Green Synthesis And Energy Storage Application Of Porous Carbon Derived From Waste Face Masks

With the massive spread of infectious diseases such as COVID-19,the demand for face masks has exploded worldwidely.Face masks are becoming one of the necessities of daily infection protection for a long time in the future.However,the recycling and reuse treatment of face mask waste at this stage is complex and cumbersome,and has low recovery rate and high operating cost.Carbonization of polymer waste to prepare high value-added porous carbon materials is a new type of recycling treatment.Especially in recent years,with the development of green energy storage devices,the use of polymer waste to prepare supercapacitor electrode materials has become a promising waste treatment technique.Based on this,two types of commonly used face masks,polypropylene(PP)non-woven fabrics face masks and cotton face masks,were used as the carbon precursors to produce porous carbon materials by high-temperature carbonization in this thesis.We prepared electrode materials with different structures based on porous sulfur-doped fibrous carbon,cotton fiber-derived porous carbon and carbon/nickel-cobalt-based composites from three perspectives: doping,activation and recombination.We explored the effects of the material structure and composition on the electrochemical properties of electrodes,and revealed the corresponding energy storage mechanism of electrode materials.The main research contents are as follows:(1)In face with common problems like low carbon yield of PP and low specific capacitance of the resulted carbon electrodes,a solvothermal method was used to sulfonate PP face mask waste in the autoclave.Then the sulfonated products(MS)were mixed with different dosages of KOH to obtain sulfur-doped porous carbon materials(CMS-x)after one-step heat treatment.The results confirmed that the unique sulfur-doping process introduced sulfur atoms into the carbon skeleton and improved the carbon yield,which could be up to 38.4%(900 ?).The doping of sulfur atoms and etching with KOH endowed CMS with large specific surface area and large number of pores in micro-or nano-meter scale,which improved the electrochemical performance of the double-layer capacitor.The specific surface area of CMS-3samples could be 2220 m2·g-1,the maximum specific capacitance of samples obtained was 328.9F·g-1 at 1 A·g-1.The energy density of symmetric solid-state supercapacitor(S-CMS-3)was up to11.2 Wh·kg-1(300 W·kg-1)and exhibited good cycling performance,with capacitance retention of 81.1% after 3000 cycles.(2)Cotton face mask is also one of the commonly used face masks.In order to fully retain the fiber structure of the original cotton,cotton-derived porous carbon(GMC-x)was successfully prepared by pre-carbonization and high temperature activation.We explored the effect of activator dosage on the electrochemical properties of porous carbon materials.The results showed that the wrinkled surface together with hollow tubular structure of the carbon materials could serve as the ion buffer bands,leading into their excellent ion transport characteristics.GMC-2 electrode had a high specific capacitance of 366.6 F·g-1(1 A·g-1).The symmetric supercapacitor assembled based on GMC-2 exhibited a high energy density of 9.8Wh·kg-1(300 W·kg-1),and had good cycling performance.After 3000 cycles,its capacitance retention was 89.0%.(3)Considering the low specific capacitance of carbon electrodes and for the further structural characterization of hollow fibers,the as-prepared GMC-2 were used to prepare GMC/Ni-Co based composite electrode materials by hydrothermal vulcanization and low-temperature phosphating techniques.The results showed that the metal compounds afforded the electrode higher specific capacitance,and the carbon layer endowed the electrode with excellent conductivity,resulting in its improved energy storage performance and multiplication performance.GMC/Ni Co P had better electrochemical properties than GMC/Ni Co2S4,which proved that the phosphating process improved the comprehensive performance of electrode materials compared with the vulcanization process.The specific capacitance of the product was718.3 F·g-1(1 A·g-1),and the capacitance retention remained to be 84.1%.In summary,mask waste can be effectively transformed into high-performance electrode materials for supercapacitors by techniques of doping,activation and recombination.It provides a new scheme for the recycling treatment of face mask waste as well as a new idea for the treatment of polymer-based medical wastes.