Kinetic Properties of Aqueous Organic Redox Flow Battery Anolytes Using the Marcus–Hush Theory

Abstract

An electrochemical analysis strategy based on the Marcus–Hush approximation is presented to analyze the kinetic component of organic redox flow battery (RFB) electrolytes. The procedure was applied to aqueous solutions of methyl viologen (MV) and 2,2′-bipyridyl (diquat, DQ) derivatives as model redox-active electrolytes; although these systems are promising negolyte candidates in organic RFBs, their electrode kinetics continues to be unclear. For compound MV, the voltammetric analysis revealed an adsorption process of electrogenerated species to the glassy carbon electrode surface, so its electron transfer rate constant k s should not be estimated by applying outer sphere electron transfer formulations. For the remaining compounds studied, experimental k s values were obtained and they range from 0.22 to 0.62 cm s–1. Quantum chemical modeling was applied not only to decipher properties of the adsorption process of the MV structure but also to rationalize the kinetic differences in compounds studied through their total and inner reorganization energies. This experimental and theoretical approach allowed elucidation of the kinetic component of compounds studied, revealing that k s values for MV and DQ compound derivatives should not exhibit the reported differences of at least one order of magnitude. Finally, the experimental k s value (0.62 cm s–1) obtained for compound 5,5′-DMDQ is the largest value reported to date in the literature of aqueous organic RFBs, which makes it a strong anolyte candidate.

Concentration Effects on the First Reduction Process of Methyl Viologens and Diquat Redox Flow Battery Electrolytes

“…Before testing an organic RFB, its redox active electrolyte solutions are typically characterized at low analyte concentrations close to 0.001 mol L –1 . , Under these conditions, the electrochemistry of compounds studied (Scheme ), also exemplified by the responses obtained from methyl viologen compound derivatives (Figure ), exhibits a first reversible one-electron transfer process in aqueous media to form radical cations A +• . Based on this experimental evidence, methyl viologen and diquat RFB electrolytes are assumed to operate in line with the mechanism described by eq , upon charging and discharging the device. ,− …”

“…To rationalize how this widely proposed mechanism (eq ) affects the first electron transfer process (eq ) in compounds studied, a strategy based on performing cyclic voltammetry experiments at different initial analyte concentrations was addressed. The responses obtained from diquat solutions were taken as reference signals expected for simple one-electron transfer mechanisms since previous works have shown that electrogenerated diquat radical cations are stable in solution even at high concentrations of C * (0.03 and 0.10 mol L –1 ). , For this compound, reversible and diffusion-controlled one-electron voltammetric waves were detected in a wide range of analyte concentrations prepared (Figure A). The signals exhibited marginal changes in potential values and normalized peak current intensities versus C *, as described by the Randles–Cevcik equation in simple one electron transfer reactions; this result is disclosing the expected high stability of electrogenerated DQ +• species in solution.…”

“…For example, the mechanism of bimolecular degradation in the MV 2+ compound was based on slight changes observed (as a function of the time) in the UV–vis spectra of very dilute ( C * close to 0.0001 mol L –1 ) solutions of chemically generated A +• species in solution. , These authors cite in turn previous electrochemical and UV–vis studies performed of methyl viologens, where dimerization and also dismutation reactions are described, but the latter reactions were not considered to be irreversible at least at low analyte concentrations. − On the other hand, Geraskina et al , concluded that methyl viologen cation radicals interact via π-bonded dimers, but they also claim that the binding constant of this interaction is independent of the nature of each analyzed substituent group. Likewise, we previously demonstrated the high stability of electrogenerated radical cations from diquat compound in acetate buffer and evidenced an adsorption process of N , N ′-dimethyl-4,4′-bipyridinium bis­(hexafluorophosphate) radical cations (MV +• ) at the glassy carbon electrode. Finally, Liu et al , avoided this adsorption ability by using N , N ′-dimethyl-4,4′-bipyridinium diiodide as both negolyte and posolyte electrolytes in a symmetric cell, which suggests that counter ions significantly affect the reduction mechanism of methyl viologen.…”

“…Electron transfer reactions of organic redox-active materials are key processes for controlling the energy conversion and storage in natural and artificial systems. − One recent advancement to storage wind and solar electricity is aqueous organic redox flow batteries (RFBs), where reversible oxidation and reduction reactions at electrodes are fundamental processes to charge and discharge the redox-active electrolytes of the device. − These electrolytes can be composed of a wide range of organic redox-active molecules, so the development of organic RFBs is continuously assisted by a selection process to find the best electroactive species before being tested on a pilot scale. For simplicity, the screening process is typically carried out combining experimental and computational studies of the redox potential of different molecules in solution, − but the mechanism controlling this parameter in the system should be correctly evaluated and calculated so as not to fail in the predictions.…”

“…As RFBs work on electrochemical principles, well-defined and executed electrochemical experiments of their electroactive materials undergoing charge transfer reactions should provide crucial information in designing more efficient systems. To differentiate between adsorptive, first-, and second-order mechanisms; , these experiments should be performed as a function of different analyte concentrations. In the present work, analytic concentration experiments were performed for a series of methyl viologens and diquat structures (Scheme ) in aqueous media, as model-active components of RFB electrolytes.…”