A renaissance for alkaline fuel cells - Fuel Cell Markets

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Alkaline fuel cells: fact and fiction
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Organic materials trap hydrogen
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THIS IS ASTORYof science and technology; of discoveries in
basic research posing a challenge to applied R&D; of 100 years
of daring experiments and false starts finally leading to the first
practical fuel cell in the 1930s. This is a story of science and
politics intertwined; companies, large and small, following the
money trail; institutions and industrial enterprises lobbying,
competing for and, in the end, depending on public money to
support innovation and advancement. And this is a story of
those investments giving impetus and direction to the roadmap
for fuel-cell R&D – more often than not, to the detriment
of alkaline-fuel-cell (AFC) technology.
After their initial spectacular successes in the US and Russian
space programmes in the 1960s and 1970s, AFCs have been
largely sidelined for the past 30 years (see “A brief history of
AFC applications”, p20). The view elaborated in this article is
that a renaissance of AFC technology is long overdue. The basis
of the argument is clear enough: AFCs exhibit excellent electrochemical
performance over a range of temperatures and
pressures; they can be built from inexpensive materials – carbon,
plastic and base metals; they employ a cheap electrolyte,
start instantly and work even in deep sub-zero temperatures;
they do not depend on expensive platinum catalysts (and are
therefore tolerant of a variety of impurities); and they come
with modest balance-of-plant requirements 1–5 .
Equally, in spite of the fact that AFCs have already notched up
competitive cell and system efficiencies, there is still enormous
room for evolutionary engineering improvement. The question
is, will appropriate levels of investment and industrial support
be forthcoming?
AFCs in focus
The chemistry of the AFC is summarized in terms of the anode
and cathode reactions. Hydrogen combines with hydroxyl ions
at the anode. Subsequently, the electrons generated in this reaction
travel round an external circuit where they react with oxygen
to form new hydroxyl ions (equation 1).
Fundamentally, AFC design is based on one of four cell/stack
architectures and on the selection of appropriate electrode
materials therein. The path to a cost-effective fuel cell is the
common link between these architectures.
FEATURE: INNOVATION
A renaissance for
alkaline fuel cells
ERICH GUELZOW, JIRI K NOR, PETER K NOR AND MATHIAS SCHULZE
The alkaline fuel cell has been the poor relation for too long. In what will be seen as a contrarian take by many of
their peers in the development community, the authors argue for a re-evaluation of this versatile technology.
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They’ve got the power: while AFCs can perform on a par with
competing low-temperature systems, they promise lower capital
and operational costs if developed into a volume technology.
Equation 1: AFC chemistry
Anode: 2H 2 +4OH – → 4H 2O+4e –
Cathode: O 2 +4e – +2H 2O → 4OH –
● Free-electrolyte cells: This is the most common cell format, with
two gas chambers and an electrolyte chamber. Gas-diffusion
electrodes separate reactant gases and the free electrolyte.
● Immobilized-electrolyte cells: In space applications, AFCs with
immobilized electrolyte allow position-independent operation
in a low-gravity environment. In principle, the design is the
same as free-electrolyte cells, but in this case the electrolyte
space is filled with soaked matrix material (like a sponge).
● ELOFLUX cells: The ELOFLUX principle is based on flexible

INNOVATION
A brief history of AFC applications
An ELOFLUX fuel-cell cell stack
from German developer
Gaskatel. The ELOFLUX
principle is based on flexible
electrodes and a liquid
electrolyte. The electrodes are
extremely porous, with both
hydrophobic and hydrophilic
pores allowing the transport of
gases and electrolyte, and with
a thin separator between
(approximately 0.3 mm).
It’s within the US space programme that AFCs have enjoyed their
greatest successes. In the 1960s and early 1970s, AFCs were used on
nine Apollo Moon flights and three Sky Labs – a total of 54 missions.
They have also been employed on the Space Shuttle since its first
flight in 1975 and are still in use today.
