4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl as a model organic redox active compound for nonaqueous flow batteries

Abstract

Nonaqueous redox flow batteries (NAqRFB) that utilize redox active organic molecules are an emerging energy storage concept with the possibility of meeting grid storage requirements.Sporadic and uneven advances in molecular discovery and development, however, have stymied efforts to quantify the performance characteristics of nonaqueous redox electrolytes and flow cells. A need exists for archetypal redox couples, with well-defined electrochemical properties, high solubility in relevant electrolytes, and broad availability, to serve as probe molecules. This work investigates the 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl (AcNH-TEMPO) redox pair for such an application. We report the physicochemical and electrochemical properties of the reduced and oxidized compounds at dilute concentrations for electroanalysis, as well as moderate-to-high concentrations for RFB applications. Changes in conductivity, viscosity, and UV-Vis absorbance as a function of state-of-charge are quantified. Cyclic voltammetry is performed to investigate the redox potential, reversibility, and diffusion coefficients of dilute solutions, while symmetric flow cell cycling is employed to determine the stability of the AcNH-TEMPO redox pair over long experiment times. Finally, single electrolyte flow cell studies are performed to demonstrate the utility of this redox couple as a platform chemistry for benchmarking NAqRFB performance.

Towards Low Resistance Nonaqueous Redox Flow Batteries

“…27,41 All electrolyte conductivity measurements were performed in triplicate. Separator conductivity measurements were also performed using a two-electrode, spring-loaded conductivity cell as reported in literature.…”

“…[20][21][22][23][24][25][26] Even fewer studies have incorporated advanced flow cell designs to minimize area specific resistance (ASR) and increase area specific power density. 21,[27][28][29] At present, reported NAqRFBs exhibit inadequate performance and durability, 8 but identifying performance limiting factors can be difficult because shortcomings could be attributed to active species degradation (e.g., instability, insolubility, chemical incompatibility), insufficient separator selectivity (e.g., crossover, low conductivity), or poor cell design (e.g., slow mass transfer, resistive electrodes, high pressure drop). 27 Due to the plethora and complexity of engineering challenges facing NAqRFBs, recent reports identify a range of performance-limiting factors, including active species stability, 8 electrode catalysts, 10 separator ASR, 30 or mass transfer rates.…”

“…27 Due to the plethora and complexity of engineering challenges facing NAqRFBs, recent reports identify a range of performance-limiting factors, including active species stability, 8 electrode catalysts, 10 separator ASR, 30 or mass transfer rates. 27 Advancing NAqRFBs to a technology-readiness level competitive with aqueous redox flow batteries (AqRFBs) will require systematic studies of the performance limitations facing NAqRFBs, in concert with the ongoing, and more serendipitous, molecular discovery activities. Significant differences in the physicochemical and electrochemical properties of aqueous and nonaqueous electrolytes lead to uncertainty in how to effectively design flow cells for NAqRFBs.…”

“…7,8,27 AqRFB development has recently benefited from a series of studies implementing a single electrolyte diagnostic flow cell technique 31 to systematically evaluate cell designs 32 and to better elucidate cell-level performance limitations relating to ohmic, charge transfer, and mass transfer losses. 27,[33][34][35] A schematic of this single electrolyte technique is provided in Figure 1a, where a flow cell is connected to a single electrolyte reservoir at 50% state-of-charge (SOC). The electrolyte passes through the positive side of the cell, where the active species are oxidized, and then loops back through the negative side of the cell, where the charged species are reduced.…”

Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods

“…species. 15,16 In the present study, we further develop the method with experimental variations that provide more specific information about possible causes of capacity fade.…”