Exploring the optimal posttreatment strategy for boosting the electrochemical performances of a new bimetal–organic framework-based supercapacitor

Abstract

Metal–organic frameworks (MOFs) have attracted great interest owing to their potential application in electrochemical energy storage. However, the poor conductivity, low structural stability and specific capacitance of pristine MOFs are their critical limitations for practical applications in energy storage devices. To solve these issues, different methods of posttreatments have been applied for MOFs to get their derivations, which are expected to exhibit unique porous structures, fascinating morphology and different chemical compositions, conductivity, stability as well as fasinating electrochemical behaviors. Nevertheless, to the best of our knowledge, the systematic investigations on the effects of different posttreatment methods on electrochemical behaviors of MOF derivatives have never been reported. Here, we have synthesized a series of new monometallic and bimetallic Ni/Co-MOF with varied ratio of Ni to Co ions through self-assembly of metal ions and terephthalic acid (BDC). Four different types of posttreatment methods including sulfidation, carbonization, oxidation, and hydroxylation have been applied in the bimetallic Ni/Co-MOF with the ratio of Ni to Co equaling to 2:1 (Ni2Co1-MOF) due to its best electrochemical behavior among these MOF precursors, and the generated MOF derivatives are named as Ni2Co1-S, Ni2Co1-C, Ni2Co1-O and Ni2Co1-OH, respectively. The obtained optimized Ni2Co1-S-140-6 electrode shows the highest specific capacitance (1500 F g−1 at 1 A g−1), the best conductivity (Rs = 2.38Ω), excellent rate capability (73.3%) and the highest cycle stability (88.2% retention after 5000 cycles) in relation to that of Ni2Co1-C, Ni2Co1-O and Ni2Co1-OH electrodes, demonstrating that sulfidation is the best posttreatment strategy. Moreover, an aqueous asymmetric supercapacitor (A-ASC), assembled by the cathode of Ni2Co1-S-140-6 and the anode of PPy in-situ growing on acid-etched carbon cloth (AECC), exhibits a wide voltage window (1.7V), competitive energy density of 147 Wh kg–1 at the power density of 845 W kg–1 and ideal long-term stability with specific capacitance retention of 75.9% after 5000 cycles at 10 A g–1. This work offers full view of postsynthetic strategies of MOFs for developing high-performance electrochemical energy storage devices.