The Role of Viscosity Variations: Analyzing and Modelling Microfluidic Membrane-Free Laminar Flow Batteries

Abstract

Membraneless micro redox flow batteries prevent the advective mixing of catholyte and anolyte by operating at a small Reynolds number with flow rates of order 100µL/min or less [1] [2] [3] [4]. This laminar flow regime allows to remove the ion-exchange membrane used in conventional macro flow batteries. The absence of membrane decreases the internal electric resistance of the reactor, reduces manufacturing costs and potentially increases the performance of the system. However, removing the physical barrier between the electrolytes poses a new challenge: the accurate control of the thin region where both miscible electrolytes enter in contact. Undesired deflections of that mixing region (see Fig. 1) may lead to increased crossover losses and net liquid transfer between the tanks.In this work we propose a numerical CFD model that sheds light on the behaviour of the mixing region at different flow conditions, considering viscosity changes in the electrolytes resulting from variations in the local state of charge during the operation of the cell. Then, a simplified dynamic model that includes electric current, flow rates and electrolyte properties as input parameters is proposed and validated against the CFD simulations. The present work constitutes the first study of membraneless micro redox flow batteries in non-ideal conditions arising from electrolyte heterogeneities. Our computationally efficient model allows to consider qualitative and long-term trends in a matter of minutes to hours, covering broad operation envelopes, which can be used to guide the design process of these emerging systems. Figure 1: Undesired deflection of the mixing region Acknowledgments This work has been partially funded by the Agencia Estatal de Investigación (PID2019-106740RB-I00/AEI/10.13039/501100011033), and by Grant IND2019/AMB-17273 of the Comunidad de Madrid.