(328a) Influence of Steam Conversion Rate and Cathode Gas Composition on High Temperature Steam Electrolysis Cell Performance

             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%.