Introduction
Conclusions
Transportation Fuel Cells
The alkaline fuel cell (AFC) and the polymer electrolyte membrane (PEM) cells are better suited for transportation applications than the larger phosphoric acid, molten carbonate, and solid oxide cells.
Overview
Alkaline fuel cells have been used since the 1960’s to provide power for the Apollo and space shuttle programs. They have a very high efficiency of 70% because of their low operating temperature, and are among the most efficient electricity producers. AFC's use an aqueous solution of potassium hydroxide soaked in a porous stabilized matrix and the potassium hydroxide concentration varies with operating temperature (65 – 220 ˚ C).
In the AFC, the hydroxyl ion migrates from the cathode to the anode where it reacts with hydrogen to form water and electrons. Water from the anode then migrates back to the cathode to regenerate hydroxyl ions. As a result, electricity and heat are produced.
Source: www.fctec.com
Alkaline fuel cells are the cheapest cells to manufacture because the catalyst can be a number of different materials and are rather inexpensive. The catalysts, however, are very sensitive to poisoning by carbon monoxide, water, and methane.
Carbon dioxide reacts with the electrolyte to form carbonates, which poison the cell and degrades its performance. As a result of carbon dioxide reacting with the hydroxide ions, there is reduced hydroxide concentration, an increase in electrolyte viscosity that leads to lower diffusion rates, carbonate salts precipitate and reduce mass transport , and oxygen solubility and electrolyte conductivity are reduced.
Because of its sensitivity, current AFC fuel cells have been limited to closed environments and were not really considered for automobile applications. Cracked ammonia, however, does not contain carbon, and could be fed directly to the cell to form hydrogen, this would not require purification, as is necessary when hydrogen is produced from carbon containing fuel sources. Alternatively, if liquid hydrogen is used as the fuel, heat exchangers could condense the carbon dioxide out of the cell.
Design
AFC’s using immobilized electrolytes are considered fully developed with the exception of increasing the lifespan of the power plants. Potassium hydroxide has the highest conductance among the alkaline hydroxides, and is the preferred electrolyte.
The electrolyte is retained in a matrix (usually asbestos) and electro-catalysts are used to promote reaction. The lifespan limiting factor is currently KOH corrosion of the cell support. Sodium hydroxide is cheaper and increases the electrode lifetime, however, it does not perform well enough for implementation.
Increased operating pressure, as in other fuel cells, leads to enhanced performance, and combining high pressure systems with increased operating temperatures achieves faster kinetics. For the space shuttle, which already has compressed gas, there is no energy loss.
For other applications, however, compressors are noisy and use power, and it is unlikely that there would be an increase in overall efficiency.
Future
One approach uses circulating electrolytes with an external absorber to remove carbon dioxide from the fuel stream. During operation, the electrolyte would circulate continuously, which would prevent drying out of the cell, provide heat management, diminish the hydroxide concentration gradient, prevent build up of gas bubbles, and accumulated impurities, such as carbonates, would be concentrated in the circulating stream and easily removed. Additionally, the cell would have high reactivity without the need of noble metal catalysts.
Proton Exchange Membrane Fuel Cell (PEM)
Overview
The proton exchange membrane fuel cell may be the cheapest fuel cell system available, especially since the platinum requirement has decreased significantly. It uses a solid electrolyte similar to Teflon, organic polymer polyperfluorosulfonic acid, so it has less corrosion and safety problems than a fuel cell with a liquid electrolyte, and operates at low operating temperature. The membrane can be handled safely and easily, and the cell has a quick start-up. For applications needing a compact power generator, for instance, automobiles, or if the excess heat is to be used for cogeneration, liquid cooling is best.
The PEM fuel cell operates between 60 and 100˚ C and about 50% maximum power is available immediately at room temperature with full operating power after three minutes. It’s operating temperature makes it ideal for supplying a home with electricity and hot water and because it is light and sturdy, it is applicable to the automobile industry.
Unlike the AFC, the PEM fuel cell can operate on reformed hydrogen fuels without removal or recirculation of the carbon dioxide. Any carbon monoxide in the fuel, however, must be converted to carbon dioxide, which can easily be done by a catalytic process that would be integrated in to a fuel supply system.
Design
Source: www.fueleconomy.gov
The membrane is an electronic insulator, but a good conductor of hydrogen ions. The sulfonic acid groups are fixed to the polymer but the protons are free to migrate through the membrane via ionic sites. Ion transport is highly dependent on the water associated with such sites.
The anode and cathode are prepared by wet-proofing a sheet of porous graphitized paper with Teflon; then a small amount of platinum black (catalyst) is applied to one side of the sheet. The electrolyte is then sandwiched between the anode and cathode and the three components are sealed together under heat and pressure. Fully assembled, the membrane/electrolyte assembly is less than a millimeter thick.
The standard electrolyte is a fully fluorinated Teflon ® and exhibits high chemical and thermal stability. However, since it is expensive, new alternative membranes are being developed.
The anode and cathode are contacted on the back side by graphite flow-field plates with ridges that contact the backs of the electrodes and conduct current to the external circuit. The channels between the ridges supply fuel to the anode and oxidant to the cathode. The reactions in the PEM are the same as those in the PAFC, however, since the PEM fuel cell operates at a lower temperature, the water is liquid and is rejected from the back of the cathode into the oxidant stream, where it is carried off with excess oxidant. The liquid water also serves to cool the cell and if it is routed to a reservoir, it can humidify the reactant gasses and prevent drying of the membrane, a common failure mode.
Most modern PEM fuel cell’s catalysts are embedded in a solution of electrolyte monomer; this provides high proton and oxygen solubility and, consequently, effective use of the catalyst surface. For pure hydrogen feed, the preferred catalyst is platinum on graphite or carbon, and for other fuels, the catalyst is usually an alloy of platinum and ruthenium, which is carbon monoxide tolerant.
Future
Source: www.siemens.com
High temperature PEM fuel cells would need new or modified ion exchange membranes, such as polybenximidizole (PBI); however, this membrane would need phosphoric acid to work, and this presents other challenges such as avoiding liquid water and corrosion protection. Another, more promising, approach is to modify the current membrane.
Membranes in present cells are expensive and limited and still require more platinum than the Department of Energy deems economical. In addition, improved cathode performance is necessary to increase current densities. There is also the challenge of reducing cell degradation, of which the causes are not fully understood.
Developers have not demonstrated a single realistic system that has a significant stack life, so before it can be implemented commercially, improvements in lifespan, simpler system integration, and cost reduction must be achieved.
Sources:
1. Bellona Foundation. www.bellona.no
2. EG&G Technical Services Under Contract No. DE-AM26-99FT40575 for U.S. Department of Energy. Fuel Cell Handbook 7th Edition. November 2004.
3. Department of Defense. www.dodfuelcell.com
4. Fuel Cell Test and Evaluation Center. www.fctec.com
5. Fuel Cell Today. www.fuelcelltoday.com
