(5k) Coupling Anion- and Cation-Exchange Membranes for Redox Flow Batteries With Mixed Ion Charges

Coupling Anion- and Cation-Exchange Membranes for Redox Flow Batteries with Mixed Ion Charges

Shuang Gu*, Ke Gong, Emily Z Yan and Yushan Yan*

Department of Chemical & Biomolecular Engineering, University of Delaware

5 Innovation Way, Newark, DE 19716, USA

*shgu@udel.edu and *yanys@udel.edu

Electrochemical rechargeable batteries have shown remarkable improvements in power and energy density in recent decades and are finding increasingly broader applications in consumer electronics, electric vehicles, and electrical grid systems^[1]. Their basic cell structure however has remained the same since their first discovery in 1859^[2]: two redox pairs residing in the solid electrodes separated by a liquid electrolyte. In 1974, the first redox flow battery (RFB)^[3] (i.e., chromium-iron RFB) was invented by introducing one ion-exchange membrane (IEM) into the cell. This IEM isolates negative electrode redox pair (negative pair) from positive electrode redox pair (positive pair) even when all of the four redox ions are freely dissolved in electrolytes; and at the same time it allows the passage of non-electroactive balancing ions. The liberation of redox pairs from the solid electrodes enables the transfer of energy storage function from electrodes to liquid electrolytes in external tanks, and this decoupling of energy storage from power delivery provides RFBs with unprecedented design flexibility and scalability. Significant efforts and progresses have been made in developing efficient and economical RFB systems in the last two decades mostly motivated by the needs of smart grid energy storage.

In the original chromium-iron RFB design, one anion-exchange membrane (AEM) was used to isolate the cation/cation negative pair (i.e.,Cr³⁺/Cr²⁺) from the cation/cation positive pair (Fe³⁺/Fe²⁺) to prevent self-discharge while allowing the conduction of the non-electroactive anions (i.e., Cl⁻ in HCl solution) to balance the electrode charges. Similarly, one cation-exchange membrane (CEM) can work for the cell with an anion/anion negative pair and an anion/anion positive pair. However, the single-IEM configuration is not able to effectively isolate redox pairs that have mixed ion charges, because neither one single AEM nor one single CEM can effectively prevent the mixing of electroactive redox ions, resulting in permanent coulombic efficiency loss. Beside low coulombic efficiency, the crossovered redox ions often interfere with the opposite electrode reaction, leading to voltage efficiency loss. As a result, the energy efficiency of those RFBs is significantly limited.

Here we present a universal design by using couple- or triple-IEMs in one cell that can accommodate any combination of ion charges for both the negative and the positive pairs. Composed of one AEM, one CEM, and a middle electrolyte in between, the couple-IEM cell provides a solution for redox pairs of all combinations of ion charges except two hybrid pairs (i.e., an anion-cation pair vs. an anion-cation pair). For two hybrid pairs, a triple-IEM cell with three membranes (CEM/AEM/CEM or AEM/CEM/AEM) and two middle electrolytes is needed. The couple- and triple-IEM designs bring unprecedented freedom in choosing redox pairs and electrolytes. Of particular importance, ultra-high cell voltage can be achieved in aqueous RFBs, e.g., 3.08 V (standard voltage hereinafter) of a zinc-cerium RFB [denoted as Zn(OH)₄²⁻/Zn||Ce₂O⁶⁺/Ce³⁺, where Zn(OH)₄²⁻/Zn and Ce₂O⁶⁺/Ce³⁺ are negative and positive pairs, respectively, and the double vertical lines are for the CEM and AEM couple]. The multi-IEM approach may also have implications in primary and other secondary battery designs.

References

[1]          Z. G. Yang, J. L. Zhang, M. C. W. Kintner-Meyer, X. C. Lu, D. W. Choi, J. P. Lemmon, J. Liu, Chem Rev 2011, 111, 3577-3613; B. Dunn, H. Kamath, J. M. Tarascon, Science 2011, 334, 928-935.

[2]          G. Planté, Comptes Rendus de l’Académie des Sciences 1860, 640-642.

[3]          L. H. Thaller, The 9th Intersociety Energy Conversion Engineering Conference Proceedings 1974, 924-928.

Key words: Coupling, Ion exchange membranes, Mixed ion charges, Redox flow batteries, Energy storage.