Engineered iron-doped MOF nanosheets: acid-induced lattice strain for enhanced rate performance in asymmetric supercapacitors†

Abstract

The development of electrode materials is the key to realizing efficient energy storage. In order to solve the problems of low conductivity and poor cycling stability of existing metal–organic frameworks (MOFs), element doping and chemical etching strategies are effective strategies. In this work, we propose a strategy to modify the surface of MOFs via Fe doping and HCl etching strategies. The Jahn–Teller effect was induced and the electronic configuration of Co was optimized by doping Fe³⁺ ions with [Fe(CN)₆]³⁻. In addition, HCl etching induces lattice strain, enhances the interaction between Fe and Co, and provides a fast charge transfer rate. This synergistic effect enhances the conductivity of the Co MOF, introduces more electrochemically active sites, and further accelerates the electrochemical reaction kinetics. In particular, the specific capacity of e-Fe-MOF CNs-30 at 1 A g⁻¹ is as high as 1431 C g⁻¹, and the capacity retention rate is 84.2%. Additionally, an e-Fe-MOF CNs-30//AC asymmetric supercapacitor was assembled, which has a high energy density of 83.75 W h kg⁻¹ and a superior cycling stability of about 91.66% after 5000 cycles. The structural design of these MOFs significantly improves the low energy density and cycle life of MOF-based supercapacitors and provides insights into the electronic structure regulation and lattice strain engineering of low conductivity MOF electrodes.