Molecular water-oxidation catalysts for photoelectrochemical cells

Photoelectrochemical cells that efficiently split water into oxygen and hydrogen, “the fuel of the future”, need to combine robust water oxidation catalysts at the anode (2H₂O → O₂ + 4H⁺ + 4e⁻) with hydrogen reduction catalysts at the cathode (2H⁺ + 2e⁻→ H₂). Both sets of catalysts will, ideally, operate at low overpotentials and employ light-driven or light-assisted processes. In this Perspective article, we focus on significant efforts to develop solid state materials and molecular coordination complexes as catalyst for water oxidation. We briefly review the field with emphasis on the various molecular catalysts that have been developed and then examine the activity of molecular catalysts in water oxidation following their attachment to conducting electrodes. For such molecular species to be useful in a solar water-splitting device it is preferable that they are securely and durably affixed to an electrode surface. We also consider recent developments aimed at combining the action of molecular catalysts with light absorption so that light driven water oxidation may be achieved.