Kaitlyn Prenger1,Sudhajit Misra2,Murali Gopal Muraleedharan2,Chaochao Dun3,Raymond Unocic2,Paul Kent2,Jeffrey Urban3,Michael Naguib1
Tulane University1,Oak Ridge National Laboratory2,Lawrence Berkeley National Laboratory3
Kaitlyn Prenger1,Sudhajit Misra2,Murali Gopal Muraleedharan2,Chaochao Dun3,Raymond Unocic2,Paul Kent2,Jeffrey Urban3,Michael Naguib1
Tulane University1,Oak Ridge National Laboratory2,Lawrence Berkeley National Laboratory3
Two-dimensional (2D) materials of different compositions can be stacked like building blocks to form unique heterostructures combining the properties of dissimilar materials to achieve new paradigms that can’t be achieved by the individual materials alone. One interesting family of 2D materials are transition metal oxides (TMO), which are relatively inexpensive and have promise in the field of electrochemical energy storage. Because 2D TMO are either insulators or semiconductors with wide bandgap, conductive additives must be used for electrochemical energy storage applications. Electrically conductive 2D transition metal carbides, MXenes, have already shown enormous potential in the field of electrochemical energy storage, due to their capabilities to host ions and protons in addition to their high electrical conductivity. The wide variety of available MXenes and TMOs with different properties allows the tuning of heterostructure architecture to optimize performance. <br/><br/>We have achieved novel supercapacitor architectures which show properties which combine and exceed the properties of either material alone. Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub>-</i>Nb<sub>2</sub>O<sub>5</sub> heterostructures demonstrate capacitance of over 200 F/g in neutral aqueous electrolytes Li<sub>2</sub>SO<sub>4</sub> and Nb<sub>2</sub>SO<sub>4</sub>. In the same electrolytes, Nb<sub>4</sub>C<sub>3</sub>T<i><sub>x</sub></i>-Nb<sub>2</sub>O<sub>5</sub> demonstrates a capacitance up to 100 F/g. These capacitances exceed what can be achieved each material alone. In case of Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub>-</i>Nb<sub>2</sub>O<sub>5</sub>, we observed multiple redox peaks, assigned mostly to Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub>. which have the effect of expanding the high-current area of the cyclic voltammagram curve. Interestingly, such redox peaks were not observed in neutral aqueous electrolyte for MXene alone. The use of high-resolution TEM, as well as allows atomic-scale resolution of the heterostructure interface, showing that MXene and TMO nanosheets are stacked to form the heterostructure interface. STEM/EELS and X-ray absorption spectroscopy shows the effect of the interface between MXene and TMO on the oxidation states of titanium and niobium, which is supported by density functional theory (DFT) calculations.