Mench M.M. Fuel Cell Engines

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Table 1.1 Fuel Cell Types, Descriptions, and Basic Data

Fuel Cell

Type

Electrolyte

Material

Operating

Temperature

(

◦

C) Major Poison Advantages Disadvantages

Most Promising

Applications

Alkaline

fuel cell

Solution of

potassium

hydroxide in

water

60–250

High efﬁciency, low

oxygen readuction

reaction losses

Must run on pure oxygen

without CO

contaminant

Space applications

with pure O

available

Phosphoric

acid fuel

cell

Solution of

phosphoric acid

in porous silicon

carbide matrix

160–220 Sulfur, high levels

of CO

1–2% CO tolerant,

good-quality waste heat,

demonstrated durability

Low power density,

expensive, platinum

catalyst used, slow

start-up, loss of

electrolyte

Premium stationary

power

Solid oxide

fuel cell

Yttria (Y

)

stabilized

zirconia (ZrO

)

600–1000 Sulfur CO tolerant, fuel ﬂexible,

high-quality waste heat,

inexpensive catalyst

Long start-up time,

durability under thermal

cycling, inactivity of

electrolyte below

∼600

◦

Stationary power

with

cogeneration,

continuous-power

applications

Molten

carbonate

fuel cell

Molten alkali metal

(Li/K or Li/Na)

carbonates in

porous matrix

600–800 Sulfur CO tolerant, fuel ﬂexible,

high-quality waste heat,

inexpensive catalyst

Electrolyte dissolves

cathode catalyst,

extremely long start-up

time, carbon dioxide

must be injected to

cathode, electrolyte

maintenance

Stationary power

with

cogeneration,

continuous-power

applications

Polymer

electrolyte

fuel cell

Flexible solid per-

ﬂuorosulfonic

acid polymer

30–100 CO, Sulfur, metal

ions, peroxide

Low-temperature

operation, high

efﬁciency, high H

power density, relatively

rapid start-up

Expensive catalyst,

durability of components

not yet sufﬁcient,

poor-quality waste heat,

Intolerance to CO,

thermal and water

manangement

Portable,

automotive, and

stationary

applications

Modern AFCs < 100

◦

Includes direct methanol fuel cell and direct alcohol fuel cells.

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1.4 Classiﬁcation of Fuel Cells 11

Figure 1.9 UTC Power supplies fuel cell bus powerplants for transit programs in the United States

and Europe. The fuel economy of transit buses powered by a UTC Power PureMotion

fuel cell

system is two times better than a diesel-powered bus. Fuel cell-powered buses also emit no harmful

tailpipe emissions and operate quietly. (Image Courtesy of UTC Power Corporation.)

is fueled either by pure hydrogen or from a diluted hydrogen mixture generated from a

fuel reformation process. A stack power density of greater than 1.3 kW/L is typical. Since

the operating temperature is from room temperature to ∼80

◦

C, a noble metal platinum

catalyst is typically used on the anode and cathode. Figure 1.9 is a picture of a PEFC engine

developed by UTC Power for city bus applications. The H

PEFC has many technical issues

that complicate performance and control. Besides issues of manufacturing, ancillary system

components, cost, and market acceptance, the main remaining technical challenges for the

fuel cell itself include (1) water and heat management, (2) durability, and (3) freeze–thaw

cycling and frozen-start capability.

Direct Methanol Fuel Cell The liquid-fed direct methanol fuel cell (DMFC) is generally

seen as the most viable alternative to lithium ion batteries in portable applications because

DMFC systems require less ancillary equipment and can therefore potentially be more

simpliﬁed compared to an H

PEFC. Additionally, the usc of a liquid fuel simpliﬁes

storage. The DMFCs can potentially compete favorably with advanced Li ion batteries

(which currently power many wireless portable applications) in terms of gravimetric energy

density of ∼120–160 Wh/kg and volumetric energy density of ∼230–270 Wh/L. While both

PEFCs and DMFCs are strictly PEFCs (both use the same ﬂexible polymer electrolyte),

the DMFC feeds a liquid solution of methanol and water to the anode as fuel. The additional

complexities of the low-temperature methanol oxidation reaction prevent the DMFC from

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12 Introduction to Fuel Cells

(a)

(b)

Figure 1.10 Photograph of (a) PDA/smart phone concept model and (b) a handheld entertainment

system concept model. Both are powered by Mobion

fuel cell technology, which uses a direct

methanol fuel cell for power. (Image Courtesy of Mechanical Technology, Inc. (MTI).)

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1.4 Classiﬁcation of Fuel Cells 13

obtaining the same level of fuel cell power density as the H

PEFC. Figure 1.10 is a picture

of a portable DMFC developed by MTI Micro for hand-held power application.

