Electrochemical Study of Interaction of Nickel and Alkaline Solutions

Kurzfassung

Nickel is an important material used as catalyst in electrochemical devices, e.g., as anode and cathode catalyst in alkaline electrolysers, as anode catalyst in alkaline fuel cells, in different batteries or as current collector grid in alkaline electrolyser. The reaction steps of heterogeneous electro-catalytic processes run at the solid-liquid interface. Therefore the understanding of the interaction of the electrolyte, especially of alkaline solutions, with the nickel surface is important. Depending on the technical application, Ni and different Ni alloys are used and therefore different oxide films are formed on the metal surface. To reduce the performance losses of the electrochemical device given by the low conductivity of such oxide films, much attention was paid to the understanding of the conduction mechanism of oxide films. The conductivity of nickel oxide surfaces is influenced by the thickness and the chemical composition of the oxide layer, as well as the gradients of the composition, depending on the applied potential and composition (purity) of the electrolyte. In particular, the influence of different contents of chloride ions (10 ppm, 100 ppm and 500 ppm) in alkaline solution (10 N NaOH) at 85°C was investigated. For the investigation of the electrochemical behaviour of nickel electrodes in 10 N NaOH with different Cl- content at 85°C, a polished rotating disk nickel electrode (>99,2% Ni) with 1 cm2 geometric surface was used. The used counter electrode was a platinum foil and a RHE (Hydroflex from Gaskatel-Kassel, Germany) as reference electrode. A first characterization of the Ni electrode was performed by recording cyclic voltammograms (CV) in the potential range from open cell potential (OCP) up to 1.5 V and down to 0 V with a scan rate of 100 mV/s. In addition, electrochemical impedance spectra (EIS) at OCP and impedance series measurements (18 spectra) over 3 h at 1.15 V in the frequency range from 50 mHz to 100 kHz were measured. From the impedance spectra one can conclude that with increasing Cl- content in the electrolyte the impedance modulus in the low frequency range decreases, corresponding to an increase of the corrosion current. The same conclusion can also be drawn from CV measurements, proving that the addition of Cl- ions enhances the formation of an oxide layer, especially in the potential range between 1 V and 1.4 V.