Citrus fruits catalyze hydrolysis of sodium borohydride for accelerating hydrogen (H2) production

Abstract

The exploration of natural agricultural products for the efficient hydrolysis of sodium borohydride (NaBH<inf>4</inf>) is crucial for environmentally benign hydrogen (H<inf>2</inf>) generation in water and expanding the industrial use of crop-derived materials. However, the direct use of natural materials to enhance H<inf>2</inf> production has been rarely investigated. This study investigates H<inf>2</inf> production using a commercial lemon juice and freshly squeezed citrus juices from lemons, oranges, limes, and mandarins. It is found that the amount of H<inf>2</inf> is close to theoretical maximum value when the molar ratio of carboxyl groups [COOH] to [NaBH<inf>4</inf>] is 1 or higher. Since it is known that many citrus fruits contain citric acid as a key chemical, a commercial lemon juice is studied as a catalysis for NaBH<inf>4</inf> hydrolysis. An aqueous solution containing 50 vol% juice and 10 mmol NaBH<inf>4</inf> produces 930.3 ± 23.4 mL of H<inf>2</inf> for 60 min. The amount enables a 200 mL bottle of lemon juice to theoretically generate 18.6 L of H<inf>2</inf>. Additionally, freshly squeezed juices from the four citrus fruits (with or without pulp) also catalyze the reaction. Particularly, lime juice with pulp produced 601.0 ± 8.2 mL of H<inf>2</inf> for 60 min when the aqueous suspension contained 50 wt% juice and 6.5 mmol NaBH<inf>4</inf>, corresponding to an estimated H<inf>2</inf> yield of ∼1.574 L per lime fruit. Overall, these findings demonstrate that citrus fruits can act as natural, effective catalysts for NaBH₄ hydrolysis, offering a promising dual benefit for sustainable energy production and agricultural resource utilization.