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There are several well-known, conventional techniques for the structural characterization of carbon aerogels. However, from the point of view of possible applications as electrode materials, where these materials are mainly immersed in liquid, characterizing the size, shape and accessibility of the pores, as well as studying the solid-liquid interface reactions or the description of the structural changes occurring under the influence of the liquid, are of primary importance. The complex use of liquid phase nuclear magnetic resonance (NMR) spectroscopy methods, like NMR relaxometry, cryoporometry, diffusometry, offers a joint solution for this, providing the opportunity for the non-destructive examination of the solid phase through the liquid medium. The behaviour of the liquid that partially or completely fills the pores provides information about the solid structure, the wetting of the pore surface, and last but not least, the mobility of the liquid in the pore system.
By measuring the T$_2$ relaxation times of the confined water in carbon aerogels during the saturation, the T$_2$ – filling factor curves provides information on the mechanism of wetting. Through a k parameter one can conclude if the surface is covered by liquid in a layer-by-layer way, or the pores are step-by-step saturated because of the poor wetting. When NMR cryoporometry provides pore-size data in the same liquid, the surface relaxation can be determined. This way the morphological changes of the porous carbon in different liquids can be detected, or the effect of different synthesis conditions can be studied in depth from a novel point of view. [1]
In case ionic liquid (IL) is mixed with water in the precursor solution in the first step of the synthesis, the IL strongly interacts with the monomers before the polymerization and solvates the formed polymer. This alters the morphology and pore size of the formed polymer and carbon aerogels. NMR relaxometry data on the carbon aerogels pointed out that IL modified their wetting mechanism, due to the formation of ultramicropores, which enhances the hydrophilicity.
The separate detection of similar pore sizes can be carried out with the use of test liquids of different polarity by NMR cryoporometry, while their unlike hydrophobicity can be revealed from T$_2$ relaxation measurements. This way even the displacement of immiscible liquids in the pores can be followed.
Keywords: carbon aerogels, nuclear magnetic resonance spectroscopy, pore morphology, wetting mechanism, restricted diffusion.
| References | [1] M. Kéri, D. Nyul, K. László, L. Novák, I. Bányai, Carbon, 189, 57-70, 2022. |
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| Country | Hungary |
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