3D MOF conducts both protons and electrons

For the first time, researchers have reported a 3D metal-organic framework (MOF) that can conduct both electrons and protons (J. Am. Chem. Soc. 2024, DOI: 10.1021/jacs.4c13792). Materials mixing electronic and ionic conductivity could increase the energy density of batteries as well as find applications in membranes, sensors, and bioelectronics.

MOFs are porous networks of metal atoms or ions connected by organic linkers, creating a scaffold-like molecular structure. Electron and ion conduction have been individually observed in MOFs, but “coupled ion-electron conductivity in MOFs has been very rarely reported,” says Manuel Souto, lead author of the study and a materials scientist at the Center for Research in Biological Chemistry and Molecular Materials (CiQUS) at the University of Santiago de Compostela.

Until now, the few examples of MOFs that showed this dual conductivity were all two-dimensional materials, which are restricted in terms of structure and charge transport. A 3D MOF is essentially a bunch of 2D MOFs layered on top of one another, but “it’s hard to get the electroactive fragments properly stacked,” Souto says. And “it’s almost impossible to predict the packing and structure” of 3D MOFs, which hinders their rational design.

To pull off the feat, Souto’s team “cleverly” combined electron- and proton-conductive building blocks, according to Natalia B. Shustova, an expert in energy materials at the University of South Carolina who wasn’t involved in the study. The new MOF’s stability relies on tetrathiafulvalene (TTF) fragments and phosphonate groups, which have distinctive ways of conducting electrons and protons, respectively.

TTF rings in each layer of the MOF pile flat on top of one another, forming columns. The rings pack closely enough that the neighboring molecules’ orbitals overlap and interact, enabling electrons to easily “hop” from one TTF unit to another, which results in the observed conductivity, Souto says.

Simultaneously, the MOF conducts protons via a phenomenon known colloquially as proton jumping, Shustova says. To jump, the protons diffuse through a network of hydrogen bonds between the uncoordinated phosphonate groups.

Measuring these various mechanisms of conductivity involved a slew of techniques, Souto says, which were carried out in collaboration with Mircea Dincă’s group at the Massachusetts Institute of Technology.

The observed electron and proton conductivities (7.2 × 10^–6 S/cm and 4.9 × 10^–5 S/cm, respectively) are low compared with those of other MOFs, which reach almost metallic conductivity for electrons and conductivity three orders of magnitude higher for protons. But Shustova stresses that “the breakthrough lies in combining electron and proton conductivity within the same platform, as these represent two entirely distinct pathways.”

Although alternative materials already showcase combined conductivity of electrons and ions, even with better performance, “this serves as a proof of concept to demonstrate the possibilities of MOFs with electroactive linkers,” Souto says. Moreover, MOFs have several significant advantages over other conductive competitors, according to Shustova. “MOFs possess an unprecedentedly high surface area, which is critical for designing porous electrodes, membranes, or sensors,” she says.

Plus, these materials are highly modular, since small structural tweaks to metal centers and organic linkers may modify their properties and provide new applications. Overall, “conductive MOFs, which combine porosity and conductivity, could be critical for the next generation of electrode materials,” Shustova says.