Hydrogen gas is a viable sustainable alternative fuel, but only if it is produced from renewable feedstocks and energy sources, such as water and sunlight. Additionally, an economically feasible system for light-driven hydrogen generation requires light-absorbers and catalysts comprised of earth-abundant elements. Inspired by nature, we use chlorophyll mimics (porphyrins) as photosensitizers and synthetic mimics of the all-iron hydrogenase ([FeFe]H₂ase) enzyme as proton reduction catalysts. These building blocks self-assemble into supramolecular photocatalytic architectures which can be used to construct an artificial leaf – a man-made leaf that produces hydrogen fuel instead of sugar.

One of the key features that make the hydrogenase enzyme such an effective catalyst is its ferredoxin cluster, which acts as an electron reservoir during the catalytic cycle. The resulting redox-potential leveling is essential for the two electron transfers required for H₂ formation to occur with similar driving force. Here, we use the same strategy in a synthetic hydrogen-generation catalyst by coupling a benzenedithiolato-based [FeFe]H₂ase mimic to a redox-active phosphole ligand [1]. The phosphole ligand is equipped with two pyridyl groups which can be supramolecularly anchored to the central metal atoms of zinc porphyrins [1]. Using time-resolved infrared spectroscopy with femtosecond resolution, we show that on visible-light excitation of the porphyrin an electron is transferred to the catalyst. The resulting electron density is delocalized over the phosphole ligand and the iron atoms of the catalyst’s active site, implying that the redox-active ligand functions like the electron transport chain present in the enzyme. Indeed, a combination of (spectro)electrochemical experiments combined with DFT calculations shows that the phosphole ligand acts as an electron reservoir during catalysis, effectively donating an electron into the active site when needed.

Two remarkable properties make this catalyst a good candidate for hydrogen-fuel production: its high turnover frequency and its operation in an aqueous environment. Through protonation of the pyridyl side-arms, the complex is soluble in dilute sulfuric acid. Analysis of the electrocatalytic waves shows that the catalyst operates with a turnover frequency of 7.0·10⁴ s^–1 at an overpotential of 0.66 V. In contrast to the enzyme, this catalyst is tolerant to the presence of oxygen, an essential feature for any catalyst operating in a real-life environment.

[1] R. Becker, S. Amirjalayer, P. Li, S. Woutersen, J. N. H. Reek, Science Advances 2016 (2), 1, e1501014.