Hydrolysis and Ball Mill Regeneration of NaBH4
Sodium borohydride (NaBH4) hydrolysis is a promising approach for hydrogen generation, but it is limited by high costs, low efficiency of recycling the by-product, and a lack of effective gravimetric storage methods. Here we demonstrate the regeneration of NaBH4 by ball milling the by-product, NaBO2·2H2O or NaBO2·4H2O, with MgH2 at room temperature and atmospheric pressure without any further post-treatment. Record yields of NaBH4 at 90.0% for NaBO2·2H2O and 88.3% for NaBO2·4H2O are achieved. This process also produces hydrogen from the splitting of coordinate water in hydrated sodium metaborate. This compensates the need for extra hydrogen for generating MgH2. Accordingly, we conclude that our unique approach realizes an efficient and cost-effective closed loop system for hydrogen production and storage.
[researchgate.net] – Hydrolysis and regeneration of sodium borohydride (NaBH4) – A combination of hydrogen production and storage (Aug 2017)
[wikipedia.org] – Ball mill
[researchgate.net] – The Processing of NaBH4 from Na2B4O7 by Mechano-chemical Synthesis and Its Catalytic Dehydrogenation (2012)
Sodium borohydride (NaBH4) is a specialty reducing agent used in fuel cell systems. Feasible NaBH4 production techniques are required to reduce the costs of NaBH4, making its usage attractive. At present, anhydrous borax (Na2B4O7) is a promising reactant in NaBH4 synthesis. However, further improvements to optimize the yield of the NaBH4 production process operating with MgH2 and Na2B4O7 in ball milling are necessary. In order to obtain optimum reaction conditions, experiments were performed under inert atmosphere. First, we attempted to compensate Na insufficiency with the addition of Na, NaH, and Na2CO3. Next, the reaction period and stoichiometric ratio of our experiments were studied with Na2B4O7 and MgH2. Reaction times were varied from 200 to 600 min, and regulated the excess MgH2 addition at 0, 10, 30, 50, and 70%. The synthesized products were characterized by FT-IR and XRD analysis and checked our results using iodimetry and dehydrogenation tests. We were able to achieve yield levels of approximately 84% of NaBH4 in certain purification processes with ethylene diamin after 400 min of reaction time and at excess MgH2 levels of 10% or more.
US government approach (2003):
[hydrogen.energy.gov] – Process for Regeneration of Sodium Borate to Sodium Borohydride for Use as a Hydrogen Storage Source
Current efforts have led to a cost reduction pathway for NaBH4 leading to its use as a hydrogen carrier for use as a transportation fuel. As more information is obtained on each of the processes to generate either sodium metal or borohydride directly the economic models will be refined to reflect the improved knowledge. NaBO2 has been recycled into Na and acidified borate solutions, completing a closed mass balance recycle of products into borohydride. Future work will endeavor to show complete conversion of borate to boric acid, and experimentation on direct synthesis of trimethyl borate in the electrolysis cell. Current density of 100 mA/cm2 through the ion selective membrane is adequate for scaled-up sodium synthesis from NaOH. Future work will be to duplicate this feat from NaBO2, and to improve the membrane lifetime, particularly in NaOH (aq.) electrochemical systems. Pending the go/no-go decision, a task is planned to build the 1 kg of Na/day electrochemical reactor. The knowledge gained from this work will be fed back into the economic model, and used to model the scale-up of the reaction. Two of three cathode compartment reactions have been demonstrated for one-pot borohydride ion synthesis. Future work will be to prove the third reaction and complete synthesis of the BH4 – species.
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