Surface Modification Of MXenes By NaBH₄ Treatment: A Strategy To Improve MXenes Electrochemical Energy Storage Performance In Supercapacitors

Renewable energy will play a major role in the future.Owing to the intermittent nature of renewable energy sources,to meet the growing power supply demand of this era,a rapid development of high-performance and high-quality electrochemical energy storage system is instantly required.Research and development in the synthesis of electrode materials and study on the electrode energy storage mechanism are the foundation for building an electrochemical energy storage system to meet the demand.In particular,supercapacitors are being considered as next-generation energy storage devices for their high power density,fast charge discharge ability,safe operation,and long cycling life.Therefore,the development of new electrode materials for supercapacitors has caught increasing attention.MXenes,a family of new two-dimensional transition metal carbides,nitrides,or carbonitrides,contain two-dimensional lamellar carbon atom layer.Hence,it exhibits good conductivity and hydrophilic surface which make it a promising pseudocapacitive material.MXenes,prepared by removing the A elements of their corresponding MAX phases by hydrofluoric acid（HF）or Li F-HCl etching,possess abundant terminal groups like F,OH,and O groups.Recent studies have proved that the MXene with few F terminal groups and more O terminal groups could have a higher pseudocapacitive performance.Therefore,the purpose of this study is to develop new methods to modify the surface terminal groups of MXenes and investigate the performance of the modified MXenes in supercapacitors.In specific,this research aims to conduct a surface modification of two kinds of MXenes;Ti₃C₂T_xand V₂CT_xvia Na BH₄treatment method and study the electrochemical energy storage properties.Few-layer Ti₃C₂T_xMXene and V₂CT_xMXene were prepared by Li F HCl and HF etching methods,respectively.The as-prepared and surface modified MXenes have been studied systematically as electrode materials in supercapacitors.A comparison with the traditional alkaline（Na OH）treatment method was also carried out.The results revealed that the Na BH₄-treated few-layer Ti₃C₂T_xdoes not show improved performance in supercapacitor applications in comparison with the pristine few-layer Ti₃C₂T_xMXene,or the alkaline treated counterpart,though it still retains its pseudocapacitance property.However,the Na BH₄-treated V₂CT_xMXene shows promising application in supercapacitor.An enhanced specific capacitance of 391.2 F/g at2 m V/s was obtained after the treatment,which is the highest among all the samples taken.The methods here may be also modified to change the surface chemistry of other MXene for new energy storage systems.