In terms of more down-to-earth applications, a number of isolated
and diverse projects have relied on the AFC to deliver power from
hydrogen. In the mid-1960s, for example, the US tractor maker Allis
Chalmers pioneered the first “civilian” usage: a 15 kW power module
for one of its vehicles. Also in the US, Union Carbide developed a
number of initial uses, including a General Motors “Electrovan”
powered by a 125 kW AFC in 1967. Three years later, one of Carbide’s
inventors, Karl Kordesch, completed an Austin A-40 hybrid vehicle
that relied on a 96 V, air-breathing AFC in combination with a leadacid
battery. Kordesch operated the Austin A-40 for three years in city
traffic, the fuel cell being shut-down and restarted as required by
normal operation.
In terms of European industrial activity, Elenco of Belgium
completed a number of AFC systems during the 1970s and 80s,
including a 52 kW system for the Belgian Geological Service. In 1994,
the same group completed work on a 200 kW AFC system for a
hybrid bus. The following year Elenco was closed, continuing
operations as Zevco and ZeTek (both now defunct as well). These
companies demonstrated an AFC-powered London taxi in 1999 and
contributed to the completion of the Hydra AFC-powered boat,
which was launched in Germany in the summer of 2000. Elsewhere in
Europe, Siemens of Germany dedicated considerable effort to
developing a fuel-cell system for submarines, work that was
completed in the early 1990s.
Back in the here and now there are several small companies and
research institutes currently active in AFC technology. They include:
electrodes and a liquid electrolyte. The electrodes are extremely
porous, with both hydrophobic and hydrophilic pores allowing
the transport of gases and electrolyte, and with a thin separator
between (approximately 0.3 mm). In contrast with the
other cell concepts, the electrolyte is transported through the
separator and the electrodes in the direction perpendicular to
the electrode plane.
This AFC demonstrator from
Hydrocell is equipped with an
internal metal-hydride storage
tank. The Finnish company is
also developing portable AFC
units to supply power to
summer residences and lowenergy
buildings. Because of
the internal hydrogen storage,
the device is sometimes called a
“fuel-cell battery”, as it is as
easy to use as a lead acid battery.
● Astris Energi (Mississauga, Ontario) develops systems (ranging in
output power from a few hundred watts to several kilowatts) for light
utility vehicles, portable applications and stationary systems
targeting backup power and cogeneration. One such generator, the
Model E8, is a 2.4 kW system that uses two 1.2 kW stacks; system
efficiency is >50% and rated life is 2000 h. Astris has established an
R&D laboratory and manufacturing facility in the Czech Republic.
● Deutsches Zentrum für Luft- und Raumfahrt (DLR) is a leading German
aerospace organization. Its Institute of Technical Thermodynamics in
Stuttgart has a long-running R&D programme on low-temperature
fuel cells, including PEMFCs and AFCs. There are currently more than
a dozen researchers working on various aspects of AFC technology,
including catalysis, gas-diffusion electrodes, electrode production
methodology, development of stacks, modelling and testing.
● Gaskatel (Kassel, Germany) works on cells, stacks and systems based
on the ELOFLUX principle. The ELOFLUX cells are based on
inexpensive materials – e.g. carbon and nickel – and deliver a power
output up to 0.5 kW/l.
● Ovonic Fuel Cell Company (Rochester Hills, Michigan, US) is
developing a metal-hydride fuel cell that uses alkaline electrolyte.
Published benefits include instant start, intrinsic operation down to
–20 ºC and operation without hydrogen fuel for several minutes.
● Oy Hydrocell (Finland) manufactures portable AFC systems for the
supply of power to summer residences and low-energy buildings.
The fuel cells are normally equipped with internal metal-hydride
hydrogen storage (up to 40 dm 3 of hydrogen). There are two storage
units: HC-MH200 is the size of a small cola can and contains 430 Wh
(12 V DC, 36 Ah) of energy and the HC-MH1200 is the size of a small
fire extinguisher and contains 2600 Wh (12 V DC, 220 Ah) of energy.
● ZAO Independent Power Technologies (Moscow, Russia) is a designer
and manufacturer of AFC-based electrochemical generators.
