The two-phase flow at gas-evolving electrodes: Bubble-driven and Lorentz-force-driven convection

Weier, Tom; Landgraf, S.

doi:10.1140/epjst/e2013-01816-1

Cited by 43 publications

(24 citation statements)

References 41 publications

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“…The MHD convecz E-mail: Dominik.Baczyzmalski@unibw.de tion in an electrode-parallel field is characterized by a strong shear flow with high flow velocities close to the electrode surface as shown by measurements of the velocity field in the electrode gap of an alkaline electrolyzer. 21,22 Thus, the enhanced bubble detachment for parallel fields was generally attributed to the effect of the strong MHD convection along the electrode, similar to the findings of other studies investigating the bubble detachment dynamics under forced convection. 23,24 However, the situation for a magnetic field perpendicular to the electrode is unclear.…”

supporting

confidence: 85%

On the Electrolyte Convection around a Hydrogen Bubble Evolving at a Microelectrode under the Influence of a Magnetic Field

Baczyzmalski

Karnbach

Yang

et al. 2016

J. Electrochem. Soc.

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Water electrolysis was carried out in a 1 M H 2 SO 4 solution under different potentiostatic conditions in the presence of a magnetic field oriented normal to the horizontal microelectrode (100 μm in diameter). The imposed magnetohydrodynamic (MHD) electrolyte flow around the evolving hydrogen bubble was studied to clarify the effect on the detachment of the bubble from the electrode and the mass transfer toward the electrode. Different particle imaging and tracking techniques were applied to measure the three-dimensional flow in the bulk of the cell as well as in close vicinity of the evolving bubble. The periodic bubble growth cycle was analyzed by measurements of the current oscillations and microscopic high-speed imaging. In addition, a numerical study of the flow was conducted to support the experimental results. The results demonstrate that the MHD flow imposes only a small stabilizing force on the bubble. However, the observed secondary flow enhances the mass transfer toward the electrode and may reduce the local supersaturation of dissolved hydrogen. Renewable energy technologies have become increasingly important in view of the worldwide rising CO 2 emissions. The growing application of volatile energy sources such as wind and solar power requires efficient energy storage and distribution systems in order to sustain stability in the electrical grid. Hydrogen shows great potential for long-term energy storage as well as mobile units employing fuel cells as it provides a large energy density. Moreover, high purity hydrogen (and oxygen) can be directly generated from the electricity produced by a wind turbine or solar panel by the electrolysis of water. However, one main challenge for the establishment of water electrolysis on an industrial scale is its relatively low efficiency, typically in the order of 60% for conventional alkaline water electrolyzers. 1 A considerable part of the losses is the result of hydrogen and oxygen gas bubbles evolving at, and sticking to the electrodes' surfaces, thus hindering the formation of new gas bubbles. Furthermore, they reduce the effective electrical conductivity of the electrolyte, which leads to an increased ohmic voltage drop. The accelerated removal of gas bubbles from the electrode's surface and the bulk was studied by many researchers to improve the efficiency of the process.2-6 Forced convection has been found to be able to enhance the detachment of bubbles from the electrode and therefore reduce the fractional bubble coverage significantly.7-12 A relatively simple method that has recently attracted considerable research interests in this context is the application of magnetic fields. The superposition of a magnetic field on the inherent electric field gives rise to Lorentz forces f L = j × B acting as body forces directly on the electrolyte, where j denotes the current density and B is the magnetic induction, respectively. 13 The flow generated by these Lorentz forces is often referred to as the magnetohydrodynamic (MHD) effect.It was shown that stirring with...

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supporting

confidence: 85%

On the Electrolyte Convection around a Hydrogen Bubble Evolving at a Microelectrode under the Influence of a Magnetic Field

Baczyzmalski

Karnbach

Yang

et al. 2016

J. Electrochem. Soc.

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“…For instance, a method to reduce the void fraction within the electrode gap is the use of perforated electrode plates, which allows the bubbles to move to the back side of the electrode (Weier and Landgraf 2013). In addition, forced convection has been found to be able to enhance the detachment of bubbles from the electrode (see, e.g., Hine et al 1975;Hine and Murakami 1980;Bongenaar-Schlenter et al 1985;Sillen 1983;Balzer and Vogt 2003;Eigeldinger and Vogt 2000) and thus reduce the fractional bubble coverage on the electrode surface.…”

