Preparation Of MXene （Ti₃C₂T_x）-based Composites For Capacitive Deionization And Supercapacitor

Water shortage and energy crisis are major issues affecting the sustainable development of society.Desalination technology is an effective solution to the problem of water resources.Commonly used water treatment technologies such as reverse osmosis,heat treatment and electrodialysis generally have the problems of high energy consumption,high cost and environmental pollution.Capacitive deionization(CDI)is a water treatment technology with low cost,low energy consumption,high efficiency,easy regeneration,and no secondary pollution.It is an electrosorption desalination method based on electric double layer capacitors or Faraday capacitors.In addition,with the continuous development of the global economy and society,the demand for new energy storage devices with high power,long life,large capacity and green environmental protection is increasing.Supercapacitors(SCs)are energy storage devices between traditional capacitors and batteries.They have the advantages of high energy density,high power density,high charge and discharge efficiency,and long life.Electrode materials are the core components of capacitor desalination and supercapacitors,which directly affect the desalination performance and the energy storage characteristics of supercapacitors.Therefore,exploring low-cost,high-performance electrode materials is of great significance for CDI and super-electric applications.In recent years,two-dimensional layered transition metal carbides or nitrides(MXene)have gradually been used in the research of capacitor desalination performance and supercapacitor characteristics due to their high specific capacitance,good electrical conductivity,and strong hydrophilicity.However,due to the weak interaction between MXene layers,MXene is prone to collapse and stacking,and exhibits severe swelling behavior in aqueous solution,which affects its electrochemical performance as an electrode material.The MXene-based composites can increase the layer spacing,reduce the ion diffusion resistance,effectively prevent the stacking of MXene sheets,increase the ion adsorption sites,and enhance the conductivity and effectively inhibit the swelling behavior of MXene,thereby improving its capacitive desalination performance or super Capacitance characteristics.In this study,metal ion intercalation MXene composites,quaternary ammonium salt intercalation MXene composites,and MXene/NH4CoPO4·H2O composites were prepared by intercalation reaction and solution-assisted etching methods,respectively,and MXene-based composites and activated carbon were assembled.Hybrid capacitor desalination devices and asymmetric supercapacitor device were used to study the capacitor desalination performance and superelectric performance of MXene-based composites.The main research contents are as follows:The alkali metal intercalation MXene composites were prepared using MXene(Ti3C2Tx)as the precursor by treatment with different alkali solution.XRD analysis showed that the interlayer spacing of MXene increased from 0.98 nm to 1.22 nm after lye treatment.The BET results show that the specific surface areas of the composites increase after the alkali metal intercalation of MXene.The specific surface area of M-LiOH is the largest and it’s 100.202 m2 g-1.Electrochemical tests show that the M-LiOH composite electrode material has higher specific capacitance(174.78 F g-1)and lower impedance in 1 M NaCl solution.MXene,M-LiOH,M-NaOH and M-KOH were used as the negative electrodes,and activated carbon(AC)was used as the positive electrode to assemble hybrid capacitive deionization(H-CDI)devices.The desalination performances were studied.The results showed that when the initial NaCl solution concentration was 1000 mg L-1 and the voltage was 1.2 V,the electrosorption capacity was 47.4,81.3,66.1,and 63.1 mg g-1,respectively.The M-LiOH electrode has better CDI performance,mainly because M-LiOH has a higher specific surface area,which can provide more Na+ adsorption sites.In addition,the AC//M-LiOH system has higher charge efficiency(87.2%)and lower energy loss(0.62 kWh kg-1)in the desalination process.The CDI cycle performance test of the system shows that after 10 cycles,the desalination capacity can still maintain the original 80.9%,indicating that the M-LiOH electrode has good cycle stability.With MXene(Ti3C2Tx)as the precursor,K-MXene,Ca-MXene,Al-MXene and Sn-MXene composite materials were prepared after treatment with chloride solution of different valence metal ions.XRD analysis shows that the