Facile preparation of flexible porous carbon fibers as self-supporting sulfur cathode hosts for high-performance Li-S batteries.

Lithium-sulfur batteries are expected to be prospective candidates of high-energy-storage systems due to their high theoretical specific capacity. However, poor electrical conductivity, severe polysulfide shuttle effect and low sulfur utilization generally cause inferior electrochemical performance, hence hindering the practical development.  In this study, common makeup cotton derived self-supporting porous carbon fibers (SPCFs) are prepared by a facile simultaneous activation/pyrolysis process accompanied by the effectively regulation of a KHCO3 activator. The as-prepared SPCF materials have mutually cross-linked porous skeletons with an ultrahigh specific surface area of 2124.9 m2 g-1 and a large pore volume of 1.01 cm3 g-1, whilst exhibiting robust flexibility. When directly used as a self-supporting carbon current collector for encapsulating sulfur, the interconnected and abundant porous carbon fibers can not only immobilize soluble polysulfides, but also form a highly conductive network for the favorable redox transformation of adsorbed polysulfides. Moreover, the voids between the carbon skeletons can alleviate the volume change of sulfur cathodes during charge/discharge. Owing to these structure merits, the optimized SPCF-based sulfur cathode with a sulfur loading of 3.0 mg cm-2 shows a high coulombic efficiency of approximately 99% and delivers a first discharge capacity of 778 mA h g-1 at 0.2 C. Even at a relatively high current rate of 0.5 C, the reversible capacity of 450 mA h g-1 can be obtained after 300 cycles. The above-mentioned self-supporting porous carbon current collectors provide a guidance for high-performance lithium-sulfur batteries.

Risk assessment of fluoride and arsenic in groundwater and a scenario analysis for reducing exposure in Inner Mongolia.

In contrast to Mongolia, family-owned land in Inner Mongolia is separated by fences, preventing the free movement of nomads and leading people to rely heavily on the same source of groundwater for their domestic water needs. Therefore, it is important to clarify groundwater quality and understand the associated human health concerns. To evaluate the risks of drinking groundwater to human health in Inner Mongolia, we examined groundwater quality by field surveys, a human health risk analysis, and a scenario analysis. During the summer of 2015 in Inner Mongolia, we measured the concentrations of major ions, metals, metalloids, and rare earth metals in groundwater samples (n = 32) and river water samples (n = 10), for which there were no known anthropogenic contamination sources. In addition, as part of a scenario analysis, samples of tap water (n = 1), snowmelt (n = 1), and bottled water (n = 1) were also evaluated. We used our analytical results to calculate hazard quotient (HQ) ratios by means of a probabilistic risk assessment method. The results indicated that residents who drank groundwater every day might have risk concerns for F- (mean ± standard deviation, 2.51 ± 1.80 mg L-1; range, 0.07-7.70 mg L-1) and As (6.49 ± 9.64 µg L-1; 0.31-47.0 µg L-1). We observed no relationships between well depth or any geophysical variation and groundwater quality. On the basis of the scenario analysis results, we concluded that using snow as a source of drinking water in winter could reduce health risks associated with using groundwater for this population in Inner Mongolia.