Researchers at MIT tackled the known problem of degradation suffered when perovskite oxides, promising candidates for electrodes in energy conversion devices like fuel cells, are exposed to water or gases such as oxygen or carbon dioxide at elevated temperatures.
The scientists explain that this degradation occurs as 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.
When the team tried to add another element on the surface of the electrode called hafnium, the result was unexpected. Hafnium shows almost no reactivity by itself, and so was a surprising choice for a material to add. However, it appears that as a surface treatment for the perovskite, 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. 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 such applications.
Also, the researchers state that the amount of material used is very small, so it does not change the bulk material. These findings could change the way manufacturers use perovskite oxide electrocatalysts, including those in solid oxide fuel cells. Surface degradation was a definite bottleneck in development for years, but with this new research, there have been significant steps made in dealing with the problem.