v1.0.0 / chapter 12 of 14 / 01 sep 07 / greg goebel / public domain
* One of the most widespread household chemical technologies is the electric storage cell or "battery", with a wide range of old and new technologies in common use. Another chemical system for generating electricity, the "fuel cell", is by no means new, but is only now poised for widespread application. This chapter discusses battery and fuel cell technology.
* It was discussed earlier Alessandro Volta had developed the first electrical storage cell, the "Voltaic pile", in 1800, based on a stack of zinc and silver disks separated by cloths soaked in a weak acid. Following Michael Faraday's pioneering research on electrochemistry, in 1836, the English chemist John Daniell (1790:1845) developed the first modern storage cell using Faraday's principles.
Storage cell operation is based on "reduction-oxidation (redox)" reactions. For example, Volta's scheme can be modeled by placing a bar of zinc at one side of a beaker containing a solution of weak sulfuric acid; placing a bar of silver at the other side of the beaker; and then wiring the two "electrodes" through a light bulb outside the beaker. The light bulb then starts glowing.
In solution, sulfuric acid, H2SO4, breaks down into two H+ ions and a single SO4-- ion, forming an electrolyte that can carry electric current. The SO4-- ion easily "oxidizes" zinc to form zinc sulfate (ZnSO4), which is released into the solution, eating away the negative zinc anode. As each zinc sulfate molecule leaves the electrode, it leaves behind two electrons that flow through the external wire as a current to the positive silver cathode. At the silver cathode, the electrons combine with or "reduce" the hydrogen ions in the solution to form diatomic hydrogen gas. The silver is inert and not consumed in the reaction.
All modern storage cells use similar redox schemes, though the specific implementations vary widely. Some classes of storage cells can be "recharged" by running an electric current through them backwards, which reverses the chemical reactions and more or less restores things to their original condition.
* The popular terminology for storage cells is somewhat confusing. Storage cells are almost always referred to as "batteries" in common usage, but this is not technically correct. The storage cell described above is just that, a "cell", not a "battery". It consists of one cathode and one anode in an electrolyte. A storage cell with specific electrode materials and electrolyte has a certain output voltage, and to get higher voltages with that specific technology, they must be electrically connected together in series as a "battery".
Flashlight cells are just that, cells, but the lead-acid battery used in an automobile consists of several cells packaged and chained together, so it is indeed a battery. A single cell of a lead-acid battery has a voltage of 2 volts, and so a 12-volt lead-acid battery has six cells in series. In practice, people, even manufacturers, call cells "batteries" whether they consist of one cell or many -- but the term "cell" has a way of sneaking itself back in even in popular usage. The default here is to use "cell", meaning "electrical storage cell", except when specifically referring to true "batteries" -- it may sound stuffy, but the alternative is hopeless confusion.
* There are two classes of cells: nonrechargeable or "primary" cells, for example typical cheap throwaway flashlight cells, and rechargeable or "secondary" cells, for example as used in an automotive lead-acid battery. Cells can also be classified as "wet cells", which have liquid electrolytes; "dry cells", which have electrolytes in the form of a paste; and "solid electrolyte" cells, which as their name indicates use a completely solid electrolyte. In addition, there are standardized form factors for certain classes of cells, such as AAA and AA penlight cells; C and D flashlight cells; and the standard nine-volt brick-shaped "transistor radio" battery, a package containing six individual cells. Output voltages are also more or less standardized for these products. However, cells are otherwise not highly standardized items, as shopping for a watch button-style cell quickly proves.
Many cells can maintain their output voltage at a reasonably constant level over a fairly wide range of output currents. In electrical engineering terms, they are said to have a low "internal resistance". Those that have cannot supply high currents without losing voltage have a high internal resistance. Cell specs often include the maximum useful current output, and curves that show the drop in output voltage with increasing current drain. By the way, the low internal resistance, or equivalently current capacity, of big automotive batteries makes them potentially dangerous. While their output voltages are so low that getting a shock off them is not a problem, if the output of a large automotive battery is shorted to the chassis ground the large currents flowing through the short can cause an almost explosive flash and severe burns. It is not usually a good idea to wear a watch with a metal band while servicing a vehicle, since the vehicle's chassis is ground and a short from a "hot" wire could easily lead to a nasty accident.
The total energy capacity of a cell is measured in the number of hours it can supply a given level of current, or "ampere-hours". This is a straightforward figure of merit for cells based on the same technology, since they will all have the same voltage. However, ampere-hours can be misleading for comparing different cell technologies, since the voltages may differ and the power output of a cell with a lower voltage is lower for the same level of output current. For this reason, the unit of "watt-hours" is used to compare energy storage capacity between different cell technologies.
