a fuel cell stack comprises a plurality of planar fuel cells in a spiral configuration . the fuel cells angularly offset from one another such that immediately adjacent cells only partially overlap one another . the cells are preferably of the unitized type . a manifold assembly is operatively adjacent to the fuel cells . the manifold assembly includes an inlet manifold and an outlet manifold for each of the gases that are in communication with the fuel cells . the unitized solid oxide fuel cell comprises a first planar interconnect , a planar ceramic cell adjacent the first planar interconnect , a second planar interconnect adjacent the opposite side of the planar ceramic cell , and a plurality of gas tubes adjacent the planar ceramic cell . the gas tubes are arranged to a first configuration , a second configuration , a third configuration and a fourth configuration with a cylindrical gas tube shape and a t - shape gas tube designs . these configurations provide a choice of co - flow , cross flow or counter flow patterns inside the cell to improve the thermal management and cell efficiency and provide more flexibilities to connect a manifold assembly . they also provide an option to directly combust the spent fuel just outside the fuel cell to eliminate a separate burner for residual energy recovery . these configurations thus provide means for improved fuel cell power system efficiency and lowered capital and operational costs .