Researchers at Massachusetts Institute of Technology have created both a practically and physically-based way to treat the surface of materials called perovskite oxides in order to not only make them far more durable but also offer better performance. These materials may be able to serve as energy-conversion devices similar to fuel cells and electrolyzers. This surface treatment may be able to solve a major challenge that has been hindering the widespread deployment of fuel cell technology that, when operated reversibly, can present a promising alternative to batteries for renewable-energy storage.
Perovskites have held the potential for applications in fuel cell electrodes, nonvolatile memory chips for computers and solar thermochemical fuel production through the splitting of water and carbon dioxide. Many teams are currently working on exploring different variations of perovskite composition in the search of the most promises candidates for each use. Unfortunately the material’s surface is known to be quite unstable, which is one of the reasons it has not yet been a go-to solution.
When the surface of these materials is exposed to either water or gas at high temperatures, they get covered up by a strontium-oxide related layer and this layer is used to insulate against oxygen reduction and oxygen evolution reactions which are a crucial part of the performance of fuel cells, electrolyzers and even thermochemical fuel production. This particular layer is detrimental to the efficiency and to the durability of the device, leading to slowed down reactions of more than an order of magnitude.
Yildiz and her team have uncovered a few reasons why there is so much segregation of strontium. She says researchers have discovered that it is actually controlled by enrichment of oxygen vacancies at the surface. The solution kills some of the oxygen vacancies. This idea contradicts the conventional understanding that such vacancies assist reactions with oxygen molecules at the perovskite oxide surface and improve the rate of oxygen reduction reaction in fuel cells.
Upon further analysis, the team found that there is a sweet spot when it comes to the addition of more oxidizable elements to the surface, both related to composition and concentration. In their first experiments, several different elements were used to provide the protective effects. The improvement increases up to a certain concentration and then adding more of the surface additives starts to make things downward spiral once again. This means, for any given material, an optimum amount has to be added. When using hafnium, the new treatment has been shown to reduce the rate of degradation and increase 30 times the rate of oxygen exchange reactions at the surface.
Yildiz says while nobody planned to use hafnium, it does provide a good balance between the stability of the surface and the availability of oxygen vacancies. She says the team believes 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.
Luckily, the surface treatment process is very easy and does not require large amounts of the additive elements deposited from a solution of the metal chloride. This means the bulk material is not being altered. The findings may prove to be very significant in making perovskite oxide electrocatalysts for various applications, such as solid oxide fuel cells. The team calls the bulk electronic and ionic properties of perovskite oxides to be very good, because they are have been optimized for a few decades for their use inside fuel cells. The bottleneck now is to find a way to improve the oxygen reduction reaction kinetics upon the surface. Researchers are now saying they have a handle on why the problem occurs and have uncovered a few ways to go about dealing with it.
William Chueh, an assistant professor of materials science and engineering at Stanford University has said in many catalytic materials, stability and performance do not come hand-in-hand. The most active catalysts are also the least stable ones. However, in this work, Yildiz and co-workers were able to identify a new way to substantially improve the stability of cobalt-based electrocatalysts just by adding a small amount of dopants onto the surface.
Chueh added that the most promising aspect of this work is that the material may be able to be used to substantially improve the stability of solid-oxide fuel cells. This is the key issue that leads to what the cost will be and any limits the widespread adoption of the technology may incur. The work is excellent in both fundamental insights and technological implications.