Hydrogen society, which will use hydrogen as an energy source and hardly emits carbon dioxide to environment, is proposed as one of the measures against global warming. In the near future, it is estimated that the hydrogen demand will increase several times larger than the present. In the days, technologies to produce a large amount of hydrogen inexpensively are required. Water electrolysis can generate hydrogen under lower voltage with higher operating temperature condition. So, high temperature steam electrolysis is one of the most promising methods for hydrogen production, which has the potential to be high efficiency.  The hydrogen production process combined with nuclear energy emits no carbon dioxide. Toshiba has been developing the high temperature steam electrolysis system with solid-oxide cells for hydrogen production over a temperature range from 800 to 900 degrees C.

According to the heat balance analysis, it is expected that the hydrogen production efficiency is improved by raising the steam conversion rate and decreasing the concentration of hydrogen in the cathode. The steam conversion rate is the ratio of the quantity of steam converted into hydrogen by electrolysis to the quantity of steam supplied. A purpose of this study is to obtain the suitable operating condition for hydrogen production.

The tubular cell has been chosen and developed from the view point of leak-tightness. This cell consists of yttrium-stabilized zirconium (YSZ) electrolytes intermediate, nickel-YSZ cermets steam/hydrogen electrodes inside, and mixed oxide of lanthanum, strontium and cobalt oxygen electrodes outside. The outer diameter of the cell is 12mm. Steam/hydrogen mixture is supplied to the hydrogen electrodes. The steam is decomposed into hydrogen and oxygen ion on the cathode. The oxygen ion diffuses to the anode through the inside of the electrolyte, and become to oxygen molecule on the anode with releasing on electrode.

The evaluation index for the cell performance was the current density at cell voltage of 1.3V at 800 degrees C. The gas mixture composition supplied to the anode cell was fixed as 20% of oxygen and 80% of nitrogen.  The test parameters were the flow rate and the gas composition supplied to the cathode. The steam conversion rate was set by controlling the flow rate. The measured current was divided by electrode surface area to obtain the current density.

The steam conversion rate effect on the cell performance was examined in the condition that the gas compositions of the cathode were hydrogen of 50% and steam of 50%. It was confirmed that the steam conversion rate did not affect the cell performance up to 63% and the performance decreased gradually at more than 63% of steam conversion rate. And also the effect on the cell performance by the hydrogen concentration of the cathode in a range of hydrogen concentration from 50% to 0% was examined under the condition that the steam conversion rate was 60%. The cell performance was not affected by the hydrogen concentration in the range of hydrogen concentration from 50% to 5%. The cell performance dropped when the hydrogen concentration was 0%. This deterioration might be caused by the oxidation of nickel on the cathode surface in steam.

The influence of the steam conversion rate and the cathode gas composition on the cell performance was examined. As a result, it is expected that the hydrogen production efficiency becomes highest at the operating condition of the steam conversion rate of 63% and the cathode hydrogen concentration of 5%.