Water Splitting Technique Efficiently Produces Hydrogen Fuel

An artist's conception of a commercial hydrogen production plant that uses sunlight to split water in order to produce clean hydrogen fuel. 

Credit: University of Colorado Boulder

A University of Colorado Boulder team has developed a radically new technique that uses the power of sunlight to efficiently split water into its components of hydrogen and oxygen, paving the way for the broad use of hydrogen as a clean, green fuel.

The CU-Boulder team has devised a solar-thermal system in which sunlight could be concentrated by a vast array of mirrors onto a single point atop a central tower up to several hundred feet tall.

Alan Weimer

The tower would gather heat generated by the mirror system to roughly 2,500 degrees Fahrenheit (1,350 Celsius), then deliver it into a reactor containing chemical compounds known as metal oxides, said CU-Boulder Professor Alan Weimer, research group leader.

As a metal oxide compound heats up, it releases oxygen atoms, changing its material composition and causing the newly formed compound to seek out new oxygen atoms, said Weimer.

The team showed that the addition of steam to the system -- which could be produced by boiling water in the reactor with the concentrated sunlight beamed to the tower -- would cause oxygen from the water molecules to adhere to the surface of the metal oxide, freeing up hydrogen molecules for collection as hydrogen gas.

"We have designed something here that is very different from other methods and frankly something that nobody thought was possible before," said Weimer of the chemical and biological engineering department.

"Splitting water with sunlight is the Holy Grail of a sustainable hydrogen economy."

Charles Musgrave

One of the key differences between the CU method and other methods developed to split water is the ability to conduct two chemical reactions at the same temperature, said Charles Musgrave, also of the chemical and biological engineering department.

While there are no working models, conventional theory holds that producing hydrogen through the metal oxide process requires heating the reactor to a high temperature to remove oxygen, then cooling it to a low temperature before injecting steam to re-oxidize the compound in order to release hydrogen gas for collection.

"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," said 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."

"Just like you would use a magnifying glass to start a fire, we can concentrate sunlight until it is really hot and use it to drive these chemical reactions," said Christopher Muhich.

"While we can easily heat it up to more than 1,350 degrees Celsius, we want to heat it 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."

In addition, the two-step conventional idea for water splitting also wastes both time and heat, said Weimer, also a faculty member at CU-Boulder's BioFrontiers Institute. "There are only so many hours of sunlight in a day," he said.

With the new CU-Boulder method, the amount of hydrogen produced for fuel cells or for storage is entirely 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 team is to build reactor tubes roughly a foot in diameter and several feet long, fill them with the metal oxide material and stack them on top of each other.

A working system to produce a significant amount of hydrogen gas would require a number of the tall towers to gather concentrated sunlight from several acres of mirrors surrounding each tower.

Weimer said the new design began percolating within the team about two years ago. "When we saw that we could use this simpler, more effective method, it required a change in our thinking," said Weimer.

"We had to develop a theory to explain it and make it believable and understandable to other scientists and engineers." 

Journal Reference: C. L. Muhich, B. W. Evanko, K. C. Weston, P. Lichty, X. Liang, J. Martinek, C. B. Musgrave, A. W. Weimer. Efficient Generation of H2 by Splitting Water with an Isothermal Redox Cycle. Science, 2013; 341 (6145): 540 DOI: 10.1126/science.1239454

Hydrogen Fuel: Thin Films Yield Efficient Solar Water-Splitting Catalyst

University of Oregon chemists say that ultra-thin films of nickel and iron oxides made through a solution synthesis process are promising catalysts to combine with semiconductors to make devices that capture sunlight and convert water into hydrogen and oxygen gases.

Researchers in the Solar Materials and Electrochemistry Laboratory of Shannon Boettcher, professor of chemistry, studied the catalyst material and also developed a computer model for applying catalyst thin films in solar water-splitting devices as a tool to predict the effectiveness of a wide range of catalyst materials for solar-hydrogen production.

The project has resulted in two recent papers.

Shannon Boettcher

The first, detailed last September in the Journal of the American Chemical Society, showed that films of a nickel-iron mixed oxide with an atomic structure similar to naturally occurring minerals show the highest catalytic activity for forming oxygen from water, based on a side-by-side comparison of eight oxide-based materials targeted in various research efforts.

The second paper, just published in the Journal of Physical Chemistry Letters, details the performance of the catalyst thin films when combined with semiconductor light absorbers, showing that the nickel-iron oxide catalyst was most effective with a film just 0.4 nanometers thick.

Boettcher's lab, located in the UO's Materials Science Institute, studies fundamental materials chemistry and physical concepts related to the conversion of solar photons (sunlight) into electrons and holes in semiconductors that can then be used to drive chemical processes such as splitting protons off water to make hydrogen and oxygen gases.

Multiple labs across the country are seeking effective and economical ways of taking sunlight and directly producing hydrogen gas as an alternative sustainable fuel to replace fossil fuels.

"When you want to pull the protons off a water molecule to make hydrogen gas for fuel, you also have to take the leftover oxygen atoms and make oxygen gas out of them," Boettcher said.

"It turns out that the slowest, hardest, most-energy-consuming step in the water-splitting process is actually the oxygen-making step. "

"We've been studying catalysts for making oxygen. Specifically, we're seeking catalysts that reduce the amount of energy it takes in this step and that don't use expensive precious metals."

The iron-nickel oxides, he said, have higher catalytic activity than the precious-metal-based catalytic materials that have been thought to be the best for the job.

"What we found is that when we take nickel oxide films that start out as a crystalline material with the rock-salt structure like table salt, they absorb iron impurities and spontaneously convert into materials with a layered structure during the catalysis process," Boettcher said.

Lena Trotochaud, a doctoral student and lead author on both papers, studied this process and how the films can be combined with semiconductors.

"The semiconductors absorb the light, generating electron-hole pairs which move onto the catalyst material and proceed to drive the water-splitting reaction, creating fuel," Boettcher said.

The computer modeling was used to understand how the amount of sunlight that the catalyst blocks from reaching the semiconductor can be minimized while simultaneously speeding up the reaction with water to form oxygen gas.

This basic discovery remains a lab accomplishment for now, but it could advance to testing in a prototype device, Boettcher added.

"We're now looking at the fundamental reasons why these materials are good," Trotochaud said. "We are trying to understand how the catalyst works by focusing on the chemistry that is happening, and then also recognizing how that fits into a real system.

Our research is fundamentally guiding how you would take these catalysts and incorporate them into something that is useful for everyone in society."

One such place the material could land in a prototype for testing is at the U.S. Department of Energy's Joint Center for Artificial Photosynthesis, an Energy Innovation Hub.

Kimberly Andrews Espy

"This research holds great potential for the development of more efficient, more sustainable solar-fuel generation systems and other kinds of transformative energy technology," said Kimberly Andrews Espy, vice president for research and innovation and dean of the graduate school.

"By seeking to advance carbon-neutral energy technology, Dr. Boettcher and his team are helping to establish Oregon as an intellectual and economic leader in fostering a sustainable future for our planet and its people."

The above story is reprinted from materials provided by University of Oregon.