Download A comparison between the alkaline fuel cell (AFC)

A comparison between the alkaline fuel cell (AFC)
and the polymer electrolyte membrane (PEM) fuel
cell
K. Kordesch and M. Cifrain
Volume 4, Part 4, pp 789–793
in
Handbook of Fuel Cells – Fundamentals, Technology and Applications
(ISBN: 0-471-49926-9)
Edited by
Wolf Vielstich
Arnold Lamm
Hubert A. Gasteiger
 John Wiley & Sons, Ltd, Chichester, 2003
Chapter 58
A comparison between the alkaline fuel cell (AFC)
and the polymer electrolyte membrane (PEM) fuel
cell
K. Kordesch and M. Cifrain
Graz University of Technology, Graz, Austria
1 THE TECHNICAL-HISTORICAL
BACKGROUND
At fuel cell meetings in the 1990s, one could clearly recognize the enthusiasm of the relatively few basic research
people and the successful technical people who were presenting their papers or posters. It may have concerned
small steps in the laboratory or large steps in the technology; the pride in their achievements was obvious. On
the other side we noticed the more cautious management
and commercial representatives of the companies supposed
to implement the next steps. They rightly considered the
alternatives, the competitive situations and the cost questions, showing their concerns often more critically than
necessary. However, in many cases progress was questioned
just to look experienced and environmentally responsible,
which was always a good advertisement for that company.
If something went wrong (which was more probable) they
predicted it; if research and development (R&D) succeeded,
they knew it, too. This was disheartening for young people.
Fuel cells since the time of Grove have gone through many
cycles of high expectations and failures. Also, the different types of fuel cells followed each other as a result of
new demands and the development of new materials. The
efficiency advantage of avoiding Carnot’s Law was known
to the prominent electrochemists at the begin of the 20th
century and high temperature coal fired fuel cells were proposed to power ships not long after the electric motor and
the propeller were invented!
Early fuel cells were built with liquid electrolytes, solutions of potassium or sodium hydroxide or diluted acids.
The classical example is the alkaline fuel cell by Francis
Bacon, who built his models around 1945 in a bathtub. The
advantages of hydrogen–oxygen fuel cells for space application became clear and it was also the only application
which could afford them. It led to the elimination of any
mechanical pumps, which were not reliable enough. The
use of matrices (e.g., microporous asbestos) soaked with
KOH became standard for National Aeronautics and Space
Administration (NASA) space fuel cells and it is still the
present method, in spite of the fact that better matrices have
been found. Membrane fuel cells (by General Electric Co.)
existed 30 years ago, but the possibility of pin holes and gas
cross leakage was too high at that time. Nafion membranes
have changed that.
The fact that liquid circulating electrolytes offered great
advantages for heat management and water removal requirements was believed to be negated by the disadvantage of
creating parasitic shunt currents in series connected cell
assemblies (see below). Delayed start-up procedures often
led to the reversal of cells and irreversible cell failures. The
use of combining fuel cells with rechargeable batteries in a
hybrid system and the advantages in this respect were not
recognized.
Handbook of Fuel Cells – Fundamentals, Technology and Applications, Edited by Wolf Vielstich, Hubert A. Gasteiger, Arnold Lamm.
Volume 4: Fuel Cell Technology and Applications.  2003 John Wiley & Sons, Ltd. ISBN: 0-471-49926-9.
790 Part 4: Alkaline fuel cells and systems (AFC)
As a matter of convenience and to save time, all testing
of fuel cells was (and is) done in a continuous mode
operation. The need to operate fuel cells in an interrupted
fashion, with sometimes long idle periods, did not seem to
be important. However, it is if one considers the operation
of an electric vehicle in normal use. Kordesch demonstrated
this fact the first time with his hydrogen/air city car, driving
it daily, for over 3 years on public roads in the early
1970s.[1]
Consider the use of a gasoline engine: it operates only
when the car is in use. Four thousand operating hours
(about 4 months when counted continuously) correspond to
about 200 000 miles of operation, and may be within 2 or
10 years of calendar time. Only large, special gas motors
can run without failure for thousands of hours continuously.