Four main technical issues affecting performance remain: (1) two-phase ﬂow man-

agement in the anode and cathode, (2) methanol crossover, (3) poor catalyst activity, and

(4) high catalyst loading. While signiﬁcant progress has been made by various groups

to develop optimized catalysts, total noble metal catalyst loading is still on the order of

10 mg/cm

. Typically a platinum–ruthenium catalyst is utilized on the anode for methanol

oxidation, and a platinum catalyst is utilized on the cathode as in the H

PEFC [3]. The

DMFC is discussed in greater detail in Chapter 6.

Solid Oxide Fuel Cell The SOFC represents a high-temperature fuel cell system with a

solid ceramic electrolyte. The historical operating temperature of SOFC systems is around

800–1000

◦

C, although developing technology has demonstrated 500

◦

C operation [4], where

simpliﬁed system sealing and materials solutions are feasible. Due to the elevated operating

temperature, the catalysts used are non–noble metal and other inexpensive raw materials.

High electrolyte temperature is required to ensure adequate ionic conductivity (of O

2−

)

in the solid-phase ceramic electrolyte. Operating efﬁciencies as high as 60% have been

attained for a 220-kW cogeneration system [5]. Figure 1.11 is a schematic of the Siemens

Westinghouse 100-kW tubular SOFC system. It is interesting to realize that most of the

volume of larger fuel cell systems is not in the fuel cell itself but in ancillary components,

including fuel processing and power conditioning systems.

Figure 1.11 Siemens 100 kW tubular solid oxide fuel cell and cogeneration system. The system

has a peak power of ∼140 kW, typically feeding 109 kW into the local grid and 64 kW of hot water

into the local district heating system. at an electrical efﬁciency of 46% [6]. (Image courtesy Siemens

Power Generation.)

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14 Introduction to Fuel Cells

There has been much recent development in the United States on SOFC systems,

incubated by the Department of Energy Solid State Energy Conversion Alliance (SECA)

program. The 10-year goal of the SECA program (started in the fall of 1999) is to develop

3–10-kw SOFC units at <$400/kW with rated performance achievable over the lifetime of

the application with less than 0.1% loss per 500 h operation by 2021 [7].

Besides manufacturing and economic issues, the main technical limitations of the

SOFC include achieving a reduced operating temperature, controling start-up time, dura-

bility, and proper cell-sealing. The SOFC is discussed in depth in Chapter 7.

Molten Carbonate Fuel Cell Molten carbonate fuel cells are commercially available from

several companies, including a 250-kW unit from FuelCell Energy in the United States and

several other companies in Japan. Some megawatt-sized demonstration units are installed

worldwide based on natural gas or coal-based fuel sources which can be internally reformed

within the anode of the MCFC. Figure 1.12 shows a picture of a 250-kWe net MCFC

developed by FuelCell Energy and installed at Yale University. MCFCs operate at high-

temperature (600–700

◦

C) with a molten mixture of alkali metal carbonates (e.g., lithium

and potassium) or lithium and sodium carbonates retained in a porous ceramic matrix

through a delicate balance of gas-phase and capillary pressure forces. Technical details of

the MCFC are discussed in detail in Chapter 7. A major advantage of the MCFC is the lack

of precious metal catalysts, which greatly reduce the system raw material costs. Original

development on the MCFC was mainly funded by the U.S. Army in the 1950s and 1960s.

Figure 1.12 FuelCell Energy’s DFC

300A 250 kWe net molten carbonate fuel cell system

installed at Yale University in Connecticut. (Image courtesy of FuelCell Energy, Inc.)

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1.4 Classiﬁcation of Fuel Cells 15

The U.S. Army desired operation of power sources from logistic fuel already available,

thus requiring high temperatures with internal fuel reformation that can be provided by

the MCFC. During this period, signiﬁcant advances of this liquid electrolyte alternative

to the SOFC were made [8]. Development waned somewhat after this early development,

but advances have continued and MCFC commercialization has been achieved. The main

disadvantages of MCFCs include (1) complex electrolyte management and loss through

ﬁnite vapor pressure, (2) extremely long start-up time (the MCFC is generally suitable only

for continuous power operation), (3) durability, and (4) carbon dioxide injection into the

anode is needed to maintain electrolyte stability.

Phosphoric Acid Fuel Cell The PAFC was originally developed for commercial appli-

cation in the 1960s. The PAFC has an acidic, mobile (liquid) electrolyte of high con-

centration phosphoric acid contained by a porous silicon carbide ceramic matrix and

operates at around 160–220

◦

C. A 200-kW PAFC array that powers the Verizon call

routing center in New York is shown in Figure 1.13. Like the MCFC, the electrolyte

is bound by capillary and gas pressure forces between porous electrode structures. The

PAFC is in many ways similar to the PEFC, except the acid-based electrolyte is in liq-

uid form and the operating temperature is slightly higher. Over two-hundred 200-kW

commercial PAFC units were developed and sold by United Technologies Corporation

through several different divisions and subsidiaries, and many are still in operation.

