Enhanced microwave absorption capacity of iron-based catalysts by introducing Co and Ni for microwave pyrolysis of waste COVID-19 Masks

Inexpensive iron-based catalysts are the most promising catalysts for microwave-assisted deconstruction of waste plastics. However, the microwave heating efficiency of most of the synthesized iron-based catalysts is very low, in particular, the FeAl catalyst was prepared by microwave combustion method, and its mixture with disposable medical masks (DMMs) was only heated to about 150℃ within 10 min. Here, we introduce the second-phase metals (Co or Ni) into the FeAl catalyst, resulting in the rearrangement of the catalyst structure and electrons to give the catalyst good microwave absorption ability. The mixture of the catalyst and DMMs can be quickly heated to above 900℃ in 10 min, especially after reaching the melting point of plastic, the instantaneous heating rate reaches 350 ℃·min−1. under the unique microwave hot-spot pyrolysis mechanism, DMMs can be rapidly pyrolyzed into carbon nanotubes (19.65 wt%) and gas (77.65 wt%) within 14 min due to the efficient dehydrogenation efficiency and activity of Co. The corresponding H2 yield is up to 38.66 mmolH2·g−1DMMs, and the percentage of CO and H2 in the gas is as high as 90 wt%. This work improves the microwave conversion efficiency of iron-based catalysts by introducing second phase metals, and waste DMMs were efficiently converted into CO, H2 and CNTs, which can also be extended to other polymer or biomass chemical cycles.

Discarded COVID-19 masks-derived-doped porous carbon for lithium-sulfur batteries.

Despite the high theoretical capacity and energy density of lithium-sulfur (Li-S) batteries, the development of Li-S batteries has been slow due to the poor electrical conductivity and the shuttle effect of the electrode materials, resulting in low sulfur utilization and fast long-term cycling capacity decay. The modified carbon materials are often used as sulfur hosts to significantly improve the cycling performance of the materials, but also bring high-cost issues. Here, the porous carbon materials are synthesized quickly and conveniently by the microwave cross-linking method using discarded medical masks as carbon sources and concentrated sulfuric acid as solvent. However, poor surface and structural properties limit the application of materials. The porous carbon material is modified with p-toluene disulfide and urea as the sulfur and nitrogen sources by the microwave cross-linking method, which not only improves the porosity and specific surface area of the porous carbon material, but also improved the electrical conductivity and interlayer spacing of the material. As synthesized SN-doped porous carbon is employed as the sulfur host, which exhibits a high discharge capacity (1349.3 mAh g-1) at 0.1°C, the S-porous C/S, N-porous C/S, and SN-porous C/S can maintain 78.1, 43.9, and 59.5% of the initial capacity after 500 cycles. The results indicate that the doping of S and N atoms provides sufficient active sites for the chemisorbed lithium polysulfides (LiPSs) to improve the reaction kinetics of the materials.

Preparation of multi-element doped porous carbon by microwave cross-linking stabilization of disposable surgical masks and its electrochemical properties as sulfur loading

Numerous disposable surgical masks (DSMs) were consumed with the development of COVID-19 epidemic. Non-solid products recovered by pyrolysis is more than twenty species with low added value. Therefore, the search for a reasonable carbonization method can not only alleviate the pressure of global plastic pollution, but also produce considerable economic value. Here it is found that microwave cross-linking can promote the substitution of hydrogen atom in the polymer master chain of DSMs by hydrogen atom, which can reorganize the easily cracked DSMs into sp2-hybridized aromatic carbon, it can maintain 51.2% carbon yield at 1000℃. The difference between the DSMs-based porous carbon obtained by in-situ and post-processing N doping was further compared, and it was found that the specific surface area of the activated in-situ doped sample (P-SNO@DSMs) was as high as 2278 m2·g-1, which had rich hierarchical pore structure and high heteroatoms doping rate. Benefiting from the synergistic effect of heteroatoms and hierarchical holes, P-SNO@DSMs sulfur cathode delivers a high specific capacity of 1550 mAh·g-1 at 0.1C and exhibits excellent long-term cycling performance with the smaller capacity decay of 0.13% per cycle after 400 cycles. In this work, clean and efficient microwave cross-linking not only realized the efficient recovery of waste DSMs, but also the application of the prepared materials can be broadened by adding additional heteroatomic sources in the process of microwave cross-linking.

Self-activated pyrolytic synthesis of S, N and O co-doped porous carbon derived from discarded COVID-19 masks for lithium sulfur batteries

The continuous spread of COVID-19 has produced a large number of abandoned disposable medical masks (DMMs), which have a greater negative impact on the environment and biosafety. In response to this issue, a method for rapid microwave sulfonation, nitrification and oxidation of DMMs was proposed to convert DMMs with low carbonization efficiency into aromatic carbon with good thermal stability, which not only maintained 51 wt.% of initial mass at 1000 °C, but also achieved in situ co-doping of S, N and O. Subsequently, porous carbons derived from DMMs were synthesized by self-activation pyrolysis, which avoided consumption of alkali and metal salts in the traditional activation process. It was further found that low pyrolysis temperature was not enough to produce enough active material H2 and H2O to obtain high specific surface area, while increasing pyrolysis temperature could adjust the specific surface area of as-prepared carbon, ranging from 52 m2·g−1 to as high as 890 m2 g−1. Thanks to synergistic effect of S, N, and O co-doping and hierarchical porous structure, the first discharge specific capacity of sample synthesized by self-activated pyrolysis at 900 °C was 1459.8 mAh·g−1 at 0.1 C, and the discharge specific capacity retention at 0.5 C after 400 cycles was 52.3%.

Porous carbon materials derived from discarded COVID-19 masks via microwave solvothermal method for lithium­sulfur batteries.

With the spread of COVID-19, disposable medical masks (DMMs) have become a significant source of new hazardous solid waste. Their proper disposal is not only beneficial to the safety of biological systems but also useful to achieve considerable economic value. The first step of this study was to investigate the chemical composition of DMMs. It is primarily composed of polypropylene, polyethylene terephthalate and iron, with fibrous polypropylene accounting for approximately 80% of the total weight. Then, DMMs were sulfonated and oxidised by the microwave-driven concentrated sulfuric acid within 8 min based on the fact that the concentrated sulfuric acid exhibits a good microwave absorption capacity. The co-doping of sulfur and oxygen was achieved while improving the thermal stability of DMMs. Subsequently, the self-activation pyrolysis of sulfonated and oxidised DMMs (P-SO@DMMs) was further realized in low-flow-rate argon. The specific surface area of P-SO@DMMs increased from 2.0 to 830.9 m2·g-1. P-SO@DMMs sulfur cathodes have promising electrochemical properties because of their porous structures and the synergistic effect of sulfur and oxygen co-doping. The capacity of the samples irradiated by microwave for 10 min at 0.1, 0.2, 0.5, 1, 2 and 5 C were 1313.6, 1010.9, 816.5, 634.4, 513.4 and 453.1 mAh·g-1, respectively, and after returning to 0.2 C and continuing the cycle for 50 revolutions, maintained 50.5% of the initial capacity. After 400 cycles, its capacity is 38.1% of the initial capacity at 0.5 C. It is slightly higher than the electrochemical performance of the sample treated by microwave for 8 min and significantly higher than the sample treated by 6 min. This work converts structurally complex, biohazardous DMMs into porous carbon with high specific surface area by clean and efficient microwave solvothermal and self-activating pyrolysis, which facilitates the development of carbon based materials at low cost and large scale.