Solid Oxide Fuel CellsJeff Reding
Purdue Doctoral Candidate.
Fuel cells hold a lot of promise for clean energy of the future. They use combustion reactions (oxidation/reduction) but in a mode where the two reactions are isolated which allows the generation of electric current (ions travelling one way and electrons the opposite).
Most fuel cells are named by the electrolyte used. Solid oxide fuel cells are the type that has the highest operating temperatue and have a great advantage in that no expensive catalyst is required to sustain the oxidation/reduction reaction. They also can tolerate a wide variety of H2 bearing fuel sources without the need for fuel reformers. Lower temperature fuel cells generally rely on burning pure H2 and therefore require fuel reformers before introduction of the fuel. A third advantage of the solid oxide fuel cell is that heat as well as electric current is produced, and this heat can be captured and used advantageously.
There are a few solid oxide fuel cells in a prototype state (Siemens), and these show an excellent 84% chemical to electrical energy conversion factor. Unfortunately they are currently very expensive as few material are suitable for the high temperatue, and materials that are suitable are hard to manufaxcture and assemble. Typically the anode is a Ni-Ytria stabilized Zirconia, the Electrolyte is Ytria stabilized Zirconia, and the cathode is a lanthanum-strontium.
Our major effort is to try to get a solid oxide fuel cell to operatue at lower temperatures, where less expensive materials could be used for manufacture. Most problamatic is the electrolyte which must be mechanically stable, gas tight and quite conductive. If electrolyte can be made very thin then the cell may be constructed with an "anode supported" or "cathode supported" design which should be cheaper than current "electrolyte supported" style.
Solid oxide fuel cells currently operate at 800 - 1000 C. The target for our project is to operate at about 650 C which will allow use of cheaper materials. It appears that vapor deposition can produce an electrolyte that could operate at this temperature, but the process is expensive and fails to produce a gas tight electrolyte leading to inefficiencies. Thus we are currently looking at spray deposition. An air brush technique using a colloidal spray of ytria stabilized zirconium in isopropyl alcohol shows some promise. The spray has been applied to glass and can be calcined to the oxide later. 4-6 micron thichnesses can be achieved. The colloidal coating technique may also have application for anodes and cathodes sprayed onto steel conductors as a low cost production technique. In this case porosity is desirable to allow reaction gasses to migrate through. It is hoped that goals can be achieved during the next 18 months as I finish my reasearch project at Purdue.