A PEM modular unit fuel cell assembly, comprising: a membrane electrode assembly having an anode side and a cathode side,; a conductive bipolar separator plate having a first side and a second side; and a spring fin heat transfer structure having a first side and a second side, the first side of the heat transfer structure being juxtaposed with the second side of the conductive bipolar separator plate; the assembly being characterised by: an anode porous transport layer having a first and second side, comprising a three-dimensional open structure suitable for gas diffusion as well as electrical and heat conduction comprising a metal foam with a cell size of between 0.040 and 1.0 mm with a thickness of 0.1 to 3.0 mm; the first side of anode porous transport layer being juxtaposed with and in electrical contact with the anode side of the membrane electrode assembly; the first side of the conductive separator plate being juxtaposed and in electrical contact with the second side of the anode porous transport layer; and a cathode porous transport layer having a first and second side, comprising a three-dimensional open structure suitable for gas diffusion as well as electrical and heat conduction comprising a metal foam with a cell size of between 0.040 and 1.0 mm with a thickness of 0.1 to 3.0 mm; wherein: the first side of cathode porous transport layer is juxtaposed with and in electrical contact with the second side of the spring fin heat transfer structure; or the second side of the cathode porous transport layer is juxtaposed with and in electrical contact with the cathode side of the membrane electrode assembly; where the heat transfer structure also provides a degree of spring compliance when stacked and pressed together with one or more other modular fuel cell assemblies. a membrane electrode assembly having an anode side and a cathode side,; a conductive bipolar separator plate having a first side and a second side; and a spring fin heat transfer structure having a first side and a second side, the first side of the heat transfer structure being juxtaposed with the second side of the conductive bipolar separator plate; the assembly being characterised by: an anode porous transport layer having a first and second side, comprising a three-dimensional open structure suitable for gas diffusion as well as electrical and heat conduction comprising a metal foam with a cell size of between 0.040 and 1.0 mm with a thickness of 0.1 to 3.0 mm; the first side of anode porous transport layer being juxtaposed with and in electrical contact with the anode side of the membrane electrode assembly; the first side of the conductive separator plate being juxtaposed and in electrical contact with the second side of the anode porous transport layer; and a cathode porous transport layer having a first and second side, comprising a three-dimensional open structure suitable for gas diffusion as well as electrical and heat conduction comprising a metal foam with a cell size of between 0.040 and 1.0 mm with a thickness of 0.1 to 3.0 mm; an anode porous transport layer having a first and second side, comprising a three-dimensional open structure suitable for gas diffusion as well as electrical and heat conduction comprising a metal foam with a cell size of between 0.040 and 1.0 mm with a thickness of 0.1 to 3.0 mm; the first side of anode porous transport layer being juxtaposed with and in electrical contact with the anode side of the membrane electrode assembly; the first side of the conductive separator plate being juxtaposed and in electrical contact with the second side of the anode porous transport layer; and a cathode porous transport layer having a first and second side, comprising a three-dimensional open structure suitable for gas diffusion as well as electrical and heat conduction comprising a metal foam with a cell size of between 0.040 and 1.0 mm with a thickness of 0.1 to 3.0 mm; wherein: the first side of cathode porous transport layer is juxtaposed with and in electrical contact with the second side of the spring fin heat transfer structure; or the second side of the cathode porous transport layer is juxtaposed with and in electrical contact with the cathode side of the membrane electrode assembly; where the heat transfer structure also provides a degree of spring compliance when stacked and pressed together with one or more other modular fuel cell assemblies. the first side of cathode porous transport layer is juxtaposed with and in electrical contact with the second side of the spring fin heat transfer structure; or the second side of the cathode porous transport layer is juxtaposed with and in electrical contact with the cathode side of the membrane electrode assembly; where the heat transfer structure also provides a degree of spring compliance when stacked and pressed together with one or more other modular fuel cell assemblies.