Abstract
Mg-based hydrides are one of the most promising hydrogen storage materials because of their relatively high storage capacity, abundance, and low cost. However, slow kinetics and stable thermodynamics hinder their practical application. In contrast to the substantial progress in the enhancement of the hydrogenation/dehydrogenation kinetics, thermodynamic tuning is still a great challenge for Mg-based alloys. At present, the main strategies to alter the thermodynamics of Mg/MgH2 are alloying, nanostructuring, and changing the reaction pathway. Using these approaches, thermodynamic tuning has been achieved to some extent, but it is still far from that required for practical application. In this article, we summarize the advantages and disadvantages of these strategies. Based on the current progress, finding reversible systems with high hydrogen capacity and effectively tailored reaction enthalpy offers a promising route for tuning the thermodynamics of Mg-based hydrogen storage alloys.
Highlights
Modern civilization would not be sustainable without sufficient energy and a clean environment.severe challenges will arise from the shortage of fossil energy resources and environmental pollution, owing to massive and long-term use of fossil fuels
The above mentioned metal hydrides have suitable hydrogen sorption thermodynamics and fast kinetics, which means that hydrogen sorption can take place with a fast rate at suitable temperatures and hydrogen pressure conditions
[59], first-principles DFT calculations gave the dehydrogenation enthalpies for the MgH2 bulk, nanowires, and single molecule as 74.0, 37.6, and −16.4 kJ/mol, respectively. These results indicate that decreasing the diameter leads to thermodynamic destabilization of MgH2 nanowires, and that hydrogen desorption is possible at room temperature for MgH2 nanowire of a diameter of 0.85 nm [59]
Summary
Modern civilization would not be sustainable without sufficient energy and a clean environment. The above mentioned metal hydrides have suitable hydrogen sorption thermodynamics and fast kinetics, which means that hydrogen sorption can take place with a fast rate at suitable temperatures and hydrogen pressure conditions They all have low gravimetric hydrogen storage density and do not satisfy the application requirements as energy sources, in particular the goal for the storage of at least 5.5 wt % by the year 2017 set for onboard automotive hydrogen storage systems by the United States Department of Energy (DOE) [14]. Mg-based hydrides have relatively high storage capacity (theoretically 7.6 wt % for MgH2), are abundant and inexpensive, and are considered to be the most promising metallic hydrogen storage materials. Tuning of the thermodynamic properties is still a great challenge, and we will review the progress in detail
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