Mesopores in Metal-Organic Frameworks Rendering Enhanced Rate Capability of Pore-Confined Polyaniline for...

Polyaniline (PANI) has been extensively studied as a pseudocapacitive material for electrochemical energy storage, and the facile mass transfer of counterions in PANI during the electrochemical process is crucial to achieve the excellent rate capability of the corresponding supercapacitor. Nanoporous materials with rigid and interconnected pores are thus considered as attractive scaffolds of PANI to facilitate mass transfer. Herein, soft-template-assisted approaches are employed to synthesize a water-stable zirconium-based metal-organic framework (MOF), UiO-66-NH₂, with distinct hierarchical pore sizes. Three kinds of UiO-66-NH₂ nanocrystals with almost the same microporosity and specific surface area, but possessing large mesopores (15 nm), small mesopores (5-6 nm) and no mesopores, respectively, are synthesized. These MOFs are employed as additives during the in situ polymerization of aniline to synthesize a series of MOF-PANI nanocomposites with tunable porosity and electrical conductivity. Electrochemical performances of all MOF-confined PANI are tested in acidic aqueous solutions, where the MOF is chemically stable. PANI confined in the MOF with large mesopores can achieve a specific capacitance of 421 F/g at 0.25 mA/cm², outperforming the pristine PANI (378 F/g). In addition, all the three PANI confined within MOFs exhibit better rate capability compared to the pristine PANI, and the trend of rate capability follows that of the pore sizes. Findings here highlight the importance of the hierarchical porosity and large mesopores in MOFs when MOFs are employed in electrochemical applications, and shed light on utilizing such a mesopore-promoting effect in various MOFs and MOF-based composites for a range of energy-storage and electrocatalytic applications.