Sun + Water = Fuel

By Kevin Bullis
MIT Tech Review, November/December 2008

Edited by Andy Ross

Daniel Nocera, a professor of chemistry at MIT, speaking this May to an auditorium filled with scientists and U.S. government energy officials, demonstrated a reaction that generates oxygen from water much as green plants do during photosynthesis. The reaction is the most difficult step in splitting water to make hydrogen gas.

Solar power has a unique potential to generate vast amounts of clean energy that doesn't contribute to global warming. But without a cheap means to store this energy, solar power can't replace fossil fuels on a large scale. In Nocera's scenario, sunlight would split water to produce hydrogen fuel.

Nocera has devised an inexpensive catalyst that produces oxygen from water at room temperature and without caustic chemicals. Several other promising catalysts could be used to complete the process and produce hydrogen gas.

Nocera sees two ways to take advantage of his breakthrough. In the first, a conventional solar panel would capture sunlight to produce electricity. That electricity would power an electrolyzer to split water. The second approach would deploy the catalysts with special dye molecules to drive the water-splitting reaction. Either way, solar energy would be converted into hydrogen fuel.

Sunlight is the world's largest potential source of renewable energy. But solar panels do not work at night. Today most solar panels are connected to the electrical grid. During sunny days, when solar panels are operating at peak capacity, homeowners and companies can sell their excess power to utilities. But they generally have to rely on the grid at night, or when clouds shade the panels.

For solar to become a primary source of electricity, vast amounts of affordable storage will be needed. And today's options for storing electricity just aren't practical on a large enough scale. Batteries are expensive, and at best store about one megajoule per kilogram, while gasoline stores 50 times more. Hydrogen stores about three times more energy per kilogram than gasoline.

Initially, Nocera didn't tackle the biggest challenge, pulling oxygen out from water. He began with the reverse reaction: combining oxygen with protons and electrons to form water. He found that certain complex compounds based on cobalt were good catalysts for this reaction.

Nocera tested the catalytic activity of dissolved cobalt, with some phosphate added to the water to help the reaction. When a current was applied to an electrode immersed in the solution, cobalt and phosphate accumulated on it in a thin film, and a dense layer of bubbles started forming. The bubbles were oxygen released by splitting the water.

Nocera's discovery has garnered a lot of attention. Many chemists find his claims overstated. Nocera's lab setup cannot split water nearly as rapidly as commercial electrolyzers do. Nocera reports generating a current density of up to 100 amps per square meter. But commercial electrolyzers typically generate 100 times more.

Other experts question the efficiency of converting sunlight into electricity, then into a chemical fuel, and then back into electricity again. Batteries may store far less energy than chemical fuels, but they are far more efficient.

In 1991, Michael Grätzel, a professor of chemistry and chemical engineering at the École Polytechnique Fédérale in Lausanne, Switzerland, invented a solar cell that uses a dye containing ruthenium to absorb light and release electrons. In this cell, the electrons are collected by a film of titanium dioxide and directed through an external circuit, generating electricity. Grätzel now thinks that he can integrate his solar cell and Nocera's catalyst into a single device that uses sunlight to split water.

Grätzel's dye would take the place of the electrode on which the catalyst forms in Nocera's system. The dye can generate the voltage needed to assemble the catalyst. Once the catalyst is formed, the sunlight absorbed by the dye drives the reactions that split water. Grätzel says that the device could be more efficient and cheaper than using a separate solar panel and electrolyzer.

Nocera is investigating is whether his catalyst can be used to split seawater. In initial tests, it performs well in the presence of salt, and he is now testing it to see how it handles other compounds found in the sea.

Artificial leaves and fuel-producing desalination systems seem closer.