To shed more light onto the RDS, we show the ORR profiles at 1.23 V (vs SHE). The first thing to notice, as pointed previously, is that both monometallic Ptn and bimetallic PtGe catalysts over phosphorene suffer from an overbinding for O* and OH*. Interestingly, in Pt4-Pt6 the RDS is the OH* formation, third (H + + e -) step, whereas in Pt7 and Pt8 the OH* hydrogenation, the fourth (H + + e -) step, becomes the RDS. Consequently, a size dependency is found on the RDS which will limit the overall overpotential. In the nanoalloys, the fourth (H + + e -) step is the RDS regardless of the Ge concentration. The computed overpotentials (ηORR) are reported in Table . All catalysts present overpotentials greater than 1.0 V. Pt4 shows the greater overpotential (ηORR = 1.67 V), while Pt7 has the smallest one (ηORR = 1.17 V). Among the nanoalloys, Pt5Ge3 is the one with the smallest overpotential (ηORR = 1.22 V). These values suggest that Pt and PtGe clusters deposited on phosphorene may not be suitable ORR catalysts.