Solid-state hydrogen storage in atomic layer deposited α−MoO3 thin films

Hydrogen is an energy vector capable of storing and supplying large amounts of energy, maximising 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, 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 a 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. Hydrogen has been shown to locate itself in the van der Waals gap along the [010] oriented α−MoO3 film. The hydrogen absorption process has been found to be totally reversible, with desorption of hydrogen effective at 350 °C / 4 hours under nitrogen atmosphere, and recoverable after repeated cycles. Furthermore, a nominal 13 nm AlxOy 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 pressurised 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.