A similar situation is obvious in fuel cell operation of an
electric vehicle. It must be able to shut down completely
for longer time periods, hours, days, weeks, etc., it must be
safe in the garage, with the gas supply turned off at ambient
and even low temperatures.
The alkaline fuel cell with circulating and removable
electrolyte can do it. It should also be mentioned that on
activated stand, without load, fuel cell electrodes and catalysts degrade more than under load. The high voltage on
open circuit is the reason for carbon oxidation processes,
catalyst changes, etc. Unfortunately, the alkaline matrix
fuel cells with immobilized KOH electrolyte combined all
possible disadvantages: the electrolyte had to stay in the
cells, residual carbonate (from any incomplete air cleaner)
accumulated, separators (matrices) deteriorated, gas cross
leakage started during drying out or crystallization periods
during storage times without careful maintenance. Therefore it is not surprising that the alkaline fuel cells (AFCs)
were discredited.
Polymer electrolyte membrane (PEM) fuel cells operate
perfectly under 45 ◦ C. They can be shut down. They are
ideal for small specific applications which can also afford
the high cost. Above 55 ◦ C the water balance and membrane
drying-out difficulties start and require carefully controlled
accessories (see below).
2 LIQUID CIRCULATING
ELECTROLYTE CONTRA MATRIX
SYSTEMS
It is useful to take another closer look into these different
systems. Everybody who decides to work on fuel cells
is confronted with the choice of which system would be
the better one. Both have advantages depending on the
planned use.
Contrary to matrix cells, fuel cells with circulating electrolyte establish a reliable barrier against reactant leakage
from the electrodes and can use the electrolyte as a cleansing medium. Accumulated impurities and carbonates can
easily be removed. The main advantage of the circulating
electrolyte is its use as a cooling liquid and for reaction
water removal. Even dead-ended reactant gas supply (pure
hydrogen and pure oxygen) is possible, if external water
vaporizing units are installed in the electrolyte circulation loop.
Normally, reaction water leaves an AFC on the hydrogen side. Some water is consumed at the cathode side, but
produced twice as fast on the anode side. This leads to a
concentration gradient of KOH inside the cell. Inside the
cathode the concentration of the potassium hydroxide is
much higher (sometimes of a factor of two) than the bulk
concentration. On the anode, a strong dilution occurs, causing the water to evaporate due to the higher water vapour
pressure. This also cools the cell from the anode side.
For optimized operation, an external hydrogen circulation
including a water condenser is necessary.
The concentration gradient in the cell depends on the
power density taken from the cell. The more power is
drained, the stronger the gradient. This gradient has a
negative influence on the electrode reactions, as on both
electrodes a lack of essential reaction partners occur (OH−
on anode and H2 O on cathode); a drop in cell voltage is
the result (concentration losses). Hence, the gradient should
be as small as possible, which is strongly supported by a
liquid circulating electrolyte and always causes problems
in matrix cells, in alkaline as well as acid systems.
Nevertheless, circulating systems also have disadvantages. Besides the fact that additional tubing, pumps and a
tank are necessary, which all have to be resistant against the
highly corrosive hot alkaline electrolyte (although all necessary parts are commercially available and low cost, nickel
and many plastics like polytetrafluoroethylene (PTFE),
polysulfone (PSU) and epoxy resins can resist hot alkaline electrolytes), parasitic currents occur in a multi-cell
stack. As electrolyte has to be fed into each of the cells,
the ionically conductive liquid bridges the cells, and high
voltages can occur between two electrodes. One can visualise a simple seven cell stack. Each cell is connected
in series to the next one (bipolar). In matrix systems no
manifolds would be there and each cell is insulated. In
case of a liquid system however, manifolds are necessary.
Each of the electrodes is electrically connected by the electrolyte, not only to the counter electrode of the same cell,
but also to all other counter electrodes. Hence, assuming
an open circuit voltage of 0.95 V per cell, the voltage of
the outermost electrodes of this stack would be as high as
0.95 × 7 = 6.65 V. This high voltage would certainly harm
A comparison between the alkaline fuel cell (AFC) and the polymer electrolyte membrane (PEM) fuel cell 791
weak cells in between. Fortunately the electrolyte resistance
is quite high (≈1 cm). Using long thin tubes as manifolds greatly increases the ohmic resistance to several 10s
of ohms, while the electrode distance in a cell is lower
than 0.1 cm, decreasing the resistance under 0.1 . Hence,
the ohmic resistance outside the cell is nearly three decades
higher than inside, no dangerous parasitic current can occur.
