In spite of the notorious instability of Si in alkaline solutions, Si partially covered with hemispherical Ni particles show striking performances for sunlight-assisted water oxidation.

Molecular engineering of efficient HER catalysts is an attractive approach for controlling the spatial environment of specific building units selected for their intrinsic functionality required within the multistep HER process. As the {Mo 3 S 4 } core derived as various coordination complexes has been identified as one as the most promising MoS xbased HER electrocatalysts, we demonstrate that the covalent association between the {Mo 3 S 4 } core and the redox-active macrocyclic {P 8 W 48 } polyoxometalate (POM) produces a striking synergistic effect featured by high HER performance. Various experiments carried out in homogeneous conditions showed that this synergistic effect arises from the direct connection between the {Mo 3 S 4 } cluster and the toroidal {P 8 W 48 } units featured by a stoichiometry which can be tuned from two to four {Mo 4 S 4 } cores per {P 8 W 48 } unit. In addition, we report that this effect is preserved within heterogeneous photoelectrochemical devices where the {Mo 3 S 4 }-{P 8 W 48 } (thio-POM) assembly was used as co-catalyst (cocat) onto a microstructured p-type silicon. Using drop-casting procedure to immobilize cocat onto the silicon interface led to high initial HER performance under simulated sunlight achieving a photocurrent density of 10 mA.cm-2 at +0.13 V vs RHE. Furthermore, electrostatic incorporation of the thio-POM anion cocat into a poly(3,4-ethylenedioxythiophene) (PEDOT) film is demonstrated to be efficient and straightforward to durably retain the cocat at the interface of a micropyramidal silicon (SimPy) photocathode. The thio-POM/PEDOT-modified photocathode is able to produce H 2 under 1 Sun illumination at a rate of ca. 100 µmol cm-2 h-1 at 0 V vs RHE, highlighting the excellent performance of this photoelectrochemical system.