71 research outputs found
Universal Algorithm for Simulating and Evaluating Cyclic Voltammetry at Macroporous Electrodes by Considering Random Arrays of Microelectrodes
An algorithm for the simulation and evaluation of cyclic voltammetry (CV) at macroporous electrodes such as felts, foams, and layered structures is presented. By considering 1D, 2D, and 3D arrays of electrode sheets, cylindrical microelectrodes, hollowâcylindrical microelectrodes, and hollowspherical microelectrodes the internal diffusion domains of the macroporous structures are approximated. A universal algorithm providing the timedependent surface concentrations of the electrochemically active species, required for simulating cyclic voltammetry responses of the individual planar, cylindrical, and spherical microelectrodes, is presented as well. An essential ingredient of the algorithm, which is based on Laplace integral transformation techniques, is the use of a modified Talbot contour for the inverse Laplace transformation. It is demonstrated that firstâorder homogeneous chemical kinetics preceding and/or following the electrochemical reaction and electrochemically active species with nonâequal diffusion coefficients can be included in all diffusion models as well. The proposed theory is supported by experimental data acquired for a reference reaction, the oxidation of [Fe(CN)6]4â at platinum electrodes as well as for a technically relevant reaction, the oxidation of VO2+ at carbon felt electrodes. Based on our calculation strategy, we provide a powerful open source tool for simulating and evaluating CV data implemented into a Python graphical user interface (GUI)
Exact analytic results for the Gutzwiller wave function with finite magnetization
We present analytic results for ground-state properties of Hubbard-type
models in terms of the Gutzwiller variational wave function with non-zero
values of the magnetization m. In dimension D=1 approximation-free evaluations
are made possible by appropriate canonical transformations and an analysis of
Umklapp processes. We calculate the double occupation and the momentum
distribution, as well as its discontinuity at the Fermi surface, for arbitrary
values of the interaction parameter g, density n, and magnetization m. These
quantities determine the expectation value of the one-dimensional Hubbard
Hamiltonian for any symmetric, monotonically increasing dispersion epsilon_k.
In particular for nearest-neighbor hopping and densities away from half filling
the Gutzwiller wave function is found to predict ferromagnetic behavior for
sufficiently large interaction U.Comment: REVTeX 4, 32 pages, 8 figure
Multi-strange baryon production in Au+Au collisions near threshold
The centrality dependence of Xi^- and Lambda production in Au+Au interactions
at E_lab=6 AGeV is studied within a microscopic transport approach. In line
with recent data, a slight enhancement of the Xi^-/(Lambda+Sigma^0) ratio
toward central collisions is found. It is demonstrated that the observed
production of multiple strange baryons can be traced back to multi-step
meson-baryon interactions in the late stages of the collisions. Therefore, the
present analysis supports an interpretation of the observed Xi abundance in
terms of hadronic re-scattering.Comment: 5 pages, 6 figure
Improvement of Oxygen-Depolarized Cathodes in Highly Alkaline Media by Electrospinning of Poly(vinylidene fluoride) Barrier Layers
Oxygenâdepolarized cathodes (ODC) were developed for chlorâalkali electrolysis to replace the hydrogen evolution reaction (HER) by the oxygen reduction reaction (ORR) providing electrical energy savings up to 30â% under industrially relevant conditions. These electrodes consist of micro sized silver grains and polytetrafluoroethylene, forming a homogeneous electrode structure. In this work, we report on the modification of ODCs by implementing an electrospun layer of hydrophobic poly(vinylidene fluoride) (PVDF) into the ODC structure, leading to a significantly enhanced ORR performance. The modified electrodes are physically characterized by liquid flow porometry, contact angle measurements and scanning electron microscopy. Electrochemical characterization is performed by linear sweep voltammetry and chronopotentiometry. The overpotential for ORR at application near conditions could be reduced by up to 75â
mV at 4â
kAâmâ2 and 135â
mV at a higher current density of 9.5â
kAâmâ2. Consequently, we propose that modifying ODCs by electrospinning is an effective and costâefficient way to further reduce the energy demand of the ORR in highly alkaline media
Relaxation of a one-dimensional Mott insulator after an interaction quench
We obtain the exact time evolution for the one-dimensional integrable
fermionic 1/r Hubbard model after a sudden change of its interaction parameter,
starting from either a metallic or a Mott-insulating eigenstate. In all cases
the system relaxes to a new steady state, showing that the presence of the Mott
gap does not inhibit relaxation. The properties of the final state are
described by a generalized Gibbs ensemble. We discuss under which conditions
such ensembles provide the correct statistical description of isolated
integrable systems in general. We find that generalized Gibbs ensembles do
predict the properties of the steady state correctly, provided that the
observables or initial states are sufficiently uncorrelated in terms of the
constants of motion.Comment: 9 pages, 1 figure; published versio
Isosbestic points in the spectral function of correlated electrons
We investigate the properties of the spectral function A(omega,U) of
correlated electrons within the Hubbard model and dynamical mean-field theory.
