Complex ion transport in porous nanotubes

A new study by researchers from Leipzig University, Friedrich Schiller University Jena and the University of Cambridge has revealed how ions move and exchange within complex porous carbon electrodes. These electrodes play a key role in high-performance energy storage, water desalination, gas storage and other electrochemical technologies. To analyse this complex ion transport, the scientists used two-dimensional NMR exchange spectroscopy, a technique that makes it possible to track how ions move between different chemical environments and how quickly this exchange occurs. They have now published their findings in the journal Advanced Energy Materials, for which they also designed the cover image.

Their work addresses a long-standing challenge in energy materials research: although porous carbons are widely used because of their large internal surface area, high electrical conductivity and tunable pore structures, researchers have found it difficult to directly quantify how ions move through their pore networks at the microscopic level. In particular, carbon electrodes do not provide a uniform pore environment. Instead, ions move through a heterogeneous landscape.

Porous electrodes consist of a solid material containing cavities, or pores, of different sizes. Micropores have a diameter of less than two nanometres (two billionths of a metre), while mesopores range from two to 50 nanometres in diameter. The pores are filled with an electrolyte – a liquid containing freely mobile positive and negative ions that enables them to move through the material.

“To account for this complexity, our research team developed a quantitative analysis approach based on two-dimensional nuclear magnetic resonance exchange spectroscopy, known as 2D EXSY NMR. This method allows us to observe how ions exchange between different local environments over time,” explains the study’s first author, Dr Henry R. N. B. Enninful of Leipzig University’s Felix Bloch Institute for Solid State Physics, adding that the approach goes beyond conventional simplified models, providing a more realistic picture of how ions move within porous electrodes.

The study used aqueous LiTFSI electrolytes. These electrolytes, which contain the lithium salt LiTFSI and enable lithium ions to move between the electrodes, were confined within two porous carbon model materials: CMK-3, a mesoporous carbon with predominantly one-dimensional, channel-like pores, and ST-CMK-3, a hierarchically structured carbon. The analysis revealed different exchange regimes, ranging from rapid exchange near the surface to slower diffusion within the pores. In the hierarchically structured carbon, the team also identified additional transport pathways between different pore environments. “This demonstrates how pore connectivity can accelerate ion exchange and improve transport efficiency,” says the study’s senior author, Professor Rustem Valiullin of Leipzig University’s Department of Applied Magnetic Resonance.