Section: Introductionmentioning

confidence: 99%

“…In fact, there have been relatively few detailed experimental studies on the fluid dynamics of the two-phase flow at gas-evolving electrodes (Weier and Landgraf 2013). The flow in such a system is usually confined to a discernible layer or curtain of polydispersed gas bubbles adjacent to the electrode.…”

Section: Introductionmentioning

confidence: 99%

“…These bubbles are electrochemically generated at the electrode's surface and adhere to it until they are able to detach depending on their buoyancy and other conditions such as the convection in the surrounding electrolyte (Boissonneau and Byrne 2000). Detached bubbles can continue to grow due to collision and coalescence with other bubbles as they rise in the bubble curtain (Weier and Landgraf 2013). Often, the boundary of the bubble curtain has a wavy shape.…”

Section: Introductionmentioning

confidence: 99%

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Near-wall measurements of the bubble- and Lorentz-force-driven convection at gas-evolving electrodes

et al. 2015

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“…The two-phase flow originating from alkaline water electrolysis at a gas-evolving electrode is studied by Weier and Landgraf [33] by a combination of particle image velocimetry, fluorescent tracers, and high-speed imaging of bubble shadows. They show that the boundary of the hydrogen bubble curtain at the cathode is wavy because the wall-normal velocity component oscillates with a frequency in the Hertz range.…”

Section: Flow Control In Electrolytes and Magneto-electrochemistrymentioning

confidence: 99%

Electromagnetic flow control in metallurgy, crystal growth and electrochemistry

Gerbeth

Eckert

Odenbach

2013

Eur. Phys. J. Spec. Top.

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Electromagnetic fields, produced either by alternating (AC) or constant (DC) electric currents, or by simple permanent magnets, strongly influence the flow of liquid metals or electrolytes. This is caused by the Lorentz force (density) f L = j × B, resulting from the interaction between the magnetic induction B and the electric current density j, either induced by Faraday's law in liquid metals, or injected in the case of weakly conducting liquids, e.g. in electrochemical systems. The study of the feedback between B and the velocity field u, driven by f L is called magnetohydrodynamics (MHD), a discipline dating back to the discovery of the induction law in 1831 by Faraday. For the basics of MHD we refer to excellent textbooks [1][2][3] or a recent review [4]. Important early milestones in the development of MHD were the first liquid metal experiments in channels performed by Hartmann in 1937 [5] and the Nobel prize decorated discovery of incompressible waves in 1942 [6], now called Alfvén waves, that play an important role in solar physics. More recent developments consist in the rapid emergence of a new discipline, the electromagnetic processing of materials with own conferences [7], or the experimental proofs of the dynamo action, i.e. the self-excitation of B solely caused by the flowing liquid metal in specifically designed laboratory experiments [8].This remarkable diversity of phenomena is sorted in MHD by the magnetic Reynolds number Rm = μ 0 σul, where μ 0 , σ, u and l refer to vacuum permeability, electric conductivity and to the characteristic scales of velocity and length, respectively. The dynamo problem as well as other important astrophysical problems, such as phenomena in the solar atmosphere, fall into the class of high-Rm problems due to the large length scales involved. Here, the magnetic fields induced by the flow of electrically conducting liquids via the induction equation have to be taken into account, making the calculation of f L a formidable task. By contrast, nearly all laboratory and industrial problems of MHD belong to the low-Rm range in which the acting B is the applied one, which drastically simplifies the determination of f L . This richness in topics on various length scales, including geo-and astrophysical problems as well a

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The two-phase flow at gas-evolving electrodes: Bubble-driven and Lorentz-force-driven convection

Cited by 43 publications

References 41 publications

On the Electrolyte Convection around a Hydrogen Bubble Evolving at a Microelectrode under the Influence of a Magnetic Field

On the Electrolyte Convection around a Hydrogen Bubble Evolving at a Microelectrode under the Influence of a Magnetic Field

Near-wall measurements of the bubble- and Lorentz-force-driven convection at gas-evolving electrodes

Electromagnetic flow control in metallurgy, crystal growth and electrochemistry

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