Abstract
Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells. To maximize proton conduction, three-dimensional conduction pathways are preferred over one-dimensional pathways, which prevent conduction in two dimensions. Many crystalline porous solids to date show one-dimensional proton conduction. Here we report porous molecular cages with proton conductivities (up to 10−3 S cm−1 at high relative humidity) that compete with extended metal-organic frameworks. The structure of the organic cage imposes a conduction pathway that is necessarily three-dimensional. The cage molecules also promote proton transfer by confining the water molecules while being sufficiently flexible to allow hydrogen bond reorganization. The proton conduction is explained at the molecular level through a combination of proton conductivity measurements, crystallography, molecular simulations and quasi-elastic neutron scattering. These results provide a starting point for high-temperature, anhydrous proton conductors through inclusion of guests other than water in the cage pores.
Highlights
Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells
One limitation of proton conduction in metal-organic frameworks (MOFs) is the tendency for directional proton transport, which in turn arises from the low-dimension pore structures in most frameworks tested[23,24]
Even in the few 3D proton-conducting MOFs that are known, the protons were found to be transported in 1D channels in most cases25–27. 3D proton transport is more favourable for application in PEMs28,29, and there have been attempts to enhance proton mobility in MOFs by introducing defects or by decreasing the crystallinity[29,30,31]
Summary
Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells. Inspired by the need for more effective PEMs, the structural and chemical features that enhance proton conduction have been studied for wide range of materials[4,5,6,7] Porous solids such as metal-organic frameworks (MOFs)[8,9] or covalent organic frameworks[10] have been a particular focus because the proton conduction properties can be fine-tuned by controlling crystallinity, porosity and chemical functionality. We present an alternative strategy, which is to develop crystalline porous molecular solids where the proton transport occurs in 3D pathway by virtue of the native channel structure and topology We demonstrate this concept for a range of crystalline porous organic cages (Fig. 1). We explain the influence of the counter anions in the protonated cage salts (Fig. 1b,d,e), which act to ‘gate’ the proton conduction
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