Using Additives to Control the Decomposition Temperature of Sodium Borohydride

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 H₂g), 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.