arXiv subjects

Norbert Weber

Publications and source records attributed to Norbert Weber.

At least 19 recordsLinked to original sources

arXiv2024

Neutron imaging of high-temperature Na-Zn Cells: implications for cell design and fabrication

Electrochemical cells employing Sodium (Na) and Zinc (Zn) electrodes and a chloride salt electrolyte have been imaged by neutron radiography during cycling. The use of such abundant raw materials confers a very low energy-normalised cost to the Na-Zn system, but its operation requires them to be entirely molten, and therefore to be operated at 600 {\deg}C. To suppress the self-discharge that results from this all-molten configuration, porous ceramic diaphragms are used to partition the electrolyte and thereby impede the movement of the Zn2+ ions responsible towards the Na electrode. Neutron images reveal large gas bubbles trapped beneath these diaphragms, formed during the cell fabrication process due to the large volume change that accompanies melting/solidifying of the electrolyte. Cycling data confirm that these bubbles interfere with cell operation by substantially increasing ohmic resistance. They indicate the need for either a new diaphragm design, or a cell fabrication process that prevents their formation in the first instance.

cond-mat.mtrl-sci↗

arXiv2024

Yamdb: easily accessible thermophysical properties of liquid metals and molten salts

Yamdb (Yet another materials data base) addresses the need to provide thermophysical properties of liquid metals and molten salts in an easily accessible manner. Mathematical relations describing material properties - usually determined by experiment - are taken from the literature. Equations and their coefficients are stored separately. The former can be implemented in any programming language (Python and Go in this case) and the latter are kept in YAML files together with additional information (source, temperature range, composition, accuracy if available, etc).

cond-mat.mtrl-sci↗

arXiv2023

A continuous multiphase model for liquid metal batteries

Liquid metal batteries (LMBs) are a promising alternative for large-scale stationary energy storage for renewable applications. Using high-abundance electrode materials such as Sodium and Zinc is highly desirable due to their low cost and excellent cell potential. LMBs undergo multiple complex mass transport dynamics and as a result, their operation limits and other critical parameters are not fully understood yet. In this work, a multiphase numerical model was developed to resolve electrode and electrolyte components in 1D and simulate the discharge process of a Na-Zn battery including the interfacial displacement of the molten metal electrodes. The variation in electrolyte composition was predicted throughout the process, including the species distribution and its effect on the cell conductivity and capacity. Volume change and species redistribution were found to be important in predicting the maximum theoretical capacity of the cell when neglecting convective phenomena.

cond-mat.mtrl-sci↗

arXiv2022

Simulation of potential and species distribution in a Li||Bi liquid metal battery using coupled meshes

In this work a 1D finite volume based model using coupled meshes is introduced to capture potential and species distribution throughout the discharge process in a lithium bismuth liquid metal battery while neglecting hydrodynamic effects, focusing on the electrochemical properties of the cell and the mass transport in electrolyte and cathode. Interface reactions in the electrical double layer are considered through the introduction of a discrete jump of the potential modelled as periodic boundary condition to resolve interfacial discontinuities in the cell potential. A balanced-force like approach is implemented to ensure consistent calculation at the interface level. It is found that mass transport and concentration gradients have a significant effect on the cell overpotentials and thus on cell performance and cell voltage. By quantifying overvoltages in the Li Bi cell with a mixed cation electrolyte, it is possible to show that diffusion and migration current density could have counteractive effects on the cell voltage. Furthermore, the simulated limiting current density is observed to be much lower than experimentally measured, which can be attributed to convective effects in the electrolyte that need to be addressed in future simulations. The solver is based on the open source library OpenFOAM and thoroughly verified against the equivalent system COMSOL multiphysics and further validated with experimental results. It is openly available at https://doi.org/10.14278/rodare.2313.

