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A Super Material for Batteries and Other Energy Conversion Devices


Scientists typically conduct their research by carefully selecting a research problem, developing a plan to solve the problem, and putting the plan into action (or executing it). Unexpected discoveries, on the other hand, can occur along the way.


A new superconductor with unusual behavior was discovered by Mercouri Kanatzidis, a professor at Northwestern University with a joint appointment at the Department of Energy's (DOE) Argonne National Laboratory. He was searching for such a material when he made an unexpected discovery. It was a material that was only four atoms thick and allowed for the study of the motion of charged particles in only two dimensions, which was revolutionary at the time. Such investigations could pave the way for the development of novel materials for use in a wide range of energy conversion devices.


The material that Kanatzidis was aiming for was a combination of silver, potassium, and selenium (a-KAg3Se2) in a four-layered structure, similar to that of a wedding cake. Due to the fact that they are only four atoms high, these 2D materials have length and width but almost no thickness.


When superconducting materials are cooled to extremely low temperatures, they completely lose their resistance to the movement of electrons. "Much to my disappointment, this material was not a superconductor at all, and we were unable to convert it into one," said Kanatzidis, a senior scientist in the Materials Science Division at Argonne National Laboratory (MSD). To my great surprise, however, it turned out to be an excellent example of a superionic conductor."


Superionic conductors are materials in which the charged ions in a solid material roam about as freely as the charged ions in the liquid electrolytes that are found in batteries. A solid with unusually high ionic conductivity, which is a measure of the ability to conduct electricity, is produced as a result of this process. Because of the high ionic conductivity, the thermal conductivity is also low, which means that heat does not pass through easily. Superionic conductors are exceptional materials for energy storage and conversion devices because of these two characteristics.


As soon as they heated the material to between 450 and 600 degrees Fahrenheit, the team realized that they had discovered a material with unique properties. It evolved into a more symmetrical layered structure as time went on. When the team lowered the temperature and then raised it again into the high temperature zone, they discovered that the transition was reversible.


"Our analysis results revealed that the silver ions were fixed in the confined space within the two dimensions of our material prior to this transition," Kanatzidis explained. "However, following this transition, they began to wiggle around." There is a great deal of knowledge about how ions move around in three dimensions, but there is very little knowledge about how they move around in only two dimensions.


Scientists have been looking for an exemplary material to study ion movement in two-dimensional materials for quite some time. One such material appears to be this layered potassium-silver-selenium material. The team measured the rate at which the ions diffused through this solid and discovered that it was equivalent to the rate at which ions diffused through a heavily salted water electrolyte, which is one of the fastest known ionic conductors.


Despite the fact that it is too soon to tell whether or not this particular superionic material will find practical application, it could serve as a crucial platform for the development of other 2D materials with high ionic conductivity and low thermal conductivity in the near future.


According to Duck Young Chung, principal materials scientist at MSD, "These properties are extremely important for those who are designing new two-dimensional solid electrolytes for batteries and fuel cell applications."


It is possible that research with this superionic material will be useful in the development of new thermoelectrics, which convert heat to electricity in power plants and industrial processes, as well as in the conversion of exhaust gas from automobile emissions. Moreover, such studies could be used to develop membranes for environmental cleanup and desalination of water, among other applications.

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