Proton conductivity resulting from different triazole-based ligands in two new bifunctional decavanadates

Triazole-rich 3D metal-organic framework incorporated solid electrolytes for superior proton conductivity and durability in fuel cells

Insufficient proton conductivity and oxidative stability of sulfonated hydrocarbons hinder their applicability as proton exchange membrane electrolytes in fuel cells. In this regard, fabrication of proton conducting mixed-matrix membranes (PC-MMMs) can be a superior approach to obtain desirable properties. In this work, a triazole ligand (1H-1,2,4 triazole) was coordinated to a zinc metal node to create a 3D metal-organic framework (MOF) and incorporated as an additive in a sulfonated poly(ether ether ketone) matrix at 1, 3, and 5 weight percentage to fabricate PC-MMMs by the casting process. Several characterization tools such as electrochemical impedance spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy were used to characterise these membranes and study their potential application as electrolyte(s) in PEMFCs (proton exchange membrane fuel cells). Membranes were also tested for water uptake, ion-exchange capacity and oxidative stability in Fenton's reagent. The performance of the polymeric composite membrane containing a 3 wt% MOF was then assessed in a H2/O2 single cell as it demonstrated the highest proton conductivity of 0.04 S cm-1 among all the compositions and a maximum current density of 1191 mA cm-2. The membrane was also subjected to an OCV hold test for 12 hours to study the chemical durability over a period of time. This report establishes that the inclusion of a triazole based MOF enhances the proton conductivity, performance, and thermal and chemical durability of composite membranes which can be considered as a promising electrolyte material at intermediate temperatures after a proper optimisation of different cell parameters.

{Mo4}-directed structural evolution of highly reduced molybdenum red clusters for efficient proton conduction

A series of highly reduced Mo red clusters {Mo28} (1), {Mo30} (2), and {Mo40} (3) are synthesized from the rational assembly of planar {MoV4} building blocks and employed for proton conduction. 3 exhibits the best conductivity of 7.56 × 10-3 S cm-1 under optimal conditions due to the most efficient hydrogen-bonding network.

Synthesis of Phosphovanadate-Based Porous Inorganic Frameworks with High Proton Conductivity

Materials with high proton conductivity have attracted significant attention for their wide-ranging applications in proton exchange membrane fuel cells. However, the design of new and efficient porous proton-conducting materials remains a challenging task. The structure-controllable and highly stable metal phosphates can be synthesized into layer or frame networks to provide proton transport capabilities. Herein, we have successfully synthesized three isomorphic metal phosphovanadates, namely, H2(C2H10N2)2[MII(H2O)2(VIVO)8(OH)4(PO4)4(HPO4)4] (C2H8N2 = 1,2-ethylenediamine; M = Co, Ni, and Cu), by the hydrothermal method employing ethylenediamine as a template. These pure inorganic open frameworks exhibit a cavity width ranging from 6.4 to 7.5 Å. Remarkably, the proton conductivity of compounds 1-3 can reach 1 × 10-2 S·cm-1 at 85 °C and 97% relative humidity (RH), and they can remain stable at high temperatures as well as long-term stability. This work provides a novel strategy for the development and design of porous proton-conducting materials.

Functionalization of decavanadate anion by coordination to cobalt(II): Binding to proteins, cytotoxicity, and water oxidation catalysis

