Exploring the optimal post-treatment 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 limit their practical applications in energy storage devices. To solve these issues, different post-treatment methods have been applied to MOFs to obtain their derivatives, which are expected to exhibit unique porous structures, fascinating morphologies, different chemical compositions, improved conductivity and stability as well as fascinating electrochemical behaviors. Nevertheless, to the best of our knowledge, systematic investigations on the effects of different post-treatment methods on the electrochemical behaviors of MOF derivatives have never been reported. Herein, we synthesized a series of new monometallic and bimetallic Ni/Co-MOFs with varied ratios of Ni to Co ions through the self-assembly of metal ions and terephthalic acid (BDC). Four different types of post-treatment methods, namely, sulfidation, carbonization, oxidation, and hydroxylation, were applied in the bimetallic Ni/Co-MOF with a 2 : 1 ratio of Ni to Co (Ni₂Co₁-MOF) due to its best electrochemical behavior among these MOF precursors, and the generated MOF derivatives were named as Ni₂Co₁–S, Ni₂Co₁–C, Ni₂Co₁–O and Ni₂Co₁–OH, respectively. The obtained optimized Ni₂Co₁–S-140-6 electrode showed the highest specific capacitance (1500 F g⁻¹ at 1 A g⁻¹), the best conductivity (R_s = 2.38 Ω), excellent rate capability (73.3%) and the highest cycle stability (88.2% retention after 5000 cycles) compared to those of Ni₂Co₁–C, Ni₂Co₁–O and Ni₂Co₁–OH electrodes, demonstrating that sulfidation was the best post-treatment strategy. Moreover, an aqueous asymmetric supercapacitor (A-ASC) assembled with the cathode of Ni₂Co₁–S-140-6 and the anode of PPy and in situ grown on acid-etched carbon cloth (AECC) exhibited a wide voltage window (1.7 V), competitive energy density of 147 W h kg⁻¹ at a power density of 845 W kg⁻¹ and ideal long-term stability with a specific capacitance retention of 75.9% after 5000 cycles at 10 A g⁻¹. This work offers full view of the post-synthetic strategies for MOFs to develop high-performance electrochemical energy storage devices.