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Under the fuel through artificial photosynthesis scenario, nanotubes embedded within a membrane would act like green leaves, using incident solar radiation (H??) to split water molecules (H2O), freeing up electrons and oxygen (O2) that then react with carbon dioxide (CO2) to produce a fuel, shown here as methanol (CH3OH). Credit: Flavio Robles, Berkeley Lab Public Affairs. Click to enlarge.

Artificial photosynthesis for the production of liquid fuels is a potential source for renewable and carbon-neutral of transportation energy. The basic concept is to integrate light-harvesting systems that can capture solar photons and catalytic systems that can oxidize water, then to combine this water oxidation half reaction with a carbon dioxide reduction step in an artificial-leaf type system to produce a liquid hydrocarbon, such as methanol (CH3OH), that can be stored, transported, and used for transportation or other applications.

Researchers with the US Department of Energy??™s Lawrence Berkeley National Laboratory (Berkeley Lab) have now found that nano-sized crystals of cobalt oxide can effectively carry out the critical photosynthetic reaction of splitting water molecules. Heinz Frei, a chemist with Berkeley Lab??™s Physical Biosciences Division, and his postdoctoral fellow Feng Jiao 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.???

O2 evolution with different bundle sizes. Source: SERC. Click to enlarge.

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. The goal of Helios SERC is to produce carbon-neutral transportation fuels using solar energy as the source of stored energy. Frei serves as deputy director of Helios SERC.

The effort in SERC is focused on building a highly affordable system in which the entire process will take place in a single photoelectrochemical (PEC) cell that collects sunlight and then generates the fuels at catalysts powered by the light harvesting and charge separating units. SERC has 26 senior scientists focused on different aspects of this problem.

Photooxidation of water molecules into oxygen, electrons and protons (hydrogen ions) is one of the two essential half reactions of an artificial photosynthesis system??”it provides the electrons needed to reduce carbon dioxide to a fuel. 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.

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. 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.

??”Heinz Frei

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.

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 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, the results were quite different.

The yield for clusters of cobalt oxide (Co3O4) nano-sized crystals was about 1,600 times higher than for micron-sized particles, 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).

??”Heinz Frei

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 because when we produced larger bundles (65 nanometer diameters), the internal area was reduced and the bundles lost much of that efficiency gain.

??”Heinz Frei

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. 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.

??”Heinz Frei

The Helios Solar Energy Research Center is supported by the Director, Office of Science, Office of Basic Energy Sciences of the US Department of Energy. Berkeley Lab is a US Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

Resources

Feng Jiao, Heinz Frei (2009) Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts. Angewandte Chemie Volume 121, Issue 10, Pages 1873-1876 doi: 10.1002/ange.200805534

Frei research

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