Lattice vs. Surface Water: Correlation between the Proton Conductivity and Microstructure of Faujasite Zeolites upon Water Adsorptions

Abstract

Zeolites are readily available crystalline materials with intrinsic porosity, and their outstanding physical and chemical properties have been demonstrated in many areas, including catalysis, adsorption, ion exchange, and gas separation. Their insulating character has so far prevented the development of zeolite-based devices; however, given their non-toxic nature and abundance, it would be worthwhile to investigate their charge transport properties, as the inherent micro-, meso- and macropores offer specific structuring and modification options for various engineering and biomedical applications. Here, two Faujasite Na-FAU type zeolites with different particle sizes, nanosized (n-FAU) and microsized (μ-FAU) zeolite samples, were prepared to comprehensively investigate their electrical properties. To elucidate the complex interplay between morphology, degree of hydration, and overall electrical performance of studied zeolites, structural (powder X-ray diffraction under ambient and non-ambient conditions), microstructural (scanning electron microscopy, light-scattering particle size analysis, porosimetry), thermal (thermogravimetry, differential scanning calorimetry), spectroscopic (vacuum infrared spectroscopy) and electrical (impedance spectroscopy) methods were used synergistically. The research results indicate that particle size affects the ionic conductivity of zeolites (1.97 × 10–5 S cm–1 for μ-FAU compared with 3.00 × 10–7 S cm–1 for n-FAU under dry conditions), as continuous channels in larger zeolite particles provide more efficient charge transfer pathways. Another important factor for the electrical properties of zeolites is relative humidity, or the degree of hydration, which causes a 4 order of magnitude change in ionic conductivity (3.00 × 10–7 S cm–1 versus 2.75 × 10–3 S cm–1 for n-FAU in dry and humid conditions). This study demonstrates that the electrical characteristics of zeolites are highly tuneable, which may create new opportunities for the use of these porous functional materials in future sustainable applications.