H2(g) + O2 (g) → H2O(l)
ΔH=-287 kJ mol-1
A hydrogen-oxygen fuel cell
How many types of fuel cell are there?
There are five major types of fuel cells being known or used in the market. They all have the same basic design as mentioned above, but with different chemicals used as the electrolyte. These fuel cells are:
- Alkaline Fuel Cell (AFC)
- Phosphoric Acid Fuel Cell (PAFC)
- Molten Carbonate Fuel Cell (MCFC)
- Solid Oxide Fuel Cell (SOFC)
- Proton Exchange Membrane Fuel Cell (PEMFC)
All the above fuel cells require fairly pure hydrogen fuel to run. However, large amount of hydrogen gas is difficult to transport and store. Therefore, a reformer is normally equipped inside these fuel cells to generate hydrogen gas from liquid fuels such as gasoline or methanol.
Among these five types of fuel cell, PEMFC has the highest potential for widespread use. PEMFC is getting cheaper to manufacture and easier to handle. It operates at relatively low temperature when compared with other types of fuel cell. Alkaline Fuel Cell (AFC) has the highest efficiency and is therefore being used to generate electricity in space craft for more than thirty years. However, it requires very pure hydrogen and oxygen to operate and thus the running cost is very expensive. As a result, AFC is unlikely to be used extensively for general purposes, such as in vehicles and in our homes. In contrast, the Molten Carbonate Fuel Cell (MCFC) and the Solid Oxide Fuel Cell (SOFC) are specially designed to be used in power stations to generate electricity in large-scale. Nevertheless, there are still a lot of technical and safety problems associated with the use of these fuel cells (MCFC and SOFC) in the long term.
Apart from the five of types fuel cell mentioned above, there is a new type of fuel cell called Direct Methanol Fuel Cell (DMFC) which is being under vigorous on-going research. This type of fuel cell has the same operating mechanism as PEMFC, but instead of using pure hydrogen, it is able to use methanol directly as the basic fuel. A reformer is therefore not essential in this fuel cell system to reform complex hydrocarbons into pure hydrogen. Several companies around the world are presently working on DMFC to power electronic equipments. The DMFC appears to be the most promising alternative electric source to replace the battery used in portable electronics such as mobile phones and laptop computers.
Why are fuel cells being developed so fast?
With the ever increasing world consumption of fossil fuels, sooner or later the reserve of fossil fuels will be used up. We should look for a renewable energy source to resolve this problem. Besides, the burning of fossil fuels has caused serious environmental problems such as air pollution, oil spillage and global warming. Using fuel cells to replace fossil fuels as our primary energy source can solve these problems because fuel cells:
- are clean energy source
- have high efficiency, 40-50% of the chemical energy being converted to electrical energy
- use a variety of fuels e.g. hydrogen or natural gas or methanol or hydrocarbons
- are reliable, maintainable and durable
What are the applications of fuel cells?
Space Exploration
In the late 1950’s, NASA began experiments with the fuel cell technology to develop power source required for space travel. Fuel cell technology is also used in space shuttles and the new international space station. Electrical power for NASA's Space Shuttle Orbiter is provided by fuel cells. Each fuel cell is a self-contained unit with size 14x15x45 inches, weighs 260 pounds and is capable of providing 12 kW of electricity continuously, and up to 16 kW for short period of time.
Transportation
Most vehicles today use internal combustion engines that burn fossil fuels to generate energy. However, such system has many unpleasant side-effects including air-pollution and low efficiency. Therefore, in order to replace the internal combustion engine, a fuel cell has to be safe to operate with high efficiency and low cost, it must also be fairly light with high power densities. Most of the leading automotive manufacturers have been focusing on building fuel cell vehicles for some time. As a matter of fact, some major automotive companies have already begun to rent out vehicles that used fuel cell as the energy source. However, this is only limited to Japan and California at the moment. Automakers and experts speculate that it will be another few years before fuel cell vehicle can be fully commercialized. Nowadays, fuel cells are also being installed in bikes, scooters, and airplanes.
Stationary and Residential Applications
At present, thousands of fuel cell systems have been incorporated into hospitals, nursing homes, hotels, commercial buildings, schools, power stations, airports as principal electric generator or to provide backup power when necessary. Fuel cell system is ideally to be used for residential and stationary applications because it operates silently with low level of noise and air pollution, also the heat it releases during operation can be used to boil water for shower and central heating system inside the house.
Portable Power for Electronics
In the near future, it will be of no surprise that miniature fuel cell power generators suitable for use in portable electronics such as PDAs, notebook computers and cellular phones will be pervaded in the electronic world. Researchers have been trying to invent fuel cells that are small, lightweight, refillable, low operating temperature and having higher energy density to replace the lithium batteries that are now being used in notebook computers and cellular phones. The most promising fuel cell type for portable applications is the direct methanol fuel cells (DMFC). These fuel cells have been estimated by scientists to have energy storage that is ten times that of the lithium batteries.
How far has the development of fuel cell gone?
Although the development of fuel cell is fast, fuel cell has not yet reached its potential level of commercial success due to high material costs (Pt electrode) and market barriers. To tackle the electrode problem, platinum or platinum-based nanoparticles are coated on the surface of carbon black. This porous electrode used in fuel cells can give higher current densities than geometric plate electrode. More importantly, it can reduce the quantity of platinum used.
To be successful in the marketplace, fuel cells have to attain a number of requirements. Firstly, its cost of investments must not be too high. According to a report, current fuel cell power plants cost about $3000/kW. Therefore, to increase the competitiveness of the fuel cells in the market, it is necessary for this price to be reduced to $1500/kW or less in order to fit in the utility and commercial on-site market. Secondly, better designs including size and weight are essential to fit the actual market needs. Other factors, such as efficiency, high power output, ease of use and maintenance etc, are also important.
Questions
-
What are the major differences between a fuel cell and a conventional electrochemical cell like a zinc-carbon cell?
- Outline the advantages that would be gained by generating electricity in fuel cells rather than in the thermal power stations used at present.
- What are the advantages and disadvantages of using fuel cells in portable electronic devices?
- Write half-equations for the reactions that would occur at the anode and cathode in a Direct Methanol Fuel Cell. Use these half-equations to produce an overall equation showing the net reaction occurring in the cell.
- Electrical power for space shuttles is provided by fuel cells for about forty years. Write a précis on the fuel cells used and their future development.
References
http://www.fuel-cell-bus-club.de/html/body_new_generation_fuel_cell_buses.html
http://ne.nikkeibp.co.jp/english/2002/02/0130toshiba_device.html
http://www.me.umn.edu/courses/me4054/dfe/power.html
http://www.nasa.gov/missions/science/focus_fuel_cell.html
http://www.staff.ncl.ac.uk/p.a.christensen/dmfc1.htm
