Frontiers in Energy Research: Fall 2016

Collector-Generator Cells: Harvesting Oxygen from Water

Taking a microscopic look at a reaction with macroscopic potential

The Collector-Generator Cell. Two electrodes work together to generate oxygen with the generator electrode and collect oxygen at the collector electrode. The scientists developed this cell to measure on a microscopic scale when water splits into oxygen and hydrogen. The device is precise, reliable, and easy to use. Image courtesy Nathan Johnson, Pacific Northwest National Laboratory

What if you could turn water into fuel? With the right chemistry, you can. Under special conditions and using specific molecules, which chemists call catalysts, you can produce fuel by splitting water. Scientists can transform water into a fuel through a process called water oxidation. Research from the Center for Solar Fuels at the University of North Carolina (UNC), an Energy Frontier Research Center, is pioneering water oxidation through catalysis, a process that accelerates a chemical reaction.

Water splitting consists of two half-reactions, one which produces hydrogen gas and one which produces oxygen gas. The water-splitting process is interesting to researchers because there are still unanswered questions about exactly how it works with particular scientific interest in the oxygen-half reaction. Also, water splitting is interesting to the average consumer because of its potential to produce an environmentally benign fuel. At the UNC, water splitting is just one way they are working to create solar-based fuels; they are also targeting carbon-based fuels.

Producing oxygen and hydrogen gases from water is a multi-step process requiring several components to work in harmony. For UNC scientists to improve the overall process, they need to investigate each component on the microscopic scale, which is where chemists excel. Introducing precise analytical techniques enables them to investigate exactly what they need to know to improve the system and bring it closer to a process that can impact a commercial market.

When successful, water oxidation produces oxygen gas. If a large quantity of oxygen gas is produced under water, bubbles will form which can be seen with the naked eye similar to how carbonation looks in a soft drink. However, if a significantly smaller quantity of oxygen gas is produced, there are no obvious visible signs that the reaction successfully occurred.

Consequently, the scientists needed a finer probe to accurately detect how much oxygen gas has been produced. Considering that water splitting occurs on a microscopic scale where two molecules of water produce a single molecule of gaseous oxygen, it makes sense to use small detectors as well. Scientists at the UNC EFRC developed a detector for gaseous oxygen. The detector, termed a collector-generator (C-G) cell, provides a closer look into what is happening during catalytic water splitting.

When seeking accurate analysis, the team had a few targets they wanted to meet. The detection technique must be accurate, reliable, and easy to use, and the C-G cell ticks all the boxes! The cell is termed collector-generator because it consists of two working electrodes, one that generates the oxygen gas, and one that detects or “collects” it. An advantage of this detection technique is that it achieves more precise measurement of the oxygen gas than other popular methods. Previously developed methods often use a proxy, such as current, to measure the oxygen production, which can lead to false positives. However, the C-G cell can directly and accurately measure the oxygen being produced—a new development in this field of research.

Using the C-G cell to detect the exact quantities of products being generated during catalysis is critical for scientific breakthrough. Scientists and engineers heavily depend on the accuracy of their tools to advance their work. Thus, when new catalytic systems are developed, they must develop new measurement techniques. Consequently, C-G cells are a stepping stone to improved water oxidation catalysis, bringing us all one step closer to a world fueled by water.

Acknowledgments: 

The research in both studies was supported solely by the University of North Carolina Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information: 

Sherman BD, MV Sheridan, KR Wee, N Song, CJ Dares, Z Fang, Y Tamaki, A Nayak, and TJ Meyer. 2016. “Analysis of Homogeneous Water Oxidation Catalysis with Collector-Generator Cells.” Inorganic Chemistry 55(2):512-517. DOI: 10.1021/acs.inorgchem.5b02182

Sherman BD, MV Sheridan, CJ Dares, and TJ Meyer. 2016. “Two Electrode Collector-Generator Method for the Detection of Electrochemically or Photoelectrochemically Produced O2.” Analytical Chemistry 88(14):7076-7082. DOI: 10.1021/acs.analchem.6b00738

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.

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