This paper deals with an experimental study on millimetre-size electrochemically evolved hydrogen bubbles. A method to generate gas bubbles controlled in number, size at detachment and place on a flat electrode is reported.Partially wetted composite islands are implemented on a polished metal substrate.As long as the island size is lower than a limit depending on its wettability, only one bubble spreads on the island and its size at detachment is controlled by the island perimeter. The composite, a metal-polytetrafluoroethylene (Ni-PTFE), is obtained by an electrochemical co-deposition process. On the contrary to predictions of available models for co-deposition, at current densities beyond Ni 2+ limiting current density, the mass ratio of PTFE in the deposit strongly increases. A mechanism is proposed to describe co-deposition when hydrogen bubbles are co-evolved. The observation of gas evolution on fully hydrophobic electrodes highlights the fact that bubbles growth rate on such electrodes differs from growth rates when bubble growth is controlled by mass transport of dissolved gas. The more a bubble grows by coalescence the more its foot expands on the electrode the bigger its size at detachment. This triple line creeping mechanism explains why, when attached bubbles coalesce many times before detaching, their size at detachment increases with current density.2

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Aluminium is produced according with the Hall-Heroult process. During this complex electrolyte two-phase electrolysis, it is difficult to model the current distribution and to optimize the process. The flow is the result of the magneto-hydrodynamic phenomena induced by the strong imposed currents and of the bubbles evolving. The reactivity and species concentration distribution are difficult to model. Bubbles are motion sources for the electrolysis cell flow, hydrodynamic properties are strongly coupled with species transport and electrical performances. Bubbles presence modifies these global and local properties. The goal of this proposition is to present the electrochemical engineering modelling of the Hall-Heroult two-phase electrolysis properties in the inter electrode interval. The numerical simulations are performed from both the chemistry and two-phase hydrodynamic point of views.

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International audienceDuring two-phase electrolysis for hydrogen production, according with alkaline-water electrolysis process, there are bubbles which are created at electrodes which imply a great hydrodynamic acceleration in the normal earth gravity field and then a quite important electrical properties and electrochemical processes disturbance, for both transport and reaction. This disturbance can lead to the modification of the local current density and to anode effects for example. In this work, a model experimental set-up is studied. The vertical pine electrode of small electro active surface area is surrounded with a large surface counter electrode. The hydrogen production is performed at the working electrode and effort is focused here upon the global electrochemical cell electrical performances. The polarization curves intensity vs. applied voltage are experimentally measured and presented for different factors such as: the electro active species concentration, nature and counter electrode diameter factors. © 2010 American Institute of Chemical Engineers AIChE J, 201

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