Fuel cell

From Environment & Energy Wiki

Contents 1 Description 2 Detailed description 2.1 Issues 3 References

Description

A fuel cell is a device to convert chemical energy directly into electrical energy. The chemical energy may be supplied by gaseous or liquid fuel, which will oxidise with oxygen, air or any other oxidant.

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Detailed description

The above image is in the public domain and was taken from Wikipedia^[1]

As one example of a fuel cell, a hydrogen atom is catalytically converted by a platinum anode which allows the electron to flow through a circuit to the cathode. The proton passes through the membrane to the cathode, where it is oxidised by the oxygen or air and receives the returning electron to form an electrically neutral molecule of water (along with a second hydrogen proton). Of course, this is a simplification of a complex product.

Other types of fuel cell include those which are being developed for powering small portable electronic devices such as computers and mobile telephones. Instead of hydrogen, the fuel used will typically be methanol.

Issues

There are a number of problems with the exploitation of fuel cells in practice. It would seem unlikely that large scale fuel cell implementation will occur in the near future, especially for automotive applications.

The following information is an edited excerpt from Wikipedia

• Costs. In 2002, typical cells had a catalyst content of US$1000 per kilowatt of electric power output. In 2008, UTC Power has 400kw Fuel cells for $1,000,000 per 400kW installed costs. The goal is to reduce the cost in order to compete with current market technologies including gasoline internal combustion engines. Many companies are working on techniques to reduce cost in a variety of ways including reducing the amount of platinum needed in each individual cell. Ballard Power Systems have experiments with a catalyst enhanced with carbon silk which allows a 30% reduction (1 mg/cm² to 0.7 mg/cm²) in platinum usage without reduction in performance.
• If automotive applications were to become mainstream, the quantity of platinum available is unlikely to satisfy the demand. As it is, the cost of platinum has doubled from about $1000 to $2000 per troy ounce in one year, partially because the demand has increased and partially because of speculation and a hedge against the dollar losing value.
• The production costs of the PEM (proton exchange membrane). The Nafion membrane currently costs €400/m². This, and the Toyota PEM and 3M PEM membrane can be replaced with the ITM Power membrane (a hydrocarbon polymer), resulting in a price of ~€4/m². In 2005 Ballard Power Systems announced that its fuel cells will use Solupor, a porous polyethylene film patented by DSM
• Water and air management^[2] (in PEMFCs). In this type of fuel cell, the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, resistance across it increases, and eventually it will crack, creating a gas "short circuit" where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction. Methods to manage water in cells are being developed like electroosmotic pumps focusing on flow control. Just as in a combustion engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel cell operating efficiently.
• Temperature management. The same temperature must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading. This is particularly challenging as the 2H₂ + O₂ -> 2H₂O reaction is highly exothermic, so a large quantity of heat is generated within the fuel cell.
• It takes typically 10 minutes for a fuel cell to reach an efficient operating temperature from average ambient temperatures. This is not an issue for static applications but it means a wait before a driver can start to move his vehicle in automotive applications. It also means a reduction in fuel efficiency where the average trip length is short.
• Durability, service life, and special requirements for some type of cells. Stationary applications typically require more than 40,000 hours of reliable operation at a temperature of -35 °C to 40 °C, while automotive fuel cells require a 5,000 hour lifespan (the equivalent of 150,000 miles) under extreme temperatures. Automotive engines must also be able to start reliably at -30 °C and have a high power to volume ratio (typically 2.5 kW per liter).
• Means must be applied to prevent the water produced from freezing either in the exhaust pipe or on the road when operated in sub-zero temperatures
• Limited tolerance of impurities in either the fuel or the oxidant. Pollutants can reduce the efficiency of the membrane very rapidly.

References

1. ↑ Fuel cell[1]
2. ↑ Water_and_Air_Management