Mechanochemically Tailored Silicon Particles for Efficient H₂ Production: Entropy and Enthalpy Engineering

Abstract

Silicon–water reactions can generate large amounts of H2, and hence, discarded solar cells can be upcycled to produce reactants. However, concerns about silicon (Si) particle preparation, hazardous chemicals, and unknown reaction mechanisms must be addressed. Herein, we demonstrate the tailoring of high-enthalpy, high-entropy Si particles for efficient H2 production using ball milling. Well-defined Si particles, with surface and internal structures characterized by using eight parameters, were reacted with alkaline water at low temperatures (30–70 °C). Two common assumptions, (i) the higher the mechanical energy, the better and (ii) the larger the surface area, the more efficient the reaction, were proved wrong in this case. Indeed, the Si particles exhibiting the best H2 production capacity were produced by grinding for 3 min without adding any chemicals. In addition, the increases in enthalpy and entropy imparted to the Si particles and the mechanical collision energy and surface and internal structures were determined. Thus, the Gibbs energy of the H2 production reaction and its activation barrier were well-defined, and the Si–water reaction mechanism was deduced. As scaling-up increased the energy efficiency of mechanochemical H2 production, which could be comparable to that of electrolysis, sustainable H2 production in the future is possible.