# Solid-State Hydrogen Storage in Atomic Layer Deposited α‑MoO3 Thin Films

## TL;DR

This paper explores using molybdenum trioxide thin films for solid-state hydrogen storage, which could offer advantages over traditional compressed or liquid storage methods.

## Contribution

The study demonstrates that α-MoO3 thin films can reversibly store hydrogen at low pressure and room temperature, with potential for practical applications.

## Key findings

- Hydrogen plasma can effectively hydrogenate α-MoO3 thin films at room temperature and low pressure.
- Hydrogen atoms form covalent bonds with monovalent oxygen atoms in the van der Waals gaps of α-MoO3.
- The hydrogen storage process is reversible and retains capacity after multiple cycles.

## Abstract

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
x
O
y
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.