State and Prospects of Unbalanced, Compositionally Symmetric Flow Battery Cycling and Steady-State Amperometry Techniques for Electrolyte Stability Assessment: The Case of Methyl Viologen

Abstract

An increasing number of redox-active substances with improved electrochemical stability is currently being reported in the literature for potential applications in redox flow batteries (RFBs). Consequently, the demand for methods capable of estimating the long-term stability of redox couples is increasing as well. It seems to be of particular importance to understand the limitations of existing stability assessment techniques. In this work, two recently established and highly promising stability assessment techniques are directly compared for the first time to measure the capacity fade rate of a N,N′-dimethyl-4,4′-bipyridinium dichloride (methyl viologen or MV) molecule as a benchmark anolyte substance. These two methods are the unbalanced, compositionally symmetric flow cell cycling (symmetric cycling or UCSFCC) and the steady-state amperometric state of health (ASOH) measurement. The results of the symmetric cycling indicate a 3-fold higher stability for MV compared to a previous literature report as well as a high standard deviation for the measured capacity fade rate. More comprehensive investigations of the technique revealed that the influence of the MV purity, electrolyte leakage, and membrane cross-over could not cause the difference between the measured and earlier reported capacity fades and, thus, indicated insufficiency of the technique’s accuracy for the measurement of highly stable redox species. ASOH as the second technique exhibited similar capacity fade rates like the symmetric cycling as well as comparably high standard deviations. Furthermore, it is prone to temperature fluctuations and variations of the electrode radius. Thus, we demonstrate an approach for a temperature correction in this method, which enables significantly higher accuracy and reliability. Despite this, the variation of the electrode radius remains the main concern for this technique. Based on the obtained results for the MV anolyte, the state and prospects of both techniques for the stability assessment in RFB research are discussed in detail.

Characterization of an Aqueous Flow Battery Utilizing a Hydroxylated Tetracationic Viologen and a Simple Cationic Ferrocene Derivative

“…[32], but also in molecular stability studies using both the unbalanced, compositionally symmetric flow cell and the steady‐state amperometric state of health techniques. [ 33 ] Following the 20th cycle, a second capacity fade slope appears until cycle ≈210 with a fade rate of ≈0.007% cycle −1 (≈0.013 h −1 or ≈0.3% day −1 ). The “first day” effect and the second capacity fade rate are due to time‐dependent membrane–resistance changes resulting from electrolyte uptake and reaction of the redox species within the membrane.…”

“…[ 10,34 ] Indeed, representative values are to be measured afterward from a steady slope, particularly in the week range. [ 33 ]…”

“…[10,34] Indeed, representative values are to be measured afterward from a steady slope, particularly in the week range. [33] These results can be put in perspective by comparing them to similar AOFBs. For instance, the pyrrolidinium-based extended viologen recently reported by Pan et al [35] shows a capacity fade rate of 0.04% cycle À1 over 500 cycles (4.56% day À1 ) when used at 0.2 mol dm À3 in combination with a pyrrolidinium-based TEMPO posolyte.…”

A method for quantifying crossover in redox flow cells through compositionally unbalanced symmetric cell cycling

“…The current was normalized against the initial reading to calculate increases in concentration over time (Figure S2a). 30 The permeability of each species j through the membrane, Pj (m 2 s -1 ), was determined via linear regression (Figure S2b) using Equation ( 14). 6,31 ( )…”

Characterizing the Impact of Oligomerization on Redox Flow Cell Performance

“…2 % per day) observed here are on par or lower than those reported in prior literature, [30,53] although this may be due in part to differences in experimental conditions, electrochemical testbeds, and electrolyte compositions that could influence such measurements. [54] Ultimately, the decay rates do not demonstrate any significant trends that would evince a relationship between common pathways and species identity -that is, it appears that the presence of the oligomeric backbone does not influence the dominant decay pathway under the tested experimental conditions. Therefore, we sought to further characterize the decayed electrolytes via voltammetry to offer complementary information to that provided by cell cycling.…”