Turning Sunlight into Liquid Fuels: … a Nano-sized Photocatalyst for Artificial Photosynthesis

Turning Sunlight into Liquid Fuels: Berkeley Lab Researchers Create a Nano-sized Photocatalyst for Artificial Photosynthesis

Posted By lcyarris On March 10, 2009 @ 12:02 pm In Press Releases | Comments Disabled

Contact: Lynn Yarris (510) 486-5375, <1> lcyarris@lbl.gov

Cobalt oxide nanocrystals can effectively be used to split water molecules, one of the half reactions critical to an artifical photosynthesis system for producing liquid fuels from sunlight.

Berkeley, CA - For millions of years, green plants have employed photosynthesis to capture energy from sunlight and convert it into electrochemical energy. A goal of scientists has been to develop an artificial version of photosynthesis that can be used to produce liquid fuels from carbon dioxide and water. Researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have now taken a critical step towards this goal with the discovery that nano-sized crystals of cobalt oxide can effectively carry out the critical photosynthetic reaction of splitting water molecules.

In this video, an aqueous solution contains silica particles that have been embedded with photooxidizing cobalt oxide nanocrystals plus a sensitizer to allow the water-splitting reaction to be driven by visible light. When laser light hits the solution it turns from gold to blue as the sensitizer absorbs light. Bubbles soon begin to form as oxygen gas is released from the spilt water molecules.

“Photooxidation of water molecules into oxygen, electrons and protons (hydrogen ions) is one of the two essential half reactions of an artifical photosynthesis system - it provides the electrons needed to reduce carbon dioxide to a fuel,” said Heinz Frei, a chemist with Berkeley Lab’s Physical Biosciences Division, who conducted this research with his postdoctoral fellow Feng Jiao. “Effective photooxidation requires a catalyst that is both efficient in its use of solar photons and fast enough to keep up with solar flux in order to avoid wasting those photons. Clusters of cobalt oxide nanocrystals are sufficiently efficient and fast, and are also robust (last a long time) and abundant. They perfectly fit the bill.”

Frei and Jiao have reported the results of their study in the journal Angewandte Chemie, in a paper entitled: “Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts.” This research was performed through the Helios Solar Energy Research Center (Helios SERC), a scientific program at Berkeley Lab under the direction of Paul Alivisatos, which is aimed at developing fuels from sunlight. Frei serves as deputy director of Helios SERC.

Artificial photosynthesis for the production of liquid fuels offers the promise of a renewable and carbon-neutral source of transportation energy, meaning it would not contribute to the global warming that results from the burning of oil and coal. The idea is to improve upon the process that has long-served green plants and certain bacteria by integrating into a single platform light-harvesting systems that can capture solar photons and catalytic systems that can oxidize water - in other words, an artificial leaf.

“To take advantage of the flexibility and precision by which light absorption, charge transport and catalytic properties can be controlled by discrete inorganic molecular structures, we have been working with polynuclear metal oxide nanoclusters in silica,” Frei said. “In earlier work, we found that iridium oxide was efficient and fast enough to do the job, but iridium is the least abundant metal on earth and not suitable for use on a very large scale. We needed a metal that was equally effective but far more abundant.”

Green plants perform the photooxidation of water molecules within a complex of proteins called Photosystem II, in which manganese-containing enzymes serve as the catalyst. Manganese-based organometallic complexes modeled off Photosystem II have shown some promise as photocatalysts for water oxidation but some suffer from being water insoluble and none are very robust. In looking for purely inorganic catalysts that would dissolve in water and would be far more robust than biomimetic materials, Frei and Jiao turned to cobalt oxide, a highly abundant material that is an an important industrial catalyst. When Frei and Jiao tested micron-sized particles of cobalt oxide, they found the particles were inefficient and not nearly fast enough to serve as photocatalysts. However, when they nano-sized the particles it was another story.

“The yield for clusters of cobalt oxide (Co3O4) nano-sized crystals was about 1,600 times higher than for micron-sized particles,” said Frei, “and the turnover frequency (speed) was about 1,140 oxygen molecules per second per cluster, which is commensurate with solar flux at ground level (approximately 1,000 Watts per square meter).”

