• Figure 1
    Figure 1

Fuel cells often fall short when it comes to operating at temperatures beyond 100 ֯C owing to their dependence on water as a proton conduction medium.

To overcome this issue, a team of researchers from Japan designed a new hydrogen-bonded starburst-shaped metal complex consisting of ruthenium (III) ion and six imidazole-imidazolategroups.

The resulting single molecular crystal shows excellent proton conductivity even at temperatures as high as 180°C and as low as\ -70 °C.

As the world is moving towards more environment-friendly and sustainable sources of energy, fuel cells are receiving a lot of attention.

The main advantage of fuel cells is that they use hydrogen, a clean fuel, and produce only water as a by-product while generating electricity.

This new and clean source of 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.

The use of water as a 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 starts to boil, leading to a drop in proton conductivity.

Therefore, there is a need for new proton conductors that can transfer protons efficiently even at such high temperatures.

In a recent breakthrough, a team of researchers from Japan, led by Professor Makoto Tadokoro from Tokyo University of Science (TUS), reported a novel imidazole-imidazolate metal complex based high-temperature proton conductor that shows efficient proton conductivity even at 147°C (see picture).

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 single molecularcrystal was highly symmetrical and resembled a “starburst” shape. Upon investigating the proton conductivity of this starburst-type metal complex, the team found that each of the six imidazole groups attached to the Ru3+ ion acts as a proton transmitter. This made the molecule six times more potent than individual HIm molecules, which could only transport one proton at a time.

The team also explored the mechanism underlying the high-temperature proton conduction ability of the starburst molecules.

They found that at a temperature of more than –70°C, the proton conductivity resulted from individual localized rotations of the HIm and Im- groups and proton jump to other Ru(III)complexes in the crystal via hydrogen bonds.  

At temperatures beyond 147°C, however, the proton conductivity arose from whole-molecule rotation, which was also responsible for the superior proton conductivity at high temperatures. This rotation, confirmed by the team using a technique called “solid-state 2H-NMR spectroscopy,” resulted in a conductivity rate three orders of magnitude larger ( = 3.08 × 10-5 S/cm) than that for individual HIm molecules ( = 10-8 S/cm).

The team believes that their study could act as a new driving principle for proton-conducting solid-state electrolytes. The insights from their novel molecular design could be used to develop new high-temperature proton conductors as well as improve the functionality of the existing ones.


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