Fast and efficient molecular electrocatalysts for H 2 production: Using hydrogenase enzymes as guides
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troduction A secure energy future will require the development of sustainable, nonfossil energy sources such as solar, wind, geothermal, and nuclear energy.1–3 For these energy sources to ultimately replace fossil fuels, energy storage will be required. In principle, fuels can be generated from abundant and inexpensive small molecules such as H2O, N2, and CO2. The simplest of these energy conversion reactions is the splitting of water to produce H2 and O2. This article will focus on the development of electrocatalysts for the conversion of electrical energy and protons from water to produce H2 (Reaction 1) 2H + + 2 e − ⇔ H 2 .
(1)
Because the production of H2 is the simplest fuel generation reaction, there is much that we can learn from its study that is applicable to other fuel generation reactions. Although platinum is an excellent catalyst for H2 production and oxidation, its limited supply and high cost provide a significant impetus to develop alternatives based on more
abundant and less expensive metals. Nature solved this problem, evolving hydrogenase enzymes, that rapidly and reversibly catalyze Reaction 1. These enzymes use nickel and iron in conjunction with carefully selected ligands in a highly evolved protein environment to create fast (1,000 to 10,000 turnovers per second) and efficient (overpotentials of less than 100 mV) catalysts for H2 production and oxidation. From detailed studies of these enzymes,4–8 hypotheses regarding how the enzymes achieve such high catalytic rates can be developed and used to guide the design of simpler synthetic catalysts, so-called molecular catalysts. By incorporating selected structural features into molecular catalysts and evaluating their catalytic activity, it is possible to test these hypotheses and gain a deeper understanding of the enzymes and the catalytic principles involved. It is hoped these studies will lead to catalysts suitable for practical applications. Although the focus of this article is on the design of catalysts at the molecular scale, many of the principles of catalyst design at the molecular level have counterparts in the design
Jenny Y. Yang, Pacific Northwest National Laboratory, Richland, WA 99352, USA, [email protected] R. Morris Bullock, Pacific Northwest National Laboratory, Richland, WA 99352, USA, [email protected] M. Rakowski DuBois, Pacific Northwest National Laboratory, Richland, WA 99352, USA, [email protected] Daniel L. DuBois, Pacific Northwest National Laboratory, Richland, WA 99352, USA, [email protected] DOI: 10.1557/mrs.2010.8
© 2011 Materials Research Society
MRS BULLETIN • VOLUME 36 • JANUARY 2011 • www.mrs.org/bulletin
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FAST AND EFFICIENT MOLECULAR ELECTROCATALYSTS FOR H2 PRODUCTION of materials at larger length scales, and we will endeavor to emphasize some of these connections.
General design principles from hydrogenase enzymes X-ray diffraction studies of the hydrogenase enzymes have provided structural information suggesting a number of concepts for catalyst design.4–8 Figure 1 shows a schematic depiction
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