Abstract
Hydrogen (H2) shows great promise as zero-carbon emission fuel, but there are several challenges to overcome in regards to storage and transportation to make it a more universal energy solution. Gaseous hydrogen requires high pressures and large volume tanks while storage of liquid hydrogen requires cryogenic temperatures; neither option is ideal due to cost and the hazards involved. Storage in the solid state presents an attractive alternative, and can meet the U.S. Department of Energy (DOE) constraints to find materials containing > 7 % H2 (gravimetric weight) with a maximum H2 release under 125 °C.While there are many candidate hydrogen storage materials, the vast majority are metal hydrides. Of the hydrides, this review focuses solely on sodium borohydride (NaBH4), which is often not covered in other hydride reviews. However, as it contains 10.6% (by weight) H2 that can release at 133 ± 3 JK−1mol−1, this inexpensive material has received renewed attention. NaBH4 should decompose to H2(g), Na(s), and B(s), and could be recycled into its original form. Unfortunately, metal to ligand charge transfer in NaBH4 induces high thermodynamic stability, creating a high decomposition temperature of 530 °C. In an effort make H2 more accessible at lower temperatures, researchers have incorporated additives to destabilize the structure. This review highlights metal additives that have successfully reduced the decomposition temperature of NaBH4, with temperatures ranging from 522 °C (titanium (IV) fluoride) to 379 °C (niobium (V) fluoride). We describe synthetic methods employed, chemical pathways taken, and the challenges of boron derivative formation on H2 cycling. Though no trends can be found across all additives, it is our hope that compiling the data here will enable researchers to gain a better understanding of the additives’ influence and to determine how a new system might be designed to make NaBH4 a more viable H2 fuel source.
Highlights
There is an ever increasing need to develop energy efficient materials that are environmentally friendly
Kalantzopoulos et al found that the boron nuclear magnetic resonance (NMR) spectra of NaBH4 with the TiF3 additive contained NaBF4 in a low ratio of 1:150 NaBF4:NaBH4
It is fortunate that BM the Ni additives with NaBH4 does not result in the creation of many boron derivatives, which is promising for H2 cyclability, but chloride additives do tend to create Na(BH4)1-xClx
Summary
There is an ever increasing need to develop energy efficient materials that are environmentally friendly. Storing in its gaseous state is very common, but there are serious concerns about flammability and explosive hazards. Storing H2 in the liquid state requires ultralow, cryogenic temperatures, making storage expensive while adding the hazards, challenges and expense of cryogen handling [1, 2]. There is a strong need to create a regenerative hydrogen storage material that is safe and lightweight enough for easy transportation. Researchers have been exploring H2 storage methods in the solid state, trying to meet the constraints given by the U.S Department of Energy (DOE) [3], such as the recommendation to find materials containing > 7 % H2 (gravimetric weight) with a maximum H2 release rate under 125 °C
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