After 1.4 VRHE, the catalytic current rose exceptionally fast and this was reflected in the low Tafel slope of 27 mV/dec (Fig. ). A similar Tafel slope was measured if the catalyst was deposited on a glassy carbon rotating disk electrode (RDE) at 0.1 mg/cm 2 in a configuration with minimized masstransport limitations. In the same configuration, we quantified that mass-activity reached 846 A/g at 300 mV overpotential (Fig. ). Similarly, the turnover frequency (TOF) was measured to be 21 /s at this voltage, which was quantified by the redox-active Co atoms that were deduced through integration of the redox peaks in the CV assuming the Co is indeed the OER active site. If measured by the total mass loading, the TOF was approximately 0.46 /s, suggesting that approximately 1 out of every 45 Co atoms was electrochemically active. The fluorine component was deemed important for the catalytic performance of Co0.5Fe0.5O0.5F1.5 because when the catalyst was annealed at 500 o C to convert to CoO/CoFe2O4, the Tafel slope, onset potential and overall activity significantly diminished (Fig. ).