Thermodynamic Corrections. Thermal and entropic corrections were evaluated at 298.15 K. The pressure for entropic corrections was taken to be 1 bar, except for the calculation of H 2 O, which used a pressure of 0.035 bar. At this pressure, liquid and gaseous water are in equilibrium, allowing the free energy of the calculated gaseous water to be equated to that of liquid water as defined in the ORR half cell reaction. Empirical O 2 Correction. The free energy of O 2 was set such that the overall reaction free energy of the 4-electron pathway would match the theoretical value of 4.92 eV, as is standard practice in ORR studies. Theoretical Overpotentials. Theoretical overpotentials were calculated as the difference between the theoretical half-cell potential for ORR (1.23 V) and the highest potential at which all steps in the free-energy pathway remained downhill. For these calculations, binding of dioxygen (1(a))