A modular fuel cell cassette for use in assembling a fuel cell stack comprising a metal separator plate and a metal cell-mounting plate joined at their edges to form a hollow cassette. A fuel cell subassembly is attached to the mounting plate and extends through an opening in the mounting plate. The plates include openings to form chimney manifolds for supply and exhaust of fuel gas to the anode and air to the cathode. A conductive interconnect element extends from the fuel cell subassembly to make contact with the next cassette in a stack. The anode openings in the mounting plate and separator plate are separated by spacer rings such that the cassette is incompressible. A fuel cell stack comprises a plurality of cassettes, the mounting plate of one cassette being attached to, and insulated from, the separator plate of the next-adjacent cassette by a dielectric seal surrounding the interconnect.