Does the Mode of Metal-Organic Framework/Electrode Adhesion Determine Rates for Redox-Hopping-Based Charge Transport within Thin-Film Metal-Organic Frameworks?

Imparting electrical conductivity to metal-organic frameworks (MOFs) can render these otherwise insulating materials functional for applications such as electrical energy storage and release, electrochemical catalysis, and conductivity-based chemical sensing. Earlier computational and experimental studies of electrode-supported thin films of the structurally anisotropic MOF, NU-1000, indicated that its redox-hopping-based electrical conductivity is also anisotropic: electronic charges can move up to ∼3000 times faster in one direction than in others. To assess the anisotropy, the experiments took advantage of the following: (a) conductivity is most easily measured in the direction normal to the supporting-electrode surface, (b) directly solvothermally grown MOF crystallites are oriented with the crystallographic c-axis, nearly normal to the electrode, and (c) electrophoretically deposited crystallites are oriented mainly with the a, b crystallographic plane normal to the electrode. While the agreement between the experiment and theory for the degree of anisotropy of redox conductivity along the c direction vs through the a, b-plane is gratifying, we wondered if instead differences in the mode of attachment and adhesion of the MOF to the electrode might be responsible for the large apparent differences in conductivity. Here, we have evaluated the apparent conductivity of electrode-supported MOF-525, a porous compound that offers identical internal bonding connectivity and compositional periodicity in the crystallographic a versus b versus c direction and thus should display isotropic redox conductivity. We compared the behavior of crystalline films of MOF-525 directly solvothermally grown on electrodes to the ones deposited on electrodes electrophoretically. The experiments point to agreement within about a factor of two, with the remaining small difference likely arising mainly from differences in MOF/electrode contact area. Finally, the experiments revealed much smaller apparent conductivities for redox hopping through MOF-525 than through NU-1000. This finding is broadly consistent with previous computational findings that emphasize the role of linker-to-linker electronic coupling. It is also consistent with a topology-driven need for hopping-based charge transport to follow a more circuitous molecular-level path through MOF-525 than through NU-1000. The combined results have implications for the design and/or selection of MOFs for electrocatalysis and related applications.