Bubble Detachment and Convection in a Simplified Porous Electrode of an Alkaline Electrolyser

  • Mahravan, Ehsan (Aarhus University)
  • Kirkegård, Morten (Aarhus University)
  • Cattani, Michele (Aarhus University)
  • Nørgaard Jacobsen, Andreas (Aarhus University)
  • Forooghi, Pourya (Aarhus University)

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Electrolysers are projected to be one of the main tools to store renewable energy in the form of hydrogen. In an electrolyser, hydrogen and oxygen bubbles form on the interface of the electrodes and the electrolyte and reduce the electroactive area until their detachment. Therefore, it is of vital importance for the productivity of the system to find solutions to enhance bubble detachment. Using porous instead of solid electrodes increases the effective area and the number of bubble nucleation sites. Besides, the effective design of the porous structure and its material can significantly change the bubble detachment, evolution, and transfer to the outlet. However, there is little known about the process inside a porous electrode, owing to the extremely hard observation of bubbles in experiments and the focus of numerical literature on simple plane surfaces. We use a numerical solver within the OpenFOAM framework to solve for the concentration of the dissolved gas, mass transfer across the liquid-gas interface, and electrical potential and current density. The ultimate goal of solving the real foam geometry, which is challenging because of the small size of the domain and bubbles leading to non-physical oscillations and numerical stability. Therefore, to examine and enhance the numerical approach, we consider a simplified two-dimensional porous medium constructed of circular ligaments is considered to investigate the effects of a wide range of parameters, ligament surface curvature, confinement, and flow velocity on the detachment size of the bubble and its immediate rise as well as distribution. The findings of this study can be used, on the one hand, to select more effective porous electrode designs and materials and, on the other hand, to estimate the bubble coverage ratio in engineering simulations.