33 research outputs found
LITHIUM SALTS OF FLUORINATED BORATE ESTERS FOR LITHIUM-ION BATTERIES
Lithium salts with fluorinated chelated orthoborate anions are prepared and used as electrolytes or electrolyte additives in lithium-ion batteries. The lithium salts have two chelate rings formed by the coordination of two bidentate ligands to a single boron atom. In addition, each chelate ring has two oxygen atoms bonded to one boron atom, methylene groups bonded to the two oxygen atoms, and one or more fluorinated carbon atoms bonded to and forming a cyclic bridge between the methylene groups
Susceptibility functions for slow relaxation processes in supercooled liquids and the search for universal relaxation patterns
In order to describe the slow response of a glass former we discuss some
distribution of correlation times, e.g., the generalized gamma distribution
(GG) and an extension thereof (GGE), the latter allowing to reproduce a simple
peak susceptibility such as of Cole-Davidson type as well as a susceptibility
exhibiting an additional high frequency power law contribution (excess wing).
Applying the GGE distribution to the dielectric spectra of glass formers
exhibiting no beta-process peak (glycerol, propylene carbonate and picoline) we
are able to reproduce the salient features of the slow response (1e-6 Hz - 1e9
Hz). A line shape analysis is carried out either in the time or frequency
domain and in both cases an excess wing can be identified. The latter evolves
in a universal way while cooling and shows up for correlation times tau_alpha >
1e-8 s. It appears that its first emergence marks the break down of the high
temperature scenario of mode coupling theory. - In order to describe a glass
former exhibiting a beta-process peak we have introduced a distribution
function which is compatible with assuming a thermally activated process in
contrast to some commonly used fit functions. Together with the GGE
distribution this function allows in the frame of the Williams-Watts approach
to completely interpolate the spectra, e.g. of fluoro aniline (1e-6 Hz - 1e9
Hz). The parameters obtained indicate an emergence of both the excess wing and
the beta-process again at tau_alpha > 1e-8s.Comment: 22 pages, 12 figure
The effect of ionic aggregates on the transport of charged species in lithium electrolyte solutions
In this investigation we focus on the problem of modelling the transport of the charged species (lithium ions) in electrolyte solutions with moderate and high salt concentrations (0.1M to >2M), and consider the Nernst-Planck equation as a model of such processes. First, using a combination of magnetic resonance imaging (MRI) and inverse modelling (IM) we demonstrate that at higher concentrations the Nernst- Planck equation requires negative transference numbers in order to accurately describe the concentration profiles obtained from experiments. The need for such a physically inconsistent constitutive relation indicates the loss of validity of the Nernst-Planck equation as a model for this process. Next we consider the formation of ion pairs and clusters as a possible effect responsible for the appearance of negative transference numbers and derive an extended version of the Nernst-Planck system which accounts for these additional species. However, a careful analysis of this model reveals that incorporation of ion-pairing effects into the modelling will not change the transference numbers inferred from the experimental data via inverse modelling. This demonstrates that physical effects other than formation of ion pairs and clusters must be incorporated into the Nernst-Planck model in order for it to correctly describe ion transport at higher salt concentrations. One prime candidate for such effects is the motion of the reaction surface resulting from dendrite growth
Critical Decay at Higher-Order Glass-Transition Singularities
Within the mode-coupling theory for the evolution of structural relaxation in
glass-forming systems, it is shown that the correlation functions for density
fluctuations for states at A_3- and A_4-glass-transition singularities can be
presented as an asymptotic series in increasing inverse powers of the logarithm
of the time t: , where
with p_n denoting some polynomial and x=ln (t/t_0). The results are
demonstrated for schematic models describing the system by solely one or two
correlators and also for a colloid model with a square-well-interaction
potential.Comment: 26 pages, 7 figures, Proceedings of "Structural Arrest Transitions in
Colloidal Systems with Short-Range Attractions", Messina, Italy, December
2003 (submitted
Dynamics in Colloidal Liquids near a Crossing of Glass- and Gel-Transition Lines
Within the mode-coupling theory for ideal glass-transitions, the mean-squared
displacement and the correlation function for density fluctuations are
evaluated for a colloidal liquid of particles interacting with a square-well
potential for states near the crossing of the line for transitions to a gel
with the line for transitions to a glass. It is demonstrated how the dynamics
is ruled by the interplay of the mechanisms of arrest due to hard-core
repulsion and due to attraction-induced bond formation as well as by a nearby
higher-order glass-transition singularity. Application of the universal
relaxation laws for the slow dynamics near glass-transition singularities
explains the qualitative features of the calculated time dependence of the
mean-squared displacement, which are in accord with the findings obtained in