A related rating is the "specific energy" of a cell, which gives the its storage capacity relative to its mass. For example, a cell could be said to have a given number of watt-hours per kilogram. There is also "energy density", which gives its storage capacity relative to its volume, for example in watt-hours per liter. Specific energy and energy density are used in comparisons between different classes of cells, particularly for automotive propulsion applications. Electric-powered automobiles have always suffered from the limited energy capacity of batteries cells compared to gasoline and other chemical fuels, and so obtaining storage cells with greater specific energy has been one of the most important goals of electric-automobile designers.
* As mentioned, rechargeable cells can be run backwards and more or less restored to their original, charged state. The "more or less" is important. The restored state is not a perfect replica of the original state, and so rechargeable cells degrade slightly every time until their storage capability fades out. For this reason, rechargeable cells are also described by the number of "charging cycles" they will tolerate. The number of cycles tends to be lower with greater average depth of discharge. Manufacturers may also provide curves showing how the cell's capacity slowly falls as the number of cycles increases. Other important specs include shelf life, temperature limits, and physical dimensions.
* The carbon-zinc cell was invented in the 1860s by a French chemist named Georges Leclanche (1839:1882), and is sometimes called the "Leclanche cell". It is also sometimes called the "dry cell", but this term is somewhat misleading since it can be just as legitimately applied to similar storage cell technologies. The carbon-zinc cell consists of zinc cylindrical "cup" that makes up the anode -- which is separated from the external casing of the battery by an insulating spacer -- and a central carbon rod. The electrolyte is powdered ammonium chloride (NH4Cl), an acid, in water, mixed with powdered manganese dioxide (MnO2) and graphite to form a paste. Zinc chloride (ZnCl2) can also be used in place of ammonium chloride, providing longer service life at higher cost, and in fact carbon-zinc cells often use a small amount of zinc chloride along with the ammonium chloride. However, for simplicity this discussion assumes that only ammonium chloride is used.
The anode reaction of the carbon-zinc cell involves the double oxidation of a
zinc atom, releasing two electrons into the external circuit:
Zn --> Zn++ + 2e-
The cathode reaction is supported by the manganese dioxide. The graphite
powder is mixed with the manganese dioxide powder to improve the conductivity
of the paste. While some sources call the central carbon rod the cathode, it
is more properly the "cathode collector", since it is inert and simply
provides a conductive path to the positive contact. In fact, this type of
cell should more properly be known as a "manganese-zinc acidic" cell, but
that's not the usage that was adopted.
The cathode reaction involves reduction of MnO2 to Mn2O3 on the inert carbon
cathode:
2NH4+ + 2MnO2 + 2e- --> Mn2O3 + 2OH-
Carbon-zinc batteries will go dead prematurely if discharged too quickly, due
to the buildup of reaction products around the carbon cathode, but they will
"rejuvenate" if allowed to rest for a while, allowing the reaction products
to disperse.
* The alkaline cell operates on similar principles, with a zinc anode and
manganese dioxide mixed with graphite for the cathode. However, the
electrolyte is potassium hydroxide (KOH), which is alkaline rather than
acidic, again mixed with manganese dioxide and graphite. It should be
properly known as the "manganese zinc alkaline" cell, but once more that's
not the usage that was adopted. The anode reaction is:
Zn + 2OH- --> ZnO + H2O + 2e-
The cathode reaction is;
2MnO2 + H2O + 2e- --> Mn2O3 + 2OH-
Sodium hydroxide (NaOH) can also be used as the electrolyte. This is the
case for almost all cells that use potassium hydroxide as an electrolyte.
Despite the similarity in operation, the alkaline cell's structure is very different from that of the carbon-zinc cell. The alkaline cell is enclosed in a nickel-plated steel can that forms the positive cathode contact, which is separated from the bottom cap, which is the negative anode contact, by a cardboard spacer. The can contains the potassium hydroxide / manganese dioxide / graphite paste for the cathode reaction, separated from a core of powdered zinc by a fabric separator. A tin-plated brass "nail" connected to the bottom cap is inserted up into the powdered zinc to conduct current to the cap. A plastic plug seals the bottom of the can and supports the fabric separator.
