Mechanism of fast hydrogen generation from pure water using Al–SnCl2 and bi-doped Al–SnCl2 composites

The mechanism of fast hydrogen generation from pure water using selectively activated Al–SnCl2 composites was elucidated with the help of experimental data using combined XRD, SEM, EDX, DSC and calorimetric techniques. It is found that H2 is produced from two different but simultaneous routes specific to the Al–SnCl2 composite stoichiometry achieved after ball milling the precursor that readily yields, besides the excess of Al, Sn and AlCl3. Hydrogen is simultaneously produced from the reaction of the so-formed Al–Sn alloy with water, and from the reaction of the in situ generated AlCl3 with water, yielding HCl (protons) that further again react with Al, both reactions significantly increasing the hydrogen production rate. The effect of Bi on the hydrogen conversion yield on the Al–SnCl2 composite was also investigated. The electrochemical activity of Al is further enhanced by doping Bi into Al–SnCl2 composite. Meanwhile, DFT (density functional theory) calculations show that Bi micro domains present onto the Al (111) crystallite faces of the composite significantly reduce the adsorption energy of the OH groups while, Mg- or Cr-doped Al–SnCl2 composites increase this adsorption energy. The Mulliken charge analysis indicates that Bi leads to less electron transport between Al and O atoms (weaker interaction) than pristine Al (111) surface. Bi therefore contributes to inhibit the formation of the hydroxyls on the Al metal surface, thereby allowing the clean metal to continuously react with water.