Kinetics of hydrogen storage on catalytically-modified porous silicon

Abstract

Porous silicon has been demonstrated as a hydrogen storage media with surface-bound hydrogen content as high as 6.6% by weight. Hydrogenated porous silicon is readily synthesized by electrochemical etching in a solution of hydrofluoric acid. Hydrogen gas can be released thermally at temperatures starting at 280 °C. It has been proposed that a suitable catalyst at the pore mouth can both reduce the desorption temperature and facilitate gaseous recharge of the silicon matrix. This work presents a detailed kinetic study using density functional theory (DFT) of a reversible hydrogen storage system based on porous silicon via the mechanisms of dissociation, spillover, and bond-hopping of hydrogen atoms. For each of these steps, activation energy values and vibrational frequency has been determined. Using these activation energies along with vibrational frequency values evaluated from the micro level DFT study, the kinetic performance of catalytically-modified porous silicon as a potential hydrogen storage material has been completed for the first time. The energy difference between full and empty charge is computed at the atomic scale and compared to macroscopic calculations, showing close agreement. These results show the potential for rapid recharge at 8 bar at temperatures commensurate with waste heat from a proton-exchange membrane fuel cell.