High-rate Decoupled Water Electrolysis System Integrated with α-MoO3 as a Redox Mediator with Fast Anhydrous Proton Kinetics

Hydrogen is a promising alternative to fossil fuels that can reduce greenhouse gas emissions. Decoupled water electrolysis system using a reversible proton storage redox mediator, where the oxygen evolution reaction and hydrogen evolution reaction are separated in time and space, is an effective approach to producing hydrogen gas with high purity, high flexibility, and low cost. To realize fast hydrogen production in such a system, a redox mediator capable of releasing protons rapidly is required. Herein, α-MoO₃, with an ultrafast proton transfer property that can be explained by a dense hydrogen bond network in the lattice oxygen arrays of HxMoO₃, is examined as a high-rate redox mediator for fast hydrogen production in acidic electrolytes. The α-MoO₃ redox mediator shows both a large capacity of 204 mAh g⁻¹ and fast hydrogen production at a current rate of 10 A cm⁻²(≈153 A g⁻¹), outperforming most of the previously reported solid-state redox mediators.     

A novel hidden station detection mechanism in IEEE 802.11 WLAN

The popular IEEE 802.11 wireless local area network (WLAN) is based on a carrier sense multiple access with collision avoidance (CSMA/CA), where a station listens to the medium before transmission in order to avoid collision. If there exist stations which can not hear each other, i.e., hidden stations, the potential collision probability increases, thus dramatically degrading the network throughput. The RTS/CTS (request-to-send/clear-to-send) frame exchange is a solution for the hidden station problem, but the RTS/CTS exchange itself consumes the network resources by transmitting the control frames. In order to maximize the network throughput, we need to use the RTS/CTS exchange adaptively only when hidden stations exist in the network. In this letter, a simple but very effective hidden station detection mechanism is proposed. Once a station detects the hidden stations via the proposed detection mechanism, it can trigger the usage of the RTS/CTS exchange. The simulation results demonstrate that the proposed mechanism can provide the maximum system throughput performance.     

A downlink beamforming assisted by locally positioned communication devices

Recently, multiple cell types with overlapping coverage have been deployed simultaneously to increase cellular network capacity. Cross‐tier interference is one of the key technical challenges in the use of this method. A simple and practical beamforming scheme assisted by locally positioned communication devices for the downlink of a multi‐cell wireless hierarchical cell structure system is proposed in this paper to maximize the capacity of the embedded small cell and simultaneously ensure minimal impact on the performance of existing macrocells. The locally positioned communication devices can be implemented with low complexity and at low cost and can continuously provide helpful and accurate information to base stations for the proper configuration of geographical cell coverage, allowing neighboring cells to cooperate effectively with each other. Simulation results verify that the proposed scheme can provide a significant gain over conventional systems not using the proposed algorithm. Copyright © 2013 John Wiley & Sons, Ltd.     

Improving Spectral and Temporal Efficiency of Collocated IEEE 802.15.4 LR-WPANs

The number of frequency channels specified for IEEE 802.15.4 low-rate wireless personal area networks (LR-WPANs) does not suffice to operate a variety of collocated WPAN applications that the standard is targeting. To overcome this limit, we introduce virtual channel, a novel concept to increase the number of available channels by efficiently managing given spectral and temporal resources. A virtual channel is created by scheduling a superframe and selecting a logical channel. This extends the notion of a channel from spectral domain to spectral and temporal domain. Specifically, we propose a superframe scheduler using throughput estimation (SUTE) of the IEEE 802.15.4 carrier sense multiple access with collision avoidance (CSMA/CA). In addition, nearest vacancy search (NEVS) is proposed, both of which are for temporal efficiency of the collocation. For both spectral and temporal efficiency, virtual channel selector (VCS) is proposed. The simulation results show that a remarkable improvement on the collocation efficiency of IEEE 802.15.4 can be achieved by our proposals. Moreover, this study also reveals the fundamental drawback of the current standard in terms of the collocation efficiency that the beacon interval and superframe duration are adjustable only by exponent parameters.     

Adaptive bandwidth reservation and admission control in QoS-sensitive cellular networks

How to keep the probability of hand-off drops within a prespecified limit is a very important quality-of-service (QoS) issue in cellular networks because mobile users should be able to maintain ongoing sessions even during their hand-off from one cell to another. We design and evaluate predictive and adaptive schemes for bandwidth reservation for the hand-offs of ongoing sessions and the admission control of new connections. We first develop a method to estimate user mobility based on an aggregate history of hand-offs observed in each cell. This method is then used to probabilistically predict mobiles' directions and hand-off times in a cell. For each cell, the bandwidth to be reserved for hand-offs is calculated by estimating the total sum of tractional bandwidths of the expected hand-offs within a mobility-estimation time window. Three different admission-control schemes for new connection requests using this bandwidth reservation are proposed. We also consider variations that utilize the path/location information available from the car navigation system or global positioning system. Finally, we evaluate the performance of the proposed schemes extensively to show that they meet our design goal and outperform the static reservation scheme under various scenarios.     

