2H2(g) + O2(g) → 2H2O(l) (5,11)
Applications of the cell
The hydrogen-oxygen fuel cell is (in terms of energy sources) a relatively new technology. This means that while it is available in some commercial situations, research is still being conducted to improve efficiency and expand potential applications. Several applications that either exist or are being researched include:
- Use in Space-Craft
- Use in new ‘clean’ automobiles
- Powering Houses
- Powering Medium sized portable devices (eg. Laptops)
The use and suitability comes down to the type of fuel cell, efficiency and cost. Space craft fuel cells are usually alkaline cells whereas most automobile prototypes are based on the PEM. (3)
The most common, current use of hydrogen-oxygen fuel cells is in spacecraft. (5,6) Some modern cars (as well as buses and trucks) are run by hydrogen fuel cells as well, however due to limited availability of hydrogen and the cost of fuel cells, this is not a common occurrence.
Uses of the cell (in relation to its properties)
Spacecraft and is the only readily accessed application (of the following) the other uses are either in research or testing (prototypes)
(3,4,7)Cost considerations
There are a variety of cost considerations that arise in the production and application of Hydrogen-Oxygen Fuel Cells:
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Cost of Catalyst – The oxidations and reduction half-reactions require the use of a precious metal catalyst, usually platinum or an alloy of platinum, palladium, or ruthenium. And as these metals are in scarce supply and high demand, the cost of obtaining them is quite high.(1)
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Cost of Hydrogen Production The production of Hydrogen can be achieved in a number of ways:
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Electrolysis of water is the most effective way of producing hydrogen gas. However, this reaction requires electricity. There are two possibilities for producing hydrogen this way: immediate cell regeneration and solar electrolysis plants. Neither of these is currently very efficient for the following reasons: To minimise the amount of energy needed and hence create an efficient regenerative cell, a catalyst is needed to lower the activation energy of the electrolysis. The catalyst used is a manufactured compound called Rubippy similar to chlorophyllwith a metal centre (in chlorophyll it's a magnesium ion, in rubippy it's a ruthenium ion) and an attached system of organic rings (in chlorophyll its a porphyrin derivative, in rubippy its a pyridine derivative). (6)
Figure 6A: Rubippy Molecule
The cost of this catalyst is high, as well as it being inefficient and instable. Research is being conducted to find a more cost-efficient catalyst.(6)
The other possibility, an electrolysis solar hydrogen plant, would be ideal as it involves utilizing two renewable resources, water and sunlight. However, the cost of solar cells and of the transportation of solar energy (with current technology) means that this is just not an option
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Another method of extraction is treatment of methane. This method deals with environmental issues and will be discussed below.
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Cost of Auxiliary systems that supply reactants and remove product(s) Is another cost consideration, especially the fuel cells on Spacecraft. (5)
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The fact that Fuel Cells are a relatively new technology means they are not yet fully commercialised and hence that they are produced in small numbers. For this reason, fuel cells will be more expensive, until they are produced on a larger scale. This means that, as research is conducted and fuel cells more readily enter the market, the price will decrease. Alkaline Fuel Cells are cheaper to produce but are easily poisoned by CO2. For this reason, pure oxygen must be fed to the cell (cost of auxiliary systems that supply pure oxygen).
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Cost of liquefying and refrigerating Hydrogen is also a consideration. (10)
Environmental factors
Hydrogen-Oxygen fuel cells are a ‘clean’ source of energy; the only products from the reaction are water and heat. (1,2,3,4,5,6,7,8) This alone puts it far ahead any other current fuel technology in terms of emissions and environmental impacts. Other factors include the greater fuel efficiency. Hence the actual function of the cell is ‘clean’. The negative environmental aspects of hydrogen-oxygen fuel cells are associated with current production of Hydrogen. (10) The two current methods of obtaining Hydrogen are methane treatment and electrolysis. Most hydrogen production comes from the former, treatment of methane with steam. There are two environmental impacts of this form of production. The first being the formation of poisonous carbon monoxide shown in the equation below:
CH4 (g) + H2O (g) → 3H2(g) + CO (g) (10)
The other impact comes from the steam, which is produced through the burning of fossil fuels. For this reason, Hydrogen extraction from methane (or any other hydrocarbon) is not an acceptable method as it defeats or undermines the ‘clean’ functioning of the fuel cell. (10) Large scale electrolysis with non-renewable energy sources undermines the fuel cell function in the same way.
For this reason, research is being conducted into another type of PEM fuel cell: the regenerative fuel cell. (2) Cost considerations above (catalyst and energy needed) are merely proof of the fact that research in this field is young. NASA and other scientific (and commercial) organizations all around the world are conducting research into this type of cell in the hope of creating a truly ‘clean’ energy source with little to no negative environmental impact. (2,4,5,10)
Another Type of fuel cell, the solid oxide fuel cell uses methanol-like fuels to extract hydrogen for fuel. This produces carbon dioxide and hence offsets the environmental benefit of fuel cells, however, the use of this type of fuel cell (as it is more convenient than a PEM, using a hydrocarbon fuel) may make fuel-cell-powered cars more common. This could eventually result in a replacement of Solid Oxide fuel cells with PEM fuel cells (in individual cars) once the technology is more developed and ‘get the ball rolling’ for fuel cell cars becoming commonplace. (4)