Perovskite oxides have for a number of years promised to be a breakthrough material in many fields in industrial chemistry. They have the potential to be applied in improved solar thermochemical fuel production, to create non-volatile memory chips for computers, and they also hold the key to extending the life of fuel cell electrodes.
However, this class of oxide materials has always had the drawback of being relatively unstable over time, and it is this instability that is preventing its further development.
Now however, a breakthrough has been made by a team at MiT which promises to drastically improve their performance and, as the MiT news website reports, will make them “…promising candidates to serve as electrodes in energy-conversion devices such as fuel cells and electrolyzers.”
The MiT team is already well-known in the industry for its 2013 work on improving fuel cells via a ‘superlattice’ that increased oxygen reactions, as reported here, and now seem to have solved another fuel cell challenge.
When discussing her team’s discovery, Bilge Yildiz, Associate Professor of Materials Science and Engineering at MiT, explained how surfaces exposed to water or gas at high temperatures (such as those inside fuel cells and electrolyzers) “suffer from degradation because of chemical segregation and phase separation. 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.”
Publishing their results in Nature Communications, the team state that, “[reducing the concentration of positively charged oxygen vacancies on the surface] improves the oxygen surface exchange kinetics and stability significantly, albeit contrary to the well-established understanding that surface oxygen vacancies facilitate reactions with O2 molecules.”
As the online journal Phys.org reports, “The team’s analysis shows that there is a sweet spot in the addition of more oxidizable elements to the surface, both in terms of the composition and the concentration. In these initial experiments, they tried several different elements to provide the protective effect. The improvement increases up to a certain concentration, and then adding more of the surface additives starts to make things worse again. So for any given material, there will be an optimum amount that should be added, they found. Using hafnium, the new treatment has been shown to reduce the rate of degradation, and increase by 30 times the rate of oxygen exchange reactions at the surface.”
However, the chemical market for hafnium is unlikely to change overnight, as, “the surface treatment amounts to no more than a single atomic layer over the bulk material.”
Instead, what is significant about the discovery is that it opens up the possibility for other developments with perovskite oxides that could dramatically change the way we use fuel cells.
As John Kilner, a professor of energy materials at Imperial College, London, said when commenting on the work, “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.”
Whilst William Chueh, an Assistant Professor of Materials Science and Engineering at Stanford University, also saw the importance of the research, noting that, “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.”
If Yildiz and her team have really found a method to prevent the degradation of perovskite oxides by adding a simple atomic layer coating, then they will not just have changed the fuel cell industry, but may have opened the door to a whole range of improvements across numerous industries. But for now the key impact will most likely be in fuel cell technology. As Chueh makes clear when stating, “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.”
Now that the issue of cost has been mostly resolved, the industry is wondering who will be first to make use of this breakthrough by bring product to market.