Low-temperature SOFCs: (A) Functionally graded bismuth oxide (Electrolyte 1) / ceria (Electrolyte 2) can allow use of hydrocarbon fuels at the anode at reduced temperatures. (B) Estimation of power output with LT-SOFCs from a single cell to a module (upper) and schematic diagram of power requirements according to various applications (lower). Credit: E.D. Wachsman, K.T. Lee; Science.
We’ve all heard stories of college kids who drop out even though they are falling short of graduation by one or two classes. That’s what two of my best friends did despite good grades. With those two, boredom, vision (or lack thereof) and money were factors. One became a truck driver, and the other … well, he went on to invent Tofurky(!). However, aside from sheer audacity, it didn’t make any sense to me then that they would walk away from such an investment (even at 1970s tuition costs).
That’s the same point Eric Wachsman et al. made about the United State’s policy and apparently diminishing support for solid oxide fuel cell R&D in his recent paper in Energy & Environmental Science. Their EES piece mainly is a policy plea emphasizing the foolishness (my word, not theirs) iof Congress and the Administration if they withdraw support for SOFCs by yanking the funding plug for the Solid State Energy Conversion Alliance in the FY 2012 and 2013 budgets.
Their argument boils down to this:
- The US has an international strategic advantage of hydrocarbon-based fuels (coal, natural gas, etc.), which will continue to be used in the foreseeable future, plus an infrastructure for distributing those fuels.
- Fuel cells are the most efficient means to directly convert these fuels to usable electrical energy.
- US fuel cell research has made considerable progress in the past, and is on the cusp of a new generation of breakthroughs that make portable power, transportation and stationary (utility and combined heat and power) applications much closer.
The last point above has been made, however, for many years if not decades, and begs for elaboration if it is to be taken seriously.
And, take it seriously they do! In the latest issue of Science, he and Kang Taek Lee, both affiliated with the University of Maryland’s Clark School of Engineering, report on significant advancements in SOFC technology, particularly in regard to high power densities— approximately 2 watts per square centimeter—with low temperature SOFCs (≤650°C). This, according to University of Maryland news release, is the highest mark set to date in that temperature range. Wachsman and Lee even report on significant advancements for SOFCs operating ≤350°C.
Other groups have demonstrated energy densities of ~2 W/cm2, but at higher temperatures (800°C) and only in button-sized units of yttria-stabilized zirconia, and these approaches have been plagued with interconnect and other problems as upscaling attempts have been made.
Wachsman and Lee have taken a different materials path, initally using a functionally graded ceria/bismuth oxide bilayered electrolyte “where the [gadolinium cerium oxide] layer on the anode (fuel) side protects the [erbia-stablized bismuth oxide] layer from decomposing while the ESB layer on the cathode (oxidant) side blocks the leakage current through the GDC layer because of its high transference number (ratio of ionic to total conductivity).”
They then worked to optimize the thickness and composition of the bilayered GDC/ESB arrangement. The 650°C power density breakthrough happened when they fabricated “an anode-supported cell composed of a thin, dense GDC(~10 μm)/ESB(~4 μm) bilayered electrolyte with a newly developed high-performance bismuth ruthenate-bismuth oxide (BRO7-ESB) composite cathode.”
Is 2 W/cm2 significant? They note that the renowned Bloom Energy stationary SOFC units operate with only one-tenth this density. Moreover, Bloom’s units operate at approximately 900°C, a characteristic that brings a raft of other problems.
But, in a more practical sense, 2 W/cm2 b brings a wide range of real-world applications much closer than previously expected. 2 W/cm2 puts LT-SOFC’s power density, pound for pound, ahead of internal combustion engines; if specific energy is used as a yardstick, the LT-SOFC and IC are equivalent.
“Thus,” the authors say, “because our LT-SOFC has essentially the same power and energy density as an IC engine, it could potentially transform the automotive sector as, for example, a range extender for plug-in hybrid electric vehicles operating on conventional fuels. The corresponding 10-kW stack would only be a small cube of 10 cm per edge.” As the illustration above indicates, the cells packaged in various stack and module combinations deliver a hefty power output range of 200 W to 100 kW.
These achievements alone argue for continued federal SOFC support, but Wachsman and Lee are frustrated because, they say, there is a huge amount of layer and electrode microstructure optimization ahead (including work that is currently underway and planned) that is starting to demonstrate feasible performance levels at the 350°C level, including the use of materials that have high tolerance for carbon coking and other problems that enter the picture at lower temperatures.
And, if operating temperatures drop to 350°C… then the ball really starts rolling with faster start-ups (think cars and trucks), and better-performing and less expensive interconnects and sealants that can be mass produced.
Going back to the issue of federal policy related to SOFC development, Wachsman suggests in the news release that a major problem is that fuel cells and hydrogen have been linked too closely. “There is a problem in the perception of the public and policy makers, and in the funding of our fuel cell programs, that hydrogen and fuel cells are linked. Hydrogen-based fuel cells are the technology that has gotten all of the press and as a result we’re still waiting for a future hydrogen infrastructure. Yes, fuel cells can run off hydrogen, but they don’t have to.”
But that misperception, he continues, “has turned fuel cells into a ‘future technology’ and has resulted in a drastic reduction in the funding of fuel cell research by the DOE in favor of developing electric cars, when in fact fuel cells can be used right now in many stationary and mobile applications, including centralized power distribution and power generation for homes, businesses, and industry.”
He and Lee concede in Science that SOFC technology “has not fully matured.” But, given recent progress and plans for advancing the R&D work, and given the US strategic resources, LT-SOFCs have bright future for highly efficient applications that range from portable power sources to industrial-scale units. They conclude by saying, “LT-SOFC should be a technology of choice for these applications as long as we are in a hydrocarbon-based energy infrastructure.
Wachsman’s and Lee’s paper in Science is “Lowering the Temperature of Solid Oxide Fuel Cells” (doi:10.1126/science.1204090).