Toward Improved Catholyte Materials for Redox Flow Batteries: What Controls Chemical Stability of Persistent Radical Cations?

Abstract

Catholyte materials are used to store positive charge in energized fluids circulating through redox flow batteries (RFBs) for electric grid and vehicle applications. Energy-rich radical cations (RCs) are being considered for use as catholyte materials, but to be practically relevant, these RCs (that are typically unstable, reactive species) need to have long lifetimes in liquid electrolytes under the ambient conditions. Only few families of such energetic RCs possess stabilities that are suitable for their use in RFBs; currently, the derivatives of 1,4-dialkoxybenzene look the most promising. In this study, we examine factors that define the chemical and electrochemical stabilities for RCs in this family. To this end, we used rigid bisannulated molecules that by design avoid the two main degradation pathways for such RCs, viz., their deprotonation and radical addition. The decay of the resulting RCs are due to the single remaining reaction: O-dealkylation. We establish the mechanism for this reaction and examine factors controlling its rate. In particular, we demonstrate that this reaction is initiated by the nucleophile attack of the counteranion on the RC partner. The reaction proceeds through the formation of the aroxyl radicals whose secondary reactions yield the corresponding quinones. The O-dealkylation accelerates considerably when the corresponding quinone has poor solubility in the electrolyte, and the rate depends strongly on the solvent polarity. Our mechanistic insights suggest new ways of improving the RC catholytes through molecular engineering and electrolyte optimization.

A Method for Evaluating Soluble Redox Couple Stability Using Microelectrode Voltammetry

“…Then, using the ferrocene/ferrocenium redox couples as a model system, we assess the relative magnitude and impact of different sources of error, including the measurement changing the bulk concentration, unequal diffusion coefficients, electrode stability, and temperature variations from charging the solution. Finally, we apply this protocol is to a model decay compound, 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB), 11,18,30,39 to evaluate decay rates across a range of concentration, finding general agreement with prior literature.…”

“…Confident in the insight this technique provides and its systemic sources of error explored using the model ferrocene/ferrocenium redox couple, we next study DBBB which, based on previous studies, is expected to undergo moderate decay. 11,39 For the first test, 5 mM of DBBB was used, the same concentration as the ferrocene experiments (Figure 4). At 5 mM, the charged species decays to about 60 % of the initial value, but the total species only decays about 5 %, indicating that the majority of the charged DBBB decays to the neutral (reduced) species (DBBB).…”

“…11,22 Radical cations of dialkoxybenzenes decompose through multiple mechanisms, including dimerization, O-dealkylation via nucleophilic attack, and self-discharge through interactions with the solvent. 11,22,39 To our knowledge, the decay of DBBB has not been extensively studied experimentally; however, based on studies with other, structurally similar dialkoxybenzenes, self-discharge and O-dealkylation are expected to be the primary modes of decomposition; dimerization is not expected to play a significant role in the decay of DBBB as the t-butyl groups sterically hinder this pathway. 39 The presence of multiple decay pathways implies that the true observed rate law is likely more complex than those directly explored in this work; indeed, the overall decomposition order of a different dialkoxybenzene changes as a function of conversion.…”

“…21 An alternative approach, which is sometimes coupled with cell cycling, is interrogation of a particular reporter constituent by spectroscopy techniques to not only monitor decay rates but also elucidate chemical information relevant to mechanisms of decomposition. Examples of such techniques include electron paramagnetic resonance (EPR) to track the decomposition kinetics of radical species, [22][23][24] nuclear magnetic resonance (NMR) to identify decay products and monitor decomposition kinetics, 6,10,19,25 and UV-Vis spectroscopy to probe the chemical stability of charged species. 14,26 While potentially powerful, these analytical methods are often limited by their compatibility with complex multicomponent electrolyte solutions and, consequently, specialty chemicals (e.g., radiolabeling) and preparatory steps (e.g., dilutions, separations, etc.)…”

Stability of molecular radicals in organic non-aqueous redox flow batteries: A mini review

“…Substituted dialkoxybenzenes such as DBBB [39,41], DBMMB [19,36,40] or other derivatives [43][44][45] have been employed as lithium-ion redox shuttles and NA RFB catholytes because their radical cations are stable due to delocalisation, electron-donating substituents and steric protection [18]. While DBBB displays poor solubility (~0.4 M in PC) [46,47], DBMMB is a highly miscible liquid at room temperature [19,43].…”