To summarize this discussion about liquid electrolytes,
two examples should be given.
A fuel cell for a space ship can be fed with pure hydrogen
and pure CO2 free oxygen, as both gases are on board.
Liquids of any kind are not preferable, as bubbles and drops
of any kind are difficult to separate in zero gravity. The
system should be lightweight and the price of the system
is not a major concern. It is obvious that a matrix system
would meet the demands.
Assuming a backup system for elevators, which powers
the cabin during periods of grid power failure, neither
weight nor place is a major problem. Here, long life and
low price is important. Also, long periods of shutdown,
fast start-up and simple shutdown are desirable. Hence, a
system with circulating liquid electrolyte could be chosen.
In practise, a hybrid system including a rechargeable battery
(lead acid) would be selected.
Table 1 summarizes the pros and cons of the two systems.
3 SYSTEM DESIGN COMPARISON
BETWEEN AFCs (LIQUID
ELECTROLYTE) AND ACIDIC PEMFCs
A frequently mentioned point is the high current capability of the PEM fuel cells compared to other systems. In
this connection it is important to look at the efficiencies
achieved at different current densities, the cell voltages
allow a rough estimate. Alkaline hydrogen/air fuel cells
have an operating voltage of about 0.8 V at 300 mA cm−2 .
Assuming that PEM fuel cells deliver 600 mA cm−2 at
0.7 V, this is a loss of 10–15% in efficiency. Engineers calculate that doubling the current density corresponds to half
the weight or size of the fuel cell. However, only the stack
is reduced, the required system accessories and controls
may more than offset these advantages, not even considering life expectancy questions, which are not investigated
easily, and catalyst cost relations.
The high current multiplies the membrane water balance
difficulties, particularly increasing air flow requirements.
Air must be blown through (only 20% is oxygen) either at
a rather high stoichiometric rate (2–3 times stoichiometric)
or pressurized, if the current is high. However, high air
flow means a careful control of water production (load and
temperature depending) versus membrane drying-out. Air
Table 1. Comparison of matrix AFC systems and systems with
circulating electrolyte.
Pros
Matrix systems
No electrolyte pump, tank,
tubing
No parasitic current
Handling easy
Electrolyte compartment
thinner
Circulating electrolyte
systems
Gas bubbles washed away
easily
Easy heat management
Easy water management
Concentration gradient
limited
Simple start up and
shutdown
Stable during long periods
of shutdown
Monopolar and bipolar
design possible
Electrolyte exchangeable
Removes carbonates and
impurities
Low cost
Cons
Gas bubbles critical (loss of
contact)
Heat and water management
difficult
Humidifiers necessary
Cooling fluid needed in
larger stacks
Start up and shutdown
difficult
High concentration gradient
Only bipolar design possible
Revival of old stacks nearly
impossible
Pumps, tanks, tubing, etc.,
necessary
Parasitic currents possible
supplied by a controlled compressor can only solve such
problems at a considerable over-all system efficiency loss
and a high cost increase. The alkaline system produces the
reaction water at the hydrogen anode, the acidic membrane
system at the air-cathode. Hydrogen can be efficiently
circulated in a closed system with a simple condenser
arrangement.
High current densities and a high air flow are also
modeling parameters for the alkaline cells, especially if the
size of the soda-lime CO2 air cleaner is considered, but it
is not a serious system problem because a suitable cleaner
exists (which lasts for 5000 h) and the KOH can easily be
exchanged (similar to an oil change in an automobile) or
refilled.
System considerations determine the cost of the system.
Prices in the range of more than a few $100 kW−1 , eliminate the chances for general electric vehicle applications.
Catalyst questions come up. Due to the sensitivity of PEM
792 Part 4: Alkaline fuel cells and systems (AFC)
cells against CO, the level of noble metal catalysts is kept
high. Alkaline fuel cells using large surface carbon based
low level Pt/metal catalysts are not very sensitive to CO in
the hydrogen, a lower grade of purity hydrogen or converter
gas can be used. The use of non-noble metal catalysts in
PEM cells is not likely because of the acidic pH. Alkaline
fuel cells can, at least at the air electrodes, use conventional
low-cost perovskites or spinels.
