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Imidazole molecules entrapped in porous materials can exhibit high and stable proton conductivity suitable for elevated temperature (>373 K) fuel cell applications. In this study, new anhydrous proton conductors based on imidazole and mesoporous KIT-6 were prepared. To explore the impact of the acidic nature of the porous matrix on proton conduction, a series of KIT-6 materials with varying Si/Al ratios and pure silica materials were synthesized. These materials were additionally modified with cerium atoms to enhance their Brønsted acidity. TPD-NH3 and esterification model reaction confirmed that incorporating aluminum into the silica framework and subsequent modification with cerium atoms generated additional acidic sites. UV-Vis and XPS identified the presence of Ce3+ and Ce4+ in the KIT-6 materials, indicating that high-temperature treatment after cerium introduction may lead to partial cerium incorporation into the framework. EIS studies demonstrated that dispersing imidazole within the KIT-6 matrices resulted in composites showing high proton conductivity over a wide temperature range (300–393 K). The presence of weak acidic centers, particularly Brønsted sites, was found to be beneficial for achieving high conductivity. Cerium-modified composites exhibited conductivity surpassing that of molten imidazole, with the highest conductivity (1.13 × 10−3 S/cm at 393 K) recorded under anhydrous conditions for Ce-KIT-6. Furthermore, all tested composites maintained high stability over multiple heating and cooling cycles. [ABSTRACT FROM AUTHOR]

The present work concerns proton-conducting composites obtained by replacing the water molecules present in aluminophosphate and silicoaluminophosphate AFI-type molecular sieves (AlPO-5 and SAPO-5) with azole molecules (imidazole or 1,2,4-triazole). Both the introduction of azoles and the generation of Brønsted acid centers by isomorphous substitution in aluminophosphate materials were aimed at improving the proton conductivity of the materials and its stability. In the presented study, AlPO-5 and several SAPO-5 materials differing in silicon content were synthesized. The obtained porous matrices were studied using PXRD, low-temperature nitrogen sorption, TPD-NH3, FTIR, and SEM. The proton conductivity of composites was measured using impedance spectroscopy. The results show that the increase in silicon content of the porous matrices is accompanied by an increase in their acidity. However, this does not translate into an increase in the conductivity of the azole composites. Triazole composites show lower conductivity and significantly higher activation energies than imidazole composites; however, most triazole composites show much higher stability. The different conductivity values for imidazole and triazole composites may be due to differences in chemical properties of the azoles. [ABSTRACT FROM AUTHOR]

Imidazole (Im) introduced into the channels of H-MFI zeolites (with diverse framework Si/Al) shows remarkably enhanced proton conductivity in a broad temperature range (303–393 K), which increased with Im loading. Some conductivity raise is already noticed after its introduction into slilicalite-1 (i.e. SiO 2) due to escalated Im dispersion and subsequently enhanced mobility inside the channels. The increase, however, becomes much more spectacular for the Im composites based on H-forms of aluminosilicate MFI zeolite matrices. It results from additional beneficial interaction of the guest Im with the zeolite acid sites, particularly OH groups, acting as additional proton carriers. Nevertheless, there is no straight correlation between the number of acid sites and the proton conductivity. It seems that the strong Lewis acid sites, contrary to Broensted centers, hinder the conduction, because of firm bonding the Im molecules and reducing their motion. Thus, both kind of sites compete with each other. The highest conduction noticed for the composites based on the H-zeolite with Si/Al = 50 results not so much from pronounced contribution of Broensted centers, but rather from the small number of Lewis sites which are relatively weak. Both Grotthuss and vehicle conductivity modes could be considered. Image 1 • Anhydrous proton conductors. • Imidazole entrapped in H-zeolites MFI with various Si/Al. • Impact of acid sites on composite conductivity. [ABSTRACT FROM AUTHOR]

Abstract Imidazole entrapped in H-forms of FAU zeolites with various Si/Al ratios (1, 4, 40) exhibit high and stable proton conductivity (up to ∼10−3 S/cm) in a broad range of temperatures (293–393 K). The conductivity increases with temperature and imidazole loading. The number and strength of zeolite acid sites (resulting from various Si/Al) influence markedly the conductivity of composites. The samples based on zeolites with low Si/Al (1) show markedly higher conduction than these supported on the matrix with medium Si/Al (4), regardless of very similar number of the acid OH groups. The strong acid OH groups in zeolite with Si/Al = 4 result in protonation and immobilization of adsorbed imidazole, which reduce their mobility and subsequently the conduction. The low siliceous with weaker acid sites protonate imidazole and form a chain of H-bonds effective in proton propagation. Almost neutral high silica matrix provides a high mobility of dispersed imidazole, due to weak mutual interaction. The overloaded samples indicate a phase transition effect at elevated temperature due to melting of extra-matrix imidazole, which is reflected in dramatic increase in conductivity. Graphical abstract Image 1 Highlights • Proton conductivity of imidazole entrapped in H-forms of faujasites. • Role of imidazole loading, phase transition at high temperature. • Influence of Si/Al on efficiency and mechanism of conduction. [ABSTRACT FROM AUTHOR]