Flow batteries are promising for energy storage due to their high safety, high reliability, long cycle life, and high efficiency.

In order to develop commercial-scale flow batteries for long-duration energy storage, it requires to reduce the cost of flow batteries, especially ion-exchange membranes. However, nearly all of the commercially available membranes are based on perfluorosulfonic acid (PFSA), such as Nafion developed by DuPont, which are very expensive and suffer from low ionic conductivity in alkaline media.

Recently, a research group led by Prof. LI Xianfeng from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) realized pilot-scale synthesis and roll-to-roll manufacturing of hydrocarbon membranes with high-performance in alkaline-based flow batteries.

This work was published in Joule on March 21.

The hydrocarbon membranes for alkaline-based flow batteries with high performance (Image by YUAN Zhizhang)

The researchers realized the kilogram-level synthesis of sulfonated poly(ether-ether-ketone) (SPEEK) polymer and demonstrated the pilot-scale roll-to-roll synthesis of SPEEK membrane and their applications in alkaline-based flow batteries.

They found that the rigid skeleton and dispersive cation exchange groups enabled the high stability of the membrane in alkaline media, and could confine O-containing species (H₂O, OH^-, etc.) inside the membrane, resulting in the formation of continuous hydrogen-bonding networks. This favoured the dissociation of H⁺ in H–O–H (H₂O) and transfer from H₂O to adjacent OH^- ions through a Grotthuss mechanism, thus providing a high OH^- conductivity in SPEEK.

The membrane was integrated in alkaline zinc-iron flow battery stack with power up to 4 kW, which demonstrated a high energy efficiency of 85.5% operated at 80 mA/cm².

"This work illustrated a potential pathway for manufacturing and upscaling of next-generation cost-effective flow batteries based on low-cost hydrocarbon membranes developed in past decades to translate to large scale applications for grid energy storage," said Prof. LI.

This work was supported by the National Natural Science Foundation of China, CAS Engineering Laboratory for Electrochemical Energy Storage, and the Youth Innovation Promotion Association of CAS. (Text by YUAN Zhizhang)