Scientists are developing ways to use solar energy and water to create fuels. To form a fuel, you need protons and electrons. Ideally, you want them from a simple, renewable source. The challenge for solar fuels is splitting water into protons and electrons. It’s a tough reaction; it needs the right catalyst. Scientists established design principles for a new ruthenium-based catalyst. They determined how to optimally control the position of the water and other reactants in the catalyst. The result? A catalyst that’s 100 times faster under acidic conditions than the best previous catalysts.
Via photosynthesis, plants use solar energy and water to produce fuels that a plant needs. Using a similar strategy, artificial photosynthesis could create fuels to address our energy needs. A key challenge in developing solar fuels? Creating a catalyst that can operate efficiently and effectively under the appropriate environmental conditions. The design principles uncovered by this research effort are vital to moving solar fuels closer to reality.
Water oxidation provides the electrons and protons required in natural photosynthesis to convert solar energy into chemical energy that it is stored in chemical bonds. The process of water oxidation is a key area for improvement toward energy-producing devices that rely on artificial photosynthesis. The application of molecular catalysts in such a device will require their immobilization on an electrode or semiconductor surface. Further, it is advantageous for the catalyst to function in harsh acidic conditions. Therefore, in addition to being fast, the optimal catalyst will be as active when immobilized as it is in solution and will be rugged enough to operate at acidic pH.
Scientists at Brookhaven National Laboratory developed a new ruthenium-based water oxidation catalyst that works in acidic conditions with a mechanism that isn’t inhibited by immobilization. The new design improves upon previous catalysts to increase rates 100 times. In fact, the team’s catalyst is the first single-site catalyst to match the rate of the natural photosynthetic oxygen-evolving complex in green plants. Previous water oxidation catalysts (single-site model) are slow due to high oxidation potentials and high H+ loss barriers, but the team’s new ruthenium-based complex, designed with computational insights, accelerates reactions with two water molecules. A labile carboxylate group allows initial water binding, lowering the oxidation potentials by releasing H+. A basic phosphonate group accelerates oxygen-oxygen bond formation by accepting H+ from the incoming second water molecule.
The Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division funded this research.
D.W. Shaffer, Y. Xie, D.J. Szalda, and J.J. Concepcion, “Lability and basicity of bipyridine−carboxylate−phosphonate ligand-accelerated single-site water oxidation by ruthenium-based molecular catalysts.” Journal of the American Chemical Society 139(43), 15347 (2017). [DOI: 10.1021/jacs.7b06096]