Synthesis of the electrode materials of sodiumion storage devices from sustainable precursors via green methods is highly desirable. In this work, we fabricated a unique N, O dual-doped biocarbon nanosheet with hierarchical porosity by direct pyrolysis of low-cost cuttlebones and simple air oxidation activation (AOA) technique. With prolonging AOA time, thickness of the carbon sheets could be reduced controllably (from 35 to 5 nm), which may lead to tunable preparation of carbon nanosheets with a certain thickness. Besides, an unexpected increase in N-doping amount from 7.5 to 13.9 atom % was observed after AOA, demonstrating the unique role of AOA in tuning the doped heteroatoms of carbon matrix. This was also the first example of increasing N-doping content in carbons by treatment in air. More importantly, by optimizing the thickness of carbon sheets and heteroatom doping via AOA, superior sodium capacity−cycling retention−rate capability combinations were achieved. Specifically, a current state-of-the-art Na + storage capacity of 640 mAh g −1 was obtained, which was comparable with the lithium-ion storage in carbon materials. Even after charging/discharging at large current densities (2 and 10 A g −1 ) for 10 000 cycles, the as-obtained samples still retained the capacities of 270 and 138 mAh g −1 , respectively, with more than 90% retention. The assembled sodium-ion capacitors also delivered a high integrated energy− power density (36 kW h kg −1 at an ultrahigh power density of 53 000 W kg −1 ) and good cycling stability (90.5% of capacitance retention after 8000 cycles at 5 A g −1 ).
In this paper, in order to describe complex network systems, we firstly propose a general modeling framework by combining a dynamic graph with hybrid automata and thus name it Dynamic Graph Hybrid Automata (DGHA). Then we apply this framework to model traffic flow over an urban freeway network by embedding the Cell Transmission Model (CTM) into the DGHA. With a modeling procedure, we adopt a dual digraph of road network structure to describe the road topology, use linear hybrid automata to describe multi-modes of dynamic densities in road segments and transform the nonlinear expressions of the transmitted traffic flow between two road segments into piecewise linear functions in terms of multi-mode switchings. This modeling procedure is modularized and rule-based, and thus is easily-extensible with the help of a combination algorithm for the dynamics of traffic flow. It can describe the dynamics of traffic flow over an urban freeway network with arbitrary topology structures and sizes. Next we analyze mode types and number in the model of the whole freeway network, and deduce a Piecewise Affine Linear System (PWALS) model. Furthermore, based on the PWALS model, a multi-mode switched state observer is designed to estimate the traffic densities of the freeway network, where a set of observer gain matrices are computed by using the Lyapunov function approach. As an example, we utilize the PWALS model and the corresponding switched state observer to traffic flow over Beijing third ring road. In order to clearly interpret the principle of the proposed method and avoid computational complexity, we adopt a simplified version of Beijing third ring road. Practical application for a large-scale road network will be implemented by decentralized modeling approach and distributed observer designing in the future research.
The process of urbanization affects the urban warming. The change of urban warming was investigated by several urbanization factors in Changsha, China. The data of surface temperature (minimum, maximum and mean) of Changsha were analyzed to understand the possible effects of urbanization on the climate of this region owing to the population growth, built-up area expansion and energy consumption increases. The weights of these three factors were calculated by the analytical hierarchy process (AHP). Then, three weights were simulated with nonlinear method to obtain the urbanization development rate which was utilized to reveal the influence of the urbanization factors on the surface temperature. The result shows that there is a significant upward trend in the urban temperatures of Changsha. The temperature increase seems to be closely related to the rate of urbanization between 1993 and 2008, and there is an evidential increase trend of the influence on urban temperature by urbanization during 16 years. It is a quantifiable approach measuring the relationship between urbanization and urban eco-environment, and can be applied for the urban sustainability.
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