A series of five decavanadates (V₁₀) using a simple, one-pot synthesis, adhering to the model template: transition metal ion - decavanadate - ligands:(Hnicotinamide)₂{[Co(H₂O)₃(nicotinamide)₂]₂[μ-V₁₀O₂₈]}.6H₂O (1), {[Co(H₂O)₄(isonicotinamide)₂]₃}V₁₀O₂₈·4H₂O (2), {[Co(H₂O)₄]₂[Co(H₂O)₂(μ-pyrazinamide)₂][μ-V₁₀O₂₈]}·4H₂O (3) {[Co(H₂O)₄(μ-pyrazinamide)]₃.V₁₀O₂₈}·4H₂O (4), and (NH₄)₂{[Ni(H₂O)₄(2-hydroxyethylpyridine)]₂}V₁₀O₂₈·2H₂O (5) was synthesized. X-ray analysis reveals that 1 and 3 are decavanadato complexes, while 2, 4 and 5 are decavanadate complex salts. Moreover, 3 is the first example of a polymeric decavanadato complex, employing direct coordination with the metal center and the organic ligand, in toto. From the solution studies using ⁵¹V NMR spectroscopy, it was decoded that 1 and 3 stay stable in the model buffer solution and aqueous media. Binding to model proteins, cytotoxicity and water oxidation catalysis (WOC) was studied primarily for 1 and 3 and concluded that neither 1 nor 3 have an interaction with the model proteins thaumatin, lysozyme and proteinase K, because of the presence of the organic ligands in the Co(II) center, any further interplay with the proteins was blocked. Cytotoxicity studies reveal that 1 is 40% less toxic (0.05 mM) and 26% less toxic (0.1 mM) than the uncoordinated V₁₀ with human cell lines A549 and HeLa respectively. In WOC, 1 performed superior activity, by evolving 143.37 nmol of O₂ which is 700% (9-fold) increase than the uncoordinated V₁₀.

Dual-Functional Proton-Conducting and pH-Sensing Polymer Membrane Benefiting from a Eu-MOF

Porous metal-organic frameworks (MOFs) have demonstrated a great potential in proton conduction and luminescence sensing due to functionalized nodes, ligands and channels, or pores. Herein, we prepared a hydrothermally stable Eu-MOF that also resisted acid and base using a bifunctional organic ligand containing carboxylic acid groups, which are easily coordinated to Eu ions, and Eu-phobic tetrazolyl groups as potential proton-hopping sites. The hydrogen bond network, which was constructed by the uncoordinated anionic tetrazolium and the coordinated and free water molecules, endowed this Eu-MOF with the highest proton conductivity of 4.45 × 10^-2 S/cm at 373 K and 93% relative humidity. The proton conductivity of the Nafion membrane containing this Eu-MOF increased 1.74 times. More interestingly, the hybrid membrane displayed luminescence pH sensing because the changeable protonation levels of uncoordinated tetrazolium groups along with the pH tuned the emission of embedded Eu-MOFs. Such a dual-functional MOF-based hybrid membrane including proton conduction and pH sensing is reported for the first time, which could open an avenue to the more practical application for functional MOFs.

Three new high-nuclear transition-metal-substituted heteropolytungstates: syntheses, crystal structures, magnetic studies and NLO properties

Three new nickel-cluster-substituted polyoxometalates (POMs), [Ni6(H2O)9(μ3-OH)3(HSiW9O34)]2·12H2O (1), [Ni(en)(H2O)3][Ni6(H2O)3(en)2L(μ3-OH)3(HSiW9O34)]2·9H2O (2) (en = ethylenediamine; L = C7H5O2 = benzoic acid) and [Ni6(L')6(CH3COO)(H2O)3(μ3-OH)3(HPW9O34)]2·9H2O (3) (L' = C5NH5 = pyridine) were successfully isolated under hydro-/solvothermal conditions. 1-3 were structurally characterized by single-crystal XRD, elemental analyses, PXRD, IR, and TGA. Compound 1 mainly consists of a pair of {Ni6SiW9} fragments and some water molecules. Interestingly, the whole structure can be regarded as the connection of {Ni6SiW9} and another unit rotated 180° through Ni-O-W bonds, forming a dimeric structure {Ni6SiW9}2. Compounds 2 and 3 also have an {Ni6XW9}2 (X = Si, P) dimer, but there is a big difference in the connection between {Ni6SiW9} units. On this basis, mono-dentate conjugated organic ligands (benzoic acid (L) and pyridine (L')) were successfully introduced. It is noteworthy that pyridine molecules were first integrated into {Ni6SiW9}-based clusters and 3 features the highest number of organic ligands (six pyridines per Ni6) in the reported {Ni6XW9}-based clusters. The introduction of organic ligands to compounds 2 and 3 can bring about better third-order nonlinear optical properties. Magnetic research indicated the existence of ferromagnetic interactions in 2-3.