Novel Imidazole–Imidazolate Metal Complex Based Proton Conductor

Fuel cells rely on water as a proton conduction medium, and hence frequently struggle to operate at temperatures above 100 °C. To address this problem, a group of Japanese scientists created a brand-new hydrogen-bonded starburst-shaped metal complex made up of six imidazole-imidazolate groups and the ruthenium (III) ion.

Even at 180 °C and −70 °C, the final single molecular crystal exhibits excellent proton conductivity.

Fuel cells are getting a lot of attention as people turn to more sustainable and environmentally friendly energy sources. The main benefit of using fuel cells is that they can produce electricity using only water as a by-product while using clean fuel hydrogen. All current lithium-ion batteries that power modern electronics could be replaced by this new, clean source of electricity.

The Nafion membrane, an ionic membrane made of synthetic polymers, is used in most fuel cells as a solid electrolyte that conducts proton in water. However, the use of water as a proton conduction medium has a significant disadvantage for the fuel cell in that it is unable to operate properly at temperatures above 100 °C, the point at which water begins to boil, which results in a decrease in proton conductivity.

New proton conductors are therefore required, ones that can transfer protons effectively even at such high temperatures.

A team of Japanese researchers recently reported a novel imidazole–imidazolate metal complex-based high-temperature proton conductor that exhibits efficient proton conductivity even at 147 °C. The team was led by Prof. Makoto Tadokoro from the Tokyo University of Science (TUS).

The group 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, a nitrogen-containing organic compound, has gained popularity as an alternative proton conductor for its ability to operate even without water. However, it has a lower proton transfer rate than conventionally used Nafion and melts at 120 °C.

Makoto Tadokoro, Professor, Tokyo University of Science

“To overcome these issues, we introduced six imidazole moieties into a ruthenium (III) ion to design a new metal complex that operates as a multi-proton carrier and has high temperature stability,” detailed Prof. Tadokoro when asked about the rationale behind the study.

The research was published on June 27^th, 2022, in the Chemistry - A European Journal and featured on the front cover of the journal.

The group created a new molecule with a central ruthenium (III) ion (Ru³⁺) and three imidazole (HIm) and three imidazolate (Im⁻) groups attached. The resulting single molecular crystal had a “starburst” shape and was very symmetrical.

Ater examining the proton conductivity of this starburst-type metal complex, the team discovered that each of the six imidazole groups connected to the Ru³⁺ ion acts as a proton transmitter. As HIm molecules could only transport one proton at a time, this made the molecule six times more powerful.

The team also looked into the mechanism underlying the starburst molecules’ capacity for high-temperature proton conduction. The researchers discovered that the proton conductivity at temperatures greater than −70 °C was caused by distinct localized rotations of the HIm and Im⁻ groups and proton jumps to other Ru(III) complexes in the crystal via hydrogen bonds.

The superior proton conductivity at high temperatures, however, was also caused by whole-molecule rotation, which occurred at temperatures above 147 °C. The team used “solid-state 2H-NMR spectroscopy” to confirm the rotation, which led to a conductivity rate that was three orders of magnitude higher (σ = 3.08 × 10⁻⁵ S/cm) than that for individual HIm molecules (σ = 10⁻⁸ S/cm).

According to the research team, solid-state electrolytes that conduct protons may adopt a new guiding principle as a result of their findings. Their inventive molecular design has provided insights that could be used to create new high-temperature proton conductors and enhance the performance of already existing ones.

Fuel cells hold the key to a cleaner and greener tomorrow. Our study offers a roadmap for improving the performance of these carbon-neutral energy resources at high temperatures by designing and implementing molecular proton conductors that can transfer protons efficiently at such temperatures.

Makoto Tadokoro, Professor, Tokyo University of Science

Hopefully, a fuel cell and clean energy-based future will not be too far off.

Journal Reference:

Tadokoro, M., et al. (2022) Proton Conduction at High Temperature in High-Symmetry Hydrogen-Bonded Molecular Crystal of Ru(III) Complexes with Six Imidazole–Imidazolate Ligands. Chemistry - A European Journal. doi.org/10.1002/chem.202201397.