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New Nano-sized Photocatalyst for Artificial Photosynthesis; Step Toward Production of Carbon-Neutral Transportation Fuels

Frei
Under the fuel through artificial photosynthesis scenario, nanotubes embedded within a membrane would act like green leaves, using incident solar radiation (Hν) 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.”

Frei2
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

Comments

SJC

This sounds encouraging. It is basic research like this that advances the state of the art and creates potential break throughs.

ssintay

It is interesting that this work and recent work of Kanan and Nocera at MIT both have confirmed the ability of nan-sized Co particles to act as relatively efficient catalysts to enable water splitting at Low voltages. Nocera and Kanan recently showed that there methods would work with Salt water and "Dirty" water. I wonder if this setup could do the same?

Either way these are both exciting developments, and they seem to indicate that it is the surface structure and size of the Co particles that have a profound effect.

Roger Davis

Very interesting stuff. Unfortunately this, like just about every other posting I've seen on photolytic technologies (direct solar to H2 or other chemical fuel), makes no useful efficiency comparison with other solar methods. For instance, if you use solar PV to make electricity to split water vs. this artificial photosynthetic method, which technique gives you more H2, at what cost, requiring how much exposure area, etc.? The fact that I never see this comparison made leads me to believe that these direct photolytic techniques are inferior to other more indirect methods of making fuel from photons, at least at this time, although I am certainly not saying that this is not an approach worth exploring!

Someone please tell me my suspicions are unfounded, and why.

SJC

I hope for simplicity. If they can figure out a way to just take water and sun to make fuel, that would be dandy. Solar panels and electrolysis are fine, but capital intensive. Even if the area efficiency is low, if the cost per square foot is low as well, they may have something that would help.

ssintay

I read through the actual journal article (linked to in the sources at the bottom of the GCC article), and while I will readily admit that this is not my expertise, I will say that they do seem to address the efficiency question directly in the article. I will attempt to do explain...

To compare various processes they introduce a concept called Turnover Frequency or TOF for solar based water splitting. From the paper:

"Catalysts need to exhibit turnover frequency (TOF) and density (hence size) commensurate with the solar flux at ground level (1000Wm^2, airmass (AM) 1.5)[1] to avoid wasting of incident solar photons. For example, a catalyst with a TOF of 100/sec requires a density of one catalytic site per square nanometer. Catalysts with lower rates or taking up a larger space will require a high-surface-area, nanostructured support that affords tens to hundreds of catalytic sites per square nanometer. Furthermore, catalysts need to operate close to the thermodynamic potential of the redox reaction so that a maximum fraction of the solar photon energy is converted to chemical energy."

So there are essentially two values that address the efficiency of a cell designed for photon based water splitting; The density of the catalytic sites and the turn over frequency (TOF) of these sites. I believe that they are saying that for the standard solar flux of 1000 Wm^2 and a TOF of 100/sec then they would need a density of 1 catalytic site/nm^2 for 100% conversion of the photons. So the question then remains; How much energy is created from one photon? Unfortunately this can not be answered until the 2nd half of the process (conversion to methanol) is complete.

With this new process they are reporting a TOF on the order of 1000/sec. This would appear to be an order of magnitude increase in efficiency.

Until the second have of the process is complete the comparison to PV can only be speculated.

Alain

The theoretical maximal conversion of solar energy to biomass seems to be 13% (because there is only 45% visible light and there are quantum losses in the conversion process).That would be the theoretical maximum for algae. Because then you would still need to collect the biomass and convert it to fuel, the total efficiency would be even lower.
So, I can imagine that an anorganic chemistry bypassing the biological systems and working with different catalysts for different wavelengths (also UV and IR) could achieve much higher rates and even cheaper.
Definitely worth exploring !

SJC

In PV solar panels, the light is measured at 1000 watts per square meter. If the square meter panel were 100% efficient, it would produce 1000 watts, since it may be only 15% efficient, it produces 150 watts.

I was hoping that this process could lead to something that did not take up a lot of area and would not cost much per square meter. It sounds like this might help get there.

The last promising advance in solar hydrogen I read about was reversible SOFCs and concentrated solar thermal. A dish with an SOFC at the focus could use the heat and electricity for electrolysis of water into hydrogen and oxygen.

aym

From

http://www.fao.org/docrep/w7241e/w7241e05.htm

"..theoretical maximum efficiency of solar energy conversion is approximately 11%. In practice, however, the magnitude of photosynthetic efficiency observed in the field, is further decreased by factors such as poor absorption of sunlight due to its reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels. The net result being an overall photosynthetic efficiency of between 3 and 6% of total solar radiation"

Probably less, as this pertains to gluclose and not more complex molecules. The major benefit of photosynthesis would be self replication. You could get higher efficiencies by photovolaics but they would have to be manufactured. I really don't see artificial photosynthesis as a major contributor except in very specialized projects but they do have an advantage in that they can do away with other external steps and labour and in this case use up CO2. Whether it's enough is another question and whether or not any breakthoughs can be discovered.

Wonder what the optimal temperature level for the catalyst is though.

Interesting enough.

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