A discussion on the possible involvement of singlet oxygen in oxygen electrocatalysis

The dioxygen molecule has two bound states, singlet and triplet, which are different in energy, lifetime, and reactivity. In the context of oxygen electrocatalysis as applied to fuel cells and water splitting the involved O₂ is typically considered to be exclusively in its triplet ground state. However, applying spin-conservation rules for the transformation between triplet O₂ and singlet OH⁻/H₂O reaction intermediates predicts an additional free energy barrier associated with the required spin flip. As a result, for conditions under which both can form, the formation of triplet dioxygen from the singlet OH⁻/H₂O might be slower than the formation of singlet O₂. Correspondingly, singlet O₂ might be more active than triplet O₂ in the oxygen reduction reaction. Here, we discuss the possible existence and influence of singlet oxygen in oxygen electrocatalysis. Some perspectives for studying singlet oxygen in oxygen electrocatalysis are also provided.