Properties and Applications of Metal (M) dodecahydro-closo-dodecaborates (Mn=1,2B12H12) and Their Implications for Reversible Hydrogen Storage in the Borohydrides

Abstract

Hydrogen has long been proposed as a versatile energy carrier that could facilitate a sustainable energy future. For an energy economy centred around hydrogen to function, a storage method is required that is optimised for both portable and stationary applications and is compatible with existing hydrogen technologies. Storage by chemisorption in borohydride species emerges as a promising option because of the advantages of solid-state storage and the unmatched hydrogen energy densities that borohydrides attain. One of the most nuanced challenges limiting the feasibility of borohydride hydrogen storage is the irreversibility of their hydrogen storage reactions. This irreversibility has been partially attributed to the formation of stable dodecahydro-closo-dodecaborates (M n=1,2 B 12 H 12 ) during the desorption of hydrogen. These dodecaborates have an interesting set of properties that are problematic in the context of borohydride decomposition but suggest a variety of useful applications when considered independently. In this review, dodecaborates are explored within the borohydride thermolysis system and beyond to present a holistic discussion of the most important roles of the dodecaborates in modern chemistry.

Metal Borohydrides beyond Groups I and II: A Review

“…Indeed, the formation of [B 3 H 8 ] − intermediaries in Y borohydride and [B 12 H 12 ] 2− in Sc borohydride [ 235 ] (and references within) has been reported. Other possible intermediaries include [B 10 H 12 ] 2− and [B 11 H 14 ] − [ 267 , 268 ]. For its part, the rapid dissociation of a BH 4 • radical into BH 3 an H • above 250 K was predicted by means of ab-initio MO calculations [ 269 ].…”

High Yield Autoclave Synthesis of pure M2B12H12 (M = Na, K)

“…4 However, the major problem is the side reactions transforming BH 4 − anions into more stable dodecaborates, B 12 H 12 2− , that cannot be easily rehydrogenated; therefore, these compounds are often called "boron sinks" in this context. 5 On the other hand, metal dodecaborates are interesting compounds as they find applications in cancer treatment, polymer chemistry, and, more recently, as solid-state ionic conductors. 6−11 They contain closed icosahedral boron clusters with 12 terminal hydrogen atoms in each corner (as shown in Figure 1) and have remarkable chemical and thermal stability.…”

“…This includes reactive hydride composites with binary hydrides, such as LiBH 4 and MgH 2 , which are capable of storing hydrogen reversibly . However, the major problem is the side reactions transforming BH 4 – anions into more stable dodecaborates, B 12 H 12 2– , that cannot be easily rehydrogenated; therefore, these compounds are often called “boron sinks” in this context …”

Complex Metal Borohydrides: From Laboratory Oddities to Prime Candidates in Energy Storage Applications

“…While the risk of running out of fossil fuels has been advocated for decades, the timeline is approaching and humanity needs to make a change. Therefore, alternative energy carriers are long sought-after; among them, metal hydrides and complex borohydrides have a special role [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. Reaching or exceeding the U.S. Department of Energy (DOE) target is no easy task; the specific target for 2020 was 1.5 kWh/kg (4.5 wt.% hydrogen) and 1.0 kWh/L (0.03 Kg hydrogen/L), costing more than USD 10 /kWh (or USD 333/Kg of stored hydrogen capacity) [ 19 , 20 ].…”

“…In the latter, sodium borohydride is actually presented as the fuel for the future [ 18 ] and judging by the emerging reports dating from 2021–2022 one could argue that this is true. Synthetic strategies and structure characterization [ 21 ] have been complemented by in-depth studies on hydrolysis process of light borohydrides [ 22 , 24 , 25 , 27 ] or by investigation on the possible intermediates that dehydrogenation could entail [ 12 ]. Destabilization strategies employed to tame the rigid behavior of tetrahydroborates during hydrogenation cycles have been reviewed [ 30 , 31 , 32 , 33 , 34 ], and the field of ionic conductivity has been explored with aplomb especially after the discovery of LT(low temperature)–HT(high temperature) phase transition in lithium borohydride [ 36 , 37 , 38 ].…”