Stanford reseachers have come up with innovations to the solid oxide fuel cell which make it more attractive for use in cars. The most significant drawback of current solid oxide fuel cells is their operating temperature; they typically operate at 1300 F (~700 C) which makes their use in cars unlikely. One of the causes of high operation temperatures is the electolyte layer which, up to now, could not conduct negative oxygen ions without creating a lot of heat.
Stanford mechanical engineering proffessor Fritz Prinz and his collegues have improved the conductivity through the electrolyte layer—a membrane of yttria stabilized zirconia (YSZ)—by making it as thin as 50 nanometers. The results are a fuel cell that operates at 750 F (~400 C) at the same power density as earlier solid oxide fuel cells operating at high temperatures. His group has also improved the membranes conductivity by 34% by bombarding the membrane with positive argon ions and then heating the charged membrane to 1,470 F (800 C). Apparently they have not combined the techniques, pehaps because the argon treatment may weaken the YSZ.
To build a membrane as thin as they have, required solving some huge mechanical challeges. In order for the gases to pass through the membrane the support stucture for the membrane must have very high porosity and still be strong enough to withstand the pressure differances of the gas pressures on either side. They have developed a silicon mesh, similar to that used in the semiconductor industry, that is strong enough to support the membrane.
Solid oxide fuel cells differ in many respects from other fuel cell technologies. First, they are composed of all-solid-state materials – the anode, cathode and electrolyte are all made from ceramic substances. Second, because of the all-ceramic make-up, the cells can operate at temperatures as high as 1800 degrees F (1000 degrees C), significantly hotter than any other major category of fuel cell. These cells generate electricity through a pair of chemical reactions that conduct negative charge around a circuit. One side of the cell takes in oxygen in from the air and combines it with electrons to form negative oxygen ions. Those ions are conducted through a solid electrolytic layer, the YSZ membrane, in the middle to the other side of the cell, where the ions combine with hydrogen gas fuel to form water. That reaction frees up electrons, which pass through whatever the fuel cell is powering to return to the first side, completing the circuit.
Minor revisions 9:15 pm 10/20/05
Resources:
"New fuel-cell technology could help power future vehicles", PhysOrg.com, October 19, 2005
"Solid Oxide Fuel Cell Technology", DOE Office of Fossil Energy, Solid Oxide Fuel cells
More blogs on these subjects: fuel cells, energy, renewable, renewable energy, alternative energy
Couldn't the heat generated by the fuel cells be used?
It would seem rather pointless to create a fuel cell that wastes so much of it's potential output on heat. If it wasn't significantly better than other fuel cell technologies in some other ways.
Posted by: Tank | October 20, 2005 at 01:27 PM
Error in title: there is no zinc used in these fuel cells. They are SOLID oxide fuel cells, using ceramic electrolytes rather than metal fuel.
Posted by: Engineer-Poet | October 20, 2005 at 04:01 PM
Sure you can use the heat from the exhaust to run a cogenerated process. If you need industrial heat for example. Or if the unit is mounted in an apartment or office tower it can produce space heat or hot water. A particularly large solid oxide fuel cell can run a combined cycle by burning the fuel and then using the exhaust to boil water and run a steam Rankine cycle.
Posted by: Robert McLeod | October 20, 2005 at 07:43 PM
Oops, sorry about the zinc. I have edited the post to take out all references to zinc. I don't know where that came from, it must have been from the Z in Zirconia, but that is no excuse.
About the heat - The heat from these cells has always been used, or rather proposed, to be used for the purposes mentioned. These researchers have been trying to adapt the battery for use in vehicles, where the higher heat is hard to isolate. Even there it could be used to drive a Stirling engine or a turbine, but that probably would be too complex. I don't know how the weight and power density of these cells compares to PEM cells but the researchers claim they are more efficient.
Posted by: James Fraser | October 20, 2005 at 09:17 PM
It really sounds like this technology is best suited for larger applications where they can take advantage of the heat as opposed to trying to get rid of it to fit it into a car.
Granted removing the necessity for gasoline in cars is of paramount importance but using fuel cell technology to reduce the need for things like coal-electricity generation or massive hydro-electric projects does have value too.
Posted by: Tank | October 21, 2005 at 09:43 AM
Current cars get about 17% efficiency. If you can boost the prime mover from ~20% up to 60%, you don't care much about the waste heat; you're making a heck of a lot less than before. Such a fuel-cell car could dump all its waste heat out the exhaust, eliminating the need for a conventional cooling system.
It would indeed revolutionize stationary generation so long as it could tolerate impure fuel gases (too fussy and the energy cost of gas cleanup gets too high). Combined-cycle plants are already hitting 60% efficiency, but these units look like they could be home-sized. If homes could generate electricity from e.g. the operation of the furnace and water heater, the potential for efficiency gains is enormous.
Posted by: Engineer-Poet | October 21, 2005 at 02:57 PM
It's good to have options for car applications of fuel cells and if they have better power/wieght densities, I don't see why we shouldn't go with these over PEMs. I guess it also comes down to which is easier to manufacture.
However, the Solid Oxide fuel cells described in this post still use platinum as a catalyst (just like PEM cells) which could be a major problem. Platinum is in pretty short supply and is more valuable than gold. It is particularly useful as a catalyst for lowering the temperatures necessary for the reactions that go on in fuel cells which is why it is used both in PEM cells and these low-temp Solid Oxide cells. What we really need though is a cheap and readily available replacement for the platinum and until we get that, I'm not sure we'll be able to mass-produce fuel cells at a low enough cost. Anybody know more on this?
Posted by: JesseJenkins | October 23, 2005 at 02:14 PM