Recent advances in metal hydrides for clean energy applications
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Introduction Hydrogen storage will be a cornerstone technology for energy utilization in the 21st century as components of a hydrogen economy come together. Figure 1 represents the clean, nonpolluting cycle of the “hydrogen economy”1 utilizing hydrogen as an energy carrier to eventually replace fossil fuels and enable an environmentally friendly world with a sustainable infrastructure, including hydrogen production, storage, and delivery. By using renewable resources such as sunlight to split water, hydrogen may be produced cleanly, and can efficiently be stored in metals and other complex host frameworks for use in fuel cells, which could power many motive and non-motive applications, including forklifts, wheelchairs, lawn mowers, bicycles, portable power, laptops, stationary and grid applications.2,3 The product from operating a fuel cell is water, and implementing a hydrogen-based economy is therefore the cleanest long-term solution to generate power. Metal hydrides are a fascinating and important class of materials that can be used for various energy applications and devices, including batteries, hydrogen storage, thermal energy storage, heating/cooling devices, thin films for smart solar collectors, smart windows, and sensors.4–8 Several interesting physical phenomena have been observed with the introduction of hydrogen into a metal or metal alloy (e.g., electric, magnetic, optical, and mechanical transitions).
Phase transitions occur reversibly from normal conductance to superconductivity, from metals to semiconductors or insulators, or from ferromagnetic to paramagnetic under certain pressures and temperatures. Some of these phenomena are described in this issue. The last MRS Bulletin theme issue devoted to hydrogen storage in 20029 came at a time of resurgent interest brought about by Bogdanovic’s seminal paper10 illustrating reversible hydrogen storage in sodium alanate, NaAlH4 when doped with transition metals. A few alanate compounds had been previously studied, but without transition metal dopants, they were effectively irreversible. This discovery more than doubled the weight percent of hydrogen stored in solid compounds from about 1.6% for LaNi5H6 to 5.5% for NaAlH4. The research that followed this discovery, largely funded through the US Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) and Basic Energy Sciences (BES), led to the discovery and study of a great many new compounds and even compound classes for hydrogen storage. During these years, research focus shifted to hydrogen-rich complex metal hydrides, and in this issue, Graetz and Hauback review progress on alane and alanates, and Wang et al. review borohydrides and amides. It is impossible to cover all progress and phenomena, but we have selected specific areas to showcase the breadth of potential clean energy applications.
Ewa C.E. Rönnebro, Pacific Northwest National Laboratory, Energy and Environment Directory; [email protected] Eric H. Majzoub, University of Missouri–St. Louis, Department of Physics and Astrono
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