08 Fakultät Mathematik und Physik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9

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Institute: search.filters.UBSInstitut.Institut für ComputerphysikAuthor: search.filters.author.Breitsprecher, Konrad

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    Efficient algorithms for electrostatic interactions including dielectric contrasts
    (2013) Arnold, Axel; Breitsprecher, Konrad; Fahrenberger, Florian; Kesselheim, Stefan; Lenz, Olaf; Holm, Christian
    Coarse grained models of soft matter are usually combined with implicit solvent models that take the electrostatic polarizability into account via a dielectric background. In biophysical or nanoscale simulations that include water, this constant can vary greatly within the system. Performing molecular dynamics or other simulations that need compute exact electrostatic interactions between charges in those systems is computationally demanding. We review here several algorithms developped by us that perform exactly this task. For planar dielectric surfaces in partial periodic boundary conditions, the arising image charges can be either treated with the MMM2D algorithm in a very efficient and accurate way, or with the ELC term that enables the user to use his favorite 3D periodic Coulomb solver . Arbitrarily shaped interfaces can be dealt with using induced surface charges with the ICC algorithm. Finally, the local electrostatics algorithm MEMD (Maxwell Equations Molecular Dynamics) allows even to employ a smoothly varying dielectric constant in the systems. We introduce the concepts of these three algorithms, and an extension for the inclusion of boundaries that are to be held fixed at constant potential (metal conditions). For each method, we present a showcase application to highlight the importance of dielectric interfaces.
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    Simulation studies on electrodes and electrolytes for electric double layer capacitors
    (2018) Breitsprecher, Konrad; Holm, Christian (Prof. Dr.)
    In this dissertation, different capacitor systems are investigated by means of computer simulations. The basic aim of the simulation studies is to contribute towards a better understanding of the interplay of carbon electrodes in various geometries with ionic liquid-based electrolytes. A large focus lies on electrode modeling with an applied external voltage. How these models and underlying methods affect the behavior of electrolyte molecules at the solid-liquid interface as well as the performance of the systems as energy storage devices is a central question of this study.
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    How to speed up ion transport in nanopores
    (2020) Breitsprecher, Konrad; Janssen, Mathijs; Srimuk, Pattarachai; Mehdi, B. Layla; Presser, Volker; Holm, Christian; Kondrat, Svyatoslav
    Electrolyte-filled subnanometre pores exhibit exciting physics and play an increasingly important role in science and technology. In supercapacitors, for instance, ultranarrow pores provide excellent capacitive characteristics. However, ions experience difficulties in entering and leaving such pores, which slows down charging and discharging processes. In an earlier work we showed for a simple model that a slow voltage sweep charges ultranarrow pores quicker than an abrupt voltage step. A slowly applied voltage avoids ionic clogging and co-ion trapping - a problem known to occur when the applied potential is varied too quickly - causing sluggish dynamics. Herein, we verify this finding experimentally. Guided by theoretical considerations, we also develop a non-linear voltage sweep and demonstrate, with molecular dynamics simulations, that it can charge a nanopore even faster than the corresponding optimized linear sweep. For discharging we find, with simulations and in experiments, that if we reverse the applied potential and then sweep it to zero, the pores lose their charge much quicker than they do for a short-circuited discharge over their internal resistance. Our findings open up opportunities to greatly accelerate charging and discharging of subnanometre pores without compromising the capacitive characteristics, improving their importance for energy storage, capacitive deionization, and electrochemical heat harvesting.