1,720,968 research outputs found
Local potential measurement through reference electrodes in vanadium redox flow batteries: Evaluation of overpotentials and electrolytes imbalance
Full text linkVanadium redox flow battery performance is actually hindered by increased overpotentials at high current, due to poor electrochemical activity of the most commonly used carbon electrodes and to electrolyte distribution, implying local mass transport limiting conditions. Moreover, vanadium cross-contamination leads to coulombic efficiency reduction and uncontrolled electrolytes imbalance. This work presents the application of through-plate reference electrodes at inlet and outlet of both positive and negative electrodes. The utility of the electrodes potential measurement is firstly demonstrated in the identification of a relation between electrolytes potential and the corresponding state of charge. Subsequently, local overpotentials and impedance spectra at both electrodes are evaluated, evidencing that the negative electrode is kinetically dominated and presents high overpotential even at low current, while the positive exhibits mass transport effects at high current, especially at cell outlet. Finally, during cycling operation with fixed capacity reference electrode measurements permit to monitor electrolytes imbalance induced by cross-contamination, that mainly affects negative electrolyte. Moreover, additional insights into electrodes potential loss during cycling are provided
Mitigation of capacity decay in vanadium redox flow batteries through initial imbalance of electrolytes state of charge
Full text linkCapacity decay due to vanadium cross-over is a key technical challenge for Vanadium Redox Flow Batteries (VRFBs). To mitigate this effect this study investigates an operating strategy based on the imbalance of electrolyte solutions State of Charge (SoC). The strategy was conceived starting from the analysis of a VRFB during charge-discharge cycles adopting through-plate Reference Hydrogen Electrodes (RHE), which allowed to continuously monitor the evolution of the electrolytes SoCs during the operation. The obtained information, combined with a modelling analysis to estimate cross-over fluxes and net-vanadium transfer, suggested that starting the operation of the battery with positive electrolyte at a higher SoC compared to the negative one can be an effective strategy. Simulations of battery operation with different initial values of positive electrolyte SoC confirmed that this approach mitigated both battery capacity decay and net vanadium transfer. Model results were then validated by experimental testing. Starting the operation of the battery with negative and positive electrolyte SoC equal to 0 % and 15 % respectively, permitted to increase the discharged energy per cycle by 15 % and to reduce the net vanadium transfer by over 30 % without penalizing the efficiency of the battery
Design and Development of Flow Fields with Multiple Inlets or Outlets in Vanadium Redox Flow Batteries
Full text linkIn vanadium redox flow batteries, the flow field geometry plays a dramatic role on the
distribution of the electrolyte and its design results from the trade-off between high battery
performance and low pressure drops. In the literature, it was demonstrated that electrolyte
permeation through the porous electrode is mainly regulated by pressure difference between
adjacent channels, leading to the presence of under-the-rib fluxes. With the support of a 3D
computational fluid dynamic model, this work presents two novel flow field geometries that are
designed to tune the direction of the pressure gradients between channels in order to promote the
under-the-rib fluxes mechanism. The first geometry is named Two Outlets and exploits the splitting
of the electrolyte flow into two adjacent interdigitated layouts with the aim to give to the pressure
gradient a more transverse direction with respect to the channels, raising the intensity of under-therib
fluxes and making their distribution more uniform throughout the electrode area. The second
geometry is named Four Inlets and presents four inlets located at the corners of the distributor, with
an interdigitated-like layout radially oriented from each inlet to one single central outlet, with the
concept of reducing the heterogeneity of the flow velocity within the electrode. Subsequently, flow
fields performance is verified experimentally adopting a segmented hardware in symmetric cell
configuration with positive electrolyte, which permits the measurement of local current distribution
and local electrochemical impedance spectroscopy. Compared to a conventional interdigitated
geometry, both the developed configurations permit a significant decrease in the pressure drops
without any reduction in battery performance. In the Four Inlets flow field the pressure drop
reduction is more evident (up to 50%) due to the lower electrolyte velocities in the feeding channels,
while the Two Outlets configuration guarantees a more homogeneous current density distribution
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Development of a Vanadium Redox Flow Battery Operating with Thin Membrane Coupled with a Highly Selective and Stable Silica‐Based Barrier Layer
No full textVanadium redox flow battery (VRFB) is a very promising solution for large-scale energy storage, but some technical issues need to be addressed. Crossover, i.e., the undesired permeation of vanadium ions through the cell separator, causes capacity loss and self-discharge. Low-cost and highly selective separators are thus required to improve the competitiveness of this technology. This work investigates the use of silica nanoparticles in an innovative selective layer to improve membrane selectivity and reduce its thickness. 1.5 mu m thick barrier layers composed of 1100EW Nafion ionomer with silica (approximate to 3-13 nm diameter) and Vulcan XC-72R (approximate to 40 nm) nanoparticles in different proportions are directly deposited on 50 mu m thick Nafion membranes. The barrier layer composed only of silica nanoparticles reduces the self-discharge due to crossover by 5 times and increases the average efficiency of the battery. Finally, during more than 1000 h of operation, the barrier layer on a 25 mu m Nafion membrane demonstrates excellent stability, working with a constant coulombic efficiency higher than 99% and a capacity decay rate comparable with a thicker Nafion membrane, thus enabling the use of thinner membranes in VRFB, allowing an estimated 8% stack costs reduction with respect to NR212.Using silica nanoparticles in the barrier layer allows for the improvement of its selectivity, reducing the self-discharge of a vanadium redox flow battery by 5 times compared to the supporting membrane, without affecting efficiency. Due to its high selectivity and stability, the barrier can also be used coupled with a thin membrane, enhancing energy efficiency and reducing battery costs.image (c) 2024 WILEY-VCH Gmb
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