In Situ Aliovalent Nickle Substitution and Acidic Modification of Nanowalls Promoted Proton Conductivity in InOF with 1D Helical Channel

High Proton Conductivity Enhancement Obtained by a Covalent Postsynthesis Modification Approach for Two Metal-Organic Frameworks

This study demonstrates an effective strategy to enhance proton conductivity by synthesizing 2 three-dimensional metal-organic frameworks (MOFs), [Zn(DTD22)]n (MOF 1) and [Cd2(DTD22)2]n (MOF 2), (DTD22 = 4,4″-diamino-[1,1':4',1″-terphenyl]-2,2″-dicarboxylic acid). The DTD22 ligand used formed a continuous hydrogen-bonding network in the structure, constructing excellent hydrophilic channels. MOF 1 and MOF 2 were further postsynthesized and modified (PSM) by Schiff base reaction, and 4-chloro-3-formylbenzenesulfonic acid ligands containing -SO3H and -Cl were successfully introduced into the framework to form PSM-MOF 1 and PSM-MOF 2. Experiments showed that this modification significantly enhanced the proton conductivity of the materials, especially at 90 °C and 98% RH: PSM-MOF 1 (2.38 × 10-1 S·cm-1) and PSM-MOF 2 (3.50 × 10-1 S·cm-1). In comparison, the conductivities of unmodified MOF 1 and MOF 2 were 8.55 × 10-2 S·cm-1 and 9.50 × 10-5 S·cm-1, respectively. The present study demonstrates that the proton conductivity of MOFs can be effectively enhanced by the covalent postmodification method, which provides a new idea for the application of MOFs.

Uncoordinated Carboxyl Groups as Proton Sources in Polyoxometalate-Based Metal-Organic Frameworks Enhance Proton Conduction

To select appropriate organic ligands is an effective strategy to enhance the proton conductivities of polyoxometalate-based metal-organic frameworks (POMOFs). Two new Dawson-type POMOFs, named CUST-961 and CUST-962, have been designed and synthesized via combining Htzbc selected by hard and soft acid and base theory and density functional theory calculation, transition metal ions, alkali metal ions (Na+ and K+), and Dawson-type polyoxometalates ([P2W18]6-) under the hydrothermal method. Their stabilities under different temperatures and relative humidities (RHs) have been investigated through powder X-ray diffraction and thermogravimetric analysis. Both CUST-961 and CUST-962 exhibited excellent aqueous and thermal stabilities. The alternating current (AC) impedance spectrum tests revealed that the proton conductivity of CUST-961 could reach 1.4 × 10-4 S cm-1 at 95 °C and 98% RH, which is about 3 times that of CUST-962. The different proton conductivities between the two compounds are due to the fact that CUST-961 possesses more uncoordinated carboxylic acid groups, as confirmed by attenuated total reflection infrared spectroscopy and 1H solid-state nuclear magnetic resonance spectroscopy, which can not only act as the proton source but also establish a richer hydrogen bonding network to enhance proton conduction. This work provides a new strategy and insight for the design and preparation of polyoxometalate-based proton conductive materials.

Proton Conduction via Water and Ammonia Coordinated Metal Cationic Species in MOF and MHOF Platforms

Although metal-organic frameworks (MOFs) and metalo hydrogen-bonded organic frameworks (MHOFs) are designed as promising solid-state proton conductors by incorporating various protonic species intrinsically or extrinsically, design and development of such materials by employing the concept of proton conduction through coordinated polar protic solvent is largely unexplored. Herein, we have constructed two proton-conducting materials having different solvent coordinated metal cationic species: In-H2O-MOF, ({[In(H2O)6][In3(Pzdc)6] ⋅ 15H2O}n; H2Pzdc: pyrazine-2,3-dicarboxylic acid) with coordinated water molecules from hexaaquaindium cationic species, and MHOF-4, ([{Co(NH3)6}2(2,6-NDS)2(H2O)2]n; 2,6-H2NDS: 2,6-naphthalenedisulfonic acid) with coordinated ammonia from hexaammoniacobalt cationic species. Interestingly, higher proton conductivity was achieved for In-H2O-MOF (1.5×10-5 S cm-1) than MHOF-4 (6.3×10-6 S cm-1) under the extreme conditions (80 °C and 95 % RH), which could be attributed to enhanced acidity of coordinated water molecules having much lower pKa value than that of coordinated ammonia. Greater charge polarization on hydrogen atoms of In3+-coordinated water molecules than that of Co2+-coordinated ammonia led to the high conductivity of In-H2O-MOF, as evident by quantum chemical studies. Such a comparative study on metal-coordinated protic polar solvents in achieving proton conduction in crystalline solids is yet to be made.

