1,720,967 research outputs found
Pore-scale simulation of fluid flow through the electrodes of high temperature PEMFC using Lattice Boltzmann Method
Full text linkPolymer electrolyte membrane fuel cells (PEMFC) have received attention as new power sources for residential, transportation, as well as portable applications. Despite the tremendous progress in PEM fuel cell technology, namely development of the phosphoric acid doped PBI-based high temperature (> 100 oC) PEMFC with improved properties, reduced production cost, high efficiency and sufficient tolerance of Pt based hydrogen oxidation catalysts to CO impurity in hydrogen fuel (up to 2% at 180 oC) [1], degradation issues still remain. Loss of phosphoric acid by different processes, especially in high current density and elevated temperature (> 160 oC) [2], is thought to be one of the major mechanisms of degradation. Deep insight into this degradation mechanism, leading to irreversible or reversible performance loss and the relation with other degradation mechanisms and operating conditions, can come by pore-scale modelling of the mass transport phenomena, which provides detailed information at the microscopic scale.
In order to optimize the mass transport properties of the electrodes of high temperature PEMFC, firstly, the microstructure of a fiber-based gas diffusive layer (GDL) and a carbon-supported catalyst layer (CL) are reconstructed. Concerning GDL, two different commercial materials are considered and investigated by 2D Scanning Electron Microscopy images: woven GDL (Toray Graphite Paper, TGPH-120, BASF Fuel Cell) and non-woven GDL (Freudenberg Plain H2315, Freudenberg Non-wovens Technical Division). Different reconstruction techniques have been developed to deal with these materials. Woven GDL has been described by a deterministic algorithm [3], while non-woven GDL by a stochastic algorithm. On the other hand, in case of the catalyst layer reconstruction, different degrees of clustering have been investigated in order to match the actual properties of commercial materials [3].
Secondly, pore-scale flow simulations by the Lattice Boltzmann Method (PALABOS [4]) have been done to estimate the permeabilities. Rarefied gas effects are taken into account by a simplified approach, relying on a good agreement with experimental data. A model is proposed to link the permeabilities with degradation processes occurring during the high temperature PEMFC operation and, in particular, with the loss of phosphoric acid. Furthermore, the effects of the micro-morphology and the catalyst particles distribution on the durability of the electrolyte membrane are studied. Some optimization strategies are proposed in order to improve fuel cells durability.
This work is part of the on-going European ARTEMIS project, within the Fuel cells and Hydrogen Joint Undertaking (FCH-JU). The purpose of ARTEMIS is to develop and optimise alternative materials for a new generation of European membrane electrolyte assembly, while reducing cost and increasing durability.
[1] Q. Li, J.O. Jensen, R.F. Savinell, N.J. Bjerrum. High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Prog. Polym. Sci. 34 (2009) 449.
[2] S. Yu, L. Xiao, and B. C. Benicewicz. Durability Studies of PBI-based High Temperature PEMFCs. FUEL CELLS 08, 2008, No. 3–4, 165–174
[3] U. Salomov, E. Chiavazzo and P. Asinari, Pore-scale modeling of fluid flow through electrodes for high temperature proton exchange membrane (HT-PEM) fuel cells, submitted to Computers and Mathematics with Applications, 2012.
