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

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.

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.

New isoreticular phosphonate MOFs based on a tetratopic linker

The tetratopic linker 1,1,2,2-tetrakis(4-phosphonophenyl)ethylene (H₈TPPE) was used to synthesize the three new porous metal-organic frameworks of composition [M₂(H₂O)₂(H₂TPPE)]·xH₂O (M = Al³⁺, Ga³⁺, Fe³⁺), denoted as M-CAU-53 under hydrothermal reaction conditions, using the corresponding metal nitrates as starting materials. The crystal structures of the compounds were determined ab initio from powder X-ray diffraction data, revealing small structural differences. Proton conductivity measurements were carried out, indicating different conductivity mechanisms. The differences in proton conductivity could be linked to the individual structures. In addition, a thorough characterization via thermogravimetry, elemental analysis, IR-spectroscopy as well as N₂- and H₂O-sorption is given.

Humidity-Mediated Anisotropic Proton Conductivity through the 1D Channels of Co-MOF-74

Large Co-MOF-74 crystals of a few hundred micrometers were prepared by solvothermal synthesis, and their structure and morphology were characterized by scanning electron microscopy (SEM), IR, and Raman spectroscopy. The hydrothermal stability of the material up to 60 °C at 93% relative humidity was verified by temperature-dependent XRD. Proton conductivity was studied by impedance spectroscopy, using a single crystal. By varying the relative humidity (70-95%), temperature (21-60 °C), and orientation of the crystal relative to the electrical potential, it was found that proton conduction occurs predominantly through the linear, unidirectional (1D) micropore channels of Co-MOF-74, and that water molecules inside the channels are responsible for the proton mobility by a Grotthuss-type mechanism.

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.

Single-crystal-to-single-crystal transformation and proton conductivity of three hydrogen-bonded organic frameworks

In this communication, we report the SCSC transformation and proton conductivity of three H-bonded organic frameworks. The results show that H-bonded systems can improve their proton conductivity by uptaking water molecules based on the adaptability.