Towards Greener and More Sustainable Synthesis of MXenes: A Review.

The unique properties of MXenes have been deemed to be of significant interest in various emerging applications. However, MXenes provide a major drawback involving environmentally harmful and toxic substances for its general fabrication in large-scale production and employing a high-temperature solid-state reaction followed by selective etching. Meanwhile, how MXenes are synthesized is essential in directing their end uses. Therefore, making strategic approaches to synthesize greener, safer, more sustainable, and more environmentally friendly MXenes is imperative to commercialize at a competitive price. With increasing reports of green synthesis that promote advanced technologies and non-toxic agents, it is critical to compile, summarize, and synthesize the latest development of the green-related technology of MXenes. We review the recent progress of greener, safer, and more sustainable MXene synthesis with a focus on the fundamental synthetic process, the mechanism, and the general advantages, and the emphasis on the MXene properties inherited from such green synthesis techniques. The emerging use of the so-called green MXenes in energy conversion and storage, environmental remediation, and biomedical applications is presented. Finally, the remaining challenges and prospects of greener MXene synthesis are discussed.

3D hierarchical cobalt vanadate nanosheet arrays on Ni foam coupled with redox additive for enhanced supercapacitor performance.

Room-temperature synthesized 3D hierarchical cobalt vanadate (Co3V2O8) nanosheet arrays on Ni foam for use as supercapacitor electrode is presented. In a 3 M KOH electrolyte, the electrode exhibits a capacitance of 109.9 mA h g-1 (878.9 F g-1) at a current density of 1 A g-1. The capacitance is enhanced to 198.1 mA h g-1 (1584.5 F g-1) at 1 A g-1 through the addition of 0.05 M redox-additive K3[Fe(CN)6] into the KOH electrolyte. Furthermore, the Co3V2O8/activated carbon asymmetric supercapacitor cell with the advanced electrolyte outperforms most reported Co3V2O8-based electrodes with a remarkable energy density of 55.5 W h kg-1 at an 800 W kg-1 power density. Combining a facile synthetic strategy and excellent electrochemical performance, the obtained Co3V2O8 exhibits potential for practical application.

MoS2 /MoOx -Nanostructure-Decorated Activated Carbon Cloth for Enhanced Supercapacitor Performance.

MoS2 /MoOx nanostructures were grown on activated carbon cloth through a facile one-step microwave-assisted hydrothermal method. The growth of MoS2 /MoOx on activated carbon cloth creates a unique structure that favors ion intercalation. The conductive activated carbon cloth, MoO3-x , and monoclinic MoO2 provide fast electron transport, whereas the MoS2 nanosheets/MoO3-x nanoparticles structure improves the capacitance. As a result, MoS2 /MoOx -nanostructure-decorated activated carbon cloth shows a high specific capacitance of 230 F g-1 at a scan rate of 5 mV s-1 with a low contact resistance of approximately 1.91 Ω. Moreover, the activated carbon cloth acts as a template for the growth of a perpendicular MoS2 layer, which gives an excellent utilization rate of the active MoS2 /MoOx material. We also demonstrate that the MoS2 /MoOx /activated carbon cloth nanocomposite shows excellent electrochemical stability with retention up to 128 % after 1500 cycles. Finally, we show the use of a microwave-assisted hydrothermal method for the synthesis of the MoS2 /MoOx /activated carbon cloth nanocomposite as an alternative and clean route to improve the kinetics of the intercalation redox reaction.

Direct Growth of MoS2 Nanowalls on Carbon Nanofibers for Use in Supercapacitor.

Direct growth of MoS2 nanowalls on vapor grown carbon nanofibers (VGCNFs) has been achieved using a microwave-assisted hydrothermal (MAH) method under an acidic condition. The acidic condition was obtained through the addition of an HCl aqueous solution. We demonstrate that the HCl not only modifies the pH value for limiting the growth rate but also leads to the formation of NaCl, which is the key for the direct and unique growth of MoS2 on the VGCNF surface. A growth mechanism is therefore proposed. The growth of MoS2 onto the high electrically conducting VGCNF creates a unique structure that not only reduces the aggregation of MoS2 but also improves the electrical conductivity of the resulting composite electrode. The MoS2 nanowall/VGCNF composite shows Csp as high as 248 F g-1 at 5 mV s-1 and excellent electrochemical stability with a retention of 96% after 1,000 cycles at a high charge rate of 200 mV s-1. The ease of composite fabrication and electrochemical stability suggest that the MoS2 nanowall/VGCNF composite is a promising candidate electrode material for supercapacitor.