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NREL Experiments Advance Hydrogen-Production Technology

December 28, 2009

Hydrogen offers great promise as a major contributor to our nation’s clean energy portfolio.  While abundant on Earth, hydrogen is almost always found in combination with other elements, such as with oxygen (in water) and carbon (in plant matter). Pure hydrogen must be produced from hydrogen-containing compounds.

One of the cleanest ways to produce hydrogen is to use sunlight to split water into hydrogen and oxygen. The solar-powered photoelectrochemical (PEC) process uses semiconductors immersed in an aqueous electrolyte (solution that conducts electricity) to split water. 

While the PEC process is promising, no single semiconductor material meets the U.S. Department of Energy (DOE) 2013 goal of 8% solar-to-hydrogen efficiency and 1,000-hour durability. Metal oxides are stable, but not very efficient. On the other hand, highly efficient semiconductor materials have been hampered by their instability in aqueous environments.

Recent experiments by National Renewable Energy Laboratory (NREL) researcher Heli Wang, however, mark a significant step forward for this hydrogen-production technology.  

Wang’s experiments build on decades of research and collaboration in this arena. NREL researchers have been working to improve the durability of photovoltaic cells for PEC hydrogen production for quite some time. In 1998, NREL’s John Turner developed a record-breaking tandem photovoltaic cell (made of gallium-indium-phosphide/gallium-arsenide) with an impressive 12.4% solar-to-hydrogen efficiency. Unfortunately, the tandem cell demonstrated a functional lifespan of about 24 hours. Since then, research has focused on identifying materials and systems that are durable and stable against corrosion in aqueous environments. 

The original aqueous solution used in the tandem cell contained sulfuric acid. As an alternative electrolyte, Wang employed a nitrate solution in which the semiconductor suffered significantly less corrosion.  In fact, one sample showed virtually no damage after a 24-hour test.

“This research represents a major step toward achieving DOE’s efficiency and durability goals,” Wang says. “To fully understand why the nitrate solution inhibited the corrosion of the semiconductor, future experimental and theoretical work will focus on identifying the inhibition mechanism. This will help us further extend the durability of the semiconductors.”

—Sara Havig