Redox flow batteries (RFBs), which decouple the power and energy output of a battery by storing the redox active material outside the battery, are a promising grid scale energy storage technology¹. Various RFBs employing elemental, metallic actives such as Fe-Cr², all-V^3,4, Zn-Ce⁵, V-Ce^6,7 have been the subject of research over the past half century. Recent developments in utilizing water soluble, earth-abundant organic/organometallic actives, some with multiple redox centers, have demonstrated organic RFBs (ORFBs) with reasonably long cycle life and all-V RFB like capacity. Various compounds of ferrocene and methyl viologen have been examined as promising redox actives for this application. Herein, we examine the influence of the electrolyte pH and the salt anion on model redox active organic cations, bis((3-trimethylammonio)propyl)- ferrocene dichloride (BTMAP-Fc) and bis(3- trimethylammonio)propyl viologen tetrachloride (BTMAP-Vi) which have exhibited excellent cycling stability and capacity retention at ≥1.00 M concentration (Beh et. al., ACS Energy Lett. 2017, 2, 639−644). We analyzed the solvation shell around BTMAP-Fc and BTMAP-Vi at acidic and neutral pH with SO₄^2-, Cl^-, CH₃SO₃^- counterions using cyclic voltammetry and revealed their impact on the cation diffusion coefficient and first electron transfer rate constant. The rate constants were found to be much higher than those of common inorganic species and were higher than those of most other organic or organometallic reactants used in RFBs indicating BTMAP-Fc and BTMAP-Vi as promising redox actives. Extending our previous work⁸, the electrochemical Thiele Modulus (h_t), which combines the diffusion coefficient and first electron transfer rate constant, was found to be exponentially correlated to the solvent reorganization energy (λ) calculated from the Born equation in both neutral and acidic pH. Thus, we propose λ as a universal descriptor and selection criteria for organic redox flow battery electrolyte compositions. This descriptor has great advantage of initial, high throughput screening of electrolyte candidates without the need to perform any experiments. Using λ for the specific case of the BTMAP-Fc/BTMAP-Vi ORFB, low pH electrolytes with methanesulfonate or chloride counterions were identified as offering the best balance of transport and kinetic requirements.

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