Quantifying the impact of viscosity on mass-transfer coefficients in redox flow batteries

Here, a model redox-active electrolyte (RAE) is fully characterized in terms of its transport properties, and subsequent flow cell polarization experiments enable extraction of mass-transfer coefficients. Specifically, experimental manipulation of flow rate and electrolyte viscosity are coupled with a 1-D polarization model in a flow cell to quantify the mass-transfer coefficients as a function of these material and operating parameters. Both flow-through and interdigitated flow fields are used to develop dimensionless correlations that describe mass transfer rates as a function of RAE properties. Experimental results and fitted model parameters illustrate and quantify the changes in limiting current and mass-transfer coefficient as a function of electrolyte velocity and viscosity. The resulting power-law correlations for the Sherwood (Sh) number, in terms of the Peclet (Pe) and Schmidt (Sc) numbers, are Sh = 0.0040Pe(0.75)Sc(-0.24) and Sh = 0.018Pe(0.68)Sc(-0.18) for the flow-through and inter digitated flow fields, respectively. These correlations provide quantitative estimates of mass-transfer coefficients within high-performance flow cell architectures as a function of geometry and RAE properties, enabling front-end screening in future RAE development campaigns, as well as performance benchmarking for potential redox flow batteries (RFBs).