In contrast, the H 2 O* population drops almost linearly with reducing σ and this leads to an analogous drop in the total OH*+H 2 O* coverage (full markers). We find a similar picture from AIMD simulations without O 2 (SI, Figure ). We propose that this drop is responsible for freeing enough available sites to allow for our AIMD capturing O 2 * dissociation under σ <-13 µC/cm 2 at the OH*-covered Pt(111). Methods section above and SI, Section S3). As a result, we find a well-defined molecular O 2 * state and follow dissociation from left-to-right along the x-axis. The process is predicted to be overall exothermic, in line with the picture emerging from the unbiased AIMD (SI, Figure ). A small dissociation barrier of about 0.2-0.3 eV is characteristic of low 0-1/4 ML surface coverages (cf. Table of the SI for exact values and extracted reaction rates); similar to experimental estimates under ultra-high-vacuum conditions. The free energy barrier increases to 0.5 eV and 0.6 eV, however, upon simulating more densely populated surfaces of 1/3 ML and 5/12 ML OH* coverage, respectively. This picture suggests that there is a cut-off coverage below which the rate of dissociation is not really affected. While such dependence has been discussed in the existing literature, determining this cut-off value here is further complicated by the co-existence of adsorbed OH* and H 2 O* species. Our analysis shows increasing surface H 2 O* in the presence of higher OH* coverage (SI, Figure ). As a result, the potential-driven H 2 O* desorption must free enough available sites such that O 2 * dissociation is captured on AIMD timescales even at the OH*-covered Pt(111). While entirely avoiding the formation of an OOH* surface species (typical for Au(111) ), our AIMD simulations show that O 2 * dissociation is followed by spontaneous protonation to OH* (SI, Figure ), presumably that being the next intermediate along the ORR mechanism.