Solar Energy, All Night Long

MIT professor Daniel G. Nocera has long been jealous of plants. He desperately wanted to do what they do--split water into hydrogen and oxygen and use the products to do work. That, he figures, is the only way we humans can solve our energy problems; enough energy pours down from the sun in one hour to power the planet's energy needs for a year.

In January, only a month after reevaluating his methodology in the face of a frustratingly slow process, he finally found a way. "For six months now I've been looking at the leaves and saying 'I own you guys!'"

Nocera's discovery--a cheap and easy way to store energy that he thinks will be used to change solar power into a mainstream energy source--will be published in the journal Science on Friday. "This is the nirvana of what we've been talking about for years," said Nocera, the Henry Dreyfus Professor of Energy at MIT. "Solar power has always been a limited, far-off solution. Now we can seriously think about solar power as unlimited--and soon."

Plants catch light and turn it into an electric current, then use that energy to excite catalysts that split water into hydrogen and oxygen during what is called photosynthesis' light cycle. The energy is then used during the dark cycle to allow the plant to build sugars used for growth and energy storage.

Nocera and Matthew Kanan, a postdoctoral fellow in Nocera's lab, focused on the water-splitting part of photosynthesis. They found cheap and simple catalysts that did a remarkably good job. They dissolved cobalt and phosphate in water and then zapped it with electricity through an electrode. The cobalt and phosphate form a thin-film catalyst around the electrode that then use electrons from the electrode to split the oxygen from water. The oxygen bubbles to the surface, leaving a proton behind.

A few inches away, another catalyst, platinum, helps that bare proton become hydrogen. (This second reaction is a well-known one, and not part of Nocera and Kanan's study.)

The hydrogen and oxygen, separated and on-hand, can be used to power a fuel cell whenever energy is needed.

"Once you put a photovoltaic on it," he says, "you've got an inorganic leaf."

James Barber, a biochemistry professor at Imperial College London who studies artificial photosynthesis but was not involved in this research, called the discovery by Nocera and Kanan a "giant leap" toward generating clean, carbon-free energy on a massive scale.

"This is a major discovery with enormous implications for the future prosperity of humankind," he said. "The importance of their discovery cannot be overstated."

Nocera's discovery arose from frustration. Disappointed with the pace of his lab's progress, Nocera and his team decided in December to question some of the basic assumptions they had made in setting up earlier experiments.

Chemists, it turns out, are always worrying about the stability of their catalysts and end up doing backflips to try to synthesize materials that won't corrode. Photosynthesis, though, is so violently reactive that the catalysts involved break down every 30 minutes. The leaf has to constantly rebuild them. Maybe, thought Nocera, instead of fighting corrosion, he should work with it. "It's a bias a lot of scientists have. We want something to be structurally stable. But all it has to be is functionally stable."

This thinking led Nocera to try his cobalt-phosphate mixture. He knew it wouldn't hold together, but he thought it might still work. Sure enough, Nocera's catalyst breaks down whenever the electricity is cut, but it assembles itself again when electricity is reapplied.

Nocera's discovery is still a science experiment. It needs plenty of engineering before it can be a useful device. The cobalt and phosphate at the center of Nocera's work is cheap and plentiful, but the hydrogen reaction uses platinum, which is rare and expensive. The electrode needs to be improved so the oxygen-making process can speed up. And the system needs to be integrated into some kind of electricity-producing device, ideally powered by solar or wind on one end and a fuel cell on the other.

But splitting the oxygen away from the water was the hard part, and Nocera has done it. "Now we can start thinking about a totally distributed solar [photovoltaic] system," he said. "We couldn't have a solar economy unless it could produce energy 24/7. Now we can."

His hope is that because unlike traditional electrolysis devices, which are expensive and require toxic alkaline solutions, his system is so cheap, simple and benign that scientists and engineers around the world will be able to improve it quickly.

For his part, Nocera says he will work to understand and improve both sides of his new discovery. His lab will try to learn every detail about just how his catalyst is making the oxygen. And he is going to work with his engineering colleagues at MIT to try to integrate his storage device into systems that he hopes one day will power homes and cars all day and all night.

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