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Thick electrode was promising to increase the energy density in device (battery or supercapacitor) scale, but always suffered the ion polarization upon high rate discharge. We reported the use of 3D Al foam (current collector) to host graphene (with high surface area but very low packing density) to fabricate 450 ​μm thick electrode with mass loading of 10–15 ​mg ​cm−2. The space confinement effect of Al wires on graphene at any regions offered sufficient ordered diffusion channel of liquids, alleviating the ion polarization significantly, compared to the same thick graphene sheet pasted on 2D Al foil by simulation with COMSOL software. The 100 ​F pouch supercapacitor with ionic liquids at 4 ​V exhibited low contact resistance, high specific capacitance in a wide range of current densities, and simultaneously the volumetric energy density of 18 ​Wh L−1 and the power density of 5 ​kW ​kg−1, far better than those of current commercial device.

Graphene is a hot star in materials science with various potential application aspects, including in Li-ion battery and supercapacitor. The burst of scientific papers in this area seems to validate the performance of graphene, but also arouses large dispute. Herein, we share our judgment of these trends to all, encouraging the discussion and enhancing the understanding of the structure-performance relationship of graphene.

In order to adapt to time-varying wireless channels, various channel-adaptive schemes have been proposed to exploit inherent spatial diversity in mobile/wireless ad hoc networks where there are usually alternate next-hop relays available at a given forwarding node. However, current schemes along this line are designed based on heuristics, implying room for performance enhancement. To seek a theoretical foundation for improving spatial diversity gain, we formulate the selection of the next-hop as a sequential decision problem and propose a general "optimal stopping relaying (OSR)" framework for designing such next-hop diversity schemes. As a particular example, assuming Rayleigh fading channels, we implement an OSR strategy to optimize information efficiency (IE) in a protocol stack consisting of greedy perimeter stateless routing (GPSR) and IEEE 802.11 MAC protocols. We present mathematical analysis of the proposed OSR together with other strategies in literature for a single forwarding node. In addition, we perform extensive simulations (using QualNet) to evaluate the end-to-end performance of these relaying strategies in a multi-hop network. Both the mathematical and simulation results demonstrate the superiority of OSR over other existing schemes.

In order to adapt to the time-varying nature of wireless channels, various channel-adaptive schemes have been proposed to exploit inherent spatial diversity in wireless ad hoc networks where there are usually alternate forwarding nodes available at a given forwarding node. However, existing schemes along this line are designed based on heuristics, implying room for performance enhancement. Thereby, to seek a theoretical foundation for improving spatial diversity gain, we formulate the selection of the next-hop relay as a sequential decision problem and derive a general "Optimal Stopping Relaying (OSR)" framework for designing such spatial-diversity schemes. As a particular example, assuming Rayleigh fading channels, we implement an OSR strategy to optimize information efficiency (IE) in a protocol stack consisting of Greedy Perimeter Stateless Routing (GPSR) and IEEE 802.11 MAC protocols. We present an analysis of the algorithm for a single node. In addition, we perform extensive simulations (using QualNet) to evaluate the end-to-end performance of the proposed forwarding strategy. The results demonstrate the superiority of OSR over other existing schemes.

Ultra-wideband (UWB) systems spread the transmitted signal power over an extremely large frequency band, and the power spectral density of the signal is very low. Due to the wide bandwidth of the transmitted signal, UWB signal energy spreads over the frequency bands allocated to other radio systems. Evaluating the interference issues of these narrowband systems to UWB systems in its overlay band is an important issue for understanding and designing efficient UWB systems. An analysis of the power spectral density (PSD) of generic narrowband signals is presented. The theoretical and simulation results reveal that the bit-error-rate (BER) performance of a direct sequence UWB (DS-UWB) system with a minimum mean square error (MMSE) receiver is robust in the presence of narrowband interference caused by existing applications.

Ultra-wideband (UWB) technology is one of the promising solutions for short-range indoor wireless communication applications. UWB systems spread the transmitted signal power over an extremely large frequency band, and the power spectral density of the signal is very low. Due to the wide bandwidth of the transmitted signal, UWB signal energy will spread over the frequency bands allocated to other radio systems. In order to evaluate the effects of UWB system to narrowband systems (such as IEEE802.11a wireless LAN) in its overlay band, it is important to understand and quantify the power spectral density (PSD) of UWB signal. In this paper, a detailed analysis on the PSD of direct sequence coded UWB (DS-UWB) signal is presented. The theoretical and simulation results reveal that the interfering DS-UWB power can be reduced by regulating UWB chip rate, pulse shape, pulse width and scrambling codes in the frequencies of victim narrowband systems.