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Mild pH-decoupling aqueous flow battery with practical pH recovery

Nature Energy volume 9, pages 479–490 (2024)Cite this article

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Abstract

Establishing a pH difference between the two electrolytes (pH decoupling) of an aqueous redox flow battery (ARFB) enables cell voltages exceeding the 1.23 V thermodynamic water-splitting window, but acid–base crossover penalizes efficiency and lifetime. Here we employ mildly acidic and mildly alkaline electrolytes to mitigate crossover, achieving high round-trip energy efficiency with open circuit voltage >1.7 V. We implemented an acid–base regeneration system to periodically restore electrolytes to their initial pH values. The combined system exhibited capacity fade rate <0.07% per day, round-trip energy efficiency >85% and approximately 99% Coulombic efficiency during stable operation for over a week. Cost analysis shows that the tolerance of acid–base crossover could be increased if the pH-decoupling ARFB achieved a higher voltage output and lower resistance. This work demonstrates principles for improving lifespan, rate capability and energy efficiency in high-voltage pH-decoupling ARFBs and pH recovery concepts applicable for pH-decoupling systems.

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Fig. 1: Cell structure and charge carrier migration.
Fig. 2: Proton/hydroxide crossover flux through AEM/CEM under applied electric field.
Fig. 3: Long-term performance of high-voltage mild pH-decoupling battery.
Fig. 4: Performance of the BPM sub-cell and long-term ARFB cycling with pH recovery.
Fig. 5: Polarization performance of the system and comparison of pH-decoupling systems.
Fig. 6: pH-decoupling ARFBs with pH recovery.

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Data availability

The datasets analysed and generated during the current study are included in the paper, its Supplementary Information and uploaded to Figshare at https://doi.org/10.6084/m9.figshare.23816796.

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Acknowledgements

This research was supported by US Department of Energy (DOE) award DE-AC05-76RL01830 through Pacific Northwest National Laboratory (PNNL) subcontract 428977, by Harvard Climate Solution Change Funding, by the Massachusetts Clean Energy Technology Center and by the Harvard School of Engineering and Applied Sciences. A.M.A. acknowledges the Materials Science and Engineering department at King Fahd University of Petroleum and Minerals and the Ministry of Education of Saudi Arabia for doctoral scholarship. L.C.I.F acknowledges financial support from São Paulo Research Foundation (FAPESP) under the grant numbers: 2019/21089-6, 2021/14537-2. T.Y.G. acknowledges funding support from the NSF Graduate Research Fellowship Program. We thank Y. Jing, K. Amini, E. Fell and J. Sosa for helpful discussions. We thank J. MacArthur from Harvard Electronic Instrument Design Lab for developing multi-channel pH sensors. We thank M. P. Marshak from University of Colorado Boulder for providing CrPDTA sample.

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Author notes

  1. These authors contributed equally: Dawei Xi, Abdulrahman M. Alfaraidi.

Authors and Affiliations

  1. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA

    Dawei Xi, Abdulrahman M. Alfaraidi, Thomas Cochard, Luana C. I. Faria, Zheng Yang, Thomas Y. George, Taobo Wang, Roy G. Gordon & Michael J. Aziz

  2. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA

    Jinxu Gao, Roy G. Gordon & Richard Y. Liu

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Contributions

D.X. and A.M.A. designed and conducted cell tests and electrochemical experiments. D.X. developed the concept of mild pH-decoupling ARFB and acid–base recovery. A.M.A. worked on molecule synthesis and characterization. D.X. and J.G. worked on the posolyte. D.X. and T.C. worked on the design of centre chambers. A.M.A., L.C.I.F., T.Y.G. and T.W. worked on crossover characterization. D.X. and Z.Y. worked on the single-membrane pH-decoupling cell. D.X. did the techno-economic calculation. R.Y.L. and R.G.G. supervised the molecular synthesis and characterization. M.J.A. supervised the project. D.X., A.M.A. and M.J.A. drafted the manuscript. All authors edited the manuscript.

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Correspondence to Roy G. Gordon, Richard Y. Liu or Michael J. Aziz.

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Nature Energy thanks Qilei Song and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figs. 1–46 and Table 1.

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Xi, D., Alfaraidi, A.M., Gao, J. et al. Mild pH-decoupling aqueous flow battery with practical pH recovery. Nat Energy 9, 479–490 (2024). https://doi.org/10.1038/s41560-024-01474-1

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