● Falling-film cell: This format was developed by Hoechst,
Germany, originally to increase the efficiency of chlor-alkali
electrolysis. The electrolyte flows between the electrodes from
top to bottom. Owing to the unhindered gravitational movement
of the electrolyte in this way, the typical height-dependent
hydrostatic-pressure distribution does not develop – so
hydrostatic pressure in the free-falling column of liquid is the
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1. Electrode aging
potential vs. Hg/HgO (mV) at 100 mA/cm 2
100
50
0
–50
–100
–150
sample 76 (CO 2 contaminated gas)
sample 75
sample 74 (CO 2 contaminated gas)
sample 73
–200
0 1000 2000 3000 4000
time of operation of an Ag–GD electrode at 150 mA/cm 2 (h)
Aging of a silver AFC cathode with either CO 2-containing
oxygen (5%) or pure oxygen. The degradation in performance in
both modes is comparable, which means that CO 2 neither
enhances the degradation process nor does it induce a new
detrimental effect. Studies on AFC anodes using nitrogen
adsorption measurements, electron microscopy and X-ray
photoelectron spectroscopy also show no deleterious effects.
same at the inlet as at the outlet, and any place in between. The
velocity of the “free fall” is limited only by the turbulent friction
between the falling film and the electrode surface. Highly
porous electrodes are used to separate the electrolyte and the
gas phases, without the need for a dedicated separator. Cells in
excess of 1 m 2 have been built using this format, with bipolar
cell interconnections.
Common to all AFC architectures are the selection of catalyst
material(s) and the electrode production process. For example,
Raney-nickel (anode) and a silver catalyst called SILFON (cathode)
have been used for more than 20 years, though there are
plenty of variations on this theme. One method of preparation
of gas-diffusion electrodes is a rolling process in which catalysts
and polytetrafluoroethylene (PTFE) undergo an initial
reactive mixing process, typically using a knife-mill. This
approach yields a bifunctional material system with hydrophilic
and hydrophobic pores. The mixed material is subsequently
rolled onto a metallic mesh that supports the
electrodes, which are then assembled into cells.
Debunking the myths
There are all sorts of myths and misconceptions associated
with AFCs. It is said that AFCs are expensive and useful only for
applications in space; that they require very pure and costly gas
feeds; that they cannot work using air; that they are poisoned
by carbon dioxide; that the liquid electrolyte is a serious drawback;
that carbon monoxide is a problem. So what is true and
what is false? One thing at least is clear: each type of fuel-cell
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INNOVATION
chemistry has its individual advantages and disadvantages, but
the disadvantages stated here for AFCs are grossly exaggerated
or, often, simply incorrect.
Let’s start with cost. AFCs yield savings because their catalysts
need not be based on expensive noble metals – a result of
the lower corrosivity of the basic environment relative to the
acidic one. On top of this, the liquid electrolyte, usually an
aqueous solution of potassium hydroxide, is cheap – just a few
euros per kilogram. Taken together, the catalyst and electrolyte
represent a considerable cost advantage in comparison with
proton exchange membrane fuel cells (PEMFCs).
However, AFCs are not just about cheaper components. One
of the big pluses in terms of system design is the absence of
process-gas humidification – a significant design headache in
current-generation PEMFCs. Admittedly, some will see the liquid
electrolyte as a disadvantage because AFCs need a circuit
for it. In practice, though, a similar circuit is also needed in
PEMFC systems, in this case for heat management of the stack.
Effectively, the circulating electrolyte is a “self-healing” layer of
liquid in which any defect (in the form of a bubble, for example)
will not remain stationary. Consequently, failure modes (like
hot-spots) common to PEMFCs do not exist. Clearly, the AFC
electrolyte circuit poses some unique design/engineering challenges
owing to its corrosive nature, though the dual purpose of
the circuit (heat and water management) more than offsets those
demands. Additional benefits of this architecture are the elimination
of the water-management issues common to PEMFCs,
which in turn ensures the AFC’s inherent freeze-start capability.
The bogeyman: carbon dioxide
There are some 350 ppm of CO 2 in the air that we breathe.
What’s more, it is commonly accepted that CO 2 intolerance is
the most pronounced disadvantage of air-breathing AFCs. But
is this really the “show-stopper” that some would have us
believe? Karl Kordesch, a pioneering battery and fuel-cell
inventor at Union Carbide, US, noted as early as 1970 that the
issue of CO 2 intolerance in AFCs appeared to be overstated. He
found that the rate of absorption of CO 2 in the electrolyte was
negligible at low concentrations and recommended using a
simple dry scrubber with soda lime to counter any take-up. The
same observations were later published by teams at Elenco in
Belgium, Astris Energi in Canada and at the Royal Institute of
Technology in Stockholm, Sweden.
