Recent advances in the theory of hydrogen storage in complex metal hydrides
- PDF / 1,955,839 Bytes
- 11 Pages / 585 x 783 pts Page_size
- 11 Downloads / 235 Views
Introduction Among the available options for onboard storage of hydrogen in passenger vehicles powered by a proton exchange membrane (PEM) fuel cell1—compressed gas, cryogenic liquid, chemical hydrogen storage, sorbents, and solid hydrides—the latter stand out with several attractive features: low-cost materials, high volumetric and gravimetric densities, compatibility with onboard recharging at practical conditions, and high cycling stability (see the Introductory article in this issue). Waste heat from the PEM fuel cell can be used to supply the energy required for hydrogen release if the equilibrium temperature is tuned to the PEM operating temperature of approximately 80–90°C. First-generation metal hydrides based on LaNi5, a famous alloy that was developed for the NiMH-battery, offer volumetric storage densities exceeding those in cryogenic liquid H2, along with fast kinetics and highly compositiontunable thermodynamics. Unfortunately, the gravimetric density of hydrogen that can be stored in a metal hydride such as LaNi5H7 is only approximately 2%, which is too low to be economically viable for use in passenger vehicles. The fast kinetics and low gravimetric density can be traced to a common cause: hydrogen in these materials occupies interstitial sites
so that the maximum number that can be accommodated is almost never larger than one hydrogen per metal atom. The seminal discovery of reversible hydrogen storage in transition metal doped sodium alanate (NaAlH4) by Bogdanovic and co-workers2 ignited immense interest in a different class of compounds wherein hydrogen is bound chemically in ionic complexes such as AlH4, BH4, and NH2, bringing about storage capacities exceeding a single hydrogen per metal atom. Contrary to the prevailing wisdom that these complex anions are bound too strongly and the corresponding compounds are completely irreversible, Bogdanovic et al. demonstrated that hydrogen can be extracted reversibly according to the following two-step reaction: NaAlH 4 ↔ 1/3Na 3AlH 6 + 2/3Al + H 2 ,
(I)
Na 3AlH6 ↔ 3NaH + Al + 3/2H 2 .
(II)
These reactions release more than 4 wt% H2 at 1 bar pressure within a few hours at 160°C and within an hour at 180°C.2 Even though the gravimetric density of Reaction I is too low for practical applications in hydrogen fuel cell vehicles (the
Kyle Jay Michel, Department of Materials Science and Engineering, Northwestern University; [email protected] Vidvuds Ozolin¸š, Department of Materials Science and Engineering, University of California–Los Angeles; [email protected] DOI: 10.1557/mrs.2013.130
462
MRS BULLETIN • VOLUME 38 • JUNE 2013 • www.mrs.org/bulletin
© 2013 Materials Research Society
RECENT ADVANCES IN THE THEORY OF HYDROGEN STORAGE IN COMPLEX METAL HYDRIDES
2017 US Department of Energy system target is 5.5 wt% H2),3 sodium alanate has served as a model system for understanding hydrogen storage in complex hydrides with polar covalent metal-hydrogen bonds. The next important discovery was by Chen and co-workers, who found that lithium amide, Li
Data Loading...