* Of course, all cells are covered with a plastic sheath to provide protection, insulation, and labeling. Alkaline cells have about twice the power density of carbon-zinc cells, but several times the cost. The carbon-zinc cell's virtue, probably its only virtue, is that it is dirt cheap. Both carbon-zinc and alkaline cells have a cell voltage of about 1.5 volts, and are both regarded as environmentally benign, at least by the standards of storage cells. Carbon-zinc and alkaline storage cells are not in general rechargeable, though rechargeable alkaline cells have been produced.
* An Israeli company came up with an interesting variation on this technology in the form of a storage cell that can be literally printed onto cardboard boxes or similar substrates using silkscreen technology. The cell consists of a five layers of silkscreened materials:
The whole assembly is sealed under a protective layer of plastic. The cell provides 1.5 volts, but multiple layers could be used to construct a battery with higher voltage if necessary. Storage capacity is 2.5 milliampere-hours per square centimeter. It has a shelf life of two years. The technology is intended for promotional gimmicks, singing greeting cards, toys or novelties, sensor systems built into packages of perishable foods, and so on.
* Calculators, hearing aids, and other small electronics devices use tiny
nonrechargeable "button" cells. The original technology for button cells was
the mercury cell, which had a mercuric oxide (HgO) cathode, an anode made of
an amalgam of mercury and zinc, and an electrolyte consisting of potassium
hydroxide mixed with zinc hydroxide (or Zn(OH)2). The anode reaction is:
Zn + 2OH- --> ZnO + H2O + 2e-
The cathode reaction is:
HgO + H2O + 2e- --> Hg + 2OH-
Mercury cells had a highly constant cell voltage of 1.35 volts. A similar
cell could be made with cadmium instead of zinc, providing a cell voltage of
0.91 volts. Since mercury is toxic, mercury cells are now banned in the US
and some other countries and they are now only a curiosity.
* Modern zinc-air button cells are similar to alkaline cells. The anode is powdered zinc mixed in a gel, the electrolyte is a layer of potassium hydroxide, and the cathode is a carbon disk, designed to support cathode reactions through the oxygen in the air. A porous Teflon membrane allows air into the cell while preventing electrolyte from leaking out.
The anode reaction is:
Zn + 2OH- --> Zn(OH)2 + 2e-
The cathode reaction is:
O2 + 2H2O + 4e- --> 4OH-
Zinc-air batteries have a cell voltage of about 1.65 volts. They have a very
high energy density, but also have a high internal resistance and are not
well suited to high-current applications. They have to be sealed in storage
to keep the air out, but as long as they are kept sealed they have a long
shelf life. Large zinc-air cells have been used in consumer equipment, at
least on a limited basis, and very large zinc-air batteries have
experimentally used in vehicular applications.
* The silver oxide cell is similar in construction to the zinc-air type, with an anode of powdered zinc in gel with a potassium hydroxide electrolyte, except that instead of having a cathode made of carbon and exposed to the air, it is a silver screen pasted with silver oxide (Ag2O). They have a cell voltage of 1.55 volts, a flat discharge curve, and long shelf life. They can be recharged a limited number of times, but they are not generally recharged in practice.
* The modern lead-acid battery is by far the most familiar rechargeable
storage cell technology. The lead-acid cell was invented in 1859 by a French
physicist, Gaston Plante (1834:1889). It uses dilute sulfuric acid for an
electrolyte, lead for the anode, and lead oxide for the cathode. The
sulfuric acid dissociates into two hydrogen ions (protons) and a sulfate
group. The sulfate group reacts with the lead anode to form lead sulfate and
releases two electrons through the external circuit. This is the oxidation
reaction, which can be summarized as:
Pb + HSO4- --> PbSO4 + H+ + 2e-
At the cathode, the two electrons cause a reaction to create lead sulfate and
water. This is the reduction reaction, which can be summarized as:
PbO2 + HSO4- + 3H+ + 2e- --> PbSO4 + 2H2O
At full discharge, both anode and cathode are covered with lead sulfate, and
the electrolyte is mostly water. As the sulfuric acid solution is denser
than water, a "densitometer", consisting of no more than a dropper with
pellets of varying densities and different colors, can be used to examine the
cell's charge level. Reversing the current flow reverses the reactions,
recharging the cell.
