New strategy lets PEM electrolyzers use impure water

In recent years, energy engineers have been working on a wide range of technologies that could help to generate and store electrical power more sustainably. These include electrolyzers, devices that could use electricity sourced via photovoltaics, wind turbines or other energy technologies to split water (H₂O) into hydrogen (H₂) and oxygen (O₂), via a process known as electrolysis.

The hydrogen produced by electrolyzers could in turn be used in fuel cells, devices that convert the chemical energy in hydrogen into electricity without combustion and could be used to power trucks, buses, forklifts and various other heavy vehicles, or could provide back-up power for hospitals, data centers and other facilities.

Many recently designed electrolyzers prompt the splitting of water into hydrogen using a proton exchange membrane (PEM), a membrane that selectively allows protons (H⁺) to pass through, while blocking gases.

PEM electrolyzers were found to produce hydrogen with a higher purity compared to that produced by alkaline electrolyzers, which are currently the most employed. Nonetheless, they are also more expensive and require ultrapure water, as impurities (e.g., positively charged ions, negatively charged ions and other contaminants) cause the devices to degrade rapidly over time.

Researchers at Tianjin University and other institutes recently devised a strategy to improve catalysts for PEM electrolyzers, allowing them to also split impure water.

Their strategy, outlined in a paper published in Nature Energy, entails the creation of an acidic microenvironment in PEM electrolyzers, by modifying layers of cathode catalysts using a class of compounds called Brønsted acid oxides.

“PEM electrolyzers typically use ultrapure water as feedstock because trace contaminants in feedwater, especially cationic impurities, can cause their failure,” Ruguang Wang, Yuting Yang and their colleagues wrote in their paper.

“Developing PEM electrolyzers that can withstand lower-purity water could minimize water pretreatment, lower maintenance costs and extend system lifetime.

“In this context, we have developed a microenvironment pH-regulated PEM electrolyzer that can operate steadily in impure (‘tap’) water for more than 3,000 h at a current density of 1.0 A cm⁻², maintaining a performance that is comparable to state-of-the-art PEM electrolyzers that use pure water.”

To evaluate the potential of their strategy, Wang, Yang and their colleagues added the Brønsted acid oxide MoO_3-x to a cathode made of platinum and carbon (Pt/C). They found that when integrated into PEM electrolyzers as a catalyst, this cathode enhanced their performance, allowing them to reliably produce hydrogen from impure water, without rapidly degrading over time.

“Using a technique that combines a pH ultramicroelectrode with scanning electrochemical microscopy, we monitored the local pH conditions in a PEM electrolyzer in situ, finding that Brønsted acid oxides can lower the local pH,” wrote Wang, Yang and their colleagues.

“We thus introduced a Brønsted acid oxide, MoO_3-x, onto a Pt/C cathode to create a strongly acidic microenvironment that boosts the kinetics of hydrogen production, inhibits deposition/precipitation on the cathode and suppresses the degradation of the membrane.”

This study could open new exciting possibilities for the design of PEM electrolyzers, as it could help to reduce their reliance on ultra-pure water and thus make them easier to deploy in real-world settings.

In the future, other energy engineers could build on the team’s findings to develop other PEM electrolyzers that can reliably split impure water into hydrogen.

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