Why do Hydrogen and Oxygen Yields from Semiconductor-Based Photocatalyzed Water Splitting Remain Disappointingly Low? Intrinsic and Extrinsic Factors Impacting Surface Redox Reactions

Water splitting occurring on a semiconductor photocatalyst has become the Holy Grail process to produce a solar fuel, hydrogen, on irradiation with sunlight (or simulated sunlight) in heterogeneous media. Authors often claim highly efficient evolution of hydrogen and oxygen from water through water splitting or efficient hydrogen evolution in the presence of some sacrificial electron donor, whether photocatalytically or photoelectrochemically. Perusal of the scientific and patent literature reveals that yields of hydrogen are disappointingly low even after decades of remarkable advances in materials science and in strategies to achieve significant progress in water splitting. This Review identifies and discusses intrinsic and extrinsic factors (e.g., Φhν = fn{β, kr, S, D, d, s, τ, αhν}; photostability; back reactions) that impact redox reactions in general and water splitting in particular. The lack of control and handling of these various factors present a challenging, if not an impossible task in improving process efficiencies to achieve significant practical evolution of hydrogen from water splitting.