Nitrogen-doped graphene (NG) is a promising conductive matrix material for fabricating high-performance Li/S batteries. Here we report a simple, low-cost, and scalable method to prepare an additive-free nanocomposite cathode in which sulfur nanoparticles are wrapped inside the NG sheets (S@NG). We show that the Li/S@NG can deliver high specific discharge capacities at high rates, that is, ∼ 1167 mAh g(-1) at 0.2 C, ∼ 1058 mAh g(-1) at 0.5 C, ∼ 971 mAh g(-1) at 1 C, ∼ 802 mAh g(-1) at 2 C, and ∼ 606 mAh g(-1) at 5 C. The cells also demonstrate an ultralong cycle life exceeding 2000 cycles and an extremely low capacity-decay rate (0.028% per cycle), which is among the best performance demonstrated so far for Li/S cells. Furthermore, the S@NG cathode can be cycled with an excellent Coulombic efficiency of above 97% after 2000 cycles. With a high active S content (60%) in the total electrode weight, the S@NG cathode could provide a specific energy that is competitive to the state-of-the-art Li-ion cells even after 2000 cycles. The X-ray spectroscopic analysis and ab initio calculation results indicate that the excellent performance can be attributed to the well-restored C-C lattice and the unique lithium polysulfide binding capability of the N functional groups in the NG sheets. The results indicate that the S@NG nanocomposite based Li/S cells have a great potential to replace the current Li-ion batteries.
A photoelectrochemical solar cell based on porous ZnO-covered TiO 2 film has been fabricated with ruthenium bipyridyl complex as the sensitizer. The cell generated a shortcircuit photocurrent of 21.3 mA cm -2 and an open-circuit voltage of 712 mV under irradiation of 81.0 mW cm -2 white light from a xenon lamp with an overall conversion efficiency of 9.8%. Compared with the pure TiO 2 (anatase) film, the ZnO-covered TiO 2 film possesses more outstanding ability to transport electrons with an overall power conversion efficiency increase by 27.3%. Optical elctrochemical studies show that surface modification of TiO 2 with ZnO can increase the concentration of free electrons in the conduction band of TiO 2 . This result implies that the charge recombination is reduced in the process of electron transport through the porous network, which can decrease the photocurrent loss and hence improve both short-circuit photocurrent and open-circuit photovoltage.
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