1,720,988 research outputs found
Toward symmetric organic aqueous flow batteries: triarylamine‐based bipolar molecules and their characterization via an extended Koutecký‐Levich analysis
No full textSymmetric organic flow batteries (SOFBs) can potentially address membrane crossover problems by employing bipolar redox-active organic molecules (BROMs). Herein, a triarylamine (TAA) skeleton was chosen as a posolyte moiety for a new class of bipolar molecules for pH-neutral aqueous FBs. Pyridinium and viologen derivatives were tethered to the posolyte moiety, and the new compounds were characterized. Cyclic voltammetry revealed that only viologen with a highly hydrophilic substituent, connected to the TAA moiety via a Zincke reaction, could be reversibly reduced. Varying the supporting electrolyte concentration on the selected derivative revealed water solubility as a challenge for further development. The selected derivative, MeO-TPA-Vi-DMAE, was subjected to hydrodynamic voltammetry, and a modified Koutecký-Levich analysis was developed to investigate the observed potential-dependent currents at the hydrodynamically dominated region, which are often seen with redox-active organic molecules. This model discarded a purely Ohmic effect, showing a useful Levich slope at a certain overpotential before the onset of a secondary reaction. TAA-based BROMs hold promise for pH-neutral aqueous SOFBs, and the results will guide the design of new derivatives. The three-term Koutecký-Levich relation here introduced will be useful not only to develop BROM-based flow batteries but will most likely appeal to a much broader audience
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
Theorie der Zyklischen Voltammetrie an Makroporösen Elektroden
No full textInvestigating the electrokinetic performance of novel electrode materials by means of diffusional cyclic voltammetry has emerged to the standard approach in electrochemistry. The straightforward implementation of the method in a three-electrode compartment provides scientists with a feasible ex-situ technique for assessing reaction kinetics in terms of potential-dependent redox currents. Providing that well-defined diffusion conditions are complied, i.e. the experiments are conducted at planar electrodes in a semi-infinite diffusion domain, characteristic features such as the separation, symmetry and magnitude of the redox peaks can be related unambiguously to the electrode kinetics. However, as soon as non-planar electrodes or electrodes with finite diffusion domains are employed an equivocation between the measured redox peaks and the intrinsic electrode kinetics emerges. Consequently, a quantitative interpretation of cyclic voltammetry data becomes exceptionally arduous. In particular porous structures like felts and foams, predominantly utilized as electrode materials in the field of battery research, exhibit an intimidating ambiguity of the polarographic current signal. Therefore, the majority of experimentalists restrict themself to a qualitative interpretation of cyclic voltammetry data in terms of arbitrarily chosen onset-potentials. Scientists who are still targeting to quantify the electrode kinetics usually aim to exploit alternative techniques such as electrochemical impedance spectroscopy. However,
from a theoretical perspective this approach is not capable of solving the dilemma either since the experiments are subjected to the same diffusion complication, examined with a different potential perturbation only. Consequently, developing a theory of cyclic voltammetry for porous electrodes is inevitable to permit a quantitative analysis of experimental results. This thesis consists of the cumulative work on the theory of cyclic voltammetry at macroporous electrodes with emphasis on felt-like structures. It is demonstrated that linking the high sensitivity of cyclic voltammetry with a sophisticated mathematical diffusion model allows for an electrochemical and morphological characterization of porous electrodes simultaneously, promoting the so-called „electrochemists spectroscopy “ to the next level. All theoretical concepts are supported by experimental data acquired for the electrochemical redox-reactions of vanadium(II)/ vanadium(III) and oxovanadium(IV)/ dioxovanadium(V), relevant in the field of vanadium redox-flow battery research. In a first approximation, porous electrodes are treated as random arrays of microelectrodes in a finite diffusion space with a statistically fluctuating size. A systematic investigation of simulated and experimentally acquired cyclic voltammetry data for both, porous and non-porous electrodes, draws an enlightening picture on the complex interplay of electrode porosity and reaction kinetics. With this knowledge, precise values for the heterogeneous rate constant of the oxovanadium(IV)/ dioxovanadium(V) redox reaction are obtained. These values usually scatter over orders of magnitude in the recent literature, most likely due to an inconsequent interpretation of data. In another study, a strategy for real-space simulation of cyclic voltammetry at carbon felt electrodes is presented. For this purpose, in-situ micro X-ray computed tomography is exploited to construct a template of the three-dimensional diffusion domain inside a porous electrode. This renders any statistical assumptions obsolete. To perform the simulations, two self-reliant computational methods, namely digital simulation and convolutive modeling, are combined. The resulting method offers significant advantages with respect to computation time, programming effort and mathematical complexity. Since effects of electrochemical double-layer charging, nonlinear contributions of ohmic resistances, coupled chemical reactions and limited electron transfer kinetics can be accounted for readily, the novel approach covers an extraordinarily wide range of electrochemical situations. The exceptional endowment of simulating polarographic experiments at porous electrodes was finally implemented into an open source program named „Polarographica “. This software provides the experimentalists community with a straightforward way of interpreting cyclic voltammetry data of porous electrodes in terms of a fitting routine. Since many other electroanalytical techniques are supported in the environment of Polarographica as well, it will eventually lead to a more decent interpretation of cyclic voltammetry data, based on mathematical models instead of ambiguous current peaks and arbitrarily chosen onset-potentials
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
Simulating diffusional cyclic voltammetry at rough electrodes by means of the DSDC algorithm
No full textElectrode roughness is an exceptionally important, yet oftentimes overlooked factor in electrokinetic analysis and catalyst research. Essentially any electroanalytical experiment involving an electrode with a surface which is not perfectly smooth will be subjected to more or less severe effects of roughness. This includes any circumstance where electrodes are decorated with micro- or nanoparticles and is therefore relevant for a plethora of experimental scenarios. Since the roughness-related gain in surface area scales the reaction rate – and thus the current – it is an inherently non-straightforward task to assess intrinsic electrode kinetics from experimental data of rough structures. In other words, a significant overestimation of kinetic constants may arise when rough electrodes are employed and kinetics are analyzed under the assumption of perfectly planar surfaces. For this reason, it is fundamentally interesting to approach electrode roughness from a purely theoretical point of view, since surface roughness and kinetics can be tuned in a deliberate manner for separating their individual contributions.
