Preparation Of Two-Dimensional Mxene-Based Composites For Energy Conversion And Storage

Energy conversion and storage is an important technology for efficient utilization of renewable energy.MXenes,a new class of two-dimensional transition metal carbide and/or nitride materials,have been regarded as promising materials in the field of energy conversion and storage due to their rich surface chemistry,large two-dimensional surface,good hydrophilicity and high electrical conductivity.However,The poor activities of MXenes still limit the demand for high performance.An effective approach is rational construction of MXenes-based composites to reach the excellent performance.In this thesis,a series of MXenes-based composites with specific-oriented properties based on the rich groups of MXenes surface have been prepared,and the applications of the MXenes-based composites in bio-lipids catalysis,electrocatalytic hydrogen evolution（HER）and lithium ion batteries are systematically investigated.The main conclusions are as follows:（1）Based on the large two-dimensional surface of Mo₂CT_xMXene and its easy oxidation property,the Ni/Mo O₂@Mo₂CT_xcomposite with high bio-lipids hydrodeoxygenation activity is prepared.The Ni-based catalysts for lipid deoxygenation generally have the problems of C-C bond fracture and Ni particles agglomeration.Using two-dimensional Mo₂CT_xMXene as the substrate not only can solve the problem of Ni particles agglomeration,but also the formed Mo O₂from in-situ oxidation of Mo₂CT_xcan effectively promote the fission of C-O/C=O bonds to improve the carbon atom economy of lipid deoxygenation.Under the effect of the Ni/Mo O₂@Mo₂CT_xcatalyst,the conversion of palmitic acid can reach 100%,and the alkane selectivity is 97.09%.The selectivity ratio of hexadecane/pentadecane of is1.39,showing a higher carbon atom economy.This study provides a new idea for the future research of MXene-based composites in the field of thermal catalysis.（2）With Mo₂CT_xMXene as active and conductive substrate,the Co-Mo S₂/Mo₂CT_xcomposite with alkaline HER activity is synthesized by interfacial coupling and Co doping.Due to the slow water dissociation kinetics of Mo S₂for HER in alkaline conditions,the alkaline HER performance of electrocatalyst is improved by synergistically enhancing the electron conductivity and water decomposition kinetics.The Co-Mo S₂/Mo₂CT_xelectrode can deliver a lower overpotential of 112 m V at the current density of 10 m A cm^–2with the Tafel slope of 82 m V dec^–1,and exhibit excellent long-term stability of 18 h without significant activity decrease.The electrode also shows a great potential for HER in the full p H range.（3）Based on the higher conductivity and lower Li⁺diffusion barrier of Ti₃C₂T_xMXene,theγ-Fe₂O₃@Ti₃C₂T_xcomposite with excellent lithium storage property is prepared by the in-situ formation of hydrogen bonds after alkali treatment.The pureγ-Fe₂O₃as lithium anode tends to cause volume expansion during charge/discharge process resulting in structural collapse.By combining Ti₃C₂T_xMXene with alkali treatment,the volume expansion ofγ-Fe₂O₃caused by lithium insertion/extraction can be effectively mitigated,the restacking of Ti₃C₂T_xMXene can be prevented,and the structure and interface stability of the electrode materials can be further enhanced.Kinetic study shows theγ-Fe₂O₃@Ti₃C₂T_xelectrode has a fast Li⁺diffusion rate.The electrode can deliver an ultrahigh reversible capacity of 1060 m Ah g^-1at 0.5 A g^-1after 400 cycles,and no obvious capacity loss is observed even after 800 cycles at high current density of 2 Ag^-1.