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Viologens, with their unique redox‐active properties, have garnered increasing attention in the development of electrochromic devices. This review focuses on key strategies in molecular design, synthesis methodologies, and tailored functionalities that have propelled the field forward from 2019 to the present. The convergence of these approaches has led to the construction of viologen‐based electrochromic materials with enhanced performance, stability, and responsiveness. The review not only examines the current state of the field but also explores promising outlooks and opportunities, including tailored applications, environmental sustainability, and integration with emerging technologies. The synergy between molecular engineering and viologen‐based electrochromic materials holds significant promise, shaping the future of dynamic, responsive materials in diverse technological domains. This review serves as a valuable resource for researchers, offering insights into recent breakthroughs and inspiring further advancements in this rapidly evolving field. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In redox flow batteries (RFBs), energy density mainly depends on the volume and concentration of active species in electrolyte. However, the poor solubility of organic molecules limits the further development of aqueous organic redox flow batteries (AORFBs). Herein, the viologen core structure is modified with asymmetric functional groups such as propyl and triethyl ammonium propyl for enhancing the solubility up to 1.82 m in aqueous medium. Further, the modified viologen structure lowers the redox potential (−0.58 V vs Ag/AgCl) and significantly reduces radical dimerization. Interestingly, synthesized [1‐propyl‐1'‐(3‐triethylammonio)propyl‐4,4'‐bipyridinium tribromide] (PV3+) coupled with bromide/bromine (2Br−/Br2) redox species shows cell voltage (1.66 V). The tested PV3+/PV2+•||2Br−/Br2 RFB having Nafion‐117 membrane exhibits a round‐trip efficiency of 99% over 100 cycles. The self‐discharge behavior of PV3+/PV2+•||2Br−/Br2 RFB system retains 85% of the cell voltage over 20 h, indicating the lowest permeability of bromine in RFB. Thus, the present approach of viologen modification with asymmetric functionalities drastically improves the RFB performance in terms of energy density and cycle life of the system. [ABSTRACT FROM AUTHOR]