NOVELTY - The porous substrate made of metal or metal alloy by sintering, comprises a first main upper surface and a second main bottom surface, a layer of high porosity contacting the second main surface, and a layer (10) of low porosity contacting the first main surface. The substrate has a porosity gradient from the first main surface to the second main surface. The first main surface and the second main surface are flat and parallel surfaces. The porosity decreases from the second main surface to the first main surface. The high porosity layer has a porosity of 30-60%.    USE - The porous substrate is useful in a high temperature electrolyzer cell or a fuel cell (claimed).    ADVANTAGE - The porous substrate can be economically, simply, rapidly and robustly prepared with desirable size and shape, improved adhesion-, physico-chemical- and mechanical properties and sufficient thickness, provides long life and improved performance to the electrolyzer cell or fuel cell, and has a coefficient of expansion compatible with that of thermal layers of ceramic or cermet.    DETAILED DESCRIPTION - The porous substrate made of metal or metal alloy by sintering, comprises a first main upper surface and a second main bottom surface, a layer of high porosity contacting the second main surface, and a layer (10) of low porosity contacting the first main surface. The substrate has a porosity gradient from the first main surface to the second main surface. The first main surface and the second main surface are flat and parallel surfaces. The porosity decreases from the second main surface to the first main surface. The high porosity layer has a porosity of 30-60%, a diameter of 20-50 mu m and a thickness of 100 mu m to 5 mm. The low porosity layer has a porosity of 10-30% and a diameter of 1-20 mu m and a thickness of 50-100 mu m. The low and high porosity layers are made of first metal/alloy and a second metal/alloy. Intermediate layers are arranged between the high porosity layer and low porosity layer, and have a porosity as the substrate porosity decreases from the layer of high porosity to the low porosity layer. The substrate is subjected to a partial oxidation by an oxidizing gas such as oxygen or air at high temperature for short duration to obtain a partially oxidized substrate. An INDEPENDENT CLAIM is included for a process for fabricating a porous substrate.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of a high temperature electrolyzer cell or a fuel cell.    Porous support (5)    First electrode (6)    Electrolyte (7)    Second electrode (8)    Low porosity layer. (10)