Investigation of a “Two‐State” Tafel Phenomenon for the Oxygen Evolution Reaction on an Amorphous Ni‐Co Alloy

The anodic polarization for the oxygen evolution reaction of many metals and metal alloys results in the measurement of two Tafel regions, one for low overpotentials and one for high overpotentials. The polarization of the amorphous alloy resulted in the measurement of a Tafel plot which appeared to have two different states linked by a transition potential region that was dependent on electrode pretreatment. A "limiting current" phenomenon in the transition region, in which the current appears to be independent of the potential, on the amorphous alloy was observed. The limiting‐current phenomenon in the transition region may reflect changes of the electrode surface oxidation states and the oxygen reaction mechanisms in the two Tafel regions. This limiting current transition region was investigated by dc anodic polarization, electrochemical impedance spectroscopy, and x‐ray photoelectron spectroscopy (XPS). A time‐dependent impedance behavior was observed coincident with a limiting current in the Tafel plot within the same potential region. This was further characterized by means of a fixed frequency impedance measurement. XPS spectra of oxidation species of Ni were obtained from the amorphous alloy and were compared with the spectra of chemically synthesized Ni oxide and hydroxides. The XPS results from the amorphous Ni‐Co alloy electrode polarized in the two Tafel regions revealed different surface chemistries. The "limiting current" phenomenon could thus be attributed to a slow, surface place exchange between Ni and Co sites and a further oxidation of the surface from the low overpotential Tafel region to the high overpotential Tafel region.