Preparation And Electrochcmical Energy Storage Apprlication Of Biochar-based Carbon And Cation-Intercalated MXene Materials

With the depletion of fossil energy and the increasing of environmental pollution,green and sustainable energy is particularly important.A series of energy storage devices have been widely studied,such as supercapacitors,lithium-ion batteries,potassium ion batteries.In this thesis,the capacitive performance of biochar-based hierarchical porous carbons in different aqueous electrolytes and the performance of cation-intercalated Ti₃C₂ MXene in storing Li⁺and K⁺are studied.1.The development of supercapacitors with high energy density and power density is an important research topic despite many challenging issues exist.In this work,a type of porous carbon material is prepared from corn straw biochar by a method of KOH activation and used as the active electrode material for supercapacitors.It is observed that the ratio of KOH/biochar significantly affects the microstructure and porosisty of the resultant carbons,thus influences the capacitive performance of the prepared carbons.The optimized carbon material possesses typical hierarchical porosity composed of multi-leveled pores with high surface area and pore volume up to 2790.4 m²g^-11 and 2.04 cm³ g^-1,respectively.Such hierarchical micro-meso-macro porosity significantly improves the rate performance of the biochar-based carbons.The achieved maximum specific capacitance is 327 F g^-1 and maintains a high value of 205 F g^-1 at an ultrahigh current density of 100A g^-1.Meanwhile,the prepared supercapacitor presents excellent cycle stability in alkaline electrolytes for 120 000 cycles at 5 A g^-1.Moreover,the biochar-based carbon could work at a high voltage of 1.6 V in neutral Na₂SO₄,and exhibit a high specific capacitance of 227 F g^-1,thus giving an outstanding energy density of 20.2 Wh kg^-1.2.Improving the performance of electrode materials in storing Li⁺and K⁺has always been a focus topic in the energy storage field.In this work,new two-dimensional?2D?material Ti₃C₂MXenewas used as anode material.Aiming at the problem of low energy density and the difficulty of electrolyte ions inserting to the stackable MXene layer,we develop a novel“quaternary ammonium-intercalated method”for precise controlling the interlayer spacing of the MXene materials.As the ion size of the intercalated quaternary ammonium,the interlayer spacing gradually increases.The characteristics of Li⁺and K⁺storage of MXenes are intensive studied to find the matching mechanism between the interlayer spacing and the size of electrolyte ions.Finally,the big promotion in storing Li⁺and K⁺are realized.After NH₄⁺intercalated,the interlayer spacing of MXene is 1.18 nm,and the specific capacity in storing Li⁺is 152 mAh g^-1,which is 21%higher than that of Ti₃C₂(120 mAh g^-1).And,there is a 60%promotion of specific capacity for NH₄Br@Ti₃C₂at a high current density of 2 A g^-1(from 46 to 74 mAh g^-1).The energy storage mechanism is further explored by cyclic voltammetry,galvanostatic intermittent titration technique,electrochemical impedance spectroscopy,which proved that the diffusion kinetics of intercalated MXenes were improved.So the rate of lithiation/delithiation process of ions is promoted,which is the main reason for the enhancing of Li⁺storage.However,with the interlayer spacing increasing,the long carbon chains between interlayer occurred curl,which hinders the diffusion of ions.After intercalated by?C₂H₅?₄N⁺,the interlayer spacing of MXene is enlarged to be1.48 nm and the capacitance contribution accounts for 79%in storing K⁺at the scan speed of 1 mV s^-1,which shows a strong capacitive K⁺storage.The specific capacity of?C₂H₅?₄NBr@Ti₃C₂ is 81 mAh g^-11 at 0.1 A g^-1,which is 35%higher than that of Ti₃C₂(60 mAh g^-1).What is more,the improvement is up to 187%at the current density of 2 A g^-1(12.2 to 35 mAh g^-1).This work exhibits the probility of improving the energy storage performance of MXene without changing the surface chemical properties,and obtains the matching machenism between the interlayer spacing of 2D materials with the ions of electrolytes.It also has important guiding significance for the modification of other 2D materials.