Bimetallic metal-organic framework: an efficient electrocatalyst for bromine-based flow batteries

Metal-organic frameworks (MOFs) have recently emerged as an attractive class of porous materials for electrochemical energy storage applications owing to their high surface area, active metal sites, and easily controllable pore size and structure. Herein, a bimetallic metal-organic framework-modified graphite felt (NiCo-MOF@GF) was prepared by a simple hydrothermal method followed by thermal treatment. The prepared modified NiCo-MOF@GF was characterized using powder X-ray diffraction, FT-IR, and Raman spectroscopies. Morphological features were characterized using field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). NiCo-MOF@GF was tested as a positive electrode in a zinc/bromine redox flow battery (ZBRFB), and it showed excellent performance with a coulombic efficiency of 93.5%, voltaic efficiency of 91.8%, and energy efficiency of 85.9% at a current density of 20 mA cm-2. ZBRFB assembled with NiCo-MOF@GF exhibited good cycling stability with a coulombic efficiency of 97.1%, voltaic efficiency of 81.1%, and energy efficiency of 78.5% for about 200 cycles. Very interestingly, the newly designed NiCo-MOF as an electrocatalyst enhances the reversibility of bromine/bromide redox reactions to a large extent, increasing the voltaic efficiency, which reflects the overall energy efficiency. Further, the intact stability of the electrocatalytic behavior of NiCo-MOF was evidenced from the postmortem analysis of NiCo-MOF@GF with no drastic change in the architecture of the felt. Therefore, NiCo-MOF@GF will be an efficient electrocatalyst for improving the 2Br-/Br2 redox couple kinetics in bromine-based flow batteries. A newly designed NiCo-MOF@GF electrocatalyst greatly enhanced the kinetics of a Br2/2Br- redox couple, resulting in a higher voltaic efficiency (85.1%) and an overall improvement in the performance of Zn-Br2 flow cells.