Lithium–sulfur batteries are promising technologies for powering flexible devices due to their high energy density, low cost and environmental friendliness, when the insulating nature, shuttle effect and volume expansion of sulfur electrodes are well addressed. Here, we report a strategy of using foldable interpenetrated metal-organic frameworks/carbon nanotubes thin film for binder-free advanced lithium–sulfur batteries through a facile confinement conversion. The carbon nanotubes interpenetrate through the metal-organic frameworks crystal and interweave the electrode into a stratified structure to provide both conductivity and structural integrity, while the highly porous metal-organic frameworks endow the electrode with strong sulfur confinement to achieve good cyclability. These hierarchical porous interpenetrated three-dimensional conductive networks with well confined S8 lead to high sulfur loading and utilization, as well as high volumetric energy density.
needs while signifi cantly reducing battery cost. For this reason, the combination of lithium and sulfur has been considered as one of the most promising battery chemistries for full electrifi cation of vehicles. [ 3 ] Despite these advantages, Li S batteries have a few critical barriers to be overcome. Besides the insulating properties of sulfur and polysulfi des, Li S batteries also suffer from dramatic (i.e., ≈76%) volume change of sulfur during cycling and shuttling effect of polysulfi des that sulfur species transport back and forth between electrodes. These lead to the destruction of sulfur cathodes and the corrosion of lithium anode resulting in short battery life.In a traditional Li S cell, a typical sulfur electrode consists of three components, i.e., the electrochemically active sulfur material, the conductive carbon additive, and the polymeric binder. [ 4 ] Through the syntheses of nanoarchitectured carbon additives, the electrochemical performance and cycle life of Li S batteries have been successfully improved. [ 5 ] Although signifi cant achievements have been made in designing nanostructured carbon/sulfur composites for cycle life improvement of Li S batteries, these processes are commonly sophisticated, high cost, and not suitable for large-scale manufacturing.The sulfur cathode in traditional lithium-sulfur batteries suffers from poor cyclability due to polysulfi de shuttling effect as well as large volume change during charge/discharge processes. Gum arabic (GA), a low cost, nontoxic, and sustainable natural polymer from Acacia senegal , is adopted as a binder for the sulfur cathode to address these issues. The excellent mechanical properties of GA endow the cathode with high binding strength and suitable ductility to buffer volume change, while the functional groups chemically and physically confi ne sulfur species within the cathode to inhibit the shuttling effect of polysulfi des. Additionally, GA shifts the electrode fabrication process from the organic solvent process to an aqueous process, eliminates the use of toxic organic solvents, and achieves uniformly distributed electrode with lower impedance. A remarkable cycling performance, i.e., 841 mAh g −1 at low current rate of C /5, is achieved throughout 500 cycles due to the bifunctions of the GA binder.
Borohydrides in alkaline media are potential fuels for fuel cells due to their high energy and power density. In this work, we studied the anodic oxidation of borohydrides on a nickel electrode. The open-circuit potential was found to be about 0.15-0.2 V more negative than the hydrogen potential, depending on the concentration of BH 4Ϫ . The results of polarization measurements indicated that a high power density can be achieved for the BH 4 Ϫ /Ni system. However, the coulombic efficiency was found to be 50% or less due to hydrogen evolution. A further investigation showed that hydrogen gas was not only generated from the hydrolysis reaction, but also from the electrochemical reaction. The actual anodic reaction of borohydride on the Ni electrode was proved to be a four-electron process rather than an eight-electron one.Fuel cells as efficient and clean power generation devices are attracting more and more public attention in recent years. Extensive effort is being made to apply them in various places including mobile appliances and vehicles. Although fuel cell technology has made considerable improvements in the last few decades, the industrial application of fuel cells will still take some time due to the problem of cost and some technical difficulties. For example, in some fuel cell types such as the polymer-electrolyte-membrane fuel cell ͑PEMFC͒, hydrogen with relatively high purity is required. However, the existing technologies for hydrogen storage are far from meeting the requirements. Lacking the infrastructure of fuel systems to a large extent blocks the way to the industrial application of these fuel cells.Besides gaseous hydrogen, some aqueous solutions containing compounds like hydrazine (N 2 H 4 ), 1,2 methanol (CH 3 OH), 3,4 and alkali borohydrides (MBH 4 , M ϭ Li, Na, K, etc.͒ 5,6 can also be used as the fuel. Among them, the direct methanol fuel cell ͑DMFC͒ using an aqueous solution of methanol is of considerable research interest recently. However, the low reactivity of methanol and its crossover to the cathode offer more challenges to researchers. Compared with methanol, alkali borohydrides (NaBH 4 , KBH 4 , etc.͒ are much more reactive and offer equally high energy densities.The anodic oxidation of BH 4 Ϫ was supposed to be as followsIn the above reaction, each BH 4 Ϫ ion releases eight electrons, resulting in very high electrochemical capacities for borohydrides (NaBH 4 5.67 Ah/g, KBH 4 3.98 Ah/g͒. The standard potential of Reaction 1 was calculated as Ϫ1.23 V ͑vs. NHE͒ by Pecsok 7 and Ϫ1.24 V by Stockmayer et al. 8 It is 0.4 V more negative than the hydrogen electrode in an alkaline medium. Also in the 1960s, some experimental efforts were made to employ alkali borohydrides as the fuel for fuel cells. 5,6,9 Although the observed potentials of BH 4Ϫ were less negative than the theoretical value, they proved to be promising fuels offering high potential and current density, except that they were expensive. Recently, borohydrides have been attracting attentions once again as a source of hydrogen...
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