A standard automotive battery consists of a box-shaped casing with internal divider walls to separate its series-connected cells. The electrodes in each cell are built as sets of interleaved plates to provide the maximum surface area for the electrochemical reaction. Each cell in a lead-acid battery provides about two volts. Lead-acid batteries usually have large capacities, though they tend to run down quickly. They can be recharged hundreds of times until their electrodes are too eroded to allow the battery to hold a charge. They have indefinite shelf lives if stored without electrolyte. Lead-acid batteries are cheap and effective, and at present are the sole practical choice where high power capacities are required at sensible cost,
Ruggedized and sealed lead-acid storage batteries are in common use in portable equipment with large power requirements. However, lead-acid batteries are bulky, and their active materials are environmentally hazardous, demanding recycling as an environmental safety measure.
* A new type of lead-acid battery was introduced in the late 1990s that operates on the same chemical principles, but has a radically different construction. The electrodes are formed as thin plates, with the electrolyte stored in a separator sheet between the plates, and stored in a sealed can in a "wound" or "jelly-roll" configuration. The improved battery configuration provides higher energy density, though the environmental issues remain much the same. This is about the only significant innovation in lead-acid battery design in over a century of the technology's existence. To be sure, there have been improvements in packaging materials for lighter weight and greater reliability, but Gaston Plante would see little in a modern lead-acid battery that he didn't find familiar.
Trying to come up with a high-capacity rechargeable cell with a higher energy density at a reasonable cost has proven extremely difficult. This was frustrating even a century ago, and the well-known American inventor Thomas Alva Edison (1847:1939) spent a fortune trying to build a rechargeable cell that could improve on Plante's invention. The result was the "nickel-iron" cell, or "Edison cell", and though it still lives on in industrial uses, it never came close to displacing the lead-acid battery. The Edison cell uses an iron anode, a nickel oxide cathode, and a potassium hydroxide electrolyte. The Edison cell provides a voltage of about 1.15 volts per cell. Its main virtue is that it is extremely rugged, tolerating aggressive discharges that would ruin other types of storage cells, and has a very long service life.
* The "nickel-cadmium" or "nicad" cell is similar to the Edison cell, but
uses a cadmium instead of an iron anode. A nicad cell is generally a
cylinder with layers of cadmium and nickel oxide separated by absorbent
layers containing KOH electrolyte. The anode reaction is:
Cd + 2OH- --> Cd(OH)2 + 2e-
The cathode reaction is:
NiO2 + 2H2O + 2e- --> Ni(OH)2 + 2OH-
These are reversible reactions. The nicad produces about 1.2 volts per cell.
It has a low internal resistance and its cell voltage remains remarkably
constant until the cell is almost discharged.
While Edison batteries are generally built as large industrial units that physically resemble lead-acid batteries, nicads are built mostly for rechargeable consumer equipment and so have smaller form factors. Nicads were once predominant as rechargeable batteries in consumer gear, but they tended to be ruined by complete discharge, and the heavy-metal cadmium anode made them an environmental nuisance. Nicads are still in widespread use, particularly for portable power tools where their ability to provide large amounts of current on demand makes them particularly useful, but are now increasingly being replaced by improved rechargeable battery types.
* One such improved rechargeable technology is the "nickel-metal hydride
(NiMH)" cell. Most designs are similar to nicads, but replace the cadmium
anode with a "metal hydride", based on complex metallic alloys that can store
large quantities of hydrogen, The cathode is nickel oxide, the electrolyte is
a solution of potassium hydroxide, stored in a polymer separator sheet. The
anode reaction, with "(M)" representing the metal hydride, is:
(M)H + OH- --> (M) + H2O + e-
The cathode reaction is:
NiOOH + H2O + e- --> Ni(OH)2 + OH-
They have a typical cell voltage of 1.2 volts, which tends to remain flat
through the cell discharge cycle. They tend to have a high self-discharge
rate, but are environmentally benign, at least by the standards of storage
cells.
* Lithium is an excellent material for making storage cell anodes, since it gives up electrons very easily and is very light. Lithium cells can provide an order of magnitude better energy density than lead-acid cells. One of the big problems with lithium is that reacts violently with moisture, and manufacturing lithium cells requires a moisture-free environment. Lithium cells also require venting and other safety systems to keep them from exploding if moisture does infiltrate the case, or if such cells are heated. This delayed their use for a very long time. There are a bewildering range of lithium cell technologies. They can be basically divided into non-rechargeable lithium cells, and rechargeable "lithium-ion" cells.
* The conceptually simplest and most common nonrechargeable lithium cell is
the "lithium-manganese" cell. This has a lithium anode, a manganese dioxide
cathode, and a carbonate electrolyte. The anode reaction is:
Li --> Li+ + e-
The cathode reaction is:
MnO2 + Li+ + e- --> MnO2(Li)
The cell voltage is about 3 volts. Such cells are constructed in a jelly
roll configuration, with a sheet of lithium foil, a separator sheet
containing electrolytic salts, and a sheet of manganese dioxide rolled up
together. They have an indefinite shelf life.
