One recent approach to modify BiVO4 photoelectrodes is incorporation of polyoxometalates (POMs) as single-source molecular precursors. POMs are molecular metal oxide clusters formed by self-assembly in solution. Of specific interest in this context is the cobalt-functionalized polyoxometalate [Co4(H2O)2(PW9O34)2] 10-(CoPOM), where a {Co4O4} core is stabilized by two oxidatively resistant polyoxotungstate ligands. The compound has been reported as an active water oxidation catalyst (WOC) as well as precursor to highly reactive cobalt oxyhydroxies, so that operation under homogeneous or heterogenized conditions is possible. This in turn has attracted major interest for surface-deposition as cocatalyst on BiVO4 photoanodes. For example, Ryu et al. successfully designed a photoanode by depositing a thin film of cationic polyelectrolytes and anionic CoPOM-WOCs on the surface of various photoelectrode materials (e.g. Fe2O3, BiVO4 and TiO2) for improving the PEC performance as well as stability. Fan and co-workers reported CoPOM as a novel molecular co-catalyst deposited on N-doped carbon (N/C) to boost the charge separation and injection of BiVO4 photoanodes for PEC water oxidation. Some of us have recently reported a direct linking of CoPOMs to the surface of Modoped BiVO4 photoanodes. The system showed significantly enhanced PEC performance, featuring lower photocurrent onset potentials and higher charge transfer efficiency. These initial studies suggest that CoPOM is a promising co-catalyst for PEC water oxidation at BiVO4 photoanodes, as it improves the photogenerated hole extraction and enhances the oxygen evolution reaction kinetics. This inspired the present study to explore how a "double use" of bulk and surface deposition of CoPOM can be used to optimize the performance Notably, transition metal doping is an established strategy to improve the conductivity of BiVO4 and promote the PEC performance. For example, He et al. developed BiVO4 nanoflake array films co-doped with Mo and W to increase the conductivity of BiVO4 and to slightly to enhance the water oxidation kinetics. Fang et al. reported that cobalt ion-doped BiVO4 photoanodes exhibit an improved PEC performance, rational-ized by the fact that a part of Co ions are doped into the BiVO4 lattice to increase its electronic conductivity, whereby excess Co ions form a Co3O4 co-catalyst on the BiVO4 surface. Shi et al. reported that W-doped BiVO4 shows a higher donor density and a higher concentration of surface states that can act as reaction sites, leading thus to improvements of both conductivity and surface catalytic activity. To-date, using POMs as a doping agent has been reported, for example, in the fields of gas sensing application, dye-sensitized or perovskite solar cells, thiol detection, and photocatalysis. Due to the abundancy of transition metals involved in typical POMs, they also hold potential as doping agents in PEC water splitting systems. In view of enhancing the electron extraction and transport from BiVO4 to the underlying FTO conductive glass substrate, Xi et al. fabricated composite photoanodes consisting of BiVO4 and selected POMs, such as [H3PW12O40] (PW12) and K6[CoW12O40] (CoW12). However, CoPOMs have rarely been used as doping agent to manipulate the electronic structure of BiVO4 film photoanodes in a PEC water splitting system, andto the best of our knowledge -CoPOM has not been used both as a dopant as well as a co-catalyst for BiVO4 photoanodes.