Study On Energy Storage Mechanism Of Two-dimensional Layered Material Ti₃C₂T_x

In today's world,the harmonious development of energy,human and environment has become the focus of the society.With the increasing demand for sustainable and portable energy,with the depletion of resources and environmental pollution from industrial production and fuel combustion.In particular,the recent burning of fossil fuels caused by the frequent occurrence of haze weather.Haze weather has seriously affected our health and life.Therefore,research and development of safe,pollution-free green energy has become an urgent need to solve a major problem.Renewable green energy,represented by solar,wind and ocean tidal energy,despite its abundant and renewable resources,the diffusion of energy in time and power seriously limits its development.Chemical power supply with excellent performance plays an important role in energy and environmental protection as a clean and efficient energy storage.The development of high-performance chemical power has become one of the world's competing development.Supercapacitors have attracted more attention due to their greater power densities than batteries.However,electrical double layer capacitors?which are the conventional type of supercapacitors?have a limited energy density due to charge storage mechanism limited to the electrosorption of ions.The other kind of supercapacitors,called pseudocapacitors,provide higher energy densities due to redox reactions,but have generally shorter cycle life and are more expensive.Conway describes pseudocapacitance as either one of the three faradaic mechanisms that exhibit capacitive electrochemical behavior.The first pseudocapacitance mechanism is underpotential deposition,or metal ions or protons forming an adsorbed monolayer at a different metal's surface above their redox potential.The second case is redox capacitance,which is the electrochemical adsorption of ions onto the surface or near surface of a material with a concomitant faradaic charge-transfer.The third case is intercalation capacitance,which is the fast faradaic intercalation of ions into tunnels or layers of electrode materials.Recently,a new family of 2D materials called MXenes has been discovered.MXenes have a layered structure composed of multiple elements with the general formula M_n+1X_n,in which M is an early transition metal and X is carbon and/or nitrogen.As of today,more than 30 different MXenes have been synthesized.The main interest in MXenes for energy storage applications is that these materials contain a carbide core that guarantees electronic conductivity and a transition metal oxide-like surface that can undergo redox reactions.MXenes are promising materials for electrochemical energy storage.In particular,Ti₃C₂T_x?T_x=O,OH,F surface termination?as an electrode for supercapacitors exhibit high gravimetric and volumetric capacitances in a wide variety of electrolytes.In the first report on capacitive behavior of Ti₃C₂T_x in sulfuric acid,the capacitance was up to 520 F cm^-3 and 325 F g^-1.Later work further improved the performance,notably Ti₃C₂T_x hydrogel reached volumetric capacitance of up to 1,500F cm^-3 and macroporous Ti₃C₂T_x film in H₂SO₄ delivered up to 210 F g^-1 at high rate of10 V s^-1,surpassing the best carbon supercapacitors known and reaching RuO₂pseudocapacitors performance.For achieving high-performance rechargeable energy storage devices,it is vital to study and understand the physical and chemical changes in electrode materials upon cycling.The charge storage mechanism of Ti₃C₂T_x in various electrolytes was tested by several in-situ techniques.In the first report on MXenes for supercapacitors application,researchers used in-situ X-ray diffraction?XRD?.The report revealed that Na⁺,K⁺and Mg²⁺?from CH?COONa,KOH and MgSO₄ solutions,respectively?were intercalated and deintercalated between Ti₃C₂T_x layers during electrochemical reaction,as demonstrated by the expansion and contraction of the lattice.In-situ XRD later revealed the intercalation charge storage mechanisms of Ti₃C₂T_x in 1-ethly-3-methylimidazoliumbis-?trifluoromethylsulfonyl?-imide.Althoughthebest electrochemical performance of MXene was obtained in H₂SO₄,in-situ XRD analysis was never reported for this system.In this thesis,we first invented the"electrochemical in-situ reaction X-raydiffraction testing device.Make up for the technical gap that in-situ electrochemical XRD equipment can't be measured in acidic electrolyte,and then study the charge storage mechanism of Ti₃C₂T_x in acidic electrolyte.Results revealed that an 0.54?expansion and shrinkage of the c-lattice parameter of Ti₃C₂T_x occurs during cycling in a 0.9 V voltage window.Until now,researchers believed that there was no change in c-lattice parameter upon cycling in acidic electrolytes due to the small size of proton.Indeed,proton intercalation mechanism into MXene has not been thoroughly studied.Our work reveals by in-situ XRD that the pseudo-intercalation mechanism of H⁺insertion between Ti₃C₂T_x layers leads to c lattice parameter change,despite the small size of protons,and correlates it to density functional theoretical?DFT?calculations.It is proved that the energy storage mechanism of Ti₃C₂T_x in sulfuric acid is not only redox pseudo-capacitance but also intercalated pseudo-capacitance with implication for MXene use in energy storage and electrochemical actuators.In order to further improve the energy density of Ti₃C₂T_x,we chose ionic liquids?EMI-TFSI?with larger voltage windows.The change of X-ray diffraction pattern of Ti₃C₂T_x in EMI-TFSI with voltage was studied by in-situ XRD.Thus,the mechanism of energy storage of Ti₃C₂T_x in ionic liquids was revealed.To extend the practical application of Ti₃C₂T_x in the field of energy storage,We also studied the change of X-ray diffraction pattern of Ti₃C₂T_x in ionic liquid EMI-TFSI at low temperature?0??and lower temperature?-20??,thus revealing the energy storage mechanism of Ti₃C₂T_x at low temperature.