[4] www.palabos.or
Multi-scale modeling to boost fuel cell performance: From pore-scale simulations to better efficiency and durability
Full text linkAccording to the European Commission, Europe has set itself a goal to reduce CO2 emission levels by 2050 to 80% of what they were in 1990. Fuel cells and hydrogen have potential to contribute to overcoming the energy challenges that accompany this change. In particular, fuel cells based on proton-exchange membranes (PEM) and fuelled by hydrogen and air have many attractive features, including high power density, rapid start-up and high efficiency. However, among the major technology issues that must be addressed for their commercialization and widespread use, the degradation phenomena of the membrane electrode assembly (MEA) plays a key role. In this talk, we present multi-scale morphological models and simulation tools for detailed understanding of degradation phenomena. This kind of modeling techniques can take strong advantage by recent progresses in dual-beam focused ion beam scanning electron microscopy (FIB-SEM). As an example, we investigate the effects of the catalyst distribution in the electrodes on the local fluid flow and on the loss of phosphoric acid from the membran
Gas-dynamic and electro-chemical optimization of catalyst layers in high temperature polymeric electrolyte membrane fuel cells
Full text linkWe investigate the impact of catalyst (Pt) particle distribution on gas dynamics, electro-chemistry and consequently the performance of high temperature polymeric electrolyte membrane (HTPEM) fuel cells. We demonstrate that optimal distribution of catalyst can be used as an effective mitigation strategy for phosphoric acid loss and crossover of reagents through the membrane. First, we recognize that one of the reasons for performance degradation of HTPEM fuel cells originates from the gas dynamic pulling at the interface between the catalyst layer and membrane. Hence, we show that this can be greatly alleviated by choosing a proper catalyst particle distribution within the catalyst layer (CL). A simplified three-dimensional macroscopic model of the membrane electrode assembly (MEA) with catalyst layer made of three or five sublayers with different catalyst loadings, have been developed to analyze the effect of the proposed mitigation strategy on gas dynamics within the catalyst layer and the overall cell performance. This simplified macroscopic model predicts significant reduction (up to 4 times) in pulling using a feasible mitigation strategy, at the cost of only 9% efficiency reduction at high current densities
Key Management in Wireless Sensor Networks, IP-Based Sensor Networks, Content Centric Networks
Full text linkCryptographic keys and their management in network communication is considered the main building block of security over which other security primitives are based. These cryptographic keys ensure the privacy, authentication, integrity and non-repudiation of messages. However, the use of these cryptographic keys and their management in dealing with the resource constrained devices (i.e. Sensor nodes) is a challenging task. A number of key management schemes have been introduced by researchers all over the world for such resource constrained networks. For example, light weight PKI and elliptic curve cryptography schemes are computationally expensive for these resource constrained devices. So far the symmetric key approach is considered best for these constrained networks and different variants of it been developed for these networks (i.e. probabilistic key distribution approach). The probabilistic key distribution approach consumes less memory than the standard symmetric key approach but it suffers from the connectivity issues (i.e. the connectivity depends on the common shared keys between the nodes). Most of those schemes were proposed by considering static sensor networks (e.g. Industrial process monitoring, Environmental monitoring, movement detection in military applications, forests etc.). However, the use of these existing key management schemes for mobile wireless sensor networks applications introduces more challenges in terms of network connectivity, energy consumption, memory cost, communication overhead and protection of key materials against some well known attacks. Keeping these challenges in mind, previous research has proposed some key management schemes considering the mobility scenarios in ad hoc networks and wireless sensor networks (e.g. vehicular networks, health monitoring systems).However these schemes consume more resource because of a much higher communication packet exchange during the handover phase for the authentication of joining and leaving nodes than the static networks where there is no extra communication for the handover and authentication. The motivation of this research work is to investigate and propose new algorithms not only to improve the efficiency of these existing authentication and key management schemes in terms of connectivity, memory and security by considering the mobility scenario in wireless sensor networks, but also to develop new algorithms that suit these constrained networks than the existing schemes. First, we choose the existing key pool approach for authentication and key management and improve its network connectivity and resilience against some well known attacks (e.g. node capturing attacks) while reduce the memory cost by storing those key pools in each sensor node. In the proposed solution, we have divided the main key pool into two virtual mutually exclusive key pools. This division and constructing a key from two chosen keys, one from each key pool, helps to reduce the memory cost of each node by assigning fewer keys for the same level of network connectivity as the existing key pool frameworks. Although, the proposed key pool approach increases the network resilience against node compromission attacks because of the smaller number of keys assigned to each node, however it does not completely nullify the effect of the attacks. Hence we proposed an online mutual authentication and key establishment and management scheme for sensor networks that provides almost 100\% network connectivity and also nullifies the effect of node compromission attacks. In the proposed online key generation approach, the secret key is dependent on both communicating parties. Once the two communicating parties authenticate each other, they would successfully establish a secret communication key, otherwise they stop communication and inform the network manager about the intruder detection and activity. The last part of the thesis considers the integration of two different technologies (i.e. wireless sensor networks and IP networks). This is a very interesting and demanding research area because of its numerous applications, such as smart energy, smart city etc.. However the security requirements of these