A Method for Evaluating Soluble Redox Couple Stability Using Microelectrode Voltammetry

Abstract

Soluble, redox-active, organic materials hold promise as charge-storage species for flow batteries; however, their stability during extended operation remains a key challenge. While a number of spectroscopic and electrochemical techniques are currently used to probe these complex and often ill-defined decay pathways, each technique has limitations, including accessibility and direct evaluation of practical electrolytes without preparatory steps. Here, we use microelectrode voltammetry to directly observe nonaqueous flow battery electrolytes, simultaneously identifying the rate of charged materials decay (reversible material loss) and total material decay (irreversible material loss). We validate this technique using ferrocene as a stable model redox couple, examine and address sources of error, and finally, demonstrate its capability by assessing the decay of a well-studied and moderately-stable substituted dialkoxybenzene [2,5-di-tert-butyl-1,4-bis(2methoxyethoxy)benzene]. These results suggest that microelectrodes may have utility for rapid assessment of redox electrolyte state-of-charge and state-of-health, both in-operando and postmortem.

Microelectrode-Based Sensor for Measuring Operando Active Species Concentrations in Redox Flow Cells

“…With the more traditional reference electrode (Figure 3b), we only observe a vertical shift in the currents due to changing bulk concentrations which give rise to variable steady-state transport rates. 37 Under quiescent conditions, the plateau currents are related to the bulk concentration and the diffusion coefficient of the reacting species according to Eq. ( 15).…”

A Nonaqueous Redox-Matched Flow Battery with Charge Storage in Insoluble Polymer Beads

“…The difference between the current at the oxidative plateau and the current at the reductive plateau in Figure 2b is proportional to the total concentration of FcR and FcR + present, with the absolute value of the anodic current proportional to the FcR concentration and the absolute value of the cathodic current proportional to the FcR + concentration. 32 The direct proportionality of the anodic and cathodic currents to the concentrations of FcR and FcR + , respectively, holds only if FcR and FcR + have similar diffusion coefficients in the solvent/electrolyte system examined; the equivalence of the anodic and cathodic currents in the CV of a 1:1 solution of FcR and FcR + in Figure 2b confirms this to be the case. That is also true for Bn-bpy-Me 2+ and Bn-bpy-Me + , as indicated by the CVs in Figure 2d.…”

Using voltammetry augmented with physics-based modeling and Bayesian hypothesis testing to identify analytes in electrolyte solutions

“…Voltammetry is a foundational technique in electrochemical science that enables both qualitative and quantitative characterization of electroactive species-such as analytes (i.e., redoxactive compounds)-for a variety of applications. [1][2][3][4][5][6][7][8][9][10] Examples include tracking the transient behavior of solutions in electrochemical systems 8,[11][12][13] and labeling compounds within a sample. 14-species), along with the electrode surface morphology, are known prior to and remain constant throughout the experiment, established fundamental relationships can often be leveraged to discern both physical and electrochemical properties in the system of interest, 3,5,7,18 enabling mechanistic insights into electrochemical and chemical reactions of electroactive compounds.…”