Perovskite oxides are seen as promising candidates for electrodes in energy conversion devices like fuel cells due to their efficiency and versatility. However, the issue that has held the technology back is the degradation suffered when the material is exposed to water or gases such as oxygen or carbon dioxide at elevated temperatures—a condition that would be common in fuel cell applications.
As explained by MIT associate professor Bilge Yildiz, “We, as well as others in the field, have discovered in the past several years that the surfaces of these perovskites get covered up by a strontium oxide–related layer, and this layer is insulating against oxygen reduction and oxygen evolution reactions, which are critical for the performance of fuel cells, electrolyzers and thermochemical fuel production.”
She added, “This layer on the electrode surface is detrimental to the efficiency and durability of the device, causing the surface reactions to slow down by more than an order of magnitude.”
The answer was to add another element on the surface of the electrode, in this case, hafnium. “The result was quite unexpected,” Yildiz stated. “Nobody would have planned to use hafnium to improve anything in this field,” due to the fact that the element shows almost no reactivity by itself.
As a surface treatment for the perovskite, however, it caused the greatest improvement of all the elements tested, “because it provides a good balance between the stability of the surface and the availability of oxygen vacancies,” she added.
It may seem counterintuitive, but as Yildiz explained, adding a small amount of more oxidizable elements to the perovskite surface “annihilates some of the oxygen vacancies, makes the surface more oxidized and prevents the formation of insulating phases that block oxygen exchange reactions at the surface of the material.” This allows the electrode surface to retain its electronic, ionic and catalytic properties that make perovskite oxide ideal for this application.
“What we put on the surface is a very small amount, so it’s not changing the bulk material,” Yildiz said. The study showed the use of no more than a single atomic layer over the bulk material.
William Chueh, an assistant professor of materials science and engineering at Stanford University, was not connected with the research, noted, “In many catalytic materials, stability and performance do not come hand in hand—the most active catalysts are also the least stable ones.”
“In this work, Yildiz and coworkers identified a new way to improve the stability of cobalt-based electrocatalysts simply by adding a small amount of dopants on the surface,” Chueh added.
He added, “The most promising application of this work is to improve the stability of solid-oxide fuel cells. This is the key issue that controls the cost and limits the widespread adoption of this technology. The work is excellent in both fundamental insights and technological implications.”
The findings were published in the journal Nature Materials.