Abstract
AbstractIt is clear that in order to satisfy global energy demands whilst maintaining sustainable levels of atmospheric greenhouse gases, alternative energy sources are required. Due to its high chemical energy density and the benign by‐product of its combustion reactions, hydrogen is one of the most promising of these. However, methods of hydrogen storage such as gas compression or liquefaction are not suitable for portable or automotive applications due to their low hydrogen storage densities. Accordingly, much research activity has been focused on finding higher density hydrogen storage methods. One such method is to generate hydrogen via the hydrolysis of aqueous sodium borohydride (NaBH4) solutions, and this has been heavily studied since the turn of the century due to its high theoretical hydrogen storage capacity (10.8 wt%) and relatively safe operation in comparison to other chemical hydrides. This makes it very attractive for use as a hydrogen generator, in particular for portable applications. Major factors affecting the hydrolysis reaction of aqueous NaBH4 include the performance of the catalyst, reaction temperature, NaBH4 concentration, stabilizer concentration, and the volume of the reaction solution. Catalysts based on noble metals, in particular ruthenium (Ru) and platinum (Pt), have been shown to be particularly efficient at rapid generation of hydrogen from aqueous NaBH4 solutions. However, given the scarcity and expense of such metals, a transition metal‐based catalyst would be a desirable alternative, and thus much work has been conducted using cobalt (Co) and nickel (Ni)‐based materials to attempt to source a practical option. “Metal free” NaBH4 hydrolysis can also be achieved by the addition of aqueous acids such as hydrochloric acid (HCl) to solid NaBH4. This review summarizes the various catalysts which have been reported in the literature for the hydrolysis of NaBH4.
Highlights
In recent years, global energy demand has grown at an unprecedented rate, and this trend is expected to continue long into the future [1]
The solubility of sodium borohydride in water is relatively low (55 g per 100 g at 25°C), requiring more water than that required by stoichiometry to ensure the sodium borohydride remains in solution ( sodium borohydride does have a considerably higher solubility than ammonia borane (33.6 g per 100 g at 25°C) [41], and other hydrolysis materials such as aluminum [46, 47] and silicon [48], which are insoluble).This is further compounded by the even lower solubility of sodium metaborate (28 g per 100 g of water at 25°C), which means that the concentration of sodium borohydride must be kept below 16 g per 100 g of water to ensure that sodium metaborate does not precipitate from the reaction mixture and foul the catalyst and reaction vessel
The highest performing Cobalt borides (Co-B) catalyst reported to date had a hydrogen generation rate of 39,000 mL minÀ1 (g catalystÀ1), though given that this value was obtained in the absence of stabilizing sodium hydroxide and at an elevated temperature of 40°C, it is difficult to make a direct comparison with other systems [85]
Summary
Global energy demand has grown at an unprecedented rate, and this trend is expected to continue long into the future [1]. The solubility of sodium borohydride in water is relatively low (55 g per 100 g at 25°C), requiring more water than that required by stoichiometry to ensure the sodium borohydride remains in solution ( sodium borohydride does have a considerably higher solubility than ammonia borane (33.6 g per 100 g at 25°C) [41], and other hydrolysis materials such as aluminum [46, 47] and silicon [48], which are insoluble).This is further compounded by the even lower solubility of sodium metaborate (28 g per 100 g of water at 25°C), which means that the concentration of sodium borohydride must be kept below 16 g per 100 g of water to ensure that sodium metaborate does not precipitate from the reaction mixture and foul the catalyst and reaction vessel All of these considerations mean that in reality the gravimetric hydrogen storage capacity of sodium borohydride is far lower than the theoretical value of 10.8 wt%, and has led to a “no-go” recommendation from the United States Department of Energy for use in automotive applications [49]. CoCl2 solution added to a solid powder mixture of NaOH and NaBH4 Pulsed laser deposition
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