When a solid-oxide fuel cell runs out of hydrogen, it takes about 15 seconds for the electrochemical reaction to wind down. Think of it as a little dose of free energy.
Now imagine that winding-down period extended dramatically, by 14 times – possibly longer. That's a reality that could be just a couple of years down the road, say Harvard scientists, who have demonstrated that adding a layer of nanostructured vanadium oxide at the fuel cell anode – with the typical material platinum – allows for extended power generation.
The researchers believe this breakthrough – essentially turning a solid-oxide fuel cell into something of a battery – could come into play with small-scale, portable energy applications.
"Unmanned aerial vehicles, for instance, would really benefit from this," lead author Quentin Van Overmeere, a postdoctoral fellow, said in a Harvard release. "When it's impossible to refuel in the field, an extra boost of stored energy could extend the device's lifespan significantly."
The scientists point to three mechanisms with vanadium oxide that encourage this effect, at least two of which are likely to be at work simultaneously.
"There are three reactions that potentially take place within the cell due to this vanadium oxide anode," principal investigator Shriram Ramanathan, an associate professor of materials science.
"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 Harvard researchers say that during normal operation, their device more or less matches a platinum-anode solid-oxide fuel cell in power output. Using the bilayer of platinum and vanadium oxide, it sustains output for three and a half minutes – and that might be just the beginning.
Ramanathan "and his team predicts that future improvements to the composition of the VOx-platinum anode will further extend the cell's lifespan." They figure that a fuel cell of this sort could be available for testing in things like micro-air vehicles in as few as two years. These aren't the missile dropping drones we hear a lot about, but are tiny, small-bird or big-sized aircraft that can be used for remote surveillance.
According to Harvard, this research was supported by the U.S. National Science Foundation (NSF), a postdoctoral scholarship from Le Fonds de la Recherche Scientifique-FNRS, and the U.S. Department of Defense's National Defense Science and Engineering Graduate Fellowship Program. The researchers also benefited from the resources of the Harvard University Center for Nanoscale Systems (a member of the NSF-funded National Nanotechnology Infrastructure Network) and the NSF-funded MRSEC Shared Experimental Facilities at MIT.