Nissan Introduces Denki Cube EV Concept at NY Auto Show
NRC Report Says FreedomCAR Making Significant Progress; Calls for Midcourse Shift in Strategic Planning

New Ceramic Fuel Cell Membrane Could Improve Fuel Cell Efficiency

Duke1
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
Duke
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.

Resources

Comments

Rafael Seidl

Higher temps also mean the radiator can be smaller, so the whole system is easier to package in a passenger car. This is why VW chose to pursue phosphoric acid fuel cells, in spite of their inherent efficiency limitations.

Of course, any fuel cell system that requires hydrogen fuel is automatically at a huge disadvantage. Direct methanol fuel cells are closely related to PEMFCs but usually less efficient and more expensive (due to higher platinum catalyst mass).

Methanol is toxic, corrosive stuff that must be handled carefully but at least it's liquid at normal pressure and temperature. There are special bacteria strains that can produce methanol from methane, which in turn can be produced in a sustainable fashion from both wet and dry (woody) cellulosic biomass.

sjc

Making CH3OH methanol from CO and H2 synthesis gas is a matter of temperature, pressure and catalyst. It has been done successfully and efficiently for quite some time. You can make synthesis gas by gasifying biomass.

Methanol has been made by steam reforming the methane in natural gas because NG could be bought at 50 cents per therm on an industrial scale, until the prices started to rise after deregulation.

If you are going to use ceramics, you might as well make an SOFC, which requires NO platinum at all and is more efficient, can take more fuels and is less prone to contamination.

Harvey D

It seems that fuel cells (all types) is a long way from being a mature technology. They all have to be much improved + much lower cost for on board installation using readily available biofuels. By 2030+ it may become an option for use as PHEV range extender.

Meanwhile, batteries + supercaps will get much better, smaller, cheaper and may not require a very large range extender, if any.

Quick charge combo units can supply 250 to 300 Km now and will probably reach 500 to 600 Km within about 10 more years. ICE or fuel cell range extenders may not be required after 2020.

Harvey D

It seems that fuel cells (all types) is a long way from being a mature technology. They all have to be much improved + much lower cost for on board installation using readily available biofuels. By 2030+ it may become an option for use as PHEV range extender.

Meanwhile, batteries + supercaps will get much better, smaller, cheaper and may not require a very large range extender, if any.

Quick charge combo units can supply 250 to 300 Km now and will probably reach 500 to 600 Km within about 10 more years. ICE or fuel cell range extenders may not be required after 2020.

GreenPlease

@SJC

If I recall, PEM fuel cells are theoretically capable of 70% efficiency without a bottoming cycle. I believe that the oft-quoted efficiencies of SOFCs includes said bottoming cycle. A PEM, in theory, would require less on board equipment.

The platinum problem shouldn't be that big of a deal as long as advanced deposition processes are used. That's not to say this whole hydrogen economy thing is viable. As long as Big Oil is interested in selling a liquid, however, the research will exist.

sjc

I have not seen a PEM reach the efficiency of an SOFC even without a turbine. Theory is one thing, practice is another.

The comments to this entry are closed.