Natural solar cells and their geochemical implications

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Technical description:The role of photoelectrochemical processes in Earth?s early history is not well understood. Many common oxide and sulfide minerals are photoactive and capable of driving natural photoredox processes. The proposal discusses several major events in Earth history, such as banded iron formations and the rise of oxygen, to which such photochemically active minerals might have contributed. The project focuses in particular on a new concept in natural photoelectrochemistry: coupled-mineral systems.In solar energy research, there is a great deal of interest in photochemical water splitting ? the formation of O2 and H2 using sunlight and water. Such processes are usually quite inefficient, but engineered ?tandem cells? have been constructed in which two semiconductors, each absorbing a different light wavelength, greatly improve the efficiency of the overall photocatalytic process. The photochemical current densities that we measured in initial experiments with hematite-pyrite tandem cell (which can be expected to form in nature as the simple consequence of incipient pyrite oxidation) are surprisingly comparable to the deposition rates of iron in banded iron formations and the rate of water loss from Mars over time. The time seems ripe, therefore, to study a few likely naturally-occurring mineral tandem cell systems in order to quantify the extent to which mineral-based photochemistry might have driven important early-Earth and planetary processes.We propose to investigate the behavior and properties of such natural photo-electrochemical cells. We intend to understand the rates at which water can be oxidized and hydrogen produced, the rates at which other common aqueous solutes can be oxidized or reduced, the mineral properties needed for such processes, assess a small set of mineral systems that could have been important on early Earth, and investigate the effects of variable pO2, light intensity, pH, temperature, and both aqueous and solid compositions on the overall photoelectrochemical process. Non-technical explanation:While the role of semiconducting minerals in natural processes of the Earth is of geological interest, the fundamental properties of mineral semiconductors as photocatalysts are probably of greatest significance in the development of solar energy technology. If a solar photocatalytic system can be developed that produces fuels (chemically stored energy) reasonably efficiently, such technology ? if it is to truly impact the global energy picture in the long run ? must be constructed of Earth-abundant materials. Iron and manganese oxides and sulfides are common, whereas there is simply not enough platinum on Earth to be used as electrodes/catalyst for such systems. From the point of view of medium- to long-term energy security (not to mention climate change), such fuel-generating technologies as described in this proposal need to be better studied, improved, and implemented. The PI has experience both in fundamental geochemistry research as well as with scientists working on solar energy, and the improved understanding of the tandem-cell mineral systems in this proposal has applications equally in geochemistry and solar energy technology.