interlayer spacing of MXene increases from 9.68 (?) to 9.94 (?) after treatment with chloride solution.The SEM images show that the MXene composites intercalated with different valence metal ions still maintain the layered structure of MXene.Electrochemical tests show that the Sn-MXene composite electrode material has higher specific capacitance(206.06 F g-1)and lower impedance in 1 M NaCl solution.The MXene composite electrode materials with different valence metal ions intercalated as the negative electrodes and activated carbon(AC)as the positive electrode were assembled into hybrid capacitive deionization(H-CDI)devices,and their desalination performances were studied.The results showed that when the initial sodium chloride solution concentration was 1000 mg L-1 and the voltage was 1.2 V,the desalination capacity of MXene,K-MXene,Ca-MXene,Al-MXene,and Sn-MXene was 47.4,65.8,72.8,76.3 and 89.2 mg g-1,respectively.In addition,the AC//Sn-MXene system has the highest charge efficiency(95.8%)and the lowest energy loss(0.57 kWh kg-1)in the desalination process.The CDI cycle performance test of the AC//Sn-MXene system shows that after 10 cycles,the desalination capacity can still maintain the original 91.5%.In comparison,the Sn-MXene electrode has better CDI performance,which is mainly because as the valence of metal ions increases,the electrostatic adsorption force with the MXene laminate is enhanced,thereby providing more stability for the insertion and extraction of Na+.The diffusion channel of MXene effectively inhibits the swelling behavior of MXene.Using MXene(Ti3C2Tx)as the precursor and using quaternary ammonium salts with different carbon chain lengths as intercalating agents,quaternary ammonium salt intercalation MXene composites were prepared.XRD results showed that as the carbon atoms in the alkyl chain of the quaternary ammonium salt increased from 14(TTAB)to 16(CTAB)and 18(STAB),the interlayer spacing of MXene gradually increased from 0.98 nm to 1.45,1.60 and 1.74 nm,respectively.XPS test results show that the quaternary ammonium cations are inserted between the MXene layers mainly through electrostatic adsorption and ion exchange reaction.Electrochemical tests show that the MXene@STAB composite electrode material has higher specific capacitance(183.35 F g-1)and lower impedance in 1 M NaCl solution.In addition,the possible arrangements of TTA+,CTA+,and STA+between MXene layers are calculated to be single-layer flat,single-layer flat or single-layer tilt,and single-layer tilt.The quaternary ammonium salts intercalation MXene composite electrode materials were used as the negative electrodes and activated carbon(AC)was used as the positive electrode,assembled into hybrid capacitive deionization(H-CDI)devices,and their desalination performances were studied.The results showed that when the initial sodium chloride solution concentration was 1000 mg L-1 and the voltage was 1.2 V,the desalination capacity of MXene,MXene@TTAB,MXene@CTAB,and MXene@STAB was 47.4,67.2,74.0 and 82.0 mg g-1,respectively.In addition,the AC//MXene@STAB system has a higher charge efficiency(88.4%),lower energy loss(0.63 kWh kg-1)and good recycling performance.The MXene@STAB electrode has better CDI performance,which is mainly due to the larger layer spacing of MXene@STAB,which can store more Na+.In addition,the larger layer spacing can also reduce the diffusion barrier of Na+,so that sodium ions migrate quickly between layers.By etching MOF(ZIF-67)/Ti3C2Tx composite material with diammonium hydrogen phosphate solution,uniform NH4CoPO4·H2O nanoparticles were prepared on Ti3C2Tx conductive nanosheets.Due to the synergistic effect of NH4CoPO4·H2O and Ti3C2Tx,the NH4CoPO4·H2O/Ti3C2Tx nanocomposite has a shorter ion transport path and excellent electronic conductivity.The composite electrode material has excellent rate performance.When the current density is 1 A g-1,the specific capacitance can reach 601 F g-1;when the current density is 20 A g-1,the specific capacitance can still reach 535 F g-1.In addition,the NH4CoPO4·H2O/Ti3C2Tx nanocomposite electrode material has excellent cycle stability.It was cycled 5000 times at a current density of 5 A g-1,and the capacitance retention rate was 87%.A novel asymmetric supercapacitor(ASC)was assembled using NH4CoPO4·H2O/Ti3C2Tx composite material as the positive electrode,activated carbon(AC)as the negative electrode and 6 M KOH solution as the electrolyte.Electrochemical tests showed the ASC had a high energy density(47 Wh kg-1 at a power density of 1350 W kg-1 and 18 Wh kg-1 at a power density of 5000 W kg-1).