Splitting Water: Harnessing Sunlight to Split Water for Clean Hydrogen Fuel

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“We have designed something here that is very different from other methods and frankly something that nobody thought was possible before,” says Weimer.

Click the droplet to learn just what goes into clean hydrogen fuel.

Wondering how to make a clean, green hydrogen fuel? Check out what Professor Alan Weimer of chemical and biological engineering and his CU-Boulder team did - they came up with a method to harness the power of sunlight to efficiently split water into its components of oxygen and hydrogen.

Weimer and colleagues devised a solar­thermal system in which sunlight could be concentrated by a vast field of mirrors onto a single point atop a central tower as tall as an ATLAS V rocket. The tower would gather heat beamed up by the mirror system to roughly 2,500 degrees Fahrenheit, then deliver it to a solar reactor containing chemical compounds known as metal oxides.

As a metal oxide compound heats up, it releases oxygen atoms, changing its material composition and causing it to seek out new oxygen atoms, explains Weimer. The addition of steam to the system - which could be produced by boiling water in the reactor using the same concentrated sunlight collected by the tower - would cause oxygen from the water molecules to adhere to the surface of the metal oxide. The result? The freed-­up hydrogen molecules could be collected as gas fuel.

Members of Weimer's team include chemical and biological engineering professor Charles Musgrave, doctoral student Christopher Muhich, post­ doctoral researcher Janna Martinek, undergraduate Kayla Weston, and former CU researchers Paul Lichty, Xinhua Liang and Brian Evanko. The study was published in Science magazine.

"The more conventional approaches require the control of both the switching of the temperature in the reactor from a hot to a cool state and the introduction of steam into the system," says Musgrave. "One of the big innovations in our system is that there is no swing in the temperature. The whole process is driven by either turning a steam valve on or off."

Muhich, who likened the concentration of sunlight to drive the chemical reactions to using a magnifying glass to start a fire, said the team members want to heat the solar reactor to the lowest temperature possible for these chemical reactions to still occur. Hotter temperatures can cause rapid thermal expansion and contraction, potentially causing damage to both the chemical materials and to the reactors themselves, he says.

In addition, the conventional, two­-step idea for water splitting wastes both time and heat, says Weimer, executive director of the Colorado Center for Biorefining and Biofuels, noting "There are only so many hours of sunlight in a day." The research was supported by the National Science Foundation and by the U.S. Department of Energy.

With the new method, the amount of hydrogen produced for fuel cells or for storage is dependent on the amount of metal oxide - which is made  up of a combination of iron, cobalt, aluminum and oxygen - and how much steam is introduced into the system.

One of the designs proposed by the CU team is to build reactor tubes roughly a foot in diameter and 18 feet long, fill them with the metal oxide material and arrange them within a solar receiver. A working system to produce a significant amount of hydrogen gas would require a tall tower to gather concentrated sunlight from 100 acres of mirrors.

"We have designed something here that is very different from other methods and frankly something that nobody thought was possible before," explains Weimer.


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