Permanent porosity and role of sulfonate groups in coordination networks constructed from a new polyfunctional phosphonato-sulfonate linker molecule

Enhanced proton conductivity by guest molecule exchange in an acylamide-functionalized metal-organic framework

Metal-organic frameworks (MOFs) as types of proton conductive materials have attracted much attention. Here, an acylamide-functionalized 3D MOF, [Ni₃(TPBTC)₂(stp)₂(H₂O)₄]·2DMA·32H₂O, has been successfully constructed via combining Ni(NO₃)₂, TPBTC (TPBTC = benzene-1,3,5-tricarboxylic acid tris-pyridin-4-ylamide) and 2-H₂stp (2-H₂stp = 2-sulfoterephthalic acid monosodium salt) under solvothermal conditions. Single-crystal X-ray diffraction revealed that there are uncoordinated guest DMA molecules in the pores of the compound. On removal of guest DMA molecules, the proton conductivity of the compound increased to 2.25 × 10^-3 S cm^-1 at 80 °C and 98% RH which is about 110 times that of the original material. It is hoped that this work can provide essential insight for designing and obtaining improved crystalline-state proton conducting materials by considering the influences of guest molecules on proton conduction properties of porous materials.

The role of sulfonate groups and hydrogen bonding in the proton conductivity of two coordination networks

The proton conductivity of two coordination networks, [Mg(H₂O)₂(H₃L)]·H₂O and [Pb₂(HL)]·H₂O (H₅L = (H₂O₃PCH₂)₂-NCH₂-C₆H₄-SO₃H), is investigated by AC impedance spectroscopy. Both materials contain the same phosphonato-sulfonate linker molecule, but have clearly different crystal structures, which has a strong effect on proton conductivity. In the Mg-based coordination network, dangling sulfonate groups are part of an extended hydrogen bonding network, facilitating a "proton hopping" with low activation energy; the material shows a moderate proton conductivity. In the Pb-based metal-organic framework, in contrast, no extended hydrogen bonding occurs, as the sulfonate groups coordinate to Pb²⁺, without forming hydrogen bonds; the proton conductivity is much lower in this material.

Colodrero RM, Demadis KD. Exploiting the Multifunctionality of M2+/Imidazole-Etidronates for Proton Conductivity (Zn2+) and Electrocatalysis (Co2+, Ni2+) toward the HER, OER, and ORR

This work deals with the synthesis and characterization of one-dimensional (1D) imidazole-containing etidronates, [M₂(ETID)(Im)₃]·nH₂O (M = Co²⁺ and Ni²⁺; n = 0, 1, 3) and [Zn₂(ETID)₂(H₂O)₂](Im)₂, as well as the corresponding Co²⁺/Ni²⁺ solid solutions, to evaluate their properties as multipurpose materials for energy conversion processes. Depending on the water content, metal ions in the isostructural Co²⁺ and Ni²⁺ derivatives are octahedrally coordinated (n = 3) or consist of octahedral together with dimeric trigonal bipyramidal (n = 1) or square pyramidal (n = 0) environments. The imidazole molecule acts as a ligand (Co²⁺, Ni²⁺ derivatives) or charge-compensating protonated species (Zn²⁺ derivative). For the latter, the proton conductivity is determined to be ∼6 × 10^-4 S·cm^-1 at 80 °C and 95% relative humidity (RH). By pyrolyzing in 5%H₂-Ar at 700-850 °C, core-shell electrocatalysts consisting of Co²⁺-, Ni²⁺-phosphides or Co²⁺/Ni²⁺-phosphide solid solution particles embedded in a N-doped carbon graphitic matrix are obtained, which exhibit improved catalytic performances compared to the non-N-doped carbon materials. Co²⁺ phosphides consist of CoP and Co₂P in variable proportions according to the used precursor and pyrolytic conditions. However, the Ni²⁺ phosphide is composed of Ni₂P exclusively at high temperatures. Exploration of the electrochemical activity of these metal phosphides toward the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) reveals that the anhydrous Co₂(ETID)(Im)₃ pyrolyzed at 800 °C (CoP/Co₂P = 80/20 wt %) is the most active trifunctional electrocatalyst, with good integrated capabilities as an anode for overall water splitting (cell voltage of 1.61 V) and potential application in Zn-air batteries. This solid also displays a moderate activity for the HER with an overpotential of 156 mV and a Tafel slope of 79.7 mV·dec^-1 in 0.5 M H₂SO₄. Ni²⁺- and Co²⁺/Ni²⁺-phosphide solid solutions show lower electrochemical performances, which are correlated with the formation of less active crystalline phases.

THE SYNTHESIS AND PROPERTIES OF A SODIUM SUPRAMOLECULAR CRYSTAL NETWORK CONSTRUCTED WITH FUNCTIONAL PYRAZINE SULFONIC ACID

Sulfonic acid groups with C_3ν symmetry can coordinate with metal ions to produce multidimensional structures due to their flexible coordination modes. They can also penetrate into supramolecular structures through an intriguing bridging pattern and weak interactions. Herein, a novel heterocyclic sodium sulfonate supramolecular structure, namely [Na(Pyr-SO₃)(H₂O)]_n where Pyr-SO₃H is pyrazine sulfonic acid, is synthesized by utilizing NaBF₄ to coordinate with the P–SO₃H ligand through the solvent evaporation method. The single crystal X-ray diffraction (XRD) data indicate that the as-formed structure belongs to the Pbca space group. Additional properties are characterized by powder XRD, thermal analysis, and solid-state fluorescence. In particular, the introduction of a soft alkali metal ion can coordinate with the oxygen atom of the sulfonate ligand and form a Na–O bridging configuration that can not only significantly improve the pyrazine sulfonic acid ligand coordination ability, but also provide a reference for the extended study of functional sulfonate polymers in the future.

Proton Conduction in a Single Crystal of a Phosphonato-Sulfonate-Based Coordination Polymer: Mechanistic Insight

The proton conduction properties of a phosphonato‐sulfonate‐based coordination polymer are studied by impedance spectroscopy using a single crystal specimen. Two distinct conduction mechanisms are identified. Water‐mediated conductance along the crystal surface occurs by mass transport, as evidenced by a high activation energy (0.54 eV). In addition, intrinsic conduction by proton ′hopping′ through the interior of the crystal with a low activation energy (0.31 eV) is observed. This latter conduction is anisotropic with respect to the crystal structure and seems to occur through a channel along the c axis of the orthorhombic crystal. Proton conduction is assumed to be mediated by sulfonate groups and non‐coordinating water molecules that are part of the crystal structure.