Fuel Cells for Transportation and Portable Power Sources

Transportation accounts for 20% of the United States economy. The resources are divided about equally between commercial transportation and personal automobiles. These vehicles are powered by gasoline and diesel engines. Thermal engines are subject to thermodynamic (Carnot) limitations that restrict the theoretic efficiency for converting hydrocarbon fuels to useful work to less than 40%. With current technology, efficient automobiles can be designed with a maximum efficiency of about 25%. Electric engines, however, are not subject to restrictions of heat engines, and can, in theory, be 100% efficient.

Electric engines include batteries and fuel cells. The difference between a battery and a fuel cell is the means by which the electric engine is "recharged." Batteries are recharged by applying a voltage. The voltage drives the battery reaction uphill thermodynamically, and stores the energy as chemical species to be discharged later (i.e., allowed to react in the spontaneous direction) during the cycle where the battery is used as a power source. A fuel cell is similar to a conventional combustion engine in its recharging protocol. A fuel cell does not have a voltage driven recharging cycle. Rather, a fuel cell always run in discharge mode and it is "recharged" by supplying it with fuel.

Current technology allows fuel cells powered by hydrogen and atmospheric oxygen to be run at high efficiency. With careful balancing of power and recycling of waste heat, fuel cell demonstration projects with efficiencies in excess of 90% have been built. However, in automotive applications, a fuel other than hydrogen might be more appropriate for safety reasons. For example, methanol and water can be converted to hydrogen and carbon dioxide as

CH₃OH + H₂O ® 3 H₂ + CO₂

In some technologies, this is done thermally in a catalytic converter and the product is fed into the fuel cell anode for oxidation to protons. However, in any thermal process of this type, there is a loss of efficiency. Direct reformation is the ideal for low temperature fuel cells. In this process, the methanol/water mixture is fed directly into the anode. Several difficulties remain to achieving direct reformation fuel cells.

1. Carbon monoxide, a by-product of the reformation reaction, passivates the platinum catalyst in the anode.
2. The membranes used as separators in the fuel cells allow crossover of water and methanol to the cathode. This drastically reduces cell efficiency.
3. The crossover also floods the cathode and the desiccates the anode.

The Leddy group is currently working to improve membrane design and to inhibit the passivation of the anode catalyst. New membranes are tailored to exploit interfacial gradients of concentration, density, charge, viscosity, potential and magnetic field.

Separation of Lanthanides and Actinides.

The lanthanides and actinides include such environmental hazards as plutonium and uranium. Plutonium and uranium are two of the high level radioactive hazards generated by the nuclear industry. Storage of these materials is a major environmental hazard in places like Hanford, Washington, because the materials are highly radioactive and have been stored in large vats for long periods. The vats are now leaking. In addition, there are leaking storage tanks with mixed wastes. A process which would allow separation of lanthanides and actinides from other transition metals and from each other would be very useful in remediating these environmental hazards. Separation of lanthanides and actinides is difficult. Because of the large number of electrons and large size of these metals and their ions, they have very similar chemistries and physical properties. Thus, attempts to separate them by chelation has been difficult and usually requires large quantities of solvents. These solvents then become hazardous wastes. The reduction potentials of the lanthanides and actinides are high and all the same within 100 mV. The lanthanides and actinides do have significantly different and large magnetic moments. These magnetic moments have been exploited using magnetic fields at cryogenic temperatures. The process is, however, costly. The Leddy group is investigating a newly designed microstructured ion exchange composite that incorporates small magnetic particles. The nonuniform magnetic fields associated with these particles generate a force comparable to the diffusion force under appropriate conditions. This allows the motion of ions and chelated complexes to be enhanced in proportion to their magnetic moments. Enhancements as large as 2500% have been observed. This technology should allow lanthanides and actinides to be separated at room temperature, using an inexpensive, low volume technology.