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Supercritical drying: The drying process reported previously,¹ to obtain extremely high surface area pSi involved transferring the wet pSi sample from storage in IPA, to the drying chamber of a Quorum Technologies Ltd K850 critical point dryer; each sample was dried individually and always remained wet until flushing with liquid CO₂ several times before the final drying step.

Characterization: Aberration corrected scanning transmission electron microscopy (AC-STEM) analysis was conducted using a 200kV JEM 2100F equipped with a CEOS probe corrector. SCD-pSi sample was subjected to particle size reduction via grinding between two highly polished silicon wafers. The resulting particles were then blown off the wafer surface directly onto TEM grids using a nitrogen gun. The nanocrystalline particle size distribution obtained by Feret diameter analysis of the BF-STEM images using ImageJ.² Powder X-ray diffraction (XRD) analysis was carried out using a Bruker D8 Advance diffractometer at 40 kV, 40 mA for Cu Kα. Nitrogen adsorption-desorption isotherms of porous Si microparticles were recorded at 77 K using a Micromeritics TriStar 3000 volumetric apparatus. Prior to the adsorption experiment, the porous Si samples were degassed under vacuum overnight. The surface area of the sample was measured by the BET (Brunnauer-Emmett-Teller) method, which yields the amount of adsorbate corresponding to a molecular monolayer.