Water oxidation is a crucial process in artificial photosynthesis, which constitutes an attractive way to use solar energy and convert it to chemical forms. By extracting four electrons and four protons, an O2 molecule can be released. Because water oxidation is a thermodynamically uphill reaction and is kinetically slow, this reaction causes a bottleneck in large‐scale water splitting. Consequently, the development of new and efficient water oxidation catalysts (WOCs) has attracted extensive attention. Recent advances have been made in the use of single transition metal complexes as water oxidants. These transition metal complexes mainly include Ru, Ir, Mn, Fe, and Co.
Water oxidation is a four-electron transport process that takes place in multiple steps and includes the formation and breaking of multiple chemical bonds. These steps require the help of catalysts (refers to transition metal complexes and denoted by the symbol M). At present, the generally recognized water oxidation mechanism includes the formation of M-OH or M-O intermediates, but its mechanism is slightly different under acidic or alkaline conditions. Figure 1 is the possible mechanism of water oxidation in different environments. It can be seen from the figure that the biggest difference between water oxidation under acidic and alkaline conditions is the last step of oxygen generation. A possible reaction mechanism is that M-OOH intermediate is first generated, and then the intermediate is decomposed to precipitate oxygen. Another possible reaction mechanism is that two M-O intermediates are first generated and then combined to precipitate oxygen.
Figure 1. Water oxidation mechanism in acidic (blue) and alkaline (red) electrolyte
Black line: route of M-OOH intermediate
Green line: route of adjacent M-O intermediates
The possible reaction mechanisms are as follows:
|Under acidic conditions||Under alkaline conditions|
Progress has been made in the development of catalysts for electrocatalytic water oxidation, particularly those based on transition metal complexes. Among the transition metal complexes that have been reported for water oxidation catalysis, Ru, Ir, Co and Fe complexes are the most common. For example, Meyer and co‐workers reported a six‐coordinated FeIII-aqua complex [FeIII(dpaq) (H2O)]2+ (dpaq is 2-[bis(pyridine-2-ylmethyl)] amino-N-quinolin-8-yl-acetamido) as an electrocatalyst for water oxidation in propylene carbonate-water mixtures . The results of electrochemical and kinetic studies suggested that this FeIII-aqua complex was first oxidized to FeV(O)2+, which then reacted with a water molecule via a reaction that was first order with respect to both the catalyst and water. The determined ko was 0.035(4) M-1 s-1. This result is consistent with a single-site mechanism instead of a bimolecular mechanism for water oxidation. Sustained water oxidation catalysis occurs at a high surface area electrode to give O2 through at least 29 turnovers over an 15h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst. In summary, transition metal complexes play an important role for electrocatalytic water oxidation.
Figure 2. Structure of [FeIII(dpaq) (H2O)]2+
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- Coggins, M.; et al. Electrocatalytic water oxidation by a monomeric amidate-ligated Fe (III)-aqua complex. J. Am. Chem. Soc. 2014, 136: 5531-5534.