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Page 11 of 17 ACS Applied Materials & Interfaces This is an indication that the mechanism of charge storage is not limited by diffusion within the porous framework of ZTC. Evidence for Insertion of FSI‐ into ZTC. A combination of 19F and 13C solid‐state NMR experiments were carried out to verify the successful insertion of FSI‐ anions within the pore structure of ZTC during charge/discharge cycling of the above described cells. A series of samples consisting of pristine (un‐ occupied) and solution‐impregnated [FSI]@ZTC were com‐ pared to ex situ recovered samples of electrochemically‐cy‐ cled ZTC (in either the fully charged or fully discharged state). To assist in distinguishing the contributions from adsorbed FSI‐ and free FSI‐ dissolved in solution (potentially a contami‐ nant on the outer surfaces of the ex situ recovered samples), a washing regimen was established that consists of carefully rinsing the ZTC particles with EC/DMC (1:1 by weight) after which the samples could be compared with and without washing (details given in Supporting Information). Static and magic angle spinning (MAS) 19F solid‐state NMR experiments showed several common features in all spectra, including two strong signals at ‐148 ppm and ‐84 ppm that can be attributed to instrument‐related background, as shown in Figure 4 and Figure S13. In the spectra for all sam‐ ples except the pristine ZTC, a distinct signal at 52 ppm is ob‐ served, corresponding to the presence of freely tumbling FSI‐ (as confirmed by solution‐state NMR of the neat liquid elec‐ trolyte solution). Figure 4. 19F MAS NMR (10 kHz) spectra of pristine, impreg‐ nated, and electrochemically cycled ZTC. The gray shaded re‐ gion indicates signal originating from instrument background. The starred peaks surrounding the signal at 57 ppm are spin‐ ning sidebands resulting from a large chemical shift anisotropy (CSA), demonstrating immobilized FSI‐ anions. This species is present in the spectra of charged and discharged ZTC, which indicates its origin as irreversibly adsorbed FSI‐ anions in‐ serted during the first six cycles of cell operation. The very in‐ tense and broad (405 Hz) signal at 52 ppm in charged ZTC, cor‐ responding to semi‐mobile adsorbed FSI‐ anions, is evidence of the reversibly adsorbed FSI‐ species inserted during each cycle. 6 This indicates that despite the washing treatment, some sol‐ ubilized FSI‐ does remain in/on all samples, although far less significantly in the case of the washed samples (Figure S13). The linewidths of this signal in the second set of 19F solid‐state NMR experiments under MAS conditions (90°‐one‐pulse se‐ quence at 10 kHz) can be directly compared to determine the relative mobility of the solubilized FSI‐ species (see Figure 4). The narrow signal at 52 ppm (exhibiting a linewidth of 74 Hz) for the solution‐impregnated (and washed) ZTC is associated with dissolved FSI‐ as in the neat liquid electrolyte solution (likely on the outer surfaces of the ZTC particles, as consistent with the results from static NMR experiments). For fully charged ZTC, an intense signal at 52 ppm is accompanied by another centered at 57 ppm that also exhibits several spin‐ ning sidebands, both corresponding to the presence of FSI‐. The primary signal at 52 ppm, exhibiting a linewidth of 405 Hz, is significantly broader than that for solubilized FSI‐ in neat solution (5 Hz), indicating that the majority FSI‐ species in charged ZTC are less mobile than in the solution state. This serves as direct evidence of adsorption of FSI‐ within the pore network of ZTC during electrochemical charging. The second‐ ary signal at 57 ppm, accompanied by spinning sidebands, must arise from a distinct FSI‐ species that exhibits significant chemical shift anisotropy (CSA). This indicates the presence of a second, fully immobilized FSI‐ species, attributable to the irreversibly inserted FSI‐ detected during the first several electrochemical cycles, as previously mentioned. For fully dis‐ charged ZTC, the signal at 52 ppm was found to be greatly re‐ duced in intensity and once again relatively narrow (74 Hz), similarly to the impregnated (and washed) ZTC containing only highly mobile FSI‐ anions in solution on the outer sur‐ faces of the ZTC particles. The discharged sample also shows the signal at 57 ppm with obvious spinning sidebands indicat‐ ing that the portion of inserted FSI‐ that is immobilized within the ZTC pore network during charging is indeed not removed during discharge. The mobility of adsorbed FSI‐ ions within the pores of fully charged ZTC, while restricted compared to the bulk liquid electrolyte, was still relatively high for an adsorbed phase confined within a narrow, microporous network. This was further investigated by 13C MAS solid‐state NMR experiments to determine if EC or DMC were also incorporated along with FSI‐ during cycling (see Figure S14). Under the conditions employed, no signals originating from the ZTC framework it‐ self were detected, owing to very long relaxation times asso‐ ciated with the weakly conductive and rather amorphous na‐ ture of the material. For all samples exposed to the EC/DMC electrolyte mixture and then thoroughly washed (including a drying step at 25 °C under vacuum), only EC was observed in the 13C NMR spectra (at 66 and 157 ppm). This implies that, due to its higher boiling point (248 °C as compared to 90 °C for DMC), EC did not fully evaporate from all of the external surfaces prior to NMR measurements. It also serves to sup‐ port the possibility of EC being inserted into the pores of ZTC along with FSI‐ during charging. If co‐inserted during charg‐ ing, this would permit some (restricted) mobility of the FSI‐ ions adsorbed on the inner pore surfaces, and would explain the narrower linewidth than expected for adsorbed FSI‐ in fully charged ZTC and the absence of spinning sidebands as‐ sociated with the signal at 52 ppm. Finally, elemental analysis was also carried out on solution‐impregnated and electro‐ chemically cycled ZTC, and the results (Table S5 and Figure ACS Paragon Plus EnvironmentPDF Image | Zeolite-Templated Carbon as the Cathode
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