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Journal of Materials Chemistry A Paper suggesting that the actual sulfur contents in AC@S and AC/S- 155 are 54.2 wt% and 42.2 wt%, respectively. Apparently, an inevitable sulfur loss occurs during the sulfur impregnation via the routine melt-diffusion method. Meanwhile, the weight loss of the AC@S sample nally ends at 450 C, which is much higher than that of AC/S-155 (350 C) and pristine sulfur (300 C). These results reveal that SC-CO2 technology is not only a facile method for realizing the highly efficient impregnation of sulfur into carbon matrices, but can also achieve the enhanced thermal stability of sulfur and the strong affinity between sulfur and AC matrices. Fig. 3e shows N2 adsorption–desorption isotherm curves and pore size distributions of AC, AC-CO2 and AC@S samples. A joint curve of the type I N2 adsorption–desorption isotherm conrms the existence of micropores in the AC sample. The specic surface area and pore volume of the AC sample were 1501.07 m2 g1 and 0.694 cm3 g1, respectively. Aer SC-CO2 treatment, the specic surface area and pore volume of the AC-CO2 sample slightly increased to 1506.25 m2 g1 and 0.696 cm3 g1, respectively. It is worth noting that the specic surface area of the MCMB sample (<2 m2 g1) was drastically increased to 23.0 m2 g1 (MCMB-CO2 sample), as depicted in Fig. S7c,† which demonstrates that the SC-CO2 process can expand the interplanar spacings and exfoliate graphite into graphitic sheets. However, aer sulfur impregnation, it was found that the micropores and mesopores in AC matrices dis- appeared, and the specic surface area of the AC sample also sharply decreased from 1501.07 m2 g1 to 9.23 m2 g1, indi- cating that sulfur was successfully impregnated into the AC matrices. Meanwhile, the specic surface area of MCMB@S (11.9 m2 g1) was larger than that of pristine MCMB (<2 m2 g1), but smaller than that of MCMB-CO2 (23.0 m2 g1). The distinct surface area changes further conrmed that SC-CO2 can tune peaks arising from the sulfur phase can be clearly detected in MCMB@S (Fig. S7a†) and MWCNTs@S (Fig. S4d†) samples, implying that some sulfur particles may still attach to the surface of MCMB@S and MWCNTs@S samples. Thus, it can be deduced that amorphous AC as sulfur hosts may be better than MCMB and MWCNTs for fabricating C@S composites via SC- CO2 method. The degrees of graphitization of AC, AC-CO2, AC@S and AC/ S-155 samples were identied by Raman spectroscopy analysis. As shown in Fig. 3c, all the samples exhibited two broad peaks located at 1350 and 1595 cm1, conrming the coexistence of disordered graphite (D band) and crystalline graphite (G band).16 Generally, the intensity ratio of the D band to the G band (ID/IG) is an important index for evaluating the graphiti- zation degree of carbon materials. A small ID/IG value means a high degree of graphitic crystallinity. Aer careful compar- ison, ID/IG ratios of AC-CO2 and AC@S samples were 1.05 and 1.06, respectively, which are slightly larger than that of AC and AC/S-155 samples, suggesting that some defects emerge in AC aer the SC-CO2 process. The same tendency also can be found in MCMB based samples (Fig. S7b†). The small value of the ID/IG ratio in the AC@S sample implies good electrical conductivity, which is favorable for boosting the electron transfer of elec- trochemical reactions. Moreover, there was no sulfur related peak in the, AC@S sample, whereas two sharp peaks of sulfur located at 220 and 470 cm1, were detected in the MCMB@S sample (Fig. S7b†). This result is consistent with XRD results, which demonstrates that many sulfur particles remain on the surface of carbon matrices, and are not completely embedded into pores, interlayers and voids of carbon matrices. To quantify the sulfur content in AC@S and AC/S-155 samples, TGA measurements are carried out. As shown in Fig. 3d, large weight losses exist in AC@S and AC/S-155 samples, View Article Online Fig. 4 (a) CV curves of the AC@S sample at a scan rate of 0.1 mV s1. (b) Charge–discharge profiles of the AC@S sample at current density of 0.1 A g1 in the potential range from 1.8 to 2.6 V. (c) Long-term cycling performance of AC@S and AC/S-155 at a current density of 0.1 A g1. (d) Charge–discharge profiles of the AC@S sample at various current densities. (e) Rate performance comparison between AC@S and AC/S-155. (f) Charge/discharge capacity and capacity retention of different carbon matrices after 100 cycles. J. Mater. Chem. A This journal is © The Royal Society of Chemistry 2017 Published on 24 November 2017. Downloaded by University of Texas Libraries on 08/12/2017 20:16:36.PDF Image | Supercritical CO2 Mediated Incorporation of Sulfur into Carbon Matrix
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