physics.chem-ph↗

arXiv2022

Layer coupling between solutal and thermal convection in liquid metal batteries

For longer than one decade, liquid metal batteries (LMBs) are developed with the primary aim to provide economic stationary energy storage. Featuring two liquid metal electrodes separated by a molten salt electrolyte, LMBs operate at elevated temperature as simple concentration cells. Therefore, efficient mass transfer is a basic prerequisite for their economic operation. Understanding these mechanisms cannot be limited at the single layer level. With this motivation, the effects of solutal- and thermally-driven flow are studied, as well as the flow coupling between the three liquid layers of the cell. It is shown that solutal convection appears first and thermal convection much later. While the presence of solutal flow depends on the mode of operation (charge or discharge), the occurrence of thermal convection is dictated by the geometry (thickness of layers). The coupling of the flow phenomena between the layers is intriguing: while thermal convection is confined to its area of origin, i.e. the electrolyte, solutal convection is able to drive flow in the positive electrode and in the electrolyte.

physics.flu-dyn↗

arXiv2022

Transient behaviour of electrovortex flow in a cylindrical container

This study is a continuation of the combined experimental and numerical investigation [1] of the flow of the eutectic GaInSn alloy inside a cylindrical vessel exposed to a constant electrical current. The emerging electrovortex flow driven by the interaction of the current, which is applied through a tapered electrode, with its own magnetic field might have both detrimental and advantageous effects in liquid metal batteries. While the former work [1] was mainly concerned with time-averaged results, this paper focuses on the transient behaviour of the electrovortex flow which becomes most relevant under the influence of an external axial field. The additional Lorentz force generated by the interaction of the imposed current with the vertical component of the geomagnetic field bz drives the ordinary electrovortex jet flow into a swirling motion. Velocity distributions and motion characteristics, such as spiral streamlines and shortened and irregularly swinging jet regions, are investigated. The mechanism is analysed in detail for bz = 25.5 {\mu}T. The maximum angular velocity of the rotating jet is basically linearly dependent on bz, at least for the values studied here. A good agreement between the transient simulation and the experimental result is shown.

physics.flu-dyn↗

arXiv2022

Cell voltage model for Li-Bi liquid metal batteries

Lithium-bismuth bimetallic cells are amongst the best explored liquid metal batteries. A simple and fast quasi-one-dimensional cell voltage model for such devices is presented. The equilibrium cell potential is obtained from a complex two-dimensional fit of data drawn from multiple studies of equilibrium cell potential and rendered congruent with the phase diagram. Likewise, several analytical and fit functions for the ohmic potential drop across the electrolyte are provided for different battery geometries. Mass transport overpotentials originating from the alloying of Li into Bi are modelled by solving a diffusion equation, either analytically or numerically, and accounting for the volume change of the positive electrode. The applicability and limitations of the model are finally illustrated in three distinct experimental settings.

cond-mat.mtrl-sci↗

arXiv2021

Self-discharge mitigation in a liquid metal displacement battery

Recently, a disruptive idea was reported about the discovery of a new type of battery named Liquid Displacement Battery (LDB) comprising liquid metal electrodes and molten salt electrolyte. This cell featured a novel concept of a porous electronically conductive faradaic membrane instead of the traditional ion-selective ceramic membrane. LDBs are attractive for stationary storage applications but need mitigation against self-discharge. In the instant battery chemistry, Li|LiCl-PbCl$_2$|Pb, reducing the diffusion coefficient of lead ions can be a way forward and a solution can be the addition of PbO to the electrolyte. The latter acts as a supplementary barrier and complements the function of the faradaic membrane. The remedial actions improved the cell's coulombic efficiency from 92% to 97% without affecting the voltage efficiency. In addition, the limiting current density of a 500 mAh cell increased from 575 to 831 mA cm$^{-2}$ and the limiting power from 2.53 to 3.66 W. Finally, the effect of PbO on the impedance and polarization of the cell was also studied.

physics.chem-ph↗

arXiv2021

Anode-metal drop formation and detachment mechanisms in liquid metal batteries

We study numerically localised short circuits in Li||Bi liquid metal batteries. In the prototype of a classical, three liquid-layer system, we assume a perceptible local deformation of the Li-salt interface. We find that there always exists a critical current at which a Li-droplet is cut off from this hump, and transferred to the Bi-phase. In a second case, we assume that the molten Li is contained in a metal foam, and that a small Li-droplet emerges below this foam due to insufficient wetting. This droplet is deformed by Lorentz forces, until eventually being pinched off. Here, the critical current is slightly lower than in the three-layer system, and both, a droplet transfer and complete short circuits are observed. Finally, we discuss the relevance of our simulations for experimentally observed short circuits and non-faradaic Li-transfer.

