SMOOTH RANEY-NICKEL COATINGS FOR CATHODIC HYDROGEN EVOLUTION BY CHEMICAL GAS-PHASE REACTION OF NICKEL ELECTRODE SURFACES

Two different processes to prepare smooth Raney nickel coatings are i nvestigated. Both are based on preparing a precursor coating by gas-ph ase react-ion of nickel at temperatures of around 400 degrees C. Shera rdizing of nickel electrodes, that is, reaction with zinc vapour, lead s to formation of the it-phase of Ni-Zn alloy with the approximate com position of 75 to 85 wt% Zn (balance: Ni), and gas-phase sulfidizing o f nickel electrodes with hydrogen sulfide produces nickel sulfide coat ings composed of the phases Ni3S2, alpha Ni7S6, and alpha NiS. Both ty pes of coating are transformed in situ, the first by caustic leaching and the second during cathodic hydrogen evolution by relatively slow: cathodic reduction into smooth, almost closed nanoporous Raney nickel coatings. The comparison of the catalysts from the two different metho ds reveals the superiority of the second. The hydrogen evolution overp otential is lowest and, during more than 3000 h of continuous operatio n, the catalyst does not deteriorate. Electrodeposited nickel sulfide gives a poorer and less stable RN catalyst than gas phase sulfidized n ickel sulfide.