Catalytic Systems for Water Splitting

Power source: The water-splitting reaction is the most important reaction for the conversion of solar energy to fuel. In this special issue our Guest Editors have gathered contributions highlighting recent advances in the field. Pictured is a Yangtze River waterfall. The global awareness of climate change as a result of human activity has resulted in significant efforts to increase the proportion of energy from sustainable, carbon-neutral sources. Despite these efforts, most energy is still based on fossil fuels, and it is clear that our transformation to a society that runs on mostly sustainable energy is associated with great challenges. The sun provides sufficient energy to power the planet and solar energy can be converted to electrical current using solar panels. But it arrives on earth in a diluted fashion, and also is subject to significant fluctuation which leads to instability of the electricity grid. Next to this, it is foreseen that fuels will continue to be required for heavy transportation. For these reasons, the direct conversion of solar energy to fuel would be an interesting means to complete the palette of technologies for our future energy system. Whereas solar energy has been well developed over the past 30 years, and can now be used to provide electricity at commercially competitive prices, solar-to-fuel research is still in its infancy. The overall process is more complex, as in addition to the photon capture and charge separation; it also requires redox reactions to store the energy in chemical bonds. Water is by far the most interesting source for electrons and protons, which become available after oxidation to oxygen, as it is cheap and abundant. For the reduction reaction, several options are in principle available, but it should at least involve a reaction with protons that are extracted from water. As such, the water-splitting reaction is the most important reaction for the solar fuel field. In the past decade, many scientists from different backgrounds have studied the water-splitting reaction, which has resulted in an exponential growth of papers in the area. At the International Chemical Congress of the Pacific Basin Societies 2015, a symposium was organized entitled “Molecular catalysis of water splitting reactions”. The detailed mechanism of water oxidation and proton reduction was discussed for a variety of homogeneous and heterogeneous catalysts. For water oxidation, iridium- and ruthenium-based molecular catalysts are currently providing the highest reaction rates at the lowest over-potentials, but from the symposium it is clear that cobalt-, copper-, and iron-based complexes can provide good catalysts, too. Key issues yet to be solved include the stability of the catalyst and further lowering the overpotential at which these catalysts start to provide significant rates. Light-driven water oxidation is still in its infancy and most molecular catalysts for water oxidation have been studied by using chemical or electrochemical oxidation. Another challenge that receives current attention is the implementation of molecular components into the device. Various strategies were reported at the conference. Obviously, when heterogeneous catalysts are involved, the implementation generally involves electrodeposition of the catalyst on the electrode. Several interesting new materials and insights were reported at the conference. Proton-reduction catalysis was the other main topic of the symposium. This is in principle an easier reaction as it only involves two electrons and two protons, and platinum is basically an ideal catalyst for this reaction as it reversibly converts protons and electrons to molecular hydrogen at zero overpotential. However, there are several reasons to develop (molecular) catalysts based on abundant metals, and Nature provides an interesting blueprint in the form of hydrogenase. Hydrogenase enzymes also reversibly convert protons and electrons to molecular hydrogen at zero overpotential, and their active sites are based on diiron, iron–nickel, or iron complexes. Interestingly, many mimics of these active sites have been reported, and most of them show activity in organic solvents. However, the reaction rate is often lower, and the overpotential is much higher than that of the natural enzyme. Next to iron-based complexes, also cobalt- and nickel-based complexes have been reported. Whereas electrochemical proton reduction has been demonstrated to be very efficient, examples of efficient light-driven proton reduction are scarce. This special issue on catalytic systems for water splitting reflects to some extent the research that is centered on the current challenges. It contains some contributions on heterogeneous catalysts on electrodes for light-driven water oxidation as well as solution studies on light-driven water oxidation. In the Communication by F. Li et al., the ruthenium-based chromophore and the catalysts are pre-organized on the electrode using