Chemists recycle shrimp waste as a catalyst for hydrogen generation
(Nanowerk News) Biomolecular chitosan flexible balls, made from shrimp waste, can be used for catalysts that produce hydrogen gas from borohydride salts. In the paper at Green Chemistry (“From shrimp meatballs to hydrogen bubbles: Borohydride hydrolysis catalyzed by flexible cobalt chitosan balls”), a team of researchers at the University of Amsterdam (UvA) demonstrated how a ball can “exhale” hydrogen bubbles without bursting. This is an important step towards practical and safe hydrogen storage and release units.
Since 2020, the Heterogeneous Catalysis & Continuous Chemistry group at the Van ‘t Hoff Institute for Molecular Sciences UVA has been working on using alkali metal borohydride salts as hydrogen carriers in the future. This solid salt can be safely stored in air under ambient conditions and releases hydrogen gas only when it reacts with water. However, controlling the release of hydrogen, and thus preventing runaway reactions, is a challenge. One solution is to stabilize the solution with a base, and control the release of hydrogen using a catalyst. The UVA team, led by Prof. Gadi Rothenberg, is developing the catalyst in collaboration with the Austrian Competence Center for Tribology (AC2T) and the company Electriq Global.
Hydrogen destroys the catalyst particles
Finding potential catalysts is easy, but getting them to work long enough to be commercially viable is not so easy. The combination of the high reaction pH and the continuous release of hydrogen bubbles destroys traditional catalysts within days. For example, the team managed to design catalyst particles containing highly active and selective cobalt. The high activity, however, results in a high volume of hydrogen which rapidly disintegrates the particles.
The breakthrough came during the so-called Friday Afternoon experiment when MSc student Jeffrey Jonk and PhD student Fran Pope decided to try and encapsulate cobalt particles in chitosan balls. Chitosan is a natural polymer that can be produced from chitin, the main component of the exoskeleton of insects and crustacean shells. It is a widely available biodegradable, biocompatible material on multi-tonne scales, produced mostly from shrimp and crab shell waste.
The repeated amine groups on the chitosan backbone make it highly soluble in aqueous acidic solutions but slightly soluble in alkaline solutions. Therefore, chitosan balls can be produced relatively easily by dropping molten chitosan into an alkaline solution. An important property of chitosan balls is their flexibility which allows them to expand during the formation of hydrogen. Thus they can “breathe” hydrogen bubbles without bursting. And because it’s made at a high pH, the basicity of borohydride solutions doesn’t pose a problem.
Real-life potential for chitosan-based catalysts
The team tested the new catalyst in batch and continuous modes, monitoring reactions by measuring the flow of hydrogen produced. A few mm-sized spheres filled with 7% cobalt are sufficient to produce 40 mL of hydrogen per minute in a continuous reactor for two days, demonstrating the real-life potential of this new catalyst.
According to Rothenberg’s work highlighting the importance of catalyst stability as a research focus. “Many papers have focused on activity and selectivity, as journals have focused on publishing spectacular results,” he said. “But if you look at the chemical industry, none of these “spectacular” catalysts are used in practice. The reason is that running a successful reaction for a few hours, or even a few days, means nothing for a large-scale process. Real catalysts have to work for months and years to be economically viable. We haven’t arrived yet.”
Hydrogen may be the energy carrier of the future, but it has its challenges. When stored as a compressed gas or in liquid form, molecular hydrogen, H2, is very energy intensive. This is an advantage in some applications, but a security concern in others. For medium-scale storage and discharge in mobile installations, such as cranes, ships and generators, other modes of hydrogen storage are preferred. There are many forms of hydrogen carriers. Examples of high hydrogen storage capacities include ammonia, methanol, formic acid, and others. But each has its pros and cons. Methanol has a high capacity (12.5 wt%), but dehydrogenation requires high temperatures and can also emit CO2. Ammonia can contaminate the generated H2 stream, and is itself a toxic gas under ambient conditions. Alternatively, alkaline borohydrides can provide a safe source of hydrogen, chemically bonding it as a solid salt. Reaction with water releases hydrogen, and the resulting metaborate salt by-product can be reprocessed and reused for hydrogen storage.
