A Low-Potential and Stable Bis-Dimethylamino-Substituted Anthraquinone for pH-Neutral Aqueous Redox Flow Batteries

Quinone-based aqueous redox flow batteries (RFBs) have drawn much attention due to their high safety, two-electron involvement, rapid reaction kinetics, property tunability, and potentially low cost. RFBs operating in neutral solution feature a wide electrochemical window of around 2.5 V, which provides much more space for the design of redox-active materials (RAMs) to realize a high voltage. However, it is still challenging to achieve low potential in the neutral condition for quinone-based RAMs, owing to inherently pH-dependent behaviors and deep LUMO (the lowest unoccupied molecular orbital) energy level. Herein, we report three low-potential quinone-based RAMs (1,4-BDPAQCl(2), 1,5-BDPAQCl(2), and 1,8-BDPAQCl(2)) by bis-dimethylamino substitution. The half-wave potential of the quinones in 0.5 M KCl is approximately -0.55 to -0.57 V versus a normal hydrogen electrode. The low potential is ascribed to the introduced functional groups with two effects. First, the intramolecular hydrogen bonds formed between C=O and H-N can weaken the association between protons and dianion Q(2-), resulting in a favorable distribution of products. Second, the functional groups can effectively increase the LUMO over 0.22 eV, compared with anthraquinone. Paired with Fe(glycine)(2)Cl-2, the theoretical open-circuit voltage of full RFBs is achieved at 1.27-1.29 V. We test full batteries using these quinones as negative RAMs at a lower concentration (0.1 M). The results show that 1,8-BDPAQCl(2) displays stability during 300 charge-discharge cycles. In contrast, the other two quinones exhibit poor cycling stability due to side reactions. We further execute a higher concentration (0.4 M) for 1,8-BDPAQCl(2). The cycling stability of the quinone-iron RFBs is outstanding, with 0.048 % capacity decay per cycle and 0.88 % capacity decay per day. Our finding offers a feasible strategy to design low-potential quinone molecules for the neutral RFBs.