TED-AJ03-165 EFFECTS OF GENERATED BUBBLES BETWEEN ELECTRODES ON EFFICIENCY OF ALKALINE WATER ELECTROLYSIS

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Hydrogen energy is expected to build up dispersive energy system together with electric power by combining water electrolysis and fuel cell. This paper is on the efficiency of alkaline water electrolysis. In water electrolyzers, hydrogen bubbles are generated at cathode plates. The authors found out in the former report that there is an optimum condition on water electrolysis efficiency under high current density : i.e. the efficiency basically increases as the electrodes space gets smaller because of the decrease of electric resistance of electrolyte. However, at the same time, the decrease of the electrodes space causes the increase of total electric resistance between electrodes because of the increase of void fraction. In this paper, in order to explain the existence of the optimum condition, a model of alkaline water electrolysis was established, incorporating the effects of generated bubbles between electrodes. In addition, using KOH as electrolyte and platinum electrodes under atmospheric pressure, rising velocity, diameter distribution of bubbles between electrodes and current density profiles on electrodes were measured in order for verifying the model. In the present model, cell voltage could be expressed as functions of current density, electrodes space, electrode height, bubble rising velocity, specific resistance, temperature and pressure, assuming that ideal gas law can be applicable and bubble rising velocity is constant. The model could express void fraction and current density profiles along electrodes, and show the existence of optimum condition by Eqs. (A1∿A2) that is qualitatively in good agreement with experimental data in the former report. δ_<op>=1.271×RTHФ/FPu (A1) α_<op>=0.3596 (A2) where δ_<op> : optimum electrodes space, α_<op> : void fraction at optimum condition, R : universal gas constant, T : system temperature, H : electrodes height, Ф : mean current density, F : Faraday constant, P : system pressure, and u : mean rising velocity of bubbles. In addition, the model predicted that there is minimum electrode space, δ_<min>, shown by Eq. (A3) : δ_<min>=0.75×RTHФ/FPu (A3) i.e. in a smaller electrodes space than δ_<min>, void fraction between electrodes reached near 100% that means electric resistance between electrodes becomes infinity. The model mentioned above shows that bubble rising velocity is an important parameter on water electrolysis efficiency, and bubble diameter is considered to be closely related to bubble rising velocity. Therefore, bubble rising velocity and diameter distribution were measured utilizing visual observation and image processing on PC. The results showed that bubble rising velocity ranges from 4∿24cm/s and becomes larger as current density increases, solution concentration decreases and temperature increases. Bubble diameter ranges from 0.01∿0.8mm, where average diameter becomes large as current density increases. Measurement results of local current density profile showed the sound validity of present model utilizing the experimental results on bubble rising velocity.
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TED-AJ03-165 EFFECTS OF GENERATED BUBBLES BETWEEN ELECTRODES ON EFFICIENCY OF ALKALINE WATER ELECTROLYSIS

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