Potassium and oxygen diffusion and segregation in nickel

Nickel is used as catalyst in alkaline electrochemical systems like batteries, electrolyzers and fuel cells. Adsorption experiments from potassium on a thin NiO(100) epitactic layer on an Ag(100)-single crystal substrate showed that potassium intrudes in that NiO-layer where at a temperature of 350 K a mixed oxide phase was built. At 120 K metallic potassium was adsorbed on the NiO surface. A symmetric K2p-peak showed that potassium is in the oxidized state compared to the asymmetric peak of metallic potassium. No potassium diffused into metallic nickel at a temperature of 450 K. The electrochemically oxidized surface layer consisted of nickel, oxygen and also potassium. After heating and ion-etching no more potassium was detectable by x-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectroscopy (EDX). But it was visible again after oxidizing and heating of the sample to 670 K. Therefore, the potassium must have been in the bulk. It diffused to the surface if there was oxygen at sufficiently high temperatures. Further heating reduces the surface and the potassium also disappeared. Received: 6 September 1998 / Revised: 1 April 1999 / Accepted: 16 April 1999