The CO 2 myth persists, however. To investigate the mechanisms
of this CO 2 “poisoning”, a team of scientists at the
Deutsches Zentrum für Luft und Raumfahrt (DLR) in Stuttgart,
Germany, has carried out an in-depth study of the phenomenon
6 . More specifically, AFC gas-diffusion electrodes were
operated in a half-cell configuration for up to several thousand
hours with a constant load of 150 mA/cm 2 . The reactants were
contaminated with 5% CO 2 – more than a hundred times
the usual aerial concentration – and the experiments were
performed with both anode and cathode. In parallel, the
researchers carried out a series of control measurements by
operating electrodes with pure reactants.
Using techniques such as scanning electron microscopy and

INNOVATION
2. Electrolyte studies
carbonate concentration (g/l)
40
30
20
10
start of operation with
CO 2 -containing gases
0
0 100 200 300
operation time (h)
400 500 600
In contrast to the electrodes, the AFC electrolyte was affected
by the CO 2-containing gases. During operation with gases
containing 5% CO 2, the carbonate concentration increases
linearly with time. However, according to the DLR team, an
overall charge transfer of 525 A/cm 2 was achieved at CO 2
concentrations some 150 times higher than that in ambient air.
In addition, the electrodes were fully operational at the end of
the experiment.
X-ray photoelectron spectroscopy, the DLR team found that
the performance of the AFC electrodes decreases over time in
both operation modes, i.e. with or without CO 2 (figure 1).
Furthermore, the decrease in both modes of operation is comparable.
In other words, CO 2 neither enhances the degradation
process nor induces any new, detrimental effect. So while carbonate
concentration in the electrolyte increases during operation
with CO 2-containing gases, there is no evidence that CO 2
affects the electrode degradation process.
Within the electrolyte, CO 2 reacts to form CO 3 2– ions, which
subsequently move to the anodes. Consequently, the DLR team
investigated whether carbonates deposits on the anode were
closing the pores in the electrode. Using nitrogen adsorption
measurements, they found that the specific surface was constant
– i.e. no observed change induced by CO 2. Also, changes
in pore size distribution with lifetime showed no link to CO 2;
the only observation was normal aging of the electrodes.
The DLR team also evaluated the effect of CO 2 on the AFC liquid
electrolyte (figure 2). Gas chromatography measurements
showed that the residual gas behind the anode contains approximately
10% of the CO 2 in the feed gas, while the residual gas of
the cathode contains approximately 20% of the CO 2 in the feed
gas. What this indicates is that most of the CO 2 is dissolved in
the alkaline solution by forming potassium carbonate, K 2CO 3.
In long-term experiments with increased K 2CO 3 concentration
up to 450 g/l, the conductivity of the electrolyte is reduced,
while the electrodes remain unaffected.
During operation with 5% CO 2, the carbonate concentration
in the electrolyte increased linearly with operation time. After
3500 h, the experiments were terminated and an overall charge
transfer of 525 Ah/cm 2 was achieved at a CO 2 concentration
3. Efficiency comparisons
voltage (V)
1.00
0.80
0.60
60
0.40
cell voltage, Orbiter
cell voltage, Astris
cell efficiency, Orbiter
40
0.20
cell efficiency, Astris
system efficiency, Astris
20
0.00
0 10 20 30 40 50 60 70 80 90
0
100 110 120
rated power output (%)
Cell voltage and efficiency curves for two types of AFCs. The top
curves represent a leading-edge noble-metal-catalysed system
(UTC Orbiter), while the lower curves show a terrestrial, airbreathing,
platinum-free system that uses a circulatory
electrolyte for heat/water management (Astris). The penalty for
displacing platinum and operating on ordinary air at
atmospheric pressure is apparent. Efficiencies stated are all
related to the lower heating value of hydrogen.
some 150 times higher than the CO 2 concentration in air.
Significantly, the electrodes were fully operational at the end of
this long-term evaluation. The only conclusion to draw from
this is that CO 2 in the air is not a problem for AFCs. Further, if a
change of electrolyte is needed, this is no more complicated or
expensive than changing the oil in an internal combustion
engine. Most importantly, what’s evident is the incorrect nature
of the term “CO 2 poisoning”. Poisoning refers to an effect where
a tiny quantity of a contaminant renders the fuel-cell catalyst
largely inoperative. There is no such effect caused by CO 2 in
AFCs, whether they rely on platinum or other catalysts.