Figure 1.13 The Verizon call routing center in Garden City, New York, is home to the largest U.S.

commercial fuel cell installation of its kind. The fuel cells from UTC Power generate 200 kilowatts

each, providing a total of 1.4 megawatts of clean power to the center. (Image Courtesy of UTC Power

Corporation.)

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16 Introduction to Fuel Cells

However, ubiquitous commercial application has not been achieved, primarily due to the

high cost of approximately $4500/kW, about ﬁve times greater than cost targets for conven-

tional stationary applications [9]. The main advantages of the PAFC include (1) the high

operating temperature allows operation with 1–2% CO in fuel stream, (2) the highly con-

centrated acid electrolyte does not need water for conductivity, making water management

very simple compared to the PEFC, and (3) the demonstrated long life and commercial

success for premium stationary power of the PAFC. Besides the high system cost, the main

technical disadvantages of the PAFC include (1) a bulky, heavy system compared to PEFC

with area-speciﬁc power less than half of the PEFC (0.2–0.3 W/cm

[10], (2) continued use

of platinum catalyst with nearly the same loading as PEFCs, (3) the relatively long warm-up

time until the electrolyte is conductive at ∼160

◦

C (although warm-up time is much less

than the MCFC or SOFC), and (4) the liquid electrolyte has ﬁnite vapor pressure, resulting

in continual loss of electrolyte in the vapor phase. Modern PAFC design includes cooling

and condensation zones to mitigate this loss. The PAFC is discussed in greater detail in

Chapter 7.

Alkaline Fuel Cell Alkaline fuel cells utilize a solution of potassium hydroxide in wa-

ter as an alkaline, mobile (liquid) electrolyte. Alkaline fuel cells were originally devel-

oped as an auxillary power unit APU for space applications by the Soviet Union and the

United States in the 1950s and served on the Apollo program as well as the Space Shut-

tle orbiter. A 12-kW AFC used to provide power and potable water for astronauts in the

Space Shuttle orbiter is shown in Figure 1.14. The AFCs were chosen for space applica-

tions for their high efﬁciency and robust operation. The AFC operates around 60–250

◦

with greatly varied design and operating conditions. Modern designs tend to operate at

the lower range of temperature and pressure near ambient conditions. The primary ad-

vantages of the AFC are the lower cost of materials and electrolyte and high operating

efﬁciency (60% demonstrated for space applications) due to use of an alkaline electrolyte.

Figure 1.14 12-kW fuel cell power plant for the Space Shuttle Orbiter. Three units power the

orbiter while in space as well as provide the drinking water for the astronauts. (Image Courtesy of

UTC Power Corporation.)

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1.5 Potential Fuel Cell Applications and Markets 17

For alkaline electrolytes, the oxidizer reduction reaction (ORR) kinetics are more efﬁ-

cient than acid-based electrolytes (e.g., PEFC, PAFC). Many space applications utilize

pure oxygen and hydrogen for chemical propulsion, so the AFC was well suited as an

APU. However, the alkaline electrolyte suffers an intolerance to even small fractions of

carbon dioxide (CO

) found in air which react to form potassium carbonate (K

)in

the electrolyte, gravely reducing performance over time. For terrestrial applications, CO

poisoning has limited lifetime of AFC systems to well below that required for commercial

application, and ﬁltration of CO

has proven too expensive for practical use. Due to this

limitation, relatively little commercial development of the AFC beyond space applications

has been realized. Some recent development of alkaline-based solid polymer electrolytes

is underway, however. The AFC is discussed in greater detail in Chapter 7.

Other Fuel Cells Many other fuel cell systems exist, and new versions are constantly

being developed. Many of these are simply existing fuel cell systems with a different fuel.

For example, PEFCs based on a direct alcohol solution offer alternatives to DMFCs for

portable power and include those based on formic acid [11], dimethyl ether [12], ethylene

glycol, dimethyl oxalate, and other so-called direct alcohol fuel cells (DAFCs) [13, 14].

A completely different concept is the biologically based fuel cell. Biologically based

fuel cells use biocatalysts for conversion of chemical to electrical energy and can be

classiﬁed into two basic categories: (1) microbial fuel cells (MFCs) and (2) enzyme-

based fuel cells. In the MFC, electricity is generated by anerobic oxidation of organic

material by bacteria. The catalytic activity and transport of protons are accomplished using

biological enzymes or exogenous mediators [15–17]. Although performance is relatively

quite low, on the order of 0.1–1 mA/cm

, the potential for generating some power, or

simply power-neutral decomposition and treatment of domestic waste matter, currently a

multibillion-dollar cost to society, is potentially quite signiﬁcant.

