A few years ago, Daniel Nocera pioneered an "artificial leaf" that—just like the real thing—uses only the sun and water to produce energy. He touted the silicon cell as a breakthrough that could allow every home to become its own power station.

His compelling invention, a cheap wafer-thin device, attracted lots of publicity but hasn't quite taken off. The leaf works well, Nocera says, but there's a key flaw.

"The problem with the artificial leaf," Nocera says, is that "it makes hydrogen. You guys don't have an infrastructure to use hydrogen." (See related profile: "Daniel Nocera: Maverick Inventor of the Artificial Leaf.")

By "you guys," Nocera means the world outside the lab. Although Toyota and others companies are making cars built