However, this quickly changes as the coverage increases; a direct consequence of the ∆ ḠO variations vs. oxygen ratio, expressed as coverage here. As the oxygen coverage, and therefore the potential increases, the descriptor shifts towards the right side of the volcano plot. At higher potentials, the descriptor ∆ ḠO is too high, and the limiting reaction is expected to be the formation of OOH * from O 2 . This is explained by the overall weaker binding on the oxide covered catalyst, which was also reported by previous DFT studies. Therefore, in this picture the significant overpotential of the ORR is linked to both site-blocking and thermodynamic considerations, both due to the presence of oxide species. The expected Gibbs free energy ∆ ḠO on the oxidized catalyst is too high, and the reaction cannot initiate. This is either caused by the inability of O 2 to adsorb, or to be reduced to OOH * . Prior studies reported the same RDS on (oxidized)-platinum catalysts. Tripkovic and Vegge 9 showed unfavorable binding of O 2 on oxide covered Pt surfaces, along with the activation energy increase for OOH * formation.