By way of a footnote, it’s worth considering CO poisoning, a
serious problem in acidic fuel cells (PEM and phosphoric acid)
relying on platinum catalysts. In this case, the OH – groups
needed for the electrochemical oxidation of CO must be
produced by dissociation of water, under the kinetically
unfavourable conditions of the acid environment. In contrast,
in the alkaline milieu, the OH – groups are a part of the electrolyte
and the CO oxidation is not hindered. This appears to
underpin the higher tolerance for catalyst poisoning in the case
of platinum-catalysed AFCs, while CO poisoning seems to be
totally absent in AFCs relying on alternative catalysts. (It’s
worth adding, though, that the CO is digested by the AFC as a
fuel and oxidized to CO 2; it therefore ultimately has the same
effect as CO 2 owing to its absorption in the electrolyte).
Achievements and challenges
Although AFCs have for some time been considered of secondary
importance by many in the development community, a
pro-AFC camp of researchers and engineers has been quietly
getting on with the task of enhancing the technology’s funda-
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100
80
efficiency (%)

Table 1: AFC catalyst combinations
AFC developer Fuel system Operating Anode Cathode
pressure (bar) catalyst catalyst
Bacon* H 2-O 2 45 Ni NiO
UTC-Apollo H 2-O 2 3.4 Ni NiO
UTC-Orbiter H 2-O 2 4 Pt/Pd Au/Pt
Elenco H 2-air atmospheric Pt Pt
Siemens H 2-O 2 2.2 Ni Ag
DLR H 2-air atmospheric Ni Ag
* AFCs developed by F T Bacon at the University of Cambridge, UK, in the 1940s and 50s.
mental performance metrics. That said, innovation is a moving
target and further improvements and investment are
needed if the AFC is to become a serious contender for mainstream
power applications.
● Power density: AFCs are often dismissed as a fringe technology
capable only of low power densities. This unfair declaration
comes from comparing apples and oranges. Put another way,
most current low-temperature fuel-cell systems are high-pressure
PEMFCs, whereas the latest AFC systems are atmosphericpressure
systems. Everything being equal, however, AFCs will
match or outperform PEMFCs at high or low pressure. That’s
because the oxygen reaction kinetics, the limiting factor in
hydrogen fuel cells, are much better in alkaline electrolytes
than acidic environments. So while both architectures benefit
from elevated pressure, to reduce the concentration polarization
losses, the effect is less pronounced in alkaline media. It’s
because of this attractive low-pressure performance that the
current generation of terrestrial AFC systems are air-breathing
systems that operate at atmospheric pressure or thereabouts.
Examples of AFC pressurized systems are those that were
deployed on the Apollo or the Space Shuttle Orbiter (SSO)
craft. The latter system (developed in 1979) operated at 90 ºC
and 4 bar; cell voltage was 0.86 V at 470 mA/cm 2 current density
and nominal power of 12 kW. The stack power density was
275 W/kg. In a later application, powerplant for the Orbital
Transfer Vehicle (OTV) reached full power of 6.75 kW at 127 ºC
and 10 bar. Cell voltage was 0.9 V at current density 1.1 A/cm 2 ,
with remarkable power density of 1.8 kW/kg for the stack and
840 W/kg for the entire system. Electrodes of the type used in
this application have been capable of supporting current densities
of 5–10 A/cm 2 .
As perhaps the ultimate example of what has been achieved
with AFC technology, UTC of the US (the manufacturer of the
SSO and OTV powerplants) announced in 1987 that it had
developed an advanced AFC system operating at 150 ºC and
13.6 bar, with cell voltage of 0.8 V at 5 A/cm 2 . Stack power density
was 24 kW/kg and the complete system power density
3.3 kW/kg (300 kW, 90 kg). To handle the heat, UTC used active
cooling with water evaporation in the gas flow fields.
