Electrochemical Properties Of Carbon-Based Nanomaterials And Their Application In Capacitive Deionization

With social progress and technological development,problems such as overexploitation of resources and environmental pollution are becoming increasingly serious.The world population has now exceeded 8 billion,and many regions no longer have enough fresh water to meet people’s daily needs,even more than 3.4 million people die every year due to the scarcity or pollution of water resources.As we all know,the earth’s oceans account for about 71%of the total surface area.Therefore,desalination has become one of the effective means to alleviate the scarcity of fresh water resources.Currently,the mainstream technologies for desalination worldwide include membrane separation technologies based on reverse osmosis and electrodialysis,and thermal separation technologies based on multi-stage foaming,multi-effect distillation and mechanical vapor compression.However,these technologies are often hampered by issues such as high energy consumption,high operating costs,low energy efficiency,fouling and environmental secondary pollution.In recent years,capacitive deionization（CDI）has received increasing attention due to its relatively high energy efficiency,low maintenance costs,adjustable water quality and environmental friendliness,which can effectively alleviate the serious problem of fresh water shortage.In this paper,a series of modification measures such as low-temperature carbonization combined with high-temperature activation and metal oxide compounding by hydrothermal reaction were used to prepare hierarchical porous carbon based materials with the coexistence of micro-,meso-and macro-pores,large specific surface area,good electrical conductivity and wettability by using soybean roots and flixweed seeds as precursors.The derived carbon based materials were employed as electrodes to construct a CDI system to achieve the goal of high desalination capacity and good stability.The main contents are as follows:1.An environmentally friendly process for the preparation of N/O/S-co-doped porous carbon with high yield using biomass waste-Descurainia Sophia seeds as precursors:N,O,S-codoped porous structural carbon（KDPC）was prepared from Descurainia Sophia seeds using plant ash and eggshells as natural activators and templates,in which KDPC-20-600 obtained by activation at 600°C had a high yield of75%,and the specific capacitance was 250.5 F g^-1 at a current density of 0.5 A g^-1 and remained 232.8 F g^-1 at 1.0 A g^-1 in 2 M KOH electrolyte,which was better than the current commercial activated carbon.In addition,KDPC-20-600 exhibited high stability after 10,000 cycles with a capacitance retention of 99.0%.Meanwhile,the symmetrical supercapacitor based on KDPC-20-600 delivered a high energy density of 24.8 Wh kg^-1 at a power density of 1050 W kg^-1 in 1 M Na₂SO₄.The ion adsorption of KDPC-20-600could reach 13.10 mg g^-1 in 500 mg g^-1 Na Cl solution,which remained about 60%retention after 20 cycles,indicating its good stability.The synthesis of in situ heteroatom-doped porous carbon with high yield can be achieved by using natural biomass as precursors and using natural materials with environmentally friendly properties（e.g.,plant ash,eggshell）instead of corrosive chemical activators（e.g.,KOH,H₃PO₄）.This cost-effective,environmentally friendly and sustainable porous carbon preparation process will provide an idea for the development of active materials for energy storage and conversion devices.2.Extraction of oil and preparation of hierarchical porous carbon using Descurainia Sophia seeds as raw material:the seeds of Descurainia Sophia were used as raw material,and different solvents were used for the extraction of oil with the highest extraction yield of 26.09%.The N/O/S/P-co-doped porous carbon SDS-E was prepared by solvothermal reaction and graded activation using Descurainia Sophia residue as the precursor,in which the porous carbon WH-SDS-E prepared with water as the solvent had a specific capacitance of 233.3 F g^-1 at a current density of 1 A g^-1 in 1 M Na Cl,which was better than most of the present porous carbons.In addition,the desalination capacity of WH-SDS-E was 32.8 mg g^-1 in 500 mg g^-1 Na Cl solution,and the desalination rate could reach0.46 mg g^-1 s^-1.Its excellent performance is mainly due to the unique three-dimensional carbon nanostructure and relatively high specific surface area of WH-SDS-E,while the self-doped N/O/P/S elements provide more pseudocapacitive active sites.Thus,the extraction of oil from Descurainia Sophia seeds for fuel and the preparation of N/O/P/S-co-doped porous carbon from the residue can realize the effective utilization of biomass waste and provide a good idea for the researchers.3.Mn O₂ supported soybean root-derived porous carbon with excellent deionization ability:In this work,soybean root was used as raw material to prepare porous carbon（SRKPC）by carbonization and KOH activation processes.The prepared SRKPC exhibited an ultra-high specific surface area(2316.8 m² g^-1)and abundant pores,which were conducive to exposing the electrochemical active sites of the material,shortening the ion/electron transfer path and improving its electrochemical performance.The composite SRKPC@Mn O₂ was obtained by synthesizing Mn O₂ nanoparticles on its surface through a low-temperature hydrothermal reaction using SRKPC as a substrate.SRKPC@Mn O₂ displayed excellent electrochemical properties with high specific capacitance(597.9 F g^-1 at 2 m V s^-1)and bimodal sodium storage properties.The dual-mode CDI system based on SRKPC@Mn O₂ delivered an effective desalination performance of 42.90 mg g^-1 with a desalination rate of 2.86 mg g^-1 min^-1 and a relatively good desalination recovery.Thus,this work not only realizes the simple preparation of low-cost,environmentally friendly and efficient dual-mode CDI electrode materials for capacitive desalination,but also provided a sustainable way to convert biomass waste into functional materials.