2D titanium carbides (MXene)
Super talent for energy storage
Like a battery, MXene stores large amounts of electrical energy - but unlike batteries, they can be charged and discharged in seconds. Researchers at Helmholtz Zentrum Berlin have now succeeded in increasing the capacity of such pseudo-capacitors by more than 50 percent.
There are different solutions for storing electrical energy: Electrochemical batteries based on lithium store large amounts of energy, but require long charging times. Supercapacitors, on the other hand, can absorb or release electrical energy extremely quickly - but store much less electrical energy.
Pseudocapacitors from MXene
A further option has been in sight since 2011: A new class of 2D materials that can store enormous amounts of charge was discovered at Drexel University in the 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.
Soft X-ray light shows what happens
In cooperation with the group around Yuri Gogotsi at Drexel University, scientists of Helmholtz Zentrum Berlin Dr. Tristan Petit and Ameer Al-Temimy have now used soft X-ray absorption spectroscopy at the electron storage ring BESSY II for the first time to investigate MXene samples at the experimental stations LiXEdrom and X-PEEM.
They were able to analyze the chemical environment of MXene surface groups in vacuum, but also directly in water. During their research, the scientists examined samples of pure MXene as well as MXene with embedded urea molecules and found dramatic differences.
Urea increases the capacity
The presence of urea molecules significantly changes the electrochemical properties of MXenes. The surface capacitance increased to 1100 mF/cm2, which is 56 percent higher than with similarly prepared pure Ti3C2Tx electrodes.
The XAS analyses at BESSY II showed that the surface chemistry is changed by the presence of the urea molecules. »At X-PEEM we could also observe the oxidation state of the Ti atoms on the Ti3C2Tx surfaces. This oxidation state increased due to the presence of urea, which could facilitate the storage of more energy,« said Ameer Al-Temimy, who carried out the measurements as part of his doctoral thesis.
The article, in which the scientists describe their research, was published in the Journal of Physical Chemistry in February 2020.
