Rehydrogenation of Sodium Borates to Close the NaBH4-H2 Cycle: A Review

Abstract

In 2007, the US Department of Energy recommended a no-go on NaBH4 hydrolysis for onboard applications; however, the concept of a NaBH4-H2-PEMFC system has the potential to become a primary source for on-demand power supply. Despite the many efforts to study this technology, most of the published papers focus on catalytic performance. Nevertheless, the development of a practical reaction system to close the NaBH4-H2 cycle remains a critical issue. Therefore, this work provides an overview of the research progress on the solutions for the by-product rehydrogenation leading to the regeneration of NaBH4 with economic potential. It is the first to compare and analyze the main types of processes to regenerate NaBH4: thermo-, mechano-, and electrochemical. Moreover, it considers the report by Demirci et al. on the main by-product of sodium borohydride hydrolysis. The published literature already reported efficient NaBH4 regeneration; however, the processes still need more improvements. Moreover, it is noteworthy that a transition to clean methods, through the years, was observed.

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

“…Recently, a series of bimetallic borohydrides of alkali metal with alkaline earth, d - or f -block metals has been reported, containing the more electropositive alkali metal with a countercharge brought about by the complex anion [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 222 , 223 ]. K 2 Mg(BH 4 ) 4 and K 3 Mg(BH 4 ) 5 have isolated [Mg(BH 4 ) 4 ] 2− tetrahedra serving as complex anions, while the metal alkali K + ensures the charge neutrality of the framework [ 224 ].…”

“…M I M II (BH 4 ) 3 (M I = K, Rb, Cs; M II = Ca, Sr, Sm) of perovskite-type 3D-networks have the divalent metal in octahedral coordination via κ 2 -bridging BH 4 units, with the alkali metal cuboctahedrally surrounded by twelve BH 4 units [ 185 , 225 , 226 , 227 ]. This perovskite-type structure is best stabilized by alkali-earth metals of smaller radii (Ca 2+ , 1.14 Å) than heavier analogues (Sr 2+ , 1.32 Å), a trend which is more apparent during heating [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 225 , 226 ].…”

“…Due to their strong reducing character, metal borohydrides have recently been investigated as electrolytes in liquid and solid form, showing conductivities σ higher than 10 −4 S/cm [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 254 , 255 ]. Solid-state electrolyte research reached new heights after the discovery that o -LiBH 4 ( Pnma , orthorhombic, ionic conductivity σ Li+ = 10 −8 S/cm) transforms at 108–110 °C into another high temperature polymorph h -LiBH 4 ( P6 3 mc , hexagonal, σ Li+ = 10 −3 at 120 °C, five orders of magnitude higher than that of o -LiBH 4 ) ( Table 2 ) [ 256 , 257 , 258 , 259 , 260 , 261 ].…”

“…Indeed, mTHRost known borohydrides fall into this category: they afford a theoretical hydrogen storage capacity superior to that imposed by the DOE (4.5 wt.%). Equation (2) also implies that the hydrogen storage capacity will be the highest for the most lightweight borohydrides, thus corresponding to low A M values: second, third, and fourth period metals are the borohydrides most used to date [ 21 , 22 , 23 , 24 , 25 , 26 , 27 ]. A summary of known borohydrides with specific hydrogen storage capacity is presented in Figure 2 , which are arranged by metal valency and atomic weight of the metal, in decreasing order of their theoretical hydrogen storage capacity (wt.%).…”

“…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 ].…”