Search for water-splitting catalysts for global usage
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Energy Sector Analysis
Catalyst development bridges bioinspiration and materials discovery.
Search for water-splitting catalysts for global usage By Melissae Fellet Feature Editor David M. Tiede
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wo widely available compounds, carbon dioxide and water, could be a source of renewable fuel in the future. In this scheme, electrocatalysts split water, generating oxygen gas along with protons and electrons used to reduce carbon dioxide into hydrocarbons. Research investigating water splitting for sunlightdriven fuels production ranges from the deciphering of structures and mechanisms in photosynthesis to the discovery of new solidstate and molecular catalytic materials. Hybrid materials that combine molecular catalysts on a solid surface could be used to build integrated devices that harvest sunlight to produce fuel. In photosynthesis, the protein complex photosystem II performs water splitting using a five metal atom Mn4CaO5 cluster housed inside the enzyme. This metalloenzyme has long-served as a paradigm for developing efficient water-splitting catalysts based on abundant first row transition metals. Crystallographic studies using x-ray-free electron lasers in Japan and the United States have resolved the structure of the Mn4CaO5 cluster photosystem II. This work also captured some of the intermediate oxidation states, leading to water splitting and O–O bond formation. These structures, combined with spectroscopy and advanced computational techniques, are providing an understanding of photosynthetic water splitting at the atomic scale. Based on earlier structures, a team comprised of researchers from China, Japan, and Germany synthesized a molecular Mn4CaO5 cluster that mimics many of the key structural features of the natural cofactor. The synthetic cluster lacks catalytic activity due, in part, to the absence of an open coordination spot for substrate water. It seems the secret to photosynthetic water splitting lies not only in the metal atom cluster, but also in the electronic communication between the cluster and the surrounding photosystem II protein, the organization of a ring of substrate water molecules, and builtin pathways for redox-coupled proton transfers. Metalloenzymes often distribute catalytic function across an individual metal center, as well as the thru-space interactions between the metal and the protein. Recreating both relationships in an artificial water-splitting catalyst is difficult because of the present lack of synthetic frameworks to create a comparably sophisticated coordination environment. However, structurally simpler catalysts—thin-film materials— have long been known to split water when used as cathodes and anodes in electrochemical cells. Vigorous present research aims to develop catalyst films that are stable, efficient, and made from
cheap earth-abundant elements. But no material or combination of materials has been found that meets all of these requirements yet, especially for direct solar-driven water splitting, said Ian Sharp, Lawrence Berkeley National Laboratory. Wh
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