two kind of networks (resource constrained and resourceful) make key management a challenging task. Hence we use an online key generation approach using elliptic curve cryptography which gives the same security level as the standard PKI approach used in IP networks with smaller key length and is suited for the sensor network packet size limitations. It also uses a less computationally expensive approach than PKI and hence makes ECC suitable to be adopted in wireless sensor networks. In the key management scheme for IP based sensor networks, we generate the public private key pair based on ECC for each individual sensor node. However the public key is not only dependent on the node's parameter but also the parameters of the network to which it belongs. This increases the security of the proposed solution and avoids intruders pretending to be authentic members of the network(s) by spreading their own public keys. In the last part of the thesis we consider Content Centric Networking (CCN) which is a new routing architecture for the internet of the future. Building on the observation that today's communications are more oriented towards content retrieval (web, P2P, etc.) than point-to-point communications (VoIP, IM, etc.), CCN proposes a radical revision of the Internet architecture switching from named hosts (TCP/IP protocols) to named data to best match its current usage. In a nutshell, content is addressable, routable, self-sufficient and authenticated, while locations no longer matter. Data is seen and identified directly by a routable name instead of a location (the address of the server). Consequently, data is directly requested at the network level not from its holder, hence there is no need for the DNS). To improve content diffusion, CCN relies on data distribution and duplication, because storage is cheaper than bandwidth: every content - particularly popular one - can be replicated and stored on any CCN node, even untrustworthy. People looking for particular content can securely retrieve it in a P2P-way from the best locations available. So far, there has been little investigation of the security of CCNs and there is no specific key management scheme for that. We propose an authentication and key establishment scheme for CCNs in which the contents are authenticated by the content generating node, using pre-distributed shares of encryption keys. The content requesting node can get those shares from any node in the network, even from malicious and intruder ones, in accordance with a key concept of CCNs. In our work we also provide means to protect the distributed shares from modification by these malicious/intruder nodes. The proposed scheme is again an online key generation approach but including a relation between the content and its encryption key. This dependency prevents the attackers from modifying the packet or the key share
Pore-scale modeling of fluid flow through gas diffusion and catalyst layers for high temperature proton exchange membrane (HT-PEM) fuel cells
Full text linkThis work represents a step towards reliable algorithms for reconstructing the micromorphology of electrode materials of high temperature proton exchange membrane fuel cells and for performing pore-scale simulations of fluid flow (including rarefaction effects). In particular, we developed a deterministic model for a woven gas diffusion layer (GDL) and a stochastic model for the catalyst layer (CL) based on clusterization of carbon particles. We verified that both of the models developed accurately recover the experimental values of the permeability, without any special ad hoc tuning. Moreover, we investigated the effect of catalyst particle distributions inside the CL on the degree of clusterization and on the microscopic fluid flow, which is relevant for the modeling of degradation (e.g. loss of phosphoric acid). The three-dimensional pore-scale simulations of the fluid flow for the direct numerical calculation of the permeability were performed by the lattice Boltzmann method (LBM
Pore- and macro-scale simulations of high temperature proton exchange fuel cells - HTPEMFC - and possible strategies for enhancing durability
No full textThe impact of catalyst particle distribution on gas dynamics, electro-chemistry and consequently the performance of polybenzimidazole-based HTPEMFC is explored. The main degradation mechanisms which shorten PEMFC durability are reviewed in details. This review shows that phosphoric acid loss and crossover of reagents highly depend on catalyst reactivity within active layer. We propose a strategy to mitigate these degradations by redistributing catalyst. The proposed optimal cathode catalyst layer (CL) configuration with regards to catalyst distribution effectively reduces gas dynamic stresses and productions of water and peroxide radicals at CL/membrane interface, hence expecting to enhance durability. At pore-scale, our morphological model of CL developed in Ref. [1] is improved and it is found that the proper catalyst redistribution significantly (in one order) reduces gas dynamic stresses. At macro-scale, we develop a 3-D model of MEA with cathode CL made of several sublayers with different catalyst loading. Adopted macroscopic model predicts stress reduction up to 69% using a feasible mitigation strategy, at the cost of only 9% efficiency reduction at high current densities. Finally, two possible experimental approaches of CL fabrication with given configuration are compared
Efficient steam generation by inexpensive narrow gap evaporation device for solar applications
Full text linkTechnologies for solar steam generation with high performance can help solving critical societal issues such as water desalination or sterilization, especially in developing countries. Very recently, we have witnessed a rapidly growing interest in the scientific community proposing sunlight absorbers for direct conversion of liquid water into steam. While those solutions can possibly be of interest from the perspective of the involved novel materials, in this study we intend to demonstrate that efficient steam generation by solar source is mainly due to a combination of efficient solar absorption, capillary water feeding and narrow gap evaporation process, which can also be achieved through common materials. To this end, we report both numerical and experimental evidence that advanced nano-structured materials are not strictly necessary for performing sunlight driven water-to-vapor conversion at high efficiency (i.e. ≥85%) and relatively low optical concentration (≈10 suns). Coherently with the principles of frugal innovation, those results unveil that solar steam generation for desalination or sterilization purposes may be efficiently obtained by a clever selection and assembly of widespread and inexpensive materials
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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