Hydrogen is an essential energy carrier to ensure energy security. The alkaline water electrolysis is one of the methods of hydrogen production and large-scale hydrogen production can be achieved with this method. Since the energy efficiency on a present water electrolysis plant is only 60% [1], improving the efficiency is required for high energy efficiency hydrogen energy system. The energy loss in water electrolysis with high current density comes from a lack of the ion on the electrode, therefore promoting the ion transportation can improve the electrolysis efficiency. The reason of the decrease in the water electrolysis efficiency have been studied; Qian [2] showed generated gas bubbles sticking to the electrodes decrease effective surface area, and Jassen [3] revealed the presence of gas bubbles prevents ion transportation in the electrolyte. Whereas, Vogt [4] reported that mass transportation phenomena are classified into three types of convections induced by gas bubbles, which can encourage ion transportation. These results indicate the dynamics of bubbles play a significant role on the ion transportation and the efficiency. As the investigation with experiments have a limitation, numerical simulations have also been carried out. Mat [5] investigated an effect of gas evolution on a vertical electrochemical cell with two-phase flow model simulating void fraction in the cell. Hreiz [6] simulated two-phase flow induced by electrogenerated bubbles using the two-way momentum coupling Euler-Lagrange CFD approach. Nevertheless, there are only a few studies that dealt with both two-phase flow and electrochemical reaction. Furthermore, no previous study has analyzed an effect of gas bubble dynamics on the electrolysis with taking bubble-induced micro-convection into consideration. From the background mentioned above, in this study, we conducted three-dimensional coupling numerical simulation of two-phase flow and electrochemical phenomena and analyze influence of the convection with micro bubbles on the ion transportation and a cell overpotential. Moreover, we discuss the influence of the bubble atomization on the efficiency under various bubble conditions.

Two-phase flow is simulated with lattice kinetic scheme (LKS) including a phase-field model proposed by Inamuro [7]. The rate of electrochemical reaction is calculated given by Butler-Volmer equation. The ion mass transportation in the electrolyte is simulated with Nernst-Planck equation. The electrical field generated by applying a voltage on the electrodes is governed by Maxwell’s equation. We use a nickel for both electrodes and 6 mol/L KOH solution for the electrolyte. The cell temperature is 298 K, and bubble size is less than 1 mm. The water electrolysis is simulated under constant current; the average of current density is 820 mA/cm2.

Time variations of the cell overpotential is shown in Fig.1. The flow is generated by bubble rising and the cell overpotential is suppressed. The overpotential suppression by the flow overcomes an overpotential increase by the presence of gas bubbles preventing ion transportation in the electrolyte. Moreover, this overpotential suppression is enhanced by bubble atomization. The overpotential suppression can be separated into two types of an overpotential; one is an ohmic loss, the other is an anodic concentration overpotential. Fig.2 illustrates time variations of drop in ohmic loss and anodic concentration overpotential. Both of the overpotential constantly decreases with time. The anodic concentration is suppressed by promoting ion transportation to the anode surface and ohmic loss is suppressed by mixing the electrolyte. Fig.3 shows three-dimensional concentration distribution, and the 2 mol/L iso-surface is also illustrated in this figure. The iso-surface shifts to the anode side with increases in the number of bubbles, and smaller bubble accelerates ion transportation to the electrode and decreases the anodic overpotential and ohmic loss. This result can be explained by the result reported by Guan [8] that a smaller bubble lowers the lift force against a wall and approaches closer to the wall.

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