The stability of vertically falling viscous liquid jets issuing from a circular orifice with a low fluid velocity where the flow is dominated by viscous effects torn down by gravity leading to a flow dominated by inertia far downstream, is studied by means of linear stability analysis as well as direct numerical simulation of the one-dimensional model. Jet stability is also studied experimentally. Results for marginal stability and critical frequencies are in excellent agreement with our theoretical predictions. A new global instability of a viscous jet is found. This instability is observable whenever a falling liquid jet exhibits a long enough viscosity dominated region. The source of instability is surface tension. In contrast to the Rayleigh capillary instability that is always present, but leads to very long disintegration lengths in highly viscous fluid jets, the instability described here only occurs when the volume flux falls below a certain limit depending on the fluid properties and the nozzle diameter.

Electrochemical impedance spectroscopy (EIS) is a powerful technique to study interface kinetics in lithium ion batteries. In order to separate the contributions of the working and counter electrode, a reference electrode (RE) is crucial. However, size and shape of the RE strongly influence the quality of the measurement data. In this study we define two criteria that enable the theoretical quality assessment of a wire-shaped RE as a function of its diameter. With help of a simplified Newman-type battery model we show that small wire diameters are essential in order to obtain meaningful EIS data, in particular when using polymer electrolytes, which have comparably low ionic conductivity. Therefore, a gold plated tungsten wire with 10 μm diameter is chosen as preferred RE. It is placed between two layers of electrolyte and lithiated before use to ensure a long-term stable reference potential. Using this setup, artifact-free EIS spectra are obtained for lithium/lithium and lithium/LFP cells with polymer electrolyte. Finally, two applications are shown for which a RE is indispensable: (I) the symmetry of anodic and cathodic lithium dissolution and deposition kinetics is characterized; (II) the growth of the anode and cathode impedance of a lithium/LFP cell during cycling is monitored.

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In this work a mathematical model for describing the performance of lithium-ion battery electrodes consisting of porous active material particles is presented. The model represents an extension of the Newman-type model, accounting for the agglomerate structure of the active material particles, here Li(Ni1/3Co1/3Mn1/3)O2 (NCM) and Li(Ni1/3Co1/3Al1/3)O2 (NCA). To this goal, an additional pore space is introduced on the active material level. The space is filled with electrolyte and a charge-transfer reaction takes place at the liquid-solid interface within the porous active material particles. Volume-averaging techniques are used to derive the model equations. A local Thiele modulus is defined and provides an insight into the potentially limiting factors on the active material level. The introduction of a liquid-phase ion transport within the active material reduces the overall transport losses, while the additional active surface area within the agglomerate lowers the charge-transfer resistance. As a consequence, calculated discharge capacities are higher for particles modeled as agglomerates. This finding is more pronounced in the case of high C-rates.

In this experimental and numerical study we analyze the polarization of the electrodes in a PEM fuel cell system. In order to measure the anode and cathode potential with respect to the electrolyte potential, a dynamic hydrogen electrode (DHE) is used as reference electrode. Two fundamental configurations for integrating a DHE into the membrane electrode assembly (MEA) are investigated: In the first configuration, the DHE is contacted to the membrane within a circular gap in the cathode catalyst layer, which is called edge configuration. In the second configuration, the DHE is placed between two membrane layers, which is called sandwich configuration. Differences between the reference electrode setups can be ascribed to an inhomogeneous electrolyte potential distribution close to the edge of the electrodes as described in literature. Our 2D fuel cell model as published in an earlier article is used to investigate this effect. We show that it is not possible to detect the electrode polarization by use of a reference electrode in edge configuration. By use of the sandwich configuration, the polarization of the anode and cathode half cells can be measured independently.