(538t) Hydrogen Release Kinetics of Thermal Hydrolysis of Sodium Borohydride and Hydrate

Due to the rapid depletion of fossil fuels and pollution from their use, it is becoming more and more important to find an alternative fuel. Hydrogen is considered one of the best contenders as its byproduct is water. To use hydrogen as an effective energy carrier or source, however, safe methods to store hydrogen must be developed [1]. Sodium borohydride (NaBH₄, SBH) has attracted great attention as a hydrogen storage material due to its high hydrogen content and inexpensive price. Hydrogen can be generated from the thermolysis or hydrolysis of SBH. Unfortunately, the thermolysis requires extremely high temperatures of 300^oC or more, while the hydrolysis results in safety concern due to the spontaneous generation of hydrogen. In our prior work, it has been demonstrated that thermal hydrolysis of solid-state SBH with boric acid offers improved safety and high hydrogen yield [2]. Despite these favorable outcomes, separation of the products for the regeneration of spent fuel remains a challenge to lower the overall cost. Here, we proposed the hydrolysis of SBH and water produced by thermal dehydration of sodium metaborate tetrahydrate (NaBO₂ 4H₂O, SMB). The safety hazards due to self-hydrolysis of SBH are eliminated as the mixture of SBH and SMB is stable at ambient temperature. In addition, SMB is a product of the hydrolysis of SBH. Therefore, since the final products are homogeneous with SMB, there is no need for post-reaction separation for the regeneration of the spent fuel. Using this approach, a rapid hydrogen release was observed at ~120^oC and a maximum H₂ equivalent of 3.62 was achieved at 200^oC with a 1:3 molar ratio of SBH:SMB. With high hydrogen yield and simplification of the regeneration process, this proposed method is promising for hydrogen storage for fuel cell applications.

References

[1] Hwang, H.T. and Varma, A., 2014. Hydrogen storage for fuel cell vehicles. Current Opinion in Chemical Engineering, 5, pp.42-48.

[2] Kim, G.J. and Hwang, H.T., 2021. Thermal hydrolysis of solid-state sodium borohydride for noncatalytic hydrogen generation. Chemical Engineering Journal, p.130445.