A Zero-Dimensional Electrochemical Model for Organic Flow Cells

Abstract

We develop and experimentally validate a zero-dimensional model of the electrochemical performance of a flow cell. The model takes into account voltage losses due to Faradaic charge transfer, ohmic and mass transport resistance, as well as the effect of spatial variations in state-of-charge between the cell and electrolyte reservoir. We validate the model by comparing voltage-current data during cycling to equivalent electrochemical measurements from a symmetric cell with organic reactants. With the exception of the mass transfer coefficient, all relevant model parameters are determined independently prior to the experiment using electrochemical impedance spectroscopy and cyclic voltammetry measurements. We find excellent agreement between the model and experiment for constant-current and constant-voltage cycling, across a wide range of current densities (40 – 200 mA/cm2) and volumetric flow rates (~ 10 – 140 mL/min). The relationship between the fitted mass transport coefficient and flow rate conforms to expectations from the literature [1-3], and reasonably approximates the analogous relationship from single-reservoir cell measurements. This work supports the notion that zero-dimensional electrochemical models can yield simple but predictive frameworks for optimizing organic flow battery performance and understanding how reactant decomposition and crossover contribute to capacity fade.