Keep going: Fuel cells

On and on and on...

 

X-ray photoelectron spectroscopy has been used to verify the science underpinning a fuel cell that keeps going even after the hydrogen runs out.

Materials scientists at Harvard University have demonstrated a solid-oxide fuel cell (SOFC) that converts hydrogen into electricity but can also store electrochemical energy like a battery. The fuel cell thus continues to provide energy for a short time after it has run out of fuel.

"This thin-film SOFC takes advantage of recent advances in low-temperature operation to incorporate a new and more versatile material," explains team leader Shriram Ramanathan who is Associate Professor of Materials Science at the Harvard School of Engineering and Applied Sciences (SEAS). He adds that, "Vanadium oxide (VOx) at the anode behaves as a multifunctional material, allowing the fuel cell to both generate and store energy."

The team describes details in the latest online edition of Nano Letters and suggests that their system will be most important for small-scale, portable energy applications, where a very compact and lightweight power supply is essential and there is a risk of the fuel supply being interrupted, the device could act then as the fuel cell equivalent of a UPS, uninterrupted power supply a device that sits between electric power outlet and device and supply power while simultaneously keeping its own batteries charged giving.

"Unmanned aerial vehicles, for instance, would really benefit from this," adds cpost-doctoral researcher Quentin Van Overmeere of SEAS. "When it's impossible to refuel in the field, an extra boost of stored energy could extend the device's lifespan significantly."

Ramanathan, Van Overmeere, and graduate student Kian Kerman usually work on thin-film SOFCs that employ platinum in the electrodes. They point out that should a platinum-anode SOFC run out of fuel it can continue to produced energy only for about 15 seconds before the electrochemical reaction subsides.

The team has incorporated a vanadium oxide layer into their novel SOFC to form a platinum-VOx bilayer for the anode. This subtle modification allows the fuel cell to carry on operating without fuel for more than three minutes; at a current density of 0.2 milliamps per square centimetre. The team concedes that this is a "proof of concept," system. Ramanathan and his team predict that they will be able to optimise the composition of the VOx-platinum anode further to extend the fuel cell's operational life time.

Three reactions

"There are three reactions that potentially take place within the cell due to this vanadium oxide anode," explains Ramanathan. "The first is the oxidation of vanadium ions, which we verified through XPS (X-ray photoelectron spectroscopy). The second is the storage of hydrogen within the VOx crystal lattice, which is gradually released and oxidized at the anode. And the third phenomenon we might see is that the concentration of oxygen ions differs from the anode to the cathode, so we may also have oxygen anions being oxidized, as in a concentration cell."

The team points out that all three reactions can feed electrons into a circuit, but they are actually unsure which of them or how, the reactions work to keep the fuel cell running. The team has, however, determined experimentally and quantitatively that at least two mechanisms operate at the same time. They anticipate optimisation will lead to a viable product for testing within about two years.

"We are currently working on getting a more detailed understanding of the mechanism taking place," Van Overmeere told SpectroscopyNOW. "We hope to ultimately demonstrate this new capability while achieving practical power outputs needed for miniature applications."