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Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. Tantalum nitride (Ta3N5) has a theoretical solar to hydrogen conversion efficiency of 15.9%, but it is not stable in the highly oxidative environment of water oxidation.

Ferrihydrite (Fh) could be used in modifying Ta3N5 photoanode for highly stable water oxidation reaction. However, the intrinsic structure responsible for the hole storage function for Fh remains unclear.

Recently a research team led by Prof. LI Can from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences unveiled the hydration structure of ferrihydrite for hole storage based on tantalum nitride photoanode system.

Their results were published in Angew. Chem. Int. Ed. and selected as hot paper.

Unveiling the hydration structure of ferrihydrite for hole storage in photoelectrochemical water oxidation (Image by WANG Pengpeng and SHI Jingying)

The researchers demonstrated that the irreversible and gradual loss of crystal water molecules in Fh led to the weakening of the hole-storge function, accompanied with the arrangement of inner structure units.

They proposed a structure evolution of the dehydration process and identified the primary active structure of Fh for HSL as the [FeO6] polyhedral units bonding with two or three molecules of crystal water.

Moreover, they found that with the successive loss of chemical crystal water, the coordination symmetry of [FeO6] hydration units underwent mutation and a more ordered structure was formed, causing the difficulty for accepting photogenerated holes as a consequence.

This study provides a new avenue for the rational design of efficient and stable solar water splitting devices based on interfacial modification strategies.

The research was supported by the National Natural Science Foundation of China and the Ministry of Science and Technology of China. (Text by WANG Pengpeng and SHI Jingying)