Photosynthesis in a beaker: In an experimental setup that duplicates the benign conditions found in photosynthetic plants, -Daniel ¬Nocera has demonstrated an easy and potentially cheap way to produce hydrogen gas. When a voltage is applied, cobalt and phosphate in solution (left) accumulate on an electrode to form a catalyst, which releases oxygen gas from the water as electrons flow out through the electrode. Hydrogen ions flow through a membrane; on the other side, hydrogen gas is produced by a nickel metal catalyst (Nocera has also used a platinum catalyst).
In real photosynthesis, green plants use chlorophyll to capture energy from sunlight and then use that energy to drive a series of complex chemical reactions that turn water and carbon dioxide into energy-rich carbohydrates such as starch and sugar. But what primarily interests many researchers is an early step in the process, in which a combination of proteins and inorganic catalysts helps break water efficiently into oxygen and hydrogen ions.
The field of artificial photosynthesis got off to a quick start. In the early 1970s, a graduate student at the University of Tokyo, Akira Fujishima, and his thesis advisor, Kenichi Honda, showed that electrodes made from titanium dioxide–a component of white paint–would slowly split water when exposed to light from a bright, 500-watt xenon lamp. The finding established that light could be used to split water outside of plants. In 1974, Thomas Meyer, a professor of chemistry at the University of North Carolina, Chapel Hill, showed that a ruthenium-based dye, when exposed to light, underwent chemical changes that gave it the potential to oxidize water, or pull electrons from it–the key first step in water splitting.
Ultimately, neither technique proved practical. The titanium dioxide couldn’t absorb enough sunlight, and the light-induced chemical state in Meyer’s dye was too transient to be useful. But the advances stimulated the imaginations of scientists. “You could look ahead and see where to go and, at least in principle, put the pieces together,” Meyer says.
Over the next few decades, scientists studied the structures and materials in plants that absorb sunlight and store its energy. They found that plants carefully choreograph the movement of water molecules, electrons, and hydrogen ions–that is, protons. But much about the precise mechanisms involved remained unknown. Then, in 2004, researchers at Imperial College London identified the structure of a group of proteins and metals that is crucial for freeing oxygen from water in plants. They showed that the heart of this catalytic complex was a collection of proteins, oxygen atoms, and manganese and calcium ions that interact in specific ways.
“As soon as we saw this, we could start designing systems,” says Nocera, who had been trying to fully understand the chemistry behind photosynthesis since 1984. Reading this “road map,” he says, his group set out to manage protons and electrons somewhat the way plants do–but using only inorganic materials, which are more robust and stable than proteins.
Initially, Nocera didn’t tackle the biggest challenge, pulling oxygen out from water. Rather, “to get our training wheels,” 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. So when it came time to try splitting water, he decided to use similar cobalt compounds.
Nocera knew that working with these compounds in water could be a problem, since cobalt can dissolve. Not surprisingly, he says, “within days we realized that cobalt was falling out of this elaborate compound that we made.” With his initial attempts foiled, he decided to take a different approach. Instead of using a complex compound, he tested the catalytic activity of dissolved cobalt, with some phosphate added to the water to help the reaction. “We said, let’s forget all the elaborate stuff and just use cobalt directly,” he says.