The nation’s energy spotlight has drifted away from solid oxide fuel cells over the last year or so (the last big splash being the Bloom Energy media fest), but that doesn’t mean researchers aren’t still working to figure out how to overcome the barriers to making SOFCs commercially successful.
One of the main engineering barriers is operating temperature. The problem is that when SOFCs operate above 850 °C, the materials or other workarounds that have to be used to prevent performance problems are either expensive or require fuel dilution. Either way, they make these fuel cells cost prohibitive under current circumstances. Go below 850 °C and coking (carbon buildup) on traditional anode materials, such as Ni-yttrium-stabilized zirconia, causes deactivation and creates sharp drop offs in fuel-to-energy conversion. It’s a pity because these fuel cells could be fueled with waste hydrocarbon sources, such as municipal wastes and biomass, or could double the energy output of coal (gasified) and consequently cut CO2 emissions in half.
But, a workgroup led by ACerS member Meilin Liu believes it has found a new way to make an SOFC anode, which can operate effectively and efficiently with carbonaceous fuels at 750 °C, using a nanostructured barium oxide/nickel interface. A paper about the group’s achievements was recently published in Nature Communications (doi:10:1038/ncomms1359). Liu, professor of MSE at Georgia Tech, and his research group collaborated with researchers at the Brookhaven National Lab, the New Jersey Institute of Technology and Oak Ridge National Lab.
The BaO possibilities were interesting to the group because the oxide is known to have been used as a promoter for reforming catalysts (the catalysts that breakdown hydrocarbon fuels into hydrogen and a byproduct). The challenge was to come up with a way of using the material that would not create a block to electrons but absorb water that would be used to remove carbon and combat coking. Their solution was to deposit (by evaporation) BaO onto Ni-YST. According to the authors, “In this process, BaO reacts with the surfaces of NiO, producing a thin film of NiO-BaO compounds on the NiO surface. On exposure to fuel, the thin film of NiO–BaO compounds is reduced to nanosized islands distributed on the Ni surface.”
Button-type test cells were made with the new anode structures. When fueled with dry C3H8, the cells attained a power density of ~0.88 Wcm-2 at 750 °C (more than 50 percent higher that traditional SOFCs operated under the same conditions). Further, cells were able to produce a stable current of 500 mA cm-2 for 100 hours, indicating the absence of coking.
Researchers also tested the new cells tolerance to CO. They used a wet (~3 v%) CO fuel stream and attained a power density of ~0.70 Wcm-2 (again, higher than what has been reported for other SOFC under the same conditions).
Finally, they used a fluidized carbon bed–SOFC arrangement to test the anode’s possible performance with something like gasified coal. They formed the fuel stream using a wet CO2 gasification technique. Here, they attained a peak power density of ~1.08 Wcm-2 at 850 °C, twice that of other SOFCs. When they lowered the temperature to 750 °C, the cell peak power density was still remarkable: ~0.65 Wcm-2 . Although cells cannot operate for long at this lower temperature, the the new anode delivered stable performance for 1000 hours.
The researchers say the performance and resistance to coking demonstrated using BaO/Ni interface “represent a vital step towards a cost-effecitve fuel cell for direct conversion of hydrocarbons and gasified carbonaceous solid fuels to electricity.”