Water oxidation: a robust all-inorganic catalyst.

Water is the most abundant molecular compound on Earth: it is omnipresent and essential for life. Water is a very stable compound because of its high formation enthalpy of 286 kJmol , and thus the splitting of water into dihydrogen and dioxygen is a high-energy process—the thermal splitting of water requires temperatures above 2500 8C, and is still incomplete. Although the electrochemical splitting of water is efficient, it needs large amounts of fuel-consuming electrical energy. An alternative is the photocatalytic splitting of water, with the aim of storing solar energy in the form of high-energy chemicals—this is, however, a challenging problem. The splitting of water into its elements [Eq. (1)] can be divided into two steps: the oxidation of water to give dioxygen [Eq. (2)] and the reduction of water to give dihydrogen [Eq. (3)], with the oxidation potential being 0.82 V for Reaction (2) and the reduction potential being 0.41 V for Reaction (3), both measured at pH 7 versus the standard hydrogen electrode. Although much progress has been made in the reduction of water, the oxidation of water is more difficult because of its implicit complexity, since it requires the loss of four electrons and four protons from two water molecules.