Assembling 2D MXenes into Highly Stable Pseudocapacitive Electrodes with High Power and Energy Densities.

Electrochemical capacitors (ECs) that store charge based on the pseudocapacitive mechanism combine high energy densities with high power densities and rate capabilities. 2D transition metal carbides (MXenes) have been recently introduced as high-rate pseudocapacitive materials with ultrahigh areal and volumetric capacitances. So far, 20 different MXene compositions have been synthesized and many more are theoretically predicted. However, since most MXenes are chemically unstable in their 2D forms, to date only one MXene composition, Ti3 C2 Tx , has shown stable pseudocapacitive charge storage. Here, a cation-driven assembly process is demonstrated to fabricate highly stable and flexible multilayered films of V2 CTx and Ti2 CTx MXenes from their chemically unstable delaminated single-layer flakes. The electrochemical performance of electrodes fabricated using assembled V2 CTx flakes surpasses Ti3 C2 Tx in various aqueous electrolytes. These electrodes show specific capacitances as high as 1315 F cm-3 and retain ≈77% of their initial capacitance after one million charge/discharge cycles, an unprecedented performance for pseudocapacitive materials. This work opens a new venue for future development of high-performance supercapacitor electrodes using a variety of 2D materials as building blocks.

Coexistance of giant tunneling electroresistance and magnetoresistance in an all-oxide composite magnetic tunnel junction.

We propose, by performing advanced ab initio electron transport calculations, an all-oxide composite magnetic tunnel junction, within which both large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist. The TMR originates from the symmetry-driven spin filtering provided by an insulating BaTiO(3) barrier to the electrons injected from the SrRuO(3) electrodes. Following recent theoretical suggestions, the TER effect is achieved by intercalating a thin insulating layer, here SrTiO(3), at one of the SrRuO(3)/BaTiO(3) interfaces. As the complex band structure of SrTiO(3) has the same symmetry as that of BaTiO(3), the inclusion of such an intercalated layer does not negatively alter the TMR and in fact increases it. Crucially, the magnitude of the TER also scales with the thickness of the SrTiO(3) layer. The SrTiO(3) thickness becomes then a single control parameter for both the TMR and the TER effect. This protocol offers a practical way to the fabrication of four-state memory cells.