Such oxygen molecule dissociation was also found by previous theoretical studies. It is noted that the two oxygen atoms were stabilized on neighbouring bridge and hollow sites of the Fe(100) surface, in agreement with earlier experimental and theoretical studies. The oxygen atoms at these sites were separately optimized, in order to assess their stability which is given in Figure of the SM. It was found that the oxygen atom adsorbed at the hollow site was more stable than at the bridge site by 0.36 eV. The enhanced stability can be mainly because the oxygen atom at the hollow site coordinates more Fe atoms than the bridge site, although oxygen atoms at bridge sites gain extra stability by forming short hydrogen bonds with adjacent water molecules. For example, the hollow site engages with five Fe atoms and the bridge site has two. In addition, Bader charge analysis revealed that both oxygen atoms gain comparable amounts of charge from the surface, with -1.11e and -1.13 e for oxygen at bridge and hollow sites, respectively. Again, the hollow site obtained more charges.