A comparative study of bandwidth reservation and admission control schemes in QoS‐sensitive cellular networks

This paper compares five different schemes – called CHOI, NAG, AG, BHARG, and NCBF – for reserving bandwidths for handoffs and admission control for new connection requests in QoS‐sensitive cellular networks. CHOI and NAG are to keep the handoff dropping probability below a target value, AG is to guarantee no handoff drops through per‐connection bandwidth reservation, and BHARG and NCBF use another type of per‐connection bandwidth reservation. CHOI predicts the bandwidth required to handle handoffs by estimating possible handoffs from adjacent cells, then performs admission control for each newly‐requested connection. On the other hand, NAG predicts the total required bandwidth in the current cell by estimating both incoming and outgoing handoffs at each cell. AG requires the set of cells to be traversed by the mobile with a newly‐requested connection, and reserves bandwidth for each connection in each of these cells. The last two schemes reserve bandwidth for each connection in the predicted next cell of a mobile where the two schemes use different admission control policies. We adopt the history‐based mobility estimation for the first two schemes. Using extensive simulations, the five schemes are compared quantitatively in terms of (1) handoff dropping probability, connection‐blocking probability, and bandwidth utilization; (2) dependence on the design parameters; (3) dependence on the accuracy of mobility estimation; and (4) complexity. The simulation results indicate that CHOI is the most desirable in that it achieves good performance while requiring much less memory and computation than the other four schemes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interference Analysis and Transmit Power Control in IEEE 802.11a/h Wireless LANs

Reducing the energy consumption by wireless communication devices is perhaps the most important issue in the widely deployed and dramatically growing IEEE 802.11 WLANs (wireless local area networks). TPC (transmit power control) has been recognized as one of the effective ways to achieve this goal. In this paper, we study the emerging 802.11a/h systems that provide a structured means to support intelligent TPC. Based on a rigorous analysis of the relationship among different radio ranges and TPC's effects on the interference, we present an optimal low-energy transmission strategy, called MiSer, which is deployed in the format of RTS-CTS(strong)-Data(MiSer)-Ack. The key idea of MiSer is to combine TPC with PHY (physical layer) rate adaptation and compute offline an optimal rate-power combination table, then at runtime, a wireless station determines the most energy-efficient transmission strategy for each data frame transmission by a simple table lookup. Simulation results show MiSer's clear superiority to other two-way or four-way frame exchange mechanisms in terms of energy conservation.     

Two-dimensional ultrasonic anemometer using the directivity angle of an ultrasonic sensor

There has been continuous research on the ultrasonic sensors [L.C. Lynnworth, Y. Liu, Ultasonic flowmeters: half-century progress report 1955–2005, Ultrasonics (2006) 1371–1378]. This study used the directivity characteristic based on phase measurement, which ultrasonic sensor has. Transmitter has wide directivity, and in its range of directivity, receivers are located to measure wind direction and velocity. The velocity experiments which are the result of this production system indicate that the system error rate is 2%. The wind direction measurements also give the same result, an error of ±3°. This result is similar to existing anemometers. However, this new anemometer could possibly have a much higher sampling rate, operating circuit and system structure, which could be much simpler than the previous anemometer.     

Effect of variation of hydrophobicity of anode diffusion media along the through-plane direction in direct methanol fuel cells

Reducing methanol crossover from the anode to cathode in direct methanol fuel cells (DMFCs) is critical for attaining high cell performance and fuel utilization, particularly when highly concentrated methanol fuel is fed into DMFCs. In this study, we present a novel design of anode diffusion media (DM) wherein spatial variation of hydrophobicity along the through-plane direction is realized by special polytetrafluoroethylene (PTFE) coating procedure. According to the capillary transport theory for porous media, the anode DM design can significantly affect both methanol and water transport processes in DMFCs. To examine its influence, three different membrane-electrode assemblies are fabricated and tested for various methanol feed concentrations. Polarization curves show that cell performance at high methanol feed concentration conditions is greatly improved with the anode DM design with increasing hydrophobicity toward the anode catalyst layer. In addition, we investigate the influence of the wettability of the anode microporous layer (MPL) on cell performance and show that for DMFC operation at high methanol feed concentration, the hydrophilic anode MPL fabricated with an ionomer binder is more beneficial than conventional hydrophobic MPLs fabricated with PTFE. This paper highlights that controlling wetting characteristics of the anode DM and MPL is of paramount importance for mitigating methanol crossover in DMFCs.     

Effect of hydrogen partial pressure on a polymer electrolyte fuel cell performance

We first investigated the effect of partial pressure of hydrogen (H₂) on the performance of polymer electrolyte fuel cells (PEFCs) by controlling the ratio of hydrogen and nitrogen (N₂). The cell performance with Pt/C anode was significantly decreased with reduction of the partial pressure of H₂ in the presence of carbon monoxide (CO), while the performance variation was negligible in the absence of CO. Severe CO poisoning on Pt/C electrode at low partial pressure of H₂ might be attributed to the hindering effect by N₂ and CO. On the other hand, PtRu/C anode showed consistent power performance even at low partial pressure of H₂.