Non-porous two-dimensional conducting metal–organic frameworks with enhanced capacitance

Abstract

The specific performance of two-dimensional conductive metal–organic frameworks (MOFs) in energy storage devices is significantly constrained by the presence of bulky redox-active centers and densely packed interlayers. Herein, we report two semi-conductive MOFs, Fe-MOF and Cr-MOF, using a small aromatic linker, pyrazine (pyz). Both MOFs demonstrated exceptional capacitive properties in an ionic electrolyte. Despite having similar layered AB-stacking geometries and non-porous structures, the single-crystalline Fe-MOF demonstrated weaker redox interactions between Fe²⁺ and pyz nodes, resulting in typical semiconducting properties with a bandgap of ∼1.07 eV. In contrast, the Cr-MOF exhibited a high conductivity, reaching 9.0 mS cm⁻¹ at 350 K. Remarkably, the Fe-MOF electrode delivered a specific capacitance of 436.7 F g⁻¹ at 0.5 A g⁻¹, almost three times higher than that of the Cr-MOF (123.5 F g⁻¹), despite its larger bandgap. Moreover, a high energy density of 98.2 W h kg⁻¹ and excellent cycling stability (retaining 95.3% after 10 000 cycles) have been achieved in the Fe-MOF electrode. In situ experimental analysis together with theoretical calculations revealed that the superior charge storage capability of the Fe-MOF originated from the participation of both cations and anions in the diffusion-controlled charge storage, even with a non-porous structure. This study enhances our understanding of energy storage mechanisms in non-porous conductive MOFs and provides valuable insights for the development of advanced MOF materials for future energy storage applications.