13C → 1H Cross-Polarization NMR in Solids at Natural 13C Abundance

Selective observation of charge storing ions in supercapacitor electrode materials

Nuclear magnetic resonance (NMR) spectroscopy has emerged as a useful technique for probing the structure and dynamics of the electrode-electrolyte interface in supercapacitors, as ions inside the pores of the carbon electrodes can be studied separately from bulk electrolyte. However, in some cases spectral resolution can limit the information that can be obtained. In this study we address this issue by showing how cross polarisation (CP) NMR experiments can be used to selectively observe the in-pore ions in supercapacitor electrode materials. We do this by transferring magnetisation from ¹³C nuclei in porous carbons to nearby nuclei in the cations (¹H) or anions (¹⁹F) of an ionic liquid. Two-dimensional NMR experiments and CP kinetics measurements confirm that in-pore ions are located within Ångströms of sp²-hybridised carbon surfaces. Multinuclear NMR experiments hold promise for future NMR studies of supercapacitor systems where spectral resolution is limited.

Towards quantitative measurements in solid-state CPMAS NMR: A Lee-Goldburg frequency modulated cross-polarization scheme

A new scheme combining a Lee-Goldburg (LG) sequence with frequency modulation is proposed for cross-polarization (LG-FMCP) in solid-state magic-angle-spinning nuclear magnetic resonance. During the CP contact time, the (1)H magnetization is spin-locked along the magic angle by the LG sequence and the irradiation offset of the S spins (e.g., (15)N) is modulated sinusoidally with a constant RF amplitude. It is shown experimentally that the LG sequence significantly lengthens the proton spin-lattice relaxation time in the tilted rotating frame and that the frequency modulation shortens the cross-polarization time for non-protonated S spins. As a result of substantially increasing the difference in these relaxation rates, the non-protonated and protonated S spins can be more efficiently and more uniformly polarized with a relatively long CP contact time, making quantitative CP measurements possible. A sample of (15)N-delta 1-L-histidine lyophilized from a solution of pH 6.3 and a (15)N-delta 1-L-His labeled transmembrane helical peptide in hydrated lipid bilayers were used to illustrate the advantages of this scheme.