Li‐rich Mn‐based layered oxides are regarded as the most promising cathode materials for advanced lithium‐ion batteries with energy density as high as 400 Wh kg−1. However, decline of capacity and ...discharge potential derived from phase transformation during cycling is still an obstacle for practical utilization of Li‐rich cathode materials. Undoubtedly, an in‐depth understanding origin and evolution of the phase transformation from bottom to top is crucial to solve the problem finally. Herein, the recent representative progress on Li‐rich cathode materials from top to bottom is summarized: starting from relationship between dimensions and performance, to evolution of phase transformation, finally to participation of anions during charge–discharge cycling. It systematically shows what happens in the different microscopic levels and how these phenomena relate to cycling of Li‐rich cathode materials with the help of emerging state‐of‐the‐art characterization techniques. On the basis of this progress, it is proposed that rational structural design can fully play its role to build high‐performance energy storage materials and enhance structural stability.
Recent research progress in Li‐rich Mn‐based layered cathode materials are summarized from dimension–performance relationship to evolution of phase transformation, and to participation of anions during cycling, showing systematically what happens in the different microscopic levels and how these phenomena relate with cycling performance.
Abstract
Recently developed solid-state catalysts can mediate carbon dioxide (CO
2
) electroreduction to valuable products at high rates and selectivities. However, under commercially relevant ...current densities of > 200 milliamperes per square centimeter (mA cm
−2
), catalysts often undergo particle agglomeration, active-phase change, and/or element dissolution, making the long-term operational stability a considerable challenge. Here we report an indium sulfide catalyst that is stabilized by adding zinc in the structure and shows dramatically improved stability. The obtained ZnIn
2
S
4
catalyst can reduce CO
2
to formate with 99.3% Faradaic efficiency at 300 mA cm
−2
over 60 h of continuous operation without decay. By contrast, similarly synthesized indium sulfide without zinc participation deteriorates quickly under the same conditions. Combining experimental and theoretical studies, we unveil that the introduction of zinc largely enhances the covalency of In-S bonds, which “locks” sulfur—a catalytic site that can activate H
2
O to react with CO
2
, yielding HCOO* intermediates—from being dissolved during high-rate electrolysis.
Operating fuel cells in alkaline environments permits the use of platinum-group-metal-free (PGM-free) catalysts and inexpensive bipolar plates, leading to significant cost reduction. Of the PGM-free ...catalysts explored, however, only a few nickel-based materials are active for catalyzing the hydrogen oxidation reaction (HOR) in alkali; moreover, these catalysts deactivate rapidly at high anode potentials owing to nickel hydroxide formation. Here we describe that a nickel-tungsten-copper (Ni
WCu
) ternary alloy showing HOR activity rivals Pt/C benchmark in alkaline electrolyte. Importantly, we achieved a high anode potential up to 0.3 V versus reversible hydrogen electrode on this catalyst with good operational stability over 20 h. The catalyst also displays excellent CO-tolerant ability that Pt/C catalyst lacks. Experimental and theoretical studies uncover that nickel, tungsten, and copper play in synergy to create a favorable alloying surface for optimized hydrogen and hydroxyl bindings, as well as for the improved oxidation resistance, which result in the HOR enhancement.
This paper investigates the physical layer security of non-orthogonal multiple access (NOMA) in large-scale networks with invoking stochastic geometry. Both single-antenna and multiple-antenna aided ...transmission scenarios are considered, where the base station (BS) communicates with randomly distributed NOMA users. In the single-antenna scenario, we adopt a protected zone around the BS to establish an eavesdropper-exclusion area with the aid of careful channel ordering of the NOMA users. In the multiple-antenna scenario, artificial noise is generated at the BS for further improving the security of a beamforming-aided system. In order to characterize the secrecy performance, we derive new exact expressions of the security outage probability for both single-antenna and multiple-antenna aided scenarios. For the single-antenna scenario, we perform secrecy diversity order analysis of the selected user pair. The analytical results derived demonstrate that the secrecy diversity order is determined by the specific user having the worse channel condition among the selected user pair. For the multiple-antenna scenario, we derive the asymptotic secrecy outage probability, when the number of transmit antennas tends to infinity. Monte Carlo simulations are provided for verifying the analytical results derived and to show that: 1) the security performance of the NOMA networks can be improved by invoking the protected zone and by generating artificial noise at the BS and 2) the asymptotic secrecy outage probability is close to the exact secrecy outage probability.
