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Associative pyridinium electrolytes for air-tolerant redox flow batteries.

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Carrington, Mark E 
Sokołowski, Kamil 
Zhao, Evan Wenbo 
Graf, Anton M 


Pyridinium electrolytes are promising candidates for flow-battery-based energy storage1-4. However, the mechanisms underlying both their charge-discharge processes and overall cycling stability remain poorly understood. Here we probe the redox behaviour of pyridinium electrolytes under representative flow battery conditions, offering insights into air tolerance of batteries containing these electrolytes while providing a universal physico-chemical descriptor of their reversibility. Leveraging a synthetic library of extended bispyridinium compounds, we track their performance over a wide range of potentials and identify the singlet-triplet free energy gap as a descriptor that successfully predicts the onset of previously unidentified capacity fade mechanisms. Using coupled operando nuclear magnetic resonance and electron paramagnetic resonance spectroscopies5,6, we explain the redox behaviour of these electrolytes and determine the presence of two distinct regimes (narrow and wide energy gaps) of electrochemical performance. In both regimes, we tie capacity fade to the formation of free radical species, and further show that π-dimerization plays a decisive role in suppressing reactivity between these radicals and trace impurities such as dissolved oxygen. Our findings stand in direct contrast to prevailing views surrounding the role of π-dimers in redox flow batteries1,4,7-11 and enable us to efficiently mitigate capacity fade from oxygen even on prolonged (days) exposure to air. These insights pave the way to new electrolyte systems, in which reactivity of reduced species is controlled by their propensity for intra- and intermolecular pairing of free radicals, enabling operation in air.


Acknowledgements: M.E.C. acknowledges support of the HRH The Prince of Wales Commonwealth Scholarship, the Trinity Henry Barlow Scholarship (Honorary) and the Engineering and Physical Sciences Research Council (EPSRC) CDT in Nanoscience and Nanotechnology. K.S. and O.A.S. acknowledge financial support from the EPSRC Programme grants NOtCH (EP/L027151/1) and RaNT (EP/R020965/1) as well as a European Research Council (ERC) grant CAM-RIG (726470). K.S. further acknowledges the European Commission for the Marie-Skłodowska Curie Fellowship ESTIMABLeNANO (706425). E.J. and C.P.G. acknowledge support from the ERC grant BATNMR (835073). We thank T. Földes and E. Rosta for helpful discussions.


40 Engineering, 34 Chemical Sciences, 3406 Physical Chemistry, 7 Affordable and Clean Energy

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Springer Science and Business Media LLC
Engineering and Physical Sciences Research Council (EP/L027151/1)
European Commission Horizon 2020 (H2020) Marie Sk?odowska-Curie actions (706425)
European Research Council (726470)
Engineering and Physical Sciences Research Council (EP/R020965/1)
European Commission Horizon 2020 (H2020) ERC (835073)