Curves of A(omega,U) vs. omega for different values of the interaction U are
found to intersect near the band-edges of the non-interacting system. For a
wide range of U the crossing points are located within a sharply confined
region. The precise location of these 'isosbestic points' depends on details of
the non-interacting band structure. Isosbestic points of dynamic quantities
therefore provide valuable insights into microscopic energy scales of
correlated systems.Comment: 16 pages, 5 figure
XâRayâComputed Radiography and Tomography Study of Electrolyte Invasion and Distribution inside Pristine and HeatâTreated Carbon Felts for Redox Flow Batteries
Porous carbon felts (CFs) are widely used electrode materials for vanadium redox flow batteries (VRFBs). These materials differ in their precursor material, thickness, or graphitization degree and demonstrate broad differences in electrochemical performance. Prior to operation, an activation step, such as acid or heat treatment (HT), is commonly performed to improve their performance. A thermal treatment in air functionalizes the surface of the electrode and improves reaction kinetics as well as the wettability of the electrode. Herein, pristine and heatâtreated CFs are compared regarding their electrolyte wetting behavior for the use in VRFB. Contact angle (CA) measurements are conducted ex situ to investigate the effect of the HT. Furthermore, the porous CFs are examined in situ with an inâhouseâbuilt flow cell regarding their invasion behavior with different types of electrolytes by Xâray radiography. Additionally, the distribution of the electrolyte inside the felts is investigated by Xâray tomography. The results demonstrate the effect of the HT and choice of electrolyte on the wetting behavior and electrolyte distribution.DFG, 276655287, FOR 2397: Multiskalen-Analyse komplexer Dreiphasensystem
Tan relations in one dimension
We derive exact relations that connect the universal -decay of the
momentum distribution at large with both thermodynamic properties and
correlation functions of two-component Fermi gases in one dimension with
contact interactions. The relations are analogous to those obtained by Tan in
the three-dimensional case and are derived from an operator product expansion
of the one- and two-particle density matrix. They extend earlier results by
Olshanii and Dunjko [Phys. Rev. Lett. 91, 090401 (2003)] for the bosonic
Lieb-Liniger gas. As an application, we calculate the pair distribution
function at short distances and the dimensionless contact in the limit of
infinite repulsion. The ground state energy approaches a universal constant in
this limit, a behavior that also holds in the three-dimensional case. In both
one and three dimensions, a Stoner instability to a saturated ferromagnet for
repulsive fermions with zero range interactions is ruled out at any finite
coupling.Comment: 8 figures, 27 pages - Updated to status of published versio
Importance of Polaronic Effects for Charge Transport in CdSe Quantum Dot Solids
We developed an accurate model accounting for electron-phonon interaction in colloidal
quantum dot supercrystals that allowed us to identify the nature of charge carriers and the
electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping
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