Title: Static Cell and Porous Electrode Model for Cycling Behavior of Aqueous Organic Redox Active Materials

Abstract

Aqueous Organic Redox Flow Batteries (AORBs) have shown growing potential as a stationary energy storage technology for regulating intermittent renewable sources. For long charge/discharge duration systems, the capital cost of a redox flow battery asymptotically approaches the cost of the electroactive material.[i] Therefore, for commercialization of this technology, proper evaluation of the redox active electrolytes is needed. The capacity retention over time and over cycling is one of the most vital criteria for evaluation. For very stable electrolytes with fade rates less than about 10% per year, disentangling cycle-denominated and time-denominated fade can be very difficult for flow batteries due to longer cycle time and noise due to pumping and splashing. Thus, we have used a sealed, static, non-flow cell to achieve capacity measurements with greater cycling frequency and reduced noise, as shown in Figure 1. To isolate decomposition from crossover through the membrane or apparent capacity fade due to changes in the resistance of the cell, we use volumetrically unbalanced, compositionally symmetric cells with potentiostatic cycling.[iii] For proper capacity fade rate evaluation, material properties of the electrode and electrolyte should be considered when setting up cycling protocols because the amount of accessed charge depends on these properties. Thus, simulations from porous electrode models that consider these material properties can inform appropriate cycling conditions.Using Newman’s porous electrode theory[iv] to model our static cell, we have simulated the spatial distribution of ions and potentials over time in response to time-varying applied potentials. From the transient behavior, we simulated the potentiostatic cycling of these cells. The simulations show how the physics inside porous electrodes and cycling performance depend on material properties. For example, the diffusivities can affect the spatial distributions of ions across a porous electrode, as shown in Figure 2 (left), as well as the minimum acceptable time for cycling and the accessed capacity for given cycling parameters, as shown in Figure 2 (right). Using the same porous electrode model for simulations, we aim to show differences between apparent and real capacity fade. [i] F. R. Brushett, M. J. Aziz, and K. E. Rodby. ACS Energy Lett. 2020, 5, 879−884 [ii] D. G. Kwabi, Y. Ji, and M. J. Aziz. C hem. Rev. 2020, 120, 14, 6467–6489 [iii] M.A. Goulet and M. J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”. Journal of The Electrochemical Society, 165 (7) A1466-A1477 (2018) [iv] J. Newman and C. W. Tobias, J. Electrochem. Soc., 1962, 109, 1183 Figure 1