Catalytic effect of H3PW12O40 on hydrogen storage of MgH2

Abstract

Developing hydrogen energy to replace carbon-rich fossil fuels is the future direction of energy technology, but there is still a lack of safe and efficient hydrogen storage technology. Hydrogen storage in solid medium is a relatively safe way to store hydrogen, among which magnesium hydride (MgH₂) is one of the most promising solid hydrogen storage materials. MgH₂ has the advantages of high hydrogen storage density, low cost and good reversibility of hydrogen absorption and release. However, improving its poor thermodynamic and slow kinetic characteristics are still challenging. Catalysts derived from polyoxometalates have been successfully used for catalyzing hydrogen evolution reaction, oxidation of organic compounds, desulfurization reaction, and so on. However, these catalysts have not been applied to the hydrogen storage materials yet. In this paper, H₃PW₁₂O₄₀ is selected as a representative of polyoxometalates and its catalytic effect on hydrogen storage is studied. MgH₂-xH₃PW₁₂O₄₀ (x ​= ​7 ​%, 10 ​%, 13 ​%, mass percentage) and pure MgH₂ samples are prepared by mechanical ball milling method. Among them, MgH₂-10H₃PW₁₂O₄₀ exhibits the optimal performance in both kinetic characteristic and hydrogen storage capacity. It can rapidly absorb 6.25 ​% hydrogen within 1 ​min at 250 ​°C and release 6.54 ​% hydrogen within 15 ​min at 300 ​°C, while ball-milled MgH₂ only releases 1.2 ​% hydrogen within 30 ​min at 300 ​°C. At the same time, the activation energy of the composite decreases to 106.08 ​kJ ​mol⁻¹, which is 46.23 ​kJ ​mol⁻¹ lower than MgH₂. The catalytic effect of H₃PW₁₂O₄₀ on the hydrogen storage properties of MgH₂ mainly comes from three aspects. Firstly, the addition of H₃PW₁₂O₄₀ helps to avoid the agglomeration of MgH₂ during the ball milling process, which makes the MgH₂ particles become smaller after ball milling, thus increasing the specific surface area of the interaction with hydrogen. Secondly, the addition of H₃PW₁₂O₄₀ makes MgH₂ produce a large number of defects and lattice distortion during ball milling, which provides more channels for hydrogen diffusion. Thirdly, the catalytic components of WO₃ and W are in situ formed during the ball milling process. They can be used as active components to accelerate the electron migration process, which promotes the cleavage of the Mg―H bond and the adsorption and dissociation of hydrogen.