Not surprisingly, performance is dramatically different in
low-pressure AFCs operating at or near atmospheric pressure,
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INNOVATION
where the partial pressure of oxygen is only 21% of the atmosphere
(figure 3). Good examples of AFCs in this regime are the
cells and systems of Astris. Unlike the space-qualified immobilized-electrolyte
systems, these terrestrial units use a circulating
electrolyte (typically 6–12 M KOH aqueous solution) as a
convenient means of heat and water management. Gas-diffusion
electrodes are capable of supporting current densities up
to 0.5–1 A/cm 2 . However, as an engineering trade-off between
power density and polarization losses, the typical operating
point is in the range 100–200 mA/cm 2 .
● Temperature: AFCs work well at low temperatures. John
Appleby, an electrochemistry specialist at Texas A&M
University, US, points out that aqueous alkaline-electrolyte systems
have low activation energy for the cell reactions – i.e. they
show a relatively small temperature coefficient of performance
7 . Consequently, AFCs can often deliver full or near full
power with a “cold start” from room temperature. Even more
significant is their performance at sub-zero temperatures. As
the electrolyte of the right concentration does not freeze
(26–40% KOH solution will not freeze above –40 ºC), AFCs are
generally capable of starting, alas at reduced power, at these
“deep freeze” temperatures. In contrast, PEMFCs are capable of
operation only above the freezing point of water, while many
PEMs may be damaged by freezing.
● Catalyst: It’s been noted that the inherently faster kinetics of the
oxygen reduction reaction in an AFC opens the way for the use
of cheaper, non-noble-metal electrocatalysts 1 . While it is
beyond the scope of this review to tackle this subject in depth, a
look at the latest trends is instructive all the same.
Platinum-group metals still work best in any media, such that
generally there is a performance penalty associated with
replacement of the noble metals. Innovative electrode engineering
must therefore be used to regain the lost ground. For
example, the catalytic activity of nickel as the anode catalyst is
about three orders of magnitude lower than that of platinum.
That performance deficit can be remedied to some extent with
a much higher electrode loading for the cheaper catalyst – by
enlarging the active area using finely divided particles (Raney
Ni) and otherwise optimizing the electrode structure. Apart
from lower cost, another advantage is the low susceptibility of
non-noble catalysts to poisoning.
Table 1 shows the catalysts used by various AFC developers
to date. It is worth noting that in the US (UTC), the evolution
went from essentially the Bacon-type cell (on the Apollo missions)
to a heavily noble-metal-loaded system (Orbiter) – i.e.
best performance, no consideration of cost. Meanwhile, in
Germany, several groups developed AFCs entirely free of noble
metals (Siemens, Varta, DLR). Variations included the use of
lower-cost noble metals (palladium, ruthenium), base metals
and metal oxides of the spinel and perovskite families, and their
combinations. A notable development in hydrogen anodes is
the use of metal hydrides that provide, in addition to catalytic
properties, a level of hydrogen-storage capacity. Such an
approach is being pursued by the Ovonic Fuel Cell Company,
US (The Fuel Cell Review Feb/Mar 2005 p30).
● Cell efficiency: Nature has granted AFCs another free gift. The

INNOVATION
faster kinetics of the peroxide reaction on the oxygen electrode
results in lower irreversible losses. In practice it means that in
strong alkali, the oxygen electrode has gained some extra
100 mV. This can be seen in the Pourbaix diagram for the oxygen–hydrogen
electrodes 8 .
● System efficiency: While the efficiency of the typical low-temperature,
low-pressure AFCs discussed earlier is by no means
remarkable, the overall system efficiency is. The latest AFC generator
designs from Astris Energi are a case in point. It was possible,
owing to the inherent simplicity of the AFC architecture,
to engineer a system with only three moving parts: an electrolyte
circulation pump, an air pump or blower and a cooling
fan. The entire budget for the ancillaries is 5% of the rated
power, which also includes stack parasitic losses and losses due
to venting and purging. This translates to less than 2.5% of the
energy of the incoming fuel. There is no compression or
humidification of gases; the pumps must overcome only the
dynamic resistance of the moving fluids. The system efficiency
rises to more than 60% at partial load, and remains above 50%
throughout the operating range from 20 to 120% of full power.
● Operating lifetime and durability: AFCs are rugged performers
made of plastic, carbon and base metals (i.e. they do not require
fragile graphite components or other exotic materials). Good
operating lifetime is a given. The UTC Orbiter, for example, had
a rated life of 2000 h, though in tests it exceeded 10 000 operating
hours and individual stacks were tested beyond 15 000 h.
