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[13.0] Fuel Cells

v1.3.0 / chapter 13 of 15 / 01 jun 16 / greg goebel

* Batteries are now being complemented by another electrochemical power-generating system, the "fuel cell". The fuel cell is by no means new, but is only now poised for widespread application. This chapter discusses fuel cell technology.

fuel cell site power unit


[13.1] FUEL CELL FUNDAMENTALS
[13.2] FUEL CELL TYPES
[13.3] FUEL CELL OUTLOOK
[13.4] FOOTNOTE: FLOW BATTERIES

[13.1] FUEL CELL FUNDAMENTALS

* 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 it 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.

general fuel cell construction

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 since 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.

a fuel cell power system

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.

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[13.2] FUEL CELL TYPES

* 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 most mature 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.

The alkaline fuel cell 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 terrestrial 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 instead of 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 perovskite 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.

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[13.3] FUEL CELL OUTLOOK

* 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. Nobody's seeing it as happening any time soon.

* 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 (MFC)" 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". Since soldiers often carry a considerable amount of electronic gear into combat, the military has backed research into small fuel cells that would reduce the weight carried by an infantryman while providing power for a longer time.

* 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.

microbial fuel cell

An even more interesting scheme is to build fuel cells driven by microbes that obtain their energy by breaking down biomass. Such fuel cells look pretty much like ordinary fuel cells, but they use anaerobic (non-oxygen breathing) bacteria act as the "catalyst" for a fuel cell anode, breaking down whatever fuel they can digest. The cathode still requires catalysts to transform oxygen and protons into water. The feedstock in lab items is often glucose, but there's work on microbial driven by waste water, to produce not only electricity but also clean water. It's not enough power to do more than run the facility, but the idea is still slick. They're still lab toys, however.

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[13.4] FOOTNOTE: FLOW BATTERIES

* As something of a footnote to the subject of fuel cells, there is a class of electrochemical power systems, known as "flow batteries", that are somewhere in between fuel cells and traditional batteries.

At the core, flow batteries look like fuel cells, with a cell stack of alternating cells -- featuring electrodes on each side of a cell and an ion-exchange membrane splitting the middle of the cell -- but instead of being fed with fuels, they are fed by two tanks of electrolyte being pumped through their respective sets of half-cells. The storage capacity is generally dependent on the size of the electrolyte tanks. While the flow battery is rechargeable, it can also be instantly recharged just by swapping out tanks of electrolyte. Rechargeability is another way flow batteries resemble batteries more than fuel cells; those who prefer to think of them as fuel cells compromise by calling them "regenerative fuel cells".

Several different electrolyte combinations have been considered for flow batteries. Zinc-bromine is one of the most familiar. A zinc-bromine flow battery features electrodes made of an inert conductive material, for example carbon a conductive plastic composite, with the chemically active component of the battery residing in the electrolyte, consisting of zinc bromide (ZnBr2) in water, with the electrolyte containing some additives in the cathode half of the system.

zinc-bromine flow battery

During charging, the zinc bromide breaks down, with the ion-exchange membrane helping to segregate the zinc on the anode side -- where it forms a thin plating on the anode -- and the bromine on the cathode side -- where it reacts with the additives to form a heavy oil that is pumped out with the electrolyte and sinks to the bottom of the tank. The additive scheme is used because bromine is a dangerously toxic halogen, and binding it up into an oil prevents it from escaping as a gas. During discharging, the electrolyte is pumped back in again, with a pump mixing up the bromine oil in the electrolyte before it's driven to the cell, where the zinc plating wears away and the bromine oil dissociates. Cell voltage is about 1.8 volts. Since the electrodes themselves are not affected by charge-discharge cycles, the flow battery has a long cycle life.

Vanadium flow batteries are also popular and have a similar architecture, but they use vanadium oxides in a sulfuric acid solution as the active electrolyte, with the chemical reactions exploiting the fact that vanadium has four oxidation states, resulting in somewhat confusing chemistry involving a set of vanadium and vanadium oxide ions. Vanadium flow batteries have a cell voltage of about 1.41 volts; are more efficient than zinc-bromine flow batteries, but they have an inferior energy density.

Flow batteries involve some complications relative to conventional battery designs, in particular a pump and plumbing system, though the flow system also helps with cooling. However, although they certainly aren't all that well-suited to powering a car and probably not a bus, they scale up very well. At large sizes they are cost-effective, several times cheaper than lithium batteries and somewhat cheaper than their toughest competitor, the sodium-sulfur battery. The flow battery is generally safe -- the electrical energy is stored in the two electrolytes, which are kept in separate tanks, with only a small portion of them coming into proximity in the cell stack at any one time.

Flow batteries are already being used for backup power at factories and cellphone towers, and are being increasingly used for power-grid backup, with units now online running to tens of megawatt-hours. Research is ongoing to make them cheaper, more reliable, and more efficient.

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