Improvements in the hydrogen storage properties of the Mg(NH2)2-LiH composite by KOH addition

G. AMICA; S. ENZO; P. ARNEODO LAROCHETTE; F. C GENNARI

Río de Janeiro

Conferencia; WHEC 2018 (World Hydrogen Energy Conference); 2018

International Association for Hydrogen Energy (IAHE)

Solid-state hydrogen storage for mobile applications is a safe and efficient method which provides improved volumetric densities at moderated pressures and temperatures. Among the complex metal hydrides storage systems developed, the Mg(NH2)2-LiH composite exhibits moderate operating temperatures, appropriate thermodynamics, good reversibility and relatively high hydrogen storage capacity (5.5 wt% H) [1]. Introducing a non-transition-metal dopant like potassium could be a strategy to enhance the dehydrogenation of this system [2]. In this work, two K-containing compounds (6wt% of KCl and KOH) were considered as additives to modify the Mg(NH2)2-2LiH composite. The KCl addition did not cause major changes in the shape of the DSC curve in comparison to the pristine, suggesting that the additive did not interact with the sample at these conditions during milling or heating. Conversely, the KOH addition was traduced in a 43°C shift of the main event towards lower temperatures with a beginning of peak rise at relatively low temperatures (~70°C), which falls into the operating temperature range for PEMs. The effect of repetitive hydrogen sorption cycling on the kinetic and thermodynamic performance at 200°C was evaluated for the KOH doped sample. The doped sample was able to reversibly absorb/desorb 4.6 wt% of hydrogen while kinetics was distinctly improved in comparison to the pristine: the four times increase in the dehydrogenation rate evidenced the beneficial utilization of potassium on the composite. Moreover, consecutive volumetric measurements showed that kinetics was improved with cycling during the first cycles, which implies that the material may be changing its composition. Pressure-composition isotherms of dehydrogenation and hydrogenation showed a notorious increase in the equilibrium dehydrogenation pressure in more than 30% at 200 °C, evidencing a thermodynamic destabilization of the Mg(NH2)2-2LiH system by the addition of a small amount of KOH. In order to clarify the role of K additive, several doped samples taken at different stages of dehydrogenation under isothermal and non-isothermal conditions were obtained. The structure and composition of these samples were determined by XRPD and FTIR as well as Rietveld analysis. Moreover, to investigate the nature of the reaction between KOH and LiNH2, MgH2 or LiH, additional measurements were performed by TG. On the basis of the collected experimental information, a proposal of dehydrogenation pathway is presented. Present study opens new opportunities for designing K-modified Mg(NH2)2?LiH composite as promissory candidates for hydrogen storage.