Revealing the effect of cobalt content and ligand exchange in the bimetallic Ni–Co MOF for stable supercapacitors with high energy density

Abstract

Metal-organic frameworks (MOF) exhibit considerable potential as electrode materials for supercapacitors. Nevertheless, the electrochemical performance of the MOF is impeded by its low electrical conductivity and chemical stability. In this study, we investigate the impact of cobalt composition and ligand exchange on the performance of bimetallic NiCo-MOF to unlock its optimal performance. Our results reveal that integrating cobalt affects the abundance of Ni3+ and Co2+, which serve as electrochemical active species. Additionally, ligand exchange with a phosphate group leads to further modifications in both abundance and surface area. The result indicates that a Ni/Co ratio of 10:1 in NiCo-MOF exhibits superior performance (437 C g−1 at 0.5 A g−1), surpassing the performance of other ratios. Furthermore, the process of ligand exchange in NiCo-MOF with a Ni/Co ratio of 10:1 yields an even higher capacity of 522 C g−1 at 0.5 A g−1, with an energy density of 22 Wh kg−1 at 363 W kg−1 when assembled in an asymmetric supercapacitor cell. The supercapacitor cell demonstrates a remarkable capacity retention of 99 % at 5 A g−1 over 5000 cycles. This study provides insight into the pivotal role of cobalt composition and ligand exchange in improving the electrochemical performance of NiCo-MOF.