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Impact: With the appropriate catalyst, water and sunlight can be used to produce hydrogen fuel efficiently, eliminating the nearly 200 million tons of carbon dioxide we currently add to our atmosphere each year here in the U.S. by the steam-reforming of natural gas. The basic science of fine tuning solid-state catalysts’ electronic structures at the level of synthesis is general, allowing us to tackle other environmentally compelling chemistry problems such as carbon dioxide reduction.
Objective: To create robust water oxidation catalysts in which the electronic energy levels in our compounds match the energies required for hydrogen and oxygen production from water.
Approach: Synthesize new solid catalysts in which magnetic transition metal ions capable of performing the multi-electron reactions needed to oxidize water are selectively incorporated. Study the fundamental relationship between atomic structure and catalytic activity in our electronically complex systems.
Impact: Development of a viable, sustained and solar-powered water electrolysis cell would revolutionize society’s approach to using and storing energy. To date, no alternative energy conversion system has emerged that can compete with the use of fossil fuels. A systematic approach for tailoring the bulk material and interfacial properties of semiconductor materials is a necessary step towards realizing clean renewable energy.
Objective: Our group is interested in preparing, understanding, and utilizing artificial photosynthetic systems that can efficiently and affordably convert solar energy into clean chemical fuels. Approach: Investigate the operative features of semiconductors as photoelectrodes for solar energy conversion. Attempt to tune and optimize inorganic and organic semiconductor materials and their interfaces for specific chemical transformation processes like H2O splitting and CO2 reduction. Hybrid research environment incorporating people in areas of chemistry, materials science, engineering, and applied physics.
Hydrogenase models: [Fe2(pdt)(dppv)2(CO)2(H)]+ terminal and bridging hydride isomers
Spectroscopy. The H-term and m-H isomers of [Fe2(pdt)(dppv)2(CO)2(H)]+ (see Figure) model the key protonated intermediate of the active site of Fe-only hydrogenase. Resonance Raman and IR spectroscopy show distinct differences between the two isomers in the n(Fe-CO) (440-520 cm-1) and n(C=O) (1800-2000 cm-1) stretching regions. However, the n(Fe-H) stretch of H-term, which is the proposed catalytically active form for H2 production, is not observed. The overall weak Raman intensity and the strong signals from the phenyl groups in IR pose serious problems in identifying the important n(Fe-H) stretching mode. DFT calculations predict n(Fe-H) at 2000 cm-1 for H-term and at 1294/1351 cm-1 for mu-H. Further spectroscopic studies of an analogous compound where the dppv ligands are replaced by PMe3, [Fe2(edt)(PMe3 )4(CO)2(H)]+, will be performed next to overcome this problem.
DFT calculations. Initial computations show that the mu-H isomer is 5 – 12 kcal/mol (depending on the DFT functional) more stable than H-term, in agreement with experiment. This suggests that the decreased reactivity of mu-H toward acids could simply be due to the distinctively lower total energy of this complex compared to H-term. A detailed analysis of the MO diagrams of both isomers indicates that the atomic charge of hydride is similar in these cases. This is surprising, since one would intuitively think that the terminal hydride is a weaker donor. This result implies that the total charge of the bound hydride does not contribute to the difference in reactivity (no charge control). However, the MO diagrams reveal one important difference between the complexes: the H-term isomer has a key molecular orbital (MO <139>, see left) at relatively high energy that shows a strong hydride(1s) contribution of 23%. No corresponding feature is present for the mu-H complex (the closest is MO <138>, see left). This indicates a possible orbital control of the reaction of the complexes with acid. This aspect requires further study.
Figure 1. Hydrogenase models: [Fe2(pdt)(dppv)2(CO)2(H)]+ terminal and bridging hydride isomers
Study of fundamental properties and behavior of electrodes employed in high-performance rechargeable batteries and fuel cells. The goal has been to synthesize and characterize new electrode materials in order to gain a fundamental understanding of the relationships among atomic and electronic structure, electrochemical performance, and long-term stability. For fuel cells the goals of his research are to synthesize new highly active electrocatalysts and characterize their kinetic and mechanist behavior.
Research Focus: Developing advanced new Li-intercalation cathode materials In-situ Structural Characterization of Li-ion cathode materials utilizing X-ray spectroscopy In-situ investigation of degradation and lifetime of advanced Li- ion battery materials Characterization of efficient electrocatalysts for fuel cells, utilizing X-ray spectroscopy Development of advanced “in-situ” diagnostic spectroscopy techniques sensitive to bulk, sensitive to surface processes and to monitor degradation processes.
J. Penner-Hahn, Aniruddha Deb
Figure 1. Cycling of the electrochemical cell using potenstiostat during X-ray spectroscopy measurements
Figure 2. Preparation of working electrodes
Figure 3. In-situ white line energy shift vs SOC
Figure 4. Combing XAS results and Faraday's Law
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