Flow batteries (FBs) attracted widespread attention in large-scale energy storage due to their high security and effectiveness. Among them, aqueous organic flow batteries (AOFBs) which utilize the structural tunable and eco-friendly redox-active molecules were getting more attention.

However, most organic molecules in AOFBs are prone to be oxidized especially by air, resulting in irreversible capacity decay, hinder their further applications.

Recently, a research group led by Prof. LI Xianfeng and Prof. ZHANG Changkun from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) developed stable kW-scale AOFB stacks with high-performance organic redox-active molecules.

"We revealed the stabilization mechanism of one redox-active molecule, methylene blue (MB), and demonstrated that the stabilizations of MB, especially in both intermediate free radical state and reduced state, played a vital role in improving redox reversibility and air-stability," said Prof. LI. "Our study may provide an important reference for the AOFB practical application," Li added.

This study was published in Energy & Environmental Science on Dec. 1.

Schematic diagram of the V-MB FB stack, in situ pseudo-2D 1H NMR spectra of MB electrolyte during the electrochemical cycling, ex-situ EPR spectra of of MB electrolyte, the stack photo, and cycling performance (Image by ZHANG Changkun and ZHANG Yonghui)

The researchers optimized electrolyte composition by tuning the interaction between different components in the electrolyte, thereby increasing the battery capacity.

They monitored MB by in-situ nuclear magnetic resonance (NMR) and found that free radicals were generated by comproportionating reactions between oxidized and reduced states in the electrolyte. What's more, they demonstrated that oxidation resistance of MB radicals and reduced states played an important role in improving redox reversibility and air-stability by combining ex-situ electron paramagnetic resonance (EPR) with theoretical calculation.

To further verify the feasibility of MB molecule in AOFBs, they assembled ten units of 1000 cm² AOFBs stacks. They have obtained the stacks with a discharge power of over 1 kW and a stable cycling capacity for 32 days.

Furthermore, they evaluated the accelerated stability of MB electrolyte at 70℃ and the degradation process of MB at high temperatures.

The above work was supported by the China Natural Science Foundation, the International Partnership Program of the Chinese Academy of Sciences, and the National Key R&D Program. (Text by ZHANG Changkun and ZHANG Yonghui)