Numerous natural systems contain surfaces or threads that enable directional water transport. This behaviour is usually ascribed to hierarchical structural features at the microscale and nanoscale, with gradients in surface energy and gradients in Laplace pressure thought to be the main driving forces. Here we study the prey-trapping pitcher organs of the carnivorous plant Nepenthes alata. We find that continuous, directional water transport occurs on the surface of the 'peristome'--the rim of the pitcher--because of its multiscale structure, which optimizes and enhances capillary rise in the transport direction, and prevents backflow by pinning in place any water front that is moving in the reverse direction. This results not only in unidirectional flow despite the absence of any surface-energy gradient, but also in a transport speed that is much higher than previously thought. We anticipate that the basic 'design' principles underlying this behaviour could be used to develop artificial fluid-transport systems with practical applications.
As one important component of sulfur cathodes, the carbon host plays a key role in the electrochemical performance of lithium‐sulfur (Li‐S) batteries. In this paper, a mesoporous nitrogen‐doped carbon (MPNC)‐sulfur nanocomposite is reported as a novel cathode for advanced Li‐S batteries. The nitrogen doping in the MPNC material can effectively promote chemical adsorption between sulfur atoms and oxygen functional groups on the carbon, as verified by X‐ray absorption near edge structure spectroscopy, and the mechanism by which nitrogen enables the behavior is further revealed by density functional theory calculations. Based on the advantages of the porous structure and nitrogen doping, the MPNC‐sulfur cathodes show excellent cycling stability (95% retention within 100 cycles) at a high current density of 0.7 mAh cm‐2 with a high sulfur loading (4.2 mg S cm‐2) and a sulfur content (70 wt%). A high areal capacity (≈3.3 mAh cm‐2) is demonstrated by using the novel cathode, which is crucial for the practical application of Li‐S batteries. It is believed that the important role of nitrogen doping promoted chemical adsorption can be extended for development of other high performance carbon‐sulfur composite cathodes for Li‐S batteries.
Electroactive polymers are a new generation of "green" cathode materials for rechargeable lithium batteries. We have developed nanocomposites combining graphene with two promising polymer cathode materials, poly(anthraquinonyl sulfide) and polyimide, to improve their high-rate performance. The polymer-graphene nanocomposites were synthesized through a simple in situ polymerization in the presence of graphene sheets. The highly dispersed graphene sheets in the nanocomposite drastically enhanced the electronic conductivity and allowed the electrochemical activity of the polymer cathode to be efficiently utilized. This allows for ultrafast charging and discharging; the composite can deliver more than 100 mAh/g within just a few seconds.
Lithium–sulfur (Li–S) batteries are recognized as promising candidates for next‐generation electrochemical energy storage systems owing to their high energy density and cost‐effective raw materials. However, the sluggish multielectron sulfur redox reactions are the root cause of most of the issues for Li–S batteries. Herein, a high‐efficiency CoSe electrocatalyst with hierarchical porous nanopolyhedron architecture (CS@HPP) derived from a metal–organic framework is presented as the sulfur host for Li–S batteries. The CS@HPP with high crystal quality and abundant reaction active sites can catalytically accelerate capture/diffusion of polysulfides and precipitation/decomposition of Li2S. Thus, the CS@HPP sulfur cathode exhibits an excellent capacity of 1634.9 mAh g−1, high rate performance, and a long cycle life with a low capacity decay of 0.04% per cycle over 1200 cycles. CoSe nanopolyhedrons are further fabricated on a carbon cloth framework (CC@CS@HPP) to unfold the electrocatalytic activity by its high electrical conductivity and large surface area. A freestanding CC@CS@HPP sulfur cathode with sulfur loading of 8.1 mg cm−2 delivers a high areal capacity of 8.1 mAh cm−2 under a lean electrolyte. This work will enlighten the rational design of structure–catalysis engineering of transition‐metal‐based nanomaterials for diverse applications.
When a liquid film of colloidal solution consisting of particles of different sizes is dried on a substrate, the colloids often stratify, where smaller colloids are laid upon larger colloids. This phenomenon is counter intuitive because larger colloids which have smaller diffusion constant are expected to remain near the surface during the drying process, leaving the layer of larger colloids on top of smaller colloids. Here we show that the phenomenon is caused by the interaction between the colloids, and can be explained by the diffusion model which accounts for the interaction between the colloids. By studying the evolution equation both numerically and analytically, we derive the condition at which the stratified structures are obtained.Drying of a colloidal film is important in many places such as in printing [1], spreading and coating [2] and material science [3,4]. An important problem is how the structure of dried film is controlled by drying conditions. It is known that the spatial distribution of colloidal particles in the drying process is determined by two competing processes. One is the Brownian motion [5][6][7] which is characterized by the diffusion constant D, and the other is evaporation [4], characterized by the speed v ev at which the surface recedes. The competition between them can be quantified by the film formation Peclet number Pe = v ev h 0 /D [8], where h 0 is the initial thickness of the film. If Pe < 1, the concentration gradient created by evaporation is quickly flattened by diffusion, and the colloid concentration remains uniform. On the other hand, if Pe > 1, the concentration gradient increases, and the colloids accumulate near the top of the film.If there are two types of colloids of different size [9-12], the above consideration predicts that the larger colloids will accumulate near the free surface (large-on-top), because larger colloids have a smaller diffusion constant, therefore a larger Peclet number. Recently, however, the opposite phenomenon has been reported by Fortini and coworkers [13]. By simulation and experiments, they have shown that smaller colloids appear on top of larger colloids (small-on-top). They argued that this is due to the osmotic pressure of smaller colloids, but no quantitative theory has been given.In this Letter, we show that the phenomenon can be explained by the standard diffusion model [14] if the interaction between colloids are taken into account. We will use a simple hard sphere model, and show that the small-on-top structure is created by the cross-interaction between colloids of different sizes. The effect of crossinteraction on colloidal motion is not symmetric: it is much stronger on larger colloids than smaller colloids and * jjzhou@buaa.edu.cn † yjiang@buaa.edu.cn ‡ masao.doi@buaa.edu.cn pushes the larger colloids towards the bottom of the film. We will give a criterion when the small-on-top structure is created, and the corresponding experimental conditions, such as the drying rate, initial colloidal concentrations, and size ratio. Evolut...
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