Solid-State Hydrogen Storage in Atomic Layer Deposited α-MoO ₃ Thin Films

Hydrogen is an energy vector capable of storing and supplying large amounts of energy, maximizing the benefits of renewable and sustainable energy sources. Hydrogen is usually stored as compressed hydrogen gas or liquid hydrogen. However, the former requires high pressure and the latter cryogenic temperatures, being a huge limit to the widespread adoption of these storage methods. Thus, new materials for solid-state hydrogen storage shall be developed. Here, we show that an α-MoO₃ thin film, grown via atomic layer deposition, is a material with potential for reversibly storing hydrogen. We found that hydrogen plasma is a convenient way to hydrogenate - at room temperature and relatively low pressures (200 mTorr) - layered α-MoO₃ thin films. Density functional theory calculations of stepwise hydrogen insertion into α-MoO₃ reveal that hydrogen atoms preferentially form covalent bonds with monovalent oxygen atoms located within the van der Waals gaps separating the [010]-oriented layers. The hydrogen absorption process has been found to be totally reversible, with desorption of hydrogen effective at 350 °C/4 h under a nitrogen atmosphere, and recoverable after repeated cycles. Furthermore, a nominal 13 nm Al _<i>x</i> O _<i>y</i> capping layer, grown via atomic layer deposition, has been shown to be efficient in preventing hydrogen release. The volumetric hydrogen storage capacity of 28 kg·m^-3 achieved in our films is comparable to that of pressurized steel cylinders, highlighting their potential for practical applications. Our essay could be a starting point to a transition from conventional (gas and liquid) to more advantageous solid-state hydrogen storage materials.