There are many other nonrechargeable lithium cell configurations, such as "lithium sulfur dioxide", "lithium thionyl chloride", and "lithium polycarbonate monofluoride", with complicated constructions and chemistries that are substantially more capable than lithium-manganese but not as cheap, and so not in as widespread use. The latest generation of nonrechargeable lithium cells uses a polymeric electrolyte. Such "lithium polymer" cells have electrical characteristics similar to those of the predecessors, but they can be more easily built in flat or rectangular configurations that are very useful for lightweight portable equipment.
* The high cell voltage of the lithium ion cell means that it is not interchangeable with standard zinc-carbon or alkaline cells. A nonrechargeable "lithium disulfide" cell or "voltage compatible lithium cell" has been introduced that does provide a cell voltage of 1.5 volts. The lithium disulfide cell is also built in a jelly roll configuration, with a lithium anode, an electrolytic separator sheet, an iron disulfide (FeS2) cathode, and an aluminum cathode collector. It is lighter than an alkaline cell, has high capacity, and has a very long shelf life.
* Lithium is easier to handle in its ionized form, and so rechargeable lithium cells, which have to deal with the hazards of being recharged, have been traditionally based on lithium compounds. Again, there are many variations, but a typical "lithium ion" cell has a carbon anode, a lithium cobalt dioxide or manganese dioxide cathode, and an electrolyte consisting of a lithium salt in solution. Lithium-ion cells have a cell voltage of about 3.6 volts. They have high internal resistance and are not suited to high current applications. They have very long cycle lives, up to a thousand cycles for single cells, and their storage capacity does not degrade significantly with cycling. They are increasingly becoming the rechargeable battery of choice for portable consumer electronics equipment, though they are expensive.
* The following table summarizes electrical storage cell technologies:
_______________________________________________________________________
CARBON-ZINC (LECLANCHE) CELL:
anode: zinc cup
cathode: manganese dioxide in graphite powder
electrolyte: ammonium chloride & zinc chloride in water
cell voltage: 1.5 volts
Non-rechargeable, poor storage density, but very cheap.
_______________________________________________________________________
ALKALINE CELL:
anode: nickel-plated steel cup
cathode: manganese dioxide in graphite powder
electrolyte: potassium hydroxide in water
cell voltage: 1.5 volts
(Generally) non-rechargeable, storage density about twice that of the
carbon-zinc cell, but several times more expensive.
_______________________________________________________________________
MERCURY BUTTON CELL:
anode: zinc
cathode: mercuric oxide
electrolyte: potassium hydroxide in paste
cell voltage: 1.35 volts
Non-rechargeable. A variation on this technology used cadmium instead of
zinc and provided a cell voltage of 0.91 volts. The first button cell
technology, now obsolete due to environmental concerns.
_______________________________________________________________________
ZINC-AIR BUTTON CELL:
anode: powdered zinc in gel
cathode: carbon disk exposed to air
electrolyte: potassium hydroxide layer
cell voltage: 1.65 volts
Non-rechargeable. Most popular current button cell technology, also some
applications in larger cell formats.
_______________________________________________________________________
SILVER OXIDE BUTTON CELL:
anode: powdered zinc in gel
cathode: silver grid pasted with silver oxide
electrolyte: potassium hydroxide layer
cell voltage: 1.55 volts
Nonrechargeable.
_______________________________________________________________________
LEAD-ACID CELL:
anode: lead
cathode: lead oxide
electrolyte: sulfuric acid
cell voltage: 2 volts
The standard large capacity battery technology. Can be recharged
hundreds of times and very cheap, but bulky and environmentally
noxious.
_______________________________________________________________________
NICKEL-IRON (EDISON) CELL:
anode: iron
cathode: nickel oxide
electrolyte: potassium hydroxide
cell voltage: 1.15 volts
Heavy-duty rechargeable unit, used in some industrial applications.
_______________________________________________________________________
NICKEL-CADMIUM (NICAD) CELL:
anode: cadmium
cathode: nickel oxide
electrolyte: potassium hydroxide
cell voltage: 1.2 volts
The original rechargeable cell for portable gear, now used mostly in
gear that needs high power levels on demand.
