Superprotonic conductivity of ketoenamine covalent-organic frameworks grafted by imidazole-based units.

The achievement of covalent organic frameworks (COFs) with high stability and exceptional proton conductivity is of tremendous practical importance and challenge. Given this, we hope to prepare the highly stable COFs carrying CN connectors and enhance their proton conductivity via a post-modification approach. Herein, one COF, TpTta, was successfully synthesized by employing 1,3,5-triformylphloroglucinol (Tp) and 4,4',4″-(1,3,5-triazine-2,4,6-triyl)-trianiline (Tta) as starting materials, which has a ß-ketoenamine structure bearing a large amount of -NH groups and intramolecular H-bonds. TpTta was then post-modified by inserting imidazole (Im) and histamine (His) molecules, yielding the corresponding COFs, Im@TpTta and His@TpTta, respectively. As a result, their proton conductivities were surveyed under changeable temperatures (30-100 °C) and relative humidities (68-98 %), revealing a degree of temperature and humidity dependence. Impressively, under identical conditions, the optimum proton conductivities of the two post-modified COFs are 1.14 × 10-2 (Im@TpTta) and 3.45 × 10-3 S/cm (His@TpTta), which are significantly greater than that of the pristine COF, TpTta (2.57 × 10-5 S/cm). Finally, their proton conduction mechanisms were hypothesized based on the computed activation energy values, water vapor adsorption values, and structural properties of these COFs. Additionally, the excellent electrochemical stability of the produced COFs was expressed, as well as the prospective application value.

Comparative Study on Proton Conductivity and Mechanism Analysis of Two Imidazole Modified Imine-Based Covalent Organic Frameworks.

This work elucidates the potential impact of intramolecular H-bonds within the pore walls of covalent organic frameworks (COFs) on proton conductivity. Employing DaTta and TaTta as representative hosts, it was observed that their innate proton conductivities (σ) are both unsatisfactory and σ(DaTta)<σ(TaTta). Intriguingly, the performance of both imidazole-loaded products, Im@DaTta and Im@TaTta is greatly improved, and the σ of Im@DaTta (0.91×10-2  S cm-1 ) even surpasses that of Im@TaTta (3.73×10-3  S cm-1 ) under 100 °C and 98 % relative humidity. The structural analysis, gas adsorption tests, and activation energy calculations forecast the influence of imidazole on the H-bonded system within the framework, leading to observed changes in proton conductivity. It is hypothesized that intramolecular H-bonds within the COF framework impede efficient proton transmission. Nevertheless, the inclusion of an imidazole group disrupts these intramolecular bonds, leading to the formation of an abundance of intermolecular H-bonds within the pore channels, thus contributing to a dramatic increase in proton conductivity. The related calculation of Density Functional Theory (DFT) provides further evidence for this inference.

Design, Preparation, and High Intrinsic Proton Conductivity of Two Highly Stable Hydrazone-Linked Covalent Organic Frameworks.

Assembling crystalline materials with high stability and high proton conductivity as a potential alternative to the Nafion membrane is a challenging topic in the field of energy materials. Herein, we concentrated on the creation and preparation of hydrazone-linked COFs with super-high stability to explore their proton conduction. Fortunately, two hydrazone-linked COFs, TpBth and TaBth, were solvothermally prepared by using benzene-1,3,5-tricarbohydrazide (Bth), 2,4,6-trihydroxy-benzene-1,3,5-tricarbaldehyde (Tp), and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (Ta) as monomers. Their structures were simulated by Material Studio 8.0 software and confirmed by the PXRD pattern, demonstrating a two-dimensional framework with AA packing. The presence of a large number of carbonyl groups as well as -NH-NH2- groups on the backbone is responsible for their super-high water stability as well as high water absorption capacity. AC impedance tests demonstrated a positive correlation between the water-assisted proton conductivity (σ) of the two COFs and the temperature and humidity. Under 100 °C/98% RH, the highest σ values of TpBth and TaBth can reach 2.11 × 10-4 and 0.62 × 10-5 S·cm-1, which are among the high σ values of the reported COFs. Their proton-conductive mechanisms were highlighted by structural analyses as well as N2 and H2O vapor adsorption data and activation energy values. Our systematic research affords ideas for the synthesis of proton-conducting COFs with high σ values.

Metal@COFs Possess High Proton Conductivity with Mixed Conducting Mechanisms.

The inherent porous structures and aligned functional units inside the skeleton of covalent organic frameworks (COFs) provide an extraordinary promise for post-modification and deservedly expand their application in the field of proton conduction. Herein, we tactfully introduced copper ions into a two-dimensional COF (TpTta) furnished with ample N,O-chelating sites by a post-modification strategy to achieve two copper(II)-modified products, namely, CuCl2@TpTta-3 and CuCl2@TpTta-10. Inspiringly, the two modified COFs demonstrated the higher conductivities of 1.77 × 10-3 and 8.81 × 10-3 S cm-1 under 100 °C and 98% relative humidity, respectively, among the previously reported COFs with higher σ values. In comparison to the pristine COFs, the σ values of CuCl2@TpTta-3 and CuCl2@TpTta-10 are boosted by 2 orders of magnitude. On the basis of structural analyses, nitrogen and water vapor adsorption tests, and proton conduction mechanism analysis, we deeply analyzed the reason why the conductivity of the modified COFs was significantly increased. To the best of our knowledge, it is the first time to employ the CuCl2-modified strategy to boost the conductivity of COFs, which offers a wise idea for the fabrication of highly conductive materials in the field of fuel cells.

High Proton Conduction in Three Highly Water-Stable Hydrogen-Bonded Ferrocene-Based Phenyl Carboxylate Frameworks.

To acquire more new crystalline proton conductive materials, three ferrocene-based phenyl carboxylate frameworks (FCFs), [FcCO(o-C6H4COOH)] (FCF 1) (Fc = (η5-C5H5)Fe(η5-C5H4)), [m-FcC6H4COOH] (FCF 2), and [p-FcC6H4COOH] (FCF 3), supported by hydrogen bonds and π···π interactions were prepared. Their structures and phase purities are clarified by single-crystal X-ray diffraction or powder X-ray diffraction (PXRD). In addition, their high thermal and water stability were confirmed by thermogravimetric analyses, PXRD, and scanning electron microscopy determinations. Proton conductivity (σ) of 1-3 was studied under different relative humidities (RHs) and temperatures, and it was found that their σ boosted with the increase of humidity and temperature. Under 100 °C and 98% RH, their optimal σ values are 0.77 × 10-3, 1.94 × 10-4, and 3.46 × 10-3 S·cm-1, respectively. Consequently, their proton conductive mechanisms were proposed by means of activation energy calculation and structural analysis. Note that they are good proton conductive materials and are expected to be used in proton exchange membrane fuel cells.