Science at the Frontier (1992) / Chapter Skim
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2 Photosynthesis: Artificial Photosynthesis: Chemical and Biological Systems for Converting Light to Electricity and Fuels
2 Photosynthesis: Artificial Photosynthesis: Chemical and Biological Systems for Converting Light to Electricity and Fuels
Pages 25-44
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From page 25... ...
However, recent advances in spectroscopy, crystallography, and molecular genetics have clarified much of the picture, and scientists like Wrighton are actively trying to transform what is known about the process into functional, efficient, synthetic systems that will tap the endless supply of energy coming from the sun. "Photosynthesis works," said Wrighton, "and on a large scale." This vast natural phenomenon occurring throughout the biosphere and producing an enormous amount of one kind of fuel food for plants and animals Wrighton described as "an existence proof that a solar conversion system can produce [a different, though]
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From page 26... ...
Notwithstanding subsequent refinements due to quantum physics and to scientists' increasing ability to probe and examine these reactions directly, Dalton's basic description of the behavior and transfer of protons and electrons among and between elements and compounds the opening salvo fired at every high school chemistry student still sets the stage for the most advanced chemical research. Photosynthesis provides a vivid example of the type of drama that is played out effortlessly in nature but reenacted elaborately in chemical laboratories with painstaking concern for the intri
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From page 27... ...
Thus the stakes for society are high, and the contrast is dramatic: a chemist works on precisely controlled and intricately choreographed reactions in a laboratory usually on a very small scale and yet the implications and applications of his or her work may lead to dramatic transformations, on a vast scale, of the material world. In the case of research on artificial photosynthesis, such work could lead to the economical production of an alternative to the dwindling supply of fossil fuels.
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From page 28... ...
Liquid hydrogen is already in use as a fuel source and has always been the primary fuel powering space vehicles, since it produces more heat per gram of weight than any other known fuel. If a photosynthetic system delivering usable hydrogen could be developed, the process would regenerate the original water source, and an entirely new recycling of natural resources could be established.
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From page 29... ...
But it is the physical chemistry that interests Wrighton and his colleagues, who hope to develop analogous systems that would Produce usable energy. -- r Harvesting Photons and Putting Them to Use Two fundamental constraints govern the system: the plant or photosynthesizing organism must possess a mechanism to register or receive the incoming photon; and since the energy content of a single photon is small, a way must also be found to collect and aggregate
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From page 30... ...
"The second critical feature of the photosynthetic apparatus," Wrighton emphasized, is that "in order to achieve high solar conversion efficiency, the formation of a single fuel molecule will involve the energy from more than one photon." The ratio of photons to electrons released will probably be one to one in any system as it is in nature but there must be a way to harness the energy from several of these freed-up electrons to produce a chemical transformation. "If a one-photon mechanism were operative in nature, the process would be doomed to low efficiency," he explained, because a single photon that would break down H2O would have to be in the blue wavelength range, and "sunlight does not contain much energy of blue light, or of shorter wavelengths." Nature's way of aggregating and using more than one photon of the energy that is abundant, throughout the entire optical wavelength spectrum, is photosynthesis.
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From page 31... ...
H2O ~ hv T l it hv , , l , ~ '- Carbohydrates ( Light Absorbers (Photosystems I and 11) CO2 + H2O FIGURE 2.1 Z-scheme representation of the photosynthetic apparatus showing, on an electrochemical potential scale, components for light absorption, charge transport, and redox processes for oxidation of H2O to O2 and reduction of CO2 and H2O to carbohydrates.
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From page 32... ...
As the electron moves through the transport chain, its negative charge and the positively charged hole are separated by ever greater distance, and potential energy is created. In essence, the reaction center acts as a tiny capacitor by separating and storing these
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From page 33... ...