_______________________________________________________________________
NICKEL-METAL HYDRIDE (NIMH) CELL:
anode: metal hydride
cathode: nickel oxide
electrolyte: potassium hydroxide solution in separator sheet
cell voltage: 1.2 volts
Greater capacity than nicads but more expensive.
_______________________________________________________________________
LITHIUM-MANGANESE DIOXIDE CELL:
anode: lithium foil
cathode: manganese dioxide
electrolyte: separator sheet impregnated with electrolytic salts
cell voltage: 3 volts
The most common non-rechargeable lithium cell.
_______________________________________________________________________
LITHIUM DISULFIDE CELL:
anode: lithium foil
cathode: iron disulfide with aluminum cathode contact
electrolyte: separator sheet impregnated with electrolytic salts
cell voltage: 1.5 volts
"Voltage compatible" lithium cell as direct replacement for carbon-zinc
or alkaline cells.
_______________________________________________________________________
LITHIUM-ION CELL:
anode: inert carbon sheet
cathode: manganese dioxide
electrolyte: electrolyte separator sheet with lithium ions
cell voltage: 3.6 volts
Rechargeable lithium cell.
_______________________________________________________________________
BACK_TO_TOP
* In 1839, the English physicist William R. Grove (1811:1896), working from the knowledge that running an electric current through water would produce hydrogen and oxygen, showed that combining hydrogen and oxygen could produce water and an electric current. Grove's demonstration opened the way to a new electrical power source, the fuel cell, but little was done with the concept for well over a century. Fuel cells were used to provide electrical power to the Apollo Moon capsule and other spacecraft, but failed to reach a wider market. They now are undergoing rapid development, however.
The fuel cell is conceptually simple. The electrolysis of water into
hydrogen and oxygen through the application of an electric current:
2H2O --> 2H2 + O2
-- can be reversed to produce water and electricity:
2H2 + O2 --> 2H2O
Fuel cells are based on reverse electrolysis. They resemble batteries in
that their DC electrical output is due to an electrochemical process.
However, unlike batteries, fuel cells operate off a continuous stream of air
as a source of oxygen, and a source of hydrogen fuel. While straight
diatomic hydrogen can be used, as discussed earlier this is not a very
convenient fuel, and so in general fossil fuels, such as methane, methanol,
naptha, coal gas, and other hydrocarbons, are broken down to provide
hydrogen. Fuel cells are also unlike batteries in that their active elements
are not consumed by the chemical reaction. This means that fuel cells in
principle have much longer service lifetimes than batteries.
In general form, a fuel cell consists of a porous anode and a porous cathode, with these two electrodes separated by a electrolyte. An oxidant is fed to the cathode to supply oxygen, while a fuel is fed to the anode to supply hydrogen. The electrolyte supports the transfer of ions between anode and cathode to support the reverse electrolysis reaction.
The anode and cathode may be patterned with channels to allow distribution of oxygen or hydrogen. An individual fuel cell generates from 0.6 to 0.8 volts DC, and large numbers of such cells have to be stacked in a fuel cell system and connected in series to provide a useful power output.
Different types of fuel cells operate at different temperatures, from under 100 degrees Celsius to over 1,000 degrees Celsius. The anode and cathode may also have channels to allow the distribution of coolants, such as water. The waste heat provided by fuel cells that operate at high temperatures can be used for heating, or the fuel cell can act as a "combustor" to drive a gas turbine for generating power. Such "cogenerating" systems can have high overall efficiencies.
A catalyst is often used to help accelerate the reverse electrolysis reaction, particularly in fuel cells that operate at low temperatures. The catalyst is platinum for some types of fuel cells, a factor that strongly influences their cost.
Although the only output of reverse electrolysis itself is water, the fact that most fuel cells break down hydrocarbon fuels to obtain hydrogen means that fuel cell systems generally exhaust carbon dioxide, some sulfur dioxide, and nitrous oxides along with the water. Nonetheless, fuel cells are relatively nonpolluting, and are in principle quiet, easy to maintain as they have no moving parts, and very efficient, with conversion efficiencies of roughly 50%. Cogenerating systems can approach overall efficiencies of up to 80% in ideal circumstances.
* A workable fuel cell system consists of more than just fuel cells. It will always include a "power conditioner" output subsystem to provide electrical power at the proper DC or AC voltages required by the equipment being driven. Fuel cells that use hydrocarbon fuels also require a "fuel processor" input subsystem to convert the hydrocarbons into hydrogen gas.
