The system cost of any SOFC plant and the chances of development of a mobile SOFC application strongly depend on the power density of the stack. Good mass transfer within the stack is one important physical requirement for high power density. The influences on the mass transfer in a tubular, a tubular-annular and a planar stack are analyzed and discussed. It results in a proposal for a tubular-helix SOFC stack to increase the mass transfer by better mixing.