Solar-driven water splitting utilizing photo(electro)catalytic systems is a possible strategy to secure the future supply of low-entropy energy in form of storable high-energy molecular fuels, such as hydrogen, alcohols or hydrocarbons. However, practically viable and sustainable systems required for large-scale applications should be preferably based on highly abundant and low-cost materials with non-critical availability, i.e., without any possible supply restrictions due to various political, economic or environmental concerns. In terms of sustainability, low cost and non-toxicity, one of the most attractive materials for photo(electro)catalytic applications are polymeric carbon nitrides (CNx). Carbon nitrides are readily available, tunable and chemically robust polymers, that have been widely utilized in a variety of light-driven chemical processes such as hydrogen evolution, CO2 reduction, selective chemical syntheses, or organic pollutants degradation. However, the studies demonstrating visible light-driven water oxidation to dioxygen using CNx-based photoelectrocatalytic systems are still rather rare. This is, on the one hand, due to the notoriously slow kinetics of the oxygen evolution reaction (OER), which typically results in accumulation of oxidizing equivalents that translates in enhanced photocorrosion of CNx. On the other hand, the very fabrication of mechanically robust CNx-based photoelectrodes is challenging, in particular due to the poor adhesion of CNx to conductive substrates, and the very low conductivity of CNx films, hindering efficient charge transport to the external circuit. In our own work, we have been developing a distinct type of hybrid CNx-based photoanodes in which a thin layer of CNx is deposited onto a porous TiO2 layer that acts as an electron-collecting scaffold, overcoming thus at the same time both the problems of low adhesion and low electronic conductivity of CNx. Such photoanodes are capable of visible light-driven oxygen evolution upon deposition of a suitable cocatalyst for water oxidation, typically in form of metal oxide (IrOx, CoOx, NiOx) nanoparticles. Notably, we found out that one of the crucial problems of the metal oxide nanoparticles as cocatalysts is their parasitic absorption of visible light, blocking thus the light absorption by the light absorber. This problem could be only partially overcome by using, for example, ultrasmall (1-2 nm) CoO(OH)x nanoparticles that exhibit a larger bandgap and correspondingly better transparency in the visible range due to quantum size effects. This led us to hypothesize that, in contrast to conventional bulk metal oxide-based water oxidation catalysts, a molecular-scale catalyst might be favorable for preventing the undesired light absorption by the catalyst and enabling also more controllable cocatalyst deposition. With this motivation in mind, we turned our attention to water oxidation catalysts based on well-defined molecular polyoxometalates (POMs), such as [Co4(H2O)2(PW9O34)2] 10─ = CoPOM, a tetra-cobalt-doped polyoxometalate, that have attracted much attention with regards to catalytic applications, and have been previously utilized as cocatalysts on various semiconducting metal oxides (e.g., TiO2, Fe2O3) to fabricate photoanodes for light-driven water splitting. However, to the best of our knowledge, no studies on CNx-based photoanodes comprising molecular POMs cocatalysts for photoelectrocatalytic water-splitting have been reported so far.