The enzyme-based biological fuel cell has signiﬁcantly greater power density

(1–10 mA/cm

) than the microbial fuel cell, although power produced is still orders of

magnitude lower than a conventional precious metal catalyzed H

PEFC [17]. However,

enzymatic fuel cells have distinct advantages in terms of potential cost and operation at

ambient temperature in near-neutral-pH environments. Enzymatic fuel cells are envisioned

as implantable power devices in humans or as using environmentally derived fuel from tree

saps for long-term remote sensor applications [18]. While biologically based fuel cells are

probably the least-developed fuel cell power source, the unique aspects of the catalytic pro-

cess and potential for natural sugar-based power are intriging. Another potentially interest-

ing application on which this author has pondered is a weight loss fuel cell where blood sugar

would be used to power an external fuel cell device, effectively burning calories with no

physical exercise required. The feasibility of this concept is further explored in Problem 22.

1.5 POTENTIAL FUEL CELL APPLICATIONS

AND MARKETS

Fuel cells have the potential to replace existing power sources for many applications. Figures

1.15 and 1.16 show the exponential progression of commercial stationary power fuel cell

installations and hydrogen refueling stations worldwide. Although the trends look promis-

ing, they currently represent only an inﬁnitesimal fraction of the fueling infrastructure

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18 Introduction to Fuel Cells

Figure 1.15 Commercial large (>10-kW) stationary power fuel cell installations under operation

versus year. (Data adapted from Ref. [19].)

required for practical widespread use. Fuel generation, storage, and delivery infrastructure

are still major barriers that must be overcome. Hydrogen generation, storage, and delivery

are discussed in Chapter 8. Hydrogen infrastructure is a vast and speculative subject, and

various summaries can be found [20, 21]. The most likely consumer applications for fuel

cells include portable (0–100-W), stationary (0–25-kW), and transportation (∼100-kW)

applications. Each market has unique demands that tend to be more suited to a particular

type of fuel cell. For portable applications, high system power density and simplicity are de-

sired over efﬁciency and cost. For stationary applications, durability and high efﬁciency are

higher priorities. For transportation applications, compact size, rapid start-up, robustness,

and high efﬁciency are the primary technical goals.

Figure 1.16 Cumulative worldwide hydrogen fueling stations. Upper bars in 2005 and 2006 are

anticipated station installations. (Data adapted from Ref. [22].)

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1.5 Potential Fuel Cell Applications and Markets 19

Figure 1.17 Toshiba 100 mWe micro direct methanol fuel cell which weighing 8.5 g (0.3 oz) [23].

(Image Courtesy Toshiba Corporation.)

Portable Applications Perhaps where fuel cells show the most promise for ubiquitous

near-term implementation is in portable power (0–100-W) applications, such as cell phones

and laptop computers. Current battery technology has not yet provided the energy density re-

quired for long-term operation, and recharging is time consuming. Additionally, the cost of

existing premium power battery systems is already on the same order as contemporary fuel

cells, with additional development anticipated. With replaceable fuel cartridges, portable

fuel cell systems have the additional advantage of instant and remote rechargeability that can

never be matched with secondary battery systems. A hand-held DMFC for portable power

developed by Toshiba is planned for production. The 8.5-g DMFC shown in Figure 1.17

is rated at 100 mW continuous power (up to 20 h) and measures 22 mm × 56 mm × 4.5

mm, including a maximum of 9.1 mm for the concentrated methanol fuel tank [24]. As the

wireless economy progresses, demand for higher power, smaller, and instantly recharge-

able technologies will undoubtedly continue to push forward development of portable

fuel cells.

Stationary and Distributed Power Applications Stationary (1–500-kW) applications in-

clude power units for homes or auxiliary and backup power generation units. Stationary

applications are designed for nearly continuous use and therefore must have far greater

lifetime than automotive units, although operation at a near-continuous steady state is

advantageous for durability. Stationary devices typically range from 1 kW temporary or

auxiliary power generator units, examples of which are the Ballard fuel cell generator units

shown in Figure 1.18, to 100-kW systems for regular power of buildings. Unlike the portable

fuel cell, where ancillary components are reduced as much as possible, the stationary fuel

cell system is not similarly constrained, and typically has an array of components to achieve

high-efﬁciency, durable operation.

A plot showing the estimated number of demonstration and commercial units in the

stationary power category from 1985 to 2002 is given in Figure 1.19. Not surprisingly,

the exponential growth in the number of online units follows a similar qualitative trend

to the available patents granted for various fuel cell technologies shown in Figure 1.1.

The early rise in stationary units in 1997 was primarily a result of PAFC systems sold

by United Technologies Corporation, although recently most additional units have been

PEFCs from various manufacturers. Data are estimated from the best available compilation