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, Ti3C2Tx, has shown stable pseudocapacitive charge storage. Here, a cation‐driven assembly process is demonstrated to fabricate highly stable and flexible multilayered films of V2CTx and Ti2CTx MXenes from their chemically unstable delaminated single‐layer flakes. The electrochemical performance of electrodes fabricated using assembled V2CTx flakes surpasses Ti3C2Tx 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.

We calculate the properties of the 4d ferromagnet SrRuO 3 in bulk and thin film form with the aim of understanding the experimentally observed metal-to-insulator transition at reduced thickness. Although the spatial extent of the 4d orbitals is quite large, many experimental results have suggested that electron-electron correlations play an important role in determining this material's electronic structure. In order to investigate the importance of correlation, we use two approaches which go beyond the conventional local-density approximation to density-functional theory ͑DFT͒: the local spin-density approximation+ Hubbard U ͑LSDA+ U͒ and the pseudopotential self-interaction correction ͑pseudo-SIC͒ methods. We find that the details of the electronic structure predicted with the LSDA do not agree with the experimental spectroscopic data for bulk and thin film SrRuO 3 . Improvement is found by including electron-electron correlations, and we suggest that bulk orthorhombic SrRuO 3 is a moderately correlated ferromagnet whose electronic structure is best described by a 0.6 eV on-site Hubbard term, or equivalently with corrections for the self-interaction error. We also perform ab initio transport calculations that confirm that SrRuO 3 has a negative spin polarization at the Fermi level, due to the position of the minority Ru 4d band center. Even with static correlations included in our calculations we are unable to reproduce the experimentally observed metal-insulator transition, suggesting that the electronic behavior of SrRuO 3 ultrathin films might be dominated by extrinsic factors, such as surface disorder and defects, or due to dynamic spin correlations which are not included in our theoretical methods.

The promise of graphene and its derivatives as next generation sensors for real-time detection of toxic heavy metals (HM) requires a clear understanding of behavior of these metals on the graphene surface and response of the graphene to adsorption events. Our calculations herein were focused on the investigation of the interaction between three HMs, namely Cd, Hg and Pb, with graphene quantum dots (GQDs). We determine binding energies and heights of both neutral and charged HM ions on these GQDs. The results show that the adsorption energy of donor-like physisorbed neutral Pb atoms is larger than that of either Cd or Hg. In contrast to the donor-like behavior of elemental HMs, the chemisorbed charged HM species act as typical acceptors. The energy barriers to migration of the neutral adatoms on GQDs are also estimated. In addition, we show how the substitution of a carbon atom by a HM adatom changes the geometric structure of GQDs and hence their electronic and vibrational properties. UV-visible absorption spectra of HM-adsorbed GQDs vary with the size and shape of the GQD. Based on our results, we suggest a route towards the development of a graphene-based sensing platform for the optical detection of toxic HMs.

MXenes, a family of layered transition metal carbides and nitrides, have shown great promise for use in emerging electrochemical energy storage devices, including batteries and supercapacitors. MXene surfaces are terminated by mixed -O, -F and -OH functional groups as a result of the chemical etching production process. These functional groups are known to be randomly distributed over the surfaces, with limited experimental control over their composition. There is considerable debate regarding the contribution of these functional groups to the properties of the underlying MXene material. For instance, their measured Li or Na capacity is far lower than that predicted by theoretical simulations, which generally assume uniformly terminated surfaces. The extent to which this structural simplification contributes to such discrepancies is unknown. We address this issue by employing first-principles calculations to compare the structural, electronic and electrochemical properties of two common MXenes, namely Ti3C2T2 and V2CT2, with both uniform terminating groups and explicitly mixed terminations. Weighted averages of uniformly-terminated layer properties are found to give excellent approximations to those of more realistic, mixed termination structures. This approximation holds for the lattice parameters, the electronic density of states and the work function. The sodium storage capacity and volume change during sodiation in the interlayer space of these MXenes with mixed surface terminations are also investigated. The redox reaction is shown to be confined to the terminating groups for low concentrations of intercalated Na, with the oxidation state of the metal atoms unaffected until higher concentrations of intercalated Na are achieved. Finally, the average open circuit voltage is shown to be very similar for both Ti3C2T2Na and V2CT2Na with mixed terminations, although it is highly sensitive to the particular composition of the terminating groups.