[NiFeSe]-Hydrogenase Chemistry

Abstract

The development of technology for the inexpensive generation of the renewable energy vector H₂ through water splitting is of immediate economic, ecological, and humanitarian interest. Recent interest in hydrogenases has been fuelled by their exceptionally high catalytic rates for H₂ production at a marginal overpotential, which is presently only matched by the non-scalable noble metal platinum. The mechanistic understanding of hydrogenase function guides the design of synthetic catalysts and selection of a suitable hydrogenase enables direct applications in electro- and photocatalysis. [FeFe]-hydrogenases display excellent H₂ evolution activity, but they are irreversibly damaged upon exposure to O₂, which currently prevents their use in full water splitting systems. O₂-tolerant [NiFe]-hydrogenases are known, but they are typically strongly biased towards H₂ oxidation, while H₂ production by [NiFe]-hydrogenases is often product (H₂) inhibited. [NiFeSe]-hydrogenases are a sub-class of [NiFe]-hydrogenases with a selenocysteine residue coordinated to the active site nickel center in place of a cysteine. They exhibit a combination of unique properties that are highly advantageous for applications in water splitting compared to other hydrogenases. They display a high H₂ evolution rate with marginal inhibition by H₂ and tolerance to O₂. [NiFeSe]-hydrogenases are therefore one of the most active molecular H₂ evolution catalysts applicable in water splitting. Herein, we summarize our recent progress in exploring the unique ‘chemistry of [NiFeSe]-hydrogenases’ through biomimetic model chemistry and the ‘chemistry with [NiFeSe]-hydrogenases’ in semi-artificial, photosynthetic systems. We gain perspective from the structural, spectroscopic, and electrochemical properties of the [NiFeSe]-hydrogenases and compare them with the chemistry of the first synthetic models of the hydrogenase active site. Our synthetic models give insight into the effects on the electronic properties and reactivity of the active site upon the introduction of selenium. We have utilized the exceptional properties of the [NiFeSe]-hydrogenase from Desulfomicrobium baculatum in a number of photocatalytic H₂ production systems, which are benchmark systems in terms of single site activity, tolerance towards O₂, and in vitro water splitting with biological molecules. Each system comprises a light-harvesting component, which allows for light-driven electron transfer to the hydrogenase in order for it to catalyze H₂ production. A system with [NiFeSe]-hydrogenase on a dye-sensitized TiO₂ nanoparticle gives an enzyme-semiconductor hybrid for visible light-driven generation of H₂ with an enzyme-based turnover frequency of 50 s⁻¹. A stable and inexpensive polymeric carbon nitride as a photosensitizer in combination with the [NiFeSe]-hydrogenase shows good activity for more than two days. Light-driven H₂ evolution with the enzyme and an organic dye under high O₂ levels demonstrates the excellent robustness and feasibility of full water splitting with a hydrogenase-based scheme. This has led, most recently, to the development of a light-driven full water splitting system with a [NiFeSe]-hydrogenase wired to the water oxidation enzyme Photosystem II in a photoelectrochemical cell. In contrast to the other systems, this photoelectrochemical system does not rely on a sacrificial electron donor and allowed us to establish the long sought after light-driven water splitting with an isolated hydrogenase.