Mechanism of corrosion and sedimentation of nickel electrodes for alkaline water electrolysis

Nickle is one of the most promising alternatives to noble-metal catalysts in alkaline water electrolysis (AWE). Although Ni exhibits superior stability in alkaline solution, the sediments which have been observed in some situations result in a limited electrolytic efficiency of Ni electrodes. This work was proposed to elucidate the sedimentation mechanism of Ni electrodes via a long-term test in KOH solution. Electrochemical tests and localized electrochemical tests were applied to assess the corrosion behavior of Ni electrodes. The micromorphology and grain orientations of electrodes were investigated by SEM, EDS, EBSD and XPS. The tested anode was found apparently roughened, leaving veinlet-like corrosion textures on the surface, meanwhile pits in several micrometers were found. The collected sediments were composed of elements such as Pb, Sn, Ni, K, O and C. The polarization impedance of the tested anode was 0.4 Ω cm2, which is far lower than that of the as-prepared anode, 56 Ω cm2. In alkaline solution, the galvanic cells formed since the tested anode was found with a positive potential (+13.8 μV) and a negative potential (−7.8 μV) and caused local corrosion. Galvanic corrosion was also observed between the anode and cathode during the electrolysis. By applying an external potential, metallic components of the Ni anode were progressively dissolved and converted to ions. Then the produced ions traversed through the test solution to the cathode side and were reduced to metals precipitating on the cathode. It is believed that anode material exfoliation and ion precipitation jointly contributed to the sedimentation.