An electrochromic redox flow battery based on an iron-complex posolyte for smart window applications

This work presents a novel electrochromic redox flow battery (RFB) based on an iron-phenanthroline complex for smart window applications. To address the limited cycling stability of previously reported phenanthroline-iron complexes, a series of 4,7-disubstituted-1,10-phenanthroline ligands with methyl (-CH₃), methoxy (-OCH₃), and hydroxymethyl (-CH₂OH) groups was designed and synthesized. Coordination of these ligands with iron yielded corresponding complexes, among which the 4,7-bis(hydroxymethyl)-1,10-phenanthroline iron complex (Fe(Bhmphen)₃) exhibited superior electrochemical performance. Energy-saving evaluations using a simulated house model showed that circulating the electrolyte could significantly reduce indoor temperature rise under simulated sunlight, translating to potential electrical energy savings. This work demonstrates a successful strategy of ligand engineering to enhance iron-complex stability for RFBs and presents a dual-functional device that combines energy storage with adaptive solar heat management, offering a promising approach for developing low-cost, multifunctional smart energy systems. A novel electrochromic RFB based on a Fe(Bhmphen) 3 iron complex is developed. Beyond energy storage, the intrinsic and reversible electrochromic property of the Fe(Bhmphen) 3 posolyte was successfully leveraged to create a dual-functional device: a smart window integrated with a flow battery. This device not only stores electrical energy but also visually indicates its state of charge through clear color changes and actively modulates solar heat gain, demonstrating tangible energy-saving potential. • A novel electrochromic RFB based on a Fe(Bhmphen) 3 iron complex is developed. • Ligand engineering enhances iron-complex stability and extends battery cycle life. • Fe(Bhmphen)₃ posolyte is successfully leveraged to create a dual-functional device. • Dual-functional device integrates energy storage with smart window. • Electrochromic posolyte achieves high optical contrast and coloration efficiency.