Scientists Achieve Impressively High Efficiency in Zn-Ce Flow Batteries

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Sep 23 2022

In an article recently published in the journal ACS Energy Letters, researchers discussed the renaissance of the Zn-Ce flow battery. The utility of dual-membrane configuration to enable exceptionally high efficiency was also illustrated.

Study: The Renaissance of the Zn-Ce Flow Battery: Dual-Membrane Configuration Enables Unprecedentedly High Efficiency. Image Credit: Illus_man/Shutterstock.com

Background

In terms of grid-scale energy storage, redox flow batteries (RFBs) are one of the most competitive contenders. Several redox couples have been explored, but zinc-based and all-vanadium RFBs are the most popular. The high vanadium species cost and low power density limit the application of VRFB. The open-circuit potential (OCP) of zinc-cerium (Zn-Ce) RFBs offers the possibility of increasing the discharge voltage, which is advantageous for high power and energy densities. In Zn-Ce RFBs, where the unresolved crucial obstacle of electrolyte incompatibility led to abnormally low Coulombic efficiency (CE) and subpar cycling performance, the advancement has been slow.

The incompatibility problem can be solved by using an anion exchange membrane (AEM) and assigning anions like SO₄^2-, NO₃^-, and Cl^- as the charge carriers, although these anions are also incompatible with cerium species. Due to the large potential window of Zn-Ce RFBs, hazardous gases like NO_x and chlorine are very likely to be produced at the positive electrode as a result of side reactions between NO₃^- and Cl^-. The energy density is mainly sacrificed due to the poor solubility of Ce³⁺ in the presence of SO₄², even though SO₄^2- is chemically stable in this environment.

About the Study

In this study, the authors discussed the employment of specifically assigned charge carriers, which prevented the infamous H⁺ poisoning on the zinc side and reduced the electrolyte incompatibility by the use of a dual-membrane cell architecture with an ion transpiration hub (iThub). To visualize the ion distribution and clarify the mechanism of the electric field regulation approach, the finite element modeling analysis of theiThub's response characteristics was carried out. The Zn-Ce battery was incredibly efficient and stable because of the system engineering, which provided the cell with a high discharge voltage plateau of 2.3 V at 20 mA cm^-2, high energy efficiency of 71.3% at 60 mA cm^-2, and a record average Coulombic efficiency of 94% throughout cycling.

The team proposed a dual-membrane architecture for Zn-Ce RFBs, where the negolyte and posolyte were separated by the iThub. With 3 M KCH₃SO₃ and a conductive spacer, the iThub had an AEM toward the Ce side and a CEM toward the Zn side. At the positive electrode, Ce³⁺ ions were transformed into Ce⁴⁺ ions during charging, and the freed electrons traveled via the outside circuit. In the interim, electrochemical zinc plating occurred at the negative electrode.

The researchers used K⁺ and CH₃SO₃ to balance charges by crossing the CEM and AEM into the Zn and Ce half-cells of the iThub at the same time. The redox processes were reversed during discharge, which caused the K⁺ and CH₃SO₃ to return to the iThub. In the proposed setup, ions compatible with the electrolytes were given specific charge carriers, and separation was achieved between uncooperative species like Zn/H⁺ and Ce/Cl. To address the dendritic issue, a mildly acidic Zn electrolyte was used. The zinc-based electrolyte's high energy density feature was retained.

Observations

R_u for the three cells was in the order: GF-cell, S-cell, and PM-cell. R_u was decreased by 29.89%, which showed that the GF-cell had a less ohmic loss. The PM-cell and S-cell had R_T values that were 16 and 5 times greater, respectively, than the GF-cell, which indicated that the conductive spacer significantly lowered voltage loss in the iThub. Higher ionic conductivity and more accessible charge carriers were produced in the iThub as a result of increasing the initial KCH₃SO₃ concentration from 1.5 to 3 M, but at the expense of a worsening of ion polarization in the iThub, which resulted in greater cell polarization.

The deep charging duration was increased to attain 75% state-of-charge (SOC) at 30 mA cm^-2. While the single-membrane Zn-Ce cell demonstrated a CE of 72.61%, which indicated the efficiency of the dual-membrane arrangement in electrolyte dissociation, both the GFcell FT and GF-cell obtained CE greater than 95.5%, similar to that of shallow charging. At 70% SOC, the GF-polarization cell's curve was recorded, and it showed a peak power density of 360.4 mW cm^-2.

Due to the ongoing transfer of H⁺ from the Ce to the Zn half-cell, which initiated hydrogen evolution reaction (HER), the CE of the single-membrane cell was significantly lower than the CE of the GF-cell and decreased to 87.33% in the subsequent cycle. The average CE and energy efficiency (EE) of the proposed dual-membrane Zn-Ce cell were 94% and 83%, respectively.

Conclusions

In conclusion, this study discussed the development of a dual-membrane Zn-Ce RFB. The proposed cell exhibited a steady and repeatable cycling performance, reviving the potential of Zn-Ce RFBs as prospective high-power-density rechargeable batteries. Additionally, the ion transport chamber's response characteristics were clarified.

The authors mentioned that the theory that conductive materials could help the local electric field by reducing cell polarization could be applied to other systems with numerous membranes or ionic conductors.

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References

Xie, X., Mushtaq, F., Wang, Q., et al. The Renaissance of the Zn-Ce Flow Battery: Dual-Membrane Configuration Enables Unprecedentedly High Efficiency. ACS Energy Letters, 7, 3484-3491 (2022). https://pubs.acs.org/doi/10.1021/acsenergylett.2c01646

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