NIST Demonstrates Improvement Solar-Powered Hydrogen Generation
Using a powerful combination of microanalytic techniques that simultaneously image photoelectric current and chemical reaction rates across a surface on a micrometer scale, researchers at the National Institute of Standards and Technology (NIST) have shed new light on what may become a cost-effective way to generate hydrogen gas directly from water and sunlight.*
Their quarry is a potentially efficient, cost-effective, photoelectrochemical (PEC) cell—essentially a solar cell that produces hydrogen gas instead of electric current. “A major challenge with solar energy is dealing with solar intermittency,” says NIST chemical engineer Daniel Esposito. “We demand energy constantly, but the sun’s not always going to be shining, so there’s an important need to convert solar energy into a form we can use when the sun’s not out. For large-scale energy storage or transportation, hydrogen has a lot of benefits.”
At its simplest, a PEC cell contains a semiconducting photoelectrode that absorbs photons and converts them into energetic electrons, which are used to facilitate chemical reactions that split water molecules into hydrogen and oxygen gases. It’s not that easy. The best PEC cell has been demonstrated with an efficiency around 12.5 percent,** says Esposito. But, “it’s been estimated that such a cell would be extremely expensive—thousands of dollars per square meter—and they also had issues with stability,” he says. One big problem is that the semiconductors used to achieve the best conversion efficiency also tend to be highly susceptible to corrosion by the cell’s water-based electrolyte. A PEC electrode that is efficient, stable and economical to produce has been elusive.
The NIST team’s proposed solution is a silicon-based device using a metal-insulator-semiconductor (MIS) design that can overcome the efficiency/stability trade-off. The key is to deposit a very thin, but very uniform, layer of silicon dioxide—an insulator—on top of the semiconductor—silicon—that is well-suited for doing the photon-gathering work. On top of that is a polka-dot array of tiny electrodes consisting of platinum-covered titanium. The stable oxide layer protects the semiconductor from the electrolyte, but it’s thin enough and transparent enough that the photons will travel through it to the semiconductor, and the photo-generated electrons will “tunnel” in the opposite direction to reach the electrodes, where the platinum catalyzes the reaction that produces hydrogen.
More: NIST Demonstrates Significant Improvement in the Performance of Solar-Powered Hydrogen Generation