An electrolyte sheet comprising two major surfaces, the electrolyte sheet including regions of differing compositions, so that (i) at least one of these regions has at least 1.5 times higher ionic conductivity than at least one other region; (ii) wherein the at least one other region has 20% more tetragonal phase zirconia per volume than the least one region with higher ionic conductivity; and (iii) when viewed in cross-section taken through said major surfaces at least one of the regions exhibits a non-uniform thickness.