This century's greatest technological challenge is the conversion of sunlight into usable energy in a sustainable, environmentally benign and carbon-free way on a global scale. For light-to-chemical energy conversion in a designed photosynthetic system, the splitting of water into its constituent elements is the key energy-storing reaction. As with natural photosynthesis, such a system relies on light absorption, charge separation, and catalysis. Recent efforts are described that focus on different components and system compatibility for the reductive side of the water splitting reaction. The light absorber in recent systems consists of water-solubilized semiconductor nanoparticles while the catalysts are complexes of cobalt, nickel and iron containing redox-active ligands. The studies provide clues to initial charge transfer steps, the mechanism(s) of H<sub>2</sub> generation and sources of system instability. An unrecognized factor in the effectiveness of these H<sub>2</sub>-generating systems involving quantum dot (QD) light absorbers regards interactions between the water-solubilizing capping agents used for the QDs and the metal complex catalysts. These interactions affect the nature of the actual catalyst for hydrogen generation. An approach currently in progress involves the development of photocathodes in which the QD light absorbers that upon excitation transfer electrons to a solution-based catalyst and are subsequently reduced by a p-type semiconductor to which they are attached.

Fuel from water: The reductive side of water splitting and the light-driven generation of hydrogen

253rd National Meeting (2017)

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