A fuel cell is an electrochemical device similar to a battery, but differing from the latter in that it is designed for continuous replenishment of the reactants consumed; i.e. it produces electricity from an external fuel supply as opposed to the limited internal energy storage capacity of a battery.
Typical reactants are hydrogen on the anode side and oxygen on the cathode side. In contrast, conventional batteries consume solid reactants and, once these reactants are depleted, must be discarded, recharged with electricity by running the chemical reaction backwards, or, at least in theory, having their electrodes replaced. Typically in fuel cells, reactants flow in and reaction products flow out, and continuous long-term operation is feasible virtually as long as these flows are maintained.
Fuel cells are also attractive in some applications for their high efficiency and low pollution. Some applications that have been suggested include
- baseload utility power plants,
- emergency backup power,
- off-grid power storage,
- portable electronics, and
- electrically-powered vehicles.
Types of fuel cells
There are five generally recognised types of fuel cells, of which two are the main subject of intensive research.
PEM fuel cells have a disputed acronym, meaning either proton-exchange membrane or polymer-electrolyte membrane, which both are in fact a good description. In this fuel cell, hydrogen is split at the membrane surface in protons, that travel through the membrane, and electrons, that travel through our external electric circuit, and provide our power. The hydrogen ions travel through the water that is entrained in the membrane to the other side, where they are combined with oxygen to form water. Unfortunately, while the splitting of the hydrogen molecule is relatively easy, splitting the stronger oxygen molecule is more difficult, and this causes significant losses that result in a sharp decrease of performance of the fuel cell. The PEM fuel cell is a prime candidate for vehicle and other mobile applications of all sizes down to mobile phones, because of its compactness. However, the water-entraining membrane is crucial to performance: too much water will flood the membrane, too little will dry it; in both cases, power output will drop; water management is a very difficult subject in PEM fuel cells. Furthermore, the platinum catalyst on the membrane is easily poisoned by CO.
PEM systems that use reformed methanol were proposed, as in Daimler Chrysler Necar 5; reforming methanol, i.e. making it react to obtain hydrogen, is however a very complicated process, that requires also purification from the CO the reaction produces. A platinum-ruthenium catalyst is necessary as some CO will unavoidably reach the membrane. The level should not exceed 10 parts per million.
A subcategory of PEM is the DMFC, or direct methanol fuel cell; here, the methanol is not reformed, but fed directly to the fuel cell. One does not need complicated reforming, and storage of methanol is much easier than that of hydrogen. However, efficiency is low, due to the high permeation of methanol through the membrane, and the dynamic behaviour is sluggish.
The main manufacturer of PEM is Ballard Power Systems of Vancouver, Canada. Efficiencies of PEM are in the range of 40-50%.
Solid oxide fuel cells, or SOFC, are intended mainly for stationary applications (power plants). They work at very high temperatures (some at 1000ºC), and their off-gases can be used to fire a secondary gas turbine to improve efficiency. Efficiency could reach as much as 70% in these hybrid systems. This time, it's oxygen being transferred through a solid oxide at high temperature to react with hydrogen on the other side. SOFC have such a high temperature that they can be fed (provided some modifications) with natural gas, that will react to give hydrogen in the fuel cell itself. SOFC are very resistant to poisoning, and can indeed be run on CO, which is a poison for PEM.
Since SOFC are made of ceramic materials, they tend to be brittle; they are therefore unsuited for mobile applications. Furthermore, thermal expansion demands a uniform and slow heating process at startup, that will cause very long startup times: typically, 8 hours are to be expected. Research is going now in the direction of lower-temperature SOFC (600ºC), which will enable the use of metallic materials with better mechanical properties and heat conductivity.
Molten-carbonate fuel cells (MCFC) are also high-temperature, but in the range of 600ºC. Their main problem is corrosion, and the need to operate a high-temperature liquid rather than a solid as in the SOFC.
Phosphoric-acid fuel cells (PAFC) are a mature technology that is commercially available. Unfortunately, the phosphoric acid solidifies under 40ºC, making startup very difficult. They have been used for stationary applications with an efficiency of about 40%, and many believe they do not offer much potential for further development.
The alkaline fuel cell (AFC) is the cell that brought the Man to the Moon. Used in Apollo-series missions and on the Space Shuttle, it is a very good fuel cell but for the fact that it is poisoned by CO2. This means that the cell will require pure oxygen, or at least purified air. As this process is relatively expensive, not much development is being done on AFC. NASA has decided they will shift to PEM for the next generation of Space Shuttles.
Fuel cells are electrochemical devices, so they are not constrained by the maximum thermal (Carnot) efficiency as combustion engines are. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy.