Then every cellular component gets to a common free energy, and you have a death by entropy." Genetic engineering has also been employed to create mutant reaction centers, where certain proteins and cofactors have been deleted or altered. Rees reported that, "rather surprisingly, in many of these mutant forms the reaction center still works," though with a reduced quantum efficiency.
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From page 34... ...
Thus far, the most promising synthetic systems have exploited the chemistry and physics of liquid-semiconductor junctions. Excited-state Electron Transfer in Synthetic Systems Quantum physics explains how the light energy of a photon is absorbed by the right sort of receptor, kicking an electron loose and commencing the process of photosynthesis by creating a source of potential energy between the separated electron and the hole it formerly occupied.
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From page 35... ...
In nature, the movement of the carriers away from one another is crucial to high efficiency. The unidirectional movement of electrons in the Z-scheme is a consequence of the components of the charge transport chain, and how they are arranged, both geometrically and energetically," Wrighton explained.
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From page 36... ...
The fastest rate occurs when the reaction free energy exactly equals the reorganization energy. If less energy is available, the bonds can't be stretched enough for reaction to occur." Conversely, continued McLendon, "if too much energy is available, the system must wait for this extra energy to 'dribble' away, since at the instant of transfer, the energy of the starting materials and products must be equal, by the law of conservation of energy." McLendon provided an example: "Consider a system containing only ferrous ions (dipositive charged ion molecules)
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From page 37... ...
" Because the cost of materials and the complexity of assembly are crucial determinants of the viability of a commercial system, Mallouk said, "we look for ways where we can get these things to self-assemble, hopefully into a matrix that will teach them how to line up usefully, even if we just chuck them all in there randomly." Several strategies are combined, including using electric field gradients to propel and direct freed electrons, creating a gate whose size will selectively admit only the desired species of molecule, and employing molecules whose electrochemical potentials are easily controlled. (Figures 2.3 and 2.4~.
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From page 38... ...
2 ,=N N ~ CH3 CH3 _ _ _ _ Ru(bpy) 3 +-2D~+ FIGURE 2.4 Analogy between natural and zeolite-based photosynthetic reaction centers.
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From page 39... ...
Mallouk's team actually "slowed down the forward rate," which, he conceded, is "kind of a macho thing in electron transfer chemistry, where the basic rule is to get your forward rates as fast as possible." But, he said, citing McLendon's thesis, "it really doesn't matter as long as your forward rate is fast enough compared to the competing back transfer pathway." "There are many steps in a row, but the point is to get the electron safely away from where it came. With so much empty space in the zeolite, we can load up the rest of the channel with a molecule that is easier to reduce.
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From page 40... ...
If the design of photoelectrochemical cells could be improved to make the cost per kilowatt-hour of their generated electricity competitive, or to yield a fuel product from ubiquitous raw materials, the world's energy future could be reconsidered altogether.
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From page 41... ...
Thus, when an electron is released and creates a hole, each of these particles will be maximally positioned to begin their respective journeys through the respective semiconductors for which they have an inherent electrical affinity. Propelled by the electric field and their own charge, they migrate through the semiconductor layers to the metal contacts at top and bottom, from which a current can be tapped.
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From page 42... ...
"If you try to match their surfaces with atomic precision, you will pay a price" to do so, said Lewis, and thus drive up the economic cost of the system. "When you miss at certain spots, those spots become recombination sites," and some of the free charge meant to be collected as electricity is drained into these surface reactions.
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From page 43... ...
"Short of changing the sun itself," said Wrighton, the problem remains to find a stable solution that will provide a complete cycle of oxidation and reduction, and a donor to complement it that will react to visible light. A real frontier in research on PV cells is finding or constructing materials and solvents that will balance all of these considerations: bandgap, passivation to counter deficits at the surface junctions, ionic acceleration of particles to avoid back transfer, and a material that will provide both sides of the redox reaction so as to produce a usable fuel.
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From page 44... ...
1990. Artificial photosynthesis in zeolite-based molecular assemblies.
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