Knowledge and foundational understanding of phenomena associated with the behavior of materials at the nanoscale is one of the key scientific challenges toward a sustainable energy future. Size reduction from bulk to the nanoscale leads to a variety of exciting and anomalous phenomena due to enhanced surface-to-volume ratio, reduced transport length, and tunable nanointerfaces. Nanostructured metal hydrides are an important class of materials with significant potential for energy storage applications. Hydrogen storage in nanoscale metal hydrides has been recognized as a potentially transformative technology, and the field is now growing steadily due to the ability to tune the material properties more independently and drastically compared to those of their bulk counterparts. The numerous advantages of nanostructured metal hydrides compared to bulk include improved reversibility, altered heats of hydrogen absorption/desorption, nanointerfacial reaction pathways with faster rates, and new surface states capable of activating chemical bonds. This review aims to summarize the progress to date in the area of nanostructured metal hydrides and intends to understand and explain the underpinnings of the innovative concepts and strategies developed over the past decade to tune the thermodynamics and kinetics of hydrogen storage reactions. These recent achievements have the potential to propel further the prospects of tuning the hydride properties at nanoscale, with several promising directions and strategies that could lead to the next generation of solid-state materials for hydrogen storage applications.

Li 2 B 12 H 12 , Na 2 B 12 H 12 , and their closo-borate relatives exhibit unusually high ionic conductivity, making them attractive as a new class of candidate electrolytes in solid-state Li-and Na-ion batteries. However, further optimization of these materials requires a deeper understanding of the fundamental mechanisms underlying ultrafast ion conduction. To this end, we use ab initio molecular dynamics simulations and density-functional calculations to explore the motivations for cation diffusion. We find that superionic behavior in Li 2 B 12 H 12 and Na 2 B 12 H 12 results from a combination of key structural, chemical, and dynamical factors that introduce intrinsic frustration and disorder. A statistical metric is used to show that the structures exhibit a high density of accessible interstitial sites and site types, which correlates with the flatness of the energy landscape and the observed cation mobility. Furthermore, cations are found to dock to specific anion sites, leading to a competition between the geometric symmetry of the anion and the symmetry of the lattice itself, which can facilitate cation hopping. Finally, facile anion reorientations and other low-frequency thermal vibrations lead to fluctuations in the local potential that enhance cation mobility by creating a local driving force for hopping. We discuss the relevance of each factor for developing new ionic conductivity descriptors that can be used for discovery and optimization of closo-borate solid electrolytes, as well as superionic conductors more generally.

Polyborane salts based on B12H12 2–, B10H10 2–, CB11H12 –, and CB9H10 – demonstrate high Li and Na superionic conductivity that makes them attractive as electrolytes in all-solid-state batteries. Their chemical and structural diversity creates a versatile design space that could be used to optimize materials with higher conductivity at lower temperatures; however, many mechanistic details remain enigmatic, including reasons why certain known modifications lead to improved performance. We use extensive ab initio molecular dynamics simulations to explore the dependence of ionic conductivity on cation/anion pair combinations for Li and Na polyborane salts. Further simulations are used to probe the influence of local modifications to chemistry, stoichiometry, and composition. Carbon doping, anion alloying, and cation off-stoichiometry are found to favorably introduce intrinsic disorder, facilitating local deviation from the expected cation population. Lattice expansion likewise has a positive effect by aiding anion reorientations that are critical for conduction. Implications for engineering polyboranes for improved ionic conductivity are discussed.

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We dedicate this paper to Natchapol "Golf" Poonyayant, who died tragically at the age of 25 during the writing of this manuscript. Golf's interest in nanoconfined materials inspired this work, and he performed the experimental synthesis and much of the analysis. The world has lost a talented young man, and we've lost a dear friend.