Improved kinetic behaviour of Mg(NH2)2-2LiH doped with nanostructured K-modified-LixTiyOz for hydrogen storage

Abstract

the system Mg(nH 2 ) 2 + 2LiH is considered as an interesting solid-state hydrogen storage material owing to its low thermodynamic stability of ca. 40 kJ/mol H 2 and high gravimetric hydrogen capacity of 5.6 wt.%. However, high kinetic barriers lead to slow absorption/desorption rates even at relatively high temperatures (>180 °C). In this work, we investigate the effects of the addition of K-modified Li x ti y o z on the absorption/ desorption behaviour of the Mg(nH 2 ) 2 + 2LiH system. In comparison with the pristine Mg(NH 2 ) 2 + 2LiH, the system containing a tiny amount of nanostructured K-modified Li x ti y o z shows enhanced absorption/ desorption behaviour. The doped material presents a sensibly reduced (∼30 °C) desorption onset temperature, notably shorter hydrogen absorption/desorption times and reversible hydrogen capacity of about 3 wt.% H 2 upon cycling. Studies on the absorption/desorption processes and micro/nanostructural characterizations of the Mg(nH 2 ) 2 + 2LiH + K-modified Li x ti y o z system hint to the fact that the presence of in situ formed nanostructure K 2 tio 3 is the main responsible for the observed improved kinetic behaviour. Scientific RepoRtS |(2020) 10:8 | https://doi.org/10.1038/s41598-019-55770-y www.nature.com/scientificreports www.nature.com/scientificreports/ According to the calculated thermodynamic properties of the Mg(NH 2 ) 2 + 2LiH stoichiometric mixture, operating temperatures of 90 °C can be achieved at 1 bar 18 . However, sufficient dehydrogenation rates, even after intense ball milling treatment, can be obtained only at temperatures above 180 °C due to harsh kinetic constraints 19 . Several attempts have been made in order to improve the sluggish dehydrogenation behaviour of the Mg(NH 2 ) 2 + 2LiH composite system . Potassium containing additives effectively reduce the dehydrogenation peak temperature down to 130 °C, which is ∼50 °C lower than that of pristine Mg(NH 2 ) 2 + 2LiH 40-42 . However, due to segregation phenomena that occurs at high-temperature (≥180 °C) upon cycling, the inhomogeneous distribution of the K-species reduces their catalytic activity 43 . Therefore, the design/synthesis of new additives is mandatory in order to achieve long-lasting absorption/desorption properties. TiO 2 is one of the low-cost additives which enhance the hydrogen storage properties of the 2LiBH 4 + MgH 2 reactive hydride composite (RHC) system [44][45][46] . Puszkiel et al. showed that 2LiH + MgB 2 /2LiBH 4 + MgH 2 RHC system doped with core-shell Li x TiO 2 nanoparticles shows improved the kinetic and cycling behaviour 44 . It was found that the core-shell Li x TiO 2 nanoparticles act as Li + pumps, increasing Li + mobility, hence accounting for the observed enhanced hydrogen storage properties. Studies on reaction mechanism of Mg(NH 2 ) 2 + LiH system showed that diffusion of small ions (e.g., Li + , Mg +2 , and H + ) might account for the improved reaction kinetics [47][48][49][50] . In this work, we investigate the effect of Li x Ti y O z and potassium-modified Li x Ti ...

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