A recipe for solar energy: learning from nature
Using sunlight to split water isn’t so easy—unless you’re a leaf. Photosynthesis is Nocera’s model for success. “Light that goes into a leaf sets up an electrical current without wires,” he said. “The current splits the water into oxygen and hydrogen inside the leaf, and then the hydrogen reacts with carbon dioxide to form sugar.”
So Nocera set out to make a special molecule—a photocatalyst—that would initiate that first step when mixed with water and zapped by sunlight. But designing such a molecule requires a fundamental understanding of the chemistry inside a leaf, and nobody fully understands photosynthesis.
Half the equation
Undeterred, Nocera began delving into the hydrogen side of the process. Drawing on the latest knowledge in chemistry, biology and physics, he developed a theoretical basis on which to design his molecule. Three years ago, he and then-graduate student Alan F. Heyduk dissolved a special molecule—a compound based on the metal rhodium—in an acidic solution, shone sunlight on it, and out came hydrogen gas
The accomplishment was seen by many as a breakthrough. “We got a lot of press for that work, but we were never super-excited because hydrogen is only half the story,” said Nocera. “We had to get the other half too—the oxygen.” In his ideal system, the captured hydrogen and oxygen recombine in the fuel cell to replenish the tub of water. “If I only make hydrogen from the water and I don’t get the oxygen out too, then I’ll need more water,” said Nocera. “In a way, water would become a non-renewable hydrogen source.”