Meanwhile, terrestrial AFCs have a rated life of 2000 h (Astris),
with individual cells and electrodes tested to 5000–8000 h.
● Failure modes and safety: AFCs tend not to fail catastrophically,
but rather decline gradually in performance terms. Most manufacturers
declare the end-of-life when performance falls below
a predetermined level, usually caused by the corrosion and
agglomeration of a catalyst and loss of repellence of the gas-diffusion
electrodes. However, as long as the electrodes are in the
wet state, fuel and oxidizer never meet and there is no danger
of internal stack failure. Even if the electrolyte is removed
before the reactants, wet electrodes will remain impervious to
gases for many hours. This inherent safety should be considered
an advantage over solid-electrolyte cells – PEMFCs and
solid-oxide fuel cells (SOFCs) – where cracks, pinholes or
hotspots in the electrolyte may lead to dangerous flash
through, internal fire and stack destruction.
● System design, balance-of-plant and materials of construction: One
look inside an AFC generator built with the philosophy of
“good performance at low cost” reveals inexpensive plastic
parts, fittings, pipes and tubes. What’s more, the inherent simplicity
of the balance-of-plant is striking. The fuel-delivery circuit
uses a low-pressure regulator that feeds hydrogen to the
stacks and a simple provision for purging of impurities from
the stack. Fuel may be recirculated but often is “dead-ended” in
the stack. A pump or a blower provides an air supply to the
stacks in excess of the stoichiometric value. The electrolyte circuit
with a pump, heat exchanger(s), filter and a cooling fan is
usually the most complicated subsystem.
● Capital cost: Five years ago, the cost of source materials for an
AFC stack was just over €1100/kW. More recently, perform-
ance has been enhanced and the cost of materials brought
down to €430–460/kW 9 . Meanwhile, Astris Energi reported a
source material cost of $275/kW in 2004. With further evolution
in performance, increased production volumes, reduced
labour content and enhanced yields, it is conceivable that the
cost of stacks will drop below the $400/kW level in annual
quantities of 10 000 kW. If appropriate investment is forthcoming,
the authors believe that, given sufficient volumes, with
AFC stacks in the few hundred dollars range, the complete system
selling price can break through the magic barrier of
$1000/kW, with a healthy margin over the manufacturing cost.
● Operating cost: While AFC technology has the potential to
become the cheapest low-temperature fuel cell – and apart
from high-temperature SOFCs, probably the cheapest fuel cell
on the market – it also comes with a very low operational cost.
Versatile performance is achieved with the least expensive
industrial-grade hydrogen (99.9%) and even with unpurified,
raw electrolytic hydrogen. In the platinum-free embodiment
with circulating electrolyte, AFCs are insensitive to trace carbon
oxides, and inert to most other impurities. Cost of the consumables
(soda lime and, in some cases, potassium hydroxide)
may add some 0.2–2 cents/kWh 1 .
Commercial opportunities
Right now, fuel cells of all types are being corralled into applications
where they can potentially displace incumbent powergeneration
schemes. As a result, the approach to market entry is
largely based on the cost dynamic that the fuel cell can achieve
compared with the existing technology of choice. In most
cases, comparing only the capital or purchase cost is not currently
sufficient to prove the business case positive for fuel cells.
In many cases, the benefits of fuel-cell operation must be
proved based on a total cost of ownership owing to higher reliability
and elimination of peripheral equipment (e.g. because
of quiet or emission-free operation). The value proposition
delivered by AFCs is similar to other low-temperature fuel cells,
namely PEMFCs, with the added advantage that both capital
cost and operations cost are considerably lower. Initial applications
are expected to be as backup power sources and in light
mobility, though declining AFC price points could in time open
up additional markets.
