Research On The Application Of Energy Conversion Based On Novel Two-dimensional MXenes Composite Materials

As fossil energy consumption has increased year by year and the greenhouse effect caused by environmental pollution has intensified,the exploration of clean energy with high energy density,pollution-free,and convenient storage and transportation is becoming a vital research hotspot.Using clean energy such as solar energy and hydrogen energy is often accompanied by energy conversions,such as electrical energy,thermal energy,and chemical energy.Therefore,it is of great significance to realize the efficient conversion of the above multiple energy forms through suitable carriers.As a new member of the two-dimensional material family,transition metal carbonitrides(MXenes)have a high specific surface area,high electronic conductivity,hydrophilicity and surface functional group controllability,and other physical and chemical properties.They are used in the development of lithium batteries,catalysis,and solar energy.Utilization and other fields involving multi-energy conversion have been widely used.This paper designs and synthesizes a series of composite materials based on two-dimensional MXenes and an in-depth study of its mechanism in the lithium-sulfur battery(LSB),high-efficiency hydrogen production catalysis(HER)solar thermal.The research content of this thesis is as follows:1.VO₂(p)-V₂C(MXene)as a high-performance sulfur-carrying anode for lithium-sulfur batteriesReasonable lithium-sulfur battery cathode material design can significantly improve the battery's energy density and working stability.In this study,VO₂(p)nanorod clusters were grown on the surface of two-dimensional V₂C(MXene)nanosheets by hydrothermal method.VO₂(p)nanorods can cooperate with V₂C to enhance the adsorption capacity of the matrix material for polysulfides.At the same time,it can reduce the redox reaction barrier for the conversion of polysulfides to short-chain sulfides.In addition,the high specific surface area and structural stability of the matrix material can improve the redox reaction kinetics and cycle reversibility of the electrode.VO₂(p)-V₂C/S positive electrode exhibits excellent electrochemical performance,with high reversible discharge capacity(1250 m Ah·g^-1,0.2 C),long-term cycle stability(residual capacity 69.1%after 500 cycles of charge and discharge),and high sulfur loading cycle performance(the initial unit area capacity reached 9.3m Ah·cm^-2 after 200 cycles at 0.2 C).2.Vanadium-based carbide-oxide heterogeneous V₂O₅@V₂C nanotube arrays for high-performance lithium-sulfur batteriesIn order to solve the impact of the shuttle effect on the high-sulfur battery and increase the electrode oxidation-reduction reaction rate as much as possible,a V₂O₅nanotube array grown vertically on V₂C-MXenes was prepared using vacuum freezing and hydrothermal methods.V₂O₅@V₂C exhibits extreme adsorption capacity for polysulfides,and V₂O₅ has excellent catalytic performance for oxidation reactions,which significantly reduces the reaction barriers for the conversion of polysulfides to Li₂S and Li₂S₂.In addition,the larger specific surface area and tubular structure of the composite matrix material can increase the sulfur loading of the positive electrode while ensuring the stability of the electrode structure.As a result,the V₂O₅@V₂C/S positive electrode exhibits a higher initial capacity(1173 m Ah·g^-1,0.2 C),a longer cycle life(more than 1000 cycles),and a higher sulfur load(8.4 mg·cm^-2).3.Study on the catalytic hydrogen production of VS₂@V₂C and other composite materials from seawaterTwo-dimensional materials are the current research hotspot of non-precious metal catalysts.In this study,a two-dimensional material/sulfide structure electrolysis strategy was proposed,and VS₂@V₂C and other composite materials were successfully prepared.The vanadium disulfide nanosheets are evenly and vertically embedded on the few-layer V₂C nanosheets,reducing the influence of inter-sheet agglomeration and structural collapse on the structure of the catalyst material.The special structure of the composite material is conducive to the transfer of electrons between them.In seawater with different p H values,the composite material exhibits a higher current density and lower hydrogen evolution overpotential comparable to platinum-based catalysts,and provides higher current in solar photovoltaic cells.In the case of density,a higher Faraday efficiency and hydrogen evolution stability are achieved.In addition,the hydrogen evolution performance of Co₃O₄@VS₂ prepared based on the MOF template in acid electrolytes was also explored,which proved that the electrolysis water strategy has general applicability in guiding the synthesis and application of other hydrogen evolution catalysts.4.Cu₃BiS₃/MXenes composite material for efficient solar thermal seawater desalinationAffected by the Fresnel effect,two-dimensional materials often have light reflection losses at the solid-liquid-gas interface.In order to improve the light absorption performance of the two-dimensional material,this study used the thermal injection method to grow Cu₃BiS₃ on the surface of the Ti₃C₂ nanosheets uniformly and constructed a new composite material,CBS-Ti₃C₂.The composite material achieves broadband absorption of light(the visible light region is greater than 90%,and the near-infrared region is greater than 80%)and has been theoretically verified.The steady-state temperature can reach 62.3°C under the incident light intensity of 1 sun.Even at a lower thickness(0.48 mg·cm^-2),it still guarantees a faster steam production rate(1.32kg·m^-2·h^-1)And extremely high light-to-heat conversion efficiency(greater than 90%).In addition,CBS-Ti₃C₂ also exhibits the characteristics of resisting surface salt accumulation,which is conducive to improving work stability.5.NiS₂@Ti₃C₂ with excellent photothermal performance for long-term solar seawater desalinationTwo-dimensional materials are emerging in light-to-heat conversion,but it is urgent to find a simple and effective method to reduce the light reflectivity of the surface to improve the light-to-heat conversion efficiency.In this study,a simple hydrothermal method was used to construct a NiS₂ nano-square crystal and two-dimensional Ti₃C₂nano-coated structure(NiS₂@Ti₃C₂).Its multilayer structure can carry out a multi-stage diffuse reflection of light so that the composite material has a light absorption rate of>90%in a wide spectral range.The composite material was verified theoretically through optical simulation and first-principles,and a higher light-to-heat conversion performance was achieved experimentally.The assembled MXene seawater steam generator achieved an excellent steam generation rate(1.27 kg m^-2·h^-1)and light-to-heat conversion efficiency(83.67%)under one sunlight intensity.