It appears that the type of fuel cell to be used in street
vehicles will ultimately be decided by economic questions
and answers.[2] Applying the teachings of the historical
developments to predictions about electric vehicle fuel cell
developments, the AFC may even have a better chance than
the PEM systems due to the following facts:
•
•
•
•
•
The best performing fuel cells are the alkaline space
vehicle systems. Since 1970 they have been performing reliably on oxygen and hydrogen at high current
densities (1 A cm−2 , at 0.75 V), but using a high amount
of noble metals (see also Space-shuttle fuel cell, Volume 4).
The Russian PHOTON needed 40 mg Pt cm−2 . Catalyzed carbon substrates have been successfully used
in Air electrodes, which since 1960 have delivered
current densities between 100 and 150 mA cm−2 at
0.75 V. Only PEM electrodes can do better now, but
at a tremendous increase in cost and complexity of
the system.
Practically all systems have arrived at the thin electrode and/or bipolar construction of the battery stacks.
The alkaline OXY was an excellent prototype system,
already geared for mass production before the company
went out of business. The ELENCO alkaline system
is still the only publicly demonstrated fuel cell which
looks economically affordable. In this connection it
should also be mentioned that no large funding has
gone into the development of alkaline systems for street
vehicles during the last 15 or 20 years.
The lifetime question is always brought up with the
goal set at 40 000 h. This is only a requirement for
power stations. It is questionable if fuel cells which
should operate in the field and not under strictly controlled conditions will be able to approach such time
periods without the possibilities of replacement of components (electrode stacks, electrolytes or accessories).
The automobiles of the 1960s may provide be a good
comparison: oil changes were frequent, lubrication
needs were apparent and the exchange of components
(pumps, fans) was regularly anticipated. Still, today in
the 1990s we do not expect a gasoline engine to operate
for more than 2000 h without an overhaul. Why should
a street vehicle fuel cell stack be required to do much
better, especially if it is built mainly from essentially
low-cost materials (like carbon) and a small amount of
catalysts, which can be easily recovered.
Professor Kordesch planned the use of liquid ammonia as fuel for his car. Unfortunately, at that time only
2 kW size NH3 converters were available.[3] Some years
before, a huge military program including liquid ammonia
as logistic (energy) fuel was abandoned in favor of jet fuels,
petrol and diesel. It may be interesting to note that heat and
combustion engines do operate on ammonia, with no nitrogen oxides produced. Only the start-up is difficult; a little
propane gas injection helps. Alkaline hydrogen/air fuel cells
with circulating KOH-electrolyte could operate directly on
the catalytically dissociated ammonia and the effluent gas,
containing a few percent of hydrogen used to heat the
converter. Filters soaked with Cu-salt solution remove any
smell from the exhaust gas. Air-operation of carbon electrodes is not difficult, a soda lime cleaner is sufficient and
the old belief that complete removal of CO2 is required
has been proven wrong. It depends on the repellency of the
electrode to what extent the electrolyte penetrates the electrode structure; small amounts of CO2 , diffuse right through
the porous system and end up as carbonate in the KOH.
With a circulating electrolyte, periodic exchange (like an oil
change) and the removal of many other impurities is easy;
the heat exchange and the water removal is no problem.
There is no gas cross-over – a big advantage compared with
matrices, which were declared to be more convenient and
simpler to use (and nearly everyone believes it up to now).
The interrupted operation mode of fuel cells was mentioned earlier to be a possible way to increase life expectancy of a fuel cell. The comparison was made with an
automobile engine which works only a few thousand hours
in its many years of life. There are good indications that
this is true if the optimal way of emptying the liquid electrolyte and the corresponding gas shut off mechanisms are
employed. Fuel cells on open circuit do deteriorate faster
than on load! Also, with such a complete shutdown system
one can sleep without any worries of an accident. This was
the reason why I selected this shutdown operational mode
for my City Car.[4, 5] After the starting of the circulating
pump, the fuel cell was operational within 1 min and the
hydrogen gas could be admitted by the control system. The
car was used in public traffic for over 3 years and the electrodes were supposed to have a lifetime of only 2000 h.
