|Proton conductivity measured at room temperature and at various relative humidities for ceramic ferroxane-derived membranes and Nafion (polymer) membrane. Click to enlarge.|
Researchers at Duke’s Pratt School of Engineering are developing a new ceramic membrane that allows fuel cells to operate at low humidity and theoretically at higher temperatures. They reported their most recent findings online in the Journal of Membrane Science.
Most PEM (proton exchange membrane) fuel cells use a polymer membrane, such as Nafion. To operate efficiently, the membrane requires humidity and a low operating temperature. As the temperature rises, the polymer becomes unstable and the membrane dehydrates, leading to a loss of performance. However, operation of PEM fuel cells at higher temperatures would be more desirable, as the reaction rates at the catalysts are increased while potential poisoning of the catalysts is reduced.
Ceramics have been proposed as good candidates for use in PEM fuel cells because of their thermal, chemical, and mechanical stability, and lower material costs. However, the Duke team notes in their paper, “the protonic conductivity of mineral membranes produced to date has been low in comparison with the perfluorosulfonate-based membranes.”
The Duke researchers are developing a ceramic membrane derived from iron nanoparticles (carboxylic acid-stabilized nanoparticles with a lepidocrocite core—a metal-oxane referred to as ferroxane)—to overcome the limitations of humidity and heat while delivering comparable performance to a Nafion polymer membrane.
The current gold standard membrane is a polymer that needs to be in a humid environment in order to function efficiently. If the polymer membrane dries out, its efficiency drops. We developed a ceramic membrane made of iron nanoparticles that works at much lower humidities. And because it is a ceramic, it should also tolerate higher temperatures.
The efficiency of current membranes drops significantly at temperatures over 190 degrees Fahrenheit. However, the chemical reactions that create the electricity are more efficient at high temperatures, so it would be a big improvement for fuel cell technology to make this advance.
If the next series of tests proves that fuel cells with these new membranes perform well at high temperatures, we believe it might attract the type of investment needed to bring this technology to the market.—Mark Wiesner, Ph.D., senior author of the paper
|Voltage-current curve of fuel cell using ceramic ferroxane compared with untreated Nafion, measured at 24 ºC, atmospheric pressure and 100% relative humidity. Click to enlarge.|
The Duke researchers examined several types of ferroxane membranes in this paper.
Of the ceramic membranes tested, the ferroxane-derived membrane with PVA (polyvinyl alcohol) calcined at 500° C (PVA Fe500) showed the best performance, producing a power density of 5.21 mW cm-2 and a current density of 16.5 mA cm-2.
This power density is lower, yet comparable to the power density of the untreated Nafion membrane (6.78 mWcm-2). This result was unexpected given that the protonic conductivity of the ferroxane-derived membrane is approximately 10 times less than that of the Nafion when compared at room temperature and 100% humidity. [See first figure.]
The unsupported ceramic membranes calcined at 300° C had much higher protonic conductivities and would be expected to perform better than the PVA Fe500. However, without the PVA binder it was not possible to produce membranes that were large enough for testing in the MEA due to the very brittle nature of the material and tendency to crack during the drying process.
...improved fuel cell performance may be possible using membranes calcined at the lower temperatures if membrane processing limitations involving the use of binders can be overcome.
The Duke membrane would also be much less expensive to produce, according to Wiesener. Membranes make up as much as 40% of the current overall cost of fuel cells.
The research was funded by the National Science Foundation and US Office of Naval Research.
E.M. Tsui, M.R. Wiesner, Fast proton conducting ceramic membranes derived from ferroxane nanoparticle-precursors as fuel cell electrolytes, Journal of Membrane Science (2007), doi:10.1016/j.memsci.2008.02.025