Hydrogen generation via photoelectrochemical (PEC) water splitting represents one of the promising strategies to secure the future supply of free (low-entropy) energy in an environmentally sustainable manner. Compared to the hydrogen evolution reaction (HER), which is a proton-coupled two-electron process, the oxygen evolution reaction (OER) is a typical kinetic bottleneck of PEC devices since the mechanism of oxygen generation is much more complex, involving concerted transfer of four electrons and four protons. Accordingly, the development of efficient, stable and lowcost photoanodes for water oxidation is of paramount importance for the realization of economically viable PEC water splitting devices. Some of the most intensely investigated photoanodes are based on passivated high-quality semiconductors (e.g., Si or III-V compounds), hybrid inorganic-organic architectures, or on low-cost metal oxides, such as Fe2O3, CuWO4 or BiVO4. In particular, BiVO4 is an attractive material owing to its bandgap energy of ~2.4-2.6 eV, which translates to the maximum theoretically achievable photocurrents of 6.4-8.9 mA cm -2 and the solar-tohydrogen (STH) efficiencies of 8-11% under AM 1.5G (1 sun) illumination. However, the PEC performance of pristine BiVO4 photoanodes is typically far below the theoretical limit, both in terms of maximum photocurrents and achievable photovoltages, the latter manifested by rather positive photocurrent onset potentials (with respect to the reversible hydrogen electrode, RHE) and generally poor fill-factors of the photocurrent density-potential (J-V) curves. The reasons for this suboptimal performance have been identified as (i) poor bulk electronic conductivity of BiVO4 and the presence of surface intragap states, both of which result in severe electron-hole recombination, and (ii) ineffective hole extraction and slow rate of water oxidation at pristine BiVO4 surfaces. Concerning the former, significant improvements in performance have been demonstrated, for example, by doping BiVO4 with molybdenum, whereby the beneficial effect of Mo doping has been ascribed to increased electronic conductivity and/or passivation of surface recombination centers. In addition, further improvements have been reported after modification of BiVO4 with various additional hole extractors and/or OER catalysts, such as rhodium oxide, vanadium oxide, iron oxyhydroxide, cobalt oxide, cobalt hexacyanoferrate, iron/nickel oxide, or cobalt/nickel sulfide, whereby the question of whether the improvement should be ascribed to enhanced water oxidation kinetics or to other factors (e.g., enhanced charge extraction or passivation of surface recombination centers) is still a source of intense debates and seems to depend on specific catalysts and photoanode architectures. Well-defined polyoxometalates (POMs)molecular metal oxide clusterscomprising redox-active transition metals, e.g. Co or Mn, are known as effective catalysts in various (photo)electrocatalytic systems. A prototypical example is the complex CoPOM (Na10[Co4(H2O)2(PW9O34)2]) with a Co4O4 core stabilized by oxidatively resistant polytungstate ligands. Notably, this CoPOM has been reported to be an efficient homogeneous water oxidation catalyst, though there is also some evidence thatdepending on specific experimental conditions (e.g., pH and composition of electrolyte, catalyst concentration, applied electrode potential)this CoPOM can also undergo conversion to cobalt oxide, and thus behave rather as a pre-catalyst for the formation of a heterogeneous water oxidation catalyst (CoOx) which then often accounts for the major part of activity in water oxidation. Very recently, there have been several reports of successful immobilization of CoPOM onto various photoanodes (e.g., TiO2, Fe2O3, BiVO4, or TiO2/carbon nitride hybrids ), typically using diverse cationic polyelectrolytes (e.g., polyethyleneimine) or N-doped carbon as an interlayer or linker. However, the scope of CoPOM-modified photoanodes investigated so far is rather limited, and our knowledge of the factors governing the PEC performance of CoPOM-containing photoelectrochemical architectures is still underdeveloped, which makes the rational design of highperformance Co-POM-based photoanodes very challenging.