On-line measurement of hydrogen isotopes in corrosive environments is essential for the development of
liquid blanket systems for fusion reactors. In the present study, a Pd membrane electrode for a solid electrolyte
hydrogen sensor was developed. An electrode fabricated onthe sensor surface was metallurgically analyzed and
was found to be a compact layer with much lower porositythan conventional Pt electrodes. This compact elec-trode can serve as a protective layer against corrosion ofthe sensor material during measurement. The response
and stability of the sensor with the Pd membrane electrode was investigated in an Ar-hydrogen gas mixture
environment. The electromotive force of the sensor with the Pd compact electrode agreed with a theoretical
calculation, assuming the same three-phase boundary as in the sensor with a porous Pt electrode.