Nickel as anode material for the electrochemical production of polysulfides in white liquor

This paper is concerned with new anode materials for the electrochemical oxidation of white liquor. This electrosynthesis process is run in a membrane cell where polysulfide ions are produced in the white liquor of the anode chamber, while a pure sodium hydroxide solution is generated in the cathode chamber: If the noble metal oxide coated anodes that were used in a previous investigation could be successfully replaced by nickel, important reductions in investment costs would be possible. The stability and catalytic activity of nickel as anode material were investigated using cyclic voltammetry and chronoamperometry on rotating disc electrodes, combined with surface analysis. Furthermore, porous nickel was tested in a flow cell, with galvanostatic electrolysis. The results show that nickel has good activity for sulfide oxidation, but a nickel oxide/sulfide surface layer is formed which deactivates the electrode at E greater than or equal to -0.2 V, probably due to the increasing proportion of poorly conducting Ni(II) oxide. Carbon precipitation, and peeling of a thin film of the metal were observed with SEM. High current efficiency and an overpotential of less than 0.2 V were obtained with a porous nickel electrode at a geometric current density of 3 kA m(-2). Corrosion did not appear to be a problem, and nickel should also be suitable as substrate material for more active anode coatings.