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Comparison of the Ionic Conductivity Properties of Microporous and Mesoporous MOFs Infiltrated with a Na-Ion Containing IL Mixture

preprint
submitted on 16.06.2020 and posted on 19.06.2020 by Joshua Tuffnell, Jędrzej K. Morzy, Rui Tan, Qilei Song, Caterina Ducati, Thomas Bennett, Siân E. Dutton
IL@MOF (IL: ionic liquid; MOF: metal-organic framework) materials have been proposed as a candidate for solid-state electrolytes, combining the inherent non-flammability and high thermal and chemical stability of the ionic liquid with the host-guest interactions of the MOF. In this work, we compare the structure and ionic conductivity of a sodium ion containing IL@MOF composite formed from a microcrystalline powder of the zeolitic imidazolate framework (ZIF), ZIF-8 with a hierarchically porous sample of ZIF-8 containing both micro- and mesopores from a sol-gel synthesis. Although the crystallographic structures were shown to be the same by X-ray diffraction, significant differences in particle size, packing and morphology were identified by electron microscopy techniques which highlight the origins of the hierarchical porosity. After incorporation of Na0.1EMIM0.9TFSI (abbreviated to NaIL; EMIM = 1-ethyl-3-methylimidazolium; TFSI = bis(trifluoromethylsulfonyl)imide), the hierarchically porous composite exhibited a 40 % greater filling capacity than the purely microporous sample which was confirmed by elemental analysis and digestive proton NMR. Finally, the ionic conductivity properties of the composite materials were probed by electrochemical impedance spectroscopy. The results showed that despite the 40 % increased loading of NaIL in the NaIL@ZIF-8micro sample, the ionic conductivities at 25 °C were 8.4x10-6 and 1.6x10-5 S cm-1 for NaIL@ZIF-8meso and NaIL@ZIF-8micro respectively. These results exemplify the importance of the long range, continuous ion pathways contributed by the microcrystalline pores, as well as the detrimental effect of discontinuous and tortuous mesoporous pathways which show a limited contribution to the overall ionic conductivity.

Funding

UF150021

RSG\R1\180395

EPSRC Centre for Doctoral Training in Sustainable and Functional Nano

Engineering and Physical Sciences Research Council

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Cambridge Trust

Winton Programme for the Physics of Sustainability

China Scholarship Council

NanoMMES, 851272

Centre for Advanced Materials for Integrated Energy Systems (CAM-IES)

Engineering and Physical Sciences Research Council

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History

Email Address of Submitting Author

jmt83@cam.ac.uk

Institution

University of Cambridge

Country

United Kingdom

ORCID For Submitting Author

0000-0003-3069-1466

Declaration of Conflict of Interest

No conflict of interest.

Version Notes

First submission.

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