Fuel processing is based on methods familiar from industrial chemical plants, traditionally known as "fuel reformation". Typical fuel processing steps include:
Fuel processing can obtain heat by burning some of the hydrogen fuel, and may use a catalytic system to enhance the reaction. Some types of fuel cells are able to break down hydrocarbons in hydrogen directly at the anode using catalysts and do not need a separate fuel processing system.
* There are two general classes of fuel cells, based on whether the electrolyte is alkaline (basic) or acidic. Resistance in the electrolyte is a source of power loss, but this problem can be reduced by making the electrolyte either very alkaline or very acidic. There is only one type of alkaline fuel cell, and it is the oldest fuel cell technology. It is still in use in aerospace applications. There are four types of acidic fuel cells:
The PAFC and the PEM fuel cells are the best developed acidic fuel cells. The PAFC is in modest use as a fixed AC power source for buildings and sites, while the PEM is under intense development as a power source for automobiles. The MCFC and SOFC are also under investigation as fixed AC power sources, but their development is not as far advanced as that of the PAFC.
* The earliest modern applied fuel cell technology, the alkaline fuel cell, uses a strongly alkaline potassium hydroxide electrolyte. Since the potassium hydroxide will react with carbon dioxide to form solid potassium carbonate, the alkaline fuel cell absolutely must have a source of pure hydrogen to operate. The alkaline fuel cell operates at relatively low temperatures, in the range of 80 to 95 degrees Celsius. It uses platinum catalyst to increase the reverse electrolysis reaction rate. The alkaline fuel cell has a number of attractive features. It requires less platinum catalyst than an acidic fuel cell, and has a high power to weight ratio. Improvements in the design have resulted in reducing the electrolyte's susceptibility to carbon dioxide poisoning.
However, the alkaline fuel cell has to be supplied with pure hydrogen, since any carbon dioxide contaminants will react with the potassium hydroxide electrolyte to form solid potassium carbonate. It remains useful for aerospace applications, where its light weight is valuable and the requirement for pure hydrogen not too difficult to meet, but is not generally regarded as useful for terrestial applications.
* Of the four acidic fuel cells, the phosphoric acid fuel cell is the only one that is now in commercial use, with units installed for fixed power generation. It has also be used experimentally with large vehicles, such as buses. The PAFC uses a phosphoric acid (H3PO4) electrolyte. Most acids operate in solution, which means that a fuel cell using them must operate below the boiling point of water, reducing efficiency. Concentrated phosphoric acid does not need to be in solution and can operate at higher temperature. The phosphoric acid is contained in a matrix of silicon carbide and teflon and sandwiched by the anode and cathode, which are built as thin plates of porous graphite. Platinum catalyst laid down on these electrodes helps accelerate the electrochemical reactions. The PAFC operates at 175 to 200 degrees Celsius. Higher temperatures of course help accelerate the reaction, but above 220 degrees Celsius, the phosphoric acid tends to attack the catalyst.
* The proton exchange fuel cell, sometimes known as the polymer electrolyte fuel cell, was originally developed by General Electric in the late 1950s, but still is not in commercial use. However, there has been considerable work on its use as an automotive power source due to its relatively light weight and low operating temperature, and even some work on using it to replace batteries in portable electronic equipment such as laptop computers. One of the advantages of focusing on such applications is that both electric vehicles and portable electronics equipment run on DC electricity, reducing the requirements for power conditioning.
The operating principles of the PEM fuel cell are very similar to those of the PAFC, the main difference being that uses a polymer film, based on sulfonic acid, for an electrolyte rather than phosphoric acid. The membrane-electrode assembly of a PEM fuel cell is very thin, on the order of a few millimeters. Traditionally, the film has been a polymer produced by DuPont known as "Nafion", but new polymers are now in the works that promise more efficient PEM cells. The PEM fuel cell operates at low temperatures, similar to those of the alkaline fuel cell, in the range of 80 to 95 degrees Celsius. Also like the alkaline fuel cell, it uses platinum catalyst to increase the reverse electrolysis reaction rate. Much work has been done on reducing the amount of platinum required, and in current fuel cells small atomic clusters of platinum are deposited on fine carbon particles.
* The two remaining acidic fuel cell types, the molten carbonate and solid oxide fuel cells, remain generally experimental devices. They are being considered for fixed site power generation systems much like the PAFC systems now in use. The MCFC uses a mix of molten lithium, sodium, and potassium carbonate (K2CO3). It operates at 540 to 650 degrees Celsius, which is hot enough to keep the electrolyte molten. The high operating temperature allows the MCFC to convert hydrocarbon fuel into hydrogen without a separate reformer.
