# Sustainable Additives for the Production of Hydrogen via Sodium Borohydride Hydrolysis
In this context, the use of hydrogen as an energy vector presents a promising alternative, although there are still challenges to be overcome, especially in terms of storage for onboard applications [3]. At present, traditional systems for hydrogen storage are based on its use either in (i) gaseous form, which allows working at 273 K but requires the use of high pressures, and (ii) liquid form, which can be operated at ambient conditions of pressure but also demands cryogenic temperatures below 25 K [4]. In line with the necessity for new systems capable of achieving high storage and regeneration capacities and lower costs [5], hydrogen storage in solid materials could constitute a step forward to reach this objective. These materials must offer safe handling while providing high gravimetric and volumetric densities, low temperatures for H\({}_{2}\) release, and the possibility of operating at atmospheric pressure. Nowadays, the materials that are gaining interest for this application are adsorbents (e.g., MOF-5) [6], organic liquids (e.g., BN-methyl cyclopentane) [7], metal hydrides (e.g., MgH\({}_{2}\)) [8], ammonia boranes (e.g., NH\({}_{3}\)BH\({}_{3}\)) [9], and chemical hydrides (e.g., NaBH\({}_{4}\)) [10]. In this context, two main approaches have shown significant results for hydrogen generation using ammonia boranes and chemical hydrides: methanolysis [11; 12] and hydrolysis [13; 14]. While methanolysis offers advantages such as higher hydrogen yields, lower required temperatures, and faster reaction rates, it does have some drawbacks. For example, the production of methanol as a byproduct requires proper handling and disposal due to its flammability and toxicity. Additionally, the need to separate methanol from the hydrogen gas stream adds an extra step and cost to the process. On the other hand, hydrolysis provides simplicity as a straightforward process, requiring only water as a reactant, and also offers lower costs and is considered safe and environmentally friendly. Specifically, in recent years, increasing attention has been paid to the hydrogen generation of metal hydrides due to their high theoretical hydrogen yield [15]. Specifically, the chemical hydride sodium borohydride (NaBH\({}_{4}\)) stands out as a promising hydrogen carrier due to its inherent advantages, (i) stability under a wide range of temperatures, (ii) low working pressures, (iii) high hydrogen purity, (iv) high hydrogen storage capacity (10.8 wt%), (v) exothermic hydrolysis (\(\Delta\)H = \(-\)210 kJ\(\cdot\)mol\({}^{-1}\)), (vi) capacity of simultaneous hydrogen generation and storage, (vii) direct H\({}_{2}\) release to feed electrochemical systems, such as PEM fuel cells, to generate electricity, and (viii) non-toxic character [16; 17; 18; 19; 20; 21]. The low hydrogen generation rate from NaBH\({}_{4}\) at ambient temperature is considered a barrier to its commercialization. However, the employment of catalysts enhances the rate of hydrogen release, whereas the use of additives may help in stabilizing the process kinetics [18; 19]. Platinum has shown high catalytic activity for the hydrolysis of NaBH\({}_{4}\), however, its scarcity and high cost hinder its widespread use [22]. Thus, many efforts have been devoted to developing new catalysts to enhance the dehydrogenation process, especially based on non-noble metals due to their low price, abundant reserves, and relatively high activities [20; 21; 22; 23; 24]. The most investigated elements are cobalt, ruthenium, and nickel as heterogeneous catalysts [25]. Among them, bimetallic catalysts based on Ni-Ru have been commonly used in the hydrolysis of NaBH\({}_{4}\) because of their large specific surface area [26]. Regarding the structural form of the catalyst, different alternatives have been studied, including powder as the most common type, aerogels, and foam [25; 27].