Porous silica-pillared MXenes with controllable interlayer distances for long-life Na-ion batteries

MXenes are a recently discovered class of two-dimensional materials that have shown great potential as electrodes in electrochemical energy storage devices. Despite their promise in this area, MXenes can still suffer limitations in the form of restricted ion accessibility between the closely spaced multistacked MXene layers, causing low capacities and poor cycle life. Pillaring, a strategy where a secondary species is inserted between layers, has been used to increase interlayer spacings in clays with great success, but has had limited application in MXenes. We report a new amine-assisted pillaring methodology that successfully intercalates silica-based pillars between Ti3C2 layers. Using this technique, the interlayer spacing can be controlled with the choice of amine and calcination temperature, up to a maximum of 3.2 nm, the largest interlayer spacing reported for an MXene. Another effect of the pillaring is a dramatic increase in surface area, achieving BET surface areas of 235 m2 g-1, a sixty-fold increase over the unpillared material and the highest reported for MXenes using an intercalation-based method. The intercalation mechanism was revealed by different characterisation techniques, allowing the surface chemistry to be optimised for the pillaring process. The porous MXene was tested for Na-ion battery applications, and showed superior capacity, rate capability and remarkable stability compared with non-pillared materials, retaining 98.5% capacity between the 50th and 100th cycles. These results demonstrate the applicability and promise of pillaring techniques applied to MXenes, providing a new approach to optimising their properties for a range of applications. Porous MXenes are very promising materials for a range of applications including energy storage, conversion, catalysis and gas separations

The X-ray luminosity–temperature relation of a complete sample of low-mass galaxy clusters

International audienceWe present Chandra observations of 23 galaxy groups and low-mass galaxy clusters at 0.03 < z < 0.15 with a median temperature of ${\sim }2{{\rm keV}}$. The sample is a statistically complete flux-limited subset of the 400 deg^2 survey. We investigated the scaling relation between X-ray luminosity (L) and temperature (T), taking selection biases fully into account. The logarithmic slope of the bolometric L–T relation was found to be 3.29 ± 0.33, consistent with values typically found for samples of more massive clusters. In combination with other recent studies of the L–T relation, we show that there is no evidence for the slope, normalization, or scatter of the L–T relation of galaxy groups being different than that of massive clusters. The exception to this is that in the special case of the most relaxed systems, the slope of the core-excised L–T relation appears to steepen from the self-similar value found for massive clusters to a steeper slope for the lower mass sample studied here. Thanks to our rigorous treatment of selection biases, these measurements provide a robust reference against which to compare predictions of models of the impact of feedback on the X-ray properties of galaxy groups

The X-ray properties of weak-lensing-selected galaxy clusters

We present the results of an X-ray follow-up campaign targeting 10 weak-lensing (WL)-selected galaxy clusters from a Subaru WL survey. Archival Chandra data exist for two of the clusters, and we obtain dedicated observations of the remaining eight. The WL clusters appear to fit the same scaling relation between X-ray luminosity and temperature as X-ray-selected clusters. However, when we consider the luminosity–mass relation, the WL-selected clusters appear underluminous by a factor 2.1 ± 0.5 (or, alternatively, more massive by 1.7 ± 0.3), compared to X-ray-selected clusters with X-ray-based mass estimates. By considering the effects of the centroid offset, Eddington bias, and triaxiallity, this difference can be reconciled. We use X-ray imaging data to quantify the dynamical state of the clusters and found that one of the clusters appears dynamically relaxed, and two of the clusters host a cool core, consistent with Sunyaev–Zel'dovich-effect-selected clusters. This fraction is much lower than observed in X-ray-selected cluster samples

Pillared Mo2TiC2MXene for high-power and long-life lithium and sodium-ion batteries

In this work, we apply an amine-assisted silica pillaring method to create the first example of a porous Mo2TiC2MXene with nanoengineered interlayer distances. The pillared Mo2TiC2has a surface area of 202 m2g−1, which is among the highest reported for any MXene, and has a variable gallery height between 0.7 and 3 nm. The expanded interlayer distance leads to significantly enhanced cycling performance for Li-ion storage, with superior capacity, rate capably and cycling stability in comparison to the non-pillared analogue. The pillared Mo2TiC2achieved a capacity over 1.7 times greater than multilayered MXene at 20 mA g−1(≈320 mA h g−1) and 2.5 times higher at 1 A g−1(≈150 mA h g−1). The fast-charging properties of pillared Mo2TiC2are further demonstrated by outstanding stability even at 1 A g−1(under 8 min charge time), retaining 80% of the initial capacity after 500 cycles. Furthermore, we use a combination of spectroscopic techniques (i.e.XPS, NMR and Raman) to show unambiguously that the charge storage mechanism of this MXene occurs by a conversion reaction through the formation of Li2O. This reaction increases by 2-fold the capacity beyond intercalation, and therefore, its understanding is crucial for further development of this family of materials. In addition, we also investigate for the first time the sodium storage properties of the pillared and non-pillared Mo2TiC