Unexpected Role For Water In Energy Storage Material

North Carolina State University researchers have now discovered that water is performing a different role than anyone anticipated in a material with atomically thin layers of water that holds promise for battery to pseudo capacitor energy storage technologies.

The finding was possible due to a new atomic force microscopy (AFM) method that measures the sub-nanoscale deformation rate in the material in response to changes in the material caused by energy storage.

The researchers studied crystalline tungsten oxide dihydrate, which consists of crystalline tungsten oxide layers separated by atomically thin layers of water. The material is of interest because it holds promise for helping to store and release energy quickly and efficiently. However, it has not been clear what role the water plays in this process.

To understand what the water role is and how it works, researchers from North Carolina State University, the Oak Ridge National Laboratory (ORNL) and Texas A&M University used a new methodology. The new technique relies on AFM to track the expansion and contraction of the material at the atomic scale and in real time as an electronic instrument called a potentiostat moves charge in and out of the material. This technique allowed the team to detect even minor deformations in the material as charge moved through it.

Veronica Augustyn, an assistant professor of materials science and engineering at NC State and corresponding author of a paper on the work described what took place, “We tested both crystalline tungsten oxide dihydrate and crystalline tungsten oxide – which lacks the water layers. And we found that the water layers appear to play a significant role in how the material responds mechanically to energy storage.”

Ruocun “John” Wang, a Ph.D. student in Augustyn’s lab and lead author of the paper explained, “Specifically, we found that the water layers do two things. One, the water layers minimize deformation, meaning that the material expands and contracts less as ions move in and out of the material when there are water layers. Two, the water layers make the deformation more reversible, meaning that the material returns to its original dimensions more easily.”

“In practical terms, this means that the material with water layers is more efficient at storing charge, losing less energy,” Augustyn says.

The paper, “Operando Atomic Force Microscopy Reveals Mechanics of Structural Water Driven Battery-to-Pseudocapacitor Transition,” has been published in the journal ACS Nano. The paper was co-authored by James Mitchell and Shelby Boyd of NC State; Qiang Gao, Wan-Yu Tsai and Nina Balke of ORNL; and Matt Pharr of Texas A&M. The press release link above lists the funding sources.

The transitional technologies are often the most interesting but are most likely to be the furthest out in time. The battery to pseudocapacitor energy storage technology field, or think fast acting battery, might be one of the quicker ones. This work may just speed it along faster than thought before the work was done.