where R is the electrocatalyst. As four electrons are transferred and the overall free energy of reaction is -4.92 eV, this ideal catalyst would cause each of these step's reaction energy to be -1.23 eV. In terms of the voltage of an electrochemical cell, each step would have a current when the cell is at 1.23 V relative to the reversible hydrogen electrode, and would have zero overpotential because this is the maximum possible voltage at which the reaction could proceed. Scaling relations between intermediate reaction energies are the primary reason that the ideal ORR electrocatalyst is so elusive. In general, a scaling relation in a catalytic cycle is the correlation between the intermediate reaction energies of two steps in the cycle over a range of different catalysts. The most well-known of these scaling relations is the relatively linear relationship between the intermediate reaction energies of R + O # + H ! + e " ⇄ R -O # H and R -OH + H ! + e " ⇄ R + H # O, which is caused by the fact that active sites that bind one oxygen species strongly, such as O2H, are likely to strongly bind to other oxygen species, such as OH. This is especially apparent for these two steps because they each involve forming and breaking bonds between the catalyst and an oxygen species.