A fabrication method for fabricating a bath of electrolyte for plating a platinum-based metal underlayer on a metal substrate, the method comprising the following steps: a) providing a first system having ligands and amine functional groups, said first system being constituted by an aqueous solution of an amino ligand comprising at least one compound X-(NH 2) n, where X belongs to the group constituted by (CH 3, CH 3 -CH 2, CH 3 -(CH 2) m), or NH 3 or an x p- (NH 4) + p salt where x is an acid radical belonging to the group constituted by (PO 4 3-, HPO 4 2-, H 2 PO 4 -, HPO 4 2- and H 2 PO 4 -, SO 4 2-, HSO 4 -, and CH 3 COO -) and where n, m, and p are non-zero integers; b) providing a second system forming a buffer system; c) providing a third system providing a metallic salt, and constituted by an aqueous solution of platinum; d) providing a fourth system suitable for imparting the property of conduction to the medium; and e) mixing together the four systems so as to obtain the said electrolyte bath; the method being characterized in that the first system, the second system, and the fourth system are grouped together in a single solution forming a first solution B, in that during step c), the third system forms a second solution A constituted by an aqueous solution of platinum comprising sodium hydroxide (NaOH) and at least one salt of platinum of degree of oxidation IV, and in that during step e), the following substeps are performed: e1) covering the first solution B and raising its temperature to at least 50°C for at least 1h30; and e2) adding the second solution A to the first solution B and mixing the second solution A with the first solution B to form an electrolyte bath including a platinum amino complex. a) providing a first system having ligands and amine functional groups, said first system being constituted by an aqueous solution of an amino ligand comprising at least one compound X-(NH 2) n, where X belongs to the group constituted by (CH 3, CH 3 -CH 2, CH 3 -(CH 2) m), or NH 3 or an x p- (NH 4) + p salt where x is an acid radical belonging to the group constituted by (PO 4 3-, HPO 4 2-, H 2 PO 4 -, HPO 4 2- and H 2 PO 4 -, SO 4 2-, HSO 4 -, and CH 3 COO -) and where n, m, and p are non-zero integers; b) providing a second system forming a buffer system; c) providing a third system providing a metallic salt, and constituted by an aqueous solution of platinum; d) providing a fourth system suitable for imparting the property of conduction to the medium; and e) mixing together the four systems so as to obtain the said electrolyte bath; the method being characterized in that the first system, the second system, and the fourth system are grouped together in a single solution forming a first solution B, in that during step c), the third system forms a second solution A constituted by an aqueous solution of platinum comprising sodium hydroxide (NaOH) and at least one salt of platinum of degree of oxidation IV, and in that during step e), the following substeps are performed: e1) covering the first solution B and raising its temperature to at least 50°C for at least 1h30; and e2) adding the second solution A to the first solution B and mixing the second solution A with the first solution B to form an electrolyte bath including a platinum amino complex. e1) covering the first solution B and raising its temperature to at least 50°C for at least 1h30; and e2) adding the second solution A to the first solution B and mixing the second solution A with the first solution B to form an electrolyte bath including a platinum amino complex.