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We have developed a mixed conducting samaria-doped ceria ͑SDC͒ cathode with highly dispersed Ni catalysts in a solid oxide electrolysis cell ͑SOEC͒ for efficient hydrogen production. The Ni-dispersed SDC cathode exhibited the highest performance at 17 vol %-Ni loading due to the effective enhancement of the reaction rate by increasing the active reaction sites and lowering the electronic resistance. The cathode activity was found to increase with an increase in the ionic conductivity of zirconia electrolyte, similar to the case of SOFC electrodes. An SOEC comprised of Ni-dispersed SDC cathode, ScSZ electrolyte, and La͑Sr͒CoO 3 anode ͑with SDC interlayer͒ exhibited high performance for the short-term test; IR-free cell voltage = 1.13 V at 0.5 A/cm 2 and 900°C under the atmosphere of H 2 O + H 2 ͑p͓H 2 O͔ = 0.6 atm͒ and O 2 ͑1 atm͒.

We have developed double layer-type (catalyst layer/current collecting layer) electrodes for reversible SOFCs operating at medium-temperature. As the H2 catalyst layer (anode for SOFC and cathode for steam electrolysis) interfaced with YSZ solid electrolyte, mixed conducting samaria-doped ceria (CeO2)0.8 (SmO1.5)0.2 particles were employed in combination with Ni metal electrocatalysts (10 to 20 vol.% of nanometer-size Ni) highly dispersed on their surfaces. The current collecting porous layer of Ni-SDC cermet (60 vol.% of micrometer-size Ni) was formed on the catalyst layer. The H2-electrode with 10 vol.% Ni in the catalyst layer exhibited high performance, due to the effective enhancement of the reaction rate by increasing both the active reaction sites and the electronic conductance.

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