The most stable binding sites for the ORR intermediates on the monometallic Ptn (n = 4-8) and the Pt8-mGem (m = 0-4) series are displayed in Figure The ORR reactions profiles are given in Figures in 7b and 8b. At 0 V (vs SHE), the adsorption free energies of all intermediates are more stable with respect to the experimental standard ORR reaction potential of 4.92 eV. As noted previously, OOH* is found to be thermodynamically unstable with respect to O* + OH*. Thus, the dissociated mechanism is predominant. The second and third (H + + e -) transfer steps become very exergonic in Ptn. ΔGO* lies around ~0.20 eV, while ΔGOH* oscillates in the 0.0-0.6 eV range, very far from the ideal values of 2.46 and 1.23 eV (see Figure ). This suggests that the available active sites are too oxophilic. On the one hand, phosphorene presents a very strong binding with O*, whereas on the other, it increases too much the affinity of Pt sites towards O* and OH*. ΔGOOH* and ΔGOH* show a considerable size effect (see Figure ).