Composite electrolyte materials include composites comprising 8YSZ in a range of 50-95%, balance 3YSZ. Either the 8YSZ or the 3YSZ can be substituted with a composition having the general formula A.sub.1-x-yB.sub.xC.sub.y where: A=Zr.sub.0.84Y.sub.0.16O.sub.2; B=at least one of the following: Zr.sub.1-xD.sub.xO.sub.2 where: D=at least one of the group Mg, Ca, Sc, and Y, and x=0.03 to 0.16; and Ce.sub.1-xRE.sub.xO.sub.2 where: RE=at least one rare-earth element and x=0.05 to 0.20; C=Al.sub.2O.sub.3 where y=0 to 0.20. The composite electrolyte materials are useful in solid state electrochemical devices such as solid oxide fuel cells and electrolyzers.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
 The United States Government has rights in this invention pursuant to contract no. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC.
NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT
 This invention arose under Cooperative Research and Development Agreement No. ORNL-0720.0 between UT-Battelle, LLC and Worldwide Energy, Inc.