Novel redox-active organic molecules for flow batteries exhibit stable cycling performance

Organic redox-active molecules (ORAMs) are abundant and diverse, offering great potential for cost-effective and sustainable energy storage, especially in aqueous organic flow batteries (AOFBs). However, it is essential to ensure the stability of ORAMs during the charging and discharging process, as side reactions can deactivate them and eliminate their redox activity. Air stability remains a challenge for many ORAMs, complicating their practical application.

Recently, a research group led by Professor Li Xianfeng and Professor Zhang Changkun from the Dalian Institute of Chemical Physics (DlCP), Chinese Academy of Sciences (CAS) developed novel naphthalene derivatives with active hydroxyls and dimethylamine scaffolds that were stable in air and served as effective catholytes for AOFBs. This study, published in Nature and sustainabilitydemonstrates that these new ORAMs can achieve long-term stable cycling even under air-atmosphere conditions.

ORAMs face some instability and high cost, especially when used without inert gas protection. This can lead to irreversible capacity loss and reduced battery life.

In this study, researchers synthesized active naphthalene derivatives using a scalable approach combining in situ chemical and electrochemical methods. This approach simplified the purification process and significantly reduced the cost of molecular synthesis.

In addition, the researchers demonstrated specific structural changes in the naphthalene derivatives during the electrochemical process. The prepared naphthalene derivatives exhibit a multisubstituted structure with hydrophilic alkylamine scaffolds, which not only protect against potential side reactions but also improve their solubility in aqueous electrolytes.

The 1.5 mol/L naphthalene-based AOFB exhibited stable cycling performance for 850 cycles (approximately 40 days) with a capacity of 50 Ah L^-1. It is remarkable to note that even with continuous air flow through the catholyte, the naphthalene-based AOFB was able to operate smoothly for about 600 cycles (about 22 days) without loss of capacity and efficiency. This demonstrated that the naphthalene-based catholyte had excellent air stability.

The researchers also scaled up the preparation of naphthalene derivatives to the kilogram scale (5 kg per jar). Pilot-scale batteries containing these naphthalene derivatives achieved an average system capacity of about 330 Ah. They showed remarkable cycling stability over 270 cycles (about 27 days), with a capacity retention of 99.95% per cycle.

“This study is expected to open a new field in the design of air-stable molecular technologies for sustainable and air-stable electrochemical energy storage,” Professor Li said.