VACUUM PLASMA SPRAYED ELECTRODES FOR ADVANCED ALKALINE WATER ELECTROLYSIS

Electrode coatings for advanced alkaline water electrolysis were produced by applying vacuum Plasma spraying (VPS), The characteristics of the VPS-equipment Used which are essential for developing effective electrocatalytic electrode layers are presented. Raney nickel and Raney nickel/Mo coatings were applied for cathodic hydrogen evolution whereas Co3O4 spinel and Raney nickel/Co3O4 composite layers served as electrocatalytic coatings for anodic oxygen evolution. The spray powders and the VPS layers were studied by X-ray diffraction and high resolution scanning electron microscopy. Careful surface preparation of the substrates is required to achieve well-bonded and stable layers especially with NiAl precursor alloy which has to be activated subsequently to Raney nickel. Due to the fact that oxide electrocatalysts such as Co3O4, for example, exhibit a high tendency for oxygen loss and decomposition and, thus, loss of electrocatalytic activity at elevated temperatures special process conditions and parameters had to be developed. "Reactive plasma spraying" involving plasmachemical effects allows the processing of thermal sensitive oxides to stable and undecomposed layers. The electrocatalytic activity of the electrode coatings was investigated by performing IR-free polarization curves up to current densities of 1 A cm-2 and long-time tests under continuous current loading of 0.5 A cm-2 over a period of 3000 h. The cathode layers were found to exhibit overvoltages of 70-90 mV at 1 A cm-2 and 70-degrees-C in 25% KOH solution. Raney nickel/Co3O4 composite anodes, which are still in the initial state of development, show overvoltage values of 290 mV at 1 A cm-2.