Local microprocesses at gas-evolving electrodes and their influence on mass transfer

Vogt, Helmut; Stephan, Karl

doi:10.1016/j.electacta.2015.01.008

Cited by 87 publications

(111 citation statements)

References 35 publications

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“…The mass and heat transfer enhancement from vertical walls to liquids due to the addition of ascending gas bubbles has long been recognized (Farmer 1885; Roald and Beck 1951;Kölbel et al 1958). A recent overview is provided by Vogt and Stephan (2015) for mass transfer and by Kitagawa and Murai (2014) for heat transfer enhancement. However, detailed measurements resolving the behavior of single bubbles are relatively scarce.…”

Section: Introductionmentioning

confidence: 98%

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

et al. 2015

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Section: Introductionmentioning

confidence: 98%

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

et al. 2015

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“…Such secondary flow and also the local flow phenomena observed close to the foot of the bubble may change the concentration of dissolved hydrogen at the bubble foot and around the bubble surface 14,45 and, consequently, influence the bubble growth. 46,47 This is different to the case without a magnetic field, where the preceding bubble can only induce a convective mass transfer right after its detachment in the initial growth phase of the next bubble.…”

Section: 31mentioning

confidence: 99%

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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“…At low volume fractions this is probably true, but when the volume fraction becomes large, this approximation could yield too high rates, because most probably the bubbles will grow when the gas volume fraction increases, reducing the surface area. We expect this behavior to be somewhat similar to how some other bubble transport models based on gas diffusion at low concentrations overestimate the mass transport rate compared to more detailed models when the gas volume fraction increases [17].…”

Section: Simplified Mass Exchange -Transport Modelmentioning

confidence: 50%

“…However, with gas bubbles the mass transport of H 2 varies with the current density [16], and therefore the diffusion layer thickness would also need to be adjusted. Several models for the effect of the bubbles on mass transport exist, but they may be quite elaborate [17]. The purpose of our model is to formulate a simple mass transport model that would consistently take the effects of the gas bubbles into account, so that an accurate estimate of the mass transport near equilibrium together with the dynamics of the mass transfer from the liquid to the bubbles would correctly adjust the overall mass transport conditions to the current density.…”

mentioning

confidence: 99%

Two-phase model of hydrogen transport to optimize nanoparticle catalyst loading for hydrogen evolution reaction

Kemppainen

Halme

Hansen

et al. 2016

International Journal of Hydrogen Energy

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2016). Two-phase model of hydrogen transport to optimize nanoparticle catalyst loading for hydrogen evolution reaction. International Journal of Hydrogen Energy, 41, 7568-7581. DOI: 10.1016/j.ijhydene.2015 Two-phase model of hydrogen transport to optimize nanoparticle catalyst loading for hydrogen evolution reaction With electrocatalysts it is important to be able to distinguish between the effects of mass transport and reaction kinetics on the performance of the catalyst. When the hydrogen evolution reaction (HER) is considered, an additional and often neglected detail of mass transport in liquid is the evolution and transport of gaseous H 2 , since HER leads to the continuous formation of H 2 bubbles near the electrode.We present a numerical model that includes the transport of both gaseous and dissolved H 2 , as well as mass exchange between them, and combine it with a kinetic model of HER at platinum (Pt) nanoparticle electrodes. We study the effect of the diffusion layer thickness and H 2 dissolution rate constant on the importance of gaseous transport, and the effect of equilibrium hydrogen coverage and Pt loading on the kinetic and mass transport overpotentials. Gaseous transport becomes significant when the gas volume fraction is sufficiently high to facilitate H 2 transfer to bubbles within a distance shorter than the diffusion layer thickness. At current densities below about 40 mA/cm 2 the model reduces to an analytical approximation that has characteristics similar to the diffusion of H 2 . At higher current densities the increase in the gas volume fraction makes the H 2 surface concentration nonlinear with respect to the current density. Compared to the typical diffusion layer model, our model is an extension that allows more detailed studies of reaction kinetics and mass transport in the electrolyte and the effects of gas bubbles on them.

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Local microprocesses at gas-evolving electrodes and their influence on mass transfer

Cited by 87 publications

References 35 publications

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

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

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

Two-phase model of hydrogen transport to optimize nanoparticle catalyst loading for hydrogen evolution reaction

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