Proton Conduction in Organically Templated 3D Open-Framework Vanadium–Nickel Pyrophosphate

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.

Properties and Applications of Metal Phosphates and Pyrophosphates as Proton Conductors

We review the progress in metal phosphate structural chemistry focused on proton conductivity properties and applications. Attention is paid to structure–property relationships, which ultimately determine the potential use of metal phosphates and derivatives in devices relying on proton conduction. The origin of their conducting properties, including both intrinsic and extrinsic conductivity, is rationalized in terms of distinctive structural features and the presence of specific proton carriers or the factors involved in the formation of extended hydrogen-bond networks. To make the exposition of this large class of proton conductor materials more comprehensive, we group/combine metal phosphates by their metal oxidation state, starting with metal (IV) phosphates and pyrophosphates, considering historical rationales and taking into account the accumulated body of knowledge of these compounds. We highlight the main characteristics of super protonic CsH₂PO₄, its applicability, as well as the affordance of its composite derivatives. We finish by discussing relevant structure–conducting property correlations for divalent and trivalent metal phosphates. Overall, emphasis is placed on materials exhibiting outstanding properties for applications as electrolyte components or single electrolytes in Polymer Electrolyte Membrane Fuel Cells and Intermediate Temperature Fuel Cells.

Efficient proton conductivity of a novel 3D open-framework vanadoborate with [V6B20] architectures

A new three-dimensional (3-D) inorganic metal-oxygen network vanadoborate Na3H10[Ni(H2O)2(VO)6(B10O22)2]·NH4·19H2O (1) constructed from lantern-type {(VO)6(B10O22)2} clusters, NaO6 polyhedra and NiO6 octahedra, was successfully synthesized by a hydrothermal method. In the structure, the {V6B20} clusters are linked together through NiO6 octahedral bridges, resulting in 1-D chains along the c-axis. The 1-D chains are further connected by NaO6 polyhedra to give rise to a 3-D open-framework structure. Furthermore, lots of NH4+ and H2O molecules are accommodated in the void of the structure, and may interact with the [V6B20] system via N-HO, O-HO hydrogen bonds, constructing a complex hydrogen-bonding network system. Strikingly, compound 1 exhibited a high proton conductivity of 3.22 × 10-3 S cm-1 at 50 °C under 100% RH with an activation energy of 1.66 eV.

Proton conduction in two hydrogen-bonded supramolecular lanthanide complexes

Two hydrogen-bonded supramolecular lanthanide complexes based on imidazole dicarboxylate show different proton conductivities.

Proton conductivity studies on five isostructural MOFs with different acidity induced by metal cations

Five isostructural MOFs display very different proton conductivities despite the same proton transfer pathway. This difference is caused by the different coordination ability between the metal cations and the ligand.

Ultrahigh Proton Conduction in Two Highly Stable Ferrocenyl Carboxylate Frameworks

Nowadays, although research of proton conductive materials has been extended from traditional sulfonated polymers to novel crystalline solid materials such as MOFs, COFs, and HOFs, research on crystalline ferrocene-based carboxylate materials is very limited. Herein, we selected two hydrogen-bonded and π-π interactions-supported ferrocenyl carboxylate frameworks (FCFs), [FcCO(CH₂)₂COOH] (FCF 1) and [FcCOOH] (FCF 2) (Fc = (η⁵-C₅H₅)Fe(η⁵-C₅H₄)) to fully investigate their water-mediated proton conduction. Their excellent thermal, water, and chemical stabilities were confirmed by the means of thermogravimetric analyses, PXRD, and SEM determinations. The two FCFs indicate temperature- and humidity-dependent proton conductive features. Intriguingly, their ultrahigh proton conductivities are 1.17 × 10^-1 and 1.01 × 10^-2 S/cm, respectively, under 100 °C and 98% RH, which not only are comparable to the commercial Nafion membranes but also rank among the highest performing MOFs, HOFs, and COFs ever described. On the basis of the structural analysis, calculated E_a value, H₂O vapor adsorption, PXRD, and SEM measurements, reasonable conduction mechanisms are highlighted. Our research provides a novel inspiration for finding new high proton conducting crystalline solid materials. Importantly, the outstanding conducting performance of 1 and 2 suggests their, hopefully, potential in fuel cells and related electrochemical fields.