Energy Focus: Azobenzene-functionalized CNTs predicted to be high-energy density solar thermal fuels
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ne of the major limitations for automobile applications of lithium-ion batteries is their inability to power electric cars for long distances due to their low cycle life, low specific capacity, and low energy efficiency. Consequently, tremendous research efforts have been made to improve the energy and power density of rechargeable lithium batteries for possible use in electrical cars. Researchers at Stanford University, led by H. Dai and Y. Cui, have developed a new type of lithium battery by synthesizing a graphene-wrapped sulfurbased composite material that shows high and stable specific capacities of up to 600 mAh/g over more than 100 cycles. The researchers suggest that this type of material shows great promise as a high-performance cathode material for Li-S batteries. As reported in the July 15 issue of Nano Letters (DOI: 10.1021/nl200658a; p. 2644), the team fabricated this material by wrapping poly(ethylene glycol) (PEG) coated submicrometer-sized sulfur particles with mildly oxidized graphene oxide sheets decorated with carbon black nanoparticles. The PEG and graphene coating layers are important for accommodating volume expansion of the coated sulfur particles during discharge, thereby trapping soluble polysulfide in-
Energy Focus Azobenzene-functionalized CNTs predicted to be high-energy density solar thermal fuels
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conomical and sustainable conversion of sunlight into other useful forms of energy that are easily transportable is currently a significant research challenge. One approach that has been explored is the use of solar thermal fuels. Upon absorption of light energy, photoactive molecules switch to
Graphene oxide (GO) suspension
Carbon black (CB)
GO/CB suspension Graphene coating to render electrical conductivity
Na2S2O3 + HCl + Triton X-100
Sulfur
S/PEG particles
Graphene-sulfur composite
Graphene wrapping to trap polysulfides PEG layer to trap polysulfides and accommodate volume change
Schematic of the synthesis steps for a graphene-sulfur composite, with a proposed schematic structure of the composite. Reproduced with permission from Nano Lett. 11 (7) 2011 (DOI: 10.1021/nl200658a; p. 2644). © 2011 American Chemical Society.
termediates and rendering the sulfur particles electrically conducting. The team found that the average size of the sulfur particles was less than 1 μm and energy dispersive spectroscopy was used to confirm the structure and composition of the graphene-sulfur composite. Colorimetric titration experiments determined that the graphene-sulfur composition contained 70 wt% of sulfur (with 15% of mildly oxidized graphene sheets and 8% carbon black). The researchers fabricated coin cells to test the electrochemical performance of the graphene-sulfur composite material with a Li foil as the anode in an electrolyte of 1.0M lithium bis-trifluoromethane sulfonylyimide in 1,3 dioxilane and 1,2 dimethyoxyethane. The results demonstrated that the PEG-coated
graphene-sulfur composite can maintain a specific capacity of ~600 mAh/g for more than 100 cycles. The capacity fa
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