This invention relates to a fuel cell system comprising an outer tubular solid oxide fuel cell, a solid phase porous matrix located inside the outer fuel cell and attached to its inner electrode layer, and at least one inner tubular solid oxide fuel cell embedded in the matrix. The outer fuel cell has an inner electrode layer configured to receive a first reactant fluid, an outer electrode layer configured to receive a second reactant fluid, and an electrolyte layer sandwiched between the electrode layers. The inner fuel cell has an outer electrode layer configured to receive the first reactant fluid, an inner electrode layer configured to receive the second reactant fluid, and an electrolyte layer sandwiched between the inner and outer electrode layers. The matrix has sufficient mechanical strength to support the inner fuel cell and sufficient porosity to enable the first reactant fluid to flow through the matrix to the inner and outer fuel cell respectively.