In the archetypal example of a hydrogen/oxygen polymer electrolyte membrane (PEM) fuel cell, a proton-conducting polymer membrane separates the anode ("fuel") and cathode sides. Each side has an electrode, typically carbon paper coated with platinum catalyst.
On the anode side, hydrogen diffuses to the anode catalyst where it dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electronically insulating.
On the cathode catalyst, oxygen molecules react with the electrons (which have travelled through the external circuit) and protons to form water.
In this example, the only waste product is water vapor.
Also, there is the possible use of fuel cells at home, to store energy at the cheap off-peak electricity rates and used at peak-use hours. It may even be profitable to sell back some of the energy to the power company, like they do with windmill electric power. Peak power production reaches twice the normal level, which means that the very expensive powerplant capacity is sized for levels used for a short period of time. Also, power plants are most efficient at only one production rate and their efficiency drops off significantly at off-peak rates.
The first fuel cell was developed in the 19th century by British scientist Sir William Grove. A sketch was published in 1843. But fuel cells did not see practical application until the 1960s, where they were used in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks). Extremely expensive materials were used and the fuel cells required very pure hydrogen and oxygen. Early fuel cells tended to require inconveniently high operating temperatures that were a problem in many applications.
Further technological advances in the 1980s and 1990s, like the use of Nafion as the electrolyte, and reductions in the quantity of expensive platinum catalyst required, have made the prospect of fuel cells in consumer applications such as automobiles more realistic.
The fuel cell industry
Ballard Power Systems is a major manufacturer of fuel cells and leads the world in automotive fuel cell technology. Ford Motor Company and DaimlerChrysler are major investors in Ballard. As of 2006, the only major automobile companies pursuing internal development of fuel cells for automotive use are General Motors, Toyota and Honda; most others are customers of Ballard.
United Technologies (UTX) is a major manufacturer of large, stationary fuel cells used as co-generation power plants in hospitals and large office buildings. The company has also developed bus fleets that are powered by fuel cells.
Pros and cons of fuel cells in various applications
Their use is controversial in some applications. The hydrogen typically used as a fuel isn't a primary source of energy. It is usually only a source of stored energy that must be manufactured using energy from other sources. Some critics of the current stages of this technology argue that the energy needed to create the fuel in the first place may reduce the ultimate energy efficiency of the system to below that of highly efficient gasoline internal-combustion engines; this is especially true if the hydrogen is generated from electrolysis of water by electricity. On the other hand, hydrogen can be generated from methane (the primary component of natural gas) with approximately 80% efficiency. The methane conversion method releases greenhouse gases, however, and the ideal environmental system would be to use renewable energy sources to generate hydrogen through electrolysis. Other types of fuel cells don't face this problem. For example, biological fuel cells take glucose and methanol from food scraps and convert it into hydrogen and food for the bacteria.
There are practical problems to be overcome as well. Although the use of fuel cells for consumer products is probable in the near future, most current designs won't work if oriented upside down. They currently cannot be scaled to the small size needed by portable devices such as cell phones. Current designs require venting and therefore cannot operate under water. They may not be usable on aircraft because of the risk of fuel leaks through the vents. Technologies for safe refueling of consumer fuel cells are not yet in place.
Among the controversies in the use of hydrogen are: First, the energy used to produce the hydrogen is comparable to the energy in the hydrogen, it is inefficient therefore too expensive. If conventional powerplants were used to produce the hydrogen, at best, there would be no gains in current pollution rates. Second, some have suggested that this is a "stalking horse" to bring back nuclear power, which may be the only commercially viable way to make hydrogen and reduce pollution. Finally, the need to provide the very long, costly and vulnerable thousands of miles of gas lines makes hydrogen too costly without government help.
There are several advantages to hydrogen as well. Clean, renewable energy sources like solar and wind power are non-continuous and unreliable through the course of a day. So power from these sources is not always available at the time it is needed. The electricity produced from solar panels or wind generators could be stored in large battery complexes but this can be expensive and batteries have a limited storage capacity and lifetime. If the electricity is used to produce hydrogen however, the energy can be stored more easily. As a gas hydrogen is not hard to store until it is needed.
- 2004-02-16, ScienceDaily: New Reactor Puts Hydrogen From Renewable Fuels Within Reach Citat: "...The first reactor capable of producing hydrogen from a renewable fuel source - ethanol - efficiently enough to hold economic potential has been invented..."
- Fuel Cells Hydrogen Technology News and Events
- BBC News, 16 June, 2000: Fuel cells get even cooler
- PhysicsWorld: Fuel cells
- PhysicsWorld: Fuel cells: power for the future Quote: "... The high efficiency of fuel cells, which can range from 40% to 70%, and the possibility of improving the overall efficiency of such systems using combined heat and power (CHP) designs would significantly reduce emissions...."
- How Hydrogen Can Save America by Wired Magazine