Abstract
A solid‐state electrochemical cell of the type 
mole percent (m/o) 
(+5 m/o 
+ 6 m/o 
)/Au, 
, was composed for determining 
concentration, where 
, a lithium ion conductor, is an electrolyte, and 
is used as the solid reference electrode. Electromotive force (EMF) of the cell depended logarithmically on the 
partial pressure in 
gas mixtures at temperatures between 350 and 400°C. EMF reached a constant value within 1 min after the change of 
partial pressure at 400°C. The sensitivity to 
of this cell was not affected by coexistence of water vapor. The sensor worked stably during a test period of 30 days. The sensing mechanism of 
was discussed together with an explanation to the stability of this sensor.