Hybrid photoanodes for water oxidation based on high-valent metal complexes Corentin Pigot 1 , Igor O. Fritsky 2 , Sergii I. Shylin 1 1 Department of Chemistry - Ångström Laboratory, Uppsala University, 75120 Uppsala, Sweden, 2 Department of Chemistry, Taras Shevchenko National University of Kyiv, 01601 Kyiv, Ukraine The water oxidation half reaction is the main bottleneck in direct conversion of solar energy into renewable fuels through photoelectrochemical (PEC) water splitting. In this contribution, we present a new approach to the catalytic water oxidation, utilizing high-valent transition metals for water molecule activation [1] . We explore a new family of polyhydrazide complexes of base metals (manganese, iron, copper) obtained via metal template synthesis as water oxidation catalysts. The proposed macrocyclic ligands are remarkably strong σ-donors, which stabilize high oxidation states of the encapsulated metals that is the key to activating water molecule and facilitating subsequent O–O bond formation. Using these catalysts, we have developed next generation molecular/semiconductor photoanodes for solar light-driven water oxidation. For this purpose, we have combined wide-bandgap semiconductor materials as light harvesters with new molecular catalysts. As semiconductors, we use BiVO 4 and FeWO 4 grown on the FTO glass. For anchoring the molecular catalysts on oxide semiconductors, we have obtained the complexes with phosphonate groups (-PO 3 H 2 ).In this way, we have fabricated the hybrid assemblies and investigated themin photoelectrochemical water oxidation. While state-of-the-art electrolyzers and PEC cells still utilize noble metals as anode materials, mainly iridium, we use catalysts based on abundant, recyclable and environmentally benign metals [2-4] . Intermediates relevant to catalytic reactions have been studied using spectroelectrochemistry methods, EPR, and, in case of Fe complexes, Mössbauer spectroscopy. We have been able to trap and identify Fe(V) and Mn(V) transient species using these methods. Transient spectroscopy experiments have been carried out to monitor the electron-transfer kinetics in photoanodes. We conclude that Fe complexes with the new polyhydrazide ligands exhibit the highest activity in water oxidation reaction, followed by Cu and then Mn complexes. References 1. S. I. Shylin, M. V. Pavliuk, L. D’Amario, F. Mamedov, J. Sá, G. Berggren, I. O. Fritsky. Chem. Commun. , 2019, 55 , 3335- 3338. 2. I. Buvailo, V. G. Makhankova, V. N. Kokozay, I. V. Omelchenko, S. V. Shishkina, J. Jezierska, M. V. Pavliuk, S. I. Shylin. Inorg. Chem. Front. , 2019, 6 , 1813-1823. 3. H. I. Buvailo, V. G. Makhankova, V. N. Kokozay, I. V. Omelchenko, S. V. Shishkina, A. Bienko, M. V. Pavliuk, S. I. Shylin. RSC Adv. , 2021, 11 , 32119-32125. 4. M. O. Plutenko, S. I. Shylin, S. Shova, A. V. Blinder, I. O. Fritsky. Acta Cryst. , 2024, 80 , 25-28.
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