INVESTIGATION OF A 2-STATE TAFEL PHENOMENON FOR THE OXYGEN EVOLUTION REACTION ON AN AMORPHOUS NI-CO ALLOY

The anodic polarization for the oxygen evolution reaction of many meta ls and metal alloys results in the measurement of two Tafel regions, o ne for low overpotentials and one for high overpotentials. The polariz ation of the amorphous Ni50Co25P15B10 alloy resulted in the measuremen t of a Tafel plot which appeared to have two different states linked b y a transition potential region that was dependent on electrode pretre atment. A ''limiting current'' phenomenon in the transition region, in which the current appears to be independent of the potential, on the amorphous Ni50Co25P15B10 alloy was observed. The limiting-current phen omenon in the transition region may reflect changes of the electrode s urface oxidation states and the oxygen reaction mechanisms in the two Tafel regions. This limiting current transition region was investigate d by de anodic polarization, electrochemical impedance spectroscopy, a nd 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 ox idation species of Ni were obtained from the amorphous Ni50Co25P15B10 alloy and were compared with the spectra of chemically synthesized Ni oxide and hydroxides. The XPS results from the amorphous Ni-Co alloy e lectrode polarized in the two Tafel regions revealed different surface chemistries. The ''limiting current'' phenomenon could thus be attrib uted to a slow surface place exchange between Ni and Co sites and a fu rther oxidation of the surface from the low overpotential Tafel region to the high overpotential Tafel region.