Mesopore matters: Hierarchically porous metal–organic framework with facilitated redox-hopping and electrochemical behaviors toward the use in supercapacitors

A chemically robust zirconium-based metal–organic framework (MOF, UiO-66-NH₂) possessing both micropores from its crystalline structure and large mesopores is synthesized by employing the soft-template-assisted synthesis, and redox-active manganese ions are post-synthetically coordinated on the nodes of MOF where missing-linker defects are located. Regular UiO-66-NH₂ without mesopores and its manganese-installed material with a similar manganese loading per node are also synthesized. Crystallinity, porosity and morphologies of these materials are characterized. Thereafter, thin films of these MOFs are fabricated on conducting electrodes to measure their electrochemical behaviors in aqueous Na₂SO₄-based electrolytes, where the MOF is found structurally robust. The redox activity of immobilized manganese sites, the electrochemically addressable fraction of manganese sites in the MOF thin film, as well as the apparent diffusivity for the redox-based charge-hopping process occurring in the MOF are examined for both manganese-installed MOFs with and without mesopores. The redox-active MOFs are also blended with carbon nanotubes to form composites, aiming for the use in aqueous supercapacitors, and galvanostatic tests are performed to quantify the capacitance. Compared to the manganese-installed MOF without the hierarchical porosity, its counterpart possessing large mesopores can render more manganese sites electrochemically addressable and achieve an around 20-fold faster ion-coupled charge-hopping phenomenon occurring in the MOF thin film, resulting in a better performance when it is employed in supercapacitors. Findings here highlight the importance of creating large mesopores in redox-active MOFs in order to accelerate the mass transfer of ionic species within MOFs during electrochemical reactions and shed light on utilizing such hierarchically porous and stable MOFs in numerous electrochemical applications.