● Backup power: The list of applications that must be either shutdown
in a controlled manner or operated for long periods without
their normal power supply is a long one and includes
mechanical systems (such as elevators) and all manner of hardware
in telecommunications and computing networks. Today,
the majority of backup schemes rely heavily on banks of leadacid
batteries, sometimes supplemented by a diesel or gasoline
generator. Fuel cells that can be produced economically will be
able to deliver increased performance and offer a savings to the
operator for a variety of backup power applications owing to
decreased maintenance, simpler installation and enhanced
reliability. There is an incredible advantage to having a power
supply that is fully independent from the primary supply. In
times of extended power outage, for example, additional gas
cylinders can be delivered to power the backup power system,
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Making the breakthrough: if
they can be turned into
commercial products, AFCs are
likely to find initial applications
as backup power sources for
wireless base stations and as
primary sources in a variety of
light utility vehicles (e.g. the
golf car shown above).
an option that does not exist for batteries.
Although purchase cost is a big factor governing the take-up
of any new technology, there are many secondary benefits
associated with an AFC-based backup system. Of particular
importance are those that can be translated into savings in
infrastructure or operations/maintenance. In the case of a
backup unit that delivers 2 kW for 10 h, the existing battery
solution would weigh in excess of 2000 lb, whereas the fuel-cell
option would weigh less than 300 lb and would occupy only a
fraction of the space.
● Light utility vehicles: Increasing numbers of electric vehicles are
in use today – as “people movers” at airports, large industrial
plants and golf courses, as well as for materials handling in
warehouses. These vehicles are typically powered by an onboard
battery that has a number of limitations. Most impor-
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INNOVATION
tantly, batteries typically take hours to recharge, which means
that continuous operations force the user to have multiple vehicles
(or at a minimum, multiple batteries) on hand. The cost of
battery handling, charging and spare batteries (or even spare
vehicles in some cases) is considerable.
The fuel-cell-powered vehicle can run for a complete shift
and typically be refuelled in less than a minute, thereby eliminating
the need for spare vehicles/batteries. Furthermore, the
fuel-cell vehicle offers improved performance, owing to
reduced overall weight and elimination of “battery-fade”. The
golf car shown above is 20% lighter than the battery-powered
version and runs for three days between refuelling (based on
typical usage). It is also emissions-free and virtually silent, so
can work indoors or outdoors.
Which way now?
In summary, AFCs have the potential to become a key technology
in the low-temperature regime, one that forms the basis of
power-generation schemes that work as well as or better than
competing approaches, and do so at lower capital and operational
cost. Much of the technology’s potential still lies unexplored
and under-utilized, chiefly because the bulk of the
investment dollars have hitherto been directed towards the
R&D and commercialization of PEMFCs.
For the AFC community, this should be seen as positive not
negative, in that there is plenty of scope for incremental and dislocating
innovation with only modest levels of investment. The
commercial opportunity is there. Who will embrace that
opportunity and take advantage of it? ●
Further reading
1. G F McLean et al. 2002 An assessment of alkaline fuel cell technology.
Int. J. of Hyd. Ener. 27 507–26.
2. E Guelzow 1996 Alkaline fuel cells: a critical view. J. Power Sources 61
99–104.
3. T Burchardt et al. 2002 Alkaline fuel cells: contemporary advancement
and limitations Fuel 81 2151–55.
4. M Cifrain and K Kordesch 2003 Hydrogen/oxygen (air) fuel cells with
alkaline electrolytes in Handbook of Fuel Cells Vol. 1 4 267–80 (Wiley,
Chichester).
5. K Kordesch and G Simader 1996 Fuel Cells and their Applications
(Wiley, New York).
6. E Guelzow and M Schulze 2004 Long-term operation of AFC
electrodes with CO 2 containing gases. J. Power Sources 127 243–51.
7. A J Appleby and F R Foulkes 1989 Fuel Cell Handbook (Van Nostrand
Reinhold, New York) p262.
8. M Pourbaix 1966 Atlas of Electrochemical Equilibria (Pergamon Press,
New York).
9. P Gouerec et al. 2004 The evolution of the performance of alkaline fuel
cells with circulating electrolyte J. Power Sources 129 193–204.
Erich Guelzow is head of the low-temperature fuel-cell group at Deutsches
Zentrum für Luft und Raumfahrt (DLR) in Stuttgart, Germany. Jiri K Nor is
president and chief technology officer of Astris Energi in Toronto, Canada. Peter
K Nor is vice-president, business development and marketing at Astris Energi.
Mathias Schulze is a fuel-cell scientist responsible for characterization of fuelcell
components and investigation of their degradation mechanisms at DLR.