A recipe for solar energy: learning from nature
For the past 20 years, MIT Professor Daniel G. Nocera of chemistry has been working on a novel system for producing pollution-free energy in real time without adding fuel. In his system, a tub of water containing specially designed chemicals sits in the sunshine. Gaseous hydrogen and oxygen rise from the water, pass through a power-producing fuel cell, then recombine to replenish the water in the tub.
Sound too good to be true? Nocera thinks not. While practical systems may be another 20 years away, recent events in his MIT lab suggest that his vision is not as implausible as it may seem.
Much discussion now focuses on a cleaner energy future based on hydrogen rather than fossil fuels. But where are we going to get all the hydrogen? The obvious answer is electrolysis, which splits water into hydrogen and oxygen, the feed stocks for clean and efficient power-generating fuel cells. But conventional electrolysis is driven by electricity mostly made using fossil fuels, just what we need to get away from.
Nocera and others believe that in the long-term we must get our energy from the sun. Even if we crowd the earth with biomass farms, windmills, solar power facilities and nuclear plants, we won’t be able to satisfy the world’s appetite for energy in 2050. Yet the daily dose of sunshine on the earth’s surface is enough to power our energy needs for 30 years.
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.”
So while everybody’s talking about hydrogen, Nocera is now concerned with capturing oxygen—a far harder task. There are more electrons to worry about and getting the oxygen atoms to let go of the photocatalyst and move together neatly in pairs is tricky.
He and his colleagues are getting close with some help from biologists. “In the past two years, biologists have begun uncovering the secrets of how the so-called oxygen-evolving complex works in plants,” said Nocera. “So in our lab we started saying, ‘How can we get the essence of the chemical reactions happening in the plant—but in a beaker?’”
While practical devices won’t come soon, Nocera’s work has already yielded spin-offs in other directions. Among them are novel ideas for microscopic sensors that detect biological hazards and a new understanding of how critical enzymes inside the body work. To Nocera, energy is not a pure engineering problem but also a basic science problem that must be tackled from many angles. “There are big problems that lie at the heart of energy,” said Nocera. “And they’re so big that when you solve them, there are spin-offs everywhere for many other technologies.”