physics.flu-dyn↗

arXiv2021

Conductivity influence on interfacial waves in liquid metal batteries and related two-layer systems

Fluid flows in liquid metal batteries can be generated by a number of effects. We start with a short overview of different driving mechanisms and then address questions specific to the metal pad role instabilities in three-layer systems. We focus on the role of the conductivity distribution in the cell, noting at the same time that interfacial tension should be considered as well for smaller cells. Following this discussion, numerical results on the excitation of interfacial waves in two-layer liquid metal systems with miscibility gaps bearing an interface normal electric current are presented. Confirming recent results from the literature, we find that magnetic damping plays a decisive role for strong vertical magnetic fields. In addition, boundary conditions for the electric field strongly influence critical currents and growth rates.

physics.flu-dyn↗

arXiv2021

Effects of current distribution on mass transport in the positive electrode of a liquid metal battery

Liquid metal electrodes are one of the key components of different electrical energy storage technologies. The understanding of transport phenomena in liquid electrodes is mandatory in order to ensure efficient operation. In the present study we focus our attention on the positive electrode of the Li||Bi liquid metal battery. Starting from a real experimental setup, we numerically investigate the charge transfer in a molten salt electrolyte and the mass transport in the positive electrode. The two phenomena are tightly coupled, because the current distribution influences the concentration field in the positive electrode. The cell is studied during charging when compositional convection becomes apparent. First results of compositional convection from a non-uniform current distribution are presented, highlighting its capability to affect the flow in the positive electrode and the cell performance.

physics.flu-dyn↗

arXiv2020

Thermal neutron radiography of a passive proton exchange membrane fuel cell for portable hydrogen energy systems

A proton exchange membrane fuel cell (PEMFC) for low power and portable applications is studied with thermal neutron radiography. The PEMFC operates under full passive conditions, with an air-breathing cathode and a dead-end anode supplied with static ambient air and dry hydrogen, respectively. A columnar cathodic plate favors the mobility of water drops over the cathode surface and their elimination. Thermal neutron images show liquid water build up during operation with the cell in vertical and horizontal positions, i.e. with its main plane aligned parallel and perpendicular to the gravity field, respectively. Polarization curves and impedance spectroscopy show cell orientation dependent response that can be related with the water accumulation profiles. In vertical position, lower water contents in the cathode electrode is favored by the elimination of water drops rolling over the cathode surface and the onset of natural convection. As a consequence, oxygen transport is improved in the vertical cell, that can be operated full passive for hours under ambient conditions, providing steady peak power densities above 100 mW/cm2. Gravity and natural convection are less effective in horizontal position, leading to a 17% decrease in peak power density due to oxygen transport losses. The horizontal position is especially adverse if the upper electrode is the cathode, because of anode flooding causing cell failure after production of a small amount of water (5 mg/cm2). The combined information from thermal neutron radiography and cell response characteristics explains the important influence of cell orientation on the performance of a fully passive air-breathing PEMFC for portable applications.

physics.app-ph↗

arXiv2019

Modeling discontinuous potential distributions using the finite volume method, and application to liquid metal batteries

The electrical potential in a battery jumps at each electrode-electrolyte interface. We present a model for computing three-dimensional current and potential distributions, which accounts for such internal voltage jumps. Within the framework of the finite volume method we discretize the Laplace and gradient operators such that they account for internal jump boundary conditions. After implementing a simple battery model in OpenFOAM we validate it using an analytical test case, and show its capabilities by simulating the current distribution and discharge curve of a Li||Bi liquid metal battery.

physics.app-ph↗

arXiv2018

Thermally driven convection in Li||Bi liquid metal batteries

Liquid Metal Batteries (LMBs) are a promising concept for cheap electrical energy storage at grid level. These are built as a stable density stratification of three liquid layers, with two liquid metals separated by a molten salt. In order to ensure a safe and efficient operation, the understanding of transport phenomena in LMBs is essential. With this motivation we study thermal convection induced by internal heat generation. We consider the electrochemical nature of the cell in order to define the heat balance and the operating parameters. Moreover we develop a simple 1D heat conduction model as wellas a fully 3D thermo-fluid dynamics model. The latter is implemented in the CFD library OpenFOAM, extending the volume of fluid solver, and validated against a pseudo-spectral code. Both models are used to study a rectangular 10x10 cm Li||Bi LMB cell at three different states of charge.

