Artificial α-amino acid based on cysteine grafted natural aloe-emodin for aqueous organic redox flow batteries

Natural redox-active anthraquinone derivatives possess promising attributes for applications in aqueous organic redox flow batteries (AORFBs) due to their environmental friendliness and abundant sources. However, their limited aqueous solubility and electrochemical stability have posed significant challenges to their practical utilization. Herein, inspired by click chemistry, we report the synthesis of an artificial α-amino acid derived from cysteine-functionalized natural aloe-emodin (namely Cys-AE), which exhibits good water-solubility and redox-reversibility, particularly suited for alkaline AORFBs. The bio-inspired Cys-AE molecule exhibits a threefold increase in aqueous solubility compared to pristine aloe-emodin. Furthermore, the AORFB based Cys-AE negolyte with an electron concentration of 1.0 M demonstrates a low capacity fade rate of 0.000948% cycle^-¹ (equivalent to 0.0438% day^-¹) during 592 cycles, significantly outperforming the AORFB based on pristine aloe-emodin (0.00446% cycle^-¹, or 0.908% day^-¹) during 1564 cycles. Our investigation incorporates time-dependent density functional theory (TDDFT) simulations and detailed spectroscopic analyses reveal the essential role played by the asymmetric distribution of multiple solubilizing groups in enhancing the aqueous solubility and cycling stability of Cys-AE. This study highlights the potential of nature-inspired molecular engineering strategies in creating and crafting redox-reversible organic species poised to revolutionize large-scale and sustainable energy storage applications.