In this study, we propose a strategy for the simulation of the most common electroanalytical technique – cyclic voltammetry – at mesoscopically rough surfaces. For this purpose, our previously introduced digital simulation - deconvolution - convolution (DSDC) algorithm [1] is employed. By modifying, the Douglas--Gunn algorithm, which is used in the digital simulation step to operate on an arbitrarily incremented spatial grid, a high numerical resolution near the electrode/electrolyte boundary and a larger spatial discretization in the bulk of the electrolyte can be used. This provides a fine resolution of artifacts of surface roughness and efficiently utilizes computational power. As a consequence, simulations can be performed on real-space data of rough electrodes which is obtained from atomic force microscopy (AFM). It is demonstrated that for an ideally reversible reaction the effects of electrode roughness on the CV response are insignificant. In contrast, for practically relevant scenarios with electrochemically irreversible or quasi-reversible kinetics, the apparent rate constants are allegedly upscaled by the area-ratio Arough/Aplanar up to a certain threshold. Qualitatively, this manifests in a lower peak-to-peak separation without a distortion of the shape of the voltammetric profile. As soon as the surface profile becomes much deeper than wide, a distortion of the CV response occurs which is associated with an increasingly finite diffusion domain. This behavior is finally explained in terms of convolution-sums and mass-transfer functions and provides a quantitative interpretation of roughness effects
Frühwarnsystem für EV-Batterien: Prädiktion des Thermischen Durchgehens von Lithium-Ionen-Speichern
No full textKritisch defekte Lithium-Ionen Batterien bergen ein signifikantes Gefährdungspotenzial, da die Möglichkeit eines so genannten thermischen Durchgehens (TD) besteht. Dabei handelt es sich um eine autoamplifizierende Reaktion, welche zu einer unkontrollierten Erwärmung mit Rauch- und Flammenerscheinung oder gar einer Explosion führt. Insbesondere bei großen Batterien mit einer Kapazität von über 100 kWh, stellt ein TD ein sehr großes Schadensereignis dar. Folglich gilt es, dieses Ereignis strikt zu vermeiden. Im Rahmen der Präsentation werden aktuelle Arbeiten der BAM im Bereich der TD-Früherkennung vorgestellt
Analytical solutions for the impedance of symmetrically distributed relaxation processes
No full textDistribution of relaxation times (DRT) analysis of impedance data is a powerful tool for unravelling entangled relaxation processes. However, reconstructing the DRT function from experimental data is an ill-posed inversion problem which suffers from noisy input and, hence, assigns a pseudo-signal to any kind of noise. To circumvent the resulting overfitting, different methods have been proposed, which are usually based on regularization.
Alternatively, carefully selected distributed basis functions (e.g. Gauss-type, Cauchy-type, Matern-type) can be included in the algorithm of DRT reconstruction. This concept originates from the work of Cuicci and coworkers and has been implemented in their great software package DRT-tools. While this works exceptionally well for minimizing the demand on regularization, it must be kept in mind that the impedance of real-world relaxation processes of the constant phase element type is not represented by any of the resulting DRT functions.
Bridging this gap was therefore the motivation for the present study. We derived analytical solutions for the impedance of several different types of distribution functions. The resulting impedances are compared with classical relaxation models and special features are pinpointed. The choice of an optimized basis function for reconstructing the DRT from experimentally acquired impedance data will improve electrochemical analysis in future work. Additionally, analytical solutions may be used for assigning a physical origin to the distributed behaviour in the impedance of certain types of electrodes
Electroanalytical experiments at macroporous electrodes for redox-flow batteries
No full textMacroporous carbon felts (CFs) are the prevalently used electrode material for redox-flow batteris. Owing to their large internal surface area, these structures can efficiently drive the Faradaic reactions in a RFB. However, the electrochemical characterization of novel CFs, or modified CF-materials is a cumbersome task. This is essentially related to the complex mass-transport phenomena which are convoluted with the electrode kinetics. This talk will focus on ex-situ cyclic voltammetry and operando electrochemical impedance spectroscopy as electroanalytical techniques for porous RFB electrodes. Common pitfalls in the voltammetric analysis of CFs are elucidated and recent strategies for modeling and evaluating CV experiments on porous structures are introduced. Finally, a method based on electrochemical impedance spectroscopy is introduced which allows for an operando fingerprinting of electrochemical reactions in a vanadium-based RFB
- …
