(596g) Noncatalytic Solid-State Hydrolysis of Sodium Borohydride with Sodium Metaborate Hydrate to Form an Isomorphous Hydrates

A method for safe and efficient hydrogen generation has been heavily researched in the past decades as a sustainable energy source for a variety of applications. Among hydrogen storage materials, sodium borohydride (NaBH₄, SBH) has been extensively studied due to its relative inexpensiveness and high hydrogen content (10.8 wt%). Traditional methods of generating hydrogen from SBH include thermolysis and hydrolysis. Unfortunately, thermolysis of SBH requires high temperatures (>300°C) even in the presence of a catalyst, while hydrolysis requires an efficient catalyst and excess water. There are also safety concerns with hydrolysis due to the possible unexpected generation of hydrogen when SBH and water are stored together. Ultimately, both hydrolysis and thermolysis present distinct challenges.

Recently, we have proposed a new process: thermal hydrolysis of a solid-phase mixture of SBH and sodium metaborate tetrahydrate (NaBO₂∙4H₂O, SMB4H). The thermal decomposition of SMBH releases water that reacts with SBH to produce hydrogen. With this, safety issues arising from self-hydrolysis pose no threat, as solid-state SBH-SMB4H mixtures are stable at ambient conditions. Since a high-density fuel is suitable for practical use of this approach, the characteristics of thermal hydrolysis of SBH were systematically investigated using a compressed mixture in the form of pellets in this study. In addition, this approach was investigated over a wide range of SBH/SMBH ratios, temperatures, pressures, pretreatment temperatures, and storage periods. During the process, rapid hydrogen evolution was observed around 150°C, and a maximum overall H₂ yield of 5.76 (2.4 H₂ equivalent) was obtained at 200°C for an SBH:SMB4H ratio of 1:0.6. The only products of the reaction are sodium metaborate and hydrogen, which makes this process more sustainable, indicating that this proposed method is promising for use in hydrogen storage for various applications including energy and commercial sectors.