Towards a Design of a Complete Solar Water Splitting System

1 Feb 2013

BISfuel© :    A team of Bisfuel researchers led by Devens Gust, Ana Moore and Tom Moore has designed and characterized an artificial photosynthetic reaction center inspired by natural Photosystem II and comprising a highly oxidizing porphyrin linked to a biomimetic electron transfer relay and a porphyrin electron acceptor. Two articles with the results of the study have appeared in September special issue of PNAS “Chemical Approaches to Artificial Photosynthesis: Solar Fuels Special Feature” 

Natural Photosystem II and its mimic

One of the challenges in the design of the artificial reaction center is to ensure that the photochemically driven redox potential generates driving force that is enough to oxidize water by water splitting catalyst. A key molecule in the natural reaction center of the photosystem II is the primary electron donor, which upon absorption of light and transfer of the electron to the primary acceptor converts to a cation radical with a redox capacity to oxidize water. The structure on the left shows the arrangement of atoms in a portion of the photosynthetic PSII complex that uses light to oxidize water. The tyrosine (Tyrz) – histidine (His 190D1) pair of amino acids plays a crucial role in moving electrons from the water oxidation complex (OEC, Mn4Ca cluster) to the chlorophylls of the primary donor (green).

The Center researchers have constructed a synthetic molecular analog of PSII (right side of the Figure). The porphyrin mimics the PSII chlorophylls (shown in green), using light energy to move electrons to the electron acceptor (shown in blue), which plays the role of the photosynthetic primary acceptor, a molecule of pheophytin (PheoD1). The porphyrin then accepts electrons from the phenol-bisimidazole couple shown in yellow, which is inspired by the photosynthetic tyrosine-histidine redox relay. Ultrafast laser studies have verified that the artificial PSII construct functions in a manner similar to the natural PSII analog. In collaborative work with the laboratory of Professor Thomas Mallouk at Pennsylvania State University, the ASU Center personnel have shown that the phenol-bisimidazole structure can be incorporated into a complete solar water splitting system that uses iridium oxide nanoparticles as an analog of the photosynthetic OEC, and that illumination of this system produces oxygen and hydrogen gas from water. The complete system is inefficient and has been operated only at laboratory scale, but it is helping point the way to a viable solar fuel technology

 

Source:

Megiatto, J. D. Jr.; Antoniuk-Pablant, A.; Sherman, B. D.; Kodis, G.; Gervaldo, M.; Moore, T. A.; Moore, A. L.; and Gust, D. “Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation,” Proc. Natl. Acad. Sci. U. S. A., 39,15578-15583 (2012), doi:10.1073/pnas.1118348109.

Zhao, Y.; Swierk, J. R.; Megiatto, J. D. Jr., Sherman, B.; Youngblood, W. J.; Qin, D.; Lentz, D. M.; Moore, A. L.; Moore, T. A.; Gust, D.; and Mallouk, T. A. “Improving the efficiency of water splitting in dye-sensitized solar cells by using a biomimetic electron transfer mediator,” Proc. Natl. Acad. Sci. U. S. A., 39, 15612-15616, (2012), doi:10.1073/pnas.1118348109

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