ARTICLE pubs.acs.org/JPCC
Hydrogen Storage Properties of 3Mg(NH2)22Li3AlH6 Dongan Liu,†,‡ Andrea Sudik,† Jun Yang,*,† Patrick Ferro,§ and Chris Wolverton|| †
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Ford Motor Company, Research and Advanced Engineering, 2101 Village Road, MD 1170/RIC, Dearborn, Michigan 48121, United States ‡ Department of Mechanical Engineering, University of Michigan, 1023 H. H. Dow Building, 2350 Hayward Street, Ann Arbor, Michigan 48109, United States § Department of Mechanical Engineering, Gonzaga University, 502 East Boone Avenue, Spokane, Washington 99258, United States Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, United States
bS Supporting Information ABSTRACT: Recent efforts have been made to develop high-capacity complex hydride composites by combining alanates and amides. The hydrogen storage mechanisms in those composites are not unambiguously clarified because of chemical reactions during the sample preparation process. In this Article, we have studied the effects of sample preparation conditions on the phase stability of a mixture of 3Mg(NH2)22Li3AlH6 and identified that unlike high-energy ball-milling light mixing generates a physical mixture of the reactants without decomposition. Subsequently, the hydrogen storage properties, the desorption pathway, and the reversible reaction mechanism of the composite were investigated through a combination of kinetic measurements and phase and microstructure analyses. The results reveal that the first step of hydrogen release (initiated at 170 °C) involves decomposition of Li3AlH6 to LiH and Al. The second step of hydrogen release occurs as the temperature increases (to 230 °C) when Mg(NH2)2 reacts with LiH to form Li2Mg(NH)2. If desorption of the 3Mg(NH2)22Li3AlH6 mixture is limited to a temperature of 400 °C, then the reversible reaction takes place between Li2Mg(NH)2 (plus H2) and LiH and Mg(NH2)2. We find that the Al generated from the first hydrogen release step does not participate in the reversible hydrogen storage process and instead inhibits mass transfer that results in higher desorption temperatures (and low desorption rates) and lowers the overall reversible capacity (2.7 wt %) as compared with the neat reaction (i.e., ∼4.3 wt % without the presence of Al).
1. INTRODUCTION Complex hydrides have been studied extensively for on-board hydrogen storage applications because of their high hydrogen capacities by both weight and volume.1 However, many of the complex hydrides evaluated to-date, especially high-capacity borohydrides (i.e., tetrahydroborates) and amides, do not possess favorable reaction thermodynamics or kinetics to realize reversible hydrogen storage for vehicular applications where desired operating temperatures are 15 to 85 °C.2,3 Over the past decade, worldwide research efforts have focused on tailoring the reaction thermodynamics of complex hydrides through the discovery of new reactant or mixtures of existing high-capacity borohydrides, amides, or alanates.46 The hydrogen storage kinetics for these thermodynamically promising new mixtures have also been enhanced through catalytic doping or microstructural engineering.7 As an example, catalyst-doped sodium alanate (NaAlH4) possesses much improved lower temperature reversible properties as compared with the undoped material.812 Despite these improvements, the reversible materials-based capacity for NaAlH4 is still too low for practical applications (e.g., the 2015 r 2011 American Chemical Society
Department of Energy (DOE) on-board system-based storage target is 5.5 wt %).1 Therefore, lithium alanates were subsequently investigated given their higher theoretical materialsbased hydrogen capacities, 10.5 and 11.1 wt % for LiAlH4 and Li3AlH6, respectively. Unfortunately, neither material reversibly stores hydrogen within the desired temperaturepressure range (i.e.,
