Superior Wear-Resistance of Ti3C2Tx Multilayer Coatings.

Owing to MXenes' tunable mechanical properties induced by their structural and chemical diversity, MXenes are believed to compete with state-of-the-art 2D nanomaterials such as graphene regarding their tribological performance. Their nanolaminate structure offers weak interlayer interactions and an easy-to-shear ability to render them excellent candidates for solid lubrication. However, the acting friction and wear mechanisms are yet to be explored. To elucidate these mechanisms, 100-nm-thick homogeneous multilayer Ti3C2Tx coatings are deposited on technologically relevant stainless steel by electrospraying. Using ball-on-disk tribometry (Si3N4 counterbody) with acting contact pressures of about 300 MPa, their long-term friction and wear performance under dry conditions are studied. MXene-coated specimens demonstrate a 6-fold friction reduction and an ultralow wear rate (4 × 10-9 mm3 N-1 m-1) over 100 000 sliding cycles, outperforming state-of-the-art 2D nanomaterials by at least 200% regarding their wear life. High-resolution characterization verified the formation of a beneficial tribolayer consisting of thermally/mechanically degraded MXenes and amorphous/nanocrystalline iron oxides. The transfer of this tribolayer to the counterbody transforms the initial steel/Si3N4 contact to tribolayer/tribolayer contact with low shear resistance. MXene pileups at the wear track's reversal points continuously supply the tribological contact with fresh, lubricious nanosheets, thus enabling an ultra-wear-resistant and low-friction performance.

Design of Carbon/Metal Oxide Hybrids for Electrochemical Energy Storage.

Next generation electrochemical energy storage materials that enable a combination of high specific energy, specific power, and cycling stability can be obtained by a hybridization approach. This involves electrode materials that contain carbon and metal oxide phases linked on a nanoscopic level and combine characteristics of supercapacitors and batteries. The combination of the components requires careful design to create synergistic effects for an increased electrochemical performance. Improved understanding of the role of carbon as a substrate has advanced the power handling and cycling stability of hybrid materials significantly in recent years. This Concept outlines different design strategies for the design of hybrid electrode materials: (1) the deposition of metal oxides on readily existing carbon substrates and (2) co-synthesizing both carbon and metal oxide phase during the synthesis procedure. The implications of carbon properties on the hybrid material's structure and performance will be assessed and the impact of the hybrid electrode architecture will be analyzed. The advantages and disadvantages of all approaches are highlighted and strategies to overcome the latter will be proposed.

Binder-Free Hybrid Titanium-Niobium Oxide/Carbon Nanofiber Mats for Lithium-Ion Battery Electrodes.

Free-standing, binder-free, titanium-niobium oxide/carbon hybrid nanofibers are prepared for Li-ion battery applications. A one-pot synthesis offers a significant reduction of processing steps and avoids the use of environmentally unfriendly binder materials, making the approach highly sustainable. Tetragonal Nb2 O5 /C and monoclinic Ti2 Nb10 O29 /C hybrid nanofibers synthesized at 1000 °C displayed the highest electrochemical performance, with capacity values of 243 and 267 mAh g-1 , respectively, normalized to the electrode mass. At 5 A g-1 , the Nb2 O5 /C and Ti2 Nb10 O29 /C hybrid fibers maintained 78 % and 53 % of the initial capacity, respectively. The higher rate performance and stability of tetragonal Nb2 O5 compared to that of monoclinic Ti2 Nb10 O29 is related to the low energy barriers for Li+ transport in its crystal structure, with no phase transformation. The improved rate performance resulted from the excellent charge propagation in the continuous nanofiber network.

Sub-micrometer Novolac-Derived Carbon Beads for High Performance Supercapacitors and Redox Electrolyte Energy Storage.

Carbon beads with sub-micrometer diameter were produced with a self-emulsifying novolac-ethanol-water system. A physical activation with CO2 was carried out to create a high microporosity with a specific surface area varying from 771 (DFT) to 2237 m(2)/g (DFT) and a total pore volume from 0.28 to 1.71 cm(3)/g. The carbon particles conserve their spherical shape after the thermal treatments. The controllable porosity of the carbon spheres is attractive for the application in electrochemical double layer capacitors. The electrochemical characterization was carried out in aqueous 1 M Na2SO4 (127 F/g) and organic 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (123 F/g). Furthermore, an aqueous redox electrolyte (6 M KI) was tested with the highly porous carbon and a specific energy of 33 W·h/kg (equivalent to 493 F/g) was obtained. In addition to a high specific capacitance, the carbon beads also provide an excellent rate performance at high current and potential in all tested electrolytes, which leads to a high specific power (>11 kW/kg) with an electrode thickness of ca. 200 µm.