molecular-dynamics simulation studies by Zaccarelli et. al [Phys. Rev. E 66,
041402 (2002)]. Correlation functions found by photon-correlation spectroscopy
in a micellar system by Mallamace et. al [Phys. Rev. Lett. 84, 5431 2000)] can
be interpreted qualitatively as a crossover from gel to glass dynamics.Comment: 13 pages, 12 figure
Metastable Dynamics above the Glass Transition
The element of metastability is incorporated in the fluctuating nonlinear
hydrodynamic description of the mode coupling theory (MCT) of the liquid-glass
transition. This is achieved through the introduction of the defect density
variable into the set of slow variables with the mass density and
the momentum density . As a first approximation, we consider the case
where motions associated with are much slower than those associated with
. Self-consistently, assuming one is near a critical surface in the MCT
sense, we find that the observed slowing down of the dynamics corresponds to a
certain limit of a very shallow metastable well and a weak coupling between
and . The metastability parameters as well as the exponents
describing the observed sequence of time relaxations are given as smooth
functions of the temperature without any evidence for a special temperature. We
then investigate the case where the defect dynamics is included. We find that
the slowing down of the dynamics corresponds to the system arranging itself
such that the kinetic coefficient governing the diffusion of the
defects approaches from above a small temperature-dependent value .Comment: 38 pages, 14 figures (6 figs. are included as a uuencoded tar-
compressed file. The rest is available upon request.), RevTEX3.0+eps
Non-Equilibrium Thermodynamic Description of the Coupling between Structural and Entropic Modes in Supercooled Liquids
The density response of supercooled glycerol to an impulsive stimulated
thermal grating (q=0.63 micron^-1) has been studied in the temperature range
(T=200-340 K) where the structure rearrangement (alpha-relaxation) and thermal
diffusion occur on the same time scale. A strong interaction between the two
modes occurs giving rise to a dip in the T-dependence of the apparent thermal
conductivity and a flattening of the apparent alpha-relaxation time upon
cooling. A non-equilibrium thermodynamic (NET) model for the long time response
of relaxing fluids has been developed. The model is capable to reproduce the
experimental data and to explain the observed phenomenology.Comment: to be published in PRE Rapid Commu
Nonlinear Hydrodynamics of a Hard Sphere Fluid Near the Glass Transition
We conduct a numerical study of the dynamic behavior of a dense hard sphere
fluid by deriving and integrating a set of Langevin equations. The statics of
the system is described by a free energy functional of the
Ramakrishnan-Yussouff form. We find that the system exhibits glassy behavior as
evidenced through stretched exponential decay and two-stage relaxation of the
density correlation function. The characteristic times grow with increasing
density according to the Vogel-Fulcher law. The wavenumber dependence of the
kinetics is extensively explored. The connection of our results with
experiment, mode coupling theory, and molecular dynamics results is discussed.Comment: 34 Pages, Plain TeX, 12 PostScript Figures (not included, available
on request
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Degradation aspects of water formation and transport in Proton Exchange Membrane Fuel Cell: A review
This review paper summarises the key aspects of Proton Exchange Membrane Fuel Cell (PEMFC) degradation that are associated with water formation, retention, accumulation, and transport mechanisms within the cell. Issues related to loss of active surface area of the catalyst, ionomer dissolution, membrane swelling, ice formation, corrosion, and contamination are also addressed and discussed. The impact of each of these water mechanisms on cell performance and durability was found to be different and to vary according to the design of the cell and its operating conditions. For example, the presence of liquid water within Membrane Electrode Assembly (MEA), as a result of water accumulation, can be detrimental if the operating temperature of the cell drops to sub-freezing. The volume expansion of liquid water due to ice formation can damage the morphology of different parts of the cell and may shorten its life-time. This can be more serious, for example, during the water transport mechanism where migration of Pt particles from the catalyst may take place after detachment from the carbon support. Furthermore, the effect of transport mechanism could be augmented if humid reactant gases containing impurities poison the membrane, leading to the same outcome as water retention or accumulation.
Overall, the impact of water mechanisms can be classified as aging or catastrophic. Aging has a long-term impact over the duration of the PEMFC life-time whereas in the catastrophic mechanism the impact is immediate. The conversion of cell residual water into ice at sub-freezing temperatures by the water retention/ accumulation mechanism and the access of poisoning contaminants through the water transport mechanism are considered to fall into the catastrophic category. The effect of water mechanisms on PEMFC degradation can be reduced or even eliminated by (a) using advanced materials for improving the electrical, chemical and mechanical stability of the cell components against deterioration, and (b) implementing effective strategies for water management in the cell