Research For Our Energy Future

Solar Energy

Harnessing the Sun’s Power for Fuel and Electricity

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Artificial Photosynthesis

The average power need of the world’s energy economy is 13 terawatts — a thousand trillion watts of power — and by 2050, that amount is expected to double. Fossil fuels and other nonrenewable sources are not the answer to the world’s ever-expanding need for energy. Also, burning oil, coal or natural gas pollutes the atmosphere and contributes to global warming, which threatens the long-term viability of the earth and its inhabitants.

Efficient utilization of energy from the sun may provide a solution to this important problem. The amount of clean, renewable energy derived from the sun in just one hour would meet the world’s energy needs for a year. If we can convert solar energy to fuels with only ten percent efficiency — and use two-tenths of a percent of land in the world for that purpose — we can meet more than 60 percent of the world’s need for energy. While no single source will solve the energy problem, converting solar energy to fuels could be a large part of the solution.

Currently, solar energy is converted to electricity only for use in specialized applications such as calculators and satellites, primarily because it is more expensive than fossil-fuel based alternatives. Use of solar energy for heating homes and commercial buildings is becoming more popular with increasing fuel prices; however solar heating is still relatively expensive and not completely reliable, compared to conventional fuels.

Brookhaven scientists are working on converting solar energy to liquid fuels, improving the efficiency of solar cells using inexpensive materials, and developing new manufacturing processes that reduce and recycle the materials used in second generation, thin film PV.

Artificial Photosynthesis

In the natural process of photosynthesis, plants convert energy from sunlight, carbon dioxide, and water to carbohydrates and oxygen. Brookhaven scientists are working at converting solar energy to fuel by mimicking photosynthesis in the laboratory.

By achieving better than one-percent efficiency in “artificial photosynthesis” in the laboratory — a better efficiency than that of plants — scientists could produce significant amounts of clean, renewable energy, but the challenges to overcome are difficult. While plants use chlorophyll to absorb sunlight, scientists use a photo-electrochemical cell to absorb light and to separate charges. The separated charges initiate complex chemical reactions that store the solar energy as fuels such as hydrogen or methanol. Brookhaven chemists are working on finding stable, visible light absorbing materials for the cell’s anode.

Another challenge is finding efficient catalysts to drive the complex chemical processes needed to mimic photosynthesis in the laboratory. Catalytic processes must coordinate multiple electron, proton, and atom transfer processes to form new high-energy chemicals from thermodynamically stable precursors. This artificial photosynthesis challenge is linked to the broader chemistry challenge of using catalysts to activate stable, small molecules for chemical reactions.

Catalysts for Two Reactions

Catalysts are needed in two separate reactions in artificial photosynthesis to produce solar fuel, such as molecular hydrogen from water. A crucial step in the process is water oxidation — one part of “water splitting,” or breaking water into hydrogen and oxygen. Water splitting requires a large amount of energy from sunlight and effective metal catalysts to activate the very stable water molecules. The process occurs as two separate “half-reactions.” In one, water oxidation produces molecular oxygen along with protons and electrons; in the other, these protons and electrons are combined to make molecular hydrogen.

Brookhaven scientists, working with collaborators from the Institute for Molecular Science in Japan, have found a novel catalyst that appears promising for water oxidation: a ruthenium complex with bound quinone molecules. This compound efficiently catalyzes water oxidation to form oxygen through a very unique pathway in which the bound quinone molecules are actively involved.

Another catalyst is needed for producing molecular hydrogen. Some bacteria employ hydrogenase enzymes to catalyze the production of hydrogen from water. These very efficient enzymes contain the earth-abundant metals iron and nickel in their active sites. Currently, platinum is the metal of choice as the catalyst for hydrogen production, but Brookhaven scientists are exploring the use of less expensive and more abundant alternatives, including cobalt, inspired by the hydrogenase examples.

Brookhaven researchers are also developing an understanding of how catalysts work in the complicated processes needed to replicate photosynthesis, including carbon dioxide transformation to fuels. If they can find catalysts that are both efficient and inexpensive to do the job, then they may bring the world a big step closer to tapping the sun’s energy.

Last Modified: November 6, 2009