A novel multi-proton carrier complex as an efficient high-temperature proton conductor

Researchers are developing a highly symmetrical ruthenium(III) complex with six imidazole-imidazolate groups for efficient high-temperature proton conduction in fuel cells.

Fuel cells often fall short when it comes to operating at temperatures above 100°C due to their reliance on water as a proton conduction medium. To overcome this problem, a team of Japanese researchers designed a new hydrogen-bonded star-shaped metal complex composed of ruthenium(III) ions and six imidazole-imidazolate groups. The resulting molecular single crystal exhibits excellent proton conductivity even at temperatures as high as 180°C and as low as -70°C.

As the world moves towards more environmentally friendly and sustainable energy sources, fuel cells are getting a lot of attention. The main advantage of fuel cells is that they use hydrogen, a clean fuel, and only produce water as a by-product while generating electricity. This new source of clean electricity could replace conventional lithium-ion batteries, which currently power all modern electronic devices.

Most fuel cells use a Nafion membrane―a synthetic polymer-based ionic membrane―which serves as the water-based proton-conducting solid electrolyte. The use of water as the proton conduction medium, however, creates a major drawback for the fuel cell, namely the inability to function properly at temperatures above 100 ֯C, the temperature at which water begins to boil, causing a decrease in proton conductivity. . Therefore, there is a need for new proton conductors capable of efficiently transferring protons even at such high temperatures.

In a recent breakthrough, a team of Japanese researchers, led by Professor Makoto Tadokoro of Tokyo University of Science (TUS), presented a new high-temperature proton conductor based on an imidazole-imidazolate metal complex that shows a effective proton conductivity even at 147°C The research team included Dr. Fumiya Kobayashi from TUS, Dr. Tomoyuki Akutagawa and Dr. Norihisa Hoshino from Tohoku University, Dr. Hajime Kamebuchi from Nihon University, Dr. Motohiro Mizuno from Kanazawa University and Dr. Jun Miyazaki from Tokyo Denki University. Imidazole, an organic nitrogen-containing compound, has gained popularity as an alternative proton conductor for its ability to function even without water. However, it has a lower proton transfer rate than conventional Nafion used and melts at 120°C.To overcome these issues, we introduced six imidazole moieties into a ruthenium(III) ion to design a novel metal complex that functions as a multi-proton carrier and has high temperature stability.” , says Professor Tadokoro when asked about the rationale for their study, which was first published online June 27, 2022 in Chemistry — A European Journal and featured on the cover of the journal.

The team designed a new molecule where three imidazole (HIm) and three imidazolate (Im) groups were attached to a central ruthenium(III) ion (Ru3+). The resulting molecular single crystal was highly symmetrical and resembled a “star” shape. After studying the proton conductivity of this starburst-like metal complex, the team found that each of the six imidazole groups attached to the Ru3+ ion acts as a proton emitter. This made the molecule 6 times more powerful than individual HIm molecules, which could only carry one proton at a time.

The team also explored the mechanism underlying the high-temperature proton conduction capacity of star molecules. They found that above -70°C, proton conductivity resulted from individual localized rotations of the HIm and Im- groups and proton hopping to other Ru(III) complexes in the crystal via hydrogen bonds. However, at temperatures above 147°C, the proton conductivity came from the rotation of the whole molecule, which was also responsible for the higher proton conductivity at high temperature. This rotation, confirmed by the team using a technique called “solid-state 2H-NMR spectroscopy”, yielded a conductivity rate three orders of magnitude higher (σ = 3.08 × 10-5 S/cm) than that of individual HIm molecules (σ = 10-8 S/cm).

The team believe their study could serve as a new driving principle for proton-conducting solid-state electrolytes. Knowledge from their new molecular design could be used to develop new high-temperature proton conductors as well as improve the functionality of existing conductors. “Fuel cells hold the key to a cleaner, greener future. Our study offers a roadmap to improve the performance of these high-temperature carbon-neutral energy resources by designing and implementing molecular proton conductors capable of efficiently transferring protons at such temperatures,” concludes Professor Tadokoro.

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Material provided by Tokyo University of Science. Note: Content may be edited for style and length.