XPS analysis of electrochemically oxidized nickel surfaces

Abstract

Nickel, commonly used in electrochemical systems like batteries, fuel cells or as catalyst in alkaline water electrolysis can occur in different oxidation states. With surface analytical methods like X-ray photoelectron spectroscopy (XPS), it is possible to determine the oxidation state of metals. The application of this method requires that the sample is in an ultrahigh vacuum (UHV)-system. Therefore in-situ XPS-measurements of electrochemically oxidized samples are not possible. To avoid transfer through atmosphere one has to mount an electrochemical cell directly into a surface analytical system. By means of such an unit, nickel was polarized in potassium hydroxide solution (KOH) at different potentials. After that the different oxidation states were examined with XPS. The interpretation of these experiments is complicated because water is adsorbed after the electrochemical preparation. This can cause an additional surface layer. Ion-beam etching will not only remove this additional surface layer, it also changes the electrochemically prepared layer beyond. Comparing an electrochemically prepared single crystal with polycrystalline samples shows that the electrochemically prepared oxide layer is more clearly visible on the single crystal than on the polycrystalline material.