MIT researchers have discovered a relatively simple technique to enhance the lifespan of these devices: adjusting the pH of the system.
This research could serve as a catalyst for a variety of technologies, including fuel cells, which are essential for storing solar and wind energy.
Fuel and electrolysis cells comprised of substances called as solid metal oxides are intriguing for numerous reasons. In the electrolysis mode, for instance, they are particularly effective at converting electricity from a renewable source into a storable fuel such as hydrogen or methane that can be used in the fuel cell mode to create electricity when the sun or wind are not present. They can also be produced without the use of expensive metals such as platinum. Their commercial potential has been limited, however, by the fact that they degrade with time. Metal atoms leaking from the interconnects used to form fuel/electrolysis cell banks harm the devices over time.
“What we’ve been able to demonstrate is that we can not only reverse that degradation, but actually enhance the performance above the initial value by controlling the acidity of the air-electrode interface,” says Harry L. Tuller, the R.P. Simmons Professor of Ceramics and Electronic Materials in MIT’s Department of Materials Science and Engineering (DMSE).
The research, first financed by the U.S. Department of Energy’s Office of Fossil Energy and Carbon Management’s (FECM) National Energy Technology Laboratory, should assist the department in achieving its objective of dramatically reducing the rate of solid oxide fuel cell deterioration by 2035 to 2050.
“Extending the lifetime of solid oxide fuels cells helps deliver the low-cost, high-efficiency hydrogen production and power generation needed for a clean energy future,” says Robert Schrecengost, acting director of FECM’s Division of Hydrogen with Carbon Management. “The department applauds these advancements to mature and ultimately commercialize these technologies so that we can provide clean and reliable energy for the American people.”
“I’ve been working in this area my whole professional life, and what I’ve seen until now is mostly incremental improvements,” says Tuller, who was recently named a 2022 Materials Research Society Fellow for his career-long work in solid-state chemistry and electrochemistry. “People are normally satisfied with seeing improvements by factors of tens-of-percent. So, actually seeing much larger improvements and, as importantly, identifying the source of the problem and the means to work around it, issues that we’ve been struggling with for all these decades, is remarkable.”
Says James M. LeBeau, the John Chipman Associate Professor of Materials Science and Engineering at MIT, who was also involved in the research, “This work is important because it could overcome [some] of the limitations that have prevented the widespread use of solid oxide fuel cells. Additionally, the basic concept can be applied to many other materials used for applications in the energy-related field.”
The work was described in an article published in Energy & Environmental Science on August 11. Han Gil Seo, a DMSE postdoc; Anna Staerz, a former DMSE postdoc, now at Interuniversity Microelectronics Centre (IMEC) Belgium and soon to join the Colorado School of Mines faculty; Dennis S. Kim, a DMSE postdoc; Dino Klotz, a former DMSE visiting scientist, now at Zurich Instruments; Michael Xu, a former DMSE graduate student; and Clement Nicollet, Seo and Staerz equally contributed to the effort.
Adjusting the acidity
The primary components of a fuel/electrolysis cell are two electrodes (the cathode and anode) separated by an electrolyte. In the electrolysis mode, power from a source like as the wind can be used to produce storable fuels such as methane or hydrogen. In the reverse fuel cell process, however, this storable fuel can be used to generate energy when there is no wind.
A functional fuel/electrolysis cell is constructed of numerous separate cells that are stacked and connected by steel metal interconnects containing chromium to prevent oxidation.
But “it turns out that at the high temperatures that these cells run, some of that chrome evaporates and migrates to the interface between the cathode and the electrolyte, poisoning the oxygen incorporation reaction,” Tuller says. After a certain point, the efficiency of the cell has dropped to a point where it is not worth operating any longer.
“So if you can extend the life of the fuel/electrolysis cell by slowing down this process, or ideally reversing it, you could go a long way towards making it practical,” Tuller says.
By regulating the acidity of the cathode’s surface, the scientists demonstrated it is possible to accomplish both goals. They also discussed the current situation.
To achieve their results, the team coated the cathode of the fuel/electrolysis cell with lithium oxide, a compound that changes the surface’s relative acidity from acidic to basic.
“After adding a small amount of lithium, we were able to recover the initial performance of a poisoned cell,” Tuller says. When the engineers added even more lithium, the performance improved far beyond the initial value. “We saw improvements of three to four orders of magnitude in the key oxygen reduction reaction rate and attribute the change to populating the surface of the electrode with electrons needed to drive the oxygen incorporation reaction.”
Observing the material at the nanoscale, or billionths of a meter, with cutting-edge transmission electron microscopy and electron energy loss spectroscopy at MIT.nano, the engineers went on to explain what is occurring. “We were interested in understanding the distribution of the different chemical additives [chromium and lithium oxide] on the surface,” says LeBeau.
They discovered that lithium oxide dissolves chromium to generate a glassy substance that no longer degrades the cathode’s performance.
Applications for sensors and catalysts, amongst others
According to Tuller, numerous technologies, such as fuel cells, rely on the ability of oxide solids to rapidly breathe oxygen into and out of their crystalline structures. The MIT research demonstrates how to regain and accelerate this ability by altering the surface acidity. Engineers are optimistic that the technique can be extended to other technologies, such as sensors, catalysts, and oxygen permeation-based reactors.
The group is also investigating the impact of acidity on systems poisoned by various substances, such as silica.
Concludes Tuller: “As is often the case in science, you stumble across something and notice an important trend that was not appreciated previously. Then you test that concept further, and you discover that it is really very fundamental.”
In addition to the U.S. Department of Energy, the National Research Foundation of Korea, the MIT Department of Materials Science and Engineering through Tuller’s appointment as the R.P. Simmons Professor of Ceramics and Electronic Materials, and the U.S. Air Force Office of Scientific Research also provided funding for this research.
