A Zero-Dimensional Model for Electrochemical Behavior and Capacity Retention in Organic Flow Cells

Abstract

Comprehensively understanding the behavior of redox-active compounds in organic flow cells is essential to developing low-cost and long service life flow batteries. We develop and validate a zero-dimensional model of the electrochemical performance of an organic flow cell. The model simulates voltage losses from Faradaic charge transfer, Ohmic resistance, and mass transfer, along with the influence of spatial variations in the electrolyte’s state-of-charge between the cell and electrolyte reservoir, on the cell’s cycling behavior. The model’s predictions agree with constant current and constant voltage cycling data for a symmetric ferro-/ferricyanide cell across a wide range of current densities and electrolyte flow rates. We determine the model’s voltage loss parameters from electrochemical impedance spectroscopy and voltammetry measurements acquired prior to cycling, rather than fitted a posteriori. In operando measurements of the electrolyte’s state-of-charge demonstrate that the finite time for electrolyte flow between its reservoir and the electrochemical cell may significantly affect voltage-current behavior. By modelling active reactant decay, we demonstrate how capacity fade measured in a cell depends on the cycling protocol and reactant decay mechanism. This work shows that zero-dimensional electrochemical modeling helps in elucidating capacity fade mechanisms and optimizing the performance of chemistries under consideration for practical organic flow batteries.