Scientists efficiently dissociate water with new catalysts

Chemists at the University of Oregon have made substantial gains by improving the catalytic dissociation reaction of water in electrochemical reactors, called bipolar membrane electrolyzers, to more efficiently separate water molecules into positively charged protons and negatively charged hydroxide ions.

The discovery, published online before printing in the magazine Science, provides a roadmap for making electrochemical devices that take advantage of the key performance property of bipolar membranes: generating the protons and hydroxide ions within the device and supplying the ions directly to the electrodes to produce the final chemicals.

The technology behind bipolar membranes, which are layered ion exchange polymers that sandwich a layer of water dissociation catalyst, emerged in the 1950s. Although they have been applied industrially on a small scale, their performance is currently limited to low current density operation, making wide applications difficult.

These include devices to produce hydrogen gas from water and electricity, capture carbon dioxide from seawater, and make carbon-based fuels directly from carbon dioxide, said co-author Shannon W. Boettcher, professor at the UO Department of Chemistry and Biochemistry and founding director of the Oregon Center for Electrochemistry,

“I suspect that our findings will accelerate a revival in the development of bipolar membrane devices and research into the fundamentals of the water dissociation reaction,” said Boettcher, who is also a fellow at the Institute for Materials Science and an associate at Phil of the UO. and Penny Knight Campus to accelerate the scientific impact.

“The performance we demonstrate is high enough,” he said. “If we can improve durability and manufacture bipolar membranes with our industry partners, there should be important immediate applications.”

Typically, water-based electrochemical devices such as batteries, fuel cells, and electrolyzers operate at a single pH throughout the system, meaning the system is either acidic or basic, the study’s lead author said. Sebastian Z. Oener, a postdoctoral scholar who supported a grant from the German Research Foundation in Boettcher’s laboratory.

“This often leads to using expensive precious metals to catalyze electrode reactions, such as iridium, one of the rarest metals on earth, or to sacrifice catalyst activity, which, in turn, increases energy input. required of the electrochemical reactor, “said Oener. said. “A bipolar membrane can overcome this offset by operating each electrocatalyst locally in its ideal pH environment. This increases the breath of stable and abundant catalyst availability in the ground for each half reaction.”

The three-member team, which also included graduate student Marc J. Foster, used a membrane electrode assembly where the bipolar polymer membrane is compressed between two rigid porous electrodes. This approach allowed them to make a large number of bipolar membranes with different layers of water dissociation catalyst and to accurately measure the activity of each.

The team discovered that the exact position of each catalyst layer within the bipolar membrane junction, the interface between a hydroxide-conducting layer and the proton-conducting layer in the bipolar membrane, dramatically affects catalyst activity. This allowed them to use catalyst bilayers to obtain bipolar membranes with record performance that essentially dissociate water with a negligible contribution of extra energy.

“The biggest surprise was the understanding that performance could be substantially improved by placing different types of catalysts on top of each other,” said Boettcher. “This is simple but it had not been fully explored.”

A second key finding, Oener said, is that the water dissociation reaction that occurs within the bipolar membrane is fundamentally related to what occurs on the surfaces of the electrocatalyst, such as when protons are directly extracted from water molecules when Makes hydrogen fuel under basic pH conditions.

“This is unique because previously it was not possible to separate the individual steps that occur during an electrochemical reaction,” said Oener. “They are all linked, involve electrons and intermediates, and proceed rapidly in series. The bipolar membrane architecture allows us to isolate the chemical dissociation step from water and study it in isolation.”

That finding, he said, could also lead to improved electrocatalysts for reactions that directly produce reduced fuels from water, such as making hydrogen gas or liquid fuel from residual carbon dioxide.

The discoveries, Boettcher said, provide a tentative mechanistic model, one that could open the field and motivate many more studies.

“We are excited to see the response from the research community and see if these findings can be translated into products that reduce society’s dependence on fossil fuels,” he said.