Frei and Jiao used mesoporous silica as their scaffold, growing their cobalt nanocrystals within the naturally parallel nanoscale channels of the silica via a technique known as “wet impregnation.” The best performers were rod-shaped crystals measuring 8 nanometers in diameter and 50 nanometers in length, which were interconnected by short bridges to form bundled clusters. The bundles were shaped like a sphere with a diameter of 35 nanometers. While the catalytic efficiency of the cobalt metal itself was important, Frei said the major factor behind the enhanced efficiency and speed of the bundles was their size.

“We suspect that the comparatively very large internal area of these 35 nanometer bundles (where catalysis takes place) was the main factor behind their increased efficiency,” he said, “because when we produced larger bundles (65 nanometer diameters), the internal area was reduced and the bundles lost much of that efficiency gain.”

Frei and Jiao will be conducting further studies to gain a better understanding of why their cobalt oxide nanocrystal clusters are such efficient and high-speed photocatalysts and also looking into other metal oxide catalysts. The next big step, however, will be to integrate the water oxidation half reaction with the carbon dioxide reduction step in an artificial leaf type system.

“The efficiency, speed and size of our cobalt oxide nanocrystal clusters are comparable to Photosystem II,” said Frei. “When you factor in the abundance of cobalt oxide, the stability of the nanoclusters under use, the modest overpotential and mild pH and temperature conditions, we believe we have a promising catalytic component for developing a viable integrated solar fuel conversion system. This is the next important challenge in the field of artificial photosynthesis for fuel production.”

The Helios Solar Energy Research Center is supported by the Director, Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our Website at www.lbl.gov/

Additional Information

To read a copy of the paper “Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts” go here: <4> http://www3.interscience.wiley.com/cgi-bin/fulltext/121664121/HTMLSTART

For more information on the research of Heinz Frei, visit his Website at <5> http://pbd.lbl.gov/about/people/frei.htm

For more information about the Helios Solar Energy Research Center, visit the Website at <6> http://www.lbl.gov/LBL-Programs/helios-serc/index.html

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IV.A.2.b. Maximum Efficiency of Photosynthesis

Many environmental factors affect the performance of the complex photosynthetic machinery in microalgae, reducing its efficiency to well below the maximum at which photosynthesis can perform. That maximum is dictated by the underlying mechanisms, biophysical constraints, and physiological adaptations. One objective of applied microalgal R&D would be to develop strains and techniques that achieve productivities as close as possible to the maximum.

However, somewhat surprisingly, there is still argument about the maximum limit for photosynthetic efficiencies. The arguments boil down to the mechanisms assumed and the many possible loss factors that may or may not be considered. Most researchers agree that an absolute minimum of eight quanta (photons) of light absorbed are required by the two-photosystem mechanism (Z-scheme) of photosynthesis to reduce one molecule of CO₂ (and closer to 10 to 12 quanta if the energy needs for CO₂ fixation and cell metabolism are considered). However, there have been many reports of higher efficiencies. For example, recently Greenbaum et al. (1995) reported that some algal mutants lacking one photosystem still fixed CO₂ (and produced H₂), suggesting less than 8 (and as few as 4) quanta per CO₂ reduced. However, recent reports cast doubts on this interpretation, and the two-photosystem mechanism appears robust.

The maximum efficiency can be estimated at about 10% of total solar (Bolton 1996). Such efficiencies have been used in the projections for microalgae biodiesel production (see Section III.D.). However, high sunlight conversions are observed ony at low light intensities. Under full sunlight, typically one-third or less of this maximal efficiency, biomass productivity is obtained, because of the light saturation effect.

Light saturation is simply the fact that algae, like many plants, can use efficiently rather low levels of light, typically only 10% of full sunlight (and often even less). Above this level, light is wasted. In fact, full sunlight intensities can damage the photosynthetic apparatus, a phenomenon known as photoinhibition. Light saturation and photoinhibition result from several hundred chlorophyll molecules collaborating in light trapping, an arrangement ideally suited for dense algal cultures, where on average a cell receives little light. However, exposed to full sunlight, the photosynthetic apparatus cannot keep up with the high photon flux and most of the photons are wasted, as heat and fluorescence, and can damage the photosynthetic apparatus in the process. One possibility, suggested by Neidhardt et al. (1998), is that photosynthetic productivity and light utilization could be maximized in microalgae by reducing the size of the light-harvesting antenna through mutation or genetic engineering. This is an interesting idea that will be discussed further in the next section.

