Methods for producing oxygen-sensing element, particularly for use with internal combustion engine exhaust emission analysis

Method to make a sensing element for electro-chemical sensing of oxygen content in exhaust gases from internal combustion engines, the sensing element being an oxygen concentration cell and including a solid oxygen ion conductive electrolyte body having an inner surface covered at least in part by an inner electrode and an outer surface adapted to contact oxygen from said exhaust gases, the method comprises applying, by means of thin layer techniques, an electron-conductive catalyst layer electrode of a material which catalyzes the gas equilibrium and covering at least part of said outer surface, said material being formed with micro pores in those regions thereof where its thickness is greater than about 100 A, and heating said electron-conductive catalyst layer, after application thereof to the electrolyte body, at a temperature between 200.degree. C and two-thirds the melting temperature T in .degree. K of said electron-conductive catalyst layer to form said micro pores, and applying an outer protective layer covering all of said electron-conductive catalyst layer and all parts of said outer surface which are not covered by said electron-conductive catalyst layer, at least the part of said outer protective layer covering said electron-conductive layer being porous.