This invention relates to a solid oxide fuel cell stack comprising a plurality of tubular solid oxide fuel cells each comprising concentric inner and outer electrode layers sandwiching a concentric electrolyte layer. The fuel cells extend in the same direction and are arranged in a cluster with at least one fuel cell having an electrolyte layer with a different composition and different maximum operating temperature than another fuel cell in the cluster. The fuel cell having the electrolyte layer with a higher maximum operating temperature is located closer to the core of the cluster than the fuel cell having the electrolyte layer with a lower maximum operating temperature.