Only far later did we realize that the intermittent operation
had such a large effect on the lifetime. A detailed description of the electrodes[6] and the fuel cell – battery hybrid
system[7] is available.
Unfortunately, the shutdown of electrode production and
fuel cell testing at Union Carbide Corporation in the mid1970s prevented a more detailed analysis and more experimentation with batteries and stacks in the intermittent
A comparison between the alkaline fuel cell (AFC) and the polymer electrolyte membrane (PEM) fuel cell 793
operational mode of an electric automobile. In other companies, efforts were directed to immobilized electrolytes and
finally to phosphoric acid fuel cell (PAFC) systems.
4 SUMMARY
A developer has to be an optimist, a fuel cell developer very
much so. Lindström wrote a paper in 1966 with the title
“The fuel cell ready for its entry on the market”.[8] In fact
it was ready for the market considered at that time, Swedish
submarines. The Swedish fuel cell finally lost in the competition with the Swedish Stirling engine. Cold combustion
is unfamiliar culture compared to hot combustion, which
contributed to the outcome of this competition. The fuel
cell is facing the same cultural barrier today. The investor
has to put fuel cell on the market, not the developer. A natural law for technical evolution is that innovation is a fruit
of decision to invest, not result of invention.[2] The bets
that fuel cells will propel future electric vehicles are rather
safe in the long run. There is no other imaginable solution
to the mobility problem once fossil fuels are diminished.
Hydrogen or some hydrogen carrier will replace the liquid
or gaseous carbon derivates of today and the energy needed
by mankind and produced by renewable sources will produce less CO2 . Some prophets say it will be fusion power
which will do it ultimately, but that does not remove the
need for the most efficient electric energy producer, the fuel
cell. What fuel ultimately? We believe that besides hydrogen and alcohols, ammonia[3] has a good chance: it can be
stored at low pressure and delivers 75% hydrogen in a simple catalytic converter. It smells; that is good. Any leakage
will be detected. It also plays a major role in the hydrogen/nitrogen/oxygen scenario which is basic to life as we
know it. Even now, as starting compound for fertilizers it
is among the most produced chemicals in all our chemistry.
Hydrogen can also be stored that way for dispersed power
generation plants.
PEM fuel cells currently dominate the low temperature fuel cell literature. Expectations are very high and
supported by large companies. The competition in the
automobile and oil industry is high and only the cost
factor will finally decide the use of fuel cells. If expectations are not fulfilled, fuel cells will revert back into
the already repeatedly experienced role of the promising
power source, available in 15 years. The choice of applications will change with time and experiences. It may pay
to re-investigate alkaline fuel cells with circulating electrolyte from the view-points of actual combustion engine
duty cycles.
REFERENCES
1. K. Kordesch, International Power Sources Symposium, Brighton, UK, May 10–12 (1999).
2. O. Lindström, ‘A Critical Assessment of Fuel Cell Technology’ Dept. of Chemical Engineering and Technology, Royal
Institute of Technology Stockholm, KTR 93/94-08 (1993).
3. UCC, ‘Ammonia/Air Fuel Cell System for Vehicle Propulsion’,
Union Carbide Corp., Techn. Proposal to US army, Virginia,
May 15, (1964).
4. K. Kordesch, ‘City Car with Hydrogen/Air Fuel Cell and
Lead/Acid Battery’, in “Proceedings of the Intersociety Energy
Conversion Engineering Conference”, Boston, paper 719016
(1971).
5. K. Kordesch, ‘Power Sources for Electric Vehicles’, in “Modern Aspects of Electrochemistry”, J. Bockris and B. Conway
(Eds), Plenum Press, New York, Vol. 10, pp. 339–443
(1975).
6. K. Kordesch, J. Gsellmann, S. Jahangir and M. Schautz, ‘The
Technology of PTFE Bonded Carbon Electrodes’, “Proceedings of the Symposium on Porous Electrodes”, H. Maru (Ed),
The Electrochemical Society, Pennington, NJ (1984–1988).
7. K. Kordesch, ‘Brennstoffbatterien’ Springer-Verlag, New York
(1984).
8. O. Lindström, ASEAs Tidning, 58, 171 (1966).