Noncatalytic Solid-State Hydrolysis of Sodium Borohydride with Sodium Metaborate Hydrate

Abstract

Due to the rapid depletion of fossil fuels and pollution from their use, finding an alternative fuel is becoming increasingly important. Hydrogen is considered one of the best contenders, as its by-product is water. However, safe methods to store hydrogen must be developed to use hydrogen as an effective energy carrier or source. Sodium borohydride (NaBH₄, SBH) has been attracting great attention as a hydrogen storage material because of its relatively high hydrogen content and low material cost. Hydrogen can be produced through thermolysis or hydrolysis of SBH. Unfortunately, thermolysis requires extremely high temperatures (> 300 °C), while hydrolysis needs an excessive amount of water because the solubilities of SBH and the spent fuel in water are low. In addition, when SBH is present with water, it can cause problems in safety and storage due to the spontaneous generation of hydrogen. In our prior work, it has been demonstrated that solid-state hydrolysis of SBH offers improved safety and high hydrogen yield. Despite these favorable outcomes, separating the products to regenerate spent fuel to lower the overall cost remains challenging. Here, we proposed the hydrolysis of SBH and water formed by thermal dehydration of sodium metaborate tetrahydrate (NaBO₂ 4H₂O, SMB4H). Since the mixture of SBH and SMB4H is stable at ambient conditions, the safety risk due to self-hydrolysis of SBH is eliminated. Additionally, since the final products are homogeneous with sodium metaborate (NaBO₄·xH₂O, SMB), a product of the hydrolysis of SBH, it is expected that separation cost can be saved when regenerating the spent fuel. Using this approach, maximum overall H₂ yields of 4.7 and 5.1wt% were obtained at 200 and 250 °C, respectively. With high hydrogen yield and safety and potential reduction in separating process costs during regeneration, this proposed method is promising for hydrogen storage for fuel cell applications.