•A comprehensive and systematic review of the reliability of mobile wireless sensor networks (MWSNs) is presented.•The typical features of MWSNs and the current classical routing protocols are ...summarized.•The typical reliability methods of MWSNs in the references are summarized, sorted, classified and compared.•The important challenges of MWSNs reliability are pointed out.
With the development and popularization of mobile terminal technology, mobile wireless sensor networks (MWSNs) have become a new evolution trend in wireless sensor networks (WSNs) technology. The mobility of nodes introduced by MWSNs greatly expands the application range of WSNs. However, the mobility of the sink node also induces changes of the network topology, time delays, and packet loss rates, which can greatly affect the routing performance of the networks. In addition, the reliability and robustness of the networks are also greatly affected and challenged. The theories and methods applied to WSNs previously, such as data collection, reliable data transmission, network reliability, network topologies, and routing protocols are not currently available for MWSNs. The introduction of mobility features into traditional static WSNs brings new challenges for the stability, security, and reliability of WSNs. Reliability is an important index for evaluating the performance of MWSNs. Moreover, the reliability of the data transmission determines the practical value of MWSNs. The stable and reliable operation of MWSNs requires ongoing and important work. Therefore, in this article, we present a comprehensive and meticulous investigation of the reliability theory of MWSNs which was developed in recent years. This article outlines the classification and features of MWSNs. Specifically, we systematically analyze the research direction and recent progress made in the field of MWSNs. The existing methods are systematically compared and the advantages and disadvantages of the methods are presented. The main parameters of comparison include time delays, network size, energy efficiency, scalability, and reliability. Finally, some open questions and future research directions are presented at the end of this article.
A considerable challenge in the conversion of carbon dioxide into useful fuels comes from the activation of CO2 to CO2.− or other intermediates, which often requires precious‐metal catalysts, high ...overpotentials, and/or electrolyte additives (e.g., ionic liquids). We report a microwave heating strategy for synthesizing a transition‐metal chalcogenide nanostructure that efficiently catalyzes CO2 electroreduction to carbon monoxide (CO). We found that the cadmium sulfide (CdS) nanoneedle arrays exhibit an unprecedented current density of 212 mA cm−2 with 95.5±4.0 % CO Faraday efficiency at −1.2 V versus a reversible hydrogen electrode (RHE; without iR correction). Experimental and computational studies show that the high‐curvature CdS nanostructured catalyst has a pronounced proximity effect which gives rise to large electric field enhancement, which can concentrate alkali‐metal cations resulting in the enhanced CO2 electroreduction efficiency.
The needle has landed: CdS nanostructures with sharp tips can generate large electric fields that lead to increased CO2 concentrations for CO2‐to‐CO conversion. The localized electric fields are significantly enhanced when two nanoneedles are in close proximity. These advantages result in CO2 electrocatalytic reduction with a 95.5±4.0 % CO Faraday efficiency.
Platinum on carbon (Pt/C) catalyst is commercially adopted in fuel cells but it undergoes formidable active‐site poisoning by carbon monoxide (CO). In particular, given the sluggish kinetics of ...hydrogen oxidation reaction (HOR) in anion‐exchange membrane fuel cell (AEMFC), the issues of Pt poisoning and slow rate would combine mutually, notably worsening the device performances. Here we overcome these challenges through incorporating cobalt (Co) into molybdenum‐nickel alloy (MoNi4), termed Co‐MoNi4, which not only shows superior HOR activity over the Pt/C catalyst in alkali, but more intriguingly exhibits excellent CO tolerance with only small activity decay after 10 000 cycles in the presence of 500 parts per million (ppm) CO. When feeding with CO (250 ppm)/H2, the AEMFC assembled by this catalyst yields a peak power density of 394 mW cm−2, far exceeding the Pt/C catalyst. Experimental and computational studies reveal that weakened CO chemisorption originates from the electron‐deficient Ni sites after Co incorporation that suppresses d→CO 2π* back‐donation.