The carbonate electrolyte is contained in a porous board of lithium aluminate. The anode is made of nickel and the cathode is nickel oxide, to which silver is sometimes added. The nickel and silver act as catalysts. The operating temperature of the MCFC is between 600 and 700 degrees Celsius, hot enough to keep the electrolyte molten. The major problem with the MCFC is that the molten carbonate electrolyte tends to attack the electrodes.
The SOFC is attractive because its electrolyte will not leak and is not corrosive. The electrolyte consists of solid zirconium oxide, stabilized with yttrium oxide. The SOFC operates at 980 degrees Celsius and uses titanium-based perskovite crystals for a catalyst. Like the MCFC, its high operating temperature eliminates the need for a separate fuel reformer subsystem. However, the electrolyte materials are expensive.
* Although large commercial fuel-cell power generation plants providing power in the megawatts have been built, these were isolated experiments that generally proved somewhat too ambitious. Companies developing fuel cell systems for power generation have instead turned to manufacturing smaller units, useful for power cogeneration at fixed sites, such as hospitals, or remote locations where network power is unavailable or uncertain. While costs still remain high in comparison to diesel or other backup power systems, the relatively clean nature of fuel cells and their low maintenance make them attractive. They operate at a very constant efficiency, no matter what the output power load is.
Small systems about the size of a large refrigerator and with output power in the range of 3 kilowatts are now being designed as household power supplies, using natural gas as fuel. Ironically, these small systems generally use a bank of lead-acid batteries to help meet peak power demands, with the batteries recharged by the fuel cell system when demand falls off.
* Although PAFC systems have been used to experimentally power buses and other large vehicles, they are simply too big and cumbersome for use with a normal automobile. Major automobile manufacturers have built test prototypes of vehicles using PEM cells as powerplants.
An automotive fuel cell system must provide about 50 kilowatts of power, though a hybrid vehicle could use a 15 kilowatt fuel cell system along with a battery system to provide peak power. Such a hybrid system could also improve automobile efficiency by providing "regenerative braking", where the braking system feeds power back into the batteries. However, hybrid vehicles tend to be relatively complicated and expensive. Prototype automobiles powered by fuel cells operate on methanol and have ranges comparable to those of conventional gasoline powered automobiles, but costs for fuel cell powered vehicles still remain uncompetitive.
Another advantage of fuel cells for automotive applications is that they can use a variety of different fuels, such as methanol, methane, or gasoline. The ultimate dream of "clean car" advocates is a fuel cell vehicle operating directly off hydrogen fuel and exhausting little but water -- but as discussed earlier, hydrogen is a very troublesome fuel to handle.
* Work on PEM fuel cells for portable electronics equipment, such as handheld computers or cellphones, remains speculative but very interesting. Prototypes of such "micro fuel cells" (MFCs) have been built, in one case using microcircuit fabrication techniques to pattern the components. Other research has focused on low-cost catalytic schemes using platinum and ruthenium to allow the anode to directly break down fuel into hydrogen, without need for a fuel processor.
MFCs potentially offer 40 or 50 times the endurance of nickel-cadmium battery packs at half the weight, though with the same volume. MFCs would be powered by a disposable methanol cartridge, allowing for an instant "recharge". How the exhaust products are handled is an interesting question, but the idea remains intriguing.
* More exotic classes of fuel cells are now being investigated in the lab. One particularly interesting item is a "biofuel cell" designed to power bioimplants, with the energy derived from the body itself. The cell combines hydrogen obtained from glucose and oxygen in the bloodstream to produce electricity. The biofuel cell consists of two carbon electrode threads, each about seven microns in diameter, linked to a cell encased in plastic. Each of the threads is coated with enzymes designed to promote the proper electrode reactions. The cell can generate a maximum of 0.8 volts and 0.6 microwatts of power, adequate to run a low-power silicon chip.
An even more interesting scheme is to build fuel cells driven by microbes that obtain their energy by breaking down biomass. Academic researchers have used a two-chamber water tank, with each chamber containing a solid graphite electrode and the two electrodes connected by a wire. One chamber was used to support a culture of a bacterium named Rhodoferrax ferrireducens, found in marine sediments, that could generate excess electrons as part of its metabolic processes. When fed sugars, the bacteria grew and coated the graphite anode on their side of the tank, pumping electrons through the circuit. Conversion efficiency was very high, about 80%, compared to the 50% obtained from experiments elsewhere along this line, though the reaction and power rate was still low. In any case, this was strictly a lab demonstration that was not remotely near practical application.
