There are various solutions for storing electrical energy: Electrochemical batteries based on lithium store large amounts of energy, but require long charging times. Supercaps, on the other hand, can absorb or release electrical energy extremely quickly - but store much less electrical energy. A further option has been in sight since 2011: A new class of 2D materials that can store enormous amounts of charge has been discovered at Drexel University, USA. These are so-called MXenes, nanosheets of Ti3C2Tx molecules that form a two-dimensional network similar to graphene. While titanium (Ti) and carbon (C) are elements, Tx denotes various chemical groups that seal the surface, for example OH groups. MXenes are highly conductive materials with hydrophilic surfaces. In water they form dispersions that resemble black ink.
Ti3C2Tx can store as much energy as a battery, but can be charged or discharged within tenths of a second. While similarly fast (or faster) supercapacitors absorb their energy by electrostatic adsorption of electrical charges, the energy in MXenes is stored in chemical bonds on their surfaces. This type of energy storage is much more efficient. In collaboration with the group around Yuri Gogotsi at Drexel University, HZB scientists Dr. Tristan Petit and Ameer Al-Temimy have now used soft X-ray absorption spectroscopy for the first time to study MXene samples at the LiXEdrom and X-PEEM experimental stations. They were able to analyse the chemical environment of MXene surface groups in vacuum, but also directly in water.