Call it a solar water knife if you will, but the solar devices that the University of Oregon has modeled could split water into hydrogen and oxygen for use as fuel. Chemists there are developing ultra-thin films of nickel and iron oxides that can catalyze water into its constituent elements. Cheap production of hydrogen has been sought after for a long time, because although it’s the most abundant element in the universe and is highly energetic in the earth its primarily in fixed form as in water or in hydrocarbons (think fossil fuels).
Now researchers in the Solar Materials and Electrochemistry Laboratory of Chemistry Professor Shannon Boettcher, have developed a computer model for applying catalyst thin films in solar water-splitting devices. The tool was developed after studying the materials and can predict the effectiveness of a wide range of catalyst materials for solar-hydrogen production, according to the university."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," said Lena Trotochaud, a doctoral student and lead author of both papers.
Boettcher’s group most recently published their research in the Journal of Physical Chemistry Letters and last year published it in the Journal of the American Chemical Society. The research in the first publication describes how 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 more recent paper details the performance of the catalysts when combined with semiconducting light absorbers. In this application the nickel-iron oxide catalyst was most effective when the film was merely 0.4 nanometers thick.While they are trying to collect the hydrogen in water as a gas, they also have to collect the oxygen, which has been more problematic. "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,” Boettcher said. “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 research determined that the iron-nickel oxides, which are more readily available, were determined to have higher catalytic activity than the precious-metal-based catalytic materials. Previously it was thought that the latter were 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.