Written by Mohit Saraf
Ekaterina Pomerantseva, Drexel University
MXene-Derived Oxides as Electrodes for Energy Storage
The prime focus of Ekaterina Pomerantseva’s talk was on using MXenes as precursors for synthesis of oxide materials. The versatile chemistry and two-dimensional morphology of MXenes make them unique precursors for the synthesis of other classes of materials. She first talked about α-V2O5 derived δ-V2O5.H2O (BVO) material, where the synthesis takes longer, involves an aging step, and requires higher temperatures. In a recent study, her group replaced α-V2O5 with V2CTx MXene that was oxidized with the help of H2O2, and the synthesis can be completed without the aging step at lower temperatures. A flower-like morphology was observed for the prepared materials. The resultant V2CTx-derived δ-V2O5.nH2O structure was intercalated with various ions such as Li and Mg and was used for electrochemical energy storage applications. Pomerantseva also discussed recent results of MXene derived K-intercalated V2O5 (KVO) showing promising results for battery applications. Pomerantseva said that this synthesis route of using MXenes as precursors can be extended onto other MXenes to produce oxides with different chemistries or mixed oxides.
Vadym Mochalin, Missouri University of Science and Technology
Chemistry of 2D Transition Metal Carbides and Carbonitrides (MXenes)
Vadym Mochalin highlighted the importance of understanding the MXenes chemistry including their synthesis and stability. MXenes have been considered as unstable in aqueous colloids due to oxidation and therefore the challenge is improving their chemical and temperature stability. However, this oxidation process can be used to prepare composites. Mochalin gave an example of MXene-titania composites that can be produced by a simple, inexpensive, and environmentally benign process of delaminating and storing MXenes under ambient environment which leads to the partial oxidation of MXene to prepare a composite. This composite demonstrates attractive properties for photoresistors with memory effect and sensitivity to the environment, as well as many other photo- and environment-sensing applications. He also showed experiments explaining that it is the hydrolysis, not oxidation, which is primarily responsible for MXene instability. According to Mochalin, acidic environment catalyzes the hydrolysis of MXenes and basic environment inhibits MXene hydrolysis. Also, high pH suppresses MXene hydrolysis and antioxidants suppress MXene oxidation. The synergistic actions of high pH and antioxidants is most beneficial for the shelf life of aqueous MXene colloids at room temperature. He also provided some examples illustrating connections between understanding MXene chemistry and potential applications. Overall, he emphasized understanding the chemistry of MXenes.
Majid Beidaghi, Auburn University
Assembling MXenes Heterostructures and 3D Printing of MXenes for Energy Storage
Majid Beidaghi says that despite having unique physicochemical properties, the application of MXenes in energy storage depends on their assembly into electrode structures. His group is developing assembly of MXenes flakes into such electrode structures that exhibit high power and energy. He also discussed the effects of MXene synthesis conditions and importance of controlling flake size on the electrochemical performance. He also demonstrated several videos of three-dimensional (3D) printing of the MXene electrode where Beidaghi’s research group successfully used MXene inks for printing on different substrates. Moreover, he emphasized on developing heterostructured MXenes for superior chemical and electrochemical stability and energy storage capability.
Michael Naguib, Tulane University
Nanoengineering MXenes Interlayer Spacing for High Performance Electrochemical Energy Storage Electrodes
Michael Naguib first introduced the MXenes and their properties such as electrical conductivity and capability of hosting multivalent ions, which make them promising candidates for electrochemical energy storage. He talked about the structural and computational diversity and tunability of MXenes and highlighted the importance of intercalation in MXenes to escalate their properties to be used in a variety of applications. He provided several examples of intercalation of cations into the MXene structure which were successfully used in supercapacitors, Na-ion capacitors, and other applications. His group also tested MXenes in several new electrolytes such as room temperature ionic liquid (RTIL) which showed great promise in electrochemical applications. He mentioned that tuning the d-spacing of MXene structures is a viable approach to unlock new applications. The prime focus of his talk was that a small change in the structure can affect the performance significantly and therefore the intercalation could be a promising strategy to enrich the MXene family. He also mentioned briefly about the recent discovery of transition metal carbochalcogenides (TMCC).
Teng Zhang, Drexel University
Electrochemical Performance of Vanadium Containing MXenes in Aqueous Electrolytes
Teng Zhang talked about the electrochemistry of vanadium-containing MXenes in aqueous electrolytes. He emphasized that since the discovery of MXenes in 2011, the primary candidate of study has been Ti3C2, but many MXenes with different compositions and structures have been synthesized but not widely explored. According to Zhang, vanadium-containing MXenes are also fascinating candidates for redox energy storage because of the presence of vanadium transition metal exhibiting multiple oxidation states. He showed the electrochemical results on two different vanadium-containing MXenes, V2C and V4C3, which were used for supercapacitors utilizing various aqueous electrolytes such as KOH and H2SO4. He mentioned that the number of layers in MXenes greatly affects the charge storage mechanism and electrochemistry. He believes that the ongoing work will clarify the structural chemistry-electrochemistry relations in MXenes; however, a proper understanding of the charge storage mechanism in difference electrolytes is important for advancing the field of MXene electrochemistry.