Hydrogen production from sodium borohydride in methanol–water mixtures

Abstract

Hydrogen production systems based on the hydrolysis of sodium borohydride loose efficiency due to the excess water needed to account for the reaction and water capture by the by-product. Solubility of NaBH 4 and sodium borates in water is also a restricting factor together with the need for stabilizers necessary for reaction control in aqueous medium. In this work, methanol was used as an alternative to water. Literature data on this subject are scarce. Methanol lowers the freezing temperature of the reactant mixture with the advantage of providing short times for the initiation of the reaction and possibility of use at low temperatures. The effect of the water fraction on the efficiency of the reaction was studied at 45 °C. Results indicated increase in the reaction rates with decreasing water fraction. Sodium tetramethoxyborate was identified as the main by-product in methanol with no added water. When using methanol with no added water the reaction follows a first order rate kinetics with respect to sodium borohydride. Activation energy is reduced by a factor of 5 in the presence of methanol with no added water, when compared to values found in 100% water solutions. Methanol can be recovered by reaction of the by-product with water, offering increased storage and energy density to the system.