Hydrogen on-Demand from Sodium Borohydride Using Reactivated Co-Foams Catalysts

Abstract

According to the World Health Organization, the use of fossil fuels has unequivocally originated climate changes with strong environmental and social impact at global scale. Simultaneously, global energy demand is predicted to rise about 30% by 2030 making the search for alternative greener fuels even more pressing. In this context, hydrogen has high potential as an energy carrier but unfortunately almost all of the H2 generation (95 %) stems from fossil fuels processing. Key factors for the development of reliable hydrogen-fueled systems comprise cost-effectiveness, greenness and safety on H2 production, storage, transportation and application [1]. Storage and transportation of H2 as a gas or in liquid state are widely used technologies. However, compressed H2 (gas state) demands high pressure and cryogenic H2 (liquid state) requires liquefaction, which turn both these options expensive (besides the volume occupied by the containers) [1]. From the point of view of the hydrogen economy, the International Energy Agency recognizes sodium borohydride (NaBH4) as a versatile route to be explored extensively. NaBH4 is a carbon free compound, nonflammable, with good stability in air and in alkaline aqueous solutions. It has a high gravimetric hydrogen storage capacity (10.8 wt.%) and can supply safe and clean H2 gas through a green approach. NaBH4 generates H2 through its hydrolysis, which is a spontaneous, exothermic reaction that can occur in ambient conditions [2]. Therefore, NaBH4 systems can solve multiple issues related with the storage, transportation and release of pure H2. However, the efficiency of borohydride hydrolysis systems depends on catalysts, to provide high enough hydrogen generation rates [2]. Noble metal-based catalysts, such as Pt and Ru, are effective for NaBH4 hydrolysis but they are scarce and expensive. On this regard, catalysts based on transition metals, such as Co and Ni, have been developed [3]. Interestingly, Co-based foam-like catalysts have been demonstrated to have better performances due to their highly porous nature with enhanced morphologies and large active surface areas [4]. Although different catalysts have been reported over the last decade, the catalyst durability is another important topic that is rarely addressed [5]. Borates by-product from borohydride hydrolysis can be strongly adsorbed at the catalyst surface, leading to its activity decrease and even deactivation [2]. So far, methods for borohydride hydrolysis catalysts reactivation involve ex-situ operations such as washing the catalyst with deionized water and acidic solutions [6], or catalyst thermal treatments [7]. Taking into account an integrated borohydride system to generate H2, these methods can be time-consuming, ineffective and not practical. The present work aims to produce tailored Co-based catalytic foams for H2 generation from borohydride source and develop in-situ electrolytic reactivation process to extend the durability of the catalysts. The optimized Co foams were submitted to several NaBH4 hydrolysis cycles to study their performance and durability (stability) before and after electro-reactivation. The foams shows highly porous three-dimensional structure and are able to catalyze borohydride hydrolysis. Reactivation of the catalytic foams through electrolytic in-situ procedure is proved to be a fast and practical approach of keeping catalysts efficiency and durability and thus for the advancement of integrated NaBH4 systems towards H2 on-demand applications. Figure 1. SEM image of the as-prepared cobalt foam.