(275a) High Temperature Electrolysis for Hydrogen Production Using Solid Oxide Electrolyte Tubular Cells Assembly Unit

Hydrogen society, which will use hydrogen as energy source and hardly discharges carbon dioxide to environment, is proposed as one of the measures against global warming. The thermochemical and electrolytic route to hydrogen production from water using nuclear energy which does not generate carbon dioxide are considered in the U.S., Japan and EU. Also in Toshiba, the hydrogen production with the thermochemical water splitting and the high temperature steam electrolysis have been being developed. High temperatue steam electrolysis with solid oxide electrolyte cells is one of the most promising methods for hydrogen production, which has the potential to be high efficiency. Combining to the nuclear energy, the hydrogen production process emits no carbon dioxide. Its most parts consist of environmentally sound and common materials. This paper describes the development of the solid oxide electrolyte tubular cell for steam electrolysis, and the performance of 15 electrolysis cells unit, which has hydrogen production capacity of more than 100 Nl/h. The solid oxide electrolyte tubular cell of 12mm in outer diameters for steam electrolysis of two kinds of different length was manufactured. The active area of each cell was 15 cm2 and 75 cm2. The electrolysis cells were hydrogen electrode-supported, with yttrium-stabilized zirconia (YSZ) electrolytes, nickel-cermet steam/hydrogen electrodes, and mixed oxide of lanthanum, strontium and cobalt oxygen electrodes. Steam/hydrogen mixed gas was supplied to the hydrogen electrodes, oxygen/nitrogen mixed gas was supplied to the oxygen electrodes, and the current-voltage characteristic was measured at 800 to 900 degree C. First, using single cells, the current-voltage characteristic was evaluated. The measured open-cell potential of 15 cm2 was 0.94V at 800 degrees C and 0.91V at 900 degrees C, and these values were mostly in agreement with the theoretical value. It was estimated that more than 0.45 A/cm2 of current density was obtained at the thermal neutral voltage of 1.3V, 800 degree C in temperature. Next, the current-voltage characteristic of 75 cm2 cell was evaluated. The 75 cm2 cell is 3 times longer than the 15 cm2 cell. The influence of the cell length on the current-voltage characteristic was measured. The current-voltage characteristic, which attaches an electric supply system to the hydrogen electrode from the both ends of a cell or from one end of a cell, was compared. In the electric supply to the hydrogen electrode from cell one end, electrolysis voltage increased rapidly with current density, and it was observed that resistance of the length direction of the hydrogen electrode itself couldn't be disregarded. The measured open-cell potential of 75 cm2 at 800 degrees C was 0.93V for one end electric supply and 0.92V for both-ends electric supply, and these values were mostly in agreement with the theoretical value. On the other hand, the area-specific resistance (ASR) was 3.3 ohm cm2 for one end electric supply and 1.2 ohm cm2 for both-ends electric supply; the influence of an electric supply system was remarkable. The hydrogen production rate was measured during the current-voltage characteristic measurement. It was confirmed that the observed hydrogen production rate was mostly in agreement with the theoretical value, which was predicted from the current density. The unit, which installed fifteen 75 cm2 tubular electrolysis cells, was manufactured. The design value of the hydrogen production rate was 100 Nl/h. Fifteen cells were divided into three blocks of every five for electric supply individually to each block and parallel conection of all the cells was carried out. Each cell is set on a pedestal via glass, which works for seal between hydrogen side and oxygen side and also works for electrical isolation. The measured open-cell potential of the unit at 800 degrees C was 0.93V for each block, and these values were mostly in agreement with the theoretical value. Moreover, ASR was observed to 1.9 ohm cm2, 1.9 ohm cm2, and 2.0 ohm cm2 for every block, and there was almost no performance variation during a block. The hydrogen production rate was measured. It was confirmed that the observed hydrogen production rate was mostly in agreement with the theoretical value, which was predicted from the current density. The hydrogen production rate of the unit of 130 Nl/h was checked to design value 100 Nl/h. It was confirmed that the unit, which consisted cells, seals, electrical isolations, electric supplies and so on, worked well from the result. From now on, the influence on the characteristic with the passage of time and temperature, and steam consumption efficiency is examined to carry out prolonged operation of the 100 Nl/h unit. And the 1Nm3/h unit is due to be developed using this result.