Unpacking SiO2 Precursor Effects on Aluminothermic Reduction in Molten NaCl-KCl Mixture to Yield Porous Silicon

Developing lower-energy routes to silicon-based materials is important for improving the sustainability of chemical processes. Porous silicon (p-Si) is a nontoxic material with applications in CO2 reduction, photocatalytic water splitting, lithium-ion batteries, and on-demand hydrogen generation, yet conventional silicon production relies on energy-intensive carbothermal reduction of SiO2 at >1500 °C. Aluminothermic reduction offers a lower-temperature alternative, but many reported methods rely on volatile AlCl3 salt. Recently, we reported a molten-salt-assisted aluminothermic reduction of fumed SiO2 using a NaCl–KCl mixture that produces p-Si within 2 h and can yield large quantities of hydrogen on-demand. Here, we evaluate the generality of this approach by applying it to additional SiO2 precursors (-325 mesh and Stöber silica) and investigate the factors governing reduction efficiency. We find that the extent of reduction is strongly dependent on precursor surface area, with high-surface-area fumed SiO2 undergoing the most complete conversion and Stöber precursor the least. Electron microscopy reveals substantial molten-salt-induced restructuring of SiO2, indicating that precursor morphology and surface area play key roles in controlling reduction kinetics. These results establish structure–reactivity relationships for molten-salt aluminothermic reduction and provide guidance for designing more efficient and scalable routes to porous silicon.