Developing
proton–conducting electrolytes that are usable
over a wide temperature range (25–150 °C) is highly desirable
to enhance the efficiency of fuel cells for on-board automotive applications.
Increasing the stability of water above its boiling point is considered
one possible method to maintain the hydrogen bond network in composite
materials for fast proton conduction. We herein propose an approach
to encapsulate LiBr into a negatively charged metal–organic
framework of (H3O)[(UO2)4(2-pmb)3(H2O)3]·0.5H2O (1) [2-pmbH3 = 2-(phosphonomethyl)benzoic acid]
to enhance the water stability at high temperatures and inhibit the
migration of Li ions by Coulombic interactions induced by anionic
skeletons. The resulting composite shows a superprotonic conductivity
of over 10–2 S cm–1 and a low
activation energy of less than 0.4 eV in an anhydrous N2 atmosphere from ambient temperature to 110 °C. Diffusion coefficient
tests confirm that protons, rather than lithium ions, are the main
contributor to conductivity.
Chiral transcription from the molecular level to the macroscopic level by self-organization has been a topic of considerable interest for mimicking biological systems. Homochiral coordination polymers (CPs) are intriguing systems...
Nanotubular
materials have garnered considerable attention since
the discovery of carbon nanotubes. Although the layer-to-tube rolling
up mechanism has been well recognized in explaining the formation
of many inorganic nanotubes, it has not been generally applied to
coordination polymers (CPs). To uncover the key factors that determine
the rolling-up of layered CPs, we have chosen the Co/R-, S-Xpemp [Xpemp = (4-X-1-phenylethylamino)methylphosphonic
acid, X = H, F, Cl, Br] systems and study how the weak interactions
influence the formation of layered or tubular structures. Four pairs
of homochiral isostructural compounds R-, S-Co(Xpemp)(H2O)2 [X = H (1H), F (2F), Cl (3Cl), Br (4Br)] were obtained with tubular structures. The inclusion of 3,3′-azobipyridine
(ABP) guest molecules led to compounds R-, S-[Co(Xpemp)(H2O)2]4·ABP·H2O with layered structures when X was Cl (5Cl)
and Br (6Br), but tubular compounds 1H and 2F when X was H and F. Layered structures were also obtained
for racemic compounds meso-Co(Xpemp)(H2O)2 [X = F (7F), Cl (8Cl), Br
(9Br)] using racemic XpempH2 as the reaction
precursor, but not when X = H. A detailed study on R-
6Br revealed that layer-to-tube transformation occurred
upon removal of ABP under hydrothermal conditions, forming R-
4Br with a tubular structure. Similar layer-to-tube
conversion did not occur in organic solvents. The results demonstrate
that weak interlayer interactions are a prerequisite but not sufficient
for the rolling-up of the layers. In the present cases, water also
provides a driving force in the layer-to-tube transformation. The
experimental results were rationalized by theoretical calculations.
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