Paulo Cesar de Mello Correa (University of Espírito Santo (UFES), Brasil)
X: @pcmspin; Bluesky: @spinplus.bsky.social
Abstract: Solid-state NMR is widely employed to study the structural properties of activated carbon and other graphenic materials. Activated carbon is extensively utilized in industrial and consumer applications, particularly in energy storage devices such as supercapacitors, which serve as an environmentally sustainable alternative for integration into electrical systems alongside conventional batteries, or potentially replacing them in the future. In supercapacitor design, activated carbon functions as an electrode material due to its highly porous network, which provides a large surface area, thereby enhancing capacitance. Another approach to improving energy storage capacity of a supercapacitor involves synthesizing activated carbon with controlled pore dimensions, confining ions near the surface for optimized charge accumulation. Solid-state NMR can measure the nucleus-independent chemical shift (NICS), defined as the chemical shift difference (in ppm) between extra-pore and intra-pore peaks in NMR spectra. In this study, solid-state ¹H and ¹⁹F NMR spectra were acquired for activated carbon samples soaked in distilled water and aqueous solutions of NaF and NaBF₄. The results reveal an increase in NICS with surface area, consistent with steam activation that introduce micropores, which leads to larger NICS values, indicating reduced ion-surface distances. Furthermore, NMR spectral deconvolution of extra-pore and intra-pore peaks enables indirect estimation of pore volume for the carbon material.
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Thank you for the presentation. Have you considered using 10/11B NMR to study the BF4- ions?
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Hello Jonas!
It would really be interesting to measure with 11B and compare the results, thank you for the suggestion. We are also working to carry out measurements with 23Na and possibly confirm some particular aspect of the BF4 ion in accessing the pores of samples B240 and B400. In addition, this analysis would provide a perspective on a possible difference between cations and anions in accessing the pores of the studied material.
thank you for the comment!!
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Nice presentation Paulo. Adding on to Jonas’ Boron suggestion, have you thought to look at 23Na or 35/37Cl to see whether these ions show similar behaviour as the ones studied? Additionally, have you performed any VT NMR to try to assess ion dynamics in the activated carbon?
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Hello Riley!
Thank you for the suggestion, we do have plans to measure with 23Na (for the NaBF4 1.0 M solution) and possibly with 35/37Cl (for the LiCl 1.0 M solution) to investigate possible relationships with the ion charge (cation or anion) for accessing the porous network of the studied carbons, thank you for the suggestion. Although the results have already shown that Li+ and BF4- ions do not access the pores of the B240 and B400 samples, a more careful analysis is necessary to draw such conclusions. Regarding VT NMR measurements, unfortunately, we do not have the apparatus for this type of measurement. However, we are conducting Exchange NMR (EXSY) measurements to investigate the dynamic aspects of ions in the exchange regimes between the bulk electrolyte and the porous network, as well as in the intra-particle exchange regime.
Thank you very much for the comment!!
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Cool! I would be curious if the Na/Li and F/Cl would have similar behaviours, and the quadrupolar parameters (particularly for Cl) could be instructive on whether the Cl ions are more immobilized in the pores vs. bulk.
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Hello, it would be very interesting to analyze all the nuclei present in NaBF4 and LiCl. We are working on taking these measurements to complement our work. Thanks again for your comment!
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Hi Paulo,
Interesting presentation!
My question is quite simple: Is there any specific reason you chose 5 kHz MAS for your experiment?-
Hello Bijaylaxmi, interesting question!
There is indeed a reason: our experiments are being conducted using an unsealed rotor, which allows for potential solution leakage during centrifugation in the MAS experiment. To mitigate this, we are operating at a spinning frequency of 5 kHz. In fact, we are actively investigating this leakage through NMR measurements, using a method that enables us to monitor the decay in intensity of both the ex-pore and in-pore peaks over time. Our results confirm the occurrence of leakage, as evidenced by a consistent decrease in the ex-pore peak intensity, particularly within the first 15 minutes of spinning. Therefore, all measurements are carefully limited to under 15 minutes and using the lower spinning frequency possible to prevent damage to the spectrometer.
Thank you for the comment!!
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