Jun 15 2016
A team of researchers from MIT has formulated a realistic and physically-based method for treating the surface of materials known as perovskite oxides, to make them highly durable, and optimize their performance. It is quite possible that these materials would act as electrodes in energy-conversion gadgets such as electrolyzers and fuel cells. This surface treatment is also a probable solution for one of the key challenges that has hampered the extensive use of fuel cell technology that, when operated reversibly, can be a potential substitute to batteries for renewable-energy storage.
This diagram depicts the way a new surface treatment can improve the efficiency and longevity of perovskite materials for use in applications such as fuel-cell electrodes. Within the bulk of the material (bottom left) oxygen vacancies shown as “holes” are distributed throughout the material, and cause intense chemical reactivity. But on the surface, many of those vacancies get filled by oxygen when atoms of another element (shown in red) are added to the surface, which significantly slows down the rate of reactions that could degrade the surface and impair its performance. (Photo credit: Courtesy of Felice Frankel)
The results of the study have been published in the Nature Materials journal, in a paper by MIT Associate Professor Bilge Yildiz of the departments of Nuclear Science and Engineering and Materials Science and Engineering, former MIT postdoc Nikolai Tsvetkov, graduate students Qiyang Lu and Lixin Sun, and Ethan Crumlin of the Lawrence Berkeley National Laboratory.
In the recent years, the field of perovskites has become an active area of research, with promising applications in the areas spanning from fuel cell electrodes, to solar thermochemical fuel production via the splitting of carbon dioxide and water, and to non-volatile memory chips for computers.
They are a wide class of oxide materials, and several teams are testing differences of perovskite composition in search of the most probable contender for varied uses. However, over time the relative volatility of the surface of the material has been one of the key limitations to use of perovskites.
The surfaces of these materials, when exposed to gases or water such as carbon dioxide or oxygen at high temperatures, as they frequently are in real-world applications, “suffer from degradation because of chemical segregation and phase separation,” Yildiz explains. She says “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. 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.”
In a previous work, Yildiz and her team found out the reasons behind such negative surface segregation of strontium.
We have found it to be governed by enrichment of oxygen vacancies at the surface.
Oxygen vacancies are atomic deformations in the lattice where oxygen atoms are absent.
“Then the solution was to kill some of those oxygen vacancies.” This idea is opposing to the traditional belief that oxygen vacancies help reactions with oxygen molecules on the perovskite oxide surface and enhance the rate of oxygen reduction reaction in fuel cells.
So, merely adding a tiny fraction of highly oxidizable elements at 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,” Yildiz says.
Similarly, the surface preserves the fundamentally first-class electronic, catalytic, and ionic properties of the perovskite oxide and facilitates rapid oxygen exchange reactions. The team’s analysis illustrates that there is a positive feature in adding more oxidizable elements to the surface, in terms of the concentration and composition.
In these preliminary experiments, they tried many different elements to offer the protective effect. The progress increases up to a definite concentration, and then accumulating more of the surface additives begin to make things inferior again. The researchers discovered that for any proposed material, there will be an optimal quantity that should be added. With hafnium, the new treatment has been able to decrease the rate of degradation, and increase by the rate of oxygen exchange reactions 30 times at the surface.
The outcome was quite unpredicted, Yildiz says. “Nobody would have planned to use hafnium to improve anything in this field,” she says, as that element or its oxide reveals nearly no reactivity on its own.
However, as a surface treatment for the perovskite, it resulted in the maximum improvement of all the elements analyzed, “because it provides a good balance between the stability of the surface and the availability of oxygen vacancies,” she explains.
We believe the value of the work is not only in having found a potential improvement to fuel cell electrode durability, but also in fundamentally proving the mechanism behind this improvement. For that, our in situ X-ray spectroscopy experiments enabled by the Advanced Light Source have been critical.
The surface treatment procedure is simple and needs only a minuscule quantity of the additive elements deposited from a solution of the metal chloride.
What we put on the surface is a very small amount, so it’s not changing the bulk material.
Yildiz
In fact, the surface treatment totals to no more than a single atomic layer over the bulk material.
The results could be pretty big in making perovskite oxide electrocatalysts for some applications, she says, including solid oxide fuel cells. “The bulk electronic and ionic properties of perovskite oxides are really good, as they have been optimized over several decades for use in fuel cells,” she explains, but “the bottleneck now was to improve the oxygen reduction reaction kinetics at the surface,” and many teams have been stymied because the behavior of the material at its surface turned out to be “not nearly as good as hoped,” or as models estimated. Currently, the researchers state that they understand the reason for occurrence of setback, and how to cope with it.
“The observations could be used to produce more robust fuel cells with lower degradation rates, which at the moment is a major target for solid oxide fuel cell developers,” says John Kilner, a professor of energy materials at Imperial College, London, who was not part of this study.
“In many catalytic materials, stability and performance do not come hand-in-hand — the most active catalysts are also the least stable ones,” says William Chueh, an assistant professor of materials science and engineering at Stanford University, who also was not connected with the research. “In this work, Yildiz and co-workers identified a new way to substantially improve the stability of cobalt-based electrocatalysts simply by adding a small amount of dopants on the surface.”
The most promising application of this work is to substantially 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.
Chueh
The research was supported by the National Science Foundation, Division of Materials Research, Ceramics Program with a CAREER Award, and from the NASA Jet Propulsion Laboratory. The team made use of the Advanced Light Source facility at the Lawrence Berkeley National Laboratory, supported by the Department of Energy.