supramolecular strategies. In their Communication, K. Sakai et al. demonstrate light-driven water oxidation using a water-soluble copper phthalocyanine. The Review by K. Tanaka and M. Yamamoto summarizes recent efforts on light-driven water oxidation and concludes with the challenges that we may want to focus on in the years to come. An interesting new direction in research using Nature not only as a source of inspiration, but also to generate hybrid materials. Combining synthetic with biological components can be achieved at different levels. Natural apo-hydrogenases have previously been loaded with synthetic catalysts, and also natural photosynthetic systems have been combined with synthetic catalysts. In the current contribution by V. Artero et al., an artificial hydrogenase has been generated by incorporation of a synthetic cobaloxime into the heme oxygenase protein environment. The catalytic efficiency can be modulated by changing the protein environment. Although such systems may not be practical for applications, they certainly provide information on how Nature uses second coordination spheres to control catalyst properties. As such, we expect a lot from the biohybrid approach. We hope that you enjoy reading this special issue and that it provides further inspiration for addressing the challenges in the area of solar-to-fuel research. Gary W. Brudvig received a BS in 1976 from the University of Minnesota, a PhD in 1981 from Caltech for research conducted with Sunney Chan and was a Miller Postdoctoral Fellow hosted by Ken Sauer at the University of California, Berkeley from 1980 to 1982. He has been on the faculty at Yale University since 1982 where he currently is the Benjamin Silliman Professor and Chair of Chemistry, Professor of Molecular Biophysics and Biochemistry, and Director of the Yale Energy Sciences Institute. His research involves study of the chemistry of solar energy conversion in photosynthesis and developing artificial bioinspired systems for solar fuel production. Joost Reek did his PhD with Prof. Nolte in the area of supramolecular chemistry (1996), and after a postdoctoral stay in Sydney (Prof. M. J. Crossley) he became lecturer at the University of Amsterdam in the group of Prof. Van Leeuwen (transition-metal catalysis). In 2006 he was appointed Full Professor (chair of supramolecular catalysis) at the University of Amsterdam. In 2013 he became Director of the van‘t Hoff Institute for Molecular Sciences. He currently heads a research group of around 40 people, working on various topics related to supramolecular chemistry and transition metal catalysis, with applications in asymmetric catalysis, conversion of biorenewables, and catalysis for green energy applications. Ken Sakai received his BS (1987), MS (1989), and PhD (1993) from Waseda University working with Kazuko Matsumoto. He was an Assistant Professor at Seikei University from 1991 to 1999, an Associate Professor at Tokyo University of Science from 1999 to 2004, and has been a Full Professor at Kyushu University since 2004. He was also appointed Principal Investigator at the Kyushu University International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) in 2012. His research focuses on improving current understanding of various important catalytic processes together with work aimed at the development of hybrid materials as artificial photosynthetic nanoreactors for solar fuel generation. Leone Spiccia obtained his PhD degree in Physical and Inorganic Chemistry from the University of Western Australia in 1984. After postdoctoral positions at the University of Calgary, l'Université de Neuchâtel, and the Australian National University, he took up an academic appointment in the School of Chemistry at Monash University in 1987, where he is currently Professor of Chemistry. His research interests include bio-inspired water oxidation catalysis, water splitting, dye-sensitized and perovskite solar cells, coordination and bio-inorganic chemistry, therapeutic and diagnostic applications of nanomaterials, and radiolabeled bioconjugates. Licheng Sun received his PhD in 1990 from Dalian University of Technology (DUT). He went to Germany as a postdoc at the Max-Planck-Institut für Strahlenchemie with Dr. Helmut Görner (1992–1993), and then as an Alexander von Humboldt fellow at the Freie Universität Berlin (1993–1995) with Prof. Dr Harry Kurreck. He moved to KTH Royal Institute of Technology, Stockholm in 1995 and became Assistant Professor in 1997, Associate Professor in 1999 (at Stockholm University), and Full Professor in 2004 (KTH). He is presently also a Distinguished Professor and the Director of the Institute of Artificial Photosynthesis at DUT. His research interests cover artificial photosynthesis, advanced materials for water oxidation, hydrogen generation, and CO2 reduction, functional devices for total water splitting, dye-sensitized and quantum dot/rod sensitized solar cells, and perovskite solar cells.

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