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IV.A.2.c. Overcoming Light Saturation, Photooxidation, and Other Limitations

The problem of light saturation has been a subject of research in photosynthesis for almost 5 decades, with the report by Kok (1953) that microalgae cultures exposed to short (milliseconds) flashes of bright light, followed by longer periods of darkness, exhibited the same light conversion efficiencies as cultures exposed to the same total photon flux averaged for the entire period. The interpretation was straightforward: only a limited number of photons can be used per unit time, and the millisecond light/dark periods allow averaging high photon fluxes. A large body of literature has developed on this subject, including laboratory work by the ASP (Terry 1984, 1986; see also Section II). The mass culture work in Hawaii (Section III.B.2.), among many others, attempted to use this phenomenon to increase algal productivities. However, practical applications are not plausible because of the very short time periods involved. Another approach, central to the Japanese microalgae program (Section IV.B.1.c.), has been to diffuse light throughout the depth of the culture, using optical fibers, thus avoiding high a surface irradiance. But this approach is also not practical for biodiesel production because of the very high cost of the system.

A potential practical solution to the light saturation problem, and also probably to photoinhibition, has been recognized for many years (e.g., Kok 1973): reduce the number of chlorophyll molecules cooperating in photosynthesis (the so-called “antenna” chlorophylls) from a few hundred to a few dozen. This would allow the photosynthetic apparatus to absorb only as much light as it can use. The benefits of reduced absorption are that it would:

• reduce waste,
• limit photooxidative damage to the photosynthetic reaction center, and
• increase the overall productivity of an algal culture, by a factor of at least 3 (see Benemann and Oswald 1996 for a recent discussion).

However, it has only recently become possible to consider achieving this objective, through the detailed understanding of photosynthesis at the molecular level, and the development of genetic engineering tools that could now allow us to redesign the photosynthetic apparatus. Recent work by Melis et al. (1998) and Neidhardt et al. (1998) demonstrated, at the physiological level, the feasibility of obtaining high efficiencies and high light saturation levels with algal cultures. Much more research is required, but the molecular and genetic tools are available to achieve the desired high photosynthetic efficiencies by algal mass cultures. Such tools can also be used to direct the flow of photosynthate to desired metabolic products, such as lipids (see Section II).

Future R&D should demonstrate the feasibility of genetically engineering an improved photosynthesis system using algae for which such genetic systems are already well established. Once proven, these techniques can then be transferred to strains suitable for mass culture.

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Optical properties of microalgae for enhanced biofuels production

Mautusi Mitra and Anastasios Melis

Optics Express, Vol. 16, Issue 26, pp. 21807-21820 doi:10.1364/OE.16.021807

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7. Conclusions

It is an objective of this work to minimize, or truncate, the chlorophyll antenna size in green microalgae in order to maximize solar-to-product conversion efficiency and photosynthetic productivity in mass culture. Further, the work seeks to identify currently unknown genes that determine the Chl antenna size in photosynthetic organisms, and to demonstrate that a truncated Chl antenna size would minimize absorption and wasteful dissipation of sunlight by individual cells, resulting in better light utilization efficiency and greater photosynthetic productivity under mass culture conditions. To achieve these objectives, a promising approach was employed, based on DNA insertional mutagenesis, screening, biochemical and molecular analyses for the isolation of “truncated Chl antenna size” strains in the green alga C. reinhardtii. In addition to the Tla1 gene, efforts are currently under way to identify and characterize other novel gene(s) that affect or define the “Chl antenna size” in the model microalga C. reinhardtii. Eventually, the work seeks to genetically manipulate these genes to generate “truncated Chl antenna size” strains in C. reinhardtii and in other green algae of interest to the alga biotechnology sector.

Current progress suggests that a partially truncated chlorophyll antenna size of the microalgae alleviates the over-absorption of incident sunlight by individual cells in a high-density culture, and minimizes the wasteful dissipation of irradiance. A truncated light-harvesting chlorophyll antenna size in such mutants diminishes the severe cell shading that occurs with normally pigmented wild type, permitting a more uniform illumination of the cells in a mass culture, and resulting in a greater solar-to-product conversion efficiency and photosynthetic productivity of the algae under high cell density and bright sunlight conditions. Accordingly, the truncated light-harvesting chlorophyll antenna size (tla) property may find application in the commercial exploitation of microalgae for the generation of biomass, biofuel, chemical feedstock, as well as nutraceutical and pharmaceutical products.

Acknowledgments

The work was supported by the DOE Hydrogen, Fuel Cells and Infrastructure Technologies Program.