Incorporating Co into MoNi4 nanocatalyst can suppress the d→CO 2π* back donation, leading to excellent CO tolerance. When feeding with CO (250 ppm)/H2, the fuel cell assembled by this catalyst yields a peak power density of 394 mW cm−2, exceeding that of 209 mW cm−2 for the Pt/C catalyst.
By transforming data into binary representation, i.e., Hashing, we can perform high-speed search with low storage cost, and thus, Hashing has collected increasing research interest in the recent ...years. Recently, how to generate Hashcode for multimodal data (e.g., images with textual tags, documents with photos, and so on) for large-scale cross-modality search (e.g., searching semantically related images in database for a document query) is an important research issue because of the fast growth of multimodal data in the Web. To address this issue, a novel framework for multimodal Hashing is proposed, termed as Collective Matrix Factorization Hashing (CMFH). The key idea of CMFH is to learn unified Hashcodes for different modalities of one multimodal instance in the shared latent semantic space in which different modalities can be effectively connected. Therefore, accurate cross-modality search is supported. Based on the general framework, we extend it in the unsupervised scenario where it tries to preserve the Euclidean structure, and in the supervised scenario where it fully exploits the label information of data. The corresponding theoretical analysis and the optimization algorithms are given. We conducted comprehensive experiments on three benchmark data sets for cross-modality search. The experimental results demonstrate that CMFH can significantly outperform several state-of-the-art cross-modality Hashing methods, which validates the effectiveness of the proposed CMFH.
A compact ±45° dual-polarized broadband antenna array is proposed in this letter for base stations. For the sake of structural simplicity, low profile, and ease of mass production, printed dipoles ...are adopted as the dual-polarized antenna element. To achieve the broadband performance, radiators are designed to be spline-edged bowties, and they are fed by tapered transmission lines. Moreover, to improve the symmetry and stability of the radiation pattern, the Pawsey stub baluns are used to balance the antenna structure. Finally, an eight-element antenna array prototype backed by a folded reflector is fabricated and measured. The compact antenna array has a broad bandwidth of 68% (1.427-2.9 GHz) with a voltage standing-wave ratio less than 1.5 and stable radiation properties to provide international mobile telecommunication services over 2G/3G/LTE systems and Land S-bands released recently.
By transferring knowledge from the abundant labeled samples of known source classes, zero-shot learning (ZSL) makes it possible to train recognition models for novel target classes that have no ...labeled samples. Conventional ZSL approaches usually adopt a two-step recognition strategy, in which the test sample is projected into an intermediary space in the first step, and then the recognition is carried out by considering the similarity between the sample and target classes in the intermediary space. Due to this redundant intermediate transformation, information loss is unavoidable, thus degrading the performance of overall system. Rather than adopting this two-step strategy, in this paper, we propose a novel one-step recognition framework that is able to perform recognition in the original feature space by using directly trained classifiers. To address the lack of labeled samples for training supervised classifiers for the target classes, we propose to transfer samples from source classes with pseudo labels assigned, in which the transferred samples are selected based on their transferability and diversity. Moreover, to account for the unreliability of pseudo labels of transferred samples, we modify the standard support vector machine formulation such that the unreliable positive samples can be recognized and suppressed in the training phase. The entire framework is fairly general with the possibility of further extensions to several common ZSL settings. Extensive experiments on four benchmark data sets demonstrate the superiority of the proposed framework, compared with the state-of-the-art approaches, in various settings.
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