Electrolysis of water to form H2 and O2 is an increasingly important process in technology for the storage of renewable electricity in synthetic fuels. Formation of oxygen is invariably the slow step and mechanistically much more difficult than H2 evolution. The difference is certainly due to involvement of 4 electrons in the reaction, but there is another reason, associated with a particular property of O2, that seems to have been largely overlooked in the electrochemistry community, namely the triplet nature of the oxygen molecule.
In simple terms, O2 is a molecule, in which the spins of the outer electrons orient in a parallel way, making the molecule magnetic. This is in contrast to H2 and almost all other simple molecules, which bear no net magnetic moment.
The natural way in which precursors to the O2 molecule approach each other, is with spins in opposite directions, implying that somewhere in the formation of the molecule an electron needs to flip its magnetic moment. Not only does this cost energy, there is also hardly any feasible excitation mechanism for such a transition on the surface of a catalyst.
A potentially better route towards triplet oxygen would be to ensure that the precursors (O-atoms, or an O-atom and an OH group) already meet with the spins in the right direction. This would incur a magnetic surface site which orients the adsorbates while they dock just before reaction.