Mi­croscopy ad­vance re­veals un­ex­pected role for wa­ter in en­ergy stor­age ma­te­rial

Chemical Industry Digest - - News & Views -

Ama­te­rial with atom­i­cally thin lay­ers of wa­ter holds prom­ise for en­ergy stor­age tech­nolo­gies, and re­searchers have now dis­cov­ered that the wa­ter is per­form­ing a dif­fer­ent role than any­one an­tic­i­pated. 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 ad­dress this ques­tion, 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.

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

“Specif­i­cally, we found that the wa­ter lay­ers do two things,” says Ruo­cun “John” Wang, a Ph.D. stu­dent in Au­gustyn’s lab and lead au­thor of the pa­per. “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.

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