physics.flu-dyn↗

arXiv2018

Electromagnetically driven convection suitable for mass transfer enhancement in liquid metal batteries

Liquid metal batteries (LMBs) were recently proposed as cheap large scale energy storage. Such devices are urgently required for balancing highly fluctuating renewable energy sources. During discharge, intermetallic phases tend to form in the cathode of LMBs. These do not only limit the up-scalability, but also the efficiency of the cells. Generating a mild fluid flow in the fully liquid cell will smoothen concentration gradients and minimise the formation of intermetallics. In this context we study electro-vortex flow numerically. We simulate a recent LMB related experiment and discuss how the feeding lines to the cell can be optimised to enhance mass transfer. The Lorentz forces have to overcome the stable thermal stratification in the cathode of the cell; we show that thermal effects may reduce electro-vortex flow velocities considerable. Finally, we study the influence of the Earth magnetic field on the flow.

physics.app-ph↗

arXiv2017

Competing forces in liquid metal electrodes and batteries

Liquid metal batteries are proposed for low-cost grid scale energy storage. During their operation, solid intermetallic phases often form in the cathode and are known to limit the capacity of the cell. Fluid flow in the liquid electrodes can enhance mass transfer and reduce the formation of localized intermetallics, and fluid flow can be promoted by careful choice of the locations and topology of a battery's electrical connections. In this context we study four phenomena that drive flow: Rayleigh-B\'enard convection, internally heated convection, electro-vortex flow, and swirl flow, in both experiment and simulation. In experiments, we use ultrasound Doppler velocimetry (UDV) to measure the flow in a eutectic PbBi electrode at 160{\deg}C and subject to all four phenomena. In numerical simulations, we isolate the phenomena and simulate each separately using OpenFOAM. Comparing simulated velocities to experiments via a UDV beam model, we find that all four phenomena can enhance mass transfer in LMBs. We explain the flow direction, describe how the phenomena interact, and propose dimensionless numbers for estimating their mutual relevance. A brief discussion of electrical connections summarizes the engineering implications of our work.

physics.flu-dyn↗

arXiv2017

Steady state detection for computational fluid dynamics

Large parameter studies of fluid dynamic instabilities can crucially be simplified, if the user does not need to specify the simulation time. For this purpose, a steady state detection is implemented in the CFD library OpenFOAM. It terminates simulations automatically if characteristic measurements do not change over time, i.e. when saturation is reached. For that purpose, recent data is compared to previous one using two selectable methods. Both calculate a value used as steady state indicator. The first method performing a two sample Student's t-test examines the difference between the means. Similarly, the second method utilises a two sample f-test checking for changes between the variances. Both methods are briefly described, compared and their specific area of application is discussed. Their usefulness is demonstrated with a simple exemplary test case.

physics.flu-dyn↗

arXiv2017

Coupling and stability of interfacial waves in liquid metal batteries

We investigate the interfacial wave coupling dynamics in liquid metal batteries and their effects to the battery's operation safety. Similar to aluminum reduction cells, liquid metal batteries can be highly susceptible to magnetohydrodynamical instabilities that excite undesired interfacial waves capable to provoke short-circuits. However, in liquid metal batteries the wave dynamics is far more complex since two metal-electrolyte interfaces are present that may step into resonance. In the first part of this paper, we present a Potential analysis of coupled gravity-capillary interfacial waves in a three-layer battery model of cylindrical shape. Analytical expressions for the amplitude ratio and the wave frequencies are derived and it is shown that the wave coupling can be completely described by two independent dimensionless parameters. We provide a decoupling criterion clarifying that wave coupling will be present in most future liquid metal batteries. In the second part, the theory is validated by comparing it with multiphase direct numerical simulations. An accompanying parameter study is conducted to analyze the system stability for differently strongly coupled interfaces. Three different coupling regimes are identified involving characteristic coupling dynamics. For strongly coupled interfaces we observe novel instabilities that may have beneficial effects on the operational safety.

physics.flu-dyn↗