be able to conduct protons effectively, and

this limits the temperature up to which a PEMFC can be operated. Even when operated under pressure,
operating temperatures are limited to below 100°C. Therefore, to achieve good performance, effective
electrocatalyst technology (Chapter 6) is required. The catalysts form thin (several microns to several
tens of microns) gas-porous electrode layers on either side of the membrane. Ionic contact with the
membrane is often enhanced by coating the electrode layers using a liquid form of the membrane ionomer.

The MEA is typically located between a pair of current collector plates with machined flow fields for

distributing fuel and oxidant to anode and cathode, respectively (compare

Fig. 4.2 in Chapter 4

). A water

jacket for cooling may be inserted at the back of each reactant flow field followed by a metallic current
collector plate. The cell can also contain a humidification section for the reactant gases, which helps to
keep the membrane electrolyte in a hydrated, proton-conduction form. The technology is given a more
thorough discussion in Chapter 4 (compare Section 10.2.3).

Having served as electric power supply in the Gemini space program, this type of fuel cell was brought

back to life by the work of Ballard Power Systems. In the early 1990s, Ballard developed the Mark 5 fuel
cell stack [

Fig. 10.4(a)

] generating 5 kW total power at a power density of 0.2 kW per liter of stack

volume. With the Mark 900 stack [

Fig. 10.4(b)

] jointly developed by Ballard and DaimlerChrysler in late

1990s, the power density had increased more than fivefold to over 1 kW/l. At a total power output of 75
kW, this stack meets the performance targets for transportation (compare Section 10.2.3).

PEMFCs are also being developed for stationary applications. In the 250-kW range, Ballard Generation

Systems is currently the only PEMFC-based developer. More recently, the micro-CHP range has been
claimed by a wide range of developers. Here, high power density is not the most crucial issue. In a

TABLE 1.1

Currently Developed Types of Fuel Cells and Their Characteristics and Applications

Fuel Cell Type

Electrolyte

Charge
Carrier

Operating

Temperature

Fuel

Electric

Efficiency

(System)

Power Range/

Application

Alkaline FC

(AFC)

KOH

OH

–

60–120°C

Pure H

2

35–55%

<5 kW, niche

markets
(military,
space)

Proton

exchange
membrane FC
(PEMFC)

a

Solid polymer

(such as
Nafion)

H

+

50–100°C

Pure H

2

(tolerates
CO

2

)

35–45%

Automotive,

CHP (5–250
kW),
portable

Phosphoric acid

FC (PAFC)

Phosphoric

acid

H

+

~220°C

Pure H

2

(tolerates CO

2

,

approx. 1%
CO)

40%

CHP (200 kW)

Molten

carbonate FC

(MCFC)

Lithium and

potassium
carbonate

CO

3

2–

~650°C

H

2

, CO, CH

4

,

other
hydrocarbons
(tolerates
CO

2

)

>50%

200 kW–MW

range, CHP
and stand-
alone

Solid oxide FC

(SOFC)

Solid oxide

electrolyte
(yttria,
zirconia)

O

2–

~1000°C

H

2

, CO, CH

4

,

other
hydrocarbons
(tolerates
CO

2

)

>50%

2 kW–MW

range, CHP
and stand-
alone

a

Also known as a solid polymer fuel cell (SPFC).

© 2003 by CRC Press LLC

(domestic) micro-CHP system, high electric efficiency and reliability count. The overall goal is the most
economic use of the fuel employed, usually natural gas, in order to generate electric power and heat.

1.4 High-Temperature Fuel Cells

Two high-temperature fuel cells, solid oxide and molten carbonate (SOFC and MCFC), have mainly been
considered for large-scale (MW) stationary power generation. In these systems, the electrolytes consist of
anionic transport materials, as O

2–

and CO

2–

3

are the charge carriers. These two fuel cells have two major

advantages over low-temperature types. First, they can achieve high electric efficiencies; prototypes have
achieved over 45%, with over 60% currently targeted. This makes them particularly attractive for fuel-
efficient stationary power generation. Second, the high operating temperatures allow direct internal
processing of fuels such as natural gas. This reduces the system complexity compared with low-temper-
ature power plants, which require hydrogen generation in an additional process step. The fact that high-
temperature fuel cells cannot easily be turned off is acceptable in the stationary sector, but most likely
only there.

A full account of the technology and the merits of fuel cells in stationary power generation is given

in Chapter 8.

1.5 Liquid Fuel: The Direct Methanol Fuel Cell

No doubt one of the most elegant solutions to the fueling problem would be to make fuel cells operate
on a liquid fuel. This is particularly so for transportation and the portable sector. The direct methanol
fuel cell (DMFC), a liquid- or vapor-fed PEM fuel cell operating on a methanol/water mix and air,
therefore deserves careful consideration. The main technological challenges are the formulation of better
anode catalysts to lower the anode overpotentials (currently several hundred millivolts at practical current
densities), and the improvement of membranes and cathode catalysts in order to overcome cathode
poisoning and fuel losses by migration of methanol from anode to cathode. Current prototype DMFCs
generate up to 0.2 Wcm

–2

(based on the MEA area) of electric power, but not yet under practical operating

conditions or with acceptable platinum loadings. However, the value is sufficiently close to what has been
estimated to be competitive with conventional fuel cell systems including reformers and reformate cleanup
stages. The current status of the DMFC is discussed in Chapter 7, and portable applications are discussed
in Chapter 9.

© 2003 by CRC Press LLC