NOVELTY - The catalyst assembly comprises an electronically conductive porous substrate material including a substrate bulk and an external surface, a two dimension extensive catalyst supported on a portion of the network of pores to provide a constant catalyst concentration profile along a thickness dimension of the substrate bulk, and a volume of electrolyte adjacent to the porous substrate material. The substrate bulk includes a network of pores for diffusing fluids. The network of pores includes an internal pore surface supporting a portion of the extensive catalyst.    USE - The catalyst assembly is useful in an electrochemical cell (claimed) such as proton exchange membrane fuel cell, which is useful in automotive applications.    ADVANTAGE - The compact catalyst assembly enhances reliability and lifespan of the electrochemical cell, and has improved catalyst stability.    DETAILED DESCRIPTION - The catalyst assembly comprises an electronically conductive porous substrate material including a substrate bulk and an external surface, a two dimension extensive catalyst supported on a portion of the network of pores to provide a constant catalyst concentration profile along a thickness dimension of the substrate bulk, and a volume of electrolyte adjacent to the porous substrate material. The substrate bulk includes a network of pores for diffusing fluids. The network of pores includes an internal pore surface supporting a portion of the extensive catalyst. The extensive catalyst has a thickness dimension of 1-50 atomic layers, contacts pores of both the first and second substrate layers, and includes spaced apart catalyst patches contacting the internal pore surface. The volume of electrolyte includes a solvent content of less than 5 wt.%. The substrate bulk includes first and second substrate layers having first and second average pore sizes, a third substrate layer that is disposed next to the second substrate layer and has a third average pore size, and wires extending longitudinally from the internal pore surface. The second substrate is positioned between the first and the third substrate layers. The first average pore size is smaller than the second average pore size, and the second average pore size is smaller than the third average pore size. The extensive catalyst further contacts a portion of the wires.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic perspective view of an electrochemical cell.    Electrochemical cell (100)    Bipolar plates (122, 124)    Flow channels (126, 128)    Fuel electrode (130)    Air electrode. (132)