New Snapshots of Proton Movement in Water Could Contribute to Better Clean Energy Technologies

PITTSBURGH—The motion of protons within water is an elusive area of science that is now of higher interest as a better understanding of how this motion occurs may lead to advancements in clean energy technologies.

Many processes, including vision, signaling in biological systems, photosynthesis, and the operation of many fuel cells, involve the motion of protons in water. Scientists are pursuing an enhanced understanding of that movement, which is needed to improve the technologies that depend on proton transfer.

An international team of scientists, including a University of Pittsburgh professor and graduate student, has obtained snapshots of the process by which a proton is relayed from one water molecule to the next. The research is published in a paper in the Dec. 2 issue of the journal Science.

“These measurements represent a major benchmark in our knowledge of how water conducts a positive electrical charge,” said co-author Kenneth Jordan. Jordan is the Richard King Mellon Professor and Distinguished Professor of Computational Chemistry in the Department of Chemistry within Pitt’s Kenneth P. Dietrich School of Arts and Sciences and co-director of Pitt’s Center for Simulation and Modeling.

Jordan’s research involved collaboration of experimental groups led by Mark Johnson, of Yale University, and Knut Asmis, of the University of Leipzig, and theory groups led by Jordan and Anne McCoy, of the University of Washington. The groups have collaborated for over a decade, focusing on the nature of excess protons in water.

The Grotthuss mechanism for proton conduction in water was introduced over two centuries ago. But the details supporting the theory have been murky due to the unpredictability of water molecules when they are near the excess proton.

A key tool in characterizing water networks is infrared spectroscopy, which determines the energies at which the atoms in molecules vibrate. These energies depend sensitively on the relative positions of the molecules.

Jordan said the rapid movement of water molecules makes it difficult to see and measure the movement of the protons when using infrared spectroscopy. That “blurring” limits the information that can be gathered using that tool.

The experimental team members solved this problem by obtaining the vibrational spectra of very cold clusters, which contain a small number of water molecules. They then switched from “regular” water to “heavy water,” which is water that contains a larger-than-normal amount of hydrogen isotope deuterium.

Under these conditions, the vibrational signatures became dramatically sharper, making it possible to obtain a series of snapshots along the proton transfer pathway.

The Jordan and McCoy groups provided calculations that illuminated the physical mechanism that determined the pathway. The calculations revealed that the electric fields imposed on the excess proton by nearby molecules play a major role in establishing the proton transfer pathway.

The understanding of the factors influencing proton transfer gained in this study can be used in designing more efficient materials and devices for energy applications.

Other authors of “Spectroscopic snapshots of the proton-transfer mechanism in water” include Tuguldur Odbadrakh (Pitt), Conrad Wolke (Yale University), Joseph Fournier (University of Chicago), Laura Dzugan (The Ohio State University), and Matias Fagiani and Harald Knorke (University of Leipzig).

Financial support for the research came from the U.S. Department of Energy, the National Science Foundation, and the Collaborative Research Center of the German Research Foundation DFG. Computational resources were provided by the Ohio Supercomputing Center and Pitt’s Center for Simulation and Modeling.

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