Electrolytic production of hydrogen by the electrochemical reduction of water at 850 degrees C. design and determination of the properties of a flat plat electrolytic cell of 50 cm2 (production electrolytique d'hydrogene par reduction electrochimique de l'eau a 850 degrees c. construction et determination des caracteristiques d'une cellule d'electrolyse plane de 50 cm2). Final report