Selective Formation of Polyaniline Confined in the Nanopores of a Metal–Organic Framework for Supercapacitors

In this study, a strategy that can result in the polyaniline (PANI) solely confined within the nanopores of a metal–organic framework (MOF) without forming obvious bulk PANI between MOF crystals is developed. A water-stable zirconium-based MOF, UiO-66-NH₂, is selected as the MOF material. The polymerization of aniline is initiated in the acidic suspension of UiO-66-NH₂ nanocrystals in the presence of excess poly(sodium 4-styrenesulfonate) (PSS). Since the pore size of UiO-66-NH₂ is too small to enable the insertion of the bulky PSS, the quick formation of pore-confined solid PANI and the slower formation of well dispersed PANI:PSS occur within the MOF crystals and in the bulk solution, respectively. By taking advantage of the resulting homogeneous PANI:PSS polymer solution, the bulk PANI:PSS can be removed from the PANI/UiO-66-NH₂ solid by successive washing the sample with fresh acidic solutions through centrifugation. As this is the first time reporting the PANI solely confined in the pores of a MOF, as a demonstration, the obtained PANI/UiO-66-NH₂ composite material is applied as the electrode material for supercapacitors. The PANI/UiO-66-NH₂ thin films exhibit a pseudocapacitive electrochemical characteristic, and their resulting electrochemical activity and charge-storage capacities are remarkably higher than those of the bulk PANI thin films.