In the quest for sustainable and clean energy carriers, hydrogen (H2) has emerged as a promising candidate. Its potential as a clean fuel lies in its ability to produce energy when combined with oxygen, releasing only water as a byproduct. However, the challenge has always been to produce hydrogen efficiently, economically, and without contributing to environmental degradation. One promising avenue for hydrogen production is photocatalytic water splitting, a process that harnesses the power of sunlight to generate hydrogen from water. This method has gained attention due to its potential to provide green hydrogen without the need for expensive electrolyzers or extensive power grid infrastructure.
Photocatalytic water splitting relies on the use of suitable catalysts to drive the hydrogen evolution reaction (HER). In recent years, researchers have been exploring biological catalysts known as hydrogenases (H2ases) as potential candidates for catalyzing HER. Unlike conventional catalysts that often rely on precious metals like platinum, H2ases offer a more sustainable and environmentally friendly approach, as they can be extracted from natural bacteria and do not require any scarce resources.
One of the most promising types of H2ases for hydrogen production is the nickel iron selenium [NiFeSe] hydrogenase due to its remarkable activity. However, a major challenge in utilizing H2ases in this context has been their sensitivity to oxygen: [NiFeSe] H2ases rapidly deactivate when they get in contact with air. Since oxygen is inevitably formed during water splitting, solving this oxygen sensitivity problem is crucial for the industrial application of H2ases. To this account, a new study published in the peer-reviewed journal Angewandte Chemie International Edition introduced a novel approach to overcoming the oxygen sensitivity of [NiFeSe] hydrogenases. Led by Dr. Moritz Kühnel who recently moved from Swansea University to the University of Hohenheim in collaboration with the teams of Dr. Christine Cavazza at CEA Grenoble and Dr. Alan Le Goff at the University Grenoble Alpes, the interdisciplinary team utilized an innovative technique called “solvent engineering.” Solvent engineering involves the manipulation of the solvent medium in which a reaction takes place to alter the performance of a catalyst rather than changing the catalyst itself which would be more expensive and time-consuming. In this study, so-called Deep Eutectic Solvents (DESs) were chosen as the reaction medium to alter the oxygen tolerance of hydrogenases due to their low oxygen solubility and diffusivity. DESs have gained attention as an alternative to conventional ionic liquids because they are low in toxicity, can be derived from readily available and affordable precursors, and have a high degree of biocompatibility.
The research team demonstrated for the first time the applicability of solvent engineering to hydrogenases. Because [NiFeSe] hydrogenase is an enzyme from a natural organism that evolved in water, the researchers first needed to establish that [NiFeSe] hydrogenase still functions in non-natural DES-based solvents. To this end, the researchers combined [NiFeSe] hydrogenase with TiO2 as a light absorber and irradiated it with simulated sunlight to produce hydrogen through photocatalysis. Amazingly, the team found that [NiFeSe] hydrogenase not only retained its photocatalytic HER activity in DES-based solvents, they even observed a superior performance compared to water as the solvent, even though it evolved in water. They reported that [NiFeSe] H2ase combined with TiO2 as a light absorber produced around 24 micromoles of H2 after irradiation for a day in a mixture of the DES glyceline (80%) with water (20%) in the absence of air, with a strong dependence of the performance on the water content in the solvent mixture. By comparing the performance in the absence and presence of air, the study revealed that [NiFeSe] H2ase exhibited a high degree of oxygen tolerance in DES-based solutions. Photocatalytic HER using [NiFeSe] H2ase and TiO2 in 80% glyceline retained nearly 90% activity under an atmosphere of air, with a total hydrogen production of 21 micromoles after irradiation for a day in air. This was in stark contrast to water, in which air caused the HER activity to drop to less than 4% and total inactivation within a few hours, whereas hydrogen production in DES continued for several days. What sets this approach apart from other work attempting to improve the oxygen tolerance of hydrogenase is that solvent engineering achieves a significant enhancement in oxygen tolerance without the need for any enzyme modification.
To understand the origin of the varying H2ase activity in different DES compositions, the researchers conducted electrochemical measurements of [NiFeSe] H2ase adsorbed onto a carbon nanotube electrode in a range of different glyceline/water mixtures. The results showed that as the glyceline content increased, the HER current increased before gradually decreasing. This phenomenon can be attributed to a decrease in H2 solubility with higher DES concentrations, caused by the “salting out” effect of solutions with high ionic strengths. This decrease in H2 solubility prevents the H2ase being blocked by H2 and thus allows for a higher HER activity in DESs compared to water.
The key to the success of solvent engineering in achieving oxygen tolerance lies in the manipulation of the behavior of dissolved oxygen in the solvent through altering its composition. The authors measured the oxygen solubilities and diffusion coefficients in glyceline and water, revealing that increasing the DES content significantly decreased oxygen diffusion while only marginally affecting oxygen solubility. When the solvent slows down the diffusion of oxygen to the hydrogenase, the detrimental interaction of oxygen with the enzyme is outcompeted by the rapid diffusion of protons, so that HER can proceed uninhibited despite the presence of oxygen. The solvent thus creates a dynamic O2 shield that provides the photocatalyst in the solvent with an enhanced oxygen tolerance.
The research by Dr. Moritz Kühnel and colleagues represents a significant advancement in the field of green hydrogen production. Solvent engineering, as demonstrated in this study, has the potential to revolutionize the use of hydrogenases, enhancing their activity and stability. This innovative approach opens up new possibilities for the design of tailored solvents that can achieve high oxygen tolerance and simultaneously increase hydrogenase activity. Further studies on the influence of key reaction parameters such as temperature, pH, and multicomponent solvent compositions on hydrogenase performance will undoubtedly provide valuable insights for the development of more efficient and environmentally friendly hydrogen production methods.
Allan MG, Pichon T, McCune JA, Cavazza C, Le Goff A, Kühnel MF. Augmenting the Performance of Hydrogenase for Aerobic Photocatalytic Hydrogen Evolution via Solvent Tuning. Angew Chem Int Ed, 2023, 62(22), e202219176. doi: 10.1002/anie.202219176.