|Cross section of ultra-thin SOFC able to operate @ 250º–-400º C with acceptable power density. Click to enlarge.|
In a series of four papers presented at an Electrochemical Society conference, a Stanford professor and his students presented work that significantly reduces the operating temperature of Solid Oxide Fuel Cells (SOFC) while maintaining power density, advances that could eventually lead to them being applied in vehicles.
To my knowledge this is the solid oxide fuel cell which delivers the highest power density and can run at the lowest temperature at the same time.—Prof. Fritz Prinz, Stanford University
Although conceptually similar (hydrogen in, electricity out) the solid oxide fuel cell is different than the PEM (Proton Exchange Membrane) fuel cells most often discussed as power plants for transportation. PEM fuel cells tend to be smaller, run at lower temperatures, produce less power and require an external supply of hydrogen.
Solid oxide fuel cells use a hard, ceramic compound of metal oxides as an electrolyte, rather than the thin, permeable polymer electrolyte sheet in a PEM. Compared to other fuel cells, solid oxide fuel cells deliver more total power with relatively high efficiency. But their high running temperature—more than 1,300º F (about 700º C)—is a huge drawback for potential use in cars, which would overheat at those temperatures.
Lowering the operation temperature is very important for making SOFCs viable for vehicles from both operational and cost points of view. But at lower operating temperatures, resistive losses across the electrolyte increase, overpotentials at electrodes are magnified and therefore the performance of the cell decreases.
One of the causes of the high operating temperature is the electrolyte layer. The best electrolytes to date have had trouble conducting the negative oxygen ions without producing a lot of heat. The research from the Stanford group is tackling that.
Professor Fritz Prinz and his research group have improved ion conductivity through the electrolyte layer—a membrane of yttria stabilized zirconia (YSZ)—by making it as thin as 50 nanometers. Platinum (Pt) is employed as catalyst for both cathode and anode.
Building a membrane that thin, yet durable enough to operate reliably, is a huge mechanical challenge. The inputs of the fuel cell are gases (hydrogen and oxygen), so as much of the membrane as possible has to be exposed to them rather than blocked off by supporting structures. Meanwhile, as thin as the membrane is, it has to be strong enough to withstand forces such as differences in gas pressures on either side of it. (While layers of platinum catalyst on each side provide little support, they are loosely packed to allow the gases to permeate.)
Prinz’s group addressed this problem by experimenting with manufacturing techniques similar to those employed by the semiconductor industry. The group has successfully built a YSZ membrane atop a silicon mesh that makes it durable enough to work, yet leaves much of the membrane exposed to the gases.
The result is a fuel cell that delivers a power density of 400 milliwatts per square centimeter at 750º F (about 400º C) in individual openings in the silicon mesh. A typical car needs 15 kW to run, but stacks of cells with a total membrane surface area of 4 square meters could produce that much power. In other words, Prinz’s group has cut the temperature almost in half without sacrificing any power.
In two other papers at the conference, Prinz and his students showed how they strove to improve the membrane’s conductivity by bombarding the membrane’s crystalline structure with positive argon ions and then heating the charged membrane to 1,470º F (800º C). This procedure has the effect of opening up or dilating the crystal structure, boosting ion conductivity by as much as 34%.
Another paper describes a method for analyzing both the structure and conductivity of a membrane.
Overall the research, supported by Honda, is part of a larger collaboration with materials science and engineering Associate Professor Paul McIntyre and chemical engineering Professor Stacey Bent to produce fuel cells usable in cars.