TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The oxide-ion mixed conductor alternately contains composition of formula: (LnaBabA1-a-b)(B1-xB'xB"y)(alpha)O(3-(~d)) or (LnaBabA1-a-b)(B1-xB'x)(alpha)O(3-(~d)), where Ln is yttrium or element()s chosen from lanthanoids, A strontium and/or calcium, B is cobalt, iron, chromium or gallium, or iron and/or cobalt (where, number of moles of chromium and gallium is 0-20% with respect to number of moles of all B elements), B' is niobium, tantalum, titanium or zirconium, or niobium and/or tantalum (where number of moles of titanium and zirconium is 0-20% with respect to number of moles of all B' elements), B" is copper, nickel, zinc, lithium and/or magnesium, a is 0-0.2, b is 0.2-0.9, x is 0.07-0.5, y is 0-0.15, (alpha) is 0.9-1.2 and (delta) is value that satisfies electric-charge neutrality. Preferred Structure: The ceramic component comprises membrane portion containing continuous layer formed on porous support. The porous support has oxide-ion mixed conductor having porosity of 10-50%. The continuous layer consists of oxide-ion mixed conductor, and has thickness of 1 microns-1 mm. Preferred Component: The metallic component of composite structure has thermal expansion coefficient of 17-23 ppm/degreesC at 100-850degreesC. The metallic component contains austenitic stainless, iron based heat-resistant alloy, nickel-based heat resistant alloy or copper alloy.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The oxide-ion mixed conductor alternately contains composition of formula: (LnaBabA1-a-b)(B1-xB'xB"y)(alpha)O(3-(~d)) or (LnaBabA1-a-b)(B1-xB'x)(alpha)O(3-(~d)), where Ln is yttrium or element()s chosen from lanthanoids, A strontium and/or calcium, B is cobalt, iron, chromium or gallium, or iron and/or cobalt (where, number of moles of chromium and gallium is 0-20% with respect to number of moles of all B elements), B' is niobium, tantalum, titanium or zirconium, or niobium and/or tantalum (where number of moles of titanium and zirconium is 0-20% with respect to number of moles of all B' elements), B" is copper, nickel, zinc, lithium and/or magnesium, a is 0-0.2, b is 0.2-0.9, x is 0.07-0.5, y is 0-0.15, (alpha) is 0.9-1.2 and (delta) is value that satisfies electric-charge neutrality. Preferred Structure: The ceramic component comprises membrane portion containing continuous layer formed on porous support. The porous support has oxide-ion mixed conductor having porosity of 10-50%. The continuous layer consists of oxide-ion mixed conductor, and has thickness of 1 microns-1 mm. Preferred Component: The metallic component of composite structure has thermal expansion coefficient of 17-23 ppm/degreesC at 100-850degreesC. The metallic component contains austenitic stainless, iron based heat-resistant alloy, nickel-based heat resistant alloy or copper alloy.