|MoO2-based SOFC using a fuel mixture consisting of n-dodecane, CO2 and air. Kwon 2013. Click to enlarge.|
Researchers at Washington State University, with colleagues at Kyung Hee University and Boeing Commercial Airplanes, have been developing liquid hydrocarbon/oxygenated hydrocarbon-fueled solid oxide fuel cells (SOFCs) for aviation (the “more electric” airplane) and other transportation applications, such as in cars. These fuel cells first internally—i.e., no external reformer—reform a complex liquid hydrocarbon fuel into carbon fragments and hydrogen, which are then electrochemically oxidized to produce electrical energy without external fuel processors. The SOFCs feature a MoO2 (molybdenum dioxide) anode with an interconnecting network of pores that exhibit excellent ion- and electron-transfer properties.
In a new paper in the journal Energy Technology, the team reports that this novel fuel cell, when directly fueled with a jet-A fuel surrogate (an n-dodecane fuel mixture), generated an initial maximum power density of 3 W cm-2 at 750 °C and maintained this high initial activity over 24 h with no coking. The addition of 500 ppm of sulfur into the fuel stream did not deactivate the cell.
The ability of the MoO2-based SOFC to operate with the direct input of complex liquid fuels makes it a promising energy converter to meet the increasing electrical power demand of future airplane designs.—Kwon et al.
For aviation applications, the researchers envision integrating their fuel cell with a battery to power auxiliary power units. These units are currently powered by gas turbines and operate lights, navigation systems and various other electrical systems. The two technologies—battery and fuel cell—complement each other’s weaknesses, says WSU Professor Su Ha.
The results of this research are a key step in the integration of fuel cell technology in aviation and the development of the more electric airplane.—Joe Breit, associate technical fellow at Boeing and co-author
The team also has a paper in press in the Journal of Power Sources on the gasoline-fueled performance of the SOFC. Fueled with premium gasoline, the SOFC demonstrated a power density of greater than 3.0 W cm-2 at 0.6 V. Over a 24 h period of operation, the open cell voltage remained stable at ∼0.9 V. At the cell voltage of 0.6 V, current density dropped over the first 7 h to a value of ∼3.0 A cm-2, where it stayed for the remaining 17 h of the test with a minor fluctuation. Power density of ∼2.0 W cm-2 at 0.6 V was still measured after 24 h on stream with a continuous feed of gasoline.
Scanning electron microscopy (SEM) examination of the anode surface pre- and post-testing showed no evidence of coking, which hints at the reason for the observed stability under the harsh cell operating conditions. The team suggested in the paper that the results of this preliminary study indicated that an SOFC using a MoO2-based anode has potential for generating electrical power from gasoline for future hybrid electric vehicles.
The work began on developing a solid-oxide fuel cell to provide electrical power on commercial airplanes began about 10 years ago. Fuel cells offer a clean and highly efficient way to convert the chemical energy in fuels into electrical energy. In addition to increasing fuel efficiency and reducing emissions of harmful pollutants, fuel cells are quiet and would be particularly helpful when a plane is at a gate and the main jet engines are turned off.
The process could be approximately four times more efficient than a combustion engine because it is based on an electrochemical reaction. The solid-oxide fuel cell is different from other fuels cells in that it is made of solid materials, and the electricity is created by oxygen ions traveling through the fuel cell.
Using jet fuel and gasoline to power their fuel cell proved tricky. To avoid the added weight of a reformer that would convert the complex fuel into syngas, the researchers wanted to be able to directly feed the liquid fuel into the fuel cell. Furthermore, they had to overcome the problems of sulfur poisoning and coking, a process in which a solid product is created from imperfect combustion. Sulfur is present in all fossil-based fuels and can quickly deactivate fuel cells.
The MoO2-based anode is fabricated on to an yttria-stabilized zirconia (YSZ) electrolyte via combined electrostatic spray deposition (ESD) and direct painting methods.
Byeong W. Kwon, Shuozhen Hu, Oscar Marin-Flores, M. Grant Norton, Jinsoo Kim, Louis Scudiero, Joe Breit and Su Ha (2014) “High-Performance Molybdenum Dioxide-Based Anode for Dodecane-Fueled Solid-Oxide Fuel Cells (SOFCs),” Energy Technol. doi: 10.1002/ente.201490009
Xiaoxue Hou, Oscar Marin-Flores, Byeong Wan Kwon, Jinsoo Kim, M. Grant Norton, Su Ha (2014) “Gasoline-Fueled Solid Oxide Fuel Cell with High Power Density, Journal of Power Sources doi: 10.1016/j.jpowsour.2014.06.038
Byeong Wan Kwon, Caleb Ellefson, Joe Breit, Jinsoo Kim, M. Grant Norton, Su Ha (2013) “Molybdenum dioxide-based anode for solid oxide fuel cell applications,” Journal of Power Sources, Volume 243, Pages 203-210 doi: 10.1016/j.jpowsour.2013.05.133