Solar goes solo: Artificial photosynthesis could provide a practical way to store energy produced by solar power, freeing people’s homes from the electrical grid. In this scheme, electricity from solar panels powers an electrolyzer, which breaks water into hydrogen and oxygen. The hydrogen is stored; at night or on cloudy days, it is fed into a fuel cell to produce electricity for lights, appliances, and even electric cars. On sunny days, some of the solar power is used directly, bypassing the hydrogen production step.
The experiment worked better than Nocera and his colleagues had expected. 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 in just a few minutes. Further tests confirmed that the bubbles were oxygen released by splitting the water. “Here’s the luck,” Nocera says. “There was no reason for us to expect that just plain cobalt with phosphate, versus cobalt being tied up in one of our complexes, would work this well. I couldn’t have predicted it. The stuff that was falling out of the compounds turned out to be what we needed.
“Now we want to understand it,” he continues. “I want to know why the hell cobalt in this thin film is so active. I may be able to improve it or use a different metal that’s better.” At the same time, he wants to start working with engineers to optimize the process and make an efficient water-splitting cell, one that incorporates catalysts for generating both oxygen and hydrogen. “We were really interested in the basic science. Can we make a catalyst that works efficiently under the conditions of photosynthesis?” he says. “The answer now is yes, we can do that. Now we’ve really got to get to the technology of designing a cell.”
Catalyzing a Debate
Nocera’s discovery has garnered a lot of attention, and not all of it has been flattering. Many chemists find his claims overstated; they don’t dispute his findings, but they doubt that they will have the consequences he imagines. “The claim that this is the answer for artificial photosynthesis is crazy,” says Thomas Meyer, who has been a mentor to Nocera. He says that while Nocera’s catalysts “could prove technologically important,” the advance is “a research finding,” and there’s “no guarantee that it can be scaled up or even made practical.”
Many critics’ objections revolve around the inability of Nocera’s lab setup to split water nearly as rapidly as commercial electrolyzers do. The faster the system, the smaller a commercial unit that produced a given amount of hydrogen and oxygen would be. And smaller systems, in general, are cheaper.
The way to compare different catalysts is to look at their “current density”–that is, electrical current per square centimeter–when they’re at their most efficient. The higher the current, the faster the catalyst can produce oxygen. Nocera reported results of 1 milliamp per square centimeter, although he says he’s achieved 10 milliamps since then. Commercial electrolyzers typically run at about 1,000 milliamps per square centimeter. “At least what he’s published so far would never work for a commercial electrolyzer, where the current density is 800 times to 2,000 times greater,” says John Turner, a research fellow at the National Renewable Energy Laboratory in Golden, CO.
Other experts question the whole principle of converting sunlight into electricity, then into a chemical fuel, and then back into electricity again. They suggest that while batteries store far less energy than chemical fuels, they are nevertheless far more efficient, because using electricity to make fuels and then using the fuels to generate electricity wastes energy at every step. It would be better, they say, to focus on improving battery technology or other similar forms of electrical storage, rather than on developing water splitters and fuel cells. As Ryan Wiser puts it, “Electrolysis is [currently] inefficient, so why would you do it?”