Symmetry related proton conductivity tunability via aliovalent metal substitution in imidazolium templated stable metal-organic framework hybrid membranes

Proton-conducting materials have gained popularity owing to their extensive applications in biologic/chemical sensors, supercapacitors, proton sieving, and proton-exchange-membrane fuel cells. To date, the most commercially used polymer membrane has been the Nafion series that exhibits conductivity exceeding 0.1 S cm-1, however, this series is expensive, has poor dimensional stability, and requires a complex synthesis process. The key criterion for selecting Nafion alternatives is to achieve the systematic integration of high proton conductivity with high stability through a simple and efficient approach. In this study, we used an aliovalent metal substitution strategy to design serial metal-organic frameworks (MOFs), including tetragonal T-Cd-BTC (CH3NH2CH3)2[Cd(BTC)](H2O) and quasi-cubic quasi-C-In-BTC (C4H7N2)[In(BTC)] and Im@quasi-C-In-BTC (C3H5N2)2[In(BTC)] frameworks, with 2-methylimidazolium and imidazolium cations as templates, respectively. Because of the aliovalent substitution of In(III) for Cd(II), both the metal-oxygen bond strength and unit cell symmetry gradually increased, resulting in an increase in the thermal stability of quasi-C-In-BTC and Im@quasi-C-In-BTC at temperatures of up to 700 K. Compared with in situ loaded 2-methylimidazolium quasi-C-In-BTC, Im@quasi-C-In-BTC prepared by incorporating the imidazolium cation into the pores of activated quasi-C-In-BTC exhibited a higher proton conductivity of 7.1 × 10-2 S cm-1 at 338 K and 95 % relative humidity. Thus, Im@quasi-C-In-BTC demonstrated real-life application. This result was confirmed by integrating Im@quasi-C-In-BTC with a poly(vinyl pyrrolidone)-poly(vinylidene fluoride) polymer matrix. Density functional theory simulations indicated that Im@quasi-C-In-BTC was strongly acidic and had high water-adsorption capacities, which contributed to extensive hydrogen-bond networks and strong host-guest interactions, in accordance with the experimental finding.

Metal Effect on the Proton Conduction of Three Isostructural Metal-Organic Frameworks and Pseudo-Capacitance Behavior of the Cobalt Analogue

Three isostructural transition metal-organic frameworks, [M(bta)0.5(bpt)(H2O)2]·2H2O (M = Co (1), Ni (2), Zn (3), H4bta = 1,2,4,5-benzenetetracarboxylic acid, bpt = 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole), were successfully constructed using different metal cations. These frameworks exhibit a three-dimensional network structure with multiple coordinated and lattice water molecules within the framework, contributing to high stability and a rich hydrogen-bond network. Proton conduction studies revealed that, at 333 K and 98% relative humidity, the proton conductivities (σ) of MOFs 1-3 reached 1.42 × 10-2, 1.02 × 10-2, and 6.82 × 10-3 S cm-1, respectively. Compared to the proton conductivity of the initial ligands, the σ values of the complexes increased by 2 orders of magnitude, with the activation energies decreasing from 0.36 to 0.18 eV for 1, 0.09 eV for 2, and 0.12 eV for 3. An in-depth analysis of the correlation between different metal centers and proton conduction performance indicated that the varying coordination abilities of the metal cations and the water absorption capacities of the frameworks might account for the differences in conductivity. Additionally, the potential of 1 as a supercapacitor electrode material was assessed. 1 exhibited a specific capacitance of 61.13 F g-1 at a current density of 0.5 A g-1, with a capacitance retention of 82.4% after 5000 cycles, making it a promising candidate for energy storage applications.

A Multifunctional Co-Based Metal-Organic Framework as a Platform for Proton Conduction and Ni trophenols Reduction

The design and development of proton conduction materials for clean energy-related applications is obviously important and highly desired but challenging. An ultrastable cobalt-based metal-organic framework Co-MOF, formulated as [Co2(btzip)2(μ2-OH2)] (namely, LCUH-103, H2btzip = 4, 6-bis(triazol-1-yl)-isophthalic acid) had been successfully synthesized via the hydrothermal method. LCUH-103 exhibits a three-dimensional framework and a one-dimensional microporous channel structure with scu topology based on the binuclear metallic cluster {Co2}. LCUH-103 indicated excellent chemical and thermal stability; peculiarly, it can retain its entire framework in acid and alkali solutions with different pH values for 24 h. The excellent stability is a prerequisite for studying its proton conductivity, and its proton conductivity σ can reach up to 1.25 × 10-3 S·cm-1 at 80 °C and 100% relative humidity (RH). In order to enhance its proton conductivity, the proton-conducting material Im@LCUH-103 had been prepared by encapsulating imidazole molecules into the channels of LCUH-103. Im@LCUH-103 indicated an excellent proton conductivity of 3.18 × 10-2 S·cm-1 at 80 °C and 100% RH, which is 1 order of magnitude higher than that of original LCUH-103. The proton conduction mechanism was systematically studied by various detection means and theoretical calculations. Meanwhile, LCUH-103 is also an excellent carrier for palladium nanoparticles (Pd NPs) via a wetness impregnation strategy, and the nitrophenols (4/3/2-NP) reduction in aqueous solution by Pd@LCUH-103 indicated an outstanding conversion efficiency, high rate constant (k), and exceptional cycling stability. Specifically, the k value of 4-NP reduction by Pd@LCUH-103 is superior to many other reported catalysts, and its k value is as high as 1.34 min-1 and the cycling stability can reach up to 6 cycles. Notably, its turnover frequency (TOF) value is nearly 196.88 times more than that of Pd/C (wt 5%) in the reaction, indicating its excellent stability and catalytic activity.

Sequential enhancement of proton conductivity by aliovalent cadmium substitution and post-synthetic esterolysis in a carboxylicate-functionalized indium framework with dimethylaminium templates

A sequential improving strategy has devised and implemented on a 3D open framework In-BQ showing 2D intersected channels filled by dimethylamine and its protonated cation constructed by −COOCH3 functionalized anilicate...