Sodium Borohydride As a Circular Hydrogen Carrier

Abstract

This thesis presents a novel method for using sodium borohydride (NaBH₄) as a circular hydrogen carrier, addressing the current lack of commercially viable approaches for hydrogen storage in NaBH₄. The traditional approach of using NaBH₄ for circular hydrogen storage through hydrolysis was reevaluated, leading to the development of a novel reaction system consisting of solely a one-pot reaction for the hydrogen release process. Alcoholysis reactions were performed using different alcohols resulting in the selective formation of borate esters. Subsequently, successful regeneration of the gained boric ester to NaBH₄ was achieved by adopting reaction conditions from the Brown-Schlesinger process, thus realizing a complete circular hydrogen storage process for the proposed system. Attempts were made to minimize the energy intensity of NaBH₄ synthesis by exploring alternate reaction pathways that could potentially lower reaction temperatures. This involved the use of radical sources such as sodium naphthalenide and sodium biphenyl, leading to the formation of boron radical anions that facilitated homolytic hydrogen cleavage which initiated borohydride formation.
To address the challenge of heat transfer during hydrogen release, a heat transfer limitation model was developed. Real-time data was collected to extrapolate temperature profiles for different NaBH4 quantities and their respective proportions of reactants and reactor volumes for predicting temperature trajectories. This data was functionalized and used for the prediction of cooling water temperature profiles and minimum cooling water flow rates upon up scaling. It was seen that heat transfer in this system has a linear correlation to the amount of reacting NaBH₄ upon scaling. Therefore, the envisioned hydrogen release process is not assumed to be forming a problem for the reactor design upon up scaling with regards to heat transfer limitations.
Furthermore, an economic assessment was conducted to evaluate the viability of the envisioned system. A process flow diagram of the alcoholysis reaction pathway was initially constructed. Subsequently, this diagram served as the foundation for establishing an energy balance for both the hydrogen release process and the NaBH₄ regeneration process, which enabled a comparative examination between the hydrolysis pathway and the proposed alcoholysis pathway.