From shrimp balls to hydrogen bubbles: borohydride hydrolysis catalysed by flexible cobalt chitosan spheres

The transition to a hydrogen economy is a must in any carbon-emissions-free future scenario. One practical challenge of this transition is the safety concern when using large amounts of compressed or liquid hydrogen. This can be averted by using solid salts, such as potassium borohydride (KBH4), as hydrogen carriers for industrial applications. Many metals and oxides catalyse this reaction, but the challenge lies in the catalysts' stability. The combination of high pH and fast hydrogen generation causes mechanical degradation. Here we show that chitosan, an abundant biobased polymer, is a practical and effective catalyst support for KBH hydrolysis. We form chitosan spheres that encapsulate the active cobalt catalyst species, all done under ambient conditions. They remain stable at the high reaction pH, and swell and shrink with the formation of hydrogen. The catalyst can be reused several times in both batch and continuous modes. A continuous test using a 5 w/w% stabilised fuel solution and only 250 mg catalyst generated hydrogen at an average rate of 32 mL min(-1) over 48 h. Studying the reaction kinetics using high-precision measurements, we observe a kink in the Arrhenius plot, reflecting a physical change in the system (the swelling of the spheres) rather than a change in mechanism. Comparison studies show that the flexible biopolymer support outperforms classic porous oxide supports such as alumina, magnesia, and alumina-magnesia. Our research shows a clear green advance, based on multi-factor comparison: First, our system enables safe hydrogen transportation and on-demand release (low hazard and enables clean energy). Second, it uses a renewable and abundant bio-based source and no noble metals (no resource depletion). Finally, it requires only simple preparation under ambient conditions and at a low cost (practical application). All these make the encapsulation in flexible chitosan spheres an attractive approach to sustainable catalysts for carbon-free fuels.