**5. Conclusions**

Hydrogen generation rate is one of the most important parameters which must be considered for the design of engineering solutions for hydrogen energy applications. Indeed, sometimes a slow and long-term production of hydrogen is requested, while for other kinds of applications the instantaneous generation of a very large amount of hydrogen must be provided. For the given technological approach, various parameters should allow for the control of the hydrogen generation rates in as large a range as possible. In our paper, precise experimental conditions, allowing the tuning of the hydrogen generation process based on the oxidation reaction of porous silicon nanoparticles in aqueous solutions, are reported. The hydrogen generation rate is dependent on chemical composition and the concentration of alkali added in the oxidizing solutions. In particular, the higher the alkali concentration is, the higher the hydrogen generation rate is, while the global amount of the released hydrogen remains constant. The size distribution of the porous silicon nanopowder also a ffects the generation rate values of produced hydrogen. For example, the smaller the nanoparticle sizes are, the more intense the oxidation reaction and, consequently, the higher the hydrogen generation rates are. Finally, hydrogen release below 0 ◦C is one of the significant advantages of such a technological way of hydrogen generation in comparison with numerous other developed technologies reported earlier. The reported experimental results confirm a huge technological potential of the hydrogen generation based on the splitting of Si–H bonds of porous silicon nanopowders and water molecules during the oxidation reaction.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/7/1413/s1, Figure S1: Photos of initial partially hydrogenated PSi nanopowder (before its reaction with water) and silicon oxide nanopowder obtained after hydrogen generation from oxidation of the PSi nanopowder in aqueous solutions, Figure S2: Nano-PSi: porosity as a function of the current density for three di fferent HF concentrations (18% (HF:Ethanol = 1:1.66), 24% (1:1), 36% (3:1)), thickness of the porous layer is 7 μm, initial substrate was p-type Si (ρ~1–10 Ω·cm), Figure S3: Meso-PSi: porosity as a function of the current density for three di fferent HF concentrations (18% (HF:Ethanol = 1:1.66), 24% (1:1), 36% (3:1)), thickness of the porous layer is 12 μm, initial substrate was p+- type Si (ρ~10-50 Ω·cm), Figure S4: Nano-PSi: porosity as a function of the current density for three different HF concentrations (18% (HF:Ethanol = 1:1.66), 24% (1:1), 36% (3:1)), thickness of the porous layer is 12 μm, initial substrate was p++- type Si (ρ~1–3 m Ω·cm), Figure S5: Dimentions of silicon nanocrystalls as function of porosity for the meso- and nano-PSi powders, Figure S6: E ffusion curves for H2 desorption from the nanoand meso-PSi powders, Figure S7: Photo of the working system used for measurements of hydrogen generation kinetics, Figure S8: Hydrogen generation rates deduced from experimental the curves shown in Figures 5 and 6, Figure S9: Hydrogen amount in terms of %mass (%m) versus the ratio between mass of the oxidizing solution (msol) and mass of meso-PSi nanopowder (mPS). Table S1: Comparison of the hydrogen generation methods, Table S2: Data summarizing the chemical reactions used to produce H2.

**Author Contributions:** Conceptualization, A.I.M., V.A.S. and V.L.; methodology, S.T., T.N. and S.V.L.; validation, G.M., S.A.A., S.T., T.N. and S.V.L.; formal analysis, all authors; investigation, G.M., S.A.A., S.T., T.N., S.V.L. and V.L.; resources, G.M., G.A., Y.S.; data curation, G.M., S.A.A., T.N. and V.L.; writing—original draft preparation, G.M., S.A.A., A.I.M. and V.L.; writing—review and editing, all authors; visualization, G.M., A.I.M. and T.N.; supervision, Y.S., G.A., S.V.L., V.A.S. and V.L.; project administration, Y.S., G.A. and V.A.S.; funding acquisition, G.A., V.A.S. and V.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by the Committee of Science of the Ministry of Education and Science of the Republic of Kazakhstan, gran<sup>t</sup> number AP05133366, and Ministry of Education and Science of Ukraine, gran<sup>t</sup> number 0119U100326.

**Acknowledgments:** G.M. is grateful to AI-Farabi Kazakh National University for financial support in the frame of the postdoctoral fellowship program.

**Conflicts of Interest:** The authors declare no conflict of interest.
