Metal Assisted Chemical Etching (MACE) of Si has attracted the attention of academy and industry during the last decades as an efficient low-cost wet etching method to produce Si nanostructures with high aspect ratios (HAR). Several noble metals are known to be effective catalysts for MACE: Ag nanoparticles (NPs), for example, provide an extremely localized etching with the formation of mesopores or Si nanowire arrays; contrarily, MACE with Pt NPs is rather delocalized, resulting in the formation of large pores whose potential application in Si surface structuration has received less attention. In this work, MACE with Pt NPs under an applied external bias is presented as a novel approach to synthesize HAR Si nanostructures of controlled size and shape, with clear application as built-in blocks for photovoltaic devices, the reflectivity being < 3 % vs. ~10 % for the state of the art texturization technique (inverted pyramids). A combination of voltammetry, impedance spectroscopy and band bending modelling allowed complete physicochemical characterization of this MACE process. This simple method allows straightforward control of the pore morphology, such that nanostructures ranging from straight mesopores to cone-shaped macropores are readily obtained as Si is biased from negative to positive potentials. Because such morphologies are difficult to obtain even with techniques like cryogenic plasma, etching MACE with Pt may have a strong potential for Si surface structuration.