The production of benign, storable, and transportable hydrogen fuel from clean and renewable sources of energy is a stimulating goal.^[1,2] In this context, the scientific community draws inspiration from nature to devise green production of hydrogen as an alternative energy carrier by the design of artificial photosynthetic systems.^[3]

Artificial photosynthesis (AP) aims to achieve the light-induced water splitting reaction (2H₂O → O₂ + 2H₂O). This reaction can be separated in two half reactions: the hydrogen evolution reaction, HER (4H⁺ + 4e^- → 2H₂), for which protons and electrons are supplied by the oxygen evolution reaction, OER (2H₂O → O₂ + 4H⁺ + 4e^-). For that, thermodynamically (E⁰_O2/H2O = 1.23 V vs RHE) and kinetically demanding water oxidation calls for effective water oxidation catalysts (WOCs), able to overcome the activation barrier of water oxidation reaction to dioxygen.^[4,5,6]

The best WOCs reported today are made of Ru and Ir. However, it is highly desirable to use more earth-abundant transition metals such as Mn, Fe, Co, Ni or Cu, whether in form of metal complexes (MC) or metal-oxide nanoparticles (MO-NPs). The latter are generally more stable electrocatalysts but are more challenging to study through mechanistic investigation protocols and pose greater demands when seeking custom optimization. On the other hand, the mechanisms in molecular catalysts are easier to study and understand, resulting in routine optimization via ligand modification. However, first-row transition MCs in OER are still a challenge because they often get deactivated by the hydrolytic behaviour and by the oxidation of the organic ligands.^[7]

In this project, we design an anodic catalytic layer to be incorporated in a water splitting electrochemical cell based on a hybrid thin film of SiO₂ containing WOC units based on first-row transition metal catalysts, whether in form of MO-NPs or MCs. This approach aims to achieve i) protection against electrodegradation, and ii) the covalent attachment of an active WOC capable of sustain operational current at low overpotentials, ensuring iii) high conductivity.^[8] Herein,  we present two candidates to achieve optimal performance activity, a family of MO-NPs and a MC based on a Cu macrocyclic unit (Cu-MAC) known to be an efficient WOC at different values of pH.^[9] The MC catalytic center has been modified with silylated groups to be incorporated into stabilizing SiO₂ matrixes (Cu-MAC^SiOx).