MIT researchers have found that changing the pH of the system can increase the lifetimes of a range of technologies including fuel cells. An open-access paper on their work is published in the RSC journal Energy & Environmental Science.
Fuel and electrolysis cells made of solid metal oxides are of interest for several reasons. For example, in the electrolysis mode, they are very efficient 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 generate electricity when the sun isn’t shining or the wind isn’t blowing. They can also be made without using costly metals like platinum.
However, their commercial viability has been hampered, in part, because they degrade over time. Metal atoms seeping from the interconnects used to construct banks of fuel/electrolysis cells slowly poison the devices.
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.—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, initially funded by the US Department of Energy through the Office of Fossil Energy and Carbon Management’s (FECM) National Energy Technology Laboratory, should help the department meet its goal of significantly cutting the degradation rate of solid oxide fuel cells by 2035 to 2050.
A fuel/electrolysis cell has three principal parts: two electrodes (a cathode and anode) separated by an electrolyte. In the electrolysis mode, electricity from, say, the wind, can be used to generate storable fuel such as methane or hydrogen. On the other hand, in the reverse fuel cell reaction, that storable fuel can be used to create electricity when the wind isn’t blowing.
A working fuel/electrolysis cell is composed of many individual cells that are stacked together and connected by steel metal interconnects that include the element chrome to keep the metal from oxidizing. But 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 explains. 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.—Harry Tuller
The team showed that you can do both by controlling the acidity of the cathode surface. They also explained what is happening.
To achieve their results, the team coated the fuel/electrolysis cell cathode with lithium oxide, a compound that changes the relative acidity of the surface from being acidic to being more basic.
After adding a small amount of lithium, the researchers were able to recover the initial performance of a poisoned cell, Tuller said. 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.—Harry Tuller
The engineers went on to explain what is happening by observing the material at the nanoscale with state-of-the-art transmission electron microscopy and electron energy loss spectroscopy at MIT.nano. They found that the lithium oxide effectively dissolves the chromium to form a glassy material that no longer serves to degrade the cathode performance.
Many technologies such as solid oxide fuel cells are based on the ability of the oxide solids to breathe oxygen in and out rapidly of their crystalline structures, Tuller says. The MIT work essentially shows how to recover—and speed up—that ability by changing the surface acidity. As a result, the engineers are optimistic that the work could be applied to other technologies including, for example, sensors, catalysts, and oxygen permeation-based reactors.
The team is also exploring the effect of acidity on systems poisoned by different elements, like silica.
In addition to the DOE, this work was also funded by the National Research Foundation of Korea, the MIT Department of Materials Science and Engineering via Tuller’s appointment as the RP Simmons Professor of Ceramics and Electronic Materials, and the US Air Force Office of Scientific Research.
Han Gil Seo, Anna Staerz, Dennis S. Kim, Dino Klotz, Clement Nicollet, Michael Xu, James M. LeBeau and Harry L. Tuller (2022) “Reactivation of chromia poisoned oxygen exchange kinetics in mixed conducting solid oxide fuel cell electrodes by serial infiltration